Indirect refrigeration system with refrigerant in cold storage
Abstract: This article will introduce how to reduce the amount of ammonia charged, and explain the safety, environmental protection and energy saving of the refrigerant-carrying indirect refrigeration system, focusing on equipment selection, energy-saving and economic optimization, defrosting system optimization, and refrigerant selection The analysis. Finally, the advantages and disadvantages of the refrigerant-carrying indirect refrigeration system relative to ammonia refrigeration are explained, for reference only.
Keywords: refrigerant carrier indirect refrigeration system ammonia
"Ammonia" as a low-cost refrigerant is widely used in cold storage and food quick freezing industries. However, ammonia is toxic and irritating. It will explode in case of open fire. If it leaks, it will cause serious personal injury and death. Established in a densely populated market area. In 2013, two large liquid ammonia leakage accidents occurred in Jilin Province and Shanghai, which caused widespread concern in the refrigeration industry. The refrigerant-carrying indirect refrigeration system effectively solves this problem.
1. Overview of indirect refrigeration system
1.1 Indirect refrigeration: The refrigerant and the refrigerant carry out low-temperature heat transfer, and the low-temperature refrigerant is pressurized by the pump to circulate in the cold storage room, so as to achieve the function of freezing and refrigeration. In locations where direct refrigerants are not easy to use or in special environments where direct refrigerants cannot be used, refrigerants are often used instead of direct refrigerants to cool the objects to be cooled (such as central air conditioning systems, ice storage systems, etc.).
1.2 Refrigerant: A substance that transfers the heat of the object to be cooled to the evaporating refrigerant in a refrigeration device that works by indirect cooling (such as salt water, glycol, water, glacial refrigerant).
2. Optimization of indirect refrigeration system configuration
2.1 Compressor
The excellent physical properties of refrigerant ammonia are the first choice, and our goal is to reduce the amount of ammonia charged, so we choose ammonia compressors. At present, most ammonia refrigeration compressors are piston and screw refrigeration compressors. The former appeared earlier and is widely used. Its advantages are: convenient use, reliable operation, mature management experience, large cooling capacity range, low power consumption per unit cooling capacity, simple processing and relatively low cost; its disadvantage is the size of the compressor Large, large metal consumption, large floor space, many vulnerable parts, high maintenance costs, single machine output cannot be too large, and energy stepless adjustment is difficult. The screw compressor has simple structure, small size, few vulnerable parts, light weight, low vibration, high volumetric efficiency, insensitive to wet compression, and can achieve stepless adjustment; its disadvantage is that the electricity consumption per unit of cold energy is higher than that of the piston type Slightly higher, the oil injection cooling makes the lubricating oil system complex and large, with high fuel consumption, high noise, and high screw machining accuracy requirements. Relatively speaking, because the screw compressor can easily control the exhaust temperature, it is better than the piston type in the ammonia refrigeration system. In the direct refrigeration system, most of the pipelines through which the refrigerant is transported to the evaporator for refrigeration are long, the pipeline loss is large, the pressure drop and superheat of the refrigerant are large, and the compressor gas transmission coefficient is reduced, which seriously affects the compressor efficiency. Adopt the glacier refrigerant indirect refrigeration system, centrally install refrigeration system equipment, the distance between the compressor and the evaporator is small, the refrigerant does not need to be transported remotely, the pressure drop and superheat are small, and the compressor efficiency is high.
2.2 Condenser
The use of high-efficiency and energy-saving evaporative condensers, the cooling tower and the shell-and-tube condenser are combined in the same equipment. The cooling tower not only uses the sensible heat of the condensed water, but also makes full use of the latent heat of the condensed water evaporating into steam; It comes with a sump, and the water is sent from the sump to the spray system through the spray circulating water pump, sprayed from the nozzle to the surface of the condensing coil, and the fan is activated to force the air through the falling water and the surface of the coil to evaporate part of the water. Take away the heat in the refrigerant, so that the refrigerant in the coil is condensed, and the remaining condensed water returns to the sump.
2.3 Heat exchanger
With the development of science and technology and the needs of production, the traditional shell-and-tube heat exchanger has not reached the requirements. This kind of heat exchanger not only has insufficient heat transfer capacity, but also has a large volume and is more laborious for transportation and maintenance. Therefore, it is recommended The plate heat exchanger has several advantages compared with: (1) Good heat transfer performance and small volume; (2) High heat transfer efficiency; (3) Large logarithmic average temperature difference; (4) Easy installation and operable Strong sex, no complicated structure.
2.4 Coil
The cold storage evaporator has many advantages such as high heat transfer coefficient, light weight, no rust, long service life, convenient installation, etc. The thermal conductivity of aluminum row tubes is 210W, which is 5 times that of copper tubes.
According to thermodynamics, the heat exchange area of the cold storage evaporator is equal to the heat exchange amount divided by the heat transfer coefficient, then the cold storage adopts aluminum tube as the evaporator, and the heat exchange area of the cold storage evaporator is 5 times smaller, which not only saves the cold storage The procurement cost of evaporation also greatly reduces the lifting and load-bearing workload of the cold storage evaporator.
The reduction in the heat transfer coefficient of the aluminum tube also reduces the total volume of the cold storage evaporator, thereby reducing the refrigerant charge of the cold storage project, thereby greatly reducing the one-time investment in the cold storage.
3, energy saving and economic optimization
One advantage of refrigerant carrier different from refrigerant is that it can store cold, which can not only offset the inevitable energy loss of indirect refrigeration, but also can efficiently use electricity for peak shifting and valley filling. During low electricity prices at night, the refrigerant temperature can be lowered for cold storage. . There are several advantages of using refrigerants for cold storage: (1) Balancing the peak and valley loads of the power grid, and slowing down the construction of power plants and power supply and distribution facilities, is energy-saving for the country, and it costs electricity and money for customers; (2) ) Can reduce the installed capacity of the refrigeration host. (3) Make full use of night temperature changes to increase the cold output of the refrigeration unit.
4, defrost system optimization
4.1 Coil
The coil type adopts high-temperature refrigerant for warm liquid defrosting. Contrary to the cold carrier system, the carrier refrigerant exchanges heat with the high-temperature refrigerant, and the high-temperature carrier refrigerant is pumped and pumped to the cold storage room by pump in reverse to perform warm liquid defrosting. Compared with electric defrosting, the temperature of warm liquid defrosting is about 10℃ lower, and energy consumption is reduced by about 9%. At the same time, the heat of high-temperature refrigeration is effectively used to save energy.
4.2 Air cooler
The air cooler uses water to flush the frost, using a water pump or a water spray device to spray water on the surface of the air cooler tube, so that the frost layer is melted by the heat of the water and removed. The water flushing operation is simple and the time is short, which is a very effective method.
5. Choice of refrigerant
5.1 brine
The freezing temperature of brine varies with concentration. The minimum freezing temperature of calcium chloride brine is -55℃; the minimum freezing temperature of sodium chloride brine is -21.2℃. Calcium chloride and sodium chloride solutions are very corrosive to equipment. The most common phenomena are formation of pitting corrosion, electrochemical corrosion of carbon steel equipment, destruction of oxide films, and formation of complexes. Over time, it will severely corrode equipment, pipes, and evaporators.
5.2 ethylene glycol
is stable in nature, miscible with water, the freezing point of its solution changes with concentration, and their aqueous solution is usually used as a refrigerant. Although the freezing point of ethylene glycol solution is low, reaching below -50°C, the viscosity of the solution rises very rapidly at low temperatures. Therefore, the general industrial application temperature is above -20°C. The aqueous solution is corrosive for three reasons:
1) ethylene glycol is oxidized into acidic substances;
2) electrochemical corrosion;
3) bubble collapse and cavitation corrosion.
5.3 Modified polyol (glacial refrigerant)
The modified polyol is added with anti-corrosion agents, solubilizers, water stabilizers, anti-fungal agents, etc., which completely solves some corrosion problems and the problem of high low-temperature viscosity. The temperature can reach -50℃, which can meet the temperature requirements of cold storage and freezer. M3 ultra-film anti-rust technology and Modify2000 composite modification technology are the core technologies of modified polyols, which are cutting-edge technologies in the field of refrigerants.
The refrigerant processed by Modify2000 composite modification technology has 5 major advantages.
① Strong fluidity at low temperature, reducing system energy consumption;
②Prevent scaling on the pipe wall and keep the system clean;
③Reduce condensed impurities and extend liquid life;
④Suppress liquid volatilization and reduce consumption;
⑤ Inhibit bacterial mildew and prevent biological contamination.
6.Schematic diagram of indirect cooling process with refrigerant
7. Summary
Carrier refrigerant refrigeration system has many advantages compared with refrigeration, reducing the amount of ammonia charged, reducing production safety risks, and effectively ensuring personal safety. Indirect refrigeration, only the carrier refrigerant enters the cold storage workshop, normal pressure circulation, simple maintenance and maintenance, industrial electric wave peak and valley cold storage, recovery of compressor heat defrost, and ammonia storage can realize continuous production transformation.
