ME340, HW 1
To be submitted by 05:00 pm, Monday, 02 September 2024
1. Refrigerant-134a enters the compressor of a refrigeration system as saturated vapor
at 0.14 MPa, and leaves as superheated vapor at 0.8 MPa and 60 oC at a rate of 0.06
kg/s. Determine the rates of energy transfers by mass into and out of the compressor.
Assume the kinetic and potential energies to be negligible. Can you deduce the work
performed by the compressor on the refrigerant?
2. An adiabatic capillary tube is used in some refrigeration systems to drop the pressure
of the refrigerant from the condenser level to the evaporator level. The refrigerant R-
134a enters the capillary tube as a saturated liquid at 50oC, and leaves at -20oC. During
the process, the capillary absorbs heat from the surroundings at the rate of 1 kJ/s. If
the mass flow rate is 0.1 kg/s, determine the quality of the refrigerant at the inlet to
the evaporator. Also, determine the change in the internal energy of the refrigerant.
3. An air-conditioning system is to be filled from a rigid container that initially contains
8 kg of liquid R-134a at 28oC. The valve connecting this container to the air-
conditioning system is now opened until the mass in the container is 0.4 kg, at which
time the valve is closed. During this time, only liquid R-134a flows from the container.
Presuming that the process is isothermal while the valve is open, determine the final
quality of the R-134a in the container and the total heat transfer.
4. A tank whose volume is unknown is divided into two parts by a partition. One side of
the tank contains 0.03 m3 of refrigerant-134a that is a saturated liquid at 0.9 MPa,
while the other side is evacuated. The partition is now removed, and the refrigerant
fills the entire tank. If the final state of the refrigerant is 20oC and 280 kPa, determine
the volume of the tank.
5. A refrigerator operates on the ideal vapor-compression refrigeration cycle and uses
refrigerant-134a as the working fluid. The condenser operates at 1.6 MPa and the
evaporator at -6 oC. If an adiabatic, reversible expansion device were available and
used to expand the liquid leaving the condenser, how much would the COP improve
by using this device instead of the throttle device?
6. A commercial refrigerator with refrigerant-134a as the working fluid is used to keep
the refrigerated space at -30oC by rejecting its waste heat to cooling water that enters
the condenser at 18oC at a rate of 0.25 kg/s and leaves at 26oC. The refrigerant enters
the condenser at 1.2 MPa and 65oC and leaves at 42oC. The inlet state of the
compressor is 60 kPa and -34oC and the compressor is estimated to gain a net heat of
450 W from the surroundings. Determine (a) the quality of the refrigerant at the
evaporator inlet, (b) the refrigeration load, (c) the COP of the refrigerator, and (d) the
theoretical maximum refrigeration load for the same power input to the compressor.
7. A simple saturation cycle using the R22 refrigerant operates between 40oC and -15
oC.Neglecting any irreversibilities, calculate the change in COP and unit power
consumption if the vapour is superheated by 30o C at the inlet to the compressor.
