By Norm Christopherson
Measuring airflow has always been one
of the more difficult and time consuming tasks a service person does. In fact, it is not generally done because it is too time
consuming, requires special tools and is not always so easy to do.
In addition, airflow measurement is not usually taught to service people
unless they are going to specialize in air balancing.
Nevertheless, knowing the airflow in cfm can be useful to
troubleshooters. Most service technicians check the airflow by measuring the
temperature drop across the cooling coil or the temperature rise across a
heating system. If the air
temperature drop or rise falls within an expected range, the assumption is the
airflow is acceptable. Here is an
easy method of measuring the airflow through a system, which uses electric
heaters. This method applies
wherever electric heat is found. Electric
heaters are often utilized for reheat as well as for emergency heat on heat
pumps.
Assume you have an electric reheat coil that
draws 25 amps at 240 volts. You
take the temperature rise of the air as it passes through the electric heater
and find that the air is heated from 50 degrees to 80 degrees.
That is a 30-degree rise in air temperature.
Now you multiply the 25 amps by the
240 volts to find that the wattage output of the heater is 6000 watts. Since there are 3.42 Btu per watt, you multiply the 6000
watts by 3.42 to find that the electric heater is adding 20,520 btuh to the air.
This amount of heat is increasing the air temperature by 30 degrees.
Now you multiply the 30 degrees by
the constant 1.08 to get a number, which can be divided, into the 20,520 Btu to
find the airflow in cfm.
Multiplying the 1.08 by 30 degrees of rise gives you a number of 32.4. Divide the 20,520 by this factor of 32.4 and you find that the cfm of air is 633 cfm.
The 1.08 multiplier is not some
mysterious magic number. This
number includes the specific heat of air (.24 btu per pound per degree F).
It takes .24 Btu of heat to change the temperature of one pound of air by
one degree Fahrenheit. The 1.08
also contains the specific density of air (.075 pounds per cubic foot).
The air is measured in cfm yet the specific heat is per pound of air.
The weight per cubic foot of air (.075 lbs) is needed to convert between
the air volume and weight. Also
contained in the 1.08 factor is the number of minutes in an hour (60 minutes per
hour). This is required to convert
between Btu per hour and cubic feet
per minute. The factor of 1.08 is the product of the specific heat (.24
btu) times the density (.075 lbs cubic foot) times the number of minutes per
hour (60 min\hour).
The factor 1.08 assumes standard air at 70 degrees F at sea level. For practical purposes on most air conditioning and heating systems, the specific heat of .24 will remain a good useable constant. Since there will always be 60 minutes in each hour, this too is a fixed constant. Should we find ourselves at an altitude other than sea level however, the density of the air may change enough to affect the accuracy of our formula. The following list of factors for different altitudes gives the adjusted factor due to the change in the air density for that altitude.
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ALTITUDE 0’ 1000’ 2000’ 3000’ 4000’ 5000’ 6000’ 7000’ 8000’ 9000’ |
FACTOR 1.08 1.04 1.00 .96 .93 .90 .86 .83 .80 .77 |
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Be careful when taking the
temperature readings so as not to allow the thermometers to be in the line of
sight of the heaters. If the
thermometers are placed in sight of the heaters, radiant heat from the heaters
will increase the temperature readings and give a false airflow rate.
It is the temperature rise of the air that we are looking for, not the
temperature of the heaters.
Remember, this method of determining
the airflow through a heat pump can be used to determine the cfm with the
heaters turned on and the cfm will be the same when the heat pump is in the
heating or cooling cycle. Whether
the air is being heated or cooled, the airflow rate in cfm is the same. The air density in pounds per cubic foot changes but the cfm
does not.
The formula used here for determining cfm comes from the sensible heat formula: Btuh = 1.08 times CFM times temperature change.
Norm is a technical writer, seminar speaker and test proctor for EPA, 410A and ESCO & NATE certifications.
He can be contacted at nchristo@juno.com