Mechanical Engineering
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# Thermodynamics

## Question

An air-to-air heat exchanger is used in some areas of the country in winter to recover the exhaust heat of the kitchen or the bathrooms to pre-heat the fresh air intake into buildings. In case of a hardware store, a heat exchanger is used to recover heat from 200 kg/s of exhaust air at 20C, to preheat incoming air from outside that is at -10C. The design engineer is asking that the 100 kg/s of incoming air be heated to 5C by just using the energy of the exhaust air.

1.Determine the temperature of the exhaust air when it is rejected to the outside and determine how much supplementary heat will it be needed in an optional heater, to raise the incoming air (100kg/s) which has been pre-heated to 5C, for an additional 15C to 20C.

Assume Cp of air=.98 kJ/kg.K for exhaust and incoming air. A sketch showing the heat exchanger and the optional heater will help you.

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