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Working principle of constant temperature and humidity chamber

Date:2016-12-09 Author: Click:

Working principle of constant temperature and humidity chamber

The refrigeration system is one of the key parts of the constant temperature and humidity chamber and also determines the price of the constant temperature and humidity chamber. In general, the cooling method of the test chamber is mechanical cooling and auxiliary liquid nitrogen refrigeration: mechanical refrigeration using steam compression refrigeration They are mainly composed of compressors, condensers, throttle bodies and evaporators. Low temperature sinks As we test the temperature to reach -55 ℃ low temperature, single-stage refrigeration is difficult to meet the requirements, so the test box of the cooling method is generally used The refrigeration system of the constant temperature and humidity chamber is composed of two parts, namely the high temperature part and the low temperature part, each part is a relatively independent refrigeration system.The evaporation of the refrigerant in the high temperature part absorbs the refrigerant from the low temperature part Heat and vaporization; the evaporation of the low-temperature part of the refrigerant from the object to be cooled (the air inside the chamber) to absorb the heat between the high temperature part and the low temperature part is connected with an evaporative condenser, it is high temperature part Of the condenser, but also the low temperature part of the condenser. STH series of high temperature part of the refrigerant using medium temperature refrigerant, low temperature part of the cooling Agent is the use of low-temperature refrigerant, the box temperature can reach -70-150 ℃.

1. Limitation of the thermal physical properties of the refrigerant.

Now the constant temperature and humidity chamber single-stage refrigeration cycle is basically used in the temperature of the refrigerant is R404A, at an atmospheric pressure of its evaporation temperature is -46.5 ℃ (R22 / -40.7 ℃). But the air-cooled condenser heat transfer temperature difference is usually Take about 10 ℃ (in the forced air cooling cycle, the evaporator and the temperature difference between the box), that is, the box can only be prepared -36.5 ℃ low temperature.Of course, by reducing the compressor evaporation pressure, you can R404A The minimum evaporation temperature of the refrigerant is reduced to -50 ° C. Therefore, it is necessary to use a medium-temperature refrigerant and a low-temperature refrigerant cascade refrigeration cycle at a low temperature of -50 ° C and below to prepare a low temperature of -50 ° C to -80 ° C , Low temperature refrigerant generally use R23 its evaporation temperature at an atmospheric pressure is -81.7 ℃.

2. Single-stage compression steam refrigeration cycle pressure ratio

The minimum evaporation temperature of the single stage steam compression type refrigerator depends primarily on its condensing pressure and the compression ratio. The condensate pressure of the refrigerant is determined by the type of refrigerant and the temperature of the ambient medium (eg air or water). It is in the range of 0.7 to 1.8 MPa, and the compression ratio is related to the condensing pressure and the evaporation pressure, and when the condensing pressure is constant, the evaporation pressure decreases as the evaporation temperature decreases, thereby increasing the compression ratio, which causes the compressor The exhaust gas temperature increases, the lubricating oil becomes thinner, so that the lubricating condition deteriorates, and even when the charring and the pulling cylinder phenomenon are serious, the increase of the compression ratio will cause the air supply coefficient of the compressor to decrease, Reduce the actual compression process deviation from the entropy process farther, the compressor power consumption increases, the cooling coefficient decreases, the economy is reduced, there will be some of the following effects.

a. The lower the evaporation temperature of any refrigerant, the lower the evaporation pressure is. The low evaporation pressure may sometimes cause the compressor to be difficult to breathe, or to allow the outside air to enter the refrigeration system.

b. When the evaporation temperature is too low, some common refrigerant has reached the solidification temperature, can not achieve the flow of refrigerant, circulation.

c. The evaporative pressure is reduced, the cooling capacity of the refrigerant is greatly reduced, the volume is increased and the mass flow rate is reduced. In order to obtain the required cooling capacity, it is necessary to increase the suction volume and make the compressor volume too large.

3. Compressor coil cooling restrictions

When the single-stage compressor is operating, it is around -35 ° C. Since the compressor coil is spinning in the middle of the compressor, this raises a problem: the low pressure of the compressor is negative at -35 ° C, that is, Resulting in a vacuum, so that the top of the coil there is no way to disperse the heat, which will make the compressor surface is very cool, but in fact the internal too high! (Because the vacuum is the best insulation medium)


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