Federation of European Heating, Ventilation and Air-conditioning Associations Address: Rue Washington 40 1050 Brussels Belgium www.rehva.eu [email protected] Tel: +32 2 514 11 71 Fax: +32 2 512 90 62 Federation of European Heating, Ventilation and Air-conditioning Associations Use of REHVA Guidebook Power Point Presentations • • • • This Power Point Presentation can be freely used for training purposes by REHVA members. It is prepared by the main author to the REHVA Guidebook. Please refer the original author always when making the presentation. Inform REHVA secretariat each time the presentation is used: [email protected] Federation of European Heating, Ventilation and Air-conditioning Associations Displacement Ventilation in Non-industrial Premises an old principle with new technology Federation of European Heating, Ventilation and Air-conditioning Associations Rehva Guidebook on Displacement Ventilation Aimed at: the practising engineer Discussing: what is displacement ventilation? what are the benefits what are the limitations? where should it be used (and not used)? The theory is illustrated by case studies showing how displacement ventilation can be utilised in practice Federation of European Heating, Ventilation and Air-conditioning Associations Contents of the book 1. 2. 3. 4. 5. 6. 7. Disp.vent in a nutshell Terminology, symbols and units Basic knowledge about displacement ventilation Air diffusers Design procedures Energy aspects Automation and control Case studies • • • • • 8. Restaurant Cell office Auditorium Meeting room Class room References Federation of European Heating, Ventilation and Air-conditioning Associations The basic idea Warm air rises due to buoyancy, and carries the pollutants from people up towards the ceiling. In industry…. ….and non-industrial premises Federation of European Heating, Ventilation and Air-conditioning Associations Man = heat source – in theory 80 – 90 W The air rises because of the heat release of the human body Federation of European Heating, Ventilation and Air-conditioning Associations Man = heat source – in practice • Warm, polluted air rises due to buoyancy. • Buoyancy is created by the heat from the body. Cigar smoke visualises the rising air, but is is the heat from the body that drives the flow. Federation of European Heating, Ventilation and Air-conditioning Associations Air supply Air supply units The air floats along the floor like water and fills the room from below Federation of European Heating, Ventilation and Air-conditioning Associations Air is extracted at ceiling level The extract opening can be located anywhere in the highest part of the room The extract air flows along the ceiling Federation of European Heating, Ventilation and Air-conditioning Associations Air supply The air is supplied from wall panels with low velocity. The supply air is colder than the room air. Federation of European Heating, Ventilation and Air-conditioning Associations Air supply can be arranged in many ways Plane, wall-mounted Circular, free-standing Semi-circular, corner-mounted Federation of European Heating, Ventilation and Air-conditioning Associations Floormounted Be aware of cold draught along the floor! • • • When the supply air is colder than the room air, it will fall to the floor, and may cause cold draught. This may avoided by choosing the right diffuser. z Typical depth 20 cm vx v z Remember: – Choose a diffuser that is suited for the purpose – Make sure that the adjacent zone (the “draught zone”) does reach places where people are located permanently. ~ 2 - 5 cm Acceleration region Federation of European Heating, Ventilation and Air-conditioning Associations Velocity decay region Diffuser types - Casino Air diffusers behind columns Federation of European Heating, Ventilation and Air-conditioning Associations Diffuser types - Atrium Air diffuser Federation of European Heating, Ventilation and Air-conditioning Associations Diffuser types - Restaurant Air diffuser Federation of European Heating, Ventilation and Air-conditioning Associations Diffuser types - Department store Air diffuser Federation of European Heating, Ventilation and Air-conditioning Associations Basic principles – Convection currents q q H q q The air that rises in the convection current must be replaced by new air. This makes a two-layer flow, where the polluted, “used” air stratifies in the upper layer. Less supply air lower the interface between the two layers. q h q Federation of European Heating, Ventilation and Air-conditioning Associations Basic principles – Thermal stratification Height above floor [m] In practice, the air will stratify in many layers, making the temperature rise from floor to ceiling. 2,5 2,0 1,5 1,0 0,5 0,0 0 0,2 0,4 0,6 0,8 Temperature ratio Federation of European Heating, Ventilation and Air-conditioning Associations 1 - s e - s 1,2 Basic principles – Thermal stratification Height above floor, z [m] Extract 100% 2,5 26 °C 2,0 1,5 Occupied zone 50 - 90% 1,0 0,5 50 - 90% Supply 0% 18 °C 0 0 20% 40% 60% 80% 100% Contaminant Height above floor, z [m] Extract 100% 2,5 26 °C 2,0 1,5 Occupied zone 90% - 100% Breathing zone < 100 % 1,0 0,5 Supply 0% 18 °C 0 0 20% 40% 60% 80% 100% Contaminants Federation of European Heating, Ventilation and Air-conditioning Associations Temperature distribution normal rooms Mixing Height above floor, z [m] Extract 24 °C 2,5 16 °C 2,0 24 °C 1,5 Occupied zone 24 °C 1,0 0,5 Less cooling is needed to obtain the desired temperature in the occupied space 24 °C Supply 16 °C 0 16 18 20 22 26 24 Temperature [°C] Height above floor, z [m] Displacement Extract 26 °C 2,5 26 °C 2,0 1,5 Occupied zone 23 °C 1,0 0,5 24 °C Supply 18 °C 18 °C 0 16 18 20 22 26 24 Temperature [°C] Federation of European Heating, Ventilation and Air-conditioning Associations Temperature distribution - Tall rooms Extract air temperature = 23°C e Mixing Extract air temperature = 27,5°C e Displacement Average temperature in the occupied space = 23°C oz 9°C 9°C 14 16 18 20 22 24 26 28 Supply air temperature =14°C s [°C] 14 16 18 20 22 24 26 28 Supply air temperature =18,5°C s The cooling advantage is most pronounced for tall rooms Federation of European Heating, Ventilation and Air-conditioning Associations [°C] Contaminant distribution in normal rooms Mixing Height above floor, z [m] Extract 100% 2,5 16 °C 2,0 24 °C 1,5 For the same ventilation rate, we may get better air quality with displacement ventilation Occupied zone 100% 1,0 100% Supply 0,5 0 0 20% 40% 60% 80% 100% Contaminant Height above floor, z [m] Displacement Extract 100% 2,5 26 °C 2,0 1,5 Occupied zone 50 - 90% 1,0 0,5 50 - 90% Supply 0% 18 °C 0 0 20% 40% 60% 80% 100% Contaminant Federation of European Heating, Ventilation and Air-conditioning Associations Little air - less benefit Height above floor, z [m] Extract 100% 2,5 26 °C 2,0 1,5 Occupied zone 50 - 90% 1,0 0,5 20 l/s per person = very good 50 - 90% Supply 0% 18 °C 0 0 20% 40% 60% 80% 100% Contaminant 10 l/s per person = acceptable Height above floor, z [m] Extract 100% 2,5 26 °C 2,0 1,5 Occupied zone 90% - 100% Breathing zone < 100 % 1,0 0,5 Supply 0% 18 °C 0 0 20% 40% 60% 80% 100% Contaminants Federation of European Heating, Ventilation and Air-conditioning Associations < 10 l/s per person = insufficient Contamination in inhaled air Concentration ratios measured at a manikin 0,6 Conc. in inhaled air Conc. in air at head level 0,5 0,4 0,3 0,2 0,1 0,0 0 5 10 15 Flow rate The rising flow around a person brings fresh air to the breathing zone Federation of European Heating, Ventilation and Air-conditioning Associations 20 litre/s person Source: Mats Sandberg, Sweden When there are many people, and insufficient air, the contaminants stratify at lower levels Height above floor [m] Better air quality: Yes – and No? 2,5 One person in the room 2,0 Two persons in the room 1,5 1,0 Three persons in the room 0,5 0,0 0 100 200 300 400 500 600 700 800 Increase in CO2-concentration Federation of European Heating, Ventilation and Air-conditioning Associations Passive smoking in crowded rooms with Displacement Ventilation 3,0 Height [m] Ambient 2,5 Exposure, seated 2,0 exposure, standing 1,5 1,0 0,5 0,0 0 1 2 3 4 5 6 Concentration ratio c/cR A standing person’s exposure may be greater, but people are still protected by the rising air current around themselves Federation of European Heating, Ventilation and Air-conditioning Associations Source: Peter V- Nielsen, Denmark For protection against tobacco smoke, see REHVA Guidebook no 4: “Ventilation and Smoking”. Federation of European Heating, Ventilation and Air-conditioning Associations Contaminant distribution in tall rooms Perfect mixing Displacement 2,5 2,0 1,5 Occupied zone 100% 1,0 0,5 0 Extract 100% Height above floor, z [m] Height above floor, z [m] Extract 100% 2,5 2,0 1,5 Occupied zone 1,0 0,5 0 0 Supply 0% 20% 40% 60% 80% 100% Contaminant 20% 40% 60% 80% 100% Contaminant Supply 0% 0 Federation of European Heating, Ventilation and Air-conditioning Associations Auditoria – air supply Air supply in front of the room Air supply under the seats Federation of European Heating, Ventilation and Air-conditioning Associations Auditoria – air flow pattern Thermal and contaminant stratification Federation of European Heating, Ventilation and Air-conditioning Associations Auditoria – air flow pattern Supply air is contained between the rows. Supply air is floating down the stairways. Federation of European Heating, Ventilation and Air-conditioning Associations Auditoria – temperature distribution Extract air temperature e =26°C 50% 50% Height above floor [m] 6 5 4 3 appr. 6K 2 1 0 16 17 18 19 20 21 22 23 24 25 26 27 Supply air temperature s =16°C Air temperature at floor level, oz21°C Temperature [°C] Federation of European Heating, Ventilation and Air-conditioning Associations Auditoria – Recirculation Outdoor air qp = 6 l/s per pers cp = 350 ppm Recirculation air qr = 4 l/s per pers cr = 1350 ppm ce = 1350 ppm Extract air qe = 10 l/s per pers ce = 1350 ppm qCO2 = 0,006 l/s per pers. Cooling unit cexp ~ 900 ppm Supply air qs = 10 l/s per pers cs = 750 ppm cs = 750 ppm Displacement ventilation gives benefits to recirculation in tall rooms – for details, see the book Federation of European Heating, Ventilation and Air-conditioning Associations Cooled ceiling - high temperature cooling Cooled ceiling Cooled ceiling is ok when the cooling output of the ceiling is less than 40% of the total cooling. 2,50 = 0 Height above floor level, z [m] = 0,6 2,00 = 0,5 1,50 = 0,4 1,00 = ratio of the cooled ceiling cooling output to the total cooling output (Tan 1998) 0,50 Cooled ceilings, or cooling convectors, decrease the air quality benefit. 0,00 0,8 1,0 1,2 1,4 Relative air temperature (relative to temp. at 0,1 m above the floor) Mixing ventilation should be considered as an alternative. Federation of European Heating, Ventilation and Air-conditioning Associations Room heating Short-circuiting: The warm, fresh air is pulled into the extract When the room is occupied: • Don't heat the room by the ventilation air ! At night: • Some people recommend heating by the ventilation air…. Federation of European Heating, Ventilation and Air-conditioning Associations Heating by radiator/convector Warm air from radiator balances meets the cold air from window/wall Federation of European Heating, Ventilation and Air-conditioning Associations Floor heating - low temperature heating Floor heating is ok as long as the floor temperature is moderate (i.e. less than appr. 25° C) Federation of European Heating, Ventilation and Air-conditioning Associations Useful or useless? Best suited for: Less suited for: • Restaurants, • • Meeting rooms, Where surplus heat is the main problem, and not air quality. • Classrooms • • Where ceiling heights are lower than approximately 2,3 metres. Tall rooms: • When the problem is cooling in low rooms (in offices, consider mixing and cooling panels) • Where disturbances to room air flow is strong. – Conference rooms, – Theatres, – Supermarkets, etc Advantages: • • Improved air quality • Most efficient in tall room • 1°C – 3°C lower temperature in the occupied space for a given supply temperature Where the contaminants are colder/denser than the ambient air. Federation of European Heating, Ventilation and Air-conditioning Associations ..and please remember: Displacement Ventilation is no miracle, and should not be used everywhere. It has lost much of its reputation in several countries due to incompetent or over-optimistic use. Problems: draught along the floor diffusers require much wall space ..but used with skill in the right places, it has definite advantages. Federation of European Heating, Ventilation and Air-conditioning Associations Thank you for your attention To order the REHVA guidebooks : www.rehva.eu / section Bookstore or through your national member Federation of European Heating, Ventilation and Air-conditioning Associations
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