Knowledge！Working principle of chiller and analysis of common faults

Posted on 2024-06-07

Chiller: This is the core part of the central air conditioning system and plays the role of "cooling heart". It compresses the refrigerant through the compressor to increase the pressure and temperature of the refrigerant. Then the refrigerant releases heat to the cooling water in the condenser and condenses into a high-pressure liquid. In this process, the cooling water absorbs heat and is dissipated into the atmosphere by the cooling tower. After that, the high-pressure liquid refrigerant is quickly decompressed through the expansion valve and enters the evaporator. At this time, the refrigerant absorbs the heat of the indoor circulating water (chilled water) and evaporates into a gas state. The chilled water is cooled and then transported to the fan coil or air handling unit in each room through the chilled water circulation system to achieve indoor air cooling.

Cooling water tower: As an auxiliary equipment of the chiller, the cooling water tower has the main function of providing cooling water and heat dissipation for the chiller. After absorbing the heat in the condenser of the chiller, the cooling water is pumped to the top of the cooling tower and sprayed down to exchange heat with the air in the tower. The heat is taken away by natural wind or forced ventilation. The cooled water is recycled to continuously support the cooling process of the chiller.

Chilled water circulation system: This system consists of a chilled water pump and a series of pipes, which are responsible for promoting the circulation of low-temperature chilled water in the system. The chilled water pump increases the pressure of the chilled water so that it can overcome the pipe resistance and reach each terminal device, such as the fan coil unit, where it completes the heat exchange with the indoor air, and the chilled water with increased temperature returns to the chiller to be cooled again, forming a closed loop.

Cooling water circulation system: The cooling water circulation system also includes pumps and pipes. Its task is to transport cooling water from the cooling tower to the condenser of the chiller, absorb the heat released by the condenser, and then return to the cooling tower to dissipate heat. The cooling water pump extracts the cooling water from the bottom of the cooling tower. After passing through the condenser, the cooling water with increased water temperature is sent back to the top of the cooling tower. After evaporation and heat dissipation, the temperature drops and participates in the circulation again. This process ensures that the chiller can continuously and effectively dissipate heat and maintain the cooling efficiency of the entire system.

Types of chillers: Generally, chillers are divided into water-cooled and air-cooled.

Air-cooled chillers contain water tanks and water pumps, and do not require cooling towers for heat dissipation. They are easy to install and move. However, they have high requirements for the environment. First of all, because it uses hot air circulation for cooling, if the ventilation effect of the installation workshop is not good, it will directly affect the cooling effect of the chiller. In addition, if you want to put the chiller in a dust-free workshop with humidity requirements, a large amount of water vapor will be generated on the top of the machine.

Water-cooled chillers are divided into open chillers, sealed types (some are called box-type chillers) and screw chillers. Water-cooled chillers must be equipped with cooling towers and water pumps to achieve better cooling effects. Open chillers also need to install a water tank, but sealed types do not need to because they have built-in water tanks. Open chillers are generally installed outside the workshop for easy maintenance, but because they are not beautiful enough, sealed water-cooled chillers are produced. Sealed water-cooled chillers are a box structure, which is easy to install and use.

Common faults of chillers

1. High cooling water temperature and poor condensation effect: The rated working condition of cooling water required by chillers is 30~35℃. High water temperature and poor heat dissipation will inevitably lead to high condensation pressure, which often occurs in high temperature seasons. The reasons for the high water temperature may be: cooling tower failure, such as the fan is not turned on or even reversed, the water distributor does not rotate, which is manifested as high cooling water temperature and rapid increase; high outside temperature, short water path, and small amount of circulating water. In this case, the cooling water temperature is generally maintained at a high level, which can be solved by adding a water storage tank.

2. Insufficient cooling water flow and failure to reach the rated water flow: The main manifestation is that the pressure difference between the inlet and outlet water of the unit becomes smaller (compared with the pressure difference at the beginning of the system operation), and the temperature difference becomes larger. The reason for insufficient water flow is that the system lacks water or there is air. The solution is to install an exhaust valve at a high point in the pipeline for exhaust; the pipeline filter is blocked or too fine, and the water permeability is limited. A suitable filter should be selected and the filter screen should be cleaned regularly; the water pump is selected to be small and not compatible with the system.

3. Condenser scaling or blockage: Condensate is generally tap water, which is easy to scale above 30°C. Moreover, since the cooling tower is open and directly exposed to the air, dust and foreign matter can easily enter the cooling water system, causing the condenser to be dirty and blocked, with a small heat exchange area, low efficiency, and also affecting the water flow. The manifestation is that the pressure difference and temperature difference between the inlet and outlet water of the unit become larger. The upper and lower temperatures of the condenser are very high when touched by hand, and the copper tube of the condenser outlet is hot. The unit should be backwashed regularly, and chemical cleaning and descaling should be performed when necessary.

4. Excessive refrigerant filling: This situation usually occurs after maintenance, and is manifested as high suction and exhaust pressure, balance pressure, and high compressor operating current. It should be vented according to the suction and exhaust pressure, balance pressure, and operating current under rated conditions until it is normal.

5. Non-condensable gases such as air and nitrogen are mixed in the refrigerant: This situation usually occurs after maintenance, and the vacuum is not completely drawn. It can only be discharged, re-vacuumed, and refilled with refrigerant.

6. False alarm caused by electrical fault: due to moisture, poor contact or damage of the high-voltage protection relay, moisture or damage of the unit electronic board, and communication failure, false alarm is caused. For this kind of false fault, the high-voltage fault indicator on the electronic board is often off or slightly lit, the manual reset of the high-voltage protection relay is invalid, the compressor running current is normal, and the suction and exhaust pressure is also normal.

7. Evaporation pressure is too low: the evaporation pressure of the water-cooled chiller may be too low due to insufficient refrigerant. Common reasons: insufficient cold water; low cooling load; throttling orifice plate failure (only low evaporation pressure); heat transfer of the evaporator heat transfer tube is deteriorated due to scale and other pollution (only low evaporation pressure); insufficient refrigerant (only low evaporation pressure).

8. Low oil pressure difference: the compressor oil pressure is too low and the compressor will stop running. Common reasons are: oil filter blockage; too much refrigerant mixed in the lubricating oil.

9. Too high oil temperature: If the refrigeration compressor oil temperature is too high and runs for a long time, the quality of the refrigeration oil will be reduced and even carbonized. Common causes: reduced cooling capacity of the oil cooler; insufficient supply of refrigerant for oil cooler cooling due to blockage of the refrigerant filter.

10. Main motor overload: unbalanced power phase voltage; large voltage drop in the power line;