Classification of screw refrigeration compressor units
1. Structure of screw refrigeration compressor unit
When the screw compressor is working, lubricating oil should be continuously sprayed into the working chamber, which plays the role of lubricating, cooling, sealing and noise reduction, as well as lubricating the lubricating oil of the main bearing, thrust bearing and shaft seal, and pushes the oil piston and balances the pressure of the piston. The oil, which is finally mixed with the high pressure gas, is discharged from the compressor. These oils must be separated, cooled, filtered, pressurized and recycled. In order to prevent impurities in the refrigeration system from entering the compressor with the suction and causing wear to the rotor and body, a suction filter must be installed.
2. Single-stage screw refrigeration compressor system, lubrication and control
2.1 Compressor system
A single-stage screw refrigeration compressor unit is generally composed of a compressor, an electric motor, a suction filter, an oil separator, an oil cooler, an oil filter, an oil pump, a check valve and an electrical console, which are installed on the same common base. Electrical starting cabinets are generally concentrated in a control room.
2.2 Lubrication and oil injection
There are two types of screw refrigeration compressors: internal oil circuit and external oil circuit. Regardless of the oil circuit, the following points should be guaranteed to be lubricated:
2.2.1 Sliding valve injection;
2.2.2 Balance piston oil injection;
2.2.3 Oil injection to slide valve guide rail;
2.2.4 The main bearing and thrust bearing at the suction and exhaust ends are sprayed with oil;
2.2.5 Shaft seal oil injection.
2.3 Control and Protection
The control of screw refrigeration compressor unit is generally divided into automatic type and manual operation plus automatic protection type. Regardless of the type, the following automatic protections should be set:
2.3.1 Main motor overload protection;
2.3.2 High exhaust pressure protection;
2.3.3 High injection temperature protection;
2.3.4 Oil pressure and exhaust pressure difference protection;
2.3.5 High differential pressure protection before and after the essential oil filter;
2.3.6 Low suction pressure protection: set according to working conditions.
3. Screw refrigeration cycle system with economizer
One of the characteristics of screw compressors is that the single-stage pressure ratio is large, but as the pressure ratio increases, the internal leakage of the compressor increases, the efficiency decreases, and the power consumption is large. Taking advantage of the one-way suction, compression, and exhaust characteristics of the screw compressor, an intermediate air supply hole is set at a certain position where the rotor rotates to start compression, and the refrigerant gas under the intermediate pressure is replenished, so that the single-stage screw presses the two poles. Compression operation is equivalent to a quasi-two-stage, that is, a screw compressor with an economizer.
The screw refrigeration cycle system with economizer is also called energy saving system.
The economizer is a shell and tube heat exchanger, the shell side is the low pressure side, and the tube side is the high pressure side. The economizer is equipped with air filter, check valve, throttle valve and other accessories. When the pressure ratio changes, the liquid supply volume of the throttle valve should be properly adjusted according to the superheat of the intermediate air supply to prevent liquid backflow, especially in the Freon system that uses a thermal expansion valve for liquid supply.
Screw compressor units with economizers are often used in refrigeration systems with an evaporation temperature of -25 to -40 °C. Compared with the original single-stage compressor, the cooling capacity and cooling efficiency have been greatly improved, and electricity is also saved. The relationship between the cooling capacity and the increase rate of the shaft power of the ammonia screw compressor with economizer and the temperature, the change of the shaft power is relatively gentle, and the increase rate of the cooling capacity increases with the decrease of the evaporating temperature, and the larger the pressure ratio, the more significant the increase effect is. .
4. Screw liquid ammonia cooling compressor unit
Compressor unit using liquid ammonia oil cooler. It is suitable for occasions with poor water quality, difficult water supply or high water cost; the oil temperature is relatively stable. The disadvantage is that the minimum temperature of the oil temperature is controlled by the condensing temperature, and the system needs to add an auxiliary liquid accumulator or an ammonia pump. When the auxiliary liquid reservoir is used, the height difference between the liquid outlet of the auxiliary liquid reservoir and the oil cooler must be at least 1 meter.
5. Screw chiller
The screw compressor unit is combined with the evaporator, the condenser, the filter drier, the solenoid valve and the thermal expansion valve on a common base, and the unit uses water as the refrigerant. The chiller often uses R22 as the refrigerant, and uses the thermal expansion valve to automatically adjust the liquid supply. At present, ammonia is also used as refrigerant, and plate heat exchangers are used as evaporators and condensers. Shell and tube heat exchangers can be used to produce air-conditioning or cooling water at 4-15°C; while plate heat exchangers can be used to produce cold water at about 1°C. In addition to the automatic protection of the normal compressor unit, the chiller also has cold water, cooling water cut-off protection and cold water temperature low protection.
Thermal expansion valve: It is composed of a temperature sensing package, a capillary tube, an elastic diaphragm, a spring, a valve core, a valve seat and a regulating device. The temperature sensor is tied to the suction line near the compressor. Using the corresponding relationship between the refrigerant saturation temperature and the saturation pressure in the temperature sensor, the temperature also changes, changing the opening of the valve core and adjusting the expansion valve. liquid supply. The adjustment device is used to change the spring force of the spring on the diaphragm, thereby changing the suction superheat.
6. Screw brine (ethylene glycol) unit
The structure is similar to the chiller, using brine (ethylene glycol solution) as the refrigerant to prepare low-temperature brine (ethylene glycol solution) at -10～-40℃. Due to the corrosive nature of salt water, the heat exchange tube in the evaporator of the salt water unit is made of high-efficiency brass tube with low temperature resistance and corrosion resistance. The evaporation temperature of the brine (ethylene glycol) unit is relatively low, so pay attention to whether the suction filter is blocked by ice during the initial operation.
