Testo AC Applications Guide & Reference
Rev 1.1
page 14
6
Understanding airflow and how to measure it
CAUTION
If the airflow is not set correctly, the system cannot operate as designed!
Airflow is one of the most overlooked yet the most important parts of verifying proper
operation of air conditioning systems. Low airflow can cause symptoms like evaporator
freezing, low system capacity, poor distribution and high-energy consumption. High airflow
can cause symptoms of poor humidity removal, higher energy costs, noise, drafts and
water/equipment damage due to water droplets blowing from the evaporator coil from
excessive air velocity. Air conditioners are designed for a nominal 400 CFM (450 for heat
pumps) of airflow per ton.
To operate with the designed capacity the airflow has to be set to the
manufacturer's design criteria at the evaporator coil.
Temperature drop across a coil
will vary with the latent load (humidity) the more humidity, the more cooling energy goes to
converting water vapor to water. The temperature drop across the evaporator can easily
be between 16 to 24 F. Therefore it is imperative to set the airflow to the proper range
and not to rely on the temperature drop across the coil to verify system performance. It is
important to understand that the conditions of the air entering the coil will not normally
affect the designed temperature difference of the coil. It will however affect the
temperature of the air leaving the coil.
As the humidity of the air entering the coil goes up, the temperature drop across the coil
decreases. If the system uses a fixed- type metering device, the evaporator superheat will
increase proportionally with an increase in humidity. System utilizing a thermal expansion
valve (TXV) under conditions of extremely excessive heat and humidity (high load) can
see an increase in suction pressure and temperature raising the temperature of the
evaporator above the design temperature difference of the coil in attempt to maintain
designed superheat to control the load. While this is not common with air type
evaporators it can and it does happen. Under these conditions the equipment would be
considered to be operating far outside of its design conditions.
CAUTION
Temperature drop across an air conditioner evaporator coil cannot be used to set
the airflow.
The most common and easiest way to verify and set airflow is to use one of the following
methods:
1) Rotating Vane Anemometer
2) Pressure drop across the dry evaporator coil
3) Total external static pressure method
4) Pitot tube and digital manometer
5) Velocity Stick (Hot Wire Anemometer)
6) The temperature rise method (Sensible heat formula)
7) RPM and manufacturers fan curve (Belt or VF Drive)
The airflow must first be set according to the equipment design not to the air
delivered at the registers. While the design of the duct system is imperative for proper air
distribution to the conditioned space, air measurements are only to be measured at the
appliance for the equipment commissioning procedure. Due to leakage inherent with all
ducting systems, airflow cannot be measured at the registers to verify correct airflow
Summary :
The most common and easiest way to verify and set airflow is to use one of the following methods: 1) Rotating Vane Anemometer 2) Pressure drop across the dry evaporator coil 3) Total external static pressure method 4) Pitot tube and digital manometer 5) Velocity Stick (Hot Wire Anemometer) 6) The temperature rise method (Sensible heat formula) 7) RPM and manufacturers fan curve (Belt or VF Drive) The airflow must first be set according to the equipment design not to the air delivered at the registers.
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eaporator,system,across,set,drop,humidity,designed,design,conditions,heat,cannot,proper,increase