Briefly talk about scroll, screw, piston, centrifugal refrigeration compressor
Piston refrigeration compressor
Piston type (ie, reciprocating) refrigeration compressors are widely used, but the displacement is not large. Generally, they are small and medium-sized compressors. The cooling capacity of general air-conditioning conditions is less than 300kW.
The main factors that affect the cooling capacity of the compressor are: the evaporation temperature and the condensation temperature, the evaporation temperature has a greater influence.
The main factors affecting the performance of piston refrigeration compressors:
1. Coefficient of performance (COP) of refrigeration compressor
2. Energy efficiency ratio (EER) of refrigeration compressors
Nominal working condition of piston refrigeration compressor:
The parameters such as the refrigeration capacity and shaft power of the compressor vary with the working conditions. In order to measure and compare the performance of the compressor, a recognized temperature condition (nominal working condition) is established as the standard for the selection and comparison of the compressor refrigeration capacity.
The refrigeration capacity and power indicated on the nameplate are generally the values under standard conditions. If they are dedicated for air conditioning, they are under air conditioning conditions.
Rotary refrigeration compressor
The rotary refrigeration compressor uses the rotary motion of the return rotor to replace the reciprocating motion of the piston refrigeration compressor, periodically increases the pressure of the low-pressure gas, and realizes high speed and miniaturization. Its operation is stable, low noise, simple structure, and volumetric efficiency. High; but the rotary compressor is a sliding seal and requires high machining accuracy.
Rotary refrigeration compressors are also positive displacement compressors. Compared with piston compressors, they have higher volumetric efficiency, stable operation, and can achieve high speed and miniaturization.
Features of scroll refrigeration compressor
1. High volumetric efficiency: The suction, compression, and exhaust of the work project are basically continuous. There is no clearance volume, and the degree of suction and harmful superheat is small; there is no suction valve, and the resistance loss is small; the pressure difference on both sides of the continuous compression chamber is small , Less air leakage; therefore, its volumetric efficiency is high, usually up to 0.95 or more;
(2) Low vibration and low noise: The torque change of eccentric shaft is only 1/10 of the reciprocating and rolling type, which makes the vibration and noise lower;
(3)Simple structure and high reliability:
Screw refrigeration compressor
The twin screw is mainly composed of two meshing screw rotors. Compared with the piston compressor, there are no suction and exhaust valves. The air delivery adjustment is realized by the unloading piston and pushing the unloading slide valve.
Three compression conditions of screw compressor
Unlike piston compressors, screw compressors have no suction and discharge valves. The pressure in the enclosed space at the end of compression is determined only by the shape of the rotating body, the position of the discharge port, the suction pressure and the nature of the refrigerant. The pressure in the tube (condensing pressure) is irrelevant. Therefore, there may be three working conditions:
At the end of compression, the closed space pressure P2 is equal to the exhaust pipe pressure Pk;
At the end of compression, the closed space pressure P2 is lower than the exhaust pipe pressure Pk; (under compression)
At the end of compression, the closed space pressure P2 is higher than the exhaust pipe pressure Pk. (Over compression)
In the case of under-compression and over-compression, the power consumption of the compressor increases. In order to achieve the same pressure between the enclosed space and the exhaust pipe at the end of compression under variable operating conditions, an internal volume ratio adjustment device can be added to the compressor to automatically adjust the internal volume ratio. In order to realize the efficient operation of the system under variable working conditions.
1—male rotor; 2—female rotor; 3—machine body; 4—sliding bearing; 5—thrust bearing; 6—balanced piston; 7—shaft seal; 8—energy regulating piston; 9—unloading slide valve; 10—row Air port; 11—air inlet
For screw compressors with a fixed internal volume ratio, the internal pressure ratio is generally slightly lower than the rated operating pressure ratio of the refrigeration or heat pump unit.
Features of screw refrigeration compressor
(1) When the screw is working, the oil lubricating oil system sprays lubricating oil on the working surface of the screw for cooling, so the exhaust temperature is low;
(2) Since there is no intake and exhaust valves, and no clearance volume, its volumetric efficiency is high; the compression ratio is large; there is no worry about wet compression;
(3) No-load start, with low motor power; complicated fuel supply system;
(4) The air intake is controlled by the slide valve to realize stepless adjustment;
(5) The efficiency of the screw machine at partial load is 8%～10% higher than that of the centrifuge
Centrifugal refrigeration compressor
The advantages of centrifugal refrigeration compressors:
Large refrigeration compressors consume huge amounts of electricity. In order to reduce various losses in the process of power generation equipment, electric motors and energy conversion, large-scale centrifugal refrigeration compressors can be directly driven by steam turbines or gas turbines, and can also be equipped with absorption chillers to achieve cascade utilization of energy or cooling and heating, Large cooling capacity of single unit of cogeneration;
There are no wearing parts; the development restriction of centrifugal refrigeration compressors: higher requirements for material strength, processing accuracy and manufacturing quality; more suitable for large-scale or special-purpose occasions.
1—impeller; 2—diffuser; 3—guide vane; 4—machine body; 5—main shaft
Basic structure: impeller, suction chamber, diffuser, curve and reflux, volute chamber.
Working principle: Generally, the airflow of centrifugal refrigeration compressor enters the impeller along the axis. The theoretical energy head generated by the rotation of the impeller is only related to the peripheral speed of the outer edge of the impeller and the flow condition, and has nothing to do with the nature of the refrigerant.
—Body; 2—Impeller; 3—Diffuser; 4—Bend; 5—Reflux; 6—Volute; 7—Main shaft; 8-Bearing; 9—Thrust bearing; 12—Inlet diversion device
Centrifugal refrigeration compressor requirements for refrigerant
In order to reduce the number of impeller stages and simplify the structure of the centrifugal compressor, the energy head of the gas is required to be small. Therefore, centrifugal compressors often use refrigerants with large molecular weights, such as R134a.
Centrifugal compressor requirements for speed
In order to obtain sufficient peripheral speed of the outer edge of the impeller, the impeller speed is required to be higher; and the smaller the diameter, the higher the speed requirement, generally 5000-15000r/min.
Centrifugal refrigeration compressor requirements for minimum cooling capacity
Due to the limitation of the processing technology, the diameter of the impeller should not be less than 200～250mm). In addition, the centrifugal compressor has a high speed. Therefore, its displacement is large. Even if a refrigerant with a small refrigeration capacity per unit volume is used, the single-stage capacity should not be less than 500kW. (Suitable for large-scale refrigeration equipment, such as centralized air-conditioning, large-scale cold storage, petrochemical industry, etc.)
The hazards of centrifugal compressor surge:
Cause periodic noise increase and vibration; high-temperature gas backflow into the compressor will cause the temperature of the compressor housing and bearings to rise. If measures are not taken in time, the compressor will be damaged.
Reasons for surge of centrifugal compressor:
The main reason is that the condensing pressure is too high or the suction pressure is too low; the refrigeration capacity of the compressor is adjusted too low (air delivery is too low, below the surge point).
Measures to prevent surge in centrifugal compressors:
① Maintain the stability of condensing pressure and evaporation pressure during compressor operation;
②Bypass adjustment method: When the refrigerant flow needs to be adjusted below the surge point, a part of the refrigerant drawn from the compressor outlet will return to the compressor suction pipe without condensation.
Factors affecting the refrigeration capacity of centrifugal compressors
The higher the condensing pressure and the lower the evaporation pressure, the greater the energy head required to compress the gaseous refrigerant.
In the working range of the centrifugal compressor, the larger the energy head obtained by the gas, the smaller the refrigerant displacement.
The influence of evaporation temperature on the refrigeration capacity of centrifugal compressor
The cooling capacity of the centrifugal compressor decreases with the decrease of the evaporation temperature;
Centrifugal compressors are more affected by the evaporation temperature than piston compressors. It can be seen that the displacement of the centrifugal compressor decreases with the increase of the condensing pressure and the decrease of the evaporation pressure. Therefore, the cooling capacity is also reduced accordingly.
Influence of condensation temperature on refrigeration capacity of centrifugal compressor
When the condensing temperature is lower than the design temperature, the condensing temperature has little effect on the cooling capacity of the centrifugal compressor;
When the condensing temperature is higher than the design temperature, the refrigeration capacity of the centrifugal compressor drops sharply as the condensing temperature rises.
