Refrigeration Principles and Applications Flashcards

In an ideal vapor-compression cycle, the process across the expansion valve is best described as:

Answer: Isenthalpic

Throttling through an expansion device occurs at constant enthalpy because no work or heat transfer takes place.

What is the primary benefit of subcooling liquid refrigerant before it enters the expansion device?

Answer: It increases the refrigerating effect per unit mass of refrigerant

Subcooling lowers the enthalpy entering the evaporator, so each pound of refrigerant absorbs more heat.

With evaporating temperature held constant, raising the condensing temperature will typically:

Answer: Decrease COP and increase compressor work per unit of cooling

A higher temperature lift raises compression work and reduces refrigerating effect, lowering COP.

A reversed Carnot refrigerator operates between an evaporator at 250 K and a condenser at 300 K. What is its COP?

Answer: 5.0

Carnot COP = Te/(Tc − Te) = 250/50 = 5.0.

One ton of refrigeration is equivalent to:

Answer: 12,000 Btu/h

One ton equals 12,000 Btu/h, derived from melting one ton of ice in 24 hours (288,000 Btu/day).

Why does the volumetric efficiency of a reciprocating compressor drop as the compression ratio increases?

Answer: More re-expansion of gas trapped in the clearance volume

At higher pressure ratios, clearance gas re-expands over a larger part of the stroke, reducing fresh suction intake.

What is the main purpose of maintaining suction superheat at the evaporator outlet in a TXV system?

Answer: To ensure only vapor returns to the compressor

Superheat confirms all liquid has evaporated, protecting the compressor from liquid slugging.