Basic principle and failure analysis of high and low temperature test chamber

  Basic principle and failure analysis of high and low temperature test chamber
1. Introduction
　　 The high-low temperature test chamber refers to a test chamber that can simultaneously apply temperature and humidity stress. With the development of China's industrial products, in recent years, China has introduced a large number of test systems from abroad, playing an important role in the development and finalization of industrial products in China. However, due to its own complexity, many problems occur in the operation of the test chamber, and problems can not be solved in time, which greatly prolongs the test cycle and affects the product development. The reason for these phenomena is that they do not understand the working principle of the integrated test chamber. For this purpose, we briefly explained the basic working principle of the test box, and introduced how to analyze and judge the failure of the comprehensive test box test equipment.
2. The principle of the test box: The comprehensive test box is composed of a refrigeration system, a heating system, a control system, a humidity system, an air circulation system, and a sensor system, which belong to two major aspects of electrical and mechanical refrigeration. The following briefly describes the working principles and working processes of several major systems:
2.1 Refrigeration system: The refrigeration system is one of the key parts of the integrated test chamber. In general, the refrigeration method of the test chamber is mechanical refrigeration and auxiliary liquid nitrogen refrigeration, and mechanical refrigeration adopts vapor compression refrigeration. This refrigeration method is one of widely used and economical refrigeration methods in artificial refrigeration. Vapor compression refrigeration models are: single-stage refrigeration, multi-stage refrigeration and cascade refrigeration, which are mainly composed of a compressor, a condenser, a throttling mechanism and an evaporator. Since the temperature and low temperature that we tested must reach -55°C , single-stage refrigeration is difficult to meet the requirements. Therefore, the integrated test chamber refrigeration method generally uses cascade refrigeration. The following is a brief introduction of the refrigeration system of an integrated test chamber produced by the Italian ACS company. Figure 2 is a simplified diagram of the refrigeration system. It consists of two parts, called the high temperature part and the low temperature part, each part being a relatively independent refrigeration system. The evaporation of the refrigerant (HP80) in the high temperature portion absorbs the heat from the refrigerant (R23) in the low temperature portion to vaporize; the evaporation of the low temperature portion of the refrigerant absorbs heat from the object to be cooled ( air in the test box ) to obtain the cooling amount. . Between the high temperature part and the low temperature part is connected with an evaporative condenser, which is both the evaporator of the high temperature part and the condenser of the low temperature part. The high-temperature part of the ACS test box uses medium- pressure refrigerant HP80 , and the low-temperature part uses low-temperature refrigerant R23 . The temperature inside the box can reach -70°C .
2.2 Heating System: The heating system of the test chamber is relatively simple compared to the cooling system. It consists mainly of high-power resistive wires. Because the required heating rate of the test box is relatively large, the power of the heating system of the test box is relatively large, and a heater is also provided on the bottom plate of the test box.
2.3 Control System: The control part is the core of the comprehensive test box, which determines the important indexes such as the temperature and the rate of the test box, and the accuracy. Most of the existing test chamber controllers use PID control. There are also a small number of control methods that combine PID and fuzzy control. Because the control system basically belongs to the category of software, and this part is not in use during the use process, therefore, this article does not make too much introduction to the control system.
2.4 Humidity system: The humidity system is divided into two subsystems: humidification and dehumidification.
The humidification method of the test chamber generally uses the steam humidification method, that is, the low pressure steam is directly injected into the test space to humidify. This kind of humidification method has strong humidifying ability, high speed, and humidification control sensitivity, and it is easy to implement forced humidification especially when cooling.
There are two ways to dehumidify the test chamber: mechanical refrigeration dehumidification and desiccant dehumidification. The dehumidification principle of mechanical refrigeration dehumidification is to cool the air to a temperature below the dew point, so that water vapor that is greater than the saturated moisture content is condensed and precipitated, thus reducing the humidity. Dryer dehumidification is the use of air pump to extract the air inside the test chamber, and the dry air is injected, and the humid air is sent to the recyclable dryer for drying. After drying, it is sent to the test chamber again. Dehumidify. Most integrated test chambers now use the former dehumidification method. The latter kind of dehumidification method can make the dew point temperature reach 0°C or lower, which is suitable for occasions with special requirements, but the cost is more expensive.
2.5 Sensor system: The sensors of the test chamber are mainly temperature and humidity sensors. Temperature sensors are more commonly used platinum resistance and thermocouples. There are two methods for measuring the humidity: the wet and dry bulb thermometer method and the solid-state electronic sensor direct measurement method. Due to the low accuracy of the wet and dry ball method, the current test chamber is gradually replacing the wet and dry ball with a solid state sensor for humidity measurement.
2.6 Air circulation system The air circulation system is generally composed of a centrifugal fan and a motor that drives it. It provides the circulation of air in the test chamber.
3. Failure analysis and judgment
Since the test chamber is a device that includes a plurality of electrical and cooling systems, once the equipment is in trouble, the entire equipment must be inspected and comprehensively analyzed. In general, the process of analysis and judgment can be " outside " after " inside. " That is, first eliminate external factors, such as cooling water, power supply, etc., after completely eliminating external factors, according to the phenomenon of failure, the system after the first system decomposition after comprehensive analysis of the system to determine, you can use the reverse method to find the cause of the failure: First, according to electrical The wiring diagram looks for problems with the electrical system and finally looks for problems with the cooling system. Table 1 provides an analysis of some common faults
Table 1 Fault Analysis Table
Failure Analysis
Troubleshooting method
Equipment does not cool
Slow cooling
The refrigeration system has insufficient refrigerant ( leakage of fluorine ) .
Dirty or ice blocking occurs in the cooling system piping.
Solenoid valve supplying liquid to the evaporator is damaged.
Expansion valve flow is too large or too small or damaged.
Check leaks and charge fluorine.
Replace the blocked device or desiccant.
Replace the solenoid valve.
Adjust or replace the expansion valve.
Slow heating of equipment
The heater's thermal fuse is blown.
Contactors that control heater operation are damaged.
Replace heat insurance.
Replace the contactor.
System does not work
Centrifugal fan is not running.
Fan insurance burnout, replacement insurance; fan thermal protection, reset protection switch.
The compressor does not operate
The compressor's insurance burned out.
The power supply voltage is not enough.
Control the compressor start contactor damage.
Replace insurance.
Increase the supply voltage.
Replace the contactor.
Exhaust pressure too high
There is air in the refrigeration system.
Insufficient cooling water or excessive temperature.
Condenser hose fouling too thick.
Put air.
Increase the amount of water supply.
Clean the condenser.
Inspiratory pressure is too low
The refrigeration system has insufficient refrigerant.
Expansion valve is blocked or damaged.
Filter plugged.
Check leaks and charge fluorine.
Dry the pipe or replace the expansion valve.
Replace the filter.
The system cannot humidify
1. Humidification boiler burned out.
2. Control contactor damage to the humidifying boiler.
3. Humidification boilers are protected due to lack of water.
1. Replace insurance.
2. Replace the contactor.
3. Replace the float switch or water supply.
The system cannot dehumidify
The compressor for dehumidification does not start.
Dehumidification solenoid valve does not work.
1. Refer to the troubleshooting method for the compressor not working.
2. Replace solenoid valve

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