1. Reducing the heat load of cold storage
1. Envelope structure of cold storage
The storage temperature of the low-temperature cold storage is generally around -25°C, while the outdoor daytime temperature in summer is generally above 30°C, that is to say, the temperature difference between the two sides of the enclosure structure of the cold storage will be about 60°C. The high solar radiant heat makes the heat load formed by the heat transfer from the wall and ceiling to the warehouse considerable, which is an important part of the heat load in the whole warehouse. Enhancing the thermal insulation performance of the envelope structure is mainly through thickening the insulation layer, applying high-quality insulation layer, and applying reasonable design schemes.
2. Thickness of insulation layer
Of course, thickening the heat insulation layer of the envelope structure will increase the one-time investment cost, but compared with the reduction of the regular operating cost of the cold storage, it is more reasonable from an economic point of view or a technical management point of view.
Two methods are commonly used to reduce the heat absorption of the outer surface
The first is that the outer surface of the wall should be white or light-colored to enhance the reflection ability. Under strong sunlight in summer, the temperature of the white surface is 25°C to 30°C lower than that of the black surface;
The second is to make sunshade enclosure or ventilation interlayer on the surface of the outer wall. This method is more complicated in actual construction and less used. The method is to set up the outer enclosure structure at a distance from the insulation wall to form a sandwich, and set vents above and below the interlayer to form natural ventilation, which can take away the solar radiation heat absorbed by the outer enclosure.
3. Cold storage door
Because the cold storage often requires personnel to enter and exit, loading and unloading goods, the warehouse door needs to be opened and closed frequently. If the heat insulation work is not done at the door of the warehouse, a certain heat load will also be generated due to the infiltration of high-temperature air outside the warehouse and the heat of personnel. Therefore, the design of the cold storage door is also very meaningful.
4. Build a closed platform
Use air cooler to cool down, the temperature can reach 1℃~10℃, and it is equipped with sliding refrigerated door and soft sealing joint. Basically not affected by the external temperature. A small cold storage can build a door bucket at the entrance.
5. Electric refrigerated door (additional cold air curtain)
The early single leaf speed was 0.3~0.6m/s. At present, the opening speed of high-speed electric refrigerator doors has reached 1m/s, and the opening speed of double leaf refrigerator doors has reached 2m/s. In order to avoid danger, the closing speed is controlled at about half of the opening speed. A sensor automatic switch is installed in front of the door. These devices are designed to shorten the opening and closing time, improve loading and unloading efficiency, and reduce the operator’s dwell time.
6. Lighting in warehouse
Use high-efficiency lamps with low heat generation, low power and high brightness, such as sodium lamps. The efficiency of high pressure sodium lamps is 10 times that of ordinary incandescent lamps, while the energy consumption is only 1/10 of inefficient lamps. At present, new LEDs are used as lighting in some more advanced cold storages, with less heat generation and energy consumption.
2. Improve the working efficiency of the refrigeration system
1. Use a compressor with an economizer
The screw compressor can be adjusted steplessly within the energy range of 20~100% to suit the load change. It is estimated that a screw-type unit with an economizer with a cooling capacity of 233kW can save 100,000 kWh of electricity a year based on 4,000 hours of annual operation.
2. Heat exchange equipment
The direct evaporative condenser is preferred to replace the water-cooled shell-and-tube condenser.
This not only saves the power consumption of the water pump, but also saves the investment in cooling towers and pools. In addition, the direct evaporative condenser requires only 1/10 of the water flow rate of the water-cooled type, which can save a lot of water resources.
3. At the evaporator end of the cold storage, the cooling fan is preferred instead of the evaporating pipe
This not only saves materials, but also has high heat exchange efficiency, and if the cooling fan with stepless speed regulation is used, the air volume can be changed to adapt to the change of the load in the warehouse. The goods can run at full speed just after they are put into the warehouse, quickly reducing the temperature of the goods; after the goods reach the predetermined temperature, the speed is reduced, avoiding the power consumption and machine loss caused by frequent start and stop.
4. Treatment of impurities in heat exchange equipment
Air separator: When there is non-condensable gas in the refrigeration system, the discharge temperature will increase due to the increase of the condensation pressure. The data shows that when the refrigeration system is mixed with air, its partial pressure reaches 0.2MPa, the power consumption of the system will increase by 18%, and the cooling capacity will decrease by 8%.
Oil separator: The oil film on the inner wall of the evaporator will greatly affect the heat exchange efficiency of the evaporator. When there is a 0.1mm thick oil film in the evaporator tube, in order to maintain the set temperature requirement, the evaporation temperature will drop by 2.5°C, and the power consumption will increase by 11%.
5. Removal of scale in the condenser
The thermal resistance of scale is also higher than that of the tube wall of the heat exchanger, which will affect the heat transfer efficiency and increase the condensation pressure. When the water pipe wall in the condenser is scaled by 1.5mm, the condensation temperature will rise by 2.8°C compared with the original temperature, and the power consumption will increase by 9.7%. In addition, the scale will increase the flow resistance of the cooling water and increase the energy consumption of the water pump.
The methods of preventing and removing scale can be descaling and anti-scaling with electronic magnetic water device, chemical pickling descaling, mechanical descaling, etc.
3. Defrost of evaporation equipment
When the thickness of the frost layer is >10mm, the heat transfer efficiency drops by more than 30%, which shows that the frost layer has such a great influence on heat transfer. It has been determined that when the measured temperature difference between the inside and outside of the pipe wall is 10°C and the storage temperature is -18°C, the heat transfer coefficient K value is only about 70% of the original value after the pipe has been operated for one month, especially the ribs in the air cooler. When the sheet tube has a frost layer, not only the thermal resistance increases, but also the flow resistance of the air increases, and in severe cases, it will be sent out without wind.
It is preferred to use hot air defrosting instead of electric heating defrosting to reduce power consumption. Compressor exhaust heat can be used as a heat source for defrosting. The temperature of the frost return water is generally 7~10°C lower than the temperature of the condenser water. After treatment, it can be used as the cooling water of the condenser to reduce the condensation temperature.
4. Evaporation temperature adjustment
If the temperature difference between the evaporating temperature and the warehouse is reduced, the evaporating temperature can be increased accordingly. At this time, if the condensing temperature remains unchanged, it means that the cooling capacity of the refrigeration compressor is increased. It can also be said that the same cooling capacity is obtained In this case, the power consumption can be reduced. According to estimates, when the evaporation temperature is lowered by 1°C, the power consumption will be increased by 2~3%. In addition, reducing the temperature difference is also extremely beneficial to reducing the dry consumption of food stored in the warehouse.
Post time: Nov-18-2022