1、 After the high and low temperature test chamber reaches a low-temperature constant temperature state, due to a significant reduction in heat load, if the cooling capacity of the evaporator does not decrease correspondingly, the heating power to balance the excess cooling capacity will be greater, resulting in poor control quality and affecting the indicators of temperature fluctuation and uniformity inside the chamber.
2、 When the temperature of the high and low temperature test chamber needs to be constant near room temperature, if the compressor is not turned on, the temperature inside the chamber cannot be maintained. If the compressor is turned on, the excess cooling capacity will be the maximum because:
1. At a constant temperature near room temperature, the heat load is minimized because the enclosure structure has the least heat leakage and requires less cooling for the test.
2. When the constant temperature is near room temperature, the refrigeration capacity greatly increases due to the high opening of the expansion valve and the high evaporation temperature. If the cooling capacity cannot be adjusted at this time, a higher heating power needs to be used to balance. If the heating power is smaller, the temperature will not remain constant and continue to decrease. When the temperature of the high and low temperature test chamber is near room temperature or even 0 ℃, the temperature still cannot be sustained. After being heated, a heating power of 1400W is used.
3、 When the temperature of the test chamber starts from high temperature (+150 or even+180 ℃), the cooling capacity of the high and low temperature test chamber also needs to be adjusted. Firstly, in order to prevent damage to the compressor due to high return air temperature, it is necessary to bypass some cold air, that is, some cold liquid that is diverted from the front of the evaporator expands to the outlet of the evaporator, mixes with the hot air from the evaporator for cooling, and then enters the compressor through the return air pipe of the compressor.
4、 When the compressor cycles at minimum load, the suction pressure will decrease. In order to prevent the suction pressure from falling below the recommended low value recommended by the compressor manufacturer, although the compressor is widely used for power outage control at low pressure, periodic power outages can seriously affect the service life of the compressor, increase temperature fluctuations inside the box, and exceed the standard. Therefore, the refrigeration capacity of the compressor also needs to decrease with the decrease of heat load, and the suction pressure also needs to be increased.
Adjusting the cooling capacity of the high and low temperature test chamber is very necessary, but adjusting the cooling power is much more difficult and complex than adjusting the heating power. Previous methods, such as using multi cylinder compressor to work in separate cylinders, reduce the Engine displacement, thus reducing the cooling capacity. This method is often used in large refrigeration systems; Changing the compressor speed to reduce cooling capacity is also feasible, but expensive variable frequency governors are required; It is also possible to use two or three small refrigeration systems in parallel, working in separate systems, but with complex structures and reduced reliability; The on/off control method of the compressor at constant temperature can also be used, but the box temperature fluctuation is too large and is not suitable for high and low temperature test boxes.
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