The cooling principle of a general refrigeration machine is that the compressor compresses low-pressure vapor into high-pressure vapor, reducing the volume of the vapor and increasing the pressure.
The compressor draws in low-pressure working fluid vapor from the evaporator, increases its pressure, and sends it to the condenser, where it condenses into high-pressure liquid. After passing through the expansion valve, it becomes low-pressure liquid and is sent to the evaporator, where it absorbs heat and evaporates into low-pressure vapor, which is then sent back to the inlet of the evaporator, thus completing the refrigeration cycle.
1. Vapor compression refrigeration principle
A single-stage vapor compression refrigeration system consists of four basic components: the refrigeration compressor, condenser, evaporator, and expansion valve. They are connected in sequence by pipes to form a closed system, where the refrigerant continuously circulates and undergoes state changes, exchanging heat with the outside.
After the liquid refrigerant absorbs heat from the object being cooled in the evaporator, it vaporizes into low-temperature, low-pressure vapor, which is drawn in by the compressor, compressed into high-pressure, high-temperature vapor, and then discharged into the condenser. In the condenser, it releases heat to the cooling medium (water or air), condensing into high-pressure liquid, which is then throttled by the expansion valve to become low-pressure, low-temperature refrigerant, and re-enters the evaporator to absorb heat and vaporize, achieving the purpose of refrigeration. In this way, the refrigerant completes a refrigeration cycle through the four basic processes of evaporation, compression, condensation, and throttling.
In the refrigeration system, the evaporator, condenser, compressor, and expansion valve are the four essential components. Among them, the evaporator is the device that delivers cooling. The refrigerant absorbs heat from the object being cooled to achieve refrigeration. The compressor acts as the heart, performing the functions of drawing in, compressing, and transporting refrigerant vapor. The condenser is the device that releases heat, transferring the heat absorbed in the evaporator along with the heat converted from the compressor's work to the cooling medium for removal. The expansion valve throttles and reduces the pressure of the refrigerant, while also controlling and regulating the amount of refrigerant liquid flowing into the evaporator, dividing the system into high-pressure and low-pressure sides. In actual refrigeration systems, in addition to the aforementioned four components, there are often auxiliary devices such as solenoid valves, distributors, dryers, heat exchangers, fusible plugs, pressure controllers, etc., which are set up to improve operational economy, reliability, and safety.
2. Main components of the refrigeration system
Air conditioning units can be divided into two types based on the condensation method: water-cooled and air-cooled. Based on the purpose of use, they can be divided into single cooling and cooling-heating types. Regardless of the type, they are composed of the following main components.
The main components of the refrigeration system include the compressor, condenser, evaporator, expansion valve (or capillary tube, subcooling control valve), four-way valve, dual valve, check valve, solenoid valve, pressure switch, fusible plug, output pressure regulating valve, pressure controller, liquid storage tank, heat exchanger, collector, filter, dryer, automatic open-close valve, shut-off valve, liquid injection plug, and other components.
The main components of the electrical system include motors (for compressors, fans, etc.), operation switches, electromagnetic contactors, interlock relays, overcurrent relays, thermal overload relays, temperature controllers, humidity controllers, and temperature switches (for defrosting, preventing freezing, etc.). The compressor crankcase heater, water cut-off relay, computer board, and other components are included.
The control system consists of multiple control devices, which are:
Refrigerant controller: expansion valve, capillary tube, etc.
Refrigerant circuit controller: four-way valve, check valve, dual valve, solenoid valve.
Refrigerant pressure controller: pressure switch, output pressure regulating valve, pressure controller.
Motor protector: overcurrent relay, thermal overload relay, temperature relay. Temperature controller: temperature position controller, temperature proportional controller. Humidity controller: humidity position controller.
Defrost controller: defrost temperature switch, defrost time relay, various temperature switches.
Cooling water control: water cut-off relay, water flow regulating valve, water pump, etc.
Alarm control: over-temperature alarm, over-humidity alarm, under-voltage alarm, fire alarm, smoke alarm, etc.
Other controls: indoor fan speed controller, outdoor fan speed controller, etc.
3 Common refrigerants and their properties
There are many types of refrigerants; here is a brief introduction to Freon 12 and 22:
a. Freon 12 (CF2Cl2), code R12. Freon 12 is a colorless, odorless, transparent, and almost non-toxic refrigerant, but when its concentration in the air exceeds 80%, it can cause suffocation. Freon 12 does not burn or explode, but when in contact with an open flame or at temperatures above 400°C, it can decompose into harmful substances such as hydrogen fluoride, hydrogen chloride, and phosgene (CoCl2). R12 is a widely used medium-temperature refrigerant suitable for small to medium refrigeration systems, such as refrigerators and freezers. R12 can dissolve various organic substances, so general rubber gaskets (rings) cannot be used; chloroprene synthetic rubber or butadiene rubber sheets or seals are typically used.
b. Freon 22 (CHF2Cl), code R22. R22 does not burn or explode, and its toxicity is slightly higher than that of R12. Although its solubility in water is greater than that of R12, it can still cause 'ice blockage' in refrigeration systems. R22 can partially dissolve in lubricating oil, and its solubility changes with the type of lubricating oil and temperature, so refrigeration systems using R22 must have oil return measures.
The corresponding evaporation temperature of R22 at standard atmospheric pressure is -40.8°C, and the condensation pressure at room temperature does not exceed 15.68×10^5 Pa, with the unit volume cooling capacity being more than 60% greater than that of R12. In air conditioning equipment, R22 refrigerant is mostly selected.
