DescriptionHomework 3

CHME 2224

Review Chapter 5 & 7 of Cengel to complete the following:

1. (Ch 5) A piston-cylinder device contains 0.27 kg of air initially at 2.1 MPa and 365°C.

The air is first expanded isothermally to 600 kPa, then compressed polytropically with a

polytrophic exponent of 1.2 to the initial pressure, and finally compressed at the constant

pressure to the initial state. Determine:

a) The boundary work for each process

b) The net work of the cycle

c) Draw a PV diagram of this process, labeling states 1, 2, and 3

2. Heat in the amount of 935 kJ is supplied to a Carnot heat engine, however there is no

thermometer in the system and the temperature is not known. This engine then rejects

415 kJ of this heat to a sink at 19°C. Determine:

a) The temperature of the source

b) The thermal efficiency of the heat engine

3. A refrigerator containing refrigerant-134a as the working fluid keeps a refrigerated space

at -35°C by rejecting waste heat to cooling water that enters the condenser at 18°C at a

rate of 0.35 kg/s and leaves at 26°C. The refrigerant enters the condenser at 1.2 MPa and

50°C and leaves at the same pressure subcooled by 5°C. if the compressor consumes 2.7

kW of power, determine:

a) The mass flow rate of the refrigerant

b) The refrigeration load

c) The COP

d) The minimum power input to the compressor for the same refrigeration load

Hint: To determine T leaving the condenser, use the pressure at this point to determine Tsat, then

remember that the actual T is subcooled by 5°C.

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