What is the working principle of evaporator?

- Aug 01, 2018-

Depending on the type of medium being cooled, evaporators can be divided into two broad categories:

(1) An evaporator that cools the liquid brine. Used to cool liquid coolant - water, brine or aqueous glycol solution. Commonly used in this type of evaporator are horizontal evaporators, riser evaporators, and spiral tube evaporators.

(2) An evaporator that cools the air. Such evaporators have cooling tubes and chillers.

First, horizontal evaporator

1. It is basically similar to the structure of a horizontal shell and tube condenser. According to the liquid supply mode, it can be divided into 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 is lowered or the brine pump is shut down for some reason, the brine may be frozen in the tube. If the refrigerant is freon, it is difficult for the lubricating oil dissolved in the freon to return to the compressor. In addition, stop working when cleaning. 

2. Dry freon evaporator. The main difference is that the refrigerant flows inside the tube and the brine flows outside the tube. After throttling, the freon liquid enters the evaporator from the lower part of the one end cap and is taken out from the upper part of the end cap after several processes. The refrigerant evaporates continuously with the flow in the tube, so that a part of the wall is occupied by the vapor, and therefore, the heat transfer effect is not as full as that of the liquid. However, its liquid column has no effect on the evaporation temperature, and since the Freon flow rate is high (≥4 m/s), the oil return is better. In addition, since the outside of the tube is filled with a large amount of brine, the risk of freezing is reduced. The amount of refrigerant in the evaporator is only 1/2 to l/3 or less of the full liquid type, so it is called a "dry evaporator". 

Second, the riser evaporator

The common point of the riser tube and the spiral tube evaporator is that the refrigerant evaporates in the tube, and the entire evaporator tube group is immersed in the tank filled with the coolant (or in the tank). In order to ensure that the brine is circulated at a certain speed in the tank, the tank is welded with a longitudinal partition and a screw stirrer. The brine flow rate is generally 0.3 to 0.7 m/s to enhance heat transfer.

These two evaporators can only be used in open circulation systems, so the coolant must be non-volatile, and brine and water are commonly used. If brine is used, the evaporator tube is easily oxidized and the brine is susceptible to moisture absorption to reduce the concentration. These two evaporators can directly observe the flow of the brine, and are widely used in brine refrigeration systems that use ammonia as a refrigerant.

Third, the cooling pipe

A cooling tube is an evaporator used to cool air. Widely used in low-temperature refrigerators, the refrigerant flows and evaporates in the cooling pipe, and the cooled air outside the pipe acts as a heat transfer medium for natural convection. 

The advantage of the cooling pipe is that the structure is simple, easy to manufacture, and the dry consumption of the unpackaged food stored in the warehouse is less. However, the heat transfer coefficient of the pipe is low, and the operation is difficult when defrosting, which is not conducive to automation. For the direct cooling system of ammonia, the seamless steel pipe is welded, and the light pipe or the finned tube is used; for the Freon system, the winding piece or the sleeve type copper tube fin tube set is mostly used.

1, Snake tube type tube

The coiled pipe jacking gravity supply or the ammonia pump can supply liquid. Single-row and double-row serpentine wall drains can be used for down-and-out ammonia pump supply systems and gravity feed systems. The single coil tube can also be used for the ammonia pump up and down liquid supply system and the thermal expansion valve supply system.

The advantage of the coiled tube is that the structure is simple, easy to manufacture, the amount of liquid storage is small, and the applicability is strong. The main disadvantage is that the steam generated in the lower part of the pipe can not be taken out in time, and must be discharged after the full length of the pipe, so the heat transfer coefficient is small, and the vapor-liquid two-phase flow resistance is large.

2. U-shaped tube. The commonly used U-shaped tube is composed of two or four layers of smooth seamless steel tubes.

The advantage of the U-shaped top tube is that the frosting is relatively uniform, the preparation and installation are convenient, and the liquid filling amount is small, accounting for about 50% of its volume. It is suitable for gravity supply system and ammonia pump to enter and exit the ammonia refrigeration system, which is widely used in cold storage. However, it has more effective space in the warehouse, and the upper tube is not easy to defrost.

Fourth, the air cooler

The air cooler is mostly a complete set of equipment consisting of an axial flow fan and a cooling exhaust pipe. It relies on the fan to force the air in the warehouse to flow through the cooling pipe in the tank for heat exchange, so that the air is cooled, thereby achieving the purpose of reducing the temperature of the reservoir.

The air cooler can be divided into three types: dry type, wet type and dry type and wet type. Among them, the refrigerant or the brine is flowing in the discharge pipe, and the air outside the pipe is cooled by the pipe wall, which is called a dry air cooler. The wet refrigerant liquid that is directly exchanged with air by the sprayed refrigerant liquid is called a wet air cooler. In addition to the cooling pipe, the hybrid air cooler has a spray device for carrying a refrigerant. The following describes the dry chillers that are widely used in cold storage.

The dry type chillers commonly used in cold storage can be divided into two types: ceiling type and floor type according to the position where they are installed. They are composed of air-cooled pipes, ventilators and defrosters, and the cooling pipes in the chillers are sleeve-type. Large dry chillers are often floor standing.