Energy-saving control and adjustment of main operating parameters of refrigeration system

Update:24 May 2021
Summary:

In the actual operation of refrigeration equipment and […]

In the actual operation of refrigeration equipment and system engineering, not only should the refrigeration system be adjusted to a reasonable operating range, meet the requirements of the refrigeration process, and maintain its safe and normal operation, but also should and can further adjust the refrigeration system to the best operating state , To achieve the purpose of high-efficiency and energy-saving operation, and improve the energy-saving level of refrigeration equipment operation.
1. Evaporating temperature and evaporation pressure
  In the design of refrigeration equipment, increasing the evaporation temperature will reduce the compression ratio and power consumption of the refrigeration system, which is very beneficial to energy saving. The problem is that the evaporation temperature depends on the object to be cooled, and the adjustment of the evaporation temperature must be based on the premise that the refrigeration process requirements of the object to be cooled are not affected. However, in the operation and adjustment of the refrigeration device, attention should be paid to observation, and corresponding measures should be taken in time, such as proper defrosting, proper increase of liquid supply, oil discharge and dirt cleaning of the evaporator, effective energy adjustment of the compressor, etc. The evaporating temperature is stabilized at the design temperature, and it is very necessary to avoid the evaporating temperature from being too low unnecessarily.
  From the perspective of energy saving, it is economical and reasonable to increase the evaporation temperature appropriately. The calculation shows that when the storage temperature of -25℃ is used instead of the storage temperature of -30℃, the increase of the evaporation temperature will save electricity by 9.8%. Therefore, for short storage periods and low quality requirements for low temperature, the evaporation temperature can be appropriately increased to achieve the effect of energy saving. In addition, the general refrigeration equipment is designed according to the full load, but the actual operation time at full load is not long, most of the time is operated under the condition of less than the design load. When the part load, that is, the cold consumption is reduced, increasing the evaporation temperature can reduce the heat transfer temperature difference of the evaporator to achieve the same cooling effect.
For example, when the condensing temperature is 38℃, the evaporation temperature of the refrigeration system is -33℃; when the cold consumption is reduced to 50% of the original design, the heat transfer temperature difference of the original evaporator is reduced from 10℃ to 5℃, and the warehouse still uses the original The equipment keeps the storage temperature at -23°C, but the evaporation temperature is increased to -28°C at this time. The calculation shows that the energy saving effect can reach 15%.
2. Condensing temperature and condensing pressure
   Too high condensing temperature will cause the compressor discharge pressure to be too high and the discharge temperature to rise, which is very unfavorable to the safe operation of the compressor and is prone to accidents; at the same time, it reduces the efficiency of the refrigeration device and increases the energy consumption. From the perspective of energy saving, a higher condensing temperature should be appropriately selected when designing refrigeration equipment, that is, a larger condensing heat exchange area should be configured to achieve the purpose of actual energy-saving operation.
  From the perspective of operation adjustment, the refrigeration equipment should be controlled to operate at the lowest possible condensing temperature to improve the refrigeration efficiency and reduce operating costs. Condensing temperature is determined by various factors that affect the heat transfer efficiency of the condenser, such as the temperature, flow rate, flow rate of the cooling medium, condensing area, compressor displacement, air humidity, oil pollution, and scale.
  To make the condensing temperature as low as possible, we mainly start with two aspects:
  Keep the heat exchange area clean, eliminate the factors that affect heat exchange, that is, timely descaling, draining oil, and removing non-condensable gas
  Control the flow and velocity of the cooling medium to ensure that the cooling medium flows evenly through the heat exchange area; also pay special attention to the uniformity of the cooling water distribution in the condenser
   When the system equipment is running under partial load, special attention should be paid to the corresponding control and adjustment of the pump or fan load of the condensing system at the same time to avoid ineffective heat exchange power consumption. Because the total energy consumption of refrigeration equipment includes the energy consumption of the compressor and the energy consumption of the heat exchanger, pumps and fans.
3. liquid subcooling degree and inhalation superheating degree
   At a certain condensing temperature and evaporating temperature, the method of subcooling the refrigerant liquid before throttling can achieve the purpose of reducing the dryness of the refrigerant after throttling and increasing the cooling capacity of the refrigeration cycle.
Under normal circumstances, it is assumed that the temperature of the outlet water of the condenser is 3~5K lower than the condensing temperature, and the temperature rise of the cooling water in the condenser is 3~8K, so the inlet temperature of the cooling water is 5~13K lower than the condensing temperature, which is enough to make the cooling The outlet temperature of the agent reaches a certain degree of subcooling. In a horizontal shell and tube condenser, if the condensed liquid is not immediately discharged from the bottom of the condenser, but accumulates inside the condenser, this part of the liquid will continue to transfer heat to the cooling water in the tube and the surrounding medium. A certain degree of supercooling can be obtained. Obtaining the degree of subcooling does not increase the power consumption of the compressor, which means that the degree of subcooling will inevitably lead to an increase in the refrigeration coefficient of the equipment system and improve the economic efficiency of refrigeration equipment operation.
  Research calculations show that under the conditions of condensation temperature of 40°C and evaporation temperature of 5°C, 5K subcooling will increase the cooling capacity of the R22 refrigeration equipment by 4.27%, the input power will not change, and the COP value will increase by 4.27%.
   suction superheat not only effectively improves the volumetric efficiency of the compressor and the refrigeration capacity per unit mass of the system, it also inevitably increases the specific volume of the compressor suction, the discharge temperature, the power consumption and the heat load of the condenser. Although its combined effect will increase the cooling capacity with the increase of superheat, the cooling coefficient of the equipment system will decrease accordingly.
   Although this seems to be contradictory to the energy-saving operation of the equipment, in refrigeration equipment, especially in low-temperature refrigeration equipment, too low suction temperature will cause serious frosting of the compressor and deterioration of lubrication conditions. Under the wet stroke, the volumetric efficiency of the compressor operation is greatly reduced, and the indicated efficiency, mechanical efficiency and electrical efficiency will all be reduced, so that the COP value of the compressor will be more drastically reduced. What's more, the wet stroke is very easy to produce liquid hammer and cause fatal mechanical damage to the compressor.
In addition, make full use of the night heat load reduction caused by the temperature difference between day and night, the reduction of condensation temperature and the low grid at night, so as to make the refrigeration equipment run at night as much as possible; optimize the design of uniform air distribution in the refrigeration environment; adopt multi-stage segmented cooling The process enables the refrigeration equipment to adopt different operating parameters in each time period, reduce the heat transfer temperature difference, and use the principle of large refrigeration coefficient during continuous temperature adjustment to achieve energy saving in the actual refrigeration and freezing process without increasing investment. It also has obvious economic benefits.

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