Federation of European Heating, Ventilation and Air-conditioning Associations

Transcription

1 Federation of European Heating, Ventilation and Air-conditioning Associations Address: Rue Washington Brussels Belgium Tel: Fax:

2 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:

3 Displacement Ventilation REHVA published 2002 the first version of displacement ventilation guide. The aim of this revised Guidebook is to give the state-of-the art knowledge of the technology. The idea of this guidebook is to simplify and improve the practical design procedure.

4 Rehva Guidebook on Displacement Ventilation Aimed at: the practising engineer Discussing: what is displacement ventilation? what are the pros and cons? How to design displacement ventilation system? What are the critical factor in design? This guide also show practical case studies in some typical applications and the latest research findings are discussed.

5 Contents of the book 1. Disp.vent in a nutshell 2. Terminology, symbols and units 3. Room air distribution 4. Performance of disp. ventilation 5. Calculation of supply airflow rate 6. Air diffusers for displ. ventilation 7. Design of disp. ventilation 8. Case studies 9. Research findings 10.References

6 The basic idea Displacement ventilation is considered to be the technique of supplying clean, cool air at floor level, letting warm air and contaminants rise to the ceiling and extracting the contaminated air at ceiling level.

7 Possible ventilation strategies in the room space Strategy DISPLACEMENT PISTON STRATIFICATION ZONING MIXING Description Unidirectional flow through the room Utilise density differences Air flow from clean zones to contaminated zones Uniform conditions in all parts of the room Room dimension Air quality; e temp, humidity RF contaminants, c s = supply e = exhaust s, RF, c Room dimension e s, RF, c Room dimension e s, RF, c Room dimension e s, RF, c Main characteristics Flow pattern controlled by low momentum supply air, strong enough to overcome disturbances Flow pattern controlled by buoyancy Flow pattern controlled partly by buoyancy and partly by supply air momentum Flow pattern controlled by high momentum supply air Ventilation effectiveness e - s c e - cs c - c - c oz s oz s 1

8 Convection flows the engines of displacement ventilation Natural convection flows are the engines of displacement ventilation. A natural convection flow is the air current that rises above warm objects like people or computers, rises along a warm wall, or descends from cold objects like windows or outer walls, due to buoyancy

9 Basic principles Convection currents H q 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. q Less supply air lower the interface between the two layers. h q q

10 Height above floor, z [m] Contamination distribution The contamination distribution in a displacement-ventilated room depends on the position of the contamination sources and if the heat sources are also the contamination sources. In the ideal case with warm concentrated contamination sources all contaminants are transported directly into the upper zone by the convection flows 2,5 2,0 1,5 1,0 0, ,2 0,4 0,6 0,8 1,0 Contamination ratio, c room /ce

11 Height above floor, z [m] Height above floor, z [m] Contaminant distribution in Mixing normal rooms Extract 100% 2,5 16 C 2,0 1,5 1,0 0,5 0 Displacement 2,5 2,0 Supply Occupied zone 100% 0 20% 40% 60% 80% 100% Contaminant Extract 100% 100% 24 C 26 C For the same ventilation rate, we may get better air quality with displacement ventilation 1,5 1,0 Occupied zone 50-90% 50-90% 0,5 0 Supply 0% 0 20% 40% 60% 80% 100% Contaminant 18 C

12 Height above floor, z [m] Height above floor, z [m] Contaminant distribution in tall rooms Perfect mixing Extract 100% Displacement Extract 100% 2,5 2,5 2,0 2,0 1,5 1,0 Occupied zone 100% 1,5 1,0 Occupied zone 0,5 0, % 40% 60% 80% 100% Contaminant Supply 0% % 40% 60% 80% 100% Contaminant Supply 0%

13 Conc. in inhaled air Conc. in air at head level Contamination in inhaled air 0,6 0,5 Concentration ratios measured at a manikin 0,4 0,3 0,2 0,1 The personal exposure index will often be larger than the local ventilation index because clean air is moved from the lower part of the room up to the breathing zone by the freeconvection boundary layer around the person 0, Flow rate litre/s person

14 Height above floor [m] Basic principles Thermal stratification 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 1 1,2 Temperature ratio - s - e s

15 Height above floor, z [m] Height above floor, z [m] Temperature distribution - Mixing normal rooms Extract 24 C 2,5 16 C 2,0 1,5 1,0 0,5 0 Displacement 2,5 2,0 Occupied zone 24 C Supply 16 C Temperature [ C] Extract 26 C 24 C 24 C 26 C Less cooling is needed to obtain the desired temperature in the occupied space 1,5 1,0 Occupied zone 23 C 24 C 0,5 Supply 18 C 18 C Temperature [ C]

16 The occupied zones the coolest part of the room In displacement ventilation, the air temperature increases from floor to ceiling. Vertical temperature profiles varied with different individual types of heat load (occupants, warm floor, warm window and warm ceiling). H(m) H(m) Occupants Temperature ratio Warm floor Temperature ratio Temperature Ratio= e H(m) H(m) s s Warm window Temperature ratio Warm ceiling Temperature ratio

17 Temperature distribution - Tall rooms Mixing Extract air temperature e = 23 C Displacement Extract air temperature e= 27,5 C 9 C Average temperature in the occupied space oz = 23 C 9 C [ C] [ C] Supply air temperature s =14 C Supply air temperature s =18,5 C The cooling advantage is most pronounced for tall rooms, but the type of heat load affects a lot on the actual vertical temperature difference.

18 Do not heat occupied rooms with displacement ventilation If warm air is supplied at floor level, in a room cooler than the supply air, it will rise due to buoyancy, and be extracted when it reaches the ceiling. Thus, the supply air will short circuit into the outlet and little of the clean and heated warm air will reach the occupied space.

19 Air supply Air supply units The air floats along the floor like water and fills the room from below

20 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

21 Air supply can be arranged in many ways Plane, wall-mounted Circular, free-standing Semi-circular, corner-mounted Floormounted

22 The air supply diffuser a crucial factor Most draught problems reported in rooms with displacement ventilation are due to high velocity in the zone adjacent to the diffuser. It is important to choose a diffuser that is suited for the application and only utilise diffusers from manufacturers that supply robust documentation together with the products.

23 Auditoria air flow pattern Thermal and contaminant stratification

24 Auditoria air flow pattern Supply air is contained between the rows. Supply air is floating down the stairways.

25 Collaboration with the architect is required The diffusers require a certain amount of wall area, or space in, or on, the floor. Close cooperation with the architect is required to find a suitable location for the air diffusers. The supply units can also be designed to fit different architectural requirements: units could be invisible or be an exposed architectural element. Air diffuser Air diffuser

26 Diffuser types - Casino Air diffusers behind columns

27 Diffuser types - Atrium Air diffuser

28 Diffuser types - Restaurant Air diffuser

29 Diffuser types - Department store Air diffuser

30 Calculation of supply air flow rate Two principal methods can be used when the supply air flow rate of displacement ventilation system is calculated: 1) temperature based design, where the design criterion is the air temperature in the occupied zone of the room and 2) air quality based design where the design criterion is the air quality in the occupied zone. In commercial buildings, the removal of the excess heat is likely to be the main concern. The cases where cooling is the main issue, the temperature based design is the most commonly applied method.

31 Temperature based design methods Two principal methods can be used when the supply air flow rate of displacement ventilation system is calculated: 1) temperature based design, where the design criterion is the air temperature in the occupied zone of the room and 2) air quality based design where the design criterion is the air quality in the occupied zone. In commercial buildings, the removal of the excess heat is likely to be the main concern. The cases where cooling is the main issue, the temperature based design is the most commonly applied method.

32 Height above floor [m] Challenge of design In the design process, the challenging task is to estimate vertical contaminant or temperature gradients in the room space. Heat sources in the upper part of the room Heat sources in the lower part of the room 2,5 2,0 1,5 1,0 0,5 0,0 0 0,2 0,4 0,6 0,8 1 1,2 Temperature ratio - s - e s

33 Linearized vertical temperature distribution calculated by Mundt model is the most common model.

34 Commonly used other In Nielsen model, a linear temperature gradient between floor and the height of mixing layer (stratification height) is predicted. Over the mixing layer, the room air temperature is assumed to be constant. The estimation accuracy of the vertical temperature distribution can be improved by modelling based on three nodes (Mateus and da Graça) methods

35 height (m) Comparison of calculated and measured vertical temperature gradients- persons Mateus and Da Graça s model works fine with occupants, computers and floor heat loads. The linear two node models cannot accurately describe heat loads that exist in the occupied zone temperature ( C) Measured Mundt Nielsen Mateus and da Graça Φ 12occupants = 900 W, θ s = 18.7 C, q s = 0.15 m 3 /s

36 height (m) Comparison of calculated and measured vertical temperature gradients- warm window The linear two node models (Mundt) works well with a warm window. However, accuracy of Mateus and Da Graça s model with the warm window and warm ceiling were not so good temperature ( C) Measured Mundt Mateus and da Graça Nielsen Φ window_3.5 = 520 W, θ s = 19.1 C, q v = 0.15 m 3 /s

37 height (m) Comparison of calculated and measured vertical temperature gradientscompination case 3.5 Mateus and Da Graça s model works fine with occupants, computers and floor heat loads temperature ( C) Measured Mundt Mateus and da Graça Nielsen Φ 10occupants = 750 W, Φ floor = 260 W, Φ light = 232 W, Φ win_3.5m = 520 W, θ s = 18.1 C, q v = 0.1 m 3 /s

38 Cooled ceiling - high temperature cooling Height above floor level, z [m] 2,50 2,00 1,50 1,00 0,50 0,00 = 0,6 = 0,5 = 0,4 Cooled ceiling = 0 = ratio of the cooled ceiling cooling output to the total cooling output (Tan 1998) 0,8 1,0 1,2 1,4 Relative air temperature (relative to temp. at 0,1 m above the floor) Cooled ceiling is ok when the cooling output of the ceiling is less than 40% of the total cooling. Cooled ceilings, or cooling convectors, decrease the air quality benefit. Mixing ventilation should be considered as an alternative.

39 Vertical air temperature profile with radiant floor at three locations (S1-S3). The radiant floor effects the temperature gradient and higher vertical air temperature differences are expected. The 50%-rule is not valid in the occupied area.

40 The dimensionless room air temperature profile in the occupied zone and the floor heating capacity A new method of calculation is proposed, using an 80%-rule as the limit, for a floor heating capacity of about 60 W/m 2.

41 Heating by radiator/convector Warm air from radiator balances meets the cold air from window/wall

42 Floor heating - low temperature heating Floor heating is ok as long as the floor temperature is moderate (i.e. less than appr. 25 C)

43 Case Study- Air distribution with four typical air supply methods in a classroom The performance of four typical air distribution methods in winter and summer conditions with different occupancy ratio was studied by physical measurements and smoke visualization in a mock-up classroom

44 Case Study- Field measurements for a multipurpose arena

45 In the guide, there is also new resarch findings Full-scale tests and CFD- simulations of indoor climate conditions Test on the performance of displacement ventilation proper simulation of occupants Airborne Cross Infection Risk in a Room with Displacement Ventilation Displacement ventilation design based on occupants response Convective boundary layer around human body

46 Best suited for: Gyms; Pros and cons of displacement Meeting rooms; Classrooms; Tall rooms: Convention centres, Lobbies, Sport arenas, Auditoriums, Theatres, Museums, Airports, Shopping centres, etc. ventilation Advantages: Less cooling needed for a given temperature in the occupied space; Longer periods with free cooling; Potential to have better air quality in the occupied spaces; The system performance is stable with all cooling load conditions. Less suited for: Where surplus heat is the main problem, and relatively low specific outdoor airflow rate is needed; Where there are space constraints for supply diffusers and duct work; When the requirement is to cool in low height rooms (in offices, consider mixing and cooling panels or chilled beams); Where there are significant disturbances to air flow near the floor (for example, furniture); Where the contaminants are cooler/denser than the ambient air

47 Thank you for your attention To order the REHVA guidebooks : / section Bookstore or through your national member