Knowledge, Installation and Maintenance of Cold Storage Systems

2024-12-29

The four main components of a cold storage refrigeration system include: refrigeration compressor, condenser, throttling device (expansion valve, capillary tube, etc.) and evaporator. These four main components and the refrigerant are essential conditions for the fluorine system to achieve the refrigeration cycle.

(I) Compressor
It is the power source of the system. Its main function is to raise the pressure of the refrigerant vapor from low pressure to high pressure and keep it circulating continuously in the system.
Common types include piston type, screw type, scroll type, centrifugal type, rotary type, etc.
Advantages of piston compressors: wide operating temperature range, high efficiency, mature and reliable technology, and suitability for various refrigerants. Currently, piston compressors are widely used in the mainstream market for units ranging from 5 to 40 horsepower.

(II) Condenser
Its main function is to cool and condense the superheated steam discharged from the compressor into high - pressure liquid.
Common types include water - cooled, air - cooled, and evaporative - cooled:

Horizontal shell - and - tube water - cooled condenser: The refrigerant vapor condenses outside the tubes in the shell, and cooling water flows inside the tubes. Generally, the cooling water enters from the bottom and exits from the top. It has a relatively high heat transfer coefficient and good condensation effect. It is often equipped with a water pump and a cooling tower.
Air - cooled condenser: It uses air as the cooling medium and a fan to accelerate the forced convection of air. It is suitable for places where water supply is difficult or it is not convenient to install a cooling tower, as well as equipment in northern regions. However, its condensation effect is slightly worse.
Evaporative - cooled condenser: It is usually used as a condenser in larger refrigeration systems. Common screw units are often equipped with evaporative - cooled condensers.

(III) Throttling Device
Main functions:

Throttle and reduce pressure to ensure the pressure difference between the condenser and the evaporator, enabling the refrigerant to evaporate and absorb heat at low pressure (low temperature) in the evaporator.
Adjust the refrigerant flow to adapt to changes in the heat load of the evaporator.
Thermal expansion valve:
Internal - balanced type: Suitable for pipe - throttling and refrigeration systems with low evaporator resistance.
External - balanced type: Suitable for refrigeration systems with high evaporator resistance.
Capillary tube: Suitable for small refrigeration devices with stable working conditions. It is commonly used in refrigerators, freezers, and small household air - conditioners.

(IV) Evaporator
The throttled liquid refrigerant vaporizes and absorbs heat (low temperature and low pressure) in the evaporator, reducing the temperature of the cooled medium.
Common types include air coolers, pipe coils, and horizontal shell - and - tube evaporators.

Air cooler: Also known as an air - cooling unit, it is generally non - full - liquid type, with a small refrigerant charge, which is beneficial for oil return, low heat loss, and rapid cooling.
Pipe coil: Common types include aluminum and steel pipe coils. They have a simple structure, are easy to manufacture, but have a relatively low heat transfer coefficient and difficult oil return.
Horizontal shell - and - tube evaporator: Generally, it is a dry - type shell - and - tube evaporator with a relatively high heat transfer coefficient, but difficult oil return. It is commonly used in water chillers.

II. Auxiliary Equipment
Auxiliary equipment includes: oil separator, gas - liquid separator, drier - filter, solenoid valve, liquid receiver, oil collector, and intermediate cooler.

Oil Separator
It is generally located behind the shock - absorbing pipe in the system, as shown in the figure below.
Function: It separates the refrigeration oil from the refrigerant in the exhaust pipeline. The refrigeration oil directly returns to the crankcase of the compressor, reducing the amount of oil carried into the refrigeration system, minimizing the impact on the heat transfer effect of the heat exchanger, and ensuring good lubrication of the compressor to maintain normal operation.
Application: It is suitable for medium - and low - temperature ranges and refrigeration systems with long pipelines (liquid supply pipes and suction pipes).
Note: If the system has poor oil return, the oil separator can only extend the running time of the compressor and cannot solve the oil return problem of the system. If the air - cooled condenser is higher than the compressor (for split - type units, when the air - cooled condenser is higher than > 1 meter), a check valve must be installed at the outlet of the oil separator to prevent the refrigerant from entering the oil separator and then the crankcase when the system stops and the refrigerant condenses, causing liquid hammer when the compressor restarts.
Gas - Liquid Separator
Function: Before the low - temperature and low - pressure refrigerant gas enters the suction valve of the compressor, it separates the unevaporated refrigerant liquid in the evaporator of the refrigeration system to prevent the compressor from experiencing a wet stroke (liquid hammer) and causing compressor failure. The gas - liquid separator is an essential component in low - temperature piston compressors.
Application: It is suitable for medium - and low - temperature working conditions.
Drier - Filter
Function: It absorbs the moisture in the liquid refrigerant and filters out dirt and impurities in the refrigerant. It is generally installed at the outlet of the liquid receiver. There are several types, such as those for filtering moisture, impurities, and acid. The interfaces are divided into welded interfaces and threaded interfaces.
Solenoid Valve
Function: It is usually normally closed (closed when not energized). When the compressor stops running, the solenoid valve closes to prevent high - pressure refrigerant from entering the low - pressure system. When the compressor restarts, it will not cause liquid hammer. The solenoid valve coils are available in 380V, 220V, and special voltages.
Installation position: It is 100mm - 200mm away from the thermal expansion valve.
Liquid Receiver
Function: There are vertical and horizontal types. When the working conditions change or the refrigeration capacity is adjusted, it can balance and stabilize the refrigerant flow in the system, enabling the normal operation of the refrigeration device.
A. Its volume should be able to accommodate all the refrigerant.
B. The maximum capacity is 80% of its container volume.
Oil Collector
Function: In a parallel system, it is commonly used to collect the oil separated from each oil separator, ensuring the normal operation of each oil separator and returning the oil to each compressor.
Intermediate Cooler (Inter - cooler, Economizer)
Function: It is mainly used in low - temperature systems with two - stage or multi - stage compression refrigeration cycles. It cools the superheated steam discharged from the low - pressure stage, ensures the intermediate pressure, and reduces the compression ratio.

III. Cold Storage Installation and Pipeline Layout
(I) Installation of the Unit

Install it at a location with the shortest connection pipes and wires, which is easy for construction and maintenance.
The ground should be solid, and the unit should be installed more than 250MM above the ground.
For air - cooled units, this prevents dust from adhering to the condenser. For water - cooled units, it is convenient for operation, maintenance, and connection of cooling water pipes and valves.
Ensure good air circulation, avoid direct sunlight, and keep it away from other heat sources.
For safety reasons, there should be measures or guardrails to prevent unauthorized personnel from entering.
There should be sufficient space around the unit for ventilation, operation, and maintenance.

(II) Height between the Unit and the Evaporator

When the evaporator is above the unit, the pressure loss caused by the height of the liquid refrigerant increases, and bubbles may be generated. Therefore, the height difference should be within 8 meters, preferably less than 5 meters, as shown in the figure above.
When the evaporator is below the unit: To facilitate oil return, an oil return bend should be set every 5 - 6 meters in the suction pipe. The height difference should be within 20 meters.

(III) Determination of System Pipeline Diameter

If the diameter of the liquid supply pipe is too small, a large pressure drop will be formed, affecting the refrigeration effect. Generally, the liquid flow velocity in the liquid supply pipe is preferably 0.8 - 1 m/s. It can also be configured according to the liquid outlet pipe of the unit.
When the evaporator is below the unit: To ensure oil return, the gas flow velocity in the vertical section of the suction pipe must be greater than 7.6 m/s, and the gas flow velocity in the horizontal section must be greater than 3.8 m/s.

(IV) Pipeline Layout
(1) Suction Pipe

The horizontal suction pipe should have a slope of not less than 2% towards the compressor to facilitate oil return. When the evaporator is higher than the compressor, the return pipe of the evaporator should first...
Bend upward to its highest point and then go downward to the compressor to prevent liquid refrigerant from flowing into the compressor when it stops.
For parallel units, an oil - gas balance pipe should be installed on the crankcase or a crankcase oil level controller should be installed. To prevent oil from flowing into the suction port of an unoperated compressor, the suction pipe of the compressor should be connected from the upper part of the suction manifold. The design of the suction manifold should consider the even oil return of each compressor.
In a refrigeration system with capacity adjustment or large load changes, a double - rising vertical pipe design can be considered to ensure oil return at low loads.
(2) Exhaust Pipe
When the unit stops, to prevent oil or condensed refrigerant from flowing back into the compressor, the horizontal exhaust pipe should have a 1% slope towards the oil separator or condenser.
If the compressor is below the condenser (more than 3 meters), to prevent the refrigerant from condensing in the exhaust pipe and the liquid from entering the compressor's exhaust cavity when the refrigeration system stops running, a check valve should be installed near the exhaust valve of the compressor's exhaust straight pipe or an anti - backflow U - shaped liquid - collecting elbow should be made, and then a liquid - collecting elbow should be set every 6 - 8M.
(3) Liquid Pipe from the Condenser to the Liquid Receiver
To ensure that the refrigerant liquid in the condenser can smoothly flow into the liquid receiver, the pipe diameter should be selected according to a liquid flow velocity of not more than 0.5M/S under full - load operation. When connecting, try to reduce the pipeline resistance loss.
The liquid receiver should be at least 200MM lower than the condenser, and the horizontal pipe between them should slope towards the liquid receiver with a slope of not less than 1%.
(4) Pipeline between the Condenser or Liquid Receiver and the Evaporator
The solenoid valve should be as close to the evaporator as possible to prevent a large amount of refrigerant liquid behind the valve from flowing into the evaporator when the unit stops, causing wet compression when the unit restarts.
The evaporator is generally selected and connected in the upper - inlet and lower - outlet mode to ensure smooth oil return.

(V) Pipeline Installation

Use internally clean copper pipes. There should be no dust, moisture, or oxide scale inside the pipes. If seamless steel pipes are used, the inside of the pipes must be derusted and degreased until the metal is exposed. Do not wipe the inside of the pipes with fibers to avoid fibers sticking to the pipe wall.
For components and valves that need to be welded on the pipeline, cool the valve body or the welding joint. At the same time, when welding, it is best to introduce some nitrogen into the pipe to prevent the generation of oxides inside the pipe.
The solenoid valve is installed 150 - 200 mm away from the expansion valve. The expansion valve should be as close to the evaporator as possible to reduce cold loss. The temperature - sensing bulb should be strapped to the upper part or the upper - side part (depending on the pipe diameter) of the horizontal pipe section at the outlet of the evaporator. The external balance pipe should also be installed in the same way.

IV. Pressure Testing and Vacuum Pumping of the Refrigeration System

When pressure testing, use dry and clean nitrogen. Keep the suction and discharge valves of the compressor closed, and manually open the solenoid valve. Slowly charge the system to 1.8MPA. Apply soapy water to all joints and welds for leak detection, and maintain the pressure for 24 hours. If the pressure does not drop (after deducting the influence of atmospheric temperature and pressure), it is considered qualified.
Drain the gas in the refrigeration system, open the suction and discharge valves of the compressor, and open all valves in the system (including the solenoid valve). Use a vacuum pump to evacuate the system to 150PA (absolute pressure) and keep it for 1 hour without change, which is considered qualified.

V. Refrigerant Charging and Inspection before Operation

Refrigerant charging: Check the oil level, close the solenoid valve, turn on the crankcase heater, and add the refrigerant in liquid form to the liquid receiver.
After starting the unit, if further refrigerant needs to be added, it can be added in gaseous form on the suction side (preferably at the inlet of the evaporator).
Pre - operation inspection:
Inspection of the protection device (inspection of the oil pressure safety controller).
Disconnect the power cable of the compressor so that the compressor does not run. Let the control circuit operate. After 90 seconds, the control circuit should cut off the power, indicating that the oil pressure switch is working. Otherwise, check the wiring method of the oil pressure switch and the oil pressure switch itself.
Inspection of the protection device (inspection of the motor protection device).
Disconnect the power cable of the compressor and check the PTC thermistor in the motor. The resistance of the PTC thermistor in the motor is in the range of 200 - 600Ω.
Inspection of the protection device (inspection of the high - pressure controller).
Adjust the protection value of the high - pressure controller to be lower than the system pressure. At this time, the control circuit should be protected. Otherwise, find out the cause of the problem.
Inspection of the protection device (inspection of the low - pressure controller).
Adjust the protection value of the low - pressure controller to be higher than the system pressure. At this time, the control circuit should be protected. Otherwise, find out the cause of the problem.
Inspection of the protection device (inspection of the crankcase heater).
Inspection of the crankcase heater (the temperature in the crankcase is 20℃ higher than the ambient temperature)
Inspection of the protection device (inspection of the split - coil starting time).
Disconnect the power cable of the compressor, start the control circuit, and check the suction time of the two contactors of the compressor. If it exceeds 1 second, adjust the time controller so that the suction time of the two contactors is within 1 second.
Inspection of the protection device (other inspections).
Check other devices in the refrigeration system when the compressor is not running. For example, check the operation of the evaporator air cooler, condenser fan, water - cooled condenser water pump, whether the oil level is normal, and whether the crankcase heater is working.

VI. Operation and Commissioning

Check the high - and low - pressure conditions and the oil return state. The oil level in the crankcase should be within the range of 1/3 - 3/4. If it is lower than 1/3 or the oil level is not visible, check the oil return situation or add refrigeration oil.
Check whether there are bubbles in the sight glass on the liquid supply pipeline. If there are bubbles, add refrigerant until there are no bubbles in the sight glass.
Check the cooling situation of the refrigeration system, check whether the frosting and dewing conditions of the pipeline after the expansion valve are normal, listen to whether there is a sound of liquid fluid flowing in the expansion valve, and adjust the corresponding evaporation temperature.
For the cooled medium of forced - circulation water or brine, the heat transfer temperature difference is generally taken as 5℃.
For the cooled medium of natural - convection air, the heat transfer temperature difference is generally taken as 10 - 15℃.
For the cooled medium of forced - circulation air, the heat transfer temperature difference is generally taken as 5 - 10℃.
Check whether the high and low pressures, suction and discharge temperatures, etc. are within the normal operating range.
Check the frosting situation of the compressor. In medium - and low - temperature systems, the best frosting position of the compressor is at the suction valve. Frosting on the motor cover is also acceptable, but not on the cylinder head.
For a two - stage compressor, the frosting position is also at the suction valve. Dew or frost can form on the suction side of the low - pressure stage cylinder head. The frosting area should not exceed 1/3 of the cylinder head.
Detect the operating current, voltage, and other parameters of the compressor.
Check the oil pressure (if it is lubricated by an oil pump), which should be 0.15 - 0.35MPa higher than the suction pressure.
After the system runs, check whether the temperature controller is reliable.

VII. Daily Maintenance of the Refrigeration System

Check the cleanliness of the refrigeration oil. If the refrigeration oil is found to be dirty, replace it after the system has run for 24 hours and clean the oil filter.
For air - cooled units, promptly clean the outer surface of the separator to prevent dust from blocking the air - cooled condenser.
For water - cooled units, check whether there is scale in the condenser.
Check whether the suction and discharge pressures are normal.
Listen for any abnormal sounds from the compressor.
Replace the refrigeration oil once after the newly installed refrigeration system has run for 100 hours. The next oil change depends on the cleanliness of the refrigeration oil or change the refrigeration oil once every 10,000 - 12,000 hours. Remember not to over - add oil.
Replace the filter element of the drier - filter once after the refrigeration system has run for more than 1 year. If the system is very clean, the filter element or filter may not need to be replaced.
Check the