Working Principle and Daily Maintenance of Water-based Machines

I. Four Major Components of the Main Unit and Basic Working Principle
The main units of refrigeration compressors for water chillers mainly use screw and centrifugal compressors, while commercial machines mainly use scroll compressors.
Refrigeration Schematic Diagram of the Water System
In our refrigeration industry, common compressors can be classified as follows:
Rotary compressor;
Reciprocating compressor;
Reciprocating compressor;
Screw compressor;
Magnetic levitation centrifugal compressor.

II. Evaporator
Dry Evaporator:
The refrigerant flows inside the pipes, and water flows outside the tube bundle.

It is composed of a shell, evaporation pipes (internal threaded pipes), baffle plates, end covers, etc.
Features:
The lubricating oil enters the compressor along with the refrigerant, and there is generally no problem of oil accumulation;
Less refrigerant needs to be charged, usually only about one-third of that of a flooded evaporator;
When the temperature is around 0°C, water will not freeze;
The heat transfer coefficient is low, and the terminal temperature difference is large;
Scale easily adheres to the surface of the copper pipes of the evaporator, and it is not easy to clean;
Flooded Evaporator:
The refrigerant flows outside the pipes, and water flows inside the pipes. It is composed of a shell, evaporation pipes (high-efficiency pipes), tube sheets, support plates, etc.
Features:
The heat transfer coefficient is high, and the terminal temperature difference is small;
Water flows inside the pipes, making it easy to clean;
The refrigerant liquid basically fills the tube bundle, and the charging amount is large;
The oil return of the flooded evaporator is difficult and unstable;
When the evaporation temperature of the refrigeration system is lower than 0°C;
The water inside the pipes is likely to freeze and damage the evaporation pipes;
The hydrostatic liquid column of the liquid in the flooded evaporator raises the saturation evaporation temperature at the bottom.
Falling Film Evaporator:
The refrigerant is evenly distributed from the distributor, forms a liquid film on the surface of the heat exchange pipes, absorbs the heat inside the pipes and evaporates, and water flows inside the pipes. It is composed of a distributor, a shell, evaporation pipes (high-efficiency pipes), tube sheets, support plates, etc.
Features:
The heat transfer coefficient is high, and the terminal temperature difference is small (better than that of the flooded evaporator and even better than that of the dry evaporator);
The lubricating oil accumulates at the bottom of the container with a high concentration, making it easy for oil return;
Avoid the situation that the hydrostatic liquid column of the liquid in the flooded evaporator raises the saturation evaporation temperature at the bottom;
Avoid the dry pipe phenomenon that is likely to occur in some falling film evaporators, with more uniform heat exchange and higher efficiency;
Water flows inside the pipes, making it easy to clean;
When the evaporation temperature of the refrigeration system is lower than 0°C, the water inside the pipes is likely to freeze and damage the evaporation pipes.
Comparison of the Three Evaporators

III. Condenser
Condenser:
The gaseous refrigerant comes from the top of the container, passes through the built-in oil separator, and is cooled on the surface of the heat exchange pipes. It is composed of a built-in oil separator, a shell, evaporation pipes (high-efficiency pipes), tube sheets, support plates, a subcooler, etc.

IV. Throttle Device
Throttle Device:
Its main function is to throttle and reduce the pressure, and control and regulate the flow rate and superheat of the refrigerant.
Double Orifice Plate:
Slow adjustment response;
Almost no adjustment ability;
Small energy adjustment range;
Low cost.
Electronic Expansion Valve:
Simple structure, small resistance;
Wide energy adjustment range;
Rapid response in energy efficiency adjustment;
High cost.

V. Daily Maintenance of the Main Unit
Unit:
To ensure that the unit can operate normally without failure or damage under the maximum load, it should be inspected regularly according to the following items.
Refrigeration System:
Check the glass sight glasses indicating the humidity on each liquid supply pipeline of the system, and confirm that they are filled with liquid and the humidity indication shows a dry state. If the humidity indication is high or there are bubbles in the sight glass, even if there is enough refrigerant charged in the unit, the core of the dry filter should be replaced.
Oil System:
The lubricating oil should be replaced regularly to ensure the normal and continuous operation of the unit. When the unit is in the shutdown state, the oil level height can be observed through the compressor oil sight glass. During the operation process, due to different system load conditions, the oil level may change in height. [Source: Refrigeration Encyclopedia] However, it is necessary to ensure that the oil level is above the lowest line of the compressor oil sight glass. The phenomenon of oil foaming in the lubricating oil mostly occurs when the load is running below 20% of the rated power of the water chiller unit. At this time, the water temperature on the condenser side is too low compared with the rated working condition. At this time, the lubricating oil is not really lost, but has entered the refrigerant system, which is most common in the evaporator. If there is an excessive amount of oil in the evaporator, it may cause unstable operation.
If the compressor discharge superheat is too low, it is possible that the compressor has a serious liquid carryover and the phenomenon of lubricating oil loss occurs. The cause needs to be found and dealt with.

Excessive refrigerant charging will also lead to a decrease in the discharge superheat of the unit, resulting in the unit's inability to load normally. [Source: Refrigeration Encyclopedia] At this time, it is necessary to adjust the refrigerant charging amount to ensure that the discharge superheat of the R22 unit is between 18 and 28 degrees and that of the R134a unit is between 12 and 20 degrees (for reference) under the rated working condition.
Water System:
In some regions, due to the relatively hard water quality, the condenser will scale, resulting in an excessively high condensing pressure in the condenser or poor heat exchange effect in the evaporator, which may lead to shutdown due to failure or uneconomical operation of the unit. Therefore, the water quality must be inspected before the water enters the unit container of the air conditioner. If the water quality does not meet the requirements of the air conditioner water, water treatment is required. The relevant water treatment can refer to the regulations in the National Standard GB50050-1995 "Design Code for Industrial Circulating Cooling Water Treatment".