A commercial laundry runs a dryer that has an exit flow of 1 lbm/s moist air at 120 F, 70% relative humidity. To reduce the heating cost, a counter flow stack heat exchanger is used to heat the incoming fresh air at 68 F with the exit flow, as shown in Fig. P11.105. Assume the outgoing flow can be cooled to 77 F and the two flows have the same mass flow rate of dry air. Is there a missing flow in the figure? Find the rate of energy recovered by this heat exchanger.

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4- 1+ Exit from dryer Exhaust 2 +3 Inlet FIGURE P11.105