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I. Reducing the heat load of the cold storage
The storage temperature of low-temperature cold storage generally ranges around -25°C, while the daytime outdoor temperature in summer is typically above 30°C. This means the temperature difference across the two sides of the enclosure structure can reach approximately 60°C. Combined with solar radiation heat during the day, the heat load generated by heat transfer from the walls and ceiling into the storage is quite significant, making it a key component of the total heat load inside the storage. Enhancing the thermal insulation performance of the enclosure structure primarily involves thickening the insulation layer, using high-quality insulation materials, and adopting reasonable design schemes.
Of course, thickening the insulation layer of the enclosure structure will increase one-time investment costs. However, compared to the reduction in the cold storage’s ongoing operating expenses, this approach remains reasonable from both economic and technical management perspectives.
Two common methods are used to reduce heat absorption on the outer surface:
First, the outer surface of the walls should preferably be painted white or light-colored to enhance reflectivity. Under intense summer sunlight, the temperature of a white surface can be 25°C to 30°C lower than that of a black surface.
Second, installing sunshade enclosures or ventilated interlayers on the outer wall surface. Although this method is more complex to construct and less commonly used in practice, it involves placing an outer enclosure structure at intervals from the insulation wall to form an interlayer. Ventilation openings are then installed at the top and bottom of the interlayer to create natural ventilation, which carries away the solar radiation heat absorbed by the outer enclosure.
Cold storage doors
Since cold storage facilities require frequent entry and exit of personnel and loading/unloading of goods, the storage doors need to be opened and closed regularly. If insulation is not properly implemented at the door, the infiltration of high-temperature air from outside and the heat brought in by personnel will generate a certain heat load. Therefore, the design of cold storage doors is also of significant importance.
Construction of enclosed platforms
By using evaporative coolers for cooling, the temperature can reach 1°C to 10°C. Equipped with sliding refrigerated doors and flexible sealing joints, refrigerated trucks can directly dock at the platform to perform door-to-door loading/unloading operations, ensuring that the entry and exit of goods are largely unaffected by external temperatures. For small cold storage facilities, a vestibule can be built at the entrance.
Electric refrigerated doors (with cold air curtains added)
In the early days, single-door speeds ranged from 0.3 to 0.6 m/s. Currently, high-speed electric refrigerated doors can open at up to 1 m/s, and double-door refrigerated doors can open at 2 m/s. To avoid hazards, the closing speed is controlled to be approximately half of the opening speed. A sensor-based automatic switch is installed in front of the door. These devices aim to shorten the door opening and closing time, improve loading/unloading efficiency, and reduce the time operators spend waiting at the door.
II. Improving the working efficiency of the refrigeration system
Use compressors with economizers
Screw compressors can perform stepless adjustment within a 20% to 100% energy range to adapt to load changes. It is estimated that a screw unit with an economizer and a cooling capacity of 233 kW, operating 4,000 hours per year, can save 100,000 kWh of electricity annually.
Heat exchange equipment
Preferably use direct evaporative condensers instead of water-cooled shell-and-tube condensers.
This not only eliminates the power consumption of water pumps but also saves on investments in cooling towers and water tanks. Additionally, the water flow rate of direct evaporative condensers is only 1/10 that of water-cooled systems, significantly conserving water resources.
Preferably use evaporative fans instead of evaporator coils at the evaporator end inside the cold storage
This approach not only saves materials and offers higher heat exchange efficiency but also allows variable-speed evaporative fans to adjust air volume according to changes in the storage load. For example, when goods are first stored, the fans can run at full speed to quickly lower the cargo temperature; once the goods reach the preset temperature, the fan speed is reduced, avoiding energy waste and mechanical wear caused by frequent start-ups and shutdowns.
Air separator: When non-condensable gases are present in the refrigeration system, the discharge temperature rises due to increased condensing pressure. Data shows that if the partial pressure of mixed air in the refrigeration system reaches 0.2 MPa, the system’s power consumption will increase by 18%, and its refrigeration capacity will decrease by 8%.
Oil separator: Oil films on the inner wall of the evaporator significantly reduce the heat exchange efficiency of the evaporator. When a 0.1 mm-thick oil film forms inside the evaporator tubes, the evaporating temperature must drop by 2.5°C to maintain the set temperature requirement, leading to an 11% increase in power consumption.
The thermal resistance of scale is higher than that of the condenser tube wall, which impairs heat transfer efficiency and elevates the condensing pressure. When 1.5 mm of scale forms on the inner wall of the condenser’s water pipes, the condensing temperature rises by 2.8°C compared to the original temperature, increasing power consumption by 9.7%. Additionally, scale increases the flow resistance of cooling water, raising the energy consumption of the water pump.
Methods to prevent and remove scale include electromagnetic water conditioners (for scale prevention and removal), chemical acid pickling, and mechanical descaling.
III. Defrosting of Evaporative Equipment
When the frost layer thickness exceeds 10 mm, its heat transfer efficiency decreases by approximately 30% or more, highlighting the significant impact of frost on heat transfer. Measurements show that when the temperature difference between the inner and outer walls of the tube is 10°C and the storage temperature is -18°C, the heat transfer coefficient (K value) of the coil drops to about 70% of its original value after one month of operation. This is especially true for finned tubes in evaporative fans: frost formation not only increases thermal resistance but also raises air flow resistance, potentially leading to a complete halt in airflow in severe cases.
Hot gas defrosting is preferred over electric heating defrosting to reduce power consumption. The waste heat from the compressor discharge can serve as a defrosting heat source. The temperature of defrost return water is generally 7–10°C lower than the condenser inlet water temperature; after treatment, this water can be reused as condenser cooling water to lower the condensing temperature.
IV. Evaporating Temperature Regulation
Narrowing the temperature difference between the evaporating temperature and the storage room temperature allows the evaporating temperature to increase accordingly. With the condensing temperature remaining constant, this effectively enhances the refrigeration capacity of the compressor. In other words, to achieve the same refrigeration effect, less electrical energy is required. Estimates show that for every 1°C decrease in evaporating temperature, power consumption increases by 2–3%. Additionally, reducing this temperature difference is highly beneficial for minimizing weight loss due to moisture evaporation in stored food products.
V. Other Energy-Saving Approaches
Using electricity during nighttime "off-peak" hours not only reduces electricity costs but also balances the power output of power plant generators, minimizing large daily fluctuations in power demand—a macro-level energy-saving measure. This practice is particularly valuable for quick-freezing and ice-making operations in cold storage.
Another option is ice storage cooling technology: ice produced at night can provide partial cooling during the day, reducing the system’s required power capacity to some extent.
VI. Automatic Control of Other Equipment
The combined energy-saving effect of these six measures can reach 15–34%.
Improving the cold chain, including precooling products, is also critical. For quick-frozen food, precooling before storage reduces freezing time by approximately 1% for every 1°C decrease in temperature during precooling.
Common precooling methods include air precooling, vacuum precooling, and cold water precooling.
I. Reducing the heat load of the cold storage
The storage temperature of low-temperature cold storage generally ranges around -25°C, while the daytime outdoor temperature in summer is typically above 30°C. This means the temperature difference across the two sides of the enclosure structure can reach approximately 60°C. Combined with solar radiation heat during the day, the heat load generated by heat transfer from the walls and ceiling into the storage is quite significant, making it a key component of the total heat load inside the storage. Enhancing the thermal insulation performance of the enclosure structure primarily involves thickening the insulation layer, using high-quality insulation materials, and adopting reasonable design schemes.
Of course, thickening the insulation layer of the enclosure structure will increase one-time investment costs. However, compared to the reduction in the cold storage’s ongoing operating expenses, this approach remains reasonable from both economic and technical management perspectives.
Two common methods are used to reduce heat absorption on the outer surface:
First, the outer surface of the walls should preferably be painted white or light-colored to enhance reflectivity. Under intense summer sunlight, the temperature of a white surface can be 25°C to 30°C lower than that of a black surface.
Second, installing sunshade enclosures or ventilated interlayers on the outer wall surface. Although this method is more complex to construct and less commonly used in practice, it involves placing an outer enclosure structure at intervals from the insulation wall to form an interlayer. Ventilation openings are then installed at the top and bottom of the interlayer to create natural ventilation, which carries away the solar radiation heat absorbed by the outer enclosure.
Cold storage doors
Since cold storage facilities require frequent entry and exit of personnel and loading/unloading of goods, the storage doors need to be opened and closed regularly. If insulation is not properly implemented at the door, the infiltration of high-temperature air from outside and the heat brought in by personnel will generate a certain heat load. Therefore, the design of cold storage doors is also of significant importance.
Construction of enclosed platforms
By using evaporative coolers for cooling, the temperature can reach 1°C to 10°C. Equipped with sliding refrigerated doors and flexible sealing joints, refrigerated trucks can directly dock at the platform to perform door-to-door loading/unloading operations, ensuring that the entry and exit of goods are largely unaffected by external temperatures. For small cold storage facilities, a vestibule can be built at the entrance.
Electric refrigerated doors (with cold air curtains added)
In the early days, single-door speeds ranged from 0.3 to 0.6 m/s. Currently, high-speed electric refrigerated doors can open at up to 1 m/s, and double-door refrigerated doors can open at 2 m/s. To avoid hazards, the closing speed is controlled to be approximately half of the opening speed. A sensor-based automatic switch is installed in front of the door. These devices aim to shorten the door opening and closing time, improve loading/unloading efficiency, and reduce the time operators spend waiting at the door.
II. Improving the working efficiency of the refrigeration system
Use compressors with economizers
Screw compressors can perform stepless adjustment within a 20% to 100% energy range to adapt to load changes. It is estimated that a screw unit with an economizer and a cooling capacity of 233 kW, operating 4,000 hours per year, can save 100,000 kWh of electricity annually.
Heat exchange equipment
Preferably use direct evaporative condensers instead of water-cooled shell-and-tube condensers.
This not only eliminates the power consumption of water pumps but also saves on investments in cooling towers and water tanks. Additionally, the water flow rate of direct evaporative condensers is only 1/10 that of water-cooled systems, significantly conserving water resources.
Preferably use evaporative fans instead of evaporator coils at the evaporator end inside the cold storage
This approach not only saves materials and offers higher heat exchange efficiency but also allows variable-speed evaporative fans to adjust air volume according to changes in the storage load. For example, when goods are first stored, the fans can run at full speed to quickly lower the cargo temperature; once the goods reach the preset temperature, the fan speed is reduced, avoiding energy waste and mechanical wear caused by frequent start-ups and shutdowns.
Air separator: When non-condensable gases are present in the refrigeration system, the discharge temperature rises due to increased condensing pressure. Data shows that if the partial pressure of mixed air in the refrigeration system reaches 0.2 MPa, the system’s power consumption will increase by 18%, and its refrigeration capacity will decrease by 8%.
Oil separator: Oil films on the inner wall of the evaporator significantly reduce the heat exchange efficiency of the evaporator. When a 0.1 mm-thick oil film forms inside the evaporator tubes, the evaporating temperature must drop by 2.5°C to maintain the set temperature requirement, leading to an 11% increase in power consumption.
The thermal resistance of scale is higher than that of the condenser tube wall, which impairs heat transfer efficiency and elevates the condensing pressure. When 1.5 mm of scale forms on the inner wall of the condenser’s water pipes, the condensing temperature rises by 2.8°C compared to the original temperature, increasing power consumption by 9.7%. Additionally, scale increases the flow resistance of cooling water, raising the energy consumption of the water pump.
Methods to prevent and remove scale include electromagnetic water conditioners (for scale prevention and removal), chemical acid pickling, and mechanical descaling.
III. Defrosting of Evaporative Equipment
When the frost layer thickness exceeds 10 mm, its heat transfer efficiency decreases by approximately 30% or more, highlighting the significant impact of frost on heat transfer. Measurements show that when the temperature difference between the inner and outer walls of the tube is 10°C and the storage temperature is -18°C, the heat transfer coefficient (K value) of the coil drops to about 70% of its original value after one month of operation. This is especially true for finned tubes in evaporative fans: frost formation not only increases thermal resistance but also raises air flow resistance, potentially leading to a complete halt in airflow in severe cases.
Hot gas defrosting is preferred over electric heating defrosting to reduce power consumption. The waste heat from the compressor discharge can serve as a defrosting heat source. The temperature of defrost return water is generally 7–10°C lower than the condenser inlet water temperature; after treatment, this water can be reused as condenser cooling water to lower the condensing temperature.
IV. Evaporating Temperature Regulation
Narrowing the temperature difference between the evaporating temperature and the storage room temperature allows the evaporating temperature to increase accordingly. With the condensing temperature remaining constant, this effectively enhances the refrigeration capacity of the compressor. In other words, to achieve the same refrigeration effect, less electrical energy is required. Estimates show that for every 1°C decrease in evaporating temperature, power consumption increases by 2–3%. Additionally, reducing this temperature difference is highly beneficial for minimizing weight loss due to moisture evaporation in stored food products.
V. Other Energy-Saving Approaches
Using electricity during nighttime "off-peak" hours not only reduces electricity costs but also balances the power output of power plant generators, minimizing large daily fluctuations in power demand—a macro-level energy-saving measure. This practice is particularly valuable for quick-freezing and ice-making operations in cold storage.
Another option is ice storage cooling technology: ice produced at night can provide partial cooling during the day, reducing the system’s required power capacity to some extent.
VI. Automatic Control of Other Equipment
The combined energy-saving effect of these six measures can reach 15–34%.
Improving the cold chain, including precooling products, is also critical. For quick-frozen food, precooling before storage reduces freezing time by approximately 1% for every 1°C decrease in temperature during precooling.
Common precooling methods include air precooling, vacuum precooling, and cold water precooling.
