The Significance of Supercooling and Superheat in Refrigeration System

The so-called “supercooling” is to recool the saturated liquid after condensation through a device (such as the supercooler) and methods (or measures), so that its temperature is lower than the saturation temperature under the condensing pressure, which is called supercooling. Comparing the temperature of the liquid before and after supercooling, the difference is “degree of supercooling”.
Supercooling is to make the refrigerant liquid before throttling reduce the flash gas produced in the throttling process, reduce the specific volume of the flash gas, and improve the unit cooling capacity; At the same time, it also improves the superheat of the return air, which has certain advantages to protect the compressor from wet stroke operation.
In the larger refrigeration system, in order to reduce the refrigerant liquid temperature into the throttle valve, reduce the flash gas generated in the throttling or throttling, improve the refrigeration efficiency, in the process design after the reservoir (the use of throttle throttle system must have the reservoir) equipped with special equipment for undercooling – undercooler.
Its structure type is sleeve type, spray type, etc., the principle is to use the cooling water that is lower than the temperature of the saturated liquid after condensation to cool again (such as deep well water), generally compared with the cooling before reducing the temperature of 3~5 degrees (namely, the degree of supercooling is 3~5 degrees). There are also some small fluorine refrigeration system, such as small cold storage, although there is no special supercooler, but the liquid supply pipe and return pipe wrapped together to keep warm, using the low temperature of the return pipe to reduce the liquid temperature in the liquid supply pipe, can also be a section of the liquid supply pipe and expansion valve directly installed in the warehouse through the cooling again to achieve the purpose of supercooling, so as to improve the refrigeration efficiency. At the same time, the temperature of the return pipe is heated to avoid the suction of damp steam by the compressor, which may produce liquid percussion.
In the system of capillary throttling, the capillary and the return pipe (suction pipe) are combined together, some are welded together, some are covered with hot glue, some are passed through the return pipe, and some are wound around the return pipe, and some pass the capillary or liquid supply pipe directly in the box. The capillary carries out heat exchange with the return pipe, so that the liquid refrigerant before throttling and the low temperature refrigerant steam in the return pipe carry out heat exchange cooling and get the supercooling, which can not only reduce the liquid liquid compressor that may be included in the return pipe, but also achieve the purpose of the liquid refrigerant supercooling before throttling. If the condenser is deliberately enlarged, it is also feasible to leave space for cooling again and supercooling. But this is not done in the design of the specification. The consideration is to minimize the overall volume and weight and reduce the manufacturing cost. For small or micro capillary throttling systems, no special subcooler is added.
Do not separate the whole condensation process to speak, the inlet or upper part of the condenser is said to be “condensation”, the lower part or the filter part is said to be “supercooling”, the difference between the initial temperature and the terminal temperature of the condensation process can not be regarded as “supercooling”, it is wrong! Gas enters from the upper part of the condenser, gradually carries out heat exchange with the outside world, and gradually reduces the condensing temperature. Finally, it is condensed (liquefied) into liquid, which is stored in the end or filter of the condenser. This is a complete condensation process. It can not be said that the upper part of the condenser is condensation, and the lower part or some part is “supercooling”; It is also not possible to subtract the inlet temperature of the condenser from the outlet temperature as the “degree of supercooling”, or to regard the difference between a certain temperature in the condensing process and the outlet temperature of the condenser as the “degree of supercooling”. This is inappropriate, or even wrong, and misunderstands the condensing process.
Under a certain pressure, the temperature is higher than the saturation temperature of steam, called superheated steam. The steam temperature at the exhaust pipe of refrigeration compressor is generally higher than the saturation temperature, so it belongs to the superheated steam, called “exhaust overheating”.
Due to the length of the return pipe (suction pipe) and the degree of heat insulation, so that the steam in the pipe and the outside world transfer heating, this phenomenon is called “suction overheating” or “pipeline overheating”. This overheating will make the suction temperature of the compressor rise, the specific capacity of the suction steam increases, resulting in the cooling capacity of the unit volume is reduced, the cooling capacity of the compressor is reduced, which is adverse to the refrigeration cycle, in this issue known as “harmful overheating”. Therefore, it is required to do a good job on the suction pipe insulation, as far as possible to shorten the length of the suction pipe, in order to reduce this harmful overheating.
In the fluorine refrigeration system using expansion valve, the application of superheat to adjust the opening degree of thermal expansion valve, this phenomenon is called “beneficial overheating”. Similarly, the overheating of fluorine steam after reheating is also beneficial.
The difference between the saturation temperature before overheating and the saturation temperature after overheating is called superheat.

(Article source: Refrigeration Encyclopedia)