A refrigeration unit is a combination of equipment for creating a low-temperature environment via a refrigeration cycle to transfer heat.
                                                                       
I. Basic Principles

1. Vapor-compression Refrigeration: Comprising a compressor, condenser, throttling device and evaporator. The refrigerant in the evaporator absorbs heat from the cooled object and vaporizes. The compressor then compresses the vaporized refrigerant into a high-temperature, high-pressure gas. This gas enters the condenser to release heat and condense into a liquid. After passing through the throttling device for pressure reduction, it returns to the evaporator to start a new cycle.
2. Absorption Refrigeration: Utilizes a binary solution (e.g., lithium bromide - water). The generator heats the solution, vaporizing the low-boiling-point component. The refrigerant vapor condenses in the condenser, passes through a throttle, and vaporizes in the evaporator to produce a refrigeration effect. The concentrated solution in the absorber absorbs the refrigerant vapor and returns to the generator.

II. Main Components

1. Compressor: The core of the unit, compressing refrigerant gas. Types include piston, screw and centrifugal compressors.
2. Condenser: Condenses refrigerant gas. Can be air-cooled (using air convection, simple but with lower efficiency) or water-cooled (using water, more efficient but requiring a water system).
3. Evaporator: Where refrigerant absorbs heat and vaporizes, such as shell-and-tube or fin-and-tube types.
4. Throttling Device: Reduces refrigerant pressure, like expansion valves (with adjustable flow) or capillary tubes (simple but with limited flow adjustment).

III. Classification

1. By Refrigeration Capacity: Small (a few to dozens of kilowatts), medium (dozens to hundreds of kilowatts) and large (hundreds of kilowatts to several megawatts).
2. By Cooling Method: Air-cooled (easy to install) and water-cooled (high efficiency).
3. By Application Field: Air-conditioning, industrial and cold storage refrigeration units.

IV. Application Scenarios

1. Commercial: In supermarkets for food preservation and in malls for air-conditioning.
2. Industrial: For cooling chemical reactions and drug storage.
3. Transportation: In refrigerated trucks and containers for transporting perishable goods.