SWEGON AB Indoor Climate Systems 2004 - Air distribution products - Rev. 5 June, 2007
www.swegon.comMeasurements and commissioning
General
Badly adjusted ventilation systems are often the reason why designs do not fulfil their intended purpose. As a rule, this is because technical installation instructions and details have not been fully carried out, and due to lack of knowledge the adjustment and measurement techniques.
Commissioning must be carefully considered at the design stage. It is also necessary to take into account any changes which may take place during construction. Commissioning can therefore be very time-consuming if it is not planned correctly.
It is important to allocate sufficient resources for this work. This should be seen as a safeguard to ensure that the installation functions as it should on completion.
Recommendations for the measurement of air flows and how this should be done are given in the report T22:1998 "Methods for the measurement of air flows in ventilation installations", which can be ordered from the Swedish Building Service.
Measurements
In a ventilation system, the deviation from prescribed values for an air flow must not exceed 15%, including measurement error. This requirement is stated in VVS AMA, Sweden.
Instruments must be calibrated using a recognised method with an acceptably small error. Calibration curves should be used where the correction is given as a function of the observed value.
Exact measurements can never be made. The measurement of air flows requires the following sources of error to be observed.
- Instrument error - denoted m1,which could be caused by friction in the instrument or other similar problems, or it could be residual system faults from calibration.
- Method error - denoted m2. For example, this could be a result of faulty measurement locations. This error is obtained for the recommended methods given in the report mentioned above.
- Reading error - denoted m3. Caused by problems such as poor resolution on a scale.
With these errors as a starting point, a probable measurement error, m, in accordance with VVS AMA regulations, can be calculated:
Measurement in ducts
There are three basic methods for measurements in ventilation ducts:
A1 - measurement with a Prandtl tube
A2 - measurement with a fixed measuring nozzle
A3 - measurement with a tracer gas
|
Method | Designation | Method error m2 |
Traversing with a Prandtl tube
in a duct with: | | |
| | |
Circular cross
section | A11 | 4-5% recommended
7% alternative
measurement plan |
| | |
Rectangular cross
section | A12 | 4% recommended
7% alternative
measurement plan |
| | |
Fixed measuring
unit | A2 | See in built size error
limits 5 and 10% |
| | |
Deciding air flow with
the aid of tracer gas | A3 | 5 and 10% |
|
Table 8.Method for measuring the air flow in duct systems.
To reduce the time needed and to increase accuracy when commissioning, Swegon recommends that permanent measuring devices (method A2) should be used in as many cases as possible. Swegon has various types of measuring units; the most recent being CIM, intended for ducts that need to be accessible for cleaning. CIM can be easily removed from the duct during cleaning.
There is also the adjustable measuring unit, CRM, where the measurement function has been complemented with a damper for easy adjustment of the air flow.
The air flow is decided by measuring the pressure drop difference over the central body. The flow can then be produced from a special adjustment diagram, (MIS).
Figure 53.Adjustable measuring unit.
Supply air terminal
Three methods are recommended for measuring the air flow in supply air terminals.
|
Method | Designation | Method error |
Pressure drop with
a permanent
measuring device | C2 | 5% |
| | |
Bag method | C5 | 3% |
| | |
Measurement with a
conventional anemo-
meter, with a extension
sleeve | C3 | 5% |
|
Table 9.Methods of measuring air flow in supply air terminals.
Swegon's air terminals are supplied with a fixed measuring device in accordance with method C2. The measuring device is either placed directly in the terminal or in the multi-function plenum box, which is available as an accessory for the terminal. A manometer can be attached to the device and a characteristic pressure measurement can be made. The air flow is a function of the characteristic pressure difference.
In the plenum box there is an adjustment damper which is easily accessible through the device. The design of the box provides good sound attenuation and an even distribution of air through the terminal, which guarantees an even spread pattern.
In all our plenum boxes, the damper is easily removable for cleaning.
There are either one or two measurement outlets on diffusers, depending on their type and the plenum box used with them.
Balancing factor (k-factor)
For every terminal device or plenum box there is a balancing factor (k-factor), which has been specially determined for each product. The air flow is calculated using the following formula:
where | q = | air volume in l/s |
| k = | balancing factor |
| Dpi = | measured pressure difference
(balancing pressure) in Pa |
This procedure for determining the air flow applies to both A2 and C2 methods.
Examples of diffusers with one or two measurement outlets.
Figue 54. One measurement outlet.
Figure 55.Two measurement outlets.
Continual checking of the commissioning data is carried out at the Swegon's laboratory.
Swegon has issues a special catalogue with instructions relating to the mounting, commissioning and maintenance of the product. These instructions are called MIS-instructions.
To simplify the commissioning procedure, the measuring contacts are lengthened using a plastic tube. The adjustment damper is supplied with a cord regulator (the white cord opens and the brown cord closes the damper), so that the adjustment can be carried out without dismantling the ceiling panels or terminal.
Commissioning of displacement terminal unit.
When dimensioning supply air diffusers for very low sound level or short throw, a low presure drop is obtained. This in turn means that the commissioning pressure Dpi is low and there can be problems measuring the pressure.
Commissioning of supply air terminal with regulating cords and plastic measurement tube.