Calculating cfm – I.C.E. 3 OSDs User Manual

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EXAMPLE FOR CALCULATING C.F.M.

Example for Calculating The Amount of Air and Gas in a Direct

Fired Make-Up Air Unit Pull Through Type

YOU WILL NEED: A pressure differential gauge (Manometer) (Magnehelic) Thermometer -30

°F - 200 °F.

All units are factory set with a profile opening around to burner for 2950 F.P.M. velocity. Due to more or less
external static pressure the velocity may not be within this range on start up of unit. The pressure drop should be
checked to insure the unit is operating around this velocity.

A pulley adjustment or change should be done to bring the velocity within operating range. If an air balance has
been done and the C.F.M. verified to be correct as stamped on the rating plate and the velocity across the burner
is not correct the profile area should be adjusted.

If velocity is higher than 2950 FPM then the profile area should be increased. This can be achieved by
readjusting the top and bottom profile plates.

The profile area is stamped on the rating plate, but to get to the free area you will have to deduct the space taken
up by the burner.

If Midco burner is used deduct .65 sq. feet for each 1 foot section, or .33 sq. feet for each 6” section.

If Maxon burner is used deduct .45 sq. feet for each 1 foot section, or .23 sq. feet for each 6” section.

By using a magnehelic differential gauge across the burner profile will give you the pressure drop and using the
burner capacity chart will tell you what velocity you have through the profile. If the unit is operating between .45
W.C. and .65 W.C. it is considered to be within operating range, as the low air switch is factory set to make at .25
W.C. and the high profile switch is set to open at .95 W.C.

This should allow for a wide leeway before the unit will lock out due to low air / high air velocity across the
profile plate.
Under normal servicing the tubes from the air switch should be checked to insure they are free of any moisture or
dirt as this could cause the burner to lockout on the flame safeguard relay as both switches are in the flame rod
circuit.

If moisture is entering the tubes they may have to be repositioned to a lower area on the cabinet. Care must be
taken to insure that they will operate to shut down the unit if the velocity is out of the operating set points.