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.
Understanding Refrigeration Cycle Reference
A refrigeration system moves thermal energy rather than creating cold. The cold region loses energy when the refrigerant boils inside the evaporator. Boiling needs energy to separate liquid molecules into vapor, so the refrigerant takes that energy from air, food, water, or another material nearby.
A fan often pushes air across the evaporator coil to improve this transfer. The coil must be colder than the space being cooled.
Heat naturally moves from a warmer object to a colder coil. This temperature difference is necessary, but making the coil much colder can reduce efficiency and may cause frost.
Pressure controls the refrigerant boiling and condensing temperatures. At low pressure, a refrigerant can boil at a low temperature and collect heat from a refrigerated space. At high pressure, the same refrigerant can condense at a higher temperature and release heat to outdoor air or cooling water.
This is why a blocked condenser or a hot outdoor day makes an air conditioner work harder. The high side pressure rises, the compressor needs more input work, and less heat may leave the condenser. Dirty coils matter because dust acts as insulation and slows heat transfer.
A pressure enthalpy diagram gives a compact picture of these changes. Enthalpy represents the energy carried by each unit mass of flowing refrigerant. The dome on this diagram marks liquid vapor mixtures.
Inside the dome, liquid and vapor exist together. The left boundary is saturated liquid, while the right boundary is saturated vapor. A point to the right of the dome is superheated vapor.
A point to the left is subcooled liquid. Students should track each state carefully before using property tables.
The pressure identifies the correct table row or saturation temperature. The phase description tells whether a saturated value, superheated table, or subcooled approximation is appropriate.
Real equipment differs from the ideal model. Refrigerant may lose pressure as it flows through pipes and heat exchangers. The compressor is not perfectly efficient, so its actual outlet state has more enthalpy than an ideal isentropic compression would predict.
Engineers often add a small amount of superheat at the evaporator outlet. This helps prevent liquid droplets from entering the compressor, where they can cause damage. They may add subcooling at the condenser outlet to make sure liquid reaches the expansion valve.
When solving problems, state the assumptions clearly. Check energy units, keep heat and work on a per unit mass basis, and use consistent sign conventions.
A result should match physical behavior. The compressor outlet should be hot vapor, the condenser outlet should be mostly liquid, and the evaporator outlet should be vapor before the compressor.