Energy - saving Control and Adjustment of Main Operating Parameters of the Refrigeration System

In the actual operation of refrigeration equipment and system engineering, not only should the refrigeration system be adjusted to a reasonable operating range to meet the requirements of the refrigeration process and maintain its safe and normal operation, but it should also be possible to further adjust the refrigeration system to the optimal operating state, achieve the goal of efficient and energy-saving operation, and improve the energy-saving level of the operation of refrigeration equipment.

In the design of refrigeration equipment, increasing the evaporation temperature will reduce the compression ratio of the refrigeration system and decrease the power consumption, which is very beneficial for energy conservation. The problem is that the evaporation temperature depends on the object to be cooled, and adjusting the evaporation temperature must be based on the premise of not affecting the refrigeration process requirements of the object to be cooled. However, during the operation and adjustment of the refrigeration unit, attention should be paid to observation, and corresponding measures should be taken in a timely manner, such as appropriate defrosting, appropriately increasing the liquid supply, draining oil and removing dirt from the evaporator, and implementing effective energy adjustment for the compressor, etc. It is very necessary to keep the evaporation temperature stable at the designed temperature and avoid the unnecessary decrease of the evaporation temperature.
From the perspective of energy conservation, it is economically reasonable to appropriately increase the evaporation temperature. Calculations show that when the storage temperature of -25°C is used instead of -30°C, due to the increase in the evaporation temperature, the power consumption can be saved by 9.8%. Therefore, for situations where the storage period is short and the quality has a low requirement for low temperature, the evaporation temperature can be appropriately increased to achieve the effect of energy conservation. In addition, generally, refrigeration units are designed according to full load, but in fact, the time of operating at full load is not long, and most of the time, they operate under conditions of less than the designed load.

When the load is partial, that is, the cooling consumption decreases, 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°C, the evaporation temperature of the refrigeration system is -33°C; when the cooling consumption is reduced to 50% of the original design, the original heat transfer temperature difference of the evaporator is reduced from 10°C to 5°C. The warehouse still uses the original equipment to keep the warehouse temperature at -23°C, but at this time, the evaporation temperature is increased to -28°C. Calculations show that the energy-saving effect can reach 15%.

If the condensing temperature is too high, it will cause the discharge pressure of the compressor to be too high and the discharge temperature to rise, which is very unfavorable for the safe operation of the compressor and is likely to cause accidents. At the same time, it will reduce the efficiency of the refrigeration unit and increase the energy consumption. From the perspective of energy conservation, a relatively high condensing temperature should be appropriately selected during the design of refrigeration equipment, that is, a larger condensing heat transfer area should be configured to achieve the goal of actual energy-saving operation.

From the perspective of operation and adjustment, the refrigeration equipment should be controlled to operate at the lowest possible condensing temperature to improve the refrigeration efficiency and reduce the operating cost. The condensing temperature is determined by various factors that affect the heat transfer efficiency of the condenser, such as the temperature, flow rate, flow velocity of the cooling medium, the condensing area, the displacement of the compressor, as well as air humidity, oil stains, and water scale.
To keep the condensing temperature as low as possible, it is mainly necessary to start from two aspects:
Keep the heat transfer area clean and eliminate the factors that affect heat exchange, that is, remove scale, drain oil, and remove non-condensable gases in a timely manner. Control the flow rate and flow velocity of the cooling medium to ensure that the cooling medium flows evenly through the heat transfer area. Special attention should also be paid to the uniformity of the distribution of cooling water in the condenser. When the system equipment operates under partial load, special attention should be paid to simultaneously controlling and adjusting the load of the water pump or fan of the condensing system to avoid ineffective heat transfer power consumption. Because the total energy consumption of the refrigeration equipment includes the energy consumption of the compressor and the energy consumption of the water pump and fan of the heat exchanger.

Under a certain condensing temperature and evaporation temperature, the method of subcooling the refrigerant liquid before throttling can be used to achieve the purpose of reducing the dryness of the refrigerant after throttling and increasing the refrigeration capacity of the refrigeration cycle.
Generally, it is assumed that the outlet water temperature 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. Therefore, the inlet temperature of the cooling water is 5-13K lower than the condensing temperature, which is sufficient to make the outlet temperature of the refrigerant reach a certain degree of subcooling. In a horizontal shell-and-tube condenser, if the liquid after condensation 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, and a certain degree of subcooling can be obtained when it is discharged. The acquisition of the subcooling degree does not increase the power consumption of the compressor, which means that the subcooling degree will inevitably lead to an increase in the refrigeration coefficient of the equipment system and improve the economic efficiency of the operation of the refrigeration equipment.
Research and calculations show that under the working conditions of a condensing temperature of 40°C and an evaporation temperature of 5°C, a subcooling degree of 5K will increase the refrigeration capacity of the R22 refrigeration equipment by 4.27%, with no change in the input power and a 4.27% increase in the COP value.
While the suction superheat degree effectively improves the volumetric efficiency of the compressor and the refrigeration capacity per unit mass of the system, it inevitably increases the specific volume of the compressor's suction, the discharge temperature, the power consumption, and the heat load of the condenser. Although the comprehensive influence will still increase the refrigeration capacity to some extent with the increase of the superheat degree, the refrigeration coefficient of the equipment system will decrease accordingly.
Although this seems to be in conflict with the energy-saving operation of the equipment, in refrigeration equipment, especially in low-temperature refrigeration equipment, too low a suction temperature will cause serious frosting of the compressor and deteriorate the lubrication conditions. Under the wet stroke, the volumetric efficiency of the compressor operation will be greatly reduced, and the indicated efficiency, mechanical efficiency, and electrical efficiency will all decrease to some extent, resulting in a greater decrease in the COP value of the compressor. Moreover, the wet stroke is extremely likely to cause liquid slugging, which will cause fatal mechanical damage to the compressor.
In addition, make full use of the reduction of the nighttime heat load caused by the temperature difference between day and night, the decrease in the condensing temperature, and the nighttime low-power grid, and try to make the refrigeration equipment operate at night as much as possible; optimize the design of a uniform air flow organization in the refrigeration environment; adopt a multi-stage and segmented refrigeration process to enable the refrigeration equipment to use different operating parameters at different times, reduce the heat transfer temperature difference, and use the principle of a large refrigeration coefficient during continuous variable temperature adjustment to achieve energy conservation in the actual refrigeration and freezing process without increasing investment, which also has obvious economic benefits.