The composition, principle, and use of chiller units

The refrigeration system of a chiller unit consists of four basic parts: a compressor, a condenser, a restrictor, and an evaporator. Connect the four major components into a closed system in a certain order using copper pipes, and fill the system with a certain amount of refrigerant.



The compressor sucks in low temperature and low pressure Freon gas from the evaporator, compresses it into high temperature and high pressure Freon gas, and then flows through a thermal expansion valve (capillary), throttling it into a low temperature and low pressure Freon liquid two-phase object. Then, the low temperature and low pressure Freon liquid absorbs heat from the indoor air in the evaporator, compressing - condensing - throttling - evaporating repeatedly.



1、 Chiller unit:

This is the "cooling source" of central air conditioning, "hidden in the heart", and the circulating water to each room is "internally exchanged" by the chiller unit, cooling to "cooling water".



2、 External heat exchange system:

Chilled water circulation system: composed of a chilled pump and chilled water pipeline. The chilled water flowing out of the chiller unit is pressurized by a refrigeration pump and sent to the chilled water pipeline for heat exchange in each room, taking away the heat in the room and causing a temperature drop in the room.

Cooling water circulation system: composed of cooling pump, cooling water pipeline, and cooling tower. The chiller unit conducts heat exchange, which not only cools the water temperature but also releases a large amount of heat. The heat is absorbed by the cooling water, causing an increase in the temperature of the cooling water. The cooling pump will feed the heated cooling water pressure into the cooling tower, allowing it to exchange heat with the atmosphere in the cooling tower. Then, the cooled water will be sent back to the chiller unit.

3、 Cooling fan:

Indoor fan: installed in the room where cooling is required, used to blow air cooled by chilled water into the room and accelerate heat exchange within the room.



Cooling tower fan: used to reduce the water temperature of the cooling tower and accelerate the heat brought back by the "return water" to dissipate into the atmosphere.

4、 Start and operation of chiller unit:

Check the oil level and temperature of each compressor: the oil level is between 1/3 and 2/3; The oil temperature is between 50 ℃ and 60 ℃, and touching the heater with your hand will make it hot.

Check the main power supply voltage and current: the power supply voltage is within the range of 340V to 440V; The three-phase voltage imbalance value is less than 2% (>2% must not be turned on); The three-phase current imbalance value is less than 10%.



Start the chilled water pump and cooling water pump: after the circulation of the two water systems is established, adjust the opening of the inlet and outlet valves of the evaporator and condenser.

Check the temperature of the chilled water supply: whether the set value is appropriate, if not, it can be changed.



Pre start inspection: Check the tightness of electrical connectors (main circuit, control circuit), and heat the oil tank for at least 24 hours to ensure that the temperature of the oil tank cannot be lower than 38 degrees.

Pre start inspection: Check the status of all valves, water pumps, pressure gauges, thermometers, filters, etc. of the unit.

Pre start inspection: Check the condition of the end of the unit. Check the condition of the cooling tower.

Pre start inspection: Start the chilled water pump and cooling water pump of the water system separately to check if the water system is running normally, ensuring that there is no gas entrainment and that the inlet and outlet water pressure drop of the water system is within the required range.

5、 Monitoring and regulation of chiller operation:



For chiller units, the following situations need to be paid attention to during operation:

1. The temperature and pressure of the inlet and outlet of the evaporator's chilled water.

2. The temperature and pressure of the inlet and outlet of the condenser cooling water.

3. The pressure and temperature of the refrigerant in the evaporator.

4. The pressure and temperature of the refrigerant in the condenser.

5. The current and voltage of the main motor.

6. The pressure and temperature of lubricating oil.

7. Whether the compressor unit operates smoothly and whether there are any abnormal noises

8. Check for any leaks in the valves of the unit.

9. Check if the joints with each water pipe are tight.

6、 Evaporation pressure and temperature:

The pressure and temperature of the refrigerant inside the evaporator are the saturation pressure and saturation temperature of the refrigerant, which can be measured by the corresponding instruments or meters set on the evaporator.

There is a close relationship between the evaporation pressure, evaporation temperature, and the heat carried by the chilled water into the evaporator. When the air conditioning cooling load is high, the return water temperature of the evaporator's chilled water increases, causing an increase in evaporation temperature and corresponding evaporation pressure. The refrigeration capacity of the chiller unit must be slightly greater than the designed cooling load of the air conditioning it bears, otherwise it will not be able to achieve satisfactory air conditioning performance during operation.



According to the JB/T7666 95 standard in China (General regulations for nominal operating conditions of refrigeration and air conditioning equipment), the nominal operating conditions of chillers are 7 ℃ for chilled water outlet temperature and 32 ℃ for cooling water return temperature. Since increasing the outlet temperature of chilled water is highly beneficial for the economy of chillers, it is necessary to increase the outlet temperature of chilled water as much as possible during operation while meeting the requirements of air conditioning usage.

In general, the evaporation temperature is usually controlled within the range of 3 ℃ to 5 ℃, which is 2 ℃ to 4 ℃ lower than the outlet temperature of chilled water. Excessive evaporation temperature often makes it difficult to achieve the required air conditioning effect, while too low evaporation temperature not only increases the energy consumption of the chiller unit, but also easily causes freezing and cracking of the evaporation pipeline.



The difference between the evaporation temperature and the outlet temperature of the chilled water increases or decreases with the increase or decrease of the cooling load of the evaporator, respectively. Under the same load conditions, as the temperature difference increases, the heat transfer coefficient decreases. In addition, the size of the temperature difference is also related to the heat transfer area, and the dirt inside the pipe and the accumulation of lubricating oil outside the pipe also have a certain impact. In order to reduce temperature difference and enhance heat transfer efficiency, it is necessary to regularly remove dirt from the evaporator water pipes and actively take measures to guide lubricating oil back into the oil tank.

7、 Condensing pressure and temperature:

The refrigerant inside the condenser is usually in a saturated state, so its pressure and temperature can be searched for each other through the corresponding thermodynamic properties table of the refrigerant.

The condensation temperature of water-cooled units is generally 2 ℃ to 4 ℃ higher than the outlet temperature of cooling water. If it is too high, the copper pipes inside the condenser should be checked for scaling and cleaned.



The high or low condensation temperature, while keeping the evaporation temperature constant, has a decisive significance for the power consumption of the chiller unit. The increase in condensation temperature leads to an increase in power consumption. On the contrary, as the condensation temperature decreases, the power consumption also decreases. When air is present in the condenser, the temperature difference between the condensation temperature and the outlet of the cooling water increases, while the temperature difference between the inlet and outlet of the cooling water actually decreases. At this time, the heat transfer effect of the condenser is not good, and the outer shell of the condenser feels hot to the touch.

Scaling and sludge on the water side of condenser tubes also play a significant role in heat transfer. Therefore, during the operation of the chiller unit, attention should be paid to ensuring that the cooling water temperature, water quantity, water quality and other indicators are within the qualified range.

8、 Pressure and temperature of chilled water:

Air conditioning chillers are generally operated under the nominal operating conditions of a chilled water return temperature of 12 ℃, a supply temperature of 7 ℃, and a temperature difference of 5 ℃.

From Q=W × △ t, it can be seen that the flow rate of chilled water through the evaporator is inversely proportional to the temperature difference between the supply and return water, that is, the larger the chilled water flow rate, the smaller the temperature difference; On the contrary, the smaller the flow rate, the greater the temperature difference.