This cheat sheet covers the basic engineering model of a vapor-compression refrigeration cycle, the most common cycle used in refrigerators, heat pumps, and air conditioners. Students need it to connect equipment parts with thermodynamic processes such as compression, condensation, expansion, and evaporation. It is useful for reading cycle diagrams, solving energy balance problems, and comparing system performance.
Key Facts
- The four main components of a vapor-compression refrigeration cycle are the compressor, condenser, expansion valve, and evaporator.
- In the evaporator, the refrigerant absorbs heat from the cold space, so q_in = h1 - h4 for an ideal steady-flow model.
- In the compressor, work is added to raise refrigerant pressure and temperature, so w_in = h2 - h1.
- In the condenser, the refrigerant rejects heat to the surroundings, so q_out = h2 - h3.
- In the expansion valve, throttling is modeled as constant enthalpy, so h3 = h4.
- The coefficient of performance for a refrigerator is COP_R = q_in / w_in = (h1 - h4) / (h2 - h1).
- The coefficient of performance for a heat pump is COP_HP = q_out / w_in = (h2 - h3) / (h2 - h1).
- For the same cycle, COP_HP = COP_R + 1 because the heat delivered equals the cooling effect plus the compressor work.
Vocabulary
- Refrigerant
- A working fluid that absorbs heat at low temperature and rejects heat at higher temperature as it circulates through the cycle.
- Compressor
- A device that increases the pressure and temperature of refrigerant vapor by adding mechanical work.
- Condenser
- A heat exchanger where high-pressure refrigerant rejects heat and usually changes from vapor to liquid.
- Expansion valve
- A throttling device that drops refrigerant pressure without producing useful work and is usually modeled with constant enthalpy.
- Evaporator
- A heat exchanger where low-pressure refrigerant absorbs heat from the refrigerated space and usually changes from liquid-vapor mixture to vapor.
- Coefficient of performance
- A measure of refrigeration or heat pump efficiency equal to useful heat transfer divided by required work input.
Common Mistakes to Avoid
- Confusing the condenser and evaporator is wrong because the evaporator absorbs heat from the cold space while the condenser rejects heat to the warm surroundings.
- Using efficiency instead of COP is wrong because refrigeration performance can be greater than 1 since it measures moved heat divided by work input, not work output divided by heat input.
- Assuming the expansion valve changes temperature only is wrong because the key ideal model is h3 = h4, with a major pressure drop and no useful work output.
- Forgetting compressor work in energy balances is wrong because the condenser heat rejection equals the evaporator heat absorption plus the compressor work input.
- Reading pressure-enthalpy diagrams backward is wrong because the cycle normally moves from evaporator outlet to compressor, condenser, expansion valve, and back to evaporator.
Practice Questions
- 1 A refrigerator has h1 = 395 kJ/kg, h2 = 430 kJ/kg, h3 = 250 kJ/kg, and h4 = 250 kJ/kg. Find q_in, w_in, and COP_R.
- 2 A heat pump has q_out = 12 kW and compressor input power of 3 kW. Find COP_HP and the heat absorbed from the cold source.
- 3 If a refrigeration system removes 6 kW of heat from a freezer and has COP_R = 3, what compressor power is required?
- 4 Explain why lowering the condenser temperature or raising the evaporator temperature usually improves the COP of a vapor-compression refrigeration cycle.