1. Why uses a two-stage compression?
2. What is the principle a two-stage compression refrigeration cycle?
3. How to work with a two-stage compression Refrigeration Cycle?
Why use a two-stage Compression?
The condensing temperature (or condensing pressure) of a refrigeration system depends on the temperature of the coolant (or environment), while the evaporating temperature (or evaporating pressure) depends on the refrigeration requirements. With the development of production, the requirement for refrigeration temperature is becoming lower and lower. Therefore, in many practical applications of refrigeration, the compressor in high pressure (condensation pressure) to low pressure (evaporation pressure) ratio (compression ratio) high conditions for work. According to the equation of state P V / T ≡ C of an ideal gas, if a single-stage compression refrigeration cycle is used, the temperature of the superheated vapor at the end of compression must be very high (V is certain, P – t).
1. The gas transmission coefficient of the compressor is greatly reduced, and when the compression ratio is ≥20, = 0.
2. For the Piston compressor, due to the existence of clearance volume, when the pressure ratio reached 20, the compressor almost no suction. The refrigerating capacity per unit volume and the refrigerating capacity per unit volume of the compressor are greatly reduced.
3.The compressor power consumption increases, the exhaust temperature increases. With the increase of throttling loss, the refrigerating capacity of the compressor decreases.
4. The exhaust temperature is high, the lubricating oil becomes thin, the lubricating condition is weakened, even the oil will appear carbon phenomenon, and must use the high ignition point lubricating oil. because the viscosity of the lubricating oil decreases with the temperature rising.
5. More lubricating oil is taken out by superheated steam, which increases the load of oil separator, makes the oil vaporize and oil vapor enter the system in large quantity and affects the heat transfer effect of the heat exchanger.
To sum up, when the compression ratio is too high, using a single-stage compression cycle is not only uneconomic, and even impossible. In order to solve these problems and meet the production requirements, the two-stage compression refrigeration cycle with intermediate cooler is often used in practice. However, the equipment investment required for a two-stage compression refrigeration cycle is much larger than that for a single-stage compression cycle, and the operation is also more complex. Therefore, the use of a two-stage compression refrigeration cycle is not in any case, is beneficial, in general, the single-stage ammonia refrigeration compressor compression ratio more than 8, fluorine more than 10, the use of two-stage compression is more economical and reasonable.
What is the principle a two-stage compression refrigeration cycle?
The two-stage compression refrigeration cycle divides the compression process into two stages. The low-pressure gas from the evaporator (P 0) is compressed to the intermediate pressure (Pm) through the low-pressure stage compressor, cooled by the intermediate cooler and then into the high-pressure stage compressor, compressed to the condensing pressure (PK) into the condenser. The compression ratios of each stage are kept below 10.
For example:-35 / + 35 °C system: evaporation temperature-35 °C, evaporation pressure (absolute pressure)0.093 MPA; condensation temperature + 35 °C, condensation pressure (absolute pressure)1.353 MPA; single-stage Cycle: Pressure Ratio 1.353 / 0.09314.55.
The middle pressure is 0.355 MPA (absolute pressure), the pressure ratio is 1.353 / 0.3553.81 for high- pressure stage and 0.355 / 0.09333.82 for low- pressure stage.
How to work with a two-stage compression Refrigeration Cycle?
The two-stage compression refrigeration cycle has a different cycle because of the different throttling modes and the different intermediate cooling degree.
There are four types of two-stage compression refrigeration cycle:
1.Complete Cooling in the middle of two throttles
2.incomplete cooling in the middle of two throttles
3.complete cooling in the middle of one throttle
4. incomplete cooling in the middle of one throttle.
Among them, two throttling refers to the refrigerant coming out of the condenser passes through two expansion valves and then enters the evaporator, that is, first throttling from the condensing pressure to the intermediate pressure, then throttling from the intermediate pressure to the evaporating pressure. While a throttling only passes through one expansion valve, most refrigerants are throttled directly from condensing pressure to evaporating pressure. In contrast, a throttling system is relatively simple And the large pressure difference can be used to realize the liquid supply in the long-distance or high-level cold storage. Therefore, the two-stage compression refrigeration system with one throttling is adopted in practice. The type of refrigerant to be used will determine the type of intermediate cooling to be used. Generally, two-stage compressed ammonia refrigeration system adopts intermediate complete cooling, while two-stage compressed Freon Refrigeration System Adopts Intermediate Incomplete Cooling.
A Two-stage cycle with complete cooling between primary throttling
The low-pressure and low-temperature refrigerant vapor produced in the evaporator is sucked up by the low-pressure compressor and compressed into the superheated vapor of the intermediate pressure, and then enters the intermediate cooler of the same pressure and is cooled into dry saturated vapor in the intercooler. The middle-pressure dry saturated vapor is sucked up by the high-pressure compressor and compressed to the superheated vapor of the condensing pressure, and then enters the condenser and is condensed into the refrigerant liquid. Then it is divided into two ways, all the way through the expansion valve throttling pressure drop into the intermediate cooler, most of the liquid from the other way into the intermediate cooler coil supercooling, but due to the existence of heat transfer temperature difference, therefore, it is impossible to be cooled to the intermediate temperature in the coil, but higher than the intermediate temperature △ t = 3-5 °C. The supercooled liquid passes through the main expansion valve to reduce the pressure to the supercooled liquid of low temperature and low pressure and finally enters into the evaporator to absorb heat and evaporate, producing the cooling effect.
A Two-stage cycle with incomplete cooling in the middle of primary throttling
A two-stage cycle with incomplete cooling in the middle of a throttling operation, in which the steam from the evaporator is sucked in by a low-pressure compressor through a regenerator Compressed to the intermediate pressure and mixed with the dry saturated steam from the intercooler in the pipeline, so that the superheated steam from the low-pressure machine is cooled and then into the high-pressure compressor, after compression to the condensation pressure and into the condenser, the condensed high-pressure refrigerant liquid enters the serpentine coil of the intercooler for re-cooling, and then enters the regenerator for heat exchange with the low-temperature and low-pressure steam from the evaporator. The supercooled liquid from the coil of the intercooler is cooled again, and finally, the supercooled liquid enters the evaporator through an expansion valve.
Complete cooling between throttles
The system features a flash intercooler. It plays two roles, one is equivalent to two throttles of the intermediate liquid separator, the other is the use of a small part of the liquid endothermic evaporation, the exhaust of the low-pressure machine for complete intermediate cooling. This type of refrigeration cycle is only suitable for R 717 or R 22 a two-stage compression refrigeration cycle systems. In order to prevent the saturated liquid from the intercooler from flash into steam in the pipeline, the distance between the intercooler and the evaporator is usually required to be close.
The adoption of two-stage compression refrigeration cycle not only reduces the exhaust temperature of the high-pressure compressor, improves the lubrication condition of the compressor, but also greatly improves the steam transmission coefficient of the compressor due to the small compression ratio of each stage. In addition, the power consumption of a two-stage compression cycle is lower than that of the single-stage compression cycle.
Please contact us for future details email@example.com