SWEGON AB Indoor Climate Systems 2004 - Air distribution products - Rev. 5 June, 2007
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Different concepts of efficiency
During the last few years we have begun to study the efficiency of ventilation systems and their role in removing air contaminants. In this connection, we talk about two different concepts:
- VENTILATION EFFICIENCY, which is a measure of how efficiently contaminants are removed.
- AIR EXCHANGE EFFICIENCY, which is a measure of how efficiently the air in a room is exchanged.
One of the main objectives for a designer therefore is to design and position supply and exhaust air terminals so that air exchange and ventilation efficiency are as high as possible.
The ventilation efficiency depends on several different parameters:
- positioning of supply and exhaust air terminals
- type of terminal
- supply air velocity
- temperature difference between supply and exhaust air
- the occurrence of disturbances, heat sources, activity etc.
According to a proposal by the Nordic Ventilation Group, the expression "specific air flow rate", (n), should be used instead of the expression "air change rate".
The specific air flow rate was called ”air change rate” with the unit airchanges/h. This however often lead to the misunderstanding that the air in a room is exchanged by the number of times per hour that the number indicates. The problem here is that the speed at which the air in a room is exchanged depends not only on the supply air flow and the volume of the room, but also to a large extent on the nature of the air currents in the room.
The surplus heat can be seen as a contaminant. The introduction of the concept of ”temperature efficiency” is therefore relevant.
Since surplus heat is considered to be a contaminant, we can replace ”concentration” with ”temperature” to obtain the temperature efficiency.
We differentiate between ”average temperature efficiency”, which applies to the whole room as an average value, and ”local temperature index” which applies to specific points in the room.Basic concepts
Ventilation efficiency, erc
Defined at a certain level of contamination, as the ratio between the concentration in the extract air and the average concentration in the room, i.e.
where Ce = equilibrium concentration in the exhaust air
where Cm = average concentration in the room at equilibrium
Local ventilation index, epc
where Cp = equilibrium concentration at point p
Air exchange efficiency, era
Defined as the ratio between the nominal time constant and the exchange time for the air in the room.
where | tn = nominal time constant
tm = average age of the air in the room
2·tm = the exchange time for the air in the room |
Note:
The average age of the air is directly related to the time it takes to exchange the air in the room.
To exchange all the air in the room takes an average time which is equal to twice the average age of the air in the room.
The average age of the air can be determined by measurements made at the exhaust air duct.
Specific air flow rate, n
where | q = the supply air flow rate (m3/h)
V = room volume (m3) |
Nominal time constant, tn
The nominal time constant (tn) is the time during which the supply, q, on average remains in the room.
where | q = the supply air flow (m3/h)
V = room volume (m3) |
Temperature efficiency, ert
(average)
where | tf = exhaust air temperature
tm = average room temperature (at equilibrium)
tt = supply air temperature |
Local temperature index, ept
where | tp = the temperature at point p at equilibrium. |
Occupied zone
The occupied zone is that part of the room, which is normally occupied by people, and should be defined together with both the builder and the architect. Its volume is determined by planes which are parallel with the walls, the ceiling and the floor of the room. The distance between the planes of the occupied zone and the room planes vary depending upon the purpose of the room.
The floor is the lowest horizontal plane of the occupied zone.
The following table provides an overview of the normal distances between the room plane and the occupied zone plane, as well as the usual distance ranges for each set of planes.
Figure 1.The marked surfaces define the occupied zone.
|
| Normal distance range from the | Normal value for distance between |
Room plane | room plane to the occupied zone | the room plane and the occupied zone |
Outer wall | 0.2 - 1.0 m | 1.0 m |
| | |
Inner wall | 0 - 0.6 m | 0.6 m |
| | |
Floor, lower limit | 0 - 0.1 m | 0.1 m |
| | |
Floor, upper limit - standing person | 1.8 - 2.0 m | 2.0 m |
|
Table 1.Limits of the occopied zone.
Affected zone
Affected zone is a concept used for low-velocity devices and is therefore of primary interest for displacement ventilation.
According to testing regulations currently in force (SS EN 122 39) Nordtest standard for Heating, Ventilation and Sanitation), the affected zone is defined by the dimensions av and bv, according to figure 2.
In the figure, dimension av represents the greatest horizontal distance from the wall (or the centre of the device in the case of a cylindrical device) to the isovel for v m/s.
Dimension bv is the greatest horizontal distance at a right angle to av between the end points of the isovel.
In the testing method it is pointed out that the isovel should be measured where the velocity is highest, i.e., not at a specific distance from the floor. In the testing method currently being proposed as an European norm (prEN 12239 Aerodynamic testing and rating for displacement flow applications), the velocity v m/s for the isovel has been defined as:
- 0.2 m/s for low-velocity devices intended
for comfort ventilation - 0.3 m/s for low-velocity devices intended
for industrial ventilation
For ceiling-mounted, low-velocity devices, the affected zone is defined according to figure 3.
It is essential that the ventilation designer notes how different manufacturers define affected zones. Various methods are used, such as:
- comfort zone, with its own special definition
- isovel on a level of 0.05 m above the floor
- isovel on a level of 0.10 m above the floor
Relatively large differences in the av and bv values can result if measurement and reporting methods deviate from those specified in the testing standard. DEVIATION FROM THE ACCEPTED METHOD ALWAYS RESULTS IN SHORTER AFFECTED ZONES!
Figure 2.Floor-mounted and wall-mounted, low-velocity devices.
Figure 3.Ceiling-mounted, low-velocity devices.
Far zone
The far zone is a concept which is used in connection with displacement ventilation. The far zone is defined as the zone outside the affected zone where density current takes place. The characteristics of density current are:
- it is powered by the difference in density between the supplied air and the room air.
- it causes a small induction of surrounding air
- it is very thin, normally around 10 cm
- it has somewhat lower velocity fluctuations (turbulence) than a jetstream.
The air velocities within a far zone are determined by:
- the heat load in the room
- the geometry of the room (width)
When an equalisation of the airflow over the width of the room has been achieved, the air velocity in the far zone can be calculated according to the following equation:
Where | q = | supply air flow (m3/s) |
| Dt = | difference between supply air temperature
and room air temperature (K) |
| b = | width of the room (m) |
| T = | absolute temperature of the room (K) |
| vf = | The air velocity within a far zone (m/s) |
Example: | |
Office of width 3,6 m | |
Semicircular section diffuser placed in rear edge | |
Supply air temperaturer | 18°C |
Room temperature | 24°C |
Air flow | 30 l/s |
This air velocity should be considered as the lowest value which can be obtained under density current conditions.
Figure 4. Far zone in the room with displacement unit.