Types of refrigeration systems

What is refrigeration?

Refrigeration is the process of absorbing heat from one fluid and transferring it to another. The fluid can be air, water, or any other gas or liquid. In all refrigeration systems, maintaining cold requires absorbing heat from materials at a lower temperature and removing this heat to an environment at a higher temperature.

Refrigeration systems

a) Food industries

• Food preservation
• Freezing foods such as meat
• Pasteurization and preservation of milk and ice cream

b) Chemical industries

• Distillation of gases
• Dehumidification of air
• Maintaining liquids at low pressure
• Heat removal in chemical reactions

c) Use in building ventilation

Generally, cold can be produced in three ways:

1. Evaporative system
2. Compression system
3. Absorption system

Evaporative refrigeration systems:

The evaporative system is known as a popular system with relatively good efficiency in dry areas due to its low cost and simple operation.

Carnot Refrigeration Cycle

The evolutions of the Carnot refrigeration cycle are as follows:

1. Adiabatic compression stage
2. Heat rejection stage at constant temperature
3. Adiabatic expansion stage
4. Heat absorption stage at constant temperature

Compression Refrigeration Systems

The working principles of compression systems or, in other words, compression refrigeration machines are as follows: some work is given to the system in the compressor, the compressor condenses the refrigerant fluid, and the fluid, which is in the form of a hot gas due to increased pressure, gives up its heat to the environment in the condenser, and after passing through the expansion valve, it enters the evaporator, and due to suction and increased volume, it evaporates, it acquires the heat of the surrounding environment, and as a result, the air around the evaporator is cooled.

Components of the compression refrigeration system cycle

All compression refrigeration systems used to create cold are composed of four main parts. These four parts are:

• Compressor
• Condenser
• Expansion valve
• Evaporator

Compressor

Compressors are actually the heart of a refrigeration system. Compressors are very diverse, and each is designed for a type of refrigeration device. The most common compressors used in the refrigeration industry are:

• Piston compressors
• Rotary compressors
• Screw compressors
• Scroll compressors
• Centrifugal compressors

Piston compressor:Piston compressors have a structure and function similar to internal combustion engines, except that they do not have a spark and fuel system.

Rotary compressors:In this compressor, the compression operation is performed by the rotational movement of the rotor relative to the compression chamber or cylinder.

Screw compressors:Due to the advances made in screw compressors in recent years, their use has become very common in the air conditioning and semi-heavy refrigeration industries.

Screw compressors:Piston compressors have a structure and function similar to internal combustion engines, except that they do not have a spark and fuel system.

Centrifugal compressors:The basis of this type of compressor is based on centrifugal force and is used for high capacities.

Evaporators

An evaporator is actually a heat exchanger that extracts heat from the cold environment as a result of the evaporation of the refrigerant inside it. Evaporators are divided into three categories in terms of structure.

Condensers

Condensers or devices that distill the refrigerant are located after the compressor and the refrigerant gas exchanges heat and is distilled after entering this device. Condensers are divided into three categories:

1. Water condenser
2. Air condenser
3. Evaporative condenser

Water condensers:

A water condenser distills the refrigerant gas using water sent from the cooling tower. Water condensers are divided into the following three groups in terms of structure:

Shell and coil water condenser

This condenser consists of a cylindrical chamber with a copper coil inside. Cold water passes through the coil and the refrigerant flows in the shell space. The hot gas inside the shell transfers its heat to the water through the surface of the copper coil.

Shell and Tube Water Condenser

This condenser consists of a cylindrical chamber with a series of horizontal tubes installed in parallel. In this type of condenser, hot refrigerant gas flows in the shell space and cold water passes through the tubes. Some of these condensers have simple, smooth tubes, while others have finned tubes to increase the heat exchange surface. Refrigeration Systems

Double-Tube Water Condenser

This type of water condenser consists of two concentric overlapping tubes, with hot gas flowing in one tube and water flowing in the other.

Cooling Tower

In a cooling tower, the water leaving the condenser is cooled and returned to the condenser. Cooling towers come in many different types.

Types of cooling towers:

By air flow direction

• Concurrent flow
• Counterflow
• Crossflow

By fan type

• Centrifugal
• Axial flow

By water circulation

• Closed
• Open

By structure

• Cubic
• Trapezoidal
• Circular or bottle-shaped

Methods for controlling cooling tower capacity

Reducing the outside air temperature on some days of the year causes excessive pressure reduction on the refrigerant before the expansion valve. To solve this problem and reduce the capacity of the cooling tower, we can change the speed of the cooling tower fan or turn it off, or reduce the capacity of the tower by bypassing the tower water using two-way and three-way valves. In each of these methods, the temperature of the water leaving the tower is sensed by a sensor.

Air-cooled condensers

An air-cooled condenser condenses the refrigerant by having one or more fans and passing air over a coil containing hot gas. Air-cooled condensers are more useful in areas with high relative humidity.

Evaporative condensers

This type of condenser is a combination of an air-cooled and water-cooled condenser, and the heat removal in this case is done by evaporating water in a device called a cooling tower. The latent heat of evaporation of the cooling tower water is taken from the water in the condenser outlet pipes and cools it.

Expansion valve

The expansion valve is installed in the liquid line and between the condenser and the evaporator inlet. The expansion valve is located opposite the compressor in a refrigeration system. While the compressor increases the pressure of the refrigerant and moves it through the system, the expansion valve reduces the pressure on the condenser side and prepares the refrigerant to enter the evaporator.

Absorption System

Example:

Suppose we have two containers, one containing water and the other containing lithium bromide solution, and assume that the air has been evacuated from these containers by a vacuum pump. The container containing water is called the evaporator and the container containing lithium bromide is called the absorber.

To solve the problem of lithium bromide dilution, we add a generator to the system.

To solve the problem of complete evaporation of water, we add a condenser to the system.

To increase efficiency, we add a heat exchanger to the system.

Classification of absorption chillers:

In terms of the heat source of the condensation operation

1. Hot water absorption chiller
2. Steam absorption chiller
3. Direct flame absorption chiller

In terms of the absorption cycle of the absorbent material

1. Single-effect absorption chiller
2. Double-effect absorption chiller

Hot water absorption chiller

These chillers, which are produced only in a single-effect type, use hot water to supply the heat required by the generator.

Steam absorption chiller

Steam absorption chillers are available in two models: double-effect steam with a high-pressure steam supply and single-effect steam with a low-pressure steam supply.

Direct flame absorption chiller

Direct flame absorption chillers have two generators (double-effect) and receive the heat energy required to produce cooling from the burner in the high-temperature generator section of the device.