Kapitel 11 (Eng)

Question 1

Why is the reversed Carnot cycle executed within the saturation dome not a realistic model for refrigeration cycles?

Answer 1

Because the compression process involves the compression of a liquid­vapor mixture which requires a compressor that will handle two phases, and the expansion process involves the expansion of high­moisture content refrigerant.

Question 2

Consider two vapor­compression refrigeration cycles. The refrigerant enters the throttling valve as a saturated liquid at 30C in one cycle and as subcooled liquid at 30C in the other one. The evaporator pressure for both cycles is the same. Which cycle do you think will have a higher COP

Answer 2

The cycle that involves saturated liquid at 30C will have a higher COP because, judging from the T­s diagram, it will require a smaller work input for the same refrigeration capacity.

Question 3

The COP of vapor­compression refrigeration cycles improves when the refrigerant is subcooled before it enters the throttling valve. Can the refrigerant be subcooled indefinitely to maximize this effect, or is there a lower limit? Explain.

Answer 3

The minimum temperature that the refrigerant can be cooled to before throttling is the temperature of the sink (the cooling medium) since heat is transferred from the refrigerant to the cooling medium.

Question 4

When selecting a refrigerant for a certain applica­tion, what qualities would you look for in the refrigerant?

Answer 4

The desirable characteristics of a refrigerant are to have an evaporator pressure which is above the atmospheric pressure, and a condenser pressure which corresponds to a saturation temperature above the temperature of the cooling medium. Other desirable characteristics of a refrigerant include being nontoxic, noncorrosive, nonflammable, chemically stable, having a high enthalpy of vaporization (minimizes the mass flow rate) and, of course, being available at low cost.

Question 5

Consider a refrigeration system using refrigerant­ 134a as the working fluid. If this refrigerator is to operate in an environment at 30C, what is the minimum pressure to which the refrigerant should be compressed? Why?

Answer 5

The minimum pressure that the refrigerant needs to be compressed to is the saturation pressure of the refrigerant at 30C, which is 0.771 MPa. At lower pressures, the refrigerant will have to condense at temperatures lower than the temperature of the surroundings, which cannot happen.