China Education Equipment Purchasing News: Constant temperature and humidity test box, also known as constant temperature and humidity test machine, constant temperature and humidity test box, constant temperature machine or constant temperature and humidity box, used to test the performance of materials in various environments The material is heat-resistant, cold-resistant, dry-resistant and moisture-resistant. It is suitable for testing the quality of electronics, electrical appliances, communications, instruments, vehicles, plastic products, metals, food, chemicals, building materials, medical, aerospace and other products. The cascade refrigeration cycle system is adopted when the refrigeration system of the constant temperature and humidity test chamber obtains a low temperature below -20 ℃. The following are the reasons for adopting a two-stage compression cascade refrigeration cycle for low temperature.

1. Restrictions on the thermophysical properties of refrigerants

At present, the medium-temperature refrigerant basically used in the single-stage refrigeration cycle of the constant temperature and humidity chamber is R404A, and its evaporation temperature at one atmospheric pressure is -46.5 ℃ (R22 / -40.7 ℃), but the heat transfer temperature difference of the air-cooled condenser is usually taken Around 10 ° C (the temperature difference between the evaporator and the inner box under forced air circulation), which means that only a low temperature of -36.5 ° C can be made in the box. Of course, by lowering the evaporation pressure of the compressor, the R404A can be cooled The minimum evaporation temperature of the agent is reduced to -50 ℃; so to obtain the low temperature of -50 ℃ and below, it is necessary to use a cascade refrigeration cycle of medium-temperature refrigerant and low-temperature refrigerant to prepare a low temperature of -50 ℃ to -80 ℃. Low-temperature refrigerants generally use R23 whose evaporation temperature at one atmospheric pressure is -81.7 ° C.

2. Limitation of single-stage compressed vapor refrigeration cycle pressure ratio

The minimum evaporation temperature of a single-stage vapor compression refrigerator is mainly determined by its condensing pressure and compression ratio. The condensing pressure of the refrigerant is determined by the type of refrigerant and the temperature of the environmental medium (such as air or water). Under normal circumstances , It is in the range of 0.7 ~ 1.8Mpa. The compression ratio is related to the condensing pressure and the evaporating pressure. When the condensing pressure is constant, as the evaporating temperature decreases, the evaporating pressure also decreases accordingly. When the exhaust temperature rises, the lubricating oil becomes thinner, which makes the lubrication conditions worse. In severe cases, carbonization and cylinder pulling may occur; on the other hand, an increase in the compression ratio will cause the compressor's air transmission coefficient to decrease, and the cooling capacity Reduced, the farther the actual compression process deviates from the isentropic process, the compressor power consumption increases, the cooling coefficient decreases, and the economy decreases. Some of the following effects will occur.

a. For any refrigerant, the lower the evaporating temperature, the lower the evaporating pressure. Too low an evaporating pressure may sometimes make it difficult for the compressor to suck in air, or allow outside air to enter the refrigeration system.

b. When the evaporating temperature is too low, some commonly used refrigerants have reached the freezing temperature, and the flow and circulation of the refrigerant cannot be achieved.

c. The evaporation pressure decreases, the specific volume of the refrigerant increases, the mass flow rate of the refrigerant decreases, and the cooling capacity greatly decreases. In order to obtain the required cooling capacity, the suction volume must be increased, making the compressor volume too large.

3. The limitation of compressor coil heat dissipation

When the single-stage compressor is working, it is about -35 ℃, because the compressor coil is swirled in the middle of the compressor, which creates a problem. At -35 ℃, the low pressure of the compressor is a negative value, that is, A degree of vacuum is generated, so that the heat at the top of the coil cannot be dissipated, so that the surface of the compressor is very cool, but in fact, its temperature is very high, (because vacuum is the best insulation medium) !

It can be seen from the above: the constant temperature and humidity test chamber can use either a single-stage refrigeration cycle or a cascade refrigeration cycle system for the -40 ℃ model, but the single-stage refrigeration cycle is reduced by opening the expansion valve of the compressor to reduce The flow rate of the small refrigerant is limited to lower the evaporation pressure (about 0.7 atmospheres) to obtain a lower evaporation temperature. This design is achieved at the expense of the cooling capacity of the system (the cooling capacity is only about 0.7 to 0.8 ), Resulting in low refrigeration efficiency and increased compressor load, and it is easy to cause the compressor coil to overheat, affecting the life of the compressor.

Leather Sofa

Leather sofa is very soft, durable and more comfortable. Leather is the representative of luxury. Leather sofa has different styles, it has the Single Leather Sofa, Loveseat Leather Sofa, 3 seat leather sofa, and corner leather sofa. You can choose what you like.

Leather Sofa,Leather Sofa Couch,Leather Sofa Furniture,Leather Sectional Couch

Kaifeng Lanwei Smart Home Co., Ltd , https://www.leather-recliner.com