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In recent years, with the international retail giants such as Walmart and other large investments in the Chinese market, as well as the rise of domestic retailers, China's retail industry is gradually transitioning from traditional department stores to supermarket chains and hypermarkets.
In the entire cold chain, the supermarket refrigeration system is the most important part, because almost all of the cold chain terminal retail is done by supermarkets. For the supermarket itself, the refrigeration system accounts for a large proportion of its operating costs. For example, the refrigeration system accounts for 40% of the total initial investment in supermarket equipment and consumes about 50% of the total energy per year. Therefore, the perfection of the supermarket refrigeration system has a profound impact on the quality of life and the social economy of the public.
Such an emerging industry presents a very urgent and challenging new topic for refrigeration technology: how to provide a stable, reliable and energy-efficient food freezing P refrigeration system for large supermarkets.
Supermarket refrigeration system brief description Compared with the comfort air conditioning system for the human body, the supermarket refrigeration system has its own distinct characteristics: (1) low evaporation temperature and high oil viscosity; (2) year-round operation, large peak-to-valley difference, part The load is obvious; (3) The frozen P frozen object is a perishable and deteriorated fresh food, and the temperature and humidity control requirements are strict; (4) The evaporator needs to be defrosted, and the system load is large after the defrost cycle.
Existing supermarket refrigeration technology has many small supermarkets, such as various community convenience stores, mainly using one-to-one system. That is, each compressed condensing unit drives a freezer or cold storage with a horsepower of 0.5 hp to 7.5 hp. In recent years, with the advent of medium-sized supermarkets, the design concept of one-to-one has obviously caused the equipment cost of supermarkets to rise sharply with the increase of total cooling demand, thus forming a design with more than one drag, that is, a single unit corresponds to more A freezer or cold storage, between 10 and 20 horsepower. The typical performance of these two programs is that a supermarket must have 515 single units.
The biggest advantage of the stand-alone system is: (1) no need for a special machine room, the unit can be placed close to the outside of the supermarket wall closest to the connected evaporator; (2) the system does not require high controllers, for the compression condensing unit, only A simple high and low pressure switch is required to control its opening and closing; (3) the systems are independent of each other, even if the leakage of individual system piping does not affect the normal operation of other systems.
However, with the continuous expansion of the scale of the supermarket, the above-mentioned single-machine system has become increasingly unable to adapt to new demands, which are mainly manifested in: (1) high cost. At present, the construction of new hypermarkets is generally around 800,15,000 square meters, the total refrigeration system needs about 200,300 horsepower, which corresponds to 1030 stand-alone systems; (2) the peaking ability is weak, and the system efficiency is low under partial load. In order to minimize costs and ensure rapid cooling after the end of the defrost cycle, only single horsepower can be increased. In non-business hours, especially in the cold winter, the freezing load is low, resulting in poor system matching performance, low efficiency of the whole machine, and frequent start and stop of the compressor.
For the new technology of large supermarkets, the system principle has different characteristics compared with convenience stores and large supermarkets: (1) the demand for cold capacity is very large, and the partial load is more obvious; (2) the mall is required to store pipes and refrigerators. The layout of the cold storage is beautiful; (3) the independent machine room can be provided, and the machine room is far away from the freezer, usually around 100 meters.
In response to this feature, the traditional extension system can be merged into a large central refrigeration unit parallel compressor unit (commonly known as Rack in foreign industries), that is, multiple compressors are connected in parallel on a common rack, sharing suction and exhaust headers, condensation Components such as a reservoir and a reservoir provide refrigerant to the evaporation ends of the entire supermarket.
Except for the condenser and evaporator connected by the dotted line, the components and pipelines represented by the solid line are all located on the parallel unit. The system flow is as follows: the refrigerant superheated steam flows from the return gas main pipe into the common gas-liquid separator, then enters the return gas header and uniformly passes through each of the return air filters into each compressor, and the exhaust gas of the compressor enters the discharge through the damping pipe The gas header flows into the common oil separator. 99% of the oil in the exhaust mixture is separated and falls to the bottom of the oil separator and evenly distributed to each compressor via the oil return system, while the exhaust gas with very little oil flows back into the condenser located outside and flows back. Reservoir. The liquid discharged from the bottom of the accumulator enters the liquid supply header and flows through the solenoid valve on each branch to enter the corresponding evaporator (display cabinet in the store, cold storage ceiling fan), and evaporates and evaporates pressure adjustment The valve enters the suction manifold to complete the closed loop.
It is pointed out that the parallel units designed and manufactured according to the principles of this paper have been successfully applied to many large supermarket chains such as Walmart, Carrefour and Auchan.
Compared with the air conditioning system, the oil system is more difficult to return to the refrigeration system.
This is because: (1) the evaporation temperature of the refrigeration system is low, and the viscosity coefficient of the pure oil rises sharply with the decrease of temperature. On the low pressure side of the system, the viscosity of the mixture of superheated refrigerant and lubricating oil reaches its maximum at its evaporation pressure. (2) The refrigeration system runs for part of the load for most of the time. (3) An additional defrost cycle is required, which means that the compressor energy is increased, and in turn, the partial load phenomenon is intensified; (4) The pipeline system of the large supermarket is longer.
It is therefore necessary to increase the oil separation system to trap the lubricating oil in the compressor as much as possible. The oil separated by the oil separator will flow into the oil reservoir, and the bottom outlet of the oil reservoir is connected to the crankcase oil level regulator of each compressor. When a compressor oil level drops, the float in the oil level regulator will automatically open the internal valve port, thereby automatically replenishing the crankcase to the crankcase until the oil level rises to the predetermined position, and the float will automatically close the valve port.
The actual use proves that in a 100 hp cryogenic system, continuous operation for one year, the parallel unit refrigeration system using the oil separation device only needs to add about 12 liters of lubricating oil during the initial commissioning cycle, and the system without the device is used. In the operation of the system, it is necessary to continuously refuel, the total amount can reach 4,080 liters, and eventually a large amount of lubricating oil accumulated in the system suddenly returns at a certain moment, resulting in damage to the compressor oil.
The special control of the end equipment is different for the storage temperature and relative humidity required for different types of food. It can be seen that except for a very small number of well-packaged meat or dried fruits, the preservation of most commodities requires a relatively high relative humidity of about 85%.
Product type storage temperature (e) Preservation relative humidity (%) Preservation period Apple pineapple (cooked) January lettuce eggplant butter January frozen fish frozen beef ice cream freezer P The relative humidity of the internal storage space is mainly controlled by the inside and outside of the evaporator The difference in thermal temperature, the greater the heat transfer temperature difference, the lower the relative humidity. After adopting the parallel unit technology, the common suction pressure must be the evaporation pressure corresponding to the end equipment with the lowest evaporation temperature in the system. If these end evaporators are directly connected to the parallel unit, the heat transfer temperature difference of some refrigerators will be caused. Large, relative humidity will be lower than the required value, thus affecting the preservation quality of the product.
An ideal measure to solve this problem is to add an evaporating pressure regulating valve EPRV to the return branch of the branch above the common back pressure. These EPRVs will automatically adjust the evaporation pressure in the branch system, thus The heat transfer temperature difference is maintained at a certain level. Obviously, increasing EPRV will result in some additional resistance drop, resulting in a reduction in system efficiency. However, it has been proved that as long as the highest evaporation temperature in all branches does not exceed the minimum 5K, there is no significant impact on system efficiency.
The performance advantages of the new technology, energy-efficient parallel units can provide more energy regulation stages, even a parallel system of three compressors, can provide up to 7 levels of energy regulation. Therefore, the parallel unit can make the cold output of the compressor more smoothly and dynamically match the actual supermarket load, thereby improving the overall efficiency of the system and thus reducing the power consumption. This is even more significant for the large supermarket systems that are partially loaded during the year.
In addition, such a system also reduces the starting current and reduces the impact on the grid.
Rasmasson has conducted a comparison test between the single unit and the parallel unit for the same system. The results show that the parallel unit system energy-saving compressor compressor energy combination energy adjustment level 5.2 reduces the temperature fluctuation of the cabinet P library and improves the food preservation quality food. Storage requires a constant temperature. If the temperature fluctuates beyond a certain level, even if the food temperature does not exceed the upper limit of the storage temperature for a long time, the food will deteriorate quickly. For example, the storage temperature fluctuation of fresh eggs is required to be controlled within 0.5e. After the compressor matches the dynamic load in real time, the evaporation pressure in the evaporator is stable, which makes the temperature control of the freezer P cold storage more stable, which significantly reduces the temperature pulsation, thereby improving the quality of food preservation.
In addition, after the end of the defrost cycle, the heat load in the refrigerated P freezer space is large and the temperature is high. Since the parallel unit has a larger margin, the temperature can be quickly lowered to a prescribed level. Figure 4 shows that a defrost cycle takes about 15 minutes, then delays for two minutes (letting the water droplets on the heat exchanger coil fall enough to flow off the water tray), and it takes only 5 minutes to enter the refrigeration cycle again. Drop to the required requirements.
The initial investment cost reduction The initial investment cost reduction includes the following parts: (1) Equipment cost reduction. Multiple compressors share an oil separator, a gas-liquid separator, a condenser, a reservoir, a control system, and a frame. (2) The installation cost is reduced. The parallel unit technology provides a new concept of modular installation for the supermarket refrigeration system, which reduces the complexity of the system, reduces the copper pipe consumption of the installation project, saves the on-site construction cycle and reduces the on-site cost. (3) It takes up a smaller room area, leaving more space for the store. Table 3 is an analysis of the initial investment of a 12,000 square meter supermarket. It can be seen that after adopting the new technology, the comprehensive initial investment can be reduced by 5.4. The reliability is improved because the frequent start and stop of the compressor is avoided, thereby greatly extending the life of the motor. In addition, even if a compressor fails to operate, it does not affect the operation of other compressors in the system, ensuring the continuous cooling effect of the system.
Equipment (compressor + condenser) installation materials (copper pipe + insulation material + wire) installation manual (working hours) single-machine technology parallel unit technology 5.5 remote monitoring, easy maintenance makes centralized control possible. Through the modem and telephone line, the supermarket chain can monitor and control the operation status of the refrigeration system in the hypermarkets scattered throughout the country in real time. It can analyze and evaluate the system efficiency and safety on a regular basis, and discover and solve system hidden dangers in advance. . While improving maintenance efficiency, it also reduces the high cost due to business trips.
Conclusion Large supermarket chains are rapidly becoming the main form of China's retail industry, but the existing refrigeration systems are no longer able to adapt to their new features. In response to this phenomenon, a new technology parallel compressor unit technology was discussed, and its advancement was proved both theoretically and practically.
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