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VTS reserves the right to implement changes without prior notice
Ventus Air Handling Units
and test measurements.
6.1 Measurement of air quantity and AHU output adjustment.
Measurement of air quantity is a primary measurement in case of:
- AHU start-up and technical acceptance,
- when the system does not operate in line with the requirements and expectations,
- periodic control of AHU operation and performance,
- replacement of fan unit elements.
Before starting measurements and adjustment make sure to:
- check if damper at all air grates or registers are adjusted in line with the design.
- set fresh- and recycled-air dampers (if applicable) in end position - 100% of fresh air or full
recirculation,
- measure the current consumed by the fan's motor. If it is necessary minimize the air flow with the
main damper or reduce the fan rotation speed.
Determining quantitative air stream is based on the measurement of average air-flow speed in the
ventilation duct test cross-section. One of the most common ways of determining average speed is
the cross-section probing method with the Prandtla pipe and measurement of the speed-related
average dynamic pressure.
Crucial factors influencing the measurement accuracy are:
- location of the measured cross-section in relation to the elements,
- quantity and location of test points in the measured cross-section,
- stable and constant air flow.
It is highly recommended NOT to locate the measurement cross-section directly after:
- network elements causing deformation of velocity speed (knees, reductions, three-way
connections, dampers, etc.), fan, because in the cross-section some reverse speed may appear.
The measurement should be carried out at the duct's fragment with parallel walls and straight
segments at least 6 times longer than the duct diameter or equivalent diameters before the test point
and not less than 3 diameters after it. In the real ventilation system finding such a long straight
fragment can be a problem. In such a case determine the measurement cross-section in a place
where the smallest distortions of air-flow are expected and intensify a network of test points. Location
of the measurement cross-section should be determined on the system design stage. Detailed
recommendations concerning air-flow measurement and location of test points are defined by the
ISO 5221 standard.
We estimate the output being measured as sufficient unless it differs more than ±10% from the
designed one. In case of bigger disproportions, the output alike the designed one can be achieved
by:
- adjustment of the ventilation ducts network,
- changing adjustment of the main damper,
- changing the fan rotary speed.
While increasing fan's rotary speed you must by all means control motor's power consumption and
do not allow to exceed designed rated current value. For durability and permissible operation
parameters it is also very important not to exceed maximal rotor speed. In justified situations, when
there is a need to increase the air-flow output, it is recommended to apply a more powerful fan's
motor.
In systems equipped with dampers which automatically change ratio of fresh, recycled and
exhausted air or by-pass flow ratio, output measurements and main damper adjustment should be
carried out in end position of the setting. Next check the air ratio and overall output in the other end
position and, if necessary, carry out appropriate adjustment in order to achieve proper ration at
constant overall output.
6.2 Heat output adjustment of water heater
Heater output adjustment is proceeded by setting appropriate amount of air passing through the
AHU.
