SWEGON AB Indoor Climate Systems 2004 - Air distribution products - Rev. 5 June, 2007
www.swegon.comDisplacement ventilation
General
Figure 42 shows a displacement system. Air is supplied with under-temperature at floor level and at ceiling level. The supply air spreads across the floor and gradually rises when it comes into contact with heat sources (e.g. people), which create convection streams.
The heat source in the figure represents a contaminant, in these case heated air, which is lighter than the surrounding air. The contaminant rises to the ceiling, and more air is drawn into the plume.
If the volume flow of air in the contaminated plume is greater than the ventilation air flow, when it reaches the ceiling, the contaminated flow may not be extracted directly with the ventilation air. Some of the polluted air is therefore recirculated downwards into the room. A front with polluted air then forms, which begins to move downwards.
The front stops at that level where the volume flow of the rising contaminated plume is equal to the ventilation flow. Two zones are then created in the room, the upper zone with contaminated air and the lower zone with "clean" air. In rooms with high ceilings and where active work is carried out, it is desirable to make the clean zone as high as possible, and preferably over the breathing zone.
The required air flows are determined on the basis of applicable standards and hygiene limits.
Figure 42.Displacement ventilation.
The air is supplied at under-temperature at floor level and is extracted at ceiling level. The supply air (qv) is distributed across the floor and gradually rises when it comes into contact with heat sources.
qv = ventilation flow l/s
qx = convection flow in the polluted plume on the level x, l/s.
A = Contaminated zone
B = Mobile front with contaminated air
C = Clean area
Design
When using ventilation based on the displacement principle, the air is supplied at a very low velocity. This means that the spread profile in the room is controlled by the density differences of the air. The spread profile can be said to be thermally controlled. This means that other aspects must be considered than are normally observed with mixing ventilation.
It is therefore important to analyse the elements that shape the final conditions in the room in the design work. The following elements can be distinguished.
1. | Process Analysis | 2. | Calculation |
| | | |
| - Activity type/type of locality | | - Air flow |
| - Activity level | | - Energy balance calculation |
| - Convection streams | | - Convection air flows |
| - Room dimensions | | - Resulting sound level |
| - Room layout | | - Affected zone |
Figure 43.Displacement ventilation.
Process analyses
Type of activity/type of locality
The use of displacement systems in contaminated rooms is very suitable, since the terminals have a very small induction rate.
Activity level
In rooms where a high level of comfort is required, it is essential to determine the comfort requirements based on the activity level in the room.
Convection curents
The size and position of prevailing convection streams in any locality determine the air movements and consequently the efficiency of the ventilation. It is therefore critical, when designing a system, to carry out a careful analysis of machine size and their convection heat, the number of people and their activity levels, the effect of the sun and the heating or cooling effect from walls, radiators etc. If the design is to be based on the generation of contamination in the room and a maximum pollution concentration in the occupied zone, the air flow must be designed according to current norms and regulations regarding hygiene limit values. See "Calculations" for more information.
Room dimensions
The height of the room greatly affects the air exchange and ventilation efficiencies obtained.
A high ceiling gives more space for contaminated air, while with a low ceiling, the opposite is the case.
Room layout / General
Since the location and size of heat sources have a considerable influence on the final ventilation result, it is important to know where they will be placed.
A careful analysis of workshop machinery positions for example, is necessary during the design stage.
In comfort ventilation systems, it is necessary to know how the room will be furnished in order to choose the best location for the supply air terminals. Guidelines for this are illustrated by the following figures
Room layout / Open-plan office
Try to find places where people do not sit regularly. Often there are distinct "walkways" in the room, and the terminals can be placed near them.
Flat or half-round terminals beside columns, or circular terminals in an office reference area or near a copying machine.
Other places which can be suitable for terminals in an office are a reference area or near a copying machine.
Figure 44.Terminals, placed in an open-plan office.
1 = Concentration of terminals far from work spaces
2 = Bookshelves
3 = Copier
4 = Alt. supply air terminals in the form of false columns
Room layout / Cell offices
In normal cell offices, the depth of the cell is often larger than the width. Try to place the device in or on a wall between the room and the corridor. This provides a suitable distance between the terminal and the person working at the desk.
Notes to Figure 45.
A.
The terminal can often be placed in a recess close to the door. It is usual that no fumiture is located along the door wall. since the light switch is normally located there.
B.
A terminal placed behind a door is not very suitable. The affected zone will be influenced and increased air velocities are created along the wall (in this case where a visitor is sitting).
C.
If the device is placed near an outer wall, it should be moved to one side or the other depending on the location of the desk. The occasional visitor is not as sensitive to draughts as the office´s occupant, who sits in the office the whole day.
Figure 45.Terminals, placed in module offices.
Flexible spread patterns, affected zones
It is in most cases a great advantage if the diffuser spread pattern can be modified to prevent seating locations from being inside the affected zone.
Swegons displacement diffusers are equipped with the VariZone system. Every diffuser has a number of circular rotatable air deflectors behind the perforated front plate. The spread pattern can be modified by turning these in various ways.
This represents a clear benefit when the layout or function of the room changes with time.
One rule of thumb is that the spread of the affected zone over the floor area (m2) which is stated in the catalogue can be changed from the standard setting to:
- Affected zone to the right
- Affected zone to the left
- Long and narrow affected zone
All these alternatives have one factor in common, namely that the spread area is the same as the standard setting.
Figure 46.Examples of alternative settings.
Figure 47.Diffusers whose affected zones have been adapted to suit the room furnishings.
Figure 48. Diffusers without any changes in their spread pattern.
Calculations
Air volume
For industrial systems, where the ventilation system must be designed for a specific, maximum pollution concentration in the occupied zone, it is important that the supply air flows are determined according to the quantity of pollution released m (mg/s) in the room and the permissible pollution concentration c (mg/m3).
The air flow q is obtained by:
According to this equation, the concentration c must always be less than the hygiene limit described in current regulations. If the pollution in question is also present in the supply air, the air flow q is calculated by:
where cin = pollution concentration in the supply air in mg/m3.
In comfort systems the minimum air flow is generally 0.35 l/s per m2 floor area. For cell offices, however, the outside air flow should not fall below 12 to 15 l/s per person.
Energy balance calculation
A calculation of the internal and external heat loads, where consideration is given to heat accumulation in the building, must be the foundation for calculation of the required cooling load. Together with the comfort requirements, this provides a suitable basis for the selection of a proper system and supply air flows. Calculation of the ability of the displacing system to supply cooling power is included in these guidelines.
Convection air flows
The convection air flow for different heat sources in industrial systems does not need to be determined if the air flows are calculated according to the instructions given above. Even in comfort systems, the size of the convection streams should be ignored when selecting air flows.
Resultant sound level
The supply air terminal usually generates very little sound. The sound attenuation is also normally quite low, and so a careful sound calculation must be made for the duct system. Correction for any room absorption is made according to the chapter "Acoustics".
Affected zone
It should be pointed out that the affected zone size should be considered during the design phase. No persons should be expected to regularly occupy the affected zone. This is particularly important to remember in high-occupancy rooms. Selection of the device should be based first on the size and shape of the affected zone, and then on the sound generation of the device.
- Min. flow according to
norms and regulations | - Resultant sound levels
- Affected zone |
- Energy balance calculation | |
The relationship between air flow rate, temperature gradient and heat loads in displacement ventilation
One method of calculating the airflow required to limit the vertical temperature gradients at various heat loads is illustrated below. The method is taken from:
Memorandum 16 from Installation Techniques KTH, March 1991.
Symbols: | | |
tlg | = | air temperature at floor level |
tt | = | supply air temperature |
tf | = | exhaust air temperature |
s | = | vertical temperature gradient, °C/M |
h | = | height of the room, m |
Dtt,1 | = | temperture difference between 1.1 m level and supply air |
The temperature gradient must be limited so that it does not exceed the limit values which are given in ”Requirements for interior climates”.
The requisite lowest ventilation airflow for a certain maximum temperature gradient is obtained from Figure 51.
The temperature difference between 1.1 m level and supply air can be determined from Figure 52.
The air temperature at floor level (tlg) is checked with the aid of Figure 50. This check is important for comfort ventilation - tlg must not fall below 20°C.
Another practical guideline is that the supply air temperature must not fall below 18°C.
Figure 50.Non-dimensioned temperature difference at floor level for various air flows.
A = akg = 5 W/(m2,K) | (heat transition coefficient due to
convection at floor surface) |
B = akg = 3 W(m2,K) | |
Figure 51.Requisite ventilation flow as a function of the cooling effect of the various products of gradient and room height.
A = Air flow (l/s,m2)
B = Cooling effect (W,m2)
Figure 52.Temperature differences between air at floor level and supply air as a function of the cooling effect of the various products of gradient and room height.
A = Cooling effect (W/m2)
The method is described with the help of the following examples:
An office with a height of 2.7 m has a cooling requirement of 25 W/m2.
The vertical temperature gradient must be limited to 1.7 °C/m.
Calculate the