Q:

# Why does the cfm per ton of air conditioning vary?

A:

The cubic foot per minute, or cfm, per ton of air conditioning varies depending on application. Approximately 400 cfm is required for normal use, while 500 cfm is needed for high sensible heat situations. High latent heat loads use 350 cfm. Sensible heat directly affects the temperature of the room, while latent heat does not affect temperature because it causes a change in state.

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An air conditioner's cfm rating describes the volume of air, in cubic feet, that flows through the unit in one minute. A/C units in high sensible heat environments run at higher cfm values because of the increased cooling load. With normal cooling loads, the A/C unit requires approximately 7.5 air changes per hour to maintain a comfortable environment.

With an increased cooling load, the required amount of air changes increases to approximately 10 per hour, which is achievable because the A/C unit moves more air per minute. An A/C's sensible capacity describes the ability to remove sensible heat and lower the temperature of the air. An A/C's latent capacity is its ability to remove latent heat from the air. This removes humidity by causing water vapor in the air to condensate. If the air speed is too fast, the accumulated condensate is blown off the fins of the A/C's evaporator and back down the duct, which is why units in environments with high latent heat run at lower cfm values.

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