Refrigeration Repair Techniques: A Must - Know - "Sub - cooling" and "Super - heating"

2025-02-25

"Sub - cooling"

Sub - cooling means re - cooling the condensed saturated liquid through certain devices (such as sub - coolers) and methods, so that its temperature is lower than the saturated temperature corresponding to the condensing pressure.
The difference between the temperature of the liquid before sub - cooling and that after sub - cooling is called the "degree of sub - cooling".

Benefits of Sub - cooling

It reduces the flash gas generated during the throttling process of the refrigerant liquid, decreases the specific volume of the flash gas in the occupied volume, and thus increases the unit refrigeration capacity.
Meanwhile, it increases the superheat of the return gas, which is beneficial for protecting the compressor from operating in a wet stroke.

How to Achieve Sub - cooling in Practical Design

Special Sub - cooler: In larger refrigeration systems, to lower 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, a special sub - cooling device, the sub - cooler, is installed after the liquid receiver (a liquid receiver is essential for a system using a throttle valve for throttling) in the process design.
- Its structural forms include double - pipe type, spray type, or plate - type heat exchanger. The principle is to use cooling water with a temperature lower than that of the condensed saturated liquid (such as deep well water) for re - cooling. Generally, the temperature can be further reduced by 3℃ - 5℃ compared to that before cooling (i.e., the degree of sub - cooling is 3℃ - 5℃).
Heat Exchange between the Liquid Supply Pipe and the Suction Pipe: In some small - scale fluorine refrigeration systems, although there is no dedicated sub - cooler, the liquid supply pipe and the suction pipe are wrapped together for insulation. The low temperature of the suction pipe is used to lower the temperature of the liquid in the liquid supply pipe. Also, a section of the liquid supply pipe and the expansion valve can be directly installed and passed through the storage room to achieve sub - cooling through re - cooling, thereby improving the refrigeration efficiency.
- At the same time, it also heats the temperature of the suction pipe, preventing the compressor from possible liquid hammer caused by inhaling overly humid steam. In a capillary - throttling system, the capillary and the suction pipe are arranged together. Some are welded together (Daikin VRV adopts this method), some are sleeved together with a heat - resistant sleeve, some pass through the suction pipe, some are wound around the suction pipe, and some capillary or liquid supply pipes are directly passed through the box.
  The capillary and the suction pipe exchange heat, so that the liquid refrigerant before throttling is cooled by exchanging heat with the low - temperature refrigerant vapor in the return gas pipeline, achieving sub - cooling. This can not only prevent the possible liquid carried in the return gas pipeline from causing liquid hammer to the compressor, but also achieve the sub - cooling of the liquid refrigerant before throttling. Of course, there are also systems using a heat regenerator, with the same principle.
Enlarging the Condenser: It is feasible to deliberately enlarge the condenser, leaving space for re - cooling to achieve sub - cooling. However, in a standardized design, this is not done. The consideration is to minimize the overall volume and weight and reduce the manufacturing cost. For small - scale or micro - scale capillary - throttling systems, a special sub - cooler is not installed.

"Super - heating"

The difference between the saturated temperature before super - heating and the saturated temperature after super - heating is called the degree of super - heat.
Under a certain pressure, steam with a temperature higher than the saturated temperature is called superheated steam. The steam temperature at the exhaust pipe of the refrigeration compressor is generally higher than the saturated temperature, so it all belongs to superheated steam, which is called "exhaust super - heating".

Harmful Super - heating

Due to the length and insulation degree of the suction pipe, the steam in the pipe exchanges heat with the outside, and this phenomenon is called "suction super - heating" or "pipeline super - heating". This kind of super - heating will increase the suction temperature of the compressor, increase the specific volume of the inhaled steam, reduce the refrigeration capacity per unit volume, and decrease the refrigeration capacity of the compressor. This is detrimental to the refrigeration cycle, and in this regard, it is called "harmful super - heating". Therefore, it is required to insulate the suction pipeline well and try to shorten the length of the suction pipe to reduce this harmful super - heating.

Refrigeration Repair Techniques: A Must - Know -

Beneficial Super - heating

In a fluorine refrigeration system using an expansion valve, the degree of super - heat is used to adjust the opening degree of the thermostatic expansion valve, and this phenomenon is called "beneficial super - heating". Similarly, the super - heating generated by fluorine steam after heat regeneration also belongs to beneficial super - heating.