# Vapor compression cycle

Basic vapor compression cycle used in refrigerators and air conditioners

# Vapor compression cycle (recuperator)

Vapor compression cycle with a recuperator. The stream before the throttle valve is precooled, using the cold stream at the evaporator outlet. This increases the compressor cooling/heating capacity of the cycle and improves slightly the COP

# Heat pump cycles

Heat pumps utilize work to move heat from colder to warmer places. This is used in cooling devices (refrigerators, freezers) and in air conditioners for either cooling or more effective heating. The same cycle design can be used either for cooling (if it realeses heat to the environment) or heating (if it absorbs heat from the environment). The measure of the efficiency of the cycle is its COP (coefficient of performance), which gives the amount of useful heating or cooling capacity per unit input work. That is:

\begin{equation*} COP_{cooling} = \frac {Q_C}{W} \end{equation*}
\begin{equation*} COP_{heating} = \frac {Q_H}{W} \end{equation*}

Typically, if thermal losses are neglected, $$COP_{heating} = COP_{cooling} + 1$$

# Vapor compression cycle

Basic vapor compression cycle

The simplest vapor compression cycle consists of four steps:

1. The refrigerant vapors are compressed nearly adiabatically.
2. The high pressure hot vapors pass through a condensor where they give off heat to the environment and cool and liquefy at constant pressure (in the case of transcritical cycle $$p_H > p_{crit}$$, no liquefaction takes place but only cooling).
3. The subcooled high pressure liquid is allowed to expand through a Joule-Thompson (throttle) valve at constant enthalpy (no work is done, no heat exchange with the environment occurs). This causes part of the liquid to vaporize, and the temperature to drop significantly, resulting in a two-phase mixture. The colder the temperature of the fluid before the valve, the bigger the fraction of the liquid.
4. The cold two-phase mixture evaporates at constant pressure at the evaporator absorbing heat from the environment.

# Vapor compression cycle with recuperator

Vapor compression cycle with recuperator

In this variation of the vapor compression cycle, a recuperator (counterflow heat exchanger) is placed between:

• the outlet of the evaporator and the inlet of the compressor, on one hand,
• the outlet of the condenser and the inlet of the throttle valve, on the other hand

The cold vapors coming from the evaporator, which still have some cooling capacity, absorb heat from the still warm liquid leaving the condensor. The subcooling of the liquid before the throttle valve shifts the two-phase mixture entering the evaporator to the liquid side, and thus provides extra cooling capacity. The temperature at the outlet of the compressor also increases, resulting in more heat rejected at the condensor.