5.7 For air-conditioning system operation in cold

weather, it is possible to introduce enough outdoor

air to eliminate cooling by the air conditioner. This

is called an economizer, and the control strategy is

to vary the amount of outdoor air entering a

plenum so that the outlet state is at the set point.

For zone conditions of 75 F and 50% RH and a

constant circulating air flow rate of 25,000 cfm,

determine the outdoor and return air flow rates

necessary to maintain a 55 F supply air leaving the

plenum. The outdoor air temperature varies

between 10 F and 50 F and the relative humidity is

always 30%.