A Complete Guide to the Operation, Malfunctions and Repair Methods of Dry Screw Refrigeration Compressors!

I. Operation of Screw Refrigeration Compressor
1. First Start-up and Shutdown
Before the first start-up, the coupling must be realigned.

For the first start-up, the working conditions of all parts of the compressor and electrical components must be checked first.
The inspection items are as follows:
l Close the power switch and set the selector switch to the manual position;
l Press the alarm button, and the alarm bell rings; press the mute button, and the alarm is cancelled;
l Press the electric heating button, the indicator light comes on. After confirming that the electric heater is working, press the heating stop button, and the heating indicator light goes off;
l Press the water pump start button, the water pump starts, the indicator light comes on. Press the water pump stop button, the water pump stops, and the indicator light goes off;
l Press the oil pump start button, the oil pump indicator light comes on, the oil pump runs and the rotation direction is correct. Adjust the oil pressure difference to 0.4~0.6MPa. Pull the four-way valve or press the load increase/decrease button to check whether the slide valve and the energy indication device are working properly. Finally, the energy level indicator is at the "0" position.
Check the set values of each automatic safety protection relay or program \ the reference values for compressor temperature and pressure protection:
High exhaust pressure protection: Exhaust pressure ≤ 1.57MPa
High oil injection temperature protection: Oil injection temperature ≤ 65°C
Low oil pressure difference protection: Oil pressure difference ≥ 0.1MPa
High differential pressure protection before and after the fine filter: Differential pressure ≤ 0.1MPa
Low suction pressure protection: Set according to the actual working conditions
After checking the above items, the compressor can be started. The start-up steps are as follows:
1) The selector switch is for manual start-up;
2) Open the compressor exhaust shut-off valve;
3) Unload the compressor to the "0" position, that is, the 10% load position;
4) Start the cooling water pump and the chilled water pump to supply water to the condenser, oil cooler and evaporator;
5) Start the oil pump;
6) 30 seconds after the oil pump starts, when the pressure difference between the oil pressure and the exhaust pressure reaches 0.4~0.6MPa, press the compressor start button, the compressor starts, and at the same time, the bypass solenoid valve A also opens automatically. After the motor runs normally, valve A closes automatically;
7) Observe the suction pressure gauge, gradually open the suction shut-off valve and manually increase the load, and pay attention that the suction pressure should not be too low. After the compressor enters normal operation, adjust the oil pressure regulating valve to make the oil pressure difference 0.15~0.3MPa.
8) Check whether the pressure and temperature of all parts of the equipment, especially the temperature of the moving parts, are normal. In case of any abnormal situation, the compressor should be stopped for inspection.
9) The initial operation time should not be too long, and it can be stopped after about half an hour. The shutdown sequence is unloading, stopping the main engine, closing the suction shut-off valve, stopping the oil pump, and stopping the water pump to complete the first start-up process. When pressing the main engine stop button, the bypass solenoid valve B opens automatically, and valve B closes automatically after shutdown.
2. Normal Start-up and Shutdown
The normal start-up process is as follows:
a. Select manual start-up, which is the same as the first start-up process.
b. Select automatic start-up
1) Open the compressor exhaust shut-off valve, and start the cooling water pump and the chilled water pump;
2) Press the compressor start button. At this time, the oil pump automatically starts to run, and the slide valve automatically returns to the "0" position. After the oil pressure difference is established, the main motor automatically starts after a delay of about 15 seconds, and at the same time, the bypass solenoid valve A opens automatically. After the motor runs normally, valve A closes automatically;
3) When the main motor starts, the suction shut-off valve should be slowly opened at the same time. Otherwise, the excessive vacuum will increase the vibration and noise of the machine.
4) The compressor will automatically increase the load to 100% and enter the normal working state. And it will automatically adjust the load position according to the pressure set value or the chilled water temperature set value.
The normal shutdown process is as follows:
a. Manual shutdown, which is the same as the shutdown process of the first start-up.
b. The selector switch is in the automatic position:
1) Press the compressor stop button, the slide valve automatically returns to the "0" position, the main motor stops automatically, and at the same time, the bypass solenoid valve B opens automatically. The oil pump stops automatically after a delay, and valve B closes automatically after shutdown;
2) Close the suction shut-off valve. If it is shut down for a long time, the exhaust shut-off valve should also be closed;
3) Turn off the power of the water pump and the power switch of the compressor.
3. Precautions during Operation
(1) During the operation of the compressor, pay attention to observing the suction and exhaust pressure, suction and exhaust temperature, oil temperature and oil pressure, and record them regularly. The instruments are required to be accurate.
(2) If the compressor stops automatically due to the action of a certain safety protection during operation, the cause of the failure must be found out before it can be started again. It is absolutely not allowed to start the compressor again by changing their set values or shielding the failure.
(3) When the main engine stops due to a sudden power outage, since the bypass solenoid valve B cannot be opened, the compressor may reverse. At this time, the suction shut-off valve should be quickly closed to reduce the reverse rotation.
(4) If it is shut down for a long time in the season with low temperature, all the water in the system should be drained to avoid freezing and damaging the equipment.
(5) If the compressor is started in the season with low temperature, first start the oil pump, and turn the coupling in the direction of the motor rotation to make the oil circulate in the compressor for sufficient lubrication. This process must be carried out in the manual start-up mode; if it is a Freon refrigerant, the oil heater should be turned on to heat the lubricating oil before starting, and the oil temperature should be guaranteed to be above 25°C.
(6) If the unit is shut down for a long time, the oil pump should be started once every about 10 days to ensure that all parts in the compressor are lubricated. Each time the oil pump is started for 10 minutes; start the compressor once every 2 to 3 months for 1 hour each time to ensure that the moving parts will not stick together.
(7) Before each start-up, it is best to turn the compressor a few turns to check whether there is any jamming in the compressor and make the lubricating oil evenly distributed in all parts.
II. Fault Analysis and Elimination of Screw Refrigeration Compressor
The temperature of the compressor body is too high
a. The compression ratio is too large;
b. The oil injection amount is insufficient;
c. The suction is severely overheated, or the bypass valve leaks;
d. There is abnormal friction of the moving parts.
The same as the too high exhaust temperature. The main reason is the abnormal friction of the moving parts. Overhaul the compressor or replace the thrust bearing.
III. Overhaul of Screw Refrigeration Compressor
1. Dismantling
Before the screw compressor must be overhauled, if possible, unload the slide valve to the "0" position, shut down normally, and cut off the power. Cut off the pipelines connected to the compressor. After discharging the refrigerant and lubricating oil in the compressor, the compressor can be dismantled. Pay attention to developing good habits during dismantling. The parts of different parts should be placed separately, and necessary assembly marks should be made.
(1) After removing the pipelines, coupling and anchor bolts connected to the compressor, hoist the compressor to the workplace.
It is best to have an operation platform.
(2) Remove the suction filter and the suction check valve.
(3) Remove the energy indicator.
(4) Remove the positioning pin, remove the suction end cover in parallel, and take out the oil piston and the balance piston.
Note that when removing the positioning pin, it can only be pulled out instead of being knocked out. Be sure to remove the positioning pin first, and then remove all the bolts to prevent the entire weight of the suction end cover from acting on the positioning pin and bending the positioning pin.
(5) Remove the shaft seal cover and take out the shaft seal.
Be careful not to damage the dynamic and static rings.
(6) Remove the positioning pin and remove the exhaust end cover.
(7) Remove the bearing gland, lock nut, washer, thrust bearing, adjusting shim, etc.
Use the special tool provided with the machine to remove the lock nut and first open the locking piece of the lock washer.
Mark at any time and place the parts separately.
(8) After removing the positioning pin, remove the exhaust end seat.
(9) Use the special eyebolt to slowly and smoothly remove the driving rotor, and do not collide with the body.
At this time, the driven rotor rotates accordingly.
(10) Take out the driven rotor.
(11) Take out the slide valve.
2. Inspection
The inspection items include:
(1) Check whether there are abnormal abrasion marks on the inner surface of the body and the surface of the slide valve, and measure the size and roundness of the inner surface with an inside micrometer.
(2) Check whether there are abrasion marks on the end faces of the driving and driven rotors and the suction and exhaust end seats.
(3) Check the wear of the outer diameters and tooth surfaces of the driving and driven rotors, and measure the outer diameters of the rotors with an outside micrometer.
(4) Measure the diameter of the main shaft of the rotor and the inner diameter of the main bearing hole, and check the wear of the main bearing.
(5) Check the wear of the shaft seal.
(6) Check the deformation and damage of all "O" rings and springs.
(7) Check all the internal oil circuits of the compressor.
(8) Check whether the energy indicator is damaged or stuck.
(9) Check whether there is abnormal wear on the oil piston and the balance piston.
(10) Check whether the transmission core or diaphragm of the coupling is damaged.
3. Repair
After the inspection, the damaged parts should be repaired or replaced. The specific contents are as follows:
(1) For less serious wear and roughening, a fitter can polish it smooth with an oilstone. There should be no rust, cracks or other defects on the surface of the rotor journal and the dynamic and static rings of the shaft seal. After the surface of the main journal is polished, its size should be carefully measured, and the main bearing should be adjusted based on this.
(2) If the main bearing is severely worn and exceeds the clearance limit for fitting with the shaft, it should be replaced. If the method of casting bearing alloy is adopted, the coaxiality of the inner surface and the bearing hole must be ensured.
(3) If the thrust bearing is damaged or the clearance is increased, it must be replaced with a new one. And the size of its vacuum clearance should be measured, and the bearing adjusting shim should be matched based on this.
(4) All gaskets and "O" rings must be replaced if they are damaged. Gaskets can be processed by oneself, but pay attention that there should be no breakage or cracks near the bolt holes to avoid leakage. High-quality materials should be selected for gaskets. The material of the "O" ring should be selected to be suitable for the refrigerant.
(5) Clean the sealant on the two end faces of the body and the plane of the suction and exhaust end seats. Be careful not to scratch the surface.
4. Assembly
After inspecting each part and repairing or replacing the damaged parts, the assembly work can be carried out. Pay attention to the marks noted during disassembly during assembly, and do not mix up the parts. During assembly, measure the assembly clearance of each part at any time. For the parts where the clearance exceeds the standard, adjust the clearance or replace the parts.
(1) Clean all parts and blow them dry with compressed air.
(2) Prepare all the required tools and clean them.
(3) Install the main bearing into the bearing hole in its original position, and measure the inner diameter of the bearing to make the inner diameter meet the clearance requirements for fitting with the rotor journal.
(4) Apply sealant evenly on the fitting surface between the suction end seat and the body.
(5) Lean the suction end seat against the suction end of the body, and insert some bolts to bear the weight. After pressing in the positioning pin, tighten the bolts.
(6) Install the slide valve and its guide support plate. The guide support plate should be positioned with the positioning pin first and then fixed with bolts.
(7) Apply clean lubricating oil of the same type as that used during operation to the main bearing hole at the suction end and the inner hole of the body, and install the male rotor and the female rotor. The later-installed rotor should be slowly screwed in and not forced into the body.
(8) Lean the male and female rotors against the suction end seat, and measure the dimensional difference between the exhaust end face of the rotor and the exhaust end face of the body.

(9) Apply sealant evenly on the fitting surface between the exhaust end seat and the body.
(10) Lean the exhaust end seat against the discharge end of the body. After pressing in the positioning pin, tighten the bolts. When installing the exhaust end seat, pay attention to the inner hole of the main bearing and do not damage the main bearing.
(11) Install the adjusting shim and the thrust bearing, and fix the inner spacer ring of the thrust bearing on the journal of the rotor with a round nut and a lock washer. The thrust bearings are installed in pairs, and pay attention to the installation direction.
(12) Install the rotor bearing gland and press the outer spacer ring of the thrust bearing against the body.
Remove the connecting bolts between the exhaust end seat and the body, pull out the exhaust end seat together with the rotor, and measure the clearance between the exhaust end face of the rotor and the exhaust end seat. This clearance is the exhaust end clearance. If the clearance is not reasonable, the thickness of the adjusting shim should be changed. Assemble after adjusting the exhaust end clearance. After installation, turn the driving rotor according to the actual operation direction, and the rotation should be flexible without jamming.
(13) Install the positioning pin on the exhaust end cover, add a gasket and fix it with bolts.
(14) Install the dynamic ring of the shaft seal, and apply refrigeration oil on the surface of the dynamic ring and the rubber ring.
(15) Install the static ring into the shaft seal gland, add a gasket and fix it on the exhaust end seat with bolts. Adjust the pre-tightening force of the shaft seal spring through the thickness of the gasket. Apply refrigeration oil on the surface of the static ring and the rubber ring of the static ring.
(16) Install the oil piston and the balance piston. Apply refrigeration oil during assembly.
(17) Install the positioning pin on the suction end seat, add a gasket and fix it with bolts.
(18) Install the energy indicator, and pay attention that the pointer corresponds to the position of the slide valve.
(19) Install the repaired suction filter and check valve on the suction end seat. Transport the assembled compressor to the unit. After finding the coaxiality, fix the compressor and the motor and connect them with a coupling.
5. Test Run after Overhaul
After the overhaul, conduct an airtightness test and a vacuum test on the compressor. The test run can be carried out only after the test is qualified. Mainly check whether the action of the slide valve, the oil pressure of the oil pump is normal, and whether the noise and vibration during operation are normal. The turning by hand after shutdown should be normal.