(1) Refrigeration principle related to cold storage application:
Refrigeration process:
1. After starting the refrigeration equipment, the compressor works to discharge high temperature and high pressure refrigerant gas; (the temperature of the exhaust pipe can be as high as 120 degrees)
2. The heat absorbed by the refrigerant in the evaporator is taken away by the air (or water) through the air-cooled condenser (or water-cooled condenser), and cooled and condensed into a supercooled liquid at room temperature and high pressure;
3. Flow through reservoir, filter, sight glass, solenoid valve;
4. After the throttling and pressure reduction of the thermal expansion valve, it becomes a low-pressure and low-temperature refrigerant liquid and then flows into the evaporator (cooler or coil);
5. The refrigerant evaporates in the evaporator to absorb the heat of the goods, so as to achieve the purpose of cooling the room;
6. The refrigerant after evaporation and heat absorption becomes a low-temperature and low-pressure gas, which is returned to the compressor to be compressed again and then discharged. This cycle is repeated, and finally the temperature in the warehouse is reduced to a specified temperature within a specified time.
(2) The composition of the cold storage:
Enclosure structure; compression refrigeration unit; solenoid valve, sight glass; expansion valve; evaporator (cooler); system connection pipeline; electrical control system (automatic computer control cabinet); refrigeration auxiliary equipment (such as cooling tower, pressure tank) , water pump, etc.). Below we describe these components one by one.
1. Enclosure structure:
According to the structure, the cold storage is divided into: civil type and prefabricated type.
Civil-built cold storage: It is mainly composed of an enclosure structure and a load-bearing structure. In addition to being able to withstand external wind and rain, its external enclosure structure must also have the functions of heat insulation and moisture resistance. Therefore, there are many construction procedures in the construction process of the civil cold storage, the workload is large, and the construction period is long.
Application: Because of its good thermal insulation effect, it is relatively less affected by the outside world. It is widely used in some large vegetable and fruit production, wholesale bases and some meat and poultry processing plants.
Assembled cold storage: The cold storage body is mainly composed of heat insulation panels, and the heat insulation panels of the cold storage are all sandwich panels. The inside and outside of the sandwich panel are mostly glass steel plates, color steel plates, and aluminum alloy plates or other plastic plates. Sandwich insulation materials are mostly rigid polyurethane or polystyrene foam.
Application: The prefabricated cold storage has the characteristics of simple structure, convenient installation, short construction period, light weight, high strength and beautiful appearance.
2. Compression refrigeration unit:
According to the different cooling methods of the condenser, it is divided into: air-cooled unit; water-cooled unit.
The composition of the compression refrigeration unit: compressor, condenser, liquid receiver, dry filter, pressure controller, high and low pressure pressure gauge, connecting pipeline.
The compressor, the heart of the refrigeration system.
The refrigerant vapor vaporized by the endothermic process in the evaporator is continuously extracted to maintain the low temperature and low pressure in the evaporator.
Continuously compress low-temperature and low-pressure refrigerant gas into high-temperature and high-pressure refrigerant gas to provide power for the refrigeration cycle, thereby realizing a refrigeration cycle of compression-condensation-expansion-evaporation (heat absorption)-compression.
Type: semi-enclosed, fully enclosed, open; rotor, scroll, piston, screw.
3. Condenser:
Condenser function - heat exchange equipment.
Transfer the heat of the high temperature and high pressure refrigerant vapor from the compressor to the surrounding medium - water or air, and cool it into a low temperature and high pressure liquid. It is a necessary device to output heat to the outside of the system.
Condenser Type:
Air-cooled (referred to as air-cooled or air-cooled), water-cooled (referred to as water-cooled), evaporative.
4. Liquid reservoir:
A container for storing and supplying high-pressure liquid refrigerant, storing the liquid refrigerant from the condenser, and adjusting and stabilizing the circulation amount of the refrigerant to adapt to changes in working conditions.
It is usually installed behind the condenser (air-cooled unit), and the water-cooled unit has a liquid storage function and generally does not install a liquid storage device.
3. Analysis of common faults in cold storage
1. Exhaust pressure is too high:
Hazards: compressor overheating, increased wear and tear, deterioration of lubricating oil, decreased cooling capacity, and increased power consumption.
Cause Analysis:
1) There are non-condensable gases such as air in the system during vacuuming;
2) The ambient temperature is too high (prone in summer) or the ventilation is poor;
3) The amount of cooling water is insufficient or the water temperature is too high (water cooling unit);
4) The water-cooled condenser has too much fouling; the air-cooled condenser has too much fouling;
5) The condenser motor or fan blades are broken.
2. Inspiratory pressure is too low
Hazards: Overheating of the compressor, increased wear and tear, decreased cooling capacity, and increased power consumption.
Cause Analysis:
1) The frost layer of the air cooler is too thick;
2) The cooling fan motor or fan blade is broken;
3) The refrigerant leaks.
3. Exhaust temperature is too high
Hazard: Deterioration of lubricating oil, wear of mechanical components inside the compressor.
cause:
1) High pressure is too high;
2) The low pressure is too low;
3) The load of the cold storage is too large.
Note: The maximum temperature of the exhaust pipe does not exceed 135℃.
4. Refrigerant liquid return
Refrigerant liquid return: Excessive refrigerant liquid enters the compressor crankcase.
Hazard: Refrigerant liquid dilutes lubricating oil. Sliding surface wear.
cause:
1) The air flow channel is blocked;
2) The fan motor is damaged or the fan blades are too dirty;
3) There is a lot of frost on the evaporator fins or coils;
4) The position of air circulation supply or return air is inaccurate;
5) Sudden change in load (hot defrost or condenser).
5. The press starts with liquid
Phenomenon: A large amount of air bubbles (oil foam) can be observed from the sight glass when the machine is turned on.
Hazards: Produces a liquid shock.
cause:
1) Long-term shutdown, the refrigerant liquid enters the crankcase;
2) The ambient temperature of the compressor is too low;
3) The crankcase heater is not powered on;
4) The refrigerant is charged too much.
6. The press is short of oil:
Hazard: The surfaces of all sliding parts inside the compressor are worn or scratched and damaged.
cause:
1) Improper design and installation of pipeline oil return;
2) The compressor starts and stops too frequently;
3) The load is too low or the frost is too much;
4) Refrigerant leakage.
7. Difficulty cooling down:
cause:
1) The incoming quantity is too much or the incoming temperature is too high.
2) The cold storage door is not closed tightly or the door is opened too many times.
3) The insulation layer of the cold storage or pipeline is damp or damaged, and the cold leakage is serious (condensation).
4) The evaporator has oil stains or frost layer, and the heat exchange efficiency decreases.
5) The refrigerant leaks or the charge is too large.
6) The compressor suction and discharge valves are damaged, resulting in a decrease in cooling capacity.
7) The thermal expansion valve adjustment fails, or the system is blocked.