【Mechanical Equipment Technology Edition】Daily Question 20170729: Design issues related to screw-type refrigeration compressors?

Single-stage compression is chosen for low pressures, while multi-stage compression is used for high pressures.

It is necessary to check whether it is isothermal compression; if so, there is no need to choose two-stage compression.

During the compression process, all the power supplied to the compressor is converted into heat. Compressing high-temperature gases requires more energy, so compressors must be equipped with cooling measures. The lower the temperature rise during compression, the more energy-efficient the compressor is. The ideal compression process is isothermal compression, in which the temperature remains constant during the compression. Piston compressors, turbine compressors, and oil-free screw compressors lack effective cooling methods during the compression process; therefore, the compression is often carried out in multiple stages. An inter-stage cooler is installed at the exit of each stage to reduce the temperature to near ambient levels, after which the condensed water is removed before the gas is sent to the next stage for further compression. This band-based compression process can keep the maximum temperature within a safe range, while keeping the average temperature relatively close to isothermal. Therefore, for piston compressors, turbine compressors, and oil-free screw compressors, multi-stage compression is beneficial for energy savings when an intercooler is available. An oil-injected screw compressor uses lubricating oil to absorb heat during the compression process. By weight, the amount of lubricating oil injected into the main engine per unit time is ten times that of the air flow. Since lubricating oil has a much greater capacity to absorb heat than air, a large amount of compression heat is absorbed by the lubricating oil, keeping the temperature rise during the compression process at a low level. The heat absorbed by the lubricating oil accounts for about 90% of the input power. Internal oil injection cooling is a key technology that has enabled the rapid development of screw compressors. It can be said that the screw compressor was able to replace piston compressors as the dominant type in the market entirely thanks to its unique cooling method. The unique cooling method of the oil-injected screw compressor means that it does not require, nor is it capable of using, multi-stage compression within normal exhaust pressure ranges, such as below 1.3 MPa exhaust pressure. Firstly, by fully leveraging the advantages of injection internal cooling, the compression process approaches an isothermal process; therefore, multi-stage compression is not necessary for injection screw compressors. Secondly, without inter-stage coolers, multi-stage compression offers no energy-saving benefits. Furthermore, if inter-stage cooling is used, the temperature of the gas entering the next stage will inevitably be lower than the dew point temperature at that pressure, resulting in significant water precipitation. It is therefore necessary to install components for oil-water separation and removal between stages; otherwise, lubricating oil containing a large amount of water, once it enters the next stage, will lead to a severe deterioration of the lubrication conditions, thereby accelerating wear on the rotor and bearings. The separation of oil from water requires several hours at rest, and the drainage element must have the ability to detect the oil-water interface. Therefore, inter-stage oil-water separation and removal is an insurmountable obstacle for multi-stage compression screw compressors equipped with inter-stage coolers. Currently, two-stage compression is used in rotary vane compressors merely to address the issue of excessive bearing load at higher discharge pressures, and it has nothing to do with energy savings. The use of two-stage compression at exhaust pressures below 1.3 MPa is purely a marketing tactic; the claim by some foreign companies that two-stage compression saves energy is based on pseudoscientific marketing gimmicks, or rather, it constitutes commercial fraud. A survey of several application examples of two-stage compression oil-injected screw compressors also shows that not only is there no energy-saving effect, but the failure rate is also significantly higher than that of single-stage compression products. The most effective measure for energy savings in screw compressors with injection is low speed. At low speeds, heat exchange between oil and gas is more efficient, allowing the technical advantages of internal fuel cooling to be fully utilized. At low speeds, the frictional loss of the bearing is lower. At low speeds, the diameter of the intake and exhaust ports on the main unit is larger, resulting in less airflow resistance. An energy-saving screw compressor must be a screw compressor with a low speed. Rotational speed is the most critical criterion for determining whether a screw compressor is energy-efficient or not.

Both single-stage and two-stage configurations are applicable to reciprocating compressors. The choice depends on the required cooling temperature: for cold storage at normal temperatures down to below -30 degrees, a two-stage compressor is preferred as it offers higher efficiency; for temperatures above 30 degrees, a single-stage compressor is suitable. If a screw compressor is used, these considerations are not relevant, as it can achieve temperatures as low as -40 degrees. Centrifugal compressors also come in single-stage and multi-stage versions, with the multi-stage type being used for even lower temperatures, though it is less common.

Evaporation temperature, compression ratio, type of refrigerant

Isentropic compression? What you’re saying doesn’t make any sense; in a refrigeration cycle, compressors basically operate under adiabatic compression conditions

　　1) The economic compression ratio for a single-stage screw refrigeration compressor is 4.7 – 5.5, as it is within this range that the efficiency is optimal. 　　2) Single-stage screw refrigeration compressors are not suitable for low-temperature operating conditions in the southern regions of China. 　　3) When the evaporation temperature is below –20°C, the operational efficiency of single-stage screw compressors is poor. The lower the evaporation temperature, the lower the efficiency and the greater the energy consumption; long-term operation leads to excessive energy use and premature damage to the compressor. 　　4) Prefer screw-type compressors equipped with an economizer. Screw compressors equipped with economizers can operate under a wide range of conditions; they have a high single-stage compression ratio, are easier to control than two-stage screw refrigeration systems, feature a simple design, and require less space ； The advantages are even more evident compared to single-stage screw compressors. 　　5) When the evaporation temperature is below –30°C and operation lasts for a long period, from an energy-saving perspective, a two-stage compression screw system should be used.