1. business and industrial
2. energy
3. oil

Master Class in Humidification
Why Humidify
Roger Palamarczuk
Chairman – Humidity Group
General Manager - Vapac Humidity Control
Master Class Why Humidify - Jul 2004.doc
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Index
Page
Regulatory introduction
Laboratory
Pharmaceutical
Printing
Textile
Mainframe computers
Organisation recommendation
British Council of Offices
Health and Safety Executive
CIBSE Design Guide
BS EN7730
Apparent Temperature Data
ASHRAE Standard
Verein Deutscher Ingenieure
Commercial Considerations
4
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Humidification and its application within the air-conditioned building
Why Humidify
Hygroscopic Material
Comfort
Health
Contact Lenses
Static Electricity
Psychrometric Process
Components of the chart
Single process function
Multiple process function
Isothermal Humidification
Adiabatic Humidification
Psychrometric load calculation
Power using steam (Isothermal)
Power using water (Adiabatic)
Energy Comparison
Humidifier types
Electrode Boiler (Atmospheric)
Resistance Heater (Atmospheric)
Gas fired
(Atmospheric)
Gas fired (Pressurised)
Steam distribution
Single pipes
Absorption distance
Pressurised pipes
Multipipe
Adiabatic
Spray
Ultrasonic
Spinning disk
Wetted media
Control
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Mechanical
Electronic
Psychrometer
Location
Calibration
Energy Usage
Comparative run costs
Water quality
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Regulations
There is a woolly area surrounding the regulations and recommendations of the operating
conditions for humidification leading to a ‘Comfort Controlled Environment’ we need to
clarify the difference between Regulation and Recommendation as a means of
proportioning ownership.
The Oxford English Dictionary states that to regulate means “Control by rule”, “Subject to
restriction”, “Adapt to requirements”. Where as to Recommend means
“Suggest as fit
for some purpose”, “Advise as a course of action”, “Make acceptable or desirable”
To ensure a regulation is adhered to implies a policing mechanism with a penalty system
for non-conformance. There is no such mechanism in place to cover the humidification
requirements of people in their own right. Codes of Best Practice are a series of
recommendations that will result, if followed, with an environmental condition that is
perceived to please the occupant and the building owner / operator, thereby increasing
productivity.
There are further recommendations covering indoor air quality, specifically ventilation,
thermal control and occupancy comfort that do make reference to Humidity control. There
is a very important link between temperature and humidity that should not be ignored. As
we move to the application and integration of humidity into air conditioning systems the link
will be reinforced.
The recommendations in the public domain cover a wide range from ‘Not required’ to ‘Must
have some humidity control’. There are industrial or process requirements that people will
benefit from if they work in that environment. Let us look at some of the process
regulations that exist and the way in which the conditions are defined.
•
•
•
•
•
Process Applications: Examples
Laboratory’s using Animals
40% to 60% RH
Pharmaceutical Process
35% to 50% RH
Printing areas
46% to 51% RH
Textile manufacturing
50% to 80% RH
Main frame computer
45% to 55% RH
Laboratory’s using Animals:
40% to 60% RH
Present regulations regarding humidification consider the welfare of animals before human
requirements. Using the care of rats, cats and rabbits for laboratory uses, as an example,
we find that these animals should be kept at humidity levels between 40% and 60% . This
is noted in the CIBSE Guide “Installation and Equipment Data”. In the case of rats this is
perfectly reasonable when you consider that at humidity levels below 40% they can
develop a disease which causes their tails to drop off leading to death.
Pharmaceutical Process Areas:
35% to 50% RH
Powder before and during manufacture of tables should be kept at 30 to 35% RH. Tablet
compression, milling, coating and packaging at 35% RH. Gelatine capsules made and
stored at 35%. Effervescent tablets and powders at 20% RH. Microanalysis and serums
maintained at 50% RH.
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Printing areas:
46% to 51% RH
Pressrooms and stock rooms are maintained at conditions between 46 to 51% RH. This
ensures clarity of multi-colour processes by ensuring the material maintains dimension as
it travels through each process.
Textile Manufacturing:
50% to 80% RH
These are many and varied; wool for instance requires higher humidity’s up to and
including spinning, than the processes that follow. Mixing and blending, combing, carding
drawing and spinning should be maintained between 60 to 75%. Winding, warping and
weaving need to be maintained between 50 to 60% RH. Cotton processes require
humidity’s between 50 to 70% where as man made material requires 50 to 65% RH. The
process would depend on the relative humidity that is required to ensure the material
maintains its workability.
Mainframe Computer Rooms:
45% to 55% RH
To maintain the integrity of the circuitry the various manufacturers have environmental
operational bands between 20 to 24oC and 45 to 55% RH. This will prevent static and
drying at low humidity and gold scavenging or arching at high. There also could be slight
variations dependent on the air circulation characteristics of the room design.
Humans are adaptable and tolerant
• Environmental levels for comfort within a working space.
• 19oC to 23oC Dry Bulb
• 45% to 55% Relative Humidity
Humans are very adaptable, especially since we lost our tails, and for our own comfort the
environment does not have to be specifically controlled to very fine tolerances. The
general understanding of good design practice criteria for the comfort and well being of
building occupancy is between 19 to 23oC Dry Bulb and 45 to 55% relative humidity.
Let us look at the various Recommendations, Codes of Practice and Design guides
that are in existence today.
Organisation Recommendations
Organisation
• British Council of Offices
• Health & Safety Executive
• CIBSE
• BS EN 7730
• ASHREA
• VDI 6022
Recommendation
None / Some
Maintain comfort
40% to 70% RH
40% to 70% RH
30% to 60% RH
30% to 65% RH
British Council of Offices (BCO) Guide 2000: Best practice in the specification for
offices.
This organisation states: “ Humification control should not be installed in the base office
scheme” and adds “ Humidification is rarely needed for general office use but space
should be allowed for a steam-based system to be added at a later date”
Somewhat of a contradiction within it self and is also contradicted by other organisations
involved in the design of environments and the operation of equipment within those areas.
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The Health and Safety Executive, ‘Display Screen Equipment Regulations 1992’ obliges
employers to maintain a level of humidity that prevents staff from suffering from general
discomfort and sore eyes when working for long periods at computer terminals.
Very few office workspaces do not have visual display equipment and most operators are
staring at it all day.
CIBSE Design Guide Volume A: Design Data Section A1-9 States “The nature of the role
played by relative humidity in the environment is less well-defined than that for
temperature. At higher air temperatures, when sweating occurs, the thermal balance is
dependent on evaporation and so relative humidity becomes a critical factor. There is a
trend for lower humidity’s being preferred at higher temperatures. However for most
applications, relative humidity should be between 40% and 70%.
The CIBSE Guide H, Building Control Systems, section 1.4 also provides a general
statement regarding the indoor environment, making reference to minimum winter heating
temperature of 19oC and the need for control of temperature and humidity for good indoor
air quality. Section 5.5.4 also covers the control recommendations to ensure the system
functions correctly. Return air sensor as well as high limit humidistat in the supply air is the
recommendation.
European Standard BS EN 7730 deals with the factors contributing to feelings of thermal
comfort and their incorporation into Predicted Mean Vote index within moderate thermal
environments.
This is a method of predicting by calculation, the degree of comfort or discomfort a number
of occupants within the space will potentially regard as acceptable. The Predicted Mean
Vote index takes into account the physical activity and clothing warn by the occupancy, air
temperature, mean radiant temperature, air velocity, and relative humidity. Using a
thermal sensation scale of –3 (Cold) to +3 (Hot) a Predicted Percentage Dissatisfied (PPD)
value can be obtained. Individual votes are scattered around this mean value and is used
to predict the number of people likely to feel uncomfortably warm or cold. A value of 0
indicates the potential for majority satisfied customers. The Predicted Percentage of
Dissatisfaction establishes a quantitative prediction of the number of thermally dissatisfied
customers.
European Standard BS EN ISO 7730:1995
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If the PMV is 0, bottom of the curve, which are the correct working conditions for this scale
we will still have a 5% dissatisfaction level. A PMV of +. 5, which can be described as very
slightly warm there will be a 10% PPD level. This goes to prove you cannot satisfy all the
people all the time.
All tests were undertaken at 50% RH and the standard nominates a Relative Humidity
band of 40 to 70% for an air conditioned office and 30 to 70 for unconditioned premises.
Apparent Temperature Data
The National Oceanic and Atmosphere Administration, Environmental Data and
Information Service and National Climate Change Centre (USA) has published a table that
relates a range of dry bulb temperatures with a range of relative humidity’s and assigns an
apparent temperature the occupant would feel. This highlights the relationship between
temperature and humidity. Take the typical design statement of 21oC DB and 50% RH, the
apparent temperature would be 2 degrees lower at 19oC. Not wishing to cause offence,
but in general this would be OK for a man and to cold for a woman assuming similar layers
of clothing.
Apparent Temperature Data
Source National Oceanic and Atmosphere Administration, Environmental Data and Information Service
and National Climate Change Centre (USA)
Room °C
24
23
22
21
20
19
18
17
16
20
22
20
19
18
17
16
16
14
13
30
22
21
19
18
18
17
16
14
14
40
23
22
20
19
18
17
16
15
14
% Relative Humidity
50
60
70
24
24
24
22
23
23
21
22
22
19
20
21
19
19
20
18
18
19
17
17
18
16
16
16
14
15
15
80
25
24
23
21
21
19
18
17
16
90
26
24
23
22
21
19
18
17
16
100
26
25
24
22
22
20
19
17
16
No change between actual and apparent temperature
-2
-1
o
2 C lower than actual room temperature
o
1 C lower than actual room temperature
1
2
o
1 C higher than actual room temperature
o
2 C higher than actual room temperature
ASHRAE Standard 62-1989, Section 5.11 states “ High humidity’s can support the growth
of pathogenic or allergenic organisms. Examples include certain species of fungi,
associated mycotoxins and dust mites….Relative humidity in habitable spaces preferably
should be maintained between 30 and 60% to minimise growth of allergenic and
pathogenic organisms
“While the primary function of a building control system has been the control of
temperature and humidity, the increased awareness of Sick Building Syndrome
(SBS) and other building related illnesses has emphasised the requirement to
ensure good indoor air quality. The demands of energy conservation and healthy
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ventilation are sometimes in conflict, necessitating better attention to the control of
ventilation to ensure a satisfactory compromise. More attention is being given to the
quality as well as the quantity of ventilation.”
The guide goes on to say in section 8.2.4
“Where full air conditioning is employed the required range of air temperature and
humidity is specified. The interaction of air temperature, relative humidity and
moisture content must be remembered. In general, the moisture content of the air in
a building is less subject to variation than the air temperature. Changes in air
temperature at constant moisture content may cause the relative humidity to go
outside the specified range. People are not very sensitive to the level of ambient
humidity and a range of 40 to 70% RH is normally found to be acceptable.”
Verein Deutscher Ingenieure. VDI 6022 Part 1 July 1998 deals with the hygiene
standards that should be adopted for Air Conditioning Systems. It revolves around
adiabatic systems where a health and safety regime should be adopted. It is further
supported by DIN1946-2 Clause 4.1.3.1 that specifies the thermal control conditions that
should be adopted. The humidity range of 30 to 65% is further clarified by the provision
that filtration measures must be introduced to combat fine dust if the level falls below
30%RH. It also confirms that allergen-producing house dust mites and fungi will be
encouraged above 65% relative humidity.
BSRIA, BRE and HEVAC generally agree that an office environment should be
maintained within the range 40 to 60% RH
Commercial Considerations
These will always colour the requirement for humidification. This can be paired
down to two distinct camps, the cost or installation of the system and the cost of
ownership. The two are not always the responsibility of the same organisation so there are
two different criteria by which they are being judged.
A building owner-occupier will take recommendations from the building design
consultant who will tailor the system to meet his client’s particular requirements. These
could involve any one or a number of key criteria
Health, Safety and Welfare of Occupancy. Keeping the work force comfortable
leads to increased productivity but no definitive studies have been undertaken to quantify
this. There are measures that can be applied but they tend to be negative rather than
positive. Examples are: the number of complaints received regarding a specific condition
such as drafts, sore eyes, dirty surfaces, static electric shocks. Also the number of sick
days taken by the workforce compared to what is considered to be normal.
Initial cost of installation. We all want the best system at the lowest price. If the bill
of quantities comes in too high the specification can suffer by down grading or deletion of
equipment. This has the effect of retrofitting systems as part of the maintenance package
based on the complaints received from the occupier.
Energy cost over the life of the plant and systems. This is becoming very current
with the Climate Change Levy being translated into commercial incentives by the
Department for Environment, Food and Rural Affairs (DEFRA) in conjunction with the
Inland Revenue. The Enhanced Capital Allowance scheme offers the building owner the
opportunity to reduce the first year costs based on energy efficient plant and systems
being included within the building structure. This can be extended to include the energy bill
as well as the maintenance and servicing costs covering the complete operation of the
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building over its predicted life span. The rules governing the Enhanced Capital Allowance
Scheme are available on the DEFRA web site.
Flexibility for change of use. The occupants can change as well as the owners of
the building. This could lead to different environmental requirements and hence extra cost.
All these considerations will be designed into the system in the knowledge of the prevailing
regulations and recommendations. Depending on the body we wish to base our design on,
this will reflect the quality of the environment that workers will be exposed to.
To conclude, perceptions are our guiding force. The lack of regulation and appropriate
policing means we must trade in Best Practice Recommendations. Health and safety of
people will lead to increased productivity. A happy worker is a productive worker. We now
enter the win/win scenario that can be attained without compromising our design integrity.
Humidification and its application within the air-conditioned building
This master class will outline the basic issues of humidification, its application within the air
conditioning system and identify the general range of equipment that is available. It also
gives a comparison of the cost of ownership for the various types of systems utilised.
Why humidification?
Hygroscopic materials
Most people understand that materials expand and contract with a rise and fall in
temperature but many do not understand that with hygroscopic materials there is a greater
rate of expansion and contraction with a rise and fall in humidification level. A dry
atmosphere can cause textiles, carpet, wood, paper, leather, plastics, etc., to shrink,
harden, crack, spoil and loose weight. Weight loss can be a particular problem when
produce is sold by the kg.
Furniture, for example, would show the typical results
of being in an atmosphere where the humidity has
gone from the extremes of damp to extremes of dry.
Panels could split and joints break. Interestingly, the
joints of most items of wooden furniture are more
susceptible to damage as the glue used to fix them is
likely to be hygroscopic in its own right and more
prone to failure than the actual material of
construction.
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Comfort
In a dry atmosphere moisture migrates from the body through the nose, eyes, mouth and
skin. Moisture migrating from the skin causes adiabatic cooling which will reduce the
comfort of room occupants.
At 21ºC, with low humidity, moisture migrates from the skin making a room occupant feel
cool. Raising the humidity level would reduce the ability of the moisture to migrate from the
skin and the occupant would feel warmer.
The normal reaction, to combat the cooling effect, would be to increase the temperature of
the space by one, two or three degrees. The amount of power required to produce the
temperature rise can be greater than the power required bringing the humidity up to a
reasonable level.
There have been instances where the temperature in a working environment has been
reduced to nominally 19ºC and the humidity increased to 60%. The comfort level was
considered acceptable and the power saving substantial.
Health
Volvic, the mineral water supplier, produced a report in 1999, which suggests that an office
with a humidity level of 25% would be as dry as the Sahara Desert.
Although it is acknowledged that low humidity is more uncomfortable than unhealthy,
airborne bacteria can travel further and faster through a dry atmosphere than through a
humid atmosphere. This may increase the possibility of contamination, in fact an ASHRAE
report of 1979 states that `dryness causes cracking of nasal tissues which can give
inhaled germs direct access to the blood-stream`.
Contact lenses
Contact lens wearers, particularly of the hard and gas permeable type, are often the first
people to complain of a dry atmosphere. The lens sits on the eye on a film of moisture.
Should this film dry out then the lens feels very coarse, little more than a piece of grit,
which could cause permanent damage to the eye and result in the wearer having to wear
the spectacles that they were trying to avoid in the first place.
Static electricity
Below 35%RH, static power will build-up in an object, which will discharge to earth through
anything that comes into contact with it, this is normally the human body.
Walking over a nylon carpet, wearing man-made soled shoes could generate a static
charge of 35,000 volts in a dry atmosphere. Raising the humidity would reduce the
discharge to about 1,500 volts, less than 5%.
Raising the humidity level above 35% allows all surfaces to become covered in a
microscopic film of moisture that dissipates the static charge harmlessly to earth.
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Psychrometric process
The psychrometric process of humidification should consider adiabatic systems, for
example sprays, ultrasonic, wetted media and spinning disc, as well as isothermal systems
which relate to steam, that is either local or centrally generation and would then be either
atmospheric steam or pressurised. Both can be powered from electricity or gas.
Identifying the main components of the psychrometric chart
The dry bulb temperature, shown in degrees centigrade, runs along the bottom horizontal
axis. The vertical axis, to the right is the moisture content of air, expressed as kg of
moisture per kg of air. There are three sets of diagonal lines running from top left to bottom
right. The first is the Wet Bulb temperatures and is represented at the 100% saturation
curve. The second is the specific volume, measured in m3/kg of air, again represented on
the 100% saturation curve but dropping more steeply than the Wet Bulb. The third is the
specific enthalpy, kJ/kg of dry air. Represented by the two outer scales of the chart. The
curved lines running top right to bottom left represents the percentage of moisture in
suspension relative to saturation. This is expressed as %relative humidity, (%RH). A
further term that is used in relation to the 100% saturation line is Dew Point. This is a
specific point within a process where by condensation or wetting out will occur.
The humidification process revolves around Wet and Dry Bulb Temperatures, Moisture
Content, Relative humidity, Specific volume and the Dew Point.
3
Specific Volume m /kg
Specific Enthalpy kj/kg
% Relative Humidity
Sensible/Total Heat Ratio
Moisture Content kg/kg
Specific Enthalpy kj/kg
Dew Point
o
Dry Bulb C
Master Class Why Humidify - Jul 2004.doc
o
Wet Bulb C
Page11
Air: Single process functions
Let us take a specific point within the psychrometric chart that represents an average
space condition. 21oC Dry Bulb (DB), 14.8oC Wet Bulb (WB), 50% Relative Humidity (RH),
.0079kg/kg Moisture Content (MC), .843m3/kg Specific Volume (SV). Applying sensible
heat follows the horizontal MC line to the right effectively increasing the Dry Bulb
temperature, decrease the relative humidity but maintaining a constant moisture content.
Dehumidification theoretically will reduce the moisture content and relative humidity whilst
maintaining the Dry Bulb temperature. In practice this process will either heat or cool the
air dependant on the type of system adopted. Diagonal lines will represent these
processes to the right bottom, heating, or to the left bottom, cooling.
Sensible cooling reduces Dry Bulb temperature and follows the horizontal moisture content
line to the left, effectively maintaining the moisture content but reducing the Wet and Dry
Bulb temperatures and increasing the relative humidity.
Humidification can follow two paths dependant on the process adopted.
Adiabatic process follows the Wet Bulb line up towards the 100% saturation curve,
ultimately arriving at the Dew Point. This process will reduce the Dry Bulb temperature
whilst increasing the moisture content and relative humidity. Very useful process for
reducing energy in net cooling applications.
Isothermal process travels, to all intense and purpose, up the Dry Bulb line. This has the
effect of increasing the moisture content and relative humidity with very little increase in
the Dry Bulb temperature. Another useful process for adding moisture without increasing
the cooling load
Air - Single Process Functions
3
kg
RH
W
B
50
%
C
m/
o
Isothermal Humidification
.843
Adiabatic Humidification &
Cooling
14
.8
.0079 kg/kg
Cooling Coil @ .7 SHR
21oC DB
Thermal or Chemical
Dehumidification.
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Multiple process Outdoor Air system: Isothermal humidification
This is an example of isothermal humidification to a full ambient-air system. Taking
outdoor air at 0ºC DB / 0oC WB (saturated) and increasing its temperature to 21ºC DB /
10.8oC WB, does not change the moisture content but would reduce the relative humidity
to approximately 24%. The addition of 0.0040 kg of moisture per kg of air (.0079 - .0039)
would be required to increase the relative humidity from 24 to 50%. During the addition of
moisture there would be very little increase or decrease of Dry Bulb temperature.
Isothermal Humidification
Multiple Process
Outdoor Air System - Isothermal Humidification
50% RH
Moisture Addition .0040 kg/kg
Sensible Heat
25% RH
Multiple process Outdoor-air system: Adiabatic humidification
The adiabatic humidification process, starting from the same 0ºC DB / 0oC WB, sensible
heated is added to increase the temperature to 21ºC DB. Applying the cold water moisture
system would follow the Wet Bulb line up to 95% RH with a Dry Bulb temperature
reduction to 11.3oC. We have added the same amount of moisture as with the Isothermal
system, .004kg/kg of air. More sensible heat must be added to bring the air to the supply
condition required. I.e. 21oC DB / 50% RH.
In practise the condition before the application of the second sensible heat, 95% RH is
very close to the Dew Point and it is difficult to control at this high level, therefore another
way would be to overheat the ambient air to 31oC DB / 14.8oC WB. Adding moisture,
following the Wet Bulb line to the supply air condition corresponding to 50% RH. The same
amount of heat has been applied but at one point rather than two. This has the effect of
reducing the capital cost, as it will be a single large heater rather than two smaller ones.
There is also a better chance of controlling the system without wetting out.
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Adiabatic Humidification
Multiple Process
Outdoor Air System - Adiabatic Humidification
14
11
.3 o
C
.8 o
C
W
B
W
B
50% RH
Moisture Addition .0040 kg/kg
Sensible Heat
25% RH
21oC DB
31oC DB
Psychrometric load calculations
Design criteria
For each example, the same design criteria are used.
Outdoor air volume: 1m³/sec.
Specific volume:
Outdoor:
0.832 m³/kg. Indoor:
Moisture differential:
0ºC DB/ 100%RH / 0.0038 kg/kg. MC
21ºC DB / 50% RH / 0.0079 kg/kg. MC
0.0079 - 0.0039 =
0.0041 kg/kg.
It is worth noting that in any calculation only the outdoor - air intake would be considered,
therefore this calculation would remain true if the total air volume was 10 m³/sec and the
outdoor air proportion was 10%.
Calculation:
1 m³/sec × 0.0041 kg/kg × 3600 sec = 17.74 kg/hr.
0.832 m³/kg
The calculation uses the air volume at (1 m³/sec), multiplied by the moisture differential
(.0041kg/kg) and the number of seconds in an hour (3600), then divided by the specific
volume (.832m3/kg). The resultant value of 17.74 kg/hr can be used as a useful yardstick
figure for the amount of moisture required for every 1 m³/sec of outdoor-air coming into a
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system. It is worth pointing out the specific volume (.832m3/kg) used in this calculation is
derived as the mean difference of the specific volumes at the start and finish of the
process.
Power using steam
Heating
Air at 0ºC/100%:
= 9 kJ/kg.
Air at 21ºC/25%:
= 31 kJ/kg.
Power differential:
31 – 9
= 22 kJ/kg.
Therefore energy used =
22 kJ ÷ 0.832(SV)
= 26.44 kW (per m³/s)
The amount of power required for the adiabatic and isothermal systems can be calculated.
Both options require the initial temperature to be increased to 21ºC DB
Power using steam (Isothermal)
Heating air from 0ºC DB/100%RH to 21ºC DB
Requires
22 kJ/kg.
Humidifying from 21ºC DB / 25% RH to 21ºC DB / 50%RH Requires
10 kJ/kg.
Energy used for heating and humidity process
32 kJ/kg
Therefore power required for whole process = 32kJ/kg x .832 m3/kg SV
26.62kW/m3
Power using water (Adiabatic)
Heating air from 0ºC DB / 100% RH to 21ºC DB
Requires
22 kJ/kg.
The adiabatic cooling effect reduces the condition to 10ºC DB / Saturated.
Therefore re-heating from 10ºC DB to 21ºC DB
Requires
11 kJ/kg.
Energy used for initial heating and re-heat
33 kJ/kg
Therefore power required for whole process = 33kJ/kg x .832 m3/kg SV
27.45kW/kg
+
Energy comparison
Energy required in isothermal process
Energy required in adiabatic process
26.62 kW.
27.45kW.
This energy balance is a generalisation for a typical air conditioning application using a
minimum quantity of outdoor air. There is little difference in the total power requirements of
the two systems so any differentiation will be within the type of energy used to heat either
the water for steam or air for reheat in the adiabatic process.
The Humidity Application Master Class later in this series will compare the variety of
applications and the processes that can be used to give the optimum answer to a humidity
problem.
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Humidifier types
Electrode Boiler
Isothermal
The Isothermal process can be further
split into Atmospheric and Pressurised
Steam raising plant. Firstly we will deal
with the most common which is the
atmospheric systems that are available
today. Atmospheric systems, whatever the
type, generate what is considered as wet
steam in so far as it akin to the domestic
kettle. Dry saturated steam can only be
produced under pressure. Open-ended
boilers, another term for this type of
atmospheric device, are not required to
deliver dry saturated steam for
humidification hence they are relatively
low technology. There are a number of devices available on the market today: Electrode,
Resistance and Gas Fired Boilers.
The Electrode boiler is the most common system adopted today. These are generally the
cheapest first cost option for producing steam for humidification. They have a tendency to
be expensive to run and maintain, partly due to the fact that they operate most effectively
on raw mains water which has a limiting affect on the cylinder life.
The polypropylene cylinder, which is the heart of this system, can be either disposable or
splitable for cleaning. The cylinder life is totally dependent on the quality of the water on
site and the operational time. Typically, water in the south of England would be mineral
rich and electrode boiler cylinders would last between 1 to 3 months. Lancashire and
Scotland, for example, have predominately soft, mineral free water and a cylinder could
last all season.
Calcium in the water is deposited onto the electrode or in the bottom of the cylinder and is
too big to be flushed away during the periodical drains. It stays and builds up until it fills the
cylinder and there is no more room for the water. The softer the water the better for
cylinder life.
The down side of this soft or low conductivity water is that it may take some time for the
water to be heated and the boiler brought up to capacity. The conductivity has to be built
up over a period of time, which would be by the action of boiling the water and leaving
whatever sediment there is within the cylinder. Sediment can be introduced; an AlkaSeltzer tablet is favoured by a lot of engineers. This will certainly increase the conductivity
immediately but if the system calls for a major drain, which then requires a re-fill of new
raw water we are back to square one. Changing the voltage configuration from single to
two phases with a new cylinder configured for this arrangement is a more permanent
Master Class Why Humidify - Jul 2004.doc
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solution. The owner / operator would need to seek the advice and help of the original
equipment manufacturer to instigate this solution.
There will always be exceptions to this example as there are a number of variables that
must be considered. Actual source of water, the actual water quality, further water
treatment that is site specific, the operational time for the cylinder, maintenance periods,
the type and design of the air conditioning system. In recent years the biggest problem has
been the variable nature of the raw water supplied to the same site. Local water authorities
will give the building owner a typical water quality analysis for the site, which will include
hardness and conductivity to be expected. This will help in selecting the right plant for
there application.
The choice of disposable or splitable cylinders is down to available time. It takes 15 to 30
minutes to change a disposable cylinder, including cleaning the pump and valves. It could
take an hour to clean the elements of a splitable cylinder in a suitable location before the
engineer tackles the pump and valves. In hard water areas this may be required once or
twice a month. Time is money and it is the choice of the plant operator / owner.
Electrode boiler devices are available as on/off, stepped and / or fully proportional control
to suit the type of application. All units’ control three basic principles, water feed, boil and
drain. Water is fed into the cylinder in response to capacity and the drainage requirement.
Drainage is controlled by the conductivity of the water. As water is boiled it leaves the
impurity behind creating a soup or high conductive cocktail. Once the conductivity reaches
a pre-set limit a drain cycle will be initiated, and, because there is still a load requirement
the cylinder will fill with fresh water. This has to be heated and boiled to generate steam
once more. Smart pulsed feed and drain cycles will allow the unit to keep boiling for
longer periods.
Capacity control is achieved by either allowing the water to boil away and not replenishing
the level or initiating a drain cycle to reduce the amount of water in contact with the
electrode. Earth leakage protection legislation requires the power to be switched off when
in drain phase. Water regulations require an air gap to be present between the cylinder or
boiler and the drain and feed lines.
Resistance Element Boiler
Isothermal
The Resistance heater type system is more
expensive to buy, than its open ended
electrode comparison. But because they use
a resistance heater element to boil water they
are not conductivity dependent and therefore
can operate using Base Exchange treated
water or even de-mineralised water.
Master Class Why Humidify - Jul 2004.doc
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Using de-mineralised water could produce nominally two years operation without the need
for anything more than a visual inspection, representing a much cheaper long-term option
as maintenance costs are comparatively low.
If fed with raw mains water as prevailing in the south of England the calcium build up
within the cylinder would be quite appreciable and necessitate frequent cleaning. For this
reason most manufacturers would recommend this device be fed with treated water to
some degree. Base exchange water softeners will exchange the solubles from calcium to
sulphite. The impurity is still there but it will not amalgamate and can be easily flushed
away.
The control of these systems would again revolve around feed, boil and drain. Power can
be kept onto the heaters during Feed and Drain cycles, as there is no fear of earth
leakage. Current is not passed through the water; therefore a finer control of the steam
production can be achieved. This devise is very useful in close control applications.
These applications would tend to use de-mineralised water, which means there would be
little or no need for a drain cycle to remove dissolved solids. The cylinder material would
be stainless steel or a suitable polypropylene so the aggressive nature of de-mineralised
water is negated.
These devises are available in on/off, stepped and fully proportional control, and can be
fed by an external control signal from, say a BMS system or a stand alone sensor /
transmitter arrangement within the duct work or the conditioned space.
Gas Fired Staem Generator
Isothermal
The gas fired atmospheric steam generator is relatively
new to the humidification market. Again, relatively
expensive in terms of capital outlay but because it is not
conductivity dependent it can use base exchange
treated water or de-mineralised. The other major factor
is that gas is approximately 20% of the cost of electrical
power, and would produce lower CO2 emissions
compared to that produce to generate electrical power.
The benefits can be accrued from the Capital Allowance
Scheme and the client can be seen to be adopting
energy efficient measures. There are a number of gas
types within Europe and each appliance will have the
correct approvals for the appropriate country. In the UK
we have the luxury of having a uniform quality of mains
gas available plus Calor and Propane in containers. Our
legislation requires the correct approvals from certified
test houses, are in place before any appliance can be
offered or installed.
As well as having a gas supply, of the right capacity available there is a need for a flue to
remove the fumes of combustion. Straightforward single stack or uncomplicated balanced
flue arrangements are available from the humidifier manufacturer or from a flue specialist.
If the flue is complicated in any way it is advisable to seek the advice if a specialist
manufacturer or supplier. A qualified gas engineer as prescribed by the CORGI
registration scheme must do any work carried out on the appliance on site.
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The same principle of operation and control will apply that is feed, boil and drain and the
equipment is available as on/off, stepped and full proportional control.
All three devices should have an antifoaming devise within its control logic. Foaming
occurs at a point when the chemical contaminates within in the cylinder get to such a high
concentration that soap type bubbles appear. If allowed to persist arching will occur within
the electrode boiler and possible water carryover into the air path. Resistance and gasfired systems will experience water carry over or heater failure due to low water levels.
Basically the foam fools the control sensor that the cylinder contains water when it doesn’t.
One way of over coming foaming is to measure the water conductivity and when it gets too
high initiate a drain and fill to dilute the contaminates. This control and monitoring strategy
is contained within the appliance control architecture.
All three devises have the ability for master / slave configuration to obtain a large capacity
in modular format. Typically there would be a fully proportional unit as the master which
would have turn down characteristics in the region of eight to 100% for electric and 20 to
100% for gas fired. Once the minimum capacity requirement has been signalled, say 20%
for the gas appliance the plant will start and the feed, boil, drain cycle will begin. As the
load increases the capacity will be ramped up proportionally until 100% is reached. If the
load carries on increasing the first slave will be started on full capacity. This would tend to
be an on/off device. The master would ramp down to minimum and again start to increase
capacity in response to the load. Typically the master slave system could have as many as
10 units configured for this operation giving an infinitely variable capacity control scenario.
Pressurised Steam Generation
Still within the Isothermal family group is the pressurised steam boilers and generators.
Boilers tend to be shell and tube type construction, the principle of which has not
dramatically altered over the years. Steam generators tend to be tube in tube type
construction and can be more efficient than conventional boilers. Over the years the water
and steam valves have developed to make all these systems deliver the quality of steam
required and the point of use.
Gas Fired Pressurised Steam Generator
Isothermal
A typical steam generator system uses Base
Exchange treated water or de-mineralised water.
They can be gas or oil fired and produces quality
steam within minutes of a cold start. Not
considered as a pressure vessel the system
highly efficient in terms of power and produces
steam at a range of pressure to suit the
application. These would suit the larger Humidity
application where long steam pipe runs are
envisages and flueing can be achieved. They
tend to be modular in design and follow the
master slave principle described previously.
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Steam would be generated at one to three bars and pass through pressure reduction
valves to deliver steam into the airflow at the design pressure. Dependant on the quality of
the steam required would depend on the need for separators and blow down devices.
Steam pipe work design is a specialist subject and help should be sort from appropriately
qualified design engineers. In this article let us assume that the correct steam train of pipe
work and valves would be used to convey the product from the generator to the dispersion
device.
The application may use pressurised steam from an existing boiler, with spare capacity,
already on site, which will greatly reduce the first cost. The steam specialist would design
the correct valves, controls and treatments to deliver the required steam quality to the
airflow.
Steam Distribution Pipes
Single Steam Pipes
Whether atmospheric or pressurised steam
there is a requirement to deliver the moisture
into the air stream. Using a single or multiple
pipe arrangement does this. Firstly let us
define the term pipe to have the same
meaning and function as a lance or dispersion
tube. Typically it would be a stainless steel
pipe with holes cut into its length, and
depending if pressurised or not a nozzle could
be inserted in each hole. They are positioned
across the full width of the air handling unit or
ductwork and would normally be self
supporting from a flange fixed to the outside
of the air handler casing.
They are inserted through a drilled hole from the outside and the steam pipe work
connected either directly or via a steam header.
The atmospheric device would typically have holes positioned on the top of the pipe so the
steam escapes at 90° to the airflow. They would be either positive or negative slope to
allow any condensate to be drained away.
Positive slope pipes point up approximately 8/10° from horizontal and allow any
condensate to run back towards the outside of the air handler or ductwork. This is very
useful in simple layouts where the generator is below the pipe position so all condensate
drains back into the cylinder or chamber.
Negative slope pipes point down and there is a drain connection on the end. This is either
piped back to the outside or into a suitable drain pan within the air handler. There is no
best solution other than the configuration that best fits the application.
It should be noted at this point that if, in the atmospheric system, if the transition pipe work
from the generator to the lance follows a path that forces a trap it will be necessary to use
a condense separator at that position. This will allow the condensate to collect and drain
away without stopping the movement of steam. The same could be said for pressurised
systems, but there a proprietary steam trap would be used.
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Absorption Chart
The moisture in the form of steam must be allowed to be absorbed into the airflow. If you
could see into the duct work this would be represented by the slow disappearance of the
wisps of steam. The length of these wisps is classed as the absorption distance.
This distance can be obtained using manufacturer’s charts and the design engineer will
need to know:
Moisture content before and after the steam pipe
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Dry Bulb temperature at the same point, normally supply air condition.
Length of Pipe and Capacity required
Velocity of the air passing over the pipe
The chart will then give a value that will be used as the base factor to calculate the
distance required from the pipe position to the first constriction element in the air path.
Let’s say this factor is 600 (mm). A Bend, Reducer, Branch, Fan or Diffuser would
correspond to the factor already obtained. A Filter or Heater battery would require the
doubling of the factor. So that’s 600 x 2 = 1200mm. An Absolute filter would require a
Master Class Why Humidify - Jul 2004.doc
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trebling. 600 x 3 = 1800mm, The sensor, be it control or high limit needs to be five times
our factor. 600 x 5 = 3500mm. It is possible to extrapolate for two or perhaps four pipes
onto one generator, but this does not mean we reduce the distance by the same number
of pipes.
Taking our theoretical example, two x pipes with the same capacity equally distributed
would show a 20% reduction of our initial factor i.e. 480mm, and four pipes would mean a
40% reduction i.e. 360mm. It would still mean the sensor must be 1800mm down stream
of the steam pipe without any bend in the ductwork. Multiple pipes would require a header
to distribute the steam evenly into each pipe. Any unevenness in the potential pressure
drop will see and uneven distribution of steam.
Single and Multiple Pressurised Steam Lances
The same principles apply for pressurised systems but the holes in the lance would be
fitted with a nozzle and would be facing the air stream. There are two types of pressure
lance or tube available, jacketed or non-jacketed. The same information is required to
calculate the absorption distance and if it is found that multiple pipes are required. It is just
as important that a steam header is used to distribute the load evenly across all the pipes.
This header would include a condensate connection and trap.
Multipipe
There will be applications when small
banks of pipes still do not satisfy the
absorption distance that is available.
The limiting factors could be the size
of the plant room, or the length of
straight duct that is available for
positioning the pipes. It could be a
retrofit application to replace an old
spray washer system that is no longer
viable and the only space available is
the 500mm between the heating and
cooling coil.
Spray washers were designed to use the cooling coil as blanket to increase the potential
surface area for absorption and eliminate any carry over. In these circumstances we must
ensure the absorption is complete before the steam hits the cooling coil as it will separate
out and the water will run to drain. In these situations there is a module that consists of
multiple pipe mounted on a steam header, each pipe has lots of holes lined with nozzles.
The concept is to blanket the full cross sectional area of the air path with steam thereby
decreasing the length required to absorb the moisture into the air. The critical factor for
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design and manufacture is the aspect ration of the number holes that can be fitted into the
area available.
Typically this concept design will give an absorption distance of 500mm or less dependant
on the number of pipes and nozzles. Unfortunately the more pipes and nozzles the more
material is used and the more expensive it becomes. The pressure drop across such a
devices is no more than one would expect with a spiral finned single row heating coil.
These multi-pipe apparatus are available in modular or all welded form, with or without a
frame. The modular form can be built piece by piece within the air handler if there is not
enough space to the side to slide the 100mm wide frame cartridge into position. The more
popular is the complete structure mounted in a frame so it can be positioned across the air
path the same way as a cooling or heating coil.
ADIABATIC
These systems revolve around the use of cold water and some form of atomisation. Some
degree of water treatment and a maintenance regime is desirable to extend the life of the
plant and deal with health and safety issues that surround these type of systems. Like all
types of system if they are designed, installed and maintained correctly they will give good
long-term operation.
The alternatives available include sprays, ultrasonic systems, spinning disc and
evaporative.
Air pressure spray system
These systems have developed out of the spray washer that is still used for some
industrial processes. These tended to operate using a simple pressurised water supply to
an atomising head and spray into the air stream normally against the cooling. This would
be prone to clogging and the performance was generally considered to be poor.
Compressed air has now been introduced which dramatically increases the agitation of the
water to create a very fine mist that does not depend on a blanket, in the form of the
cooling coil, to help absorption. The introduction of compressed air has so advanced the
atomisation process that some manufacturers have described it as ultrasonic.
The effect is that the water is broken up into a very fine aerosol. The capability of waterborne minerals to block the fine nozzles is reduced by the speed of the water passing
through the system.
These systems can use raw or base exchange treated water but would then emit
quantities of minerals into the air stream which suggests that they are more suited to a demineralised water feed. These systems can be used in ducted systems or space
application. There must be full water conditioning and control to prevent the development
of Legionella bacteria through out the system. These water treatment measures are fully
described in the Health and Safety Document ‘ Approved Code of Practice and Guidance’
for the control of Legionella in water systems. The Humidity Group, part of the HEVAC
Association has also published a Code of Practice for the design maintenance of such cold
water adiabatic systems.
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Nozzles
Air Pressure System Spray
The heart of the system is the nozzle, which
would normally have an automatic cleaning
cycle to keep the orifice clear and free of
mineral build up. The nozzle material can be
brass or stainless steel dependent on the
water quality being used in the application.
The same nozzles would be used in the
ducted and space application.
The picture above shows two types of Air/Water nozzle, which require high pressure air to
atomise the water within the head prior to injection into the air. There is also a Pressurised
Water nozzle that takes water at pressure and atomises it by passing it through a small
orifice. The size of the orifice dictates the water droplet size. The pressurised water system
is slightly cheaper to run as it does not require an air compressor although it does require
a positive displacement water pump
The ducted application mounts a number of nozzles onto a header bar positioned across
the full width of the air path. There could be multiple headers offset from each other
dependant on the capacity required. Eliminators are normally positioned downstream of
the calculated absorption distance to ensure no carry over of water occurs.
The space application would have a range of single and double nozzle header
arrangements that can be rotated for directional purposes. Each header arrangement
would be mounted at high level around the space to ensure all the area is covered. The
space application does not rely on an air path for dispersion. Water vapour will disperse
naturally form a high to a low moisture content without the aid of forced ventilation.
Control of the space system would be by a single humidistat in a designated area or an
averaging arrangement across the whole area. Which ever is used it will be relatively
crude control but the application would not warrant a high degree of sophistication.
Ultrasonic spray system
Adiabatic
The ultrasonic system use transducers
oscillating at a frequency of 1.7 MHz to
atomise water into an aerosol which is then
passed into the air-stream. Each transducer
has a nominal output of approximately 0.6
kg/hr, which requires a considerable number
to produce a reasonable output.
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There are two types of device available, one sits in the air stream and utilises the system
air movement, and the other sits outside and pipes the moisture into the space and
includes a fan arrangement within its own cabinet.
The in-duct device has aerodynamic hoods over each transducer well, the open end facing
in the direction of the airflow. The atomised water is induced out to entrain with the air prior
to the next element in the air path. The ‘fog’ produced by this device is very fine and will be
absorbed very quickly into the surrounding air.
As with all cold water systems the ultrasonic unit is best served with de-mineralised water
to ensure no mineral particles either entered the air stream or clogged the transducer well.
Again there must be conformance to the Code of practice governing the Legionella
bacteria. This would require regular draining of the well if left inoperable for lengthy
periods.
The second type is a self contained device that can be used to duct the ‘fog’ into an air
stream or into a space. Because it is a cold water ‘fog’ and heavier than surrounding air it
will have a tendancy to fall to the floor unless supported by an air flow.
These devices have been used in air conditioning plant as well as horticultural and food
humidification application. The very fine mist would be absorbed into the surrounding air
quite quickly although eliminators are sometimes used to ensure no carry over.
Relatively expensive to buy compared to the other systems but very cheap to run as a
stand alone humidifier. If water treatment such as softeners, de-mineralisation and or
ultraviolet light is required the initial cost as well as run costs will increase. As with all
adiabatic systems the re-heat, if required can offset the run cost benefit accrued with this
technology.
Spinning Disk Humidifier
Adiabatic
The spinning disc type of unit introduces droplets of water
onto the face of a rotating disc. This representation shows
a horizontal disc, centrifugal force spins the droplets to the
perimeter through the teeth and into the air stream as a
mist. There are variations to this theme where the disc is
vertical and rotates at high speed into a shallow bath of
water. Another rotates a drum at high speed around a
spray nozzle.
Control is relatively course and would normally be controlled by varying the volume of
water introduced. De-mineralised water would be the most appropriate water supply for
this type of adiabatic system.
The bench mark London water would require some serious filtration before the air is
introduced into the space or water treatment to get rid of the dusting before it becomes a
maintenance problem. Ducted and space applications can be considered from relatively
low to high capacity ratings although the system does lend itself to the larger application.
Relatively cheap as a first cost and cheap to operate and run. The expense will come with
the water treatment to prevent dusting plus the health and safety regime required with all
cold water systems. Again re-heat may also have to be considered dependant on the
application.
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Evaporative : Wetted media
Adiabatic
The evaporative system feeds water to the top cell structure
which then precipitates through the media. Air flowing
through the media evaporates the moisture and takes it up
into the air stream. This is quite an uncomplicated system
to install and operate and tends to come as a self contained
cartridge that includes the cell material, water bath, pump,
supply and drain controls
.There are a number of different cell materials available and would depend on the type of
application including the air that would pass through the structure. A good reliable media
would be glass fibre treated with wetting agents that are non-toxic and non-organic.
The water supply should be filtered as a minimum consideration as should the air to
prevent build up of contaminants within the bath or on the cell structure. An ultra violet
lamp to the supply water ensures the water is sterile when introduced into the reservoir.
Using de-mineralised water could be counter productive as it will be quite aggressive and
attract air born minerals and contaminants that could develop into long term maintenance
issues. The system would be constantly replenished with fresh water over and above the
evaporation rate and incorporate a bleed mechanism The cell is kept wet at times of
humidification, which has the effect of constantly washing the cell media to prevent a build
up of contaminants or micro-organisms.
These systems are available for medium to large applications and are best suited to the
industry process application where large amounts of air are required especially if washing
or scrubbing action is required for the supply air.
Again relatively cheap as a first cost base item, the water treatment adding considerably to
the system cost. Cheap to run although allowances should be made for major cleaning or
replacement of the cell material after a number of years operation. The intervals would
depend on the water and air quality, the manufacturer would give recommendations given
the design conditions and water / air qualities being dealt with.
Humidity control
The statement that occurs time and again is that temperature and moisture content,
whether in absolute or relative terms, are linked and cannot be seperated. We can sum
this statement up with another rule of thumb: A temperature difference of 1.5oC affects a
target humidity of 50% by about 5% RH. So when we specify 21°C DB +/- 2 degrees C
and 50% RH +/- 5% Relative Humidity we are defining quite a considerable area of control
as indicated on the Control Parameter Psychrometric Chart. We have not taken into
account any accuaracy deviations that will occure with whatever type of control sensor we
choose.
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Control Parameters
Control Parameters = 21oC db +/- 2 Deg C / 50 % RH +/- 5% RH
55%
50% RH +/- 5% RH
45%
19oC
23oC
21oC +/- 2 Deg C
To control the humidity level we must first measure the amount of water vapour present in
the air at the sensing point. There are two main types of sensor used: Dew Point Sensor
tells us at what temperature condensation will occur if the air is cooled and is a measure of
how much water vapour the air holds in absolute terms. The second is the Relative
Humidity Sensor which measures the degree of saturation of the air on a scale of 0 to
100%. This device is dependant of temperature as well as moisture content. It is more
common the use the RH Contoler in commercial applications and the Condensation
Hygrometers in very close control applications or as a reference sensors for calibration
purposes.
Mechanical controller
The oldest devise for humidity control would be with a simple humidistat. This would be a
single step mechanism that would probably utilise either horse or human hair to operate a
switch. Both these materials are hygroscopic and lengthen or shorten with changing
humidity levels. Relatively course control and are used for high or low limit safety devices
within an uncomplicated control scenario. Times have moved on and electronic devices
are more prevalent and can be split into two camps.
Electronic controller
The Capacitive device has a control band of 0 to 95% and the resistive, which has a band
of 25 to 95%. All RH controllers whether mechanical or electronic have problems
controlling at 95 to 100% RH and it is normal to use the Dew Point or Condensation
Hygrometer for these high limits.
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Psychrometer
The sling psychrometer tends to be used to measure or check internal or external
conditions. Using a wet and dry bulb thermometer, they are rotated or exposed to an air
velocity, readings taken and plotted onto a psychrometric chart. Used in weather stations
to measure maximum and minimum prevailing conditions. Electronic devices that do offer
a more accurate measurement with the added benefit of being able to be re-calibrated are
superseding these.
Location
As with temperature controllers or sensors the position is of paramount importance. The
space controller/sensor needs to be positioned to reflect the prevailing condition within the
space. This could be mid way between the supply and return air points. This can be further
complicated if there are multiple points of entry and exit, so the safe bet is to choose a
point in the return air path as close to the temperature controller as possible. Keep it away
from any external influences such as;
Solar effects - Direct sun light will give erratic performance of the system as the sun moves
around the building. Difficult to diagnose when looking for reasons for bad performance.
Draughts - Supply air diffuser that may blow air directly over the sensor. The temperature
gradient across the space will not be picked up if the control point is the supply air
condition. There will be a further effect of air velocity over the sensor, which would take it
outside its operating limits.
Equipment - Radiators, Drinks machines, Office equipment giving off heat, all of these will
create a micro climate within the space that by its shear nature will not reflect the
prevailing condition.
External Surfaces -This could be more difficult to ensure against. Insulating the sensor
from the influence of the cold or warm surface is a possible way of limiting this factor
Mounting the controller / sensor in the return air duct is another safe decision, but care
must be taken to introduce an off set for the condition gradient across the space. There
could be multiple return air points all converging on a common return air duct. A good
averaging point, but bear in mind it will be an average not a specific condition, we move
back to the science of the predicted percentage of dissatisfaction discussed earlier.
Multiple sensing points will increase our ability to average or introduce supply air control
into each space or zone. The common factor as we move through all the more elegant
solutions is that the price goes up proportionally. Not necessarily what the client wants to
hear.
High limit overrides, mounted in the supply duct, should be positioned to prevent over
humidification leading to wetting out within the air handler or supply air duct. A good rule of
thumb is minimum of four to five metres of duct from the point the moisture is introduced.
Avoid dead spots in the ductwork and if possible ensure the moisture is fully absorbed into
the supply air.
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Calibration
All sensors will drift out of calibration over a period of time typically three to five percent.
They can also suffer from some of the following ailments:
Poor repeatability in the short term.
Slow response time
Hysteresis or memory effects
Coarse resolution
Regular calibration checks will identify bad sensors and with the right action will improve
the system performance. These measures would be:
Start calibration checks with short time intervals then reduce frequency as
confidence builds in the equipment installed.
Compare with a reliable calibration reference, typically one supplied by a United
Kingdom Accreditation Service (UKAS) test laboratory. Using a UKAS laboratory ensures
the technical competence and proficiency of the service. There is traceability to national
standards that will also meet quality management standards, ISO 9000, and a
demonstrable audit trail resulting in increased confidence that the system will operate
within the design criteria laid down at the start of the project.
Energy Usage
Uncertainties in the control equipment can add up to an increase in run costs for the
system and building. These uncertainties would depend on the operation and calibration of
the measuring equipment, the fluctuation in the humidity during its cycle of operation and
the condition gradient across the space or zones. The National Physical Laboratory
calculates these uncertainties, on a theoretical base, using the following criteria
Say the calibration uncertainty at 50% RH was +/- 2% The real uncertainty in using the
device after allowing for intrinsic drift, repeatability, historisis, resolution, etc is +/- 3%
Say the room is cycling up and down by +/- 3%
Say there are differences across the space of +/- 5%
Lets say the above can be added up by taking the square root of (3)2 + (3)2 + (5)2 = 6.6.
We are reasonably sure the true humidity at any spot in the space is between 43.4% RH
and 56.6% RH.
If we take this theoretical control shift or sensing error and equate it to a theoretical
building of say 5000 m3 with a design set point of 22oC DB / 50% RH. If we apply our 6.6%
shift so the measured set point is 22oC DB / 56.6% RH there is a possible 33% increase
on run cost for an electrode boiler humidifier. One must off set the possibility the shift is in
the other direction and the measured set point is 22oC DB / 43.4% RH, in which case there
would be a 33% reduction in cost. It’s all down to whether the sensors are measuring
predominantly high or low.
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Impact on Humidification Cost
Design Set Point
Theoritical Building Design Parameters
o
Condition without Humidification
Design Set Point
Moisture addition required
C DB / %RH
22/30
22/50
MC g/m3
5.82
9.7
3.88
Measured Set Point with 6.6%
RH Shift
o
C DB / %RH
MC g/m 3
22/30
5.82
22/56.6
10.98
5.16
Building Volume m3
Air change rate / hour
Moisture addition / hr
Moisture addition / year (12 hr. day)
Humidifier output kg/hr
Power required at full output
5000
1
19.4
84972
30
22.8
5000
1
25.8
113004
30
22.8
Annual kWh Usage
Cost per kWhr
32289
0.072
42941
0.072
Total Cost per Year
2325
3092
Additional cost
766
33%
Comparative run costs
This final section covers comparative running costs over the 15-year life span one should
expect from capital plant and looks at four different capacities. The comparison of
systems within the Isothermal family include electrode and element boilers, local and
centrally generated steam from gas fired equipment units compared are electrode boilers,
element boilers, central steam generators. The wetted media and spray systems represent
adiabatic family.
Calculation data
The information used to generate comparable figures are :
15-year life span for the equipment, 30, 60, 200 and 400 kg/hr Humidification load,
Winter heat operation with the system designed to be recirculation nominally 90% with
10% outdoor-air intake. Hours run figures are based on 30 weeks per annum, with 60%
diversification, five days per week and 10 hours per day. The calculations have used
electrical power at six pence and gas at .01 pence per kW.
Each comparison includes allowances for spare part, maintenance, service and ancillary
equipment that would not be normally included in commercial office building. The costs
also include for re-heat in the case of adiabatic systems as the design criteria is 22oC DB /
50% RH
Electrode boiler Operating on raw London water and includes three replacement
cylinders per year plus the manufacturers recommended maintenance intervals.
Resistance or Element Boiler Assumes base exchange treated water is available on
the same London site and includes the normal maintenance periods for this type of unit.
Gas Fired Locally Generated Steam Assumes the same base exchange treated water
as does the Centrally Generated Steam but an allowance has been made for a coil
Master Class Why Humidify - Jul 2004.doc
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change after 10 years. Maintenance and service visits would be all as per the
manufacturer’s recommendations.
Wetted Media
Includes for a reverse osmosis plant and Ultra Violet light as part of
the recommended water treatment as laid down in the code of practice to prevent and
control Legionella bacteria from polluting the system. It also includes a media change after
eight years.
Spray System
Includes a compressor and receiver set for pressurisation of the
water, reverse osmosis and ultraviolet plants plus all the control for water and air filtration.
Comparative Run Cost – 30kg/hr
£26k
Electrode
30 kg/hr
Element
£20
Spray
£10
Wet Media
Local
Steam
10 Years
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20 Years
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Comparative Run Cost – 60kg/hr
£52k
60 kg/hr
Electrode
Element
£40
Spray
£20
Wet Media
Local
Steam
20 Years
10 Years
Comparative Run Cost – 200kg/hr
£260k
200 kg/hr
Electrode
Element
Spray
£100
Wet Media
Central Steam
Local steam
10 Years
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20 Years
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Comparative Run Cost – 400kg/hr
£520k
400 kg/hr
Electrode
Element
Spray
£200
Wet Media
Local steam
Central
Steam
20 Years
10 Years
Comparative run cost table (Alternative to above)
Whole life cost based on 15 year period.
System Type
30 kg/hr Capacity
Year 1
Year 15
60 kg/hr Capacity
Year 1
Year 15
200 kg/hr Capacity
Year 1
Year 15
400 kg/hr Capacity
Year 1
Year 15
ISOTHERMAL (£)
Electrode Boiler (Atmospheric)
1516
25024
1984
46075
7854
184398
13512
322464
Resistance Heater (Atmospheric)
3021
21489
5858
36936
21800
147744
31912
290464
Gas Fired Boiler (Atmospheric)
5733
8811
6500
12656
18763
39013
44260
84760
23370
48620
26767
68067
Gas Fired Generator (Pressurised)
ADIABATIC (£)
Spray System (Pressurised)
6700
11000
6700
26000
57000
100000
90000
180000
Wetted Media (Atmospheric)
4000
9078
9000
19156
40750
81000
65000
135500
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We can see that all the four sizes follow the same pattern in terms of whole life cost except
in the case of the 400kg system that shows a reversal between the locally generated and
centrally generated steam.
Water Quality
Through all this should be considered the incoming water supply. Water containing 500PPM temporary hardness particles in suspension would precipitate approximately 0.1
kg/hr of scale or calcium deposit when heated and boiled of as steam. Assuming no water
treatment for the cold water spray or ultrasonic system this value would still be relevant.
Using the same hours run criteria as the comparative run cost we can be calculated that
135 kg (297 lb.) per annum of solid material would be collected in the boiler, if isothermal,
or in the filter downstream of the spray if adiabatic. If no filter is fitted this dust would
precipitate out onto the workspace.
This is a startling statement as a reminder that it is better to treat the water for optimum
performance in accordance with the humidification device fitted than to add cost to
address a problem of our own creation.
Vapac Humidity Control Ltd
Steam Humidifiers / Steam Generators
Fircroft Way, Edenbridge
Kent TN8 6EZ, England
Telephone: +44 (0)1732 863447
Facsimile:
+44 (0)1732 865658
www.eaton-williams.com
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