What is the reason for the decline in performance of centrifugal chillers?

centrifugal chillerAfter long-term operation, the unit will not be able to operate at full load, surge, low evaporation temperature, high condensation temperature, and large motor current will cause its cooling capacity to drop. The main reasons for the performance decline of the unit are the changes in the chemical and thermal properties of the refrigerant, the deterioration of the lubricating oil, the excessive thermal resistance of the heat exchange surface, the poor sealing of the system, and the mutual penetration of the refrigerant system and the water system. What is the reason for the decline in performance of centrifugal chillers?

(1) Refrigerant

　　centrifugal chillerFreon used as refrigerant mainly R11, R134a, R123 and so on. The performance change of refrigerant can be described from three aspects: refrigerant and water, refrigerant and lubricating oil, refrigerant and metal and non-metal. Refrigerant and water, in a refrigeration system with Freon as the refrigerant, when the refrigeration temperature is lower than 0, the free water will freeze and block the throttle channel, causing insufficient liquid supply in the system and reducing the cooling capacity of the unit; in addition, when the water dissolves into When the refrigerant, water decomposition will occur, produce acidic substances, corrode metal materials, and reduce the electrical insulation performance of the winding. The solubility of refrigerant and lubricating oil is different from that of refrigerant and lubricating oil. When dissolved in each other, the lubricating oil can penetrate into all parts of the compressor together with the refrigerant to form good lubrication conditions and extend the service life of the unit, but mutual dissolution will reduce the viscosity of the lubricating oil and increase the evaporation of the unit under the same pressure Temperature; when insoluble, the refrigerant and lubricating oil are easily separated, but the oil film formed on the heat transfer surface and the inner surface of the unit will reduce the heat transfer coefficient. Refrigerant and metal non-metal halogenated hydrocarbon water, will be hydrolyzed to form acidic substances, corrosion of metal. Therefore, the mixture of the refrigerant containing water and the lubricating oil dissolves copper and forms a copper plating phenomenon. Copper plating will affect the sealing condition of moving parts, damage the bearing and then affect the overall sealing performance of the negative pressure unit, so that the outside air enters the unit, resulting in abnormal conditions such as excessive condensing pressure; at the same time, halogenated hydrocarbon is also a good organic solvent, Can be made into polymer materials (such as natural rubber, resin, etc.) soften, expand, or generate bubbles.

(2) Lubricating oil

Lubricating oil, also known as refrigerating machine oil, lubricates all the moving parts of the compressor, which not only reduces friction and wear, but also plays a cooling role, keeping the moving parts at a lower temperature and high efficiency. The effect of lubricating oil on refrigerant performance is mainly manifested in the following aspects. (1) Thermal performance: After the oil is dissolved in the refrigerant, the thermal performance of the refrigerant decreases. Under a certain condensing pressure, the pressure difference between the evaporator and the condenser becomes larger, and the power consumption of the compressor becomes larger. Generally, for every 1 decrease in evaporation temperature, the power consumption will increase by 4%. In addition, lubricating oil carries refrigerant into the oil system, which will reduce the circulation of refrigerant in the system, resulting in a decrease in refrigeration capacity. (2) Heat transfer effect The lubricating oil dissolves in the refrigerant, which will reduce the heat transfer coefficient. Pure refrigerant in the evaporation and condensation process is boiling heat transfer and condensation heat transfer. The existence of oil will form an oil film on the heat transfer surface, and the heat transfer mode will change from boiling heat transfer and condensation heat transfer to heat transfer mode. The thermal conductivity of oil is much smaller than the boiling heat transfer coefficient and the condensation heat transfer coefficient, which is obvious when the oil content is 6%. After the refrigerant enters the oil system, friction heat is generated at the lubrication point. The refrigerant in the oil flashes due to its low boiling point, resulting in poor local cooling and excessive heat generation. Flash evaporation may occur in the thrust bearing, forming an axial balance The change of thrust causes mechanical axial pulsation and affects sealing and carrying capacity.

(3) non-condensable gas

　　centrifugal chillerThe unit with R11 and R123 as refrigerant is a negative pressure unit, and its compressor inlet is in a vacuum state. When the unit is in operation, maintenance or shutdown, water, air or non-condensable gas will inevitably penetrate into the unit. If it is not eliminated in time, the pressure at the top of the condenser will rise sharply, resulting in a decrease in cooling capacity and an increase in power consumption.

(4) Cooling water and cold water

　　centrifugal chillerThe general cooling tower water system is an open cycle, and the water is in constant contact with the atmosphere for heat and mass transfer. The loss of CO2 in the circulating water, CaCO3 crystallization precipitation. At the same time, dust in the atmosphere mixed with water to form dirt, so that the condenser heat transfer tube wall fouling, resulting in the condenser heat transfer capacity decreased, the unit power consumption increased. Relatively speaking, the cold water system is in a closed circulation state and does not directly contact the outside world, but it will also form scale on the wall of the evaporation tube.