What do the refrigeration system overcooling and overheating mean

What do the refrigeration system overcooling and overheating mean

The so-called "supercooling" is to pass the condensed saturated liquid through a certain device (such as a subcooler) and method (or measure) to recool it so that its temperature is lower than the saturation temperature under the condensation 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".

The subcooling is to reduce the flash gas generated during the throttling of the refrigerant liquid before throttling, reduce the specific volume of the flash gas, and increase the unit refrigeration capacity; at the same time, it also increases the superheat of the return gas. Protecting the compressor from wet stroke operation has certain benefits.

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, etc. The principle is to use cooling water that is lower than the saturated liquid temperature after condensation to cool again (such as deep well water), and generally can lower the temperature by 3 to 5 degrees than before cooling ( That is, the degree of subcooling is 3~5 degrees). There are also some small fluorine refrigeration systems, such as small cold storage. 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. A section of the liquid supply pipe and expansion valve are directly installed in the warehouse and pass through, and the purpose of subcooling is achieved after recooling, 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.

The capillary throttling system. The capillary tube and the return pipe (suction pipe) are combined and run together. Some are welded together, are sleeved with a hot glue sleeve, pass through the return pipe, and are wound around the return pipe. Some of them pass the capillary tube or the liquid supply tube 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 are heat-exchanged and cooled to obtain subcooling, which can reduce the liquid impact 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. 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.

Don’t separate the entire condensation process, say that the inlet or upper part of the condenser is “condensing", and the lower part or filter part is “super-cooled". The difference between the initial temperature and the terminal temperature of the condensation process cannot be considered. As "undercooling", that is wrong! The gas enters from the upper part of the condenser and gradually exchanges heat with the outside world. The condensation temperature is gradually reduced, and finally it is condensed (liquefied) into liquid, which is accumulated at the end of the condenser or in the filter. This is the complete condensation process. It cannot be said that the upper part of the condenser is condensing, and the lower part or a certain part is "supercooling"; nor can the subtraction of the inlet temperature and the outlet temperature of the condenser be regarded as the "supercooling degree", or the temperature of a certain stage in the condensation process and condensation The difference in the outlet temperature of the condenser is regarded as the "subcooling degree", which is inappropriate, or even wrong, and misunderstands the condensation process.

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

　　 Due to the length of the return pipe (suction pipe) and the degree of heat insulation, 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.

　　 In the fluorine refrigeration system using the 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.

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