In the evolution of modern refrigeration technology, equipment performance has continued to upgrade to meet diverse low-temperature requirements. The two-stage compression refrigeration cycle stands out with its unique design and outstanding performance, becoming a key refrigeration method. This article will analyze the necessity of adopting two-stage compression in refrigeration units from dimensions such as definition, principles, types, applications, and comparisons with single-stage systems.
What is Two-Stage Compression?
The two-stage compression refrigeration cycle is an efficient and scientific refrigeration system design solution. The low-temperature, low-pressure refrigerant vapor exiting the evaporator first enters the low-pressure compressor for preliminary compression, raising its pressure and temperature to an intermediate state.
Subsequently, the preliminarily compressed refrigerant vapor enters a specially designed intermediate cooler, where the temperature of the vapor is effectively reduced through the action of a cooling medium (such as liquid refrigerant or cooling water), enabling it to enter the subsequent process under more suitable conditions.
Next, the cooled vapor enters the high-pressure compressor for secondary compression. After this stage, the refrigerant vapor reaches the high-pressure, high-temperature state required for condensation in the condenser, enabling it to smoothly enter the condenser and complete the heat release process.
This cycle centers on staged compression, breaking down a single-stage high-pressure ratio into two stages, thereby effectively reducing compressor load and power consumption. The intermediate cooler optimizes the refrigerant temperature, enabling the cycle to operate within a reasonable pressure range, significantly enhancing system efficiency and stability.
With these advantages, the two-stage compression refrigeration cycle is widely applied in medium to large-scale refrigeration equipment such as cold storage facilities and central air conditioning systems, becoming a key technology in modern industrial and commercial refrigeration.
Why Does a Refrigeration Unit Need Two-Stage Compression?
In many industrial production scenarios, to meet specific production process requirements, refrigeration cycles often need to achieve lower evaporation temperatures. However, the lowest evaporation temperature achievable by a single-stage vapor compression refrigeration cycle is not fixed; it varies significantly depending on the compressor’s operating principle and the type of refrigerant used.
For a single-stage vapor compression refrigeration cycle, when the pressure ratio (pk/po) becomes too large and exceeds a certain threshold, it leads to a series of issues:
Reduction in Gas Transmission Coefficient and Cooling Capacity
For piston-type refrigeration compressors with a fixed clearance volume, an increase in the pressure ratio is a key influencing factor. As the pressure ratio rises, the volume occupied by the gas within the clearance volume during expansion to the suction pressure significantly increases.
This change directly leads to a decrease in the volumetric efficiency of the piston-type refrigeration compressor, resulting in reduced actual gas delivery and a significant reduction in the cooling capacity of the refrigeration system. When the pressure ratio exceeds 20, the volumetric efficiency of a conventional piston-type refrigeration compressor is approximately zero, meaning the compressor can barely draw in refrigerant gas and completely loses its refrigeration capability.
Enhanced Irreversibility of Compression Process
An increase in the pressure ratio enhances the irreversibility of the refrigeration compressor’s compression process, which can be summarized from three aspects: principle, energy consumption, and efficiency, with key data highlighting the impact.
An increase in the pressure ratio significantly enhances the irreversibility of the compression process in refrigeration compressors. Thermodynamic principles indicate that an increase in the pressure ratio exacerbates friction between the refrigerant gas and components, as well as internal vortices, causing the actual compression process to deviate from the isentropic process.
This leads to two issues: first, additional energy consumption due to friction and heat transfer losses; second, changes in the rate of gas temperature increase, causing the final temperature and pressure of compression to deviate from theoretical values. Experimental data shows that after the pressure ratio exceeds the critical value, each 10% increase results in a 15%-20% increase in the compressor’s actual energy consumption.
Taking a screw compressor as an example, when the pressure ratio increases from 4 to 6, the isentropic efficiency decreases from 78% to 62%, and the coefficient of performance decreases by approximately 25%, leading to increased equipment operating costs and a decline in the overall performance of the refrigeration system.
Excessively High Discharge Temperature Leading to Failures
Excessively high discharge temperature is another issue that cannot be ignored. Excessively high discharge temperature can cause lubricating oil to thin out, disrupting proper lubrication conditions, and may even lead to carbonization of the lubricating oil and severe failures such as cylinder scoring. For example, when the condensing temperature is 40°C and the evaporating temperature is -30°C, even under ideal isentropic compression conditions, the exhaust temperature of a single-stage ammonia compressor can reach as high as 160°C, which clearly exceeds the specified maximum exhaust temperature limit of 150°C.
Pressure Ratio is Limited by Exhaust Temperature
The actual operating process of a refrigeration compressor can be approximated as a variable compression process. An excessive increase in the pressure ratio inevitably leads to an excessive rise in the exhaust temperature of the refrigeration compressor, potentially exceeding the permissible exhaust temperature limit of the compressor. Similarly, excessively high exhaust temperatures can also disrupt the normal operation of the refrigeration cycle. Based on the aforementioned reasons, particularly the exhaust temperature limitation, the pressure ratio of a single-stage refrigeration compressor is also strictly restricted.
Generally, the pressure ratio of single-stage piston-type refrigeration compressors using ammonia refrigerant is ≤8; the pressure ratio of single-stage piston-type refrigeration compressors using Freon refrigerant is ≤10; and the pressure ratio that centrifugal refrigeration compressors can achieve in a single cycle is ≤4.
In summary, single-stage compression refrigeration cycles face numerous issues under high pressure ratio conditions. High discharge temperatures accelerate lubricant carbonization and damage seals; the compression process deviates from the ideal state, leading to reduced volumetric efficiency, refrigeration capacity, and energy efficiency ratio; High pressure differentials also exacerbate mechanical loads on components, shortening equipment lifespan.
Therefore, in applications requiring temperatures below -30°C, such as cold storage and industrial freezing, when the pressure ratio exceeds the design limits of single-stage compressors (8–10), a two-stage compression refrigeration cycle becomes necessary. By reducing the pressure ratio through two-stage compression, it ensures system stability while achieving low-temperature, high-efficiency refrigeration.
Working principle of two-stage compression
Low-pressure, low-temperature refrigerant vapor generated in the evaporator is drawn into the low-pressure compressor and compressed into medium-pressure superheated vapor. This process follows the ideal gas state equation, with compressor efficiency influencing vapor state. The vapor enters a counterflow intermediate cooler, where it is evaporatively cooled by liquid refrigerant to form dry saturated vapor.
The medium-pressure dry saturated vapor is compressed by a high-pressure compressor in multiple stages to form superheated vapor at condensation pressure, which then liquefies in the condenser (air-cooled or water-cooled). The refrigerant liquid splits into two paths: one path passes through the expansion valve f and enters the intermediate cooler after throttling; the other path is subcooled in the intermediate cooler coils, with insufficient subcooling due to the temperature difference in heat transfer (3-5°C higher than the intermediate temperature).
The subcooled liquid is throttled and pressure-reduced through the main expansion valve (thermal or electronic expansion valve) before entering the evaporator to absorb heat and evaporate for cooling. The refrigerant undergoes a phase change within the evaporator, and its evaporation temperature and pressure determine the system’s cooling capacity and energy efficiency ratio.
Common Types of Two-Stage Compression Refrigeration Cycles
The two-stage compression refrigeration cycle is a more advanced refrigeration method. Its working principle involves the refrigerant vapor from the evaporator being compressed twice, first by a low-pressure compressor and then by a high-pressure compressor, before entering the condenser.
An intermediate cooler is also installed between the two compression stages to optimize refrigeration performance. The two-stage compression refrigeration cycle system has flexible configuration options, it can be either a dual-compressor system consisting of two compressors or a single-compressor two-stage system consisting of one compressor.
Two-stage compression offers numerous significant advantages. First, it effectively reduces the compression ratio per stage, minimizing the impact of factors such as clearance, heat exchange, and leakage on cooling performance, thereby improving the volumetric efficiency and enhancing the system’s operational efficiency.
Second, two-stage compression reduces discharge temperature, ensuring the compressor operates within a safe temperature range and extending its service life. Finally, by reducing the pressure difference between each stage of the compressor, it improves operational balance, reduces mechanical friction, and lowers equipment failure rates.
Two-stage compression refrigeration cycles can be classified into the following common types based on different throttling methods and cooling levels:
- Single-stage throttling, intermediate full cooling two-stage compression refrigeration cycle: The refrigerant undergoes only one throttling after the condenser and enters the intermediate cooler, achieving full cooling after low-pressure stage compression. Intermediate cooling uses saturated liquid subcooling to ensure that the high-pressure stage compressor inhales saturated vapor.
- Single-stage throttling, two-stage compression refrigeration cycle with incomplete intermediate cooling: Maintains a single-stage throttling design, with the intermediate cooler only performing partial cooling. The high-pressure stage compressor inhales superheated vapor, achieving temperature-entropy balance through intermediate pressure control.
- Two-stage throttling, two-stage compression refrigeration cycle with intermediate full cooling: The refrigerant undergoes two throttling operations after the condenser, entering the low-pressure cycle tank and evaporator respectively. Intermediate cooling uses liquid refrigerant spray cooling to ensure the high-pressure stage inhales saturated vapor.
- Two-stage throttling, two-stage compression refrigeration cycle with incomplete intermediate cooling: Combining dual throttling with partial cooling design, the intermediate cooler only reduces steam temperature, and the high-pressure stage inhales gas with moderate superheat, optimizing intermediate pressure to match load.
Applications of Two-Stage Compression
Two-stage compressors are widely used in various fields due to their high-efficiency refrigeration performance, precise temperature control capabilities, and stable operational performance. The following is a classification of their primary application scenarios:
Industrial Refrigeration and Freezing Applications
In industrial production, two-stage compressors are extensively used in various refrigeration and freezing processes, capable of meeting diverse process requirements such as cooling, freezing, refrigeration, and freeze-drying. Their specific applications span food processing, beverage production, chemical processes, pharmaceutical manufacturing, refrigerated warehousing, and refrigerated transportation. The high efficiency and low-temperature control capabilities of two-stage compressors enable them to effectively address these industrial refrigeration needs.
Commercial Refrigeration and Cold Storage Sector
In the commercial sector, two-stage compressors are widely used in various commercial refrigeration and cold storage equipment. Supermarket refrigerators, freezers, display cabinets, refrigeration facilities in the food service industry, and refrigeration equipment in hotels, restaurants, and other venues typically utilize two-stage compressors. These devices require stable temperature control and efficient refrigeration performance to ensure the freshness and quality of stored products.
Air Conditioning and Heat Pump Systems
Two-stage compressors are also widely used in air conditioning and heat pump systems. Especially in large commercial buildings, office buildings, industrial plants, and high-end residential areas, where high refrigeration capacity and precise temperature control are required, two-stage compressors provide efficient refrigeration and heat pump capabilities, ensuring stable system operation under varying load conditions.
Chemical and Petroleum Industry
In the chemical and petroleum industries, two-stage compressors play a critical role in refrigeration and compression. During petroleum refining, the dewaxing process requires a low-temperature environment. Two-stage compressors achieve this by using a two-stage compression refrigeration cycle to control temperatures between -20°C and -40°C, thereby enhancing dewaxing efficiency.
In chemical synthesis reactions, it employs staged compression with intermediate cooling and secondary compression to maintain stable raw material gas pressure, prevent side reactions, and improve product quality. In LNG production, the two-stage compressor compresses and cools natural gas to -162°C for liquefaction, reducing energy consumption costs.
Medical and Pharmaceutical Fields
In the medical and pharmaceutical fields, two-stage compressors are widely used in various refrigeration and cold storage equipment, such as hospital cold storage facilities, pharmaceutical storage cabinets, and laboratory cooling equipment. These devices require precise temperature control and reliable refrigeration capacity to maintain the quality and stability of pharmaceuticals and biological samples.
Single-Stage Compression vs. Two-Stage Compression
The Relationship Between Pressure Ratio and Compressor Performance
- In single-stage compression, the compression ratio is the ratio of condensing pressure to evaporating pressure. During the operation of a refrigeration system, when the evaporating temperature decreases (e.g., from -10°C to -30°C), the evaporating pressure decreases significantly due to its corresponding relationship with temperature, leading to a sharp increase in the compression ratio (e.g., from 5 to 15). . Under these conditions, the compressor will exhibit the following adverse phenomena:
- Sudden drop in volumetric efficiency: As the pressure ratio increases, the expansion process of the high-pressure gas remaining in the compressor’s clearance volume intensifies, occupying more cylinder space and significantly reducing the actual amount of refrigerant gas that can be drawn in.
This phenomenon not only reduces the refrigeration capacity of the refrigeration system but also increases the power consumption per unit of refrigeration capacity, thereby reducing the overall economic efficiency of the system.
- Excessively high discharge temperature: Under high compression ratio conditions, the refrigerant gas inside the compressor undergoes intense adiabatic compression during the compression process, causing the gas temperature to rise sharply.
Excessively high discharge temperatures can reduce the viscosity of the lubricating oil and even cause carbonization, severely affecting the compressor’s lubrication performance. Additionally, high temperatures may degrade the material properties of compressor components, leading to overheating damage and significantly shortening the compressor’s service life.
- Two-stage compression adopts a “stage compression + intermediate cooling” mode, dividing the total pressure ratio into two stages (e.g., a total pressure ratio of 15 is divided into a low-pressure stage of 3 and a high-pressure stage of 5), ensuring that the pressure ratio of each stage is ≤6, thereby avoiding the issues associated with single-stage high-pressure compression.
After being compressed in the low-pressure stage, the refrigerant is cooled to the intermediate pressure saturated temperature through heat exchange with the medium in the intermediate cooler, then enters the high-pressure stage for secondary compression. This design maintains high volumetric efficiency while controlling discharge temperature, ensuring stable compressor operation and enhancing refrigeration system performance.
The Role of Intermediate Cooling
The core advantage of the two-stage cycle lies in intermediate cooling: the superheated refrigerant discharged from the low-pressure stage is cooled in the intermediate cooler to the saturated temperature at intermediate pressure, reducing the temperature of the refrigerant entering the high-pressure stage and minimizing compression energy consumption in the high-pressure stage. By lowering the refrigerant’s specific enthalpy, the high-pressure stage compressor operates closer to ideal conditions, improving system energy efficiency.
Taking an R22 refrigerant system as an example, the 80°C superheated refrigerant from the low-pressure stage is cooled to 40°C saturated state, reducing the specific enthalpy value. This reduces the “starting energy” of the high-pressure stage compression, makes the compression ratio more reasonable, and lowers the system’s total energy consumption by 15%-20%, directly improving the COP.
Additionally, intermediate cooling optimizes compressor operating conditions, reduces high-pressure stage discharge temperature, prevents lubricant carbonization, extends compressor lifespan, reduces pipeline pressure loss, and enhances system stability. Suitable for low-temperature refrigeration applications with high efficiency and reliability requirements, such as food freezers operating at -30°C to -40°C.
Application Scenario Comparison
Single-Stage Compression
The single-stage compression refrigeration cycle compresses low-temperature, low-pressure refrigerant gas into high-temperature, high-pressure gas through a single compression process, followed by condensation and throttling to achieve refrigeration. This is the most basic form of refrigeration cycle. Its operating principle is based on the reverse Carnot cycle, utilizing the heat absorption during refrigerant phase change to transfer heat.
Suitable for scenarios with higher evaporation temperatures (≥-20°C) and moderate pressure ratios, such as:
- Residential air conditioners: During operation, the evaporation temperature typically ranges from 5°C to 10°C. This temperature range ensures effective cooling of indoor air while preventing excessive frost buildup on the evaporator surface. Heat is absorbed by the indoor unit’s evaporator, while the outdoor unit’s compressor and condenser release heat, enabling continuous cooling to meet household environmental regulation needs.
- Conventional refrigerators: The evaporation temperature in the refrigerator compartment is approximately -10°C to -18°C. This temperature effectively inhibits bacterial growth while maintaining food freshness and moisture content. The single-stage compression cycle features a compact design with low noise levels, making it suitable for installation in the limited space of refrigerators.
- Small-scale cold storage (refrigerated storage): The evaporation temperature ranges from -5°C to -15°C, primarily used for fruit and vegetable preservation and dairy product storage. Lower operating costs and simplified maintenance procedures make single-stage cycles the preferred choice for small commercial users.
Advantages: The system consists of only four core components—compressor, condenser, throttling device, and evaporator—with simple piping connections, reducing initial procurement costs by 20–30%. Routine maintenance requires only periodic checks of refrigerant pressure and equipment seals, with low technical barriers for repairs. When the system pressure ratio is within the reasonable range of 3-8, the compressor’s adiabatic efficiency can reach 75%-85%, resulting in excellent energy efficiency ratio (EER) performance.
Two-Stage Compression
The two-stage compression refrigeration cycle uses two compressors—a low-pressure stage and a high-pressure stage—operating in series to compress the refrigerant in two stages to the target pressure. An intermediate flash evaporator or intermediate cooler is installed to reduce irreversible losses during the compression process. This technology effectively reduces the single-stage compression ratio, lowers the compressor discharge temperature, and improves system refrigeration performance through staged compression and intermediate cooling.
It is suitable for scenarios with low evaporation temperatures (-20°C to -60°C) and excessive pressure ratios, such as:
- Low-temperature cold storage (freezer): Evaporation temperatures typically range from -25°C to -40°C, used for long-term frozen storage of meat, seafood, and other foods. Two-stage compression avoids the issue of excessively high discharge temperatures that occur in single-stage systems under ultra-high pressure ratios, ensuring a stable freezing environment and reducing food moisture loss.
- Freeze-drying equipment: At evaporation temperatures of -40°C to -50°C, it achieves low-temperature dehydration of materials. The two-stage system maintains stable refrigeration capacity output, ensuring uniformity and efficiency in the freeze-drying process, and is widely applied in fields such as biopharmaceuticals and food processing.
- Low-temperature test chambers: These simulate extremely cold environments of -50°C to -60°C for testing the cold resistance of electronic products and aerospace materials. By precisely controlling the evaporator temperature, they meet the stringent requirements for temperature stability (±0.5°C) and cooling rate (1°C/min) during testing.
Advantages: Under low evaporation temperature conditions, energy efficiency is improved by 15%-25% compared to single-stage systems. By reducing the compression ratio, the compressor discharge temperature is controlled within the safe range of 80-100°C, avoiding lubricant carbonization and seal aging caused by high temperatures, significantly extending equipment lifespan. Additionally, the staged compression design enhances the system’s adaptability to different operating conditions, allowing adjustment of the intermediate pressure
Conclusion
In summary, the two-stage compression refrigeration cycle overcomes the drawbacks of high compression ratios in single-stage systems through its staged compression and intermediate cooling design, offering significant advantages in efficiency, stability, and adaptability. It is widely applied in industrial, commercial, and medical fields, establishing its pivotal role in modern refrigeration technology. As refrigeration demands evolve, this technology will play an increasingly critical role.
