Refrigeration Systems: Ultimate Guide

A refrigeration system is a crucial component of any cold room, ensuring its effective operation. Cold rooms rely on refrigeration systems to maintain low-temperature environments, preserving the quality and safety of food, plants, pharmaceuticals, chemicals, and other perishable items. To build an effective cold room, it’s essential to understand refrigeration systems, including their principles, components, and types. This ultimate guide provides comprehensive information about refrigeration systems, helping you better understand and apply refrigeration technology.

Refrigeration System

 

What is a Refrigeration System?

What is a Refrigeration System

 

A refrigeration system uses physical and chemical principles to lower and maintain the ambient temperature below the surrounding environment. Simply put, it is a system that uses external energy to transfer heat from a cooler environment to a warmer one.

 

How Do Refrigeration Systems Work?

The working principle of a refrigeration system is based on thermodynamics and the phase change of refrigerants. It involves four main processes: compression, condensation, expansion, and evaporation. These processes form a continuous cycle that transfers heat from a low-temperature environment to a high-temperature one, allowing the system to provide ongoing cooling.

 

Components of a Refrigeration System

A refrigeration system primarily consists of a compressor, condenser, expansion valve, and evaporator.

Compressor

Compressor

 

The compressor plays a critical role in the refrigeration system by transforming the refrigerant from a low-temperature, low-pressure state to a high-temperature, high-pressure state through the process of compression.

Reciprocating Compressor 

A reciprocating compressor uses a piston or diaphragm that moves back and forth within a cylinder to compress gas. It consists mainly of a cylinder, piston or diaphragm, crankshaft, connecting rod, intake and exhaust valves, and a lubrication system. The operation involves four key processes: expansion, intake, compression, and exhaust. The piston’s continuous movement allows the cylinder to cyclically intake and discharge gas.

Advantages: High compression ratio and efficiency, capable of compressing various gases, widely used in industrial and commercial applications.

Disadvantages: Large size, complex structure, high maintenance costs, and greater vibration and noise compared to other types of compressors.

Rotary Compressor 

Rotary Compressor 

Unlike reciprocating compressors, which rely on the back-and-forth motion of a piston or diaphragm, rotary compressors use a rotating piston to compress gas. The primary processes are intake, compression, and exhaust.

Advantages: High compression efficiency, compact size, low noise, and a space-saving design, suitable for applications with limited space.

Disadvantages: Higher manufacturing and lubrication system requirements, making them relatively more expensive.

Centrifugal Compressor 

A centrifugal compressor uses aerodynamic principles to compress gas. It primarily consists of a shaft and bearings, impeller, diffuser, and volute. The impeller’s high-speed rotation increases the gas’s pressure and kinetic energy, which is then converted into pressure energy in the diffuser, further increasing the gas pressure. The main processes are intake, acceleration, compression, and exhaust.

Centrifugal Compressor 

Advantages: High efficiency, energy-saving, stable operation, low noise, low maintenance costs, and the ability to handle large gas volumes, making them ideal for large refrigeration systems.

Disadvantages: High manufacturing and installation costs.

Scroll Compressor 

A scroll compressor features a stationary scroll and a moving scroll that compresses gas through a volume reduction process. The stationary scroll is fixed within the compressor casing, while the moving scroll engages with the stationary scroll and rotates eccentrically without self-rotating. As the eccentric shaft rotates, the gas is gradually compressed. The main processes are intake, compression, and exhaust.

Scroll Compressor

 

Advantages: High compression efficiency, low noise, relatively simple structure, and easy maintenance.

Disadvantages: The absence of gas valves may lead to over-compression or under-compression within the compression chamber.

Screw Compressor 

A screw compressor compresses gas using a pair of intermeshing helical rotors rotating in opposite directions. The main components are the casing, male rotor, female rotor, bearings, and seals. The key processes are intake, compression, and exhaust.

Advantages: High compression efficiency, good stability, low noise, relatively simple structure, easy maintenance, and insensitivity to large amounts of lubricating oil in the refrigerant. It has excellent gas flow regulation capabilities, widely used in refrigeration, air conditioning, and chemical processes.

Disadvantages: High cost, unsuitable for high-pressure applications, and not ideal for small-scale applications.

Condenser

Condenser

 

The condenser cools the high-temperature, high-pressure gaseous refrigerant, converting it into a low-temperature liquid state. It works in tandem with the compressor, forming the heat compression part of the refrigeration system.

Air-Cooled Condensers

Air-cooled condensers use air as the cooling medium and are widely used in small to medium-sized air conditioning systems, refrigeration systems for small to medium-sized cold rooms, and industrial refrigeration systems. 

These condensers cool the refrigerant by allowing air to flow over the heat exchange tubes, transferring heat from the refrigerant to the air, which condenses the high-temperature, high-pressure gas into a low-temperature liquid.

Air-cooled condensers typically consist of heat exchange tubes, fins, fans, and a casing. The process involves four main steps: refrigerant entry, heat exchange, condensation, and liquid discharge.

Advantages: Easy installation, relatively simple structure, low maintenance, and high adaptability to different environments. They do not require water resources, making them ideal for areas with water scarcity and environments with significant temperature variations.

Disadvantages: Their cooling efficiency can decrease in high ambient temperatures.

Water-Cooled Condensers

Water-Cooled Condensers

 

Water-cooled condensers use water as the cooling medium and are suitable for large commercial air conditioning systems, large cold rooms, and industrial refrigeration systems requiring high-efficiency cooling. They use water to remove heat from the refrigerant, condensing the high-temperature, high-pressure gas into a low-temperature liquid.

Water-cooled condensers typically consist of heat exchange tubes, a water jacket, and a water pump. In larger systems, a cooling tower is often added. The process involves refrigerant entry, heat exchange, condensation, and liquid discharge.

Advantages: High cooling efficiency and stability, with minimal impact from ambient temperatures, providing excellent cooling even in high-temperature environments.

Disadvantages: Complex installation, heavy dependence on water resources, not suitable for water-scarce areas, and significant wastewater production that requires proper treatment to avoid environmental pollution. Additionally, regular maintenance and water treatment add to operational costs. Due to economic and environmental concerns, some countries have restricted the use of water-cooled condensers.

Evaporative Condensers

Evaporative Condenser

 

Evaporative condensers combine the benefits of both air-cooled and water-cooled systems, making them popular in industrial and commercial refrigeration applications. They cool the refrigerant by evaporating water sprayed over the coils, removing heat and condensing the gaseous refrigerant into a liquid.

Evaporative condensers usually consist of coils, a spray system, a fan, a water basin, and a water pump. To increase the contact area between water and air and improve evaporation efficiency, fill materials are often added. The process involves refrigerant entry, spraying, evaporation, condensation, and water recirculation.

Advantages: High cooling efficiency, suitable for high-temperature environments, compact structure, and reduced water usage compared to water-cooled condensers, saving space and conserving water.

Disadvantages: Higher maintenance costs.

Expansion Valves

Expansion valves regulate and control the flow of refrigerant, enhancing the efficiency and performance of the refrigeration system.

Thermostatic Expansion Valves 

Thermostatic Expansion Valves

Thermostatic expansion valves use the temperature at the evaporator outlet, specifically the superheat (the difference between the refrigerant’s temperature at the evaporator outlet and its saturation temperature), as the control signal. These valves adjust the position of the valve needle to control the flow of refrigerant, responding to temperature changes sensed by a bulb and capillary tube. A thermostatic expansion valve typically consists of a sensing bulb, capillary tube, and valve body, with some systems also incorporating an equalizer tube to balance the pressure between the evaporator outlet and the valve.

The process involves the refrigerant flowing into the valve, the sensing bulb detecting temperature changes, adjusting the needle position, controlling the refrigerant flow, and the refrigerant flowing out.

Advantages: Simple structure and low maintenance costs.

Disadvantages: Fixed superheat setting and relatively slow response time.

Electronic Expansion Valves

Electronic expansion valves control refrigerant flow by adjusting the valve opening through electrical signals. They usually consist of a valve body, motor, sensors, and a controller. The sensors monitor parameters such as the temperature and pressure at the evaporator outlet in real time, and the controller adjusts the valve opening accordingly to precisely control the refrigerant flow.

Advantages: Adjustable superheat settings, fast response time, and high control precision, making them ideal for systems requiring precise refrigerant flow adjustment.

Disadvantages: More complex structure, higher cost, requiring professional installation and calibration. Additionally, they rely on a power supply, so power outages can affect their operation.

Evaporators

 

Evaporators

Evaporators play a crucial role in refrigeration systems by converting low-temperature, low-pressure liquid refrigerant into low-pressure gas through evaporation. This process absorbs a significant amount of heat from the surrounding environment, effectively lowering the temperature and achieving the desired cooling effect.

Direct Evaporators

Direct evaporators involve direct contact between the refrigerant and the object or air to be cooled. By absorbing heat from the immediate environment, these evaporators efficiently transfer cooling directly to the target area or object.

Characteristics: High cooling efficiency due to direct heat transfer.

Applications: Commonly used in household refrigerators, air conditioners, and small cold rooms.

Indirect Evaporators

Indirect evaporators utilize a heat exchanger or an intermediate medium to separate the refrigerant from the object or air being cooled, avoiding direct contact. The cooling effect is achieved through heat conduction or convection between the refrigerant and the target medium.

Characteristics: Higher safety as the refrigerant does not directly contact the cooled object, but lower cooling efficiency compared to direct evaporators. And it can transfer cooling over long distances due to the intermediate medium.

Applications: More suitable for central air conditioning systems and large industrial refrigeration systems.

Applications

 

Types of Refrigeration Systems

Classified by Operating Principle

Compression Refrigeration Systems

Compression refrigeration systems primarily consist of a compressor, condenser, expansion valve, and evaporator. The refrigerant is compressed by the compressor into a high-temperature, high-pressure gas, which then enters the condenser. After passing through the expansion valve, it becomes a low-temperature, low-pressure liquid. This liquid refrigerant then enters the evaporator, where it evaporates into a gas and absorbs heat from the surroundings, thereby cooling the environment. The gas refrigerant is then cycled back into the compressor, continuing the refrigeration cycle.

Advantages: High cooling efficiency, mature technology, and wide application suitable for various cooling needs.

Disadvantages: Dependence on electricity, high energy consumption, potential vibrations, and noise from the compressor. Some refrigerants can negatively impact the environment, though using eco-friendly refrigerants can mitigate this to some extent.

Absorption Refrigeration Systems

Unlike compression systems, absorption refrigeration systems do not use a compressor. Instead, they rely on a heat source to heat the refrigerant, producing a high-temperature, high-pressure gas. These systems mainly comprise a generator, condenser, expansion valve, evaporator, and absorber. The refrigerant is heated to become a high-temperature, high-pressure gas, which then passes through the condenser and expansion valve to become a low-temperature, low-pressure liquid. This liquid enters the evaporator, where it evaporates and absorbs heat, cooling the environment. The gas refrigerant is then absorbed by the absorbent, forming a solution that is sent back to the generator to continue the cycle. Commonly, ammonia is used as the refrigerant and water as the absorbent.

Absorption Refrigeration Systems

Advantages: Relies on thermal energy, resulting in relatively low energy consumption and minimal noise. Ammonia, as an eco-friendly refrigerant, has a lower environmental impact.

Disadvantages: Requires more space and has a lower cooling efficiency compared to compression systems. Absorption refrigeration systems are less common in cold rooms and are more frequently used for industrial waste heat recovery.

Classification by Refrigerant Type

Freon Refrigeration Systems

Freon refrigeration systems use Freon compounds as refrigerants. Their working principle is similar to standard compression refrigeration systems. Freon refrigerants are categorized into three main types: chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs).

Chlorofluorocarbons (CFCs)

CFCs refrigerants, including R11, R12, R113, R114, R115, R500, and R502, were widely used in various refrigeration systems due to their excellent cooling performance and chemical stability. However, research has shown that these refrigerants significantly deplete the ozone layer and have high Global Warming Potential (GWP), which negatively impacts the climate. They persist in the atmosphere for long periods, causing prolonged environmental harm. Consequently, CFCs are listed as controlled substances under the Montreal Protocol, aiming to gradually phase out and eventually cease their production and use.

Hydrochlorofluorocarbons (HCFCs)

Hydrochlorofluorocarbons (HCFCs)

HCFCs refrigerants, such as R22, R123, R141b, and R142b, are an improved version of CFCs. While HCFCs have a lower impact on the ozone layer and a reduced Global Warming Potential (GWP) compared to CFCs, they still possess relatively high GWP. As a result, HCFCs are also being phased out.

Hydrofluorocarbons (HFCs)

HFCs refrigerants, including R134a, R410A, R404A, and R407C, do not contain chlorine atoms, which means they do not deplete the ozone layer. Thus, they are widely used in various refrigeration systems. R404A is particularly common in cold room systems. Although HFCs are ozone-friendly, they still have high GWP. The international community is gradually working towards phasing out HFCs as well, with environmentally friendly refrigerants expected to dominate the market in the future.

Ammonia Refrigeration Systems

Ammonia refrigeration systems use ammonia (R717) as the refrigerant. The operating principle is similar to that of typical compression refrigeration systems. Ammonia is an environmentally friendly refrigerant with zero ozone depletion potential and very low Global Warming Potential (GWP). Additionally, ammonia is cost-effective and has excellent thermodynamic properties, making these systems highly efficient and widely used in industrial refrigeration. However, ammonia is toxic, corrosive, and flammable, necessitating appropriate safety measures. Its strong, pungent odor helps in easily detecting even minor leaks, ensuring prompt corrective action.

CO₂ Refrigeration Systems

CO₂ Refrigeration Systems

CO₂ refrigeration systems utilize carbon dioxide (R744) as the refrigerant. Like traditional compression systems, CO₂ systems operate similarly but involve transcritical cycles due to CO₂’s low critical temperature. CO₂ is an eco-friendly refrigerant with no ozone depletion and a very low GWP. It is cost-effective, abundant, and has excellent thermodynamic properties, resulting in high refrigeration efficiency. Unlike ammonia, CO₂ is non-toxic and non-flammable, making it safer. However, CO₂ systems face challenges such as high operating pressures and transcritical conditions, requiring further technological advancement for broader adoption.

Hydrocarbon Refrigeration Systems

Hydrocarbon refrigeration systems use hydrocarbons like R290 (propane), R600a (isobutane), and R1150 (ethylene) as refrigerants. These systems work similarly to conventional compression systems. Hydrocarbon refrigerants are environmentally friendly, with no ozone depletion and low GWP. They are becoming more popular in certain applications due to their high efficiency and relatively low cost. However, hydrocarbons are highly flammable, which limits their use in safety-critical environments.

Water Refrigeration Systems

Water Refrigeration Systems

Water refrigeration systems use water (R718) as the refrigerant, operating similarly to absorption refrigeration systems or by using high-speed water jets for evaporative cooling. Water is an eco-friendly refrigerant, being non-toxic, non-flammable, and having zero ozone depletion and GWP. It is also abundant and inexpensive. However, water’s freezing point (0°C) limits its use in low-temperature applications, as ice formation can damage the system. Additionally, absorption refrigeration systems generally have lower efficiency compared to compression systems, restricting the use of water refrigeration systems primarily to industrial cooling applications, such as power plant cooling.

 

Choosing a Refrigeration System

How do you choose the right refrigeration system? This decision requires a thorough consideration of various factors, including cooling needs, efficiency, safety, environmental impact, cost, and regulatory requirements. Earlier sections have detailed the characteristics, advantages, and disadvantages of different refrigeration systems, which should be carefully evaluated against the specific applications.

Compression refrigeration systems due to their high efficiency and mature technology, compression refrigeration systems are often the preferred choice for most applications. However, it is essential to opt for eco-friendly refrigerants within these systems to minimize environmental impact. Absorption refrigeration systems and water refrigeration systems will be a good choice if it is used in applications with high environmental protection requirements or special heat source utilization requirements.

 

Conclusion

Conclusion

This article provides a comprehensive overview of various refrigeration systems, guiding you through the intricacies of refrigeration. Each type of system has its unique features, advantages, and disadvantages, making it crucial to understand them for making an informed choice. Whether for residential, commercial, or industrial applications, knowing the specifics of each refrigeration system helps in selecting the most suitable one. By mastering refrigeration knowledge, you can make better decisions for your refrigeration needs.

 

Scroll to Top