The evaporator is a cooling output device in the refrigerator. The refrigerant evaporates in the evaporator and absorbs the heat of the low-temperature heat source medium (water or air) to achieve the purpose of refrigeration.
Evaporator according to its cooling medium is divided into: cooling air evaporator, cooling liquid (water or other liquid refrigerant) evaporator.
Evaporator for cooling air:
The optical disc tube structure is used when the air is naturally convection
Finned tube structure is used when air is forced convection
Evaporators for cooling liquids (water or other liquid-borne coolants) :
Shell and tube type
Submerged type
According to the refrigerant liquid supply method:
Full liquid evaporator
Dry evaporator
Circulating evaporator
Spray evaporator
Full liquid evaporator
According to its structure, it is divided into horizontal shell and tube type, straight tube type of water tank, water tank type and other structural types.
Their common feature is that the evaporator is filled with liquid refrigerant, and the refrigerant vapor generated by heat-absorbing evaporation during operation is constantly separated from the liquid. Because the refrigerant is in full contact with the heat transfer surface, the boiling heat transfer coefficient is higher.
However, the disadvantage is that the amount of refrigerant charged is large, and the static pressure of liquid column will cause adverse effects on the evaporation temperature. If the refrigerant is soluble in lubricating oil, the lubricating oil is difficult to return to the compressor.
Shell and tube full liquid evaporator
Generally horizontal structure, see figure. The refrigerant evaporates outside the shell tube; The carrier coolant flows in the tube and is generally multi-program. The inlet and outlet of the refrigerant are arranged on the end cover, and the inlet and outlet direction are removed.
The refrigerant liquid enters the shell from the bottom or side of the shell, and the vapor is drawn from the upper part and returned to the compressor. The refrigerant in the shell always maintains a hydrostatic surface height of about 70% to 80% of the shell diameter.
Shell and tube full liquid evaporator should pay attention to the following problems:
① With water as the refrigerant, when the evaporation temperature is reduced to below 0 ° C, the tube may freeze, which will lead to the expansion of the heat transfer tube. At the same time, the evaporator water capacity is small, and the thermal stability is poor during operation.
When the evaporation pressure is low, the hydrostatic column of liquid in the shell will increase the bottom temperature and reduce the heat transfer temperature difference;
(3) When the refrigerant is miscible with the lubricating oil, it is difficult to return the oil using the full liquid evaporator;
④ Large amount of refrigerant is charged. At the same time, it is not suitable for the machine to work under moving conditions, the liquid level shaking will lead to the compressor cylinder accident;
In the full liquid evaporator, due to the gasification of refrigerant, a large number of bubbles are generated, so that the liquid level is raised, so the amount of refrigerant charge should not be immersed in all the heat exchange surface.
Tank evaporator
The tank evaporator can be composed of parallel straight tubes or spiral tubes (also known as vertical evaporator).
They are immersed in the liquid refrigerant work, due to the role of the agitator, the liquid refrigerant in the tank circulation flow, not full liquid evaporator
Non-full liquid evaporator
Dry evaporator is a kind of evaporator in which the refrigerant liquid can be completely vaporized in the heat transfer tube.
The cooled medium on the outside of the heat transfer tube is the refrigerant (water) or air, and the refrigerant evaporates in the tube, and its hourly flow rate is about 20%-30% of the volume of the heat transfer tube.
Increasing the mass flow rate of refrigerant can increase the wetting area of refrigerant liquid in the pipe. At the same time, the pressure difference at the inlet and outlet increases with the increase of flow resistance, so that the refrigeration coefficient is reduced.
To enhance the heat transfer effect. The refrigerant liquid evaporates and absorbs heat in the pipe to cool the refrigerant outside the pipe.
Working principle of condenser
The gas passes through a long tube (usually coiled into a solenoid), allowing heat to be lost to the surrounding air. Metals such as copper, which conduct heat, are often used to transport steam. In order to improve the efficiency of the condenser, heat sinks with excellent heat conduction performance are often attached to the pipes to increase the heat dissipation area to accelerate heat dissipation, and the air convection is accelerated through the fan to take away the heat.
The refrigeration principle of the general refrigerator is that the compressor compresses the working medium from low temperature and low pressure gas into high temperature and high pressure gas, and then condensates into medium temperature and high pressure liquid through the condenser, and becomes a low temperature and low pressure liquid after the throttle valve is throttled. The low-temperature and low-pressure liquid working medium is sent into the evaporator, which absorbs heat and evaporates into low-temperature and low-pressure steam, which is transported into the compressor again to complete the refrigeration cycle.
Single-stage steam compression refrigeration system is composed of four basic components of refrigeration compressor, condenser, throttle valve and evaporator, which are successively connected by pipes to form a closed system, and the refrigerant constantly circulates in the system, changes state, and exchanges heat with the outside world.
How the evaporator works
The heating chamber is composed of a vertical tube bundle, with a central circulation tube with a large diameter in the middle, and the other heating tubes with smaller diameter are called boiling tubes. Because the central circulation tube is larger, the heat transfer surface occupied by the unit volume solution is smaller than that occupied by the unit solution in the boiling tube, that is, the central circulation tube and other heating tube solutions are heated to different degrees, so that the density of the vapor-liquid mixture in the boiling tube is smaller than the density of the solution in the central circulation tube.
Coupled with the upward suction of the rising steam, the solution in the evaporator will form a circulating flow from the central circulation tube down and from the boiling tube up. This cycle is mainly caused by the density difference of the solution, so it is called the natural cycle. This effect is conducive to the improvement of the heat transfer effect in the evaporator.
