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[What does the chiller system overcool and overheat mean?]
Release date:[2020/9/25] Is reading[57]次

"Supercooling": It means to recool the condensed saturated liquid through a certain device (such as a supercooler) and method to make its temperature lower than the saturation temperature under the condensing pressure, which is called supercooling.

Compare the temperature of the liquid before the subcooling with the temperature after the subcooling, and the difference is the "degree of subcooling".

1. The benefits of too cold:

1. Reduce the flash gas generated by the refrigerant liquid in the throttling process, reduce the specific volume of the flash gas, and increase the unit refrigeration capacity;

2. Increasing the superheat of the return air at the same time has certain benefits to protect the compressor from running in the wet stroke.

2. How to do overcooling in actual design?

1. Special subcooler:

In a larger refrigeration system, in order to reduce the temperature of the refrigerant liquid entering the throttle valve, reduce the flash gas generated during or after throttling, and appropriately improve the refrigeration efficiency, the process design is after the accumulator (A system that uses a throttle valve for throttling must have a liquid reservoir) A special device for subcooling-a subcooler is installed.

Its structure type is casing type, spray type or plate exchange, etc. The principle is to use cooling water whose temperature is lower than that of the saturated liquid after condensation to cool again (such as deep well water), generally it can be lowered by 3℃~5 before cooling. ℃ temperature (that is, the supercooling degree is 3℃~5℃).

2. Heat exchange between liquid supply pipe and return air pipe

There are also some small fluorine refrigeration systems. Although there is no special subcooler, the liquid supply pipe and the return air pipe are wrapped together for insulation, and the low temperature of the return air pipe is used to reduce the liquid temperature in the liquid supply pipe. The expansion valve is directly installed in the warehouse to pass through, and it is cooled again to achieve the purpose of subcooling, thereby improving the cooling efficiency.

At the same time, the temperature of the return air pipe is heated to prevent the compressor from inhaling excessive moisture vapor and possible liquid hammer.

In the capillary throttling system, the capillary tube and the return pipe (suction pipe) are combined together, and some are welded together (this method is used by Daikin VRV), and the hot glue sleeve is used to put together, and there is a return pipe Some pass through the air return pipe, and some pass the capillary tube or the liquid supply pipe directly in the box. The capillary tube exchanges heat with the return air pipe, so that the liquid refrigerant before throttling and the low-temperature refrigerant vapor in the return air pipe perform heat exchange and cooling to obtain subcooling, which can reduce the liquid liquid shock compressor that may be entrained in the return air pipe. At the same time, it can achieve the purpose of subcooling the liquid refrigerant before throttling.

Of course, there are also regenerators, the principle is the same, please refer to the following figure:

3. Enlarge the condenser

If the condenser is deliberately enlarged, it is feasible to leave room for cooling again and supercooling. However, this is not done in a standardized design. The consideration is to minimize the overall volume and weight and reduce manufacturing costs.

For small or micro capillary throttling systems, no special subcooler will be added.

overheat:

The difference between the saturation temperature before overheating and the saturation temperature after overheating is called the degree of superheat.

At a certain pressure, the steam whose temperature is higher than the saturation temperature is called superheated steam. The steam temperature at the exhaust pipe of the refrigeration compressor is generally higher than the saturation temperature, so it belongs to superheated steam, which is called "exhaust superheat".

1. Harmful overheating:

Due to the length and thermal insulation of the air return pipe (suction pipe), the steam in the pipe is transferred to the outside and heated. This phenomenon is called "inhalation overheating" or "pipe overheating". This kind of overheating will increase the suction temperature of the compressor and increase the specific volume of the suction steam, resulting in a decrease in the refrigeration capacity per unit volume and a decrease in the refrigeration capacity of the compressor. This is detrimental to the refrigeration cycle. This problem is called "Harmful overheating."

Therefore, it is required that the suction pipe must be well insulated, and the length of the suction pipe should be shortened as much as possible to reduce this harmful overheating.

2. Beneficial overheating:

In a fluorine refrigeration system using an expansion valve, the degree of superheat is used to adjust the opening degree of the thermal expansion valve. This phenomenon is called "beneficial overheating." Similarly, the superheat generated by the fluorine vapor after reheating is also a beneficial superheat.


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