Which part converts the high pressure and high temperature refrigerant to low temperature and low pressure?

In my example, a compressor is located at home plate at the bottom of the baseball diamond (shown above). In a refrigeration or cooling system, compression is the first step:

• • Refrigerant enters as a low-pressure (LP), low-temperature (LT) superheated vapor and exits the compressor as a high-pressure (HP), high-temperature (HT) vapor.
  • The compressor mechanically compresses the refrigerant gas.
  • Under pressure, the refrigerant volume is reduced and the temperature is raised.

The second step involves a condenser, located at first base on the right side of the baseball diamond:

• • Hot, pressurized refrigerant gas arrives from the compressor into the condenser, which is designed to reject heat by lowering or returning the temperature of the refrigerant to its condensing temperature.
  • As it rejects heat, the condenser converts the vapor to a sub-cooled liquid.
  • In most condensers, the refrigerant gas enters at the top of the equipment and leaves at the bottom because the refrigerant in a liquid state is much heavier than the weight of refrigerant in a gas state.

In the third step, a metering device located at second base at the top of the baseball diamond regulates the amount of refrigerant released into the evaporator in response to the cooling load and causes a pressure drop.

The metering device also:

• • Measures the superheat at the evaporator outlet
  • Maintains a constant temperature by raising or lowering the amount of refrigerant flowing into the evaporator

At the fourth step, cold liquid refrigerant mixes with vapor causing the saturation temperature as it boils off or vaporizes in the evaporator, located at third base, on the left side of the baseball diamond:

• • The process allows the refrigerant to absorb heat through a series of metal coils.
  • The low-pressure superheated vapor refrigerant gas then returns to the compressor to continue the refrigeration process.

Here is the value of comparing the refrigeration process to a baseball diamond: If I draw a vertical line from home plate up to second base, everything in the system on the right side of that line is under high pressure; everything on the left side of that line is low pressure.

Likewise, if I draw a horizontal line from first base to third base, the refrigerant above the line is in a liquid state; below the line, the refrigerant is a vapor, regardless of whether it is under high or low pressure.

Liquid Injection Cools Compressor and Increases Capacity

A compressor is designed to operate at very high temperatures, so a liquid injection method has been developed to cool the compressor internally. How this works can be confusing; refrigerant is injected in a vapor state, not in a liquid state.

When necessary, liquid injection cools a compressor to enable it to run reliably under difficult high compression ratio conditions normally seen on low-temperature freezer applications.

• • Refrigerant is piped from the system liquid line, through an injector valve to the compressor; in scroll compressors, the refrigerant is injected directly into the scroll elements.
  • Without this cooling, the compression elements can get too hot and the oil breaks down, leading to compressor failures.

Another approach called enhanced vapor injection (EVI) increases refrigeration capacity and, in turn, the efficiency of the system:

• • A heat exchanger is utilized to provide subcooling to the refrigerant before it enters the evaporator.
  • A small amount of refrigerant is evaporated and superheated above its boiling point.
  • This superheated refrigerant is then injected mid-cycle into the scroll compressor and compressed to discharge pressure.

The diagram below shows how enhanced vapor injection (EVI) increases the efficiency of the system.

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Explanation:

• In a vapour compression refrigeration system refrigerant enters in a liquid state in the evaporator and it absorbs heat then becomes vapour than with the help of compressor and condenser vapour refrigerant comes back to a liquid state.
• The vapor-compression cycle is a process used to extract heat from a box or a room that underlies most refrigeration and air conditioning techniques. It consists of four separate stages:
  • Compression (1-2) Isentropic Compression
  • Condensation (2-3) Heat rejection at constant pressure
  • Expansion (3-4) Constant enthalpy expansion
  • Evaporation (4-1) Constant pressure heat addition

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1. Compressor:  In this device, the temperature of the refrigerant increases at constant entropy. It shows work input to the refrigerator.
2. Condenser:  In this device, the heat is rejected at constant pressure. The vapor refrigerant is converted into a liquid refrigerant. The maximum temperature of the cycle is at Condenser.
3. Expansion Valve: This device removes pressure from the liquid refrigerant to allow expansion or change of state from a liquid to a vapor in the evaporator. It is a constant enthalpy process.
4. Evaporator: In this device, the liquid refrigerant is expanded and evaporated. It acts as a heat exchanger that transfers heat from the substance being cooled to a boiling temperature. It shows the refrigeration effect (Cooling effect) The minimum temperature of the cycle is at the Evaporator.

• Among all four devices, Condenser operated at high pressure. 

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"Vapour compression refrigeration cycle" is the name give to describe the operation of the closed circuits used in refrigeration applications.

This exploits the evaporation of a refrigerant inside the circuit, specifically in a heat exchanger called the evaporator, which absorbs energy from the surrounding air; this is then delivered to the food storage compartment by natural or fan-forced convection (also see "MAKING IT COLD" and "PRESSURE & TEMPERATURE").

Once having evaporated, the refrigerant can no longer absorb considerable amounts of energy, and consequently it needs to be returned to the liquid state by condensation.

The problem thus arises of having an environment that's "cold" enough to absorb energy from the refrigerant, which naturally cannot be the same storage compartment that's just been cooled.

By exploiting the correlation between pressure and temperature for change of state whereby higher pressures correspond to higher temperatures, a compressor is used to compress the refrigerant to a pressure that's higher than the evaporator (up to 8-10 times!) so that the condensation process can take place at a temperature that's compatible with a readily available "cold" source, typically the outside air.

Condensation thus occurs at a high temperature (usually 35-55°C) inside a heat exchanger where the two fluids are outside air and refrigerant. The latter condenses and returns to the liquid state, while the outside air will be heated.

The liquid refrigerant is still at high pressure when it leaves the condenser. An expansion device is thus needed to expand the liquid refrigerant and reduce its pressure to the value at which evaporation occurs; the refrigerant has now returned to its initial state (liquid at low pressure and temperature) and can once again absorb energy from the air in the food storage compartment.

The main components of a refrigerant circuit are therefore:

Evaporator: this is a heat exchanger similar to a radiator when used with air (finned coil) or more compact when used with water (plate heat exchanger, tube bundle); it exchanges energy by conduction between the refrigerant that evaporates, changing state from liquid to gas, and the surrounding air (or water) that's cooled as a result. Evaporation takes place at virtually constant pressure and temperature, except for a slight pressure drop. The refrigerant leaving the evaporator is a superheated gas whose temperature is slightly higher than evaporation temperature.

Compressor: this is a device providing volumetric compression, i.e. a progressive reduction in volume, using rotating or reciprocating systems. The compressor has the function of circulating refrigerant inside the circuit, specifically drawing it in as a gas from the evaporator and then compressing it and delivering it at higher pressure to the condenser. The mechanical work performed by the compressor implies a significant increase in the temperature of the gas (at times above 100°C) as well as power consumption. Compressor power consumption depends on the difference between the two operating pressures. The refrigerant entering the compressor must be in the gaseous state, as liquids are notoriously incompressible. The compressor starts working when the unit needs to provide cooling, and is usually activated via temperature control systems.

Condenser: this is a heat exchanger that's similar to an evaporator yet slightly larger, and may also be a finned coil, plate heat exchanger or tube bundle. It exchanges energy between the outside air (or water) blown by fans and the refrigerant in the form of hot gas discharged by the compressor. The refrigerant is cooled and then condenses at a virtually constant temperature and pressure, meaning it undergoes slight subcooling. At the condenser outlet the refrigerant will be in the liquid state at high pressure and with a temperature slightly lower than the condensing temperature.

Expansion device: this consists of a calibrated opening, a thin capillary tube or a mechanical or motor-driven regulating valve with microprocessor control. The choking produced by the expansion device lowers the pressure of the liquid refrigerant leaving the condenser without exchanging energy. This exploits the Bernoulli principle whereby the speed of a fluid through a restriction increases significantly, causing a drop in pressure and a corresponding drop in temperature. In this way, the liquid refrigerant returns to low pressure and low temperature and is ready to evaporate again, repeating the cycle described above.

The expansion device also has the function of controlling refrigerant flow through the circuit. An excessive quantity risks damaging the compressor as it won't completely evaporate in the evaporator, remaining partly in liquid state. An insufficient quantity sensitively reduces unit efficiency, as the evaporator is not fully exploited.