Four major components that make up the refrigeration system
- Compressor (circulating refrigerant)
- Condenser (cooling of refrigerant)
- Expansion valve (expanding and dispersing refrigerant)
- Evaporator (releasing refrigerant energy)
Refrigeration working principle
The low-pressure end of the compressor inhales the low-pressure vapor refrigerant flowing out of the evaporator. After compression, the compressor discharges high-pressure, high-temperature refrigerant gas, which then releases heat through the condenser and becomes a normal-temperature, high-pressure liquid refrigerant. It then enters the evaporator after throttling and reducing pressure through the expansion valve and becomes a low-pressure vapor refrigerant after absorbing heat in the evaporator. This repeated absorption and release of heat completes the entire refrigeration cycle.
Working principle and auxiliary devices of refrigeration equipment
Freezing: The process of using the low-temperature source generated by refrigeration to cool the product from room temperature and then freeze it.
Refrigeration: The process of using the cold effect generated by the change in the physical state of the refrigerant to obtain a low-temperature source.
Types of refrigeration equipment: cold source production (refrigeration), freezing of materials, cooling.
Refrigeration methods: piston, screw, centrifugal refrigeration compressor units, absorption refrigeration units, steam jet refrigeration units, and liquid nitrogen.
Freezing methods: air cooling, immersion, and devices where the refrigerant contacts the metal pipe, wall, and material to transfer heat and cool down.
Applications:
- Freezing, refrigeration, and frozen transportation of food.
- Cooling, refrigeration, gas-controlled storage, and cooling transportation of agricultural products and food.
- Food processing, such as freeze drying, freeze concentration, and cooling of materials.
- Air conditioning in food processing plants.
Refrigeration cycle principle
Main devices: refrigeration compressor, condenser, expansion valve, evaporator.
Refrigeration cycle principle: When the refrigerant is in the state of low-temperature and low-pressure liquid, it absorbs heat and reaches the boiling point, then evaporates to become low-temperature and low-pressure steam. The evaporated refrigerant becomes high-temperature and high-pressure gas under the action of the compressor. This high temperature and high pressure is condensed to become high-pressure liquid. The high-pressure liquid passes through the expansion valve and becomes low-pressure and low-temperature liquid, and then absorbs heat and evaporates again, forming the refrigeration cycle of the refrigerator.
Basic concepts and principles:
Refrigeration capacity: The amount of heat that a refrigerant takes out of a frozen object per unit of time under certain operating conditions (i.e., a certain refrigerant evaporation temperature, condensation temperature, and supercooling temperature). It is also called the cooling capacity of a refrigerant. Under the same conditions, the refrigeration capacity of the same refrigerant is related to the size, speed, and efficiency of the compressor.
Direct refrigeration: In the refrigeration cycle, the evaporator that absorbs heat from the refrigerant directly exchanges heat with the object being cooled or the surrounding environment of the object being cooled. It is generally used in single refrigeration equipment that requires industrial cooling, such as ice cream freezers, small cold storage, and household refrigerators.
Refrigerant: The working substance that circulates continuously in a refrigeration device to achieve refrigeration. A vapor compression refrigeration device achieves heat transfer through changes in the physical state of the refrigerant. Refrigerant is an indispensable substance for achieving artificial refrigeration.
Indirect refrigeration: Use cheap materials as a medium to achieve heat exchange between the refrigeration device and the cold-consuming place or machine.
Refrigerant: The cold energy generated in the evaporator of the refrigeration device refrigerant is transferred to the heat absorbed by the cooled object to the refrigeration device and then to the refrigerant, which is then recycled for cooling.
Indirect evaporative cooling principle
Principle of indirect refrigeration: After the refrigerant absorbs cold air from the refrigerant in the evaporator, it enters the cold storage through the brine pump, exchanges heat with the cooled object or the medium in the workplace to absorb heat, returns to the evaporator, transfers the absorbed heat to the refrigerant, and cools itself down again for recycling.
Commonly used refrigerants
Commonly used refrigerants: air, water, brine, and organic aqueous solution.
Selection criteria: low freezing point, large specific heat capacity, no metal corrosion, chemical stability, low price, and easy to obtain. As a refrigerant used in the food industry, it often needs to be tasteless, odorless, colorless, and non-toxic.
Although air has many advantages as a refrigerant, it is used in direct contact with food in food refrigeration or freezing processing because of its small specific heat capacity and poor convective heat transfer effect when used as a gas.
Water has a large specific heat but a high freezing point, so it can only be used as a refrigerant to produce cold above 0℃. If you want to produce cold below 0℃, you can only use brine or organic solution as a refrigerant.
Aqueous solutions of sodium chloride, calcium chloride, and magnesium chloride are usually called frozen brine. The most widely used frozen brine in the food industry is sodium chloride aqueous solution. Among organic solution refrigerants, the two most representative refrigerants are aqueous solutions of ethylene glycol and propylene glycol.
Main devices of piston compression refrigeration equipment
Function: Used to compress the refrigerant to obtain energy, and then condense and expand to form a cold source that can absorb heat.
Model representation: number of cylinders, type of refrigerant used, cylinder arrangement, cylinder diameter.
Composition: cylinder block, cylinder, piston, connecting rod, crankshaft, crankcase, intake and exhaust valves, false cover, etc.
Working process: When the piston moves upward, the suction valve is opened, and the refrigerant vapor enters the cylinder above the piston through the suction valve. When the piston moves upward, the suction valve is closed, and the piston continues to move upward. The refrigerant in the cylinder is compressed. When the air pressure reaches a certain level, the exhaust valve of the false cover is opened, and the refrigerant vapor is discharged from the cylinder and pressed into the high-pressure pipeline.
Features: simple structure, easy to manufacture, strong adaptability, stable operation, and convenient maintenance.
Condenser
Function: Heat exchanger, which makes the superheated steam of refrigerant cooled and condensed into liquid.
Types: horizontal shell and tube type, vertical shell and tube type, water spray type, evaporative type, air cooling type
Working process: The superheated steam of refrigerant enters the condenser from the upper part of the shell and condenses into a liquid film on the cold surface of the tube after contacting it. Under the action of gravity, the condensate slides down along the tube wall and separates from the tube wall.
The composition of the water spray evaporator: liquid storage tank, cooling pipe, and water distribution tank.
Working process: The cooling water enters the water distribution tank from the top, flows to the outer surface of the serpentine pipe through the water distribution trough, and flows down in a film layer. Part of the water evaporates, and the rest falls into the pool, which is recycled after cooling. The bottom of the hidden pipe enters the tube, condenses when it encounters cold when rising along the tube, and flows into the liquid storage tank.
Expansion valve
Function: Reduce the pressure of the refrigerant and control the refrigerant flow. When the high-pressure liquid refrigerant passes through the expansion valve, the condensation pressure drops sharply to the evaporation pressure. At the same time, the liquid refrigerant boils and absorbs heat, and its temperature decreases.
Thermal expansion valve: Use the superheat of the steam at the outlet of the evaporator to adjust the refrigerant. Under normal operating conditions of the refrigeration unit, the pressure of the filling agent of the supply element is equal to the sum of the gas pressure under the diaphragm and the spring pressure and is in a balanced state. Insufficient refrigerant supply causes steam return at the outlet of the evaporator, the superheat increases, the temperature of the temperature-sensing package rises, the diaphragm moves down, and the opening degree of the outlet increases until the liquid supply amount is equivalent to the evaporation amount, and then the balance is achieved. Therefore, the thermal expansion valve can automatically adjust the opening degree of the valve, and the liquid supply amount automatically increases or decreases with the load size, which can ensure that the heating area of the evaporator is fully utilized.
Evaporator
Function: The refrigerant absorbs the heat of the cooled medium.
Classification: It is divided into three categories according to the properties of the cooling medium.
- Evaporator for cooling liquid coolant: such as water cooler, brine cooler, etc. The refrigerant absorbs heat outside the tube, and the liquid coolant circulates in the tube by a liquid pump. According to the structure, it is divided into horizontal tube type, vertical tube type, spiral tube type, and serpentine tube type
- Evaporator for cooling air: The refrigerant evaporates in the tube, and the air flows outside. The flow of air belongs to natural convection
- Contact evaporator for cooling frozen materials: The refrigerant evaporates on one side of the heat transfer partition, and the other side of the partition is in direct contact with the cooled or frozen material.
Features: Good heat transfer effect, simple structure, small footprint, and low corrosion to equipment due to the sealed coolant circulation system.
Disadvantages: When the brine pump stops running due to a fault, freezing may occur, causing the tube cluster to rupture.
Cooling pipe
Vertical cooling pipe
Advantages: Refrigerant is easily discharged after vaporization, and the heat transfer effect is better. However, when the pipe is high, the evaporation temperature of the lower refrigerant is higher due to the static pressure of the liquid column.
Single-row coil wall pipe:
Advantages: The amount of refrigerant is small, about 50% of the pipe volume, but the refrigerant will not be quickly discharged from the pipe after vaporization, which reduces the heat transfer effect.
Warp-fin pipe:
Advantages: Large heat dissipation area.
Auxiliary device of piston compression refrigeration equipment
Oil separator
Function: Used to separate the lubricating oil carried by the compressed liquid and gas to prevent the lubricating oil from entering the condenser and deteriorating the heat transfer conditions.
Working principle: Taking advantage of the different specific gravity of oil droplets and refrigerant vapor, the flow velocity is reduced by increasing the pipe diameter and changing the flow direction of the refrigerant; or the oil droplets are precipitated to lower the steam temperature by centrifugal force. For lubricating oil in the vapor state, washing or cooling is used to reduce the steam temperature so that it condenses into oil droplets and separates. The oil separator that uses Freon for refrigeration is a filter-type oil separator.
The function of the oil collector is to collect the refrigerant and oil mixture separated from the oil separator, condenser, and other devices of the refrigeration system, then separate the oil from the mixed refrigerant under low pressure, and then release them separately, so as to ensure the safety of oil discharge and reduce the loss of refrigerant.
The function of the liquid storage tank is to store and regulate the liquid refrigerant supplied to various parts of the refrigeration system to meet the safe operation of the equipment. The liquid storage tank is divided into high-pressure, low-pressure, drainage barrel, and circulating liquid storage barrel.
The function of the gas-liquid separator is to separate the refrigerant from the evaporator to prevent the refrigerant liquid from entering the compressor and causing cylinder knocking; to separate the invalid steam carried by the low-pressure ammonia liquid after throttling to improve the heat transfer effect of the evaporator.
The function of the air separator is to separate the non-condensable gas in the discharge system to ensure the normal operation of the refrigeration system.
Function of the intercooler: Installed in a two-stage (or multi-stage) compression refrigeration system, it is used to cool the superheated gas discharged from the low-pressure stage for inter-stage cooling to ensure the normal operation of the high-pressure stage compressor; to separate the lubricating oil carried in the low-pressure stage exhaust gas, and to cool the refrigerant so that the refrigerant obtains a larger degree of subcooling.
Cold storage
Classification:
Large cold storage (more than 5000t); medium cold storage (1500-5000t); small cold storage (less than 1500t).
According to the use requirements:
High-temperature cold storage: mainly refrigerates fruits, vegetables, fresh eggs, and other foods, the general storage temperature is 4-2℃;
Low-temperature cold storage: mainly freezes and freezes meat, aquatic products, etc., the general storage temperature is -18-30℃;
Air-conditioned storage: stores rice, noodles, medicinal materials, wine, etc. at room temperature, the general storage temperature is 10-15℃
Quick freezing equipment: suitable for freezing small packaged or unpackaged blocks, slices, grains, and other raw materials to make various quick-frozen foods such as livestock, aquatic products, vegetables, dumplings, etc. Freezing temperature -30-40℃.
Box quick freezer: several movable plates with interlayers are installed in the box wrapped with insulation material. The interlayer is equipped with an evaporating coil, and salt water can also be poured between the tubes. The refrigerant flows through the evaporating coil; the quick-frozen products are placed between the plates and the plates are moved to compact the materials and freeze them.
Tunnel quick freezer: It consists of a tunnel body, an evaporator, a fan, a material rack, or a stainless steel transmission net. The material first passes through the first-level mesh belt, which runs faster and has a thinner material layer, so that the surface is frozen; the second-level mesh belt runs slower and has a thicker material layer so that the entire material is frozen and a single-grain quick-frozen product is obtained.
Immersion quick freezer: The frozen material is directly contacted with a very low-temperature liquefied gas or liquid refrigerant to make a quick-frozen product. The food passes through the pre-cooling zone, freezing zone, and uniform temperature zone in turn. Liquid nitrogen is stored outside the tunnel and introduced into the freezing zone at a certain pressure for spraying or immersion freezing. The nitrogen formed after the liquid nitrogen absorbs heat is still at a very low temperature of -10 to -5°C. It is sent to the front section of the tunnel through the fan for pre-freezing. In the freezing zone, food is quickly frozen by contact with liquid nitrogen at -200°C.
Controlled atmosphere refrigeration system
Controlled atmosphere refrigeration: Combine refrigeration with controlled atmosphere storage, control the storage temperature and gas composition, and make the content of oxygen and carbon dioxide in the warehouse. It is mainly used for fruit and vegetable storage and can obtain good preservation effect.
The loss of products during storage is small. According to statistics, the loss rate of cold storage products is 21.3%, while the loss rate of controlled atmosphere cold storage products is 4.8%.
