Refrigeration List 10 practical applications of refrigeration systems:

The three principle types of refrigeration cycles are:

Vapor-compression

Absorption

Reversed Brayton

Vapor-compression Refrigeration Systems Most common refrigeration systems in use today. Assumptions for an ideal cycle include:

1. Steady state

2. Kinetic and potential energy changes are negligible

3. Reversible heat transfer through evaporator and condenser coils. No frictional pressure drops

so refrigerant flows through the heat exchangers at constant pressure.

4. Compression is adiabatic with negligible heat transfer to surroundings (isentropic efficiency = 1)

Evaporator

Refrigerant passes through the evaporator, heat transfer from the refrigerated space results in the

vaporization of the refrigerant.

Mass and energy balance results in the following, where �̇� is the mass flow rate of the refrigerant, and

�̇�𝑖𝑛 is the refrigerant capacity. 1 ton of refrigeration is equal to 200 Btu/min or about 211 kJ/min or

3.52 kW.

�̇�𝑖𝑛

�̇�= ℎ1 − ℎ4

Compressor

Refrigerant leaves the evaporator and is compressed to a relatively high pressure and temperature by

the compressor. Assuming adiabatic compression, the mass and energy balance gives:

�̇�𝑐

�̇�= ℎ2 − ℎ1

Condenser

Once compressed, the refrigerant passes through the condenser, where the refrigerant condenses and

there is heat transfer from the refrigerant to the cooler surroundings.

�̇�𝑜𝑢𝑡

�̇�= ℎ2 − ℎ3

Throttling Valve

Lastly, the refrigerant flows the expansion, (aka. throttling) valve and expands to evaporator pressure.

In this process, the refrigeration pressure decreases in the irreversible adiabatic expansion. Entropy

increases. Enthalpy remains constant. Leaving the valve, the refrigerant is a two-phase liquid-vapor

mixture.

ℎ3 = ℎ4 = ℎ𝑓4 + 𝑥 (ℎ𝑔4 − ℎ𝑓4)

Coefficient of Performance (COP)

𝐶𝑂𝑃 =

�̇�𝑖𝑛�̇��̇�𝑐�̇�

=ℎ1 − ℎ4

ℎ2 − ℎ1

Performance of Vapor-Compression Systems

Process 1 - 2s: Isentropic compression of the cold vapor refrigerant from state 1 to the condenser

pressure at 2s. Increases pressure of the vapor, making it a hot vapor.

Process 2s - 3: Heat is transferred from the refrigerant to the surroundings as it flows at constant

pressure through the condenser. Hot compressed vapor now becomes a warm liquid at state 3.

Process 3 - 4: Drops the pressure of the liquid using a throttling process from state 3 to a cold two-

phase liquid-vapor mixture at 4.

Process 4 - 1: Heat transfer from the surroundings to the refrigerant as it flows at constant pressure

through the evaporator to complete the cycle. Heat is transferred from the cold surroundings to boil

the even colder refrigerant. Prior to entering

Actual Vapor-Compression Cycle Heat transfer is not actually reversible.

1. The refrigerant temperature in the evaporator is less than the cold region temperature, TC

2. The refrigerant temperature in the condenser is greater than the warm region temperature, TH

COP decreases as the average temperature

of the refrigerant in the evaporator

decreases and as the average temperature

of the refrigerant in the condenser

increases.

These formulas were developed using mass

and energy balance. If enthalpies are

known throughout the cycle, these formulas

equally apply to systems when

irreversibilities are present in the

evaporator, compressor and condenser.

Refrigeration Example

Need to interpolate to find temperature and enthalpy at 2s.

Heat Pumps

Example

Example