Apr 21, 2020
Air conditioner evaporator is a kind of evaporator. The function of the air conditioner evaporator is to use liquid low-temperature refrigerant to evaporate easily at low pressure, transform into vapor and absorb the heat of the cooled medium, and achieve the purpose of refrigeration.
It relies on the fan to force the air in the warehouse to flow through the cooling pipes in the box for heat exchange to cool the air, thereby achieving the purpose of reducing the temperature of the warehouse. Among them, the refrigerant or refrigerant flowing in the discharge pipe and cooling the air outside the pipe through the pipe wall is called a dry cooler; the sprayed refrigerant liquid directly exchanges heat with the air and is called a wet cooler; In addition to the cooling exhaust pipe, the hybrid cooler also has a spray device for the coolant. Dry coolers commonly used in cold storage can be divided into two types according to their installation location: ceiling type and floor type
Characteristics of air conditioner evaporator
①When the evaporating temperature of the refrigeration system is lower than 0 ℃, the water in the tube is easy to freeze and damage the evaporating tube;
② Large refrigerant charge;
③Due to the height of the refrigerant liquid column, the evaporation temperature at the bottom of the cylinder is too high, which will reduce the heat transfer temperature difference;
④ Oil will accumulate in the lower part of the evaporator barrel, and reliable oil return measures must be taken, otherwise the safe operation of the system will be affected.
The refrigerant of the dry shell and tube evaporator flows inside the tube, and the water flows outside the tube cluster. There are usually several processes for refrigerant flow. Due to the gradual vaporization of the refrigerant liquid, the more upward, the more the number of process tubes. In order to increase the water-side heat exchange, a number of baffles are provided on the outside of the cylinder heat transfer tube to make the water flow across the tube cluster multiple times.
Advantages and disadvantages of air conditioner evaporator
The advantages are:
① Lubricating oil enters the compressor with the refrigerant, there is generally no oil accumulation problem
② There is less refrigerant to fill, generally only about 1/3 of the full liquid type;
③When it is near 0 ℃, the water will not freeze.
The disadvantages are:
①The refrigerant has multiple processes, and if the handle is not handled properly, it will generate liquid accumulation, which will make the liquid distribution into the next process uneven and affect the heat transfer effect;
② There is a leakage problem on the water side, because there is generally a 1 ~ 3mm gap between the outer edge of the baffle and the case, and a gap of about 2mm from the heat transfer tube, which will cause water leakage. Practice has proved that water leakage will cause the water side heat transfer coefficient to decrease by 20% to 30%, and the total heat transfer coefficient to decrease by 5% to 15%.
Air conditioner evaporator category
According to the different types of cooling medium, evaporators can be divided into two categories:
(1) Evaporator for cooling liquid refrigerant. It is used to cool liquid refrigerants-water, brine or glycol aqueous solution. Such evaporators are commonly used horizontal evaporator, vertical tube evaporator and spiral tube evaporator.
(2) Evaporator for cooling air. This type of evaporator has cooling exhaust pipes and chillers.
Its structure is basically similar to the horizontal shell and tube condenser. According to the liquid supply method, it can be divided into two types: shell and tube evaporator and dry evaporator. Horizontal shell and tube evaporators are widely used in closed brine circulation systems. Its main features are: compact structure, good contact between liquid and heat transfer surface, and high heat transfer coefficient. However, it needs to be filled with a large amount of refrigerant, and the liquid column will have a certain influence on the evaporation temperature. And when the brine concentration decreases or the brine pump shuts down for some reason, the brine may be frozen in the pipe. If the refrigerant is Freon, it is difficult for the lubricating oil dissolved in Freon to return to the compressor. In addition, stop working during cleaning. Dry Freon evaporator The main difference is that the refrigerant flows inside the tube, and the refrigerant flows outside the tube. The throttled Freon liquid enters the evaporator from the lower part of the end cap, and is drawn out from the upper part of the end cap after several processes. The refrigerant continuously evaporates with the flow in the tube, so part of the wall surface is occupied by steam, so , Its heat transfer effect is not as good as full liquid. However, it has no influence of the liquid column on the evaporation temperature, and due to the high flow rate of Freon (≥4m / s), the oil return is better. In addition, since the outside of the tube is filled with a large amount of refrigerant, the risk of freezing is reduced. The charging amount of refrigerant in this evaporator only needs 1/2 ～ l / 3 or less of the full liquid type, so it is called “dry evaporator”.
Vertical tube evaporator
The common point between the vertical tube and the spiral tube evaporator is that the refrigerant evaporates in the tube. The entire evaporator tube group is immersed in the tank (or pool or tank) filled with refrigerant. Circulate at a certain speed, a longitudinal partition is welded in the box, and a screw agitator is installed. The flow rate of the refrigerant is generally 0.3 ~ 0.7m / s to enhance heat transfer.
These two types of evaporators can only be used in open circulation systems, so the refrigerant must be a non-volatile substance, commonly used brine and water. If salt water is used, the evaporator tube is easily oxidized, and the salt water easily absorbs moisture and reduces the concentration. These two evaporators can directly observe the flow of the refrigerant, and are widely used in brine refrigeration systems that use ammonia as the refrigerant.
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