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%.