The air-distribution system is the critical part of an air-conditioning system. If insufficient air is circulated, proper cooling cannot be done. On the other hand, handling large quantities of air is expensive in both initial cost and operation. The amount of cool air required increases rapidly the closer its temperature is brought to the desired room temperature.
If, for example, we wish to maintain 80F DB (dry-bulb) in a room and we introduce air at 60F, the colder air when warming up to 80F will absorb an amount of sensible heat equal to qs . According to Eq. (13.30), qs = 1.08Q1(80 60) 21.6Q1, where Q1 is the required airflow in cubic feet per minute. From Eq. (13.30), it can be seen also that, if we introduce air at 70F, with a temperature rise of 10F instead of 20F, qs 10.8Q2, and we shall have to handle twice as much air to do the same amount of sensible cooling.
From a psychrometric chart, the dew point of a room at 80F DB and 50% relative humidity is found to be 59F. If the air leaving the air-conditioning unit is 59F or less, the duct will sweat and will require insulation. Even if we spend the money to insulate the supply duct, the supply grilles may sweat and drip. Therefore, theoretically, to be safe, the air leaving the air-conditioning unit should be 60F or higher.
Because there are many days when outside temperatures are less than design conditions, the temperature of the air supplied to the coil will fluctuate, and the temperature of the air leaving the coil may drop a few degrees. This will result in sweating ducts. It is good practice, therefore, to design the discharge-air temperature about 3F higher than the room dew point.
Thus, for 80F DB, 50% RH, and dew point at 59F, the minimum discharge air temperature would be 62F as insurance against sweating. The amount of air to be handled may be obtained from Eq. (13.30), with a temperature difference of 18F.