[Mechanical Equipment Technology Edition] Daily Question 201708030 – Principles of Refrigeration

Phase transition refers to a change in the aggregation state of a substance. When a substance undergoes a phase change, due to the rearrangement of molecules and changes in the speed of their thermal motion, it is inevitable that a certain amount of heat is absorbed or released; this heat is known as latent heat of phase change. Phase-change refrigeration utilizes the heat absorption effect that occurs during the phase transition of a substance from a dense state to a less dense state (melting, evaporation, sublimation) to achieve cooling. 1) Solid melting refrigeration 2) Solid sublimation refrigeration 3) Liquid vaporization refrigeration. The four key components of refrigeration systems: 1. Compressor. In a compression refrigeration cycle, the compressor has two functions. First, it extracts the refrigerant vapor from the evaporator and reduces the pressure in the evaporator, thereby maintaining an ideal evaporation temperature within it ； Second, the compressor can increase the pressure of the refrigerant vapor, raising its saturation temperature above that of the cooling medium, thereby causing the refrigerant vapor to condense. 2. Condenser: One of the main heat exchange devices in a refrigeration unit. The high-pressure, superheated refrigerant vapor, after releasing heat in the condenser, condenses into a saturated liquid or a subcooled liquid. 3. Evaporator: The evaporator is the low-pressure part of a refrigeration system, where the liquid refrigerant boils and absorbs heat in the process. It ultimately achieved the actual goal of this system—cooling. 4. Expansion valve: The commonly used device for controlling the flow rate of liquid refrigerant entering the evaporator is a thermal expansion valve. A small hole in the valve is used to limit the flow rate into the evaporator; this flow rate is adjusted by a needle valve and its seat, which can change the size of the hole. Appendix 1: Solenoid valves. The structure of refrigeration solenoid valves is divided into direct-acting and pilot-operated types based on their mode of operation. (1) Direct-acting solenoid valve (2) Pilot-operated solenoid valve 2. Fuse plug: A fuse plug is a safety device that contains metal with a specific melting point in its interior. This melting point is calibrated by the instrument. But generally, its melting point is the saturation temperature of the refrigerant at a pressure not exceeding 40% of the burst pressure of the refrigerant container, or the critical temperature of the refrigerant (usually the lower of the two values is taken). 3. Liquid reservoir: The liquid reservoir is essentially a tank for storing refrigerant that is not part of the circulation process. It is used in small systems where liquid refrigerant is supplied via capillaries; the amount of refrigerant supplied is very small. If the operating load remains relatively stable, well-designed evaporators and condensers allow the system to function without a liquid reservoir. If the condenser has sufficient volume, it can provide storage space, and a separate liquid reservoir may not be necessary. This is the design approach commonly used in water-cooled units equipped with shell-and-tube condensers. However, all air-cooled units equipped with an expansion valve require a separate liquid reservoir. 4. Regenerator: A regenerator is a device that transfers heat from one medium to another. In commercial refrigeration systems, a regenerator is commonly used to refer to a component that transfers heat from the liquid refrigerant to the refrigerant vapor. 5. Suction line gas-liquid separator: If liquid refrigerant is allowed to flow through the system and return to the compressor without being vaporized, this can lead to problems such as loss of oil in the crankcase and wear on the bearings, causing damage to the compressor due to the accumulation of liquid. To protect systems that may be damaged by liquids, such as heat pumps, vehicle cooling systems, and other similar systems, gas-liquid separators on the suction lines are often used. 6. Oil separator: Although a well-designed system can effectively address the issue of oil backflow, an oil separator is still necessary in some cases, especially in ultra-low temperature systems such as full-liquid evaporators, or in other systems that require it for their own reasons. 7. Drier: Moisture is the enemy of refrigeration systems. To prevent system failures and damage to the compressor, it is necessary to keep the moisture content at a very low level. Even with the best preventive measures in place, moisture can still enter the system whenever it is opened for on-site repairs. When moisture gets into the system without thorough vacuuming and refilling with liquid, the only other method to remove a small amount of moisture from the system is to use a dry filter. 8. Intake air filter: To protect the compressor from impurities remaining in the system during installation, intake filters are widely used. The suction filter is usually designed as a permanent component on the suction pipe; it can be completely sealed, or it can feature a replaceable filter element that allows for easy replacement when necessary. 9. Shock absorbers: To prevent the noise and vibration of the compressor from being transmitted through the refrigeration piping, shock absorbers are installed on both the suction and discharge pipes. For small units, the use of small-sized, low-diameter soft copper pipes is sufficient to provide vibration damping protection for the system. In larger compressors, metal hoses are typically used. 10. Filter: The filter is installed in the refrigeration tubes to remove impurities and metal debris from the refrigerant, thereby preventing problems with the refrigerant control components and the compressor’s operation. Although filter structures vary, they are essentially all composed of sleeves with fine mesh pores. Because there are small orifice plates in the expansion valve and solenoid valve, which are susceptible to damage from impurities, filters are generally installed on the liquid pipe, that is, upstream of the valve. 11. Liquid level gauge and humidity indicator: The liquid level gauge on the liquid tube allows the operator or maintenance technician to observe the flow of the liquid refrigerant. When bubbles or foam are observed through the sight glass, it indicates a shortage of refrigerant or an obstruction in the liquid line, which affects the proper operation of the system. When adding fluid, the sight glass is a commonly used method to ensure that the system is filled to the appropriate level. 12. Exhaust silencer: When it is necessary to minimize system noise or address the vibration problems caused by the pulses from the compressor, exhaust silencers are often used for noise reduction. A silencer is essentially a housing with baffles; its internal volume depends primarily on the compressor’s exhaust volume. Additionally, the frequency and intensity of sound waves are also factors that need to be taken into account in the design of the silencer. 13. Crankcase heater: If the ambient temperature of the compressor is lower than that of the evaporator, when the compressor stops, the pressure difference between the evaporator and the compressor causes the refrigerant to move toward the crankcase. To prevent this phenomenon from occurring, crankcase heaters are often used to keep the temperature of the oil in the crankcase high enough, so that the liquid refrigerant entering the crankcase evaporates immediately, thereby generating sufficient pressure to prevent large-scale migration.

1. Main equipment: evaporator, which facilitates evaporation at low pressure, absorbs heat, and generates cooling effect； 2. Main equipment: screw compressor, which is used to increase the pressure of the vapor after evaporation ； 3. Main equipment: Condenser, which is used to liquefy high-pressure gases for further recycling ； 4. Main equipment: Expander, which is used to reduce the pressure of high-pressure liquids

1. Evaporator – The liquid refrigerant evaporates, absorbing heat – this generates chilled water – which is used for cooling； 2-Condenser-Compresses gaseous refrigerant; heat is released during condensation-Cooling water ； 3. Expansion valve – regulates the flow of liquid refrigerant from the condenser to the evaporator ； 4-Compressor: It draws in the gaseous medium from the absorption evaporator and transports it to the condenser where it is compressed and condensed.

Phase-change refrigeration utilizes the evaporation of liquids at low temperatures, as well as the melting or sublimation of solids at low temperatures, to absorb heat from the object being cooled – that is, the cooling capacity. Therefore, phase-change refrigeration is divided into liquid vaporization refrigeration and solid melting and sublimation refrigeration; since liquids possess fluidity, liquid vaporization refrigeration is widely used. The process of liquid turning into vapor absorbs heat, thereby achieving cooling. To enable it to operate continuously in a cycle, it is necessary for the refrigerant to evaporate at low pressure, the vapor to be pressurized, the high-pressure gas to liquefy, and the high-pressure liquid to be depressurized.

Evaporation vaporization, steam pressurization, high-pressure gas liquefaction, and high-pressure liquid depressurization

Compressor, condenser, filter, evaporator

The four main components of a refrigeration system are: the evaporator, the compressor, the condenser, and the throttling device. The cooling principle is as follows: The low-pressure vapor of the refrigerant in the cooling system is drawn in by the compressor, compressed into high-pressure vapor, and then sent to the condenser. At the same time, the outdoor air drawn in by the axial flow fan flows through the condenser, carrying away the heat released by the refrigerant and causing the high-pressure refrigerant vapor to condense into a high-pressure liquid. The high-pressure liquid passes through a filter and a throttling mechanism before being sprayed into the evaporator, where it evaporates at a corresponding low pressure while absorbing heat from its surroundings.

After the condensing medium turns into a gas in the evaporator, it is drawn in at the compressor inlet. After being compressed by the compressor, it enters the condenser, where it changes from a gas to a liquid state. It then passes through a filter to have impurities removed, before returning to the evaporator, thus completing one heat exchange cycle.

What comes out of the compressor is the cold phase, which returns after absorbing heat

Compressor, evaporator, condenser, expansion valve