High and low temperature damp heat test chamber is the most sold product in the test equipment at present. In the market, the temperature and humidity of this kind of test chamber are constant. Most of them adopt thermal control, that is, the heat of the heater offsets the cold volume of the evaporator to achieve the balance of temperature and humidity. Cold control is that the controller PID calculates the on-off time of the solenoid valve in a unit cycle according to the actual situation of the equipment, and controls the amount of refrigerant entering the system through the on-off of the solenoid valve. Compared with thermal control mode, cold control mode has obvious advantages in energy saving.

1.Equipment Introduction

Next, take the cold control high and low temperature humidity test chamber developed by DGBELL. as an example to introduce its equipment technical indicators.

1)    Internal volume: W 600 mm x H 850 mm x D 800 mm;

2) Temperature range: -70~+150 ℃;

3) Humidity range: 20% – 98%;

4) Temperature uniformity: ± 2 ℃;

5) Temperature deviation: ≤± 2.0 ℃;

6) Temperature fluctuation: ± 0.5 ℃;

7) Humidity fluctuation: when humidity >75% RH, ≤± 3.0% RH, when humidity ≤ 75%RH, ≤± 5.0% RH;

8) Heating rate: 25~+85 ℃, the average temperature change rate is about 3 ℃ /min (average in the whole process, no-load);

9) Cooling rate: 25~-70 ℃, the average temperature change rate is about 1 ℃ /min (average in the whole process, no-load).

2.Equipment Structure and Working Principle

The equipment is divided into the following parts: main box, air circulation, refrigeration and electrical control. Its overall structure and working principle are as follows:

a) Main box part

Library body. The main box is composed of polyurethane plate + glass wool (thickness: 100 mm), the inner wall of the box is SUS 304 stainless steel plate (thickness: 10 mm), and the outer wall of the box is plastic sprayed steel plate (surface color: camel).

Gate. The single door has a door opening size of W 600 mm x H 850 mm. The inner plate of the door is stainless steel plate, and the outer surface is plastic sprayed steel plate with the same color and material as the outer surface of the box. Heating wires are embedded around the door and door frame. The opening of heating wires is automatically opened by the equipment according to different operating conditions, so as to ensure that the door of the equipment will not condense and frost under low temperature environment.

b) Air circulation part

This part is an important guarantee part for the uniformity, deviation and other indicators of the equipment, which is mainly composed of the following components: centrifugal fan. The fan is the power source for mixing air in the box. The fan mixes the air mixed in the box through the evaporator and heater, and then sends it out of the channel to circulate the air in the box. Finned evaporator. The main cooling source in the box is the main component for energy exchange between the cooling capacity of the refrigerating unit and the hot air in the box. Electric wire heater. This is the main heat source of the equipment, providing the heat required for equipment temperature rise and temperature point cold and heat control. Humidifier. The stainless steel jacket heater is used to heat the water in the water pan to provide the required humidity for the equipment.

c) Refrigeration part

The refrigeration part is the main part of the cold source generated by the equipment, providing the required cooling capacity for the cooling, low temperature, constant temperature and damp heat of the equipment.

Since the low temperature limit of the test chamber is -70 ℃, the refrigeration system adopts cascade refrigeration.

Working principle four cascade refrigeration is composed of two single-stage refrigeration systems using different refrigerants. Usually, the environmental refrigerant R404A with higher boiling point is used in the high-temperature system, and the environmental refrigerant R23 with lower boiling point is used in the low-temperature system. Each becomes a refrigeration system using a single refrigerant. The evaporation of refrigerant in the high-temperature system is used to condense the refrigerant in the low-temperature system. The refrigerant in the low-temperature system absorbs the heat of the cooled air during evaporation, so that the temperature in the box can reach -70 ℃. The cascade system connects the high and low temperature levels through the condensing evaporator. In the condensing evaporator, the refrigerant evaporation of the high temperature level system condenses the refrigerant of the low temperature level system into a liquid, and the refrigerant in the high temperature level system transfers heat to the environmental medium (air or water).

The PID controller of the cold control refrigeration system calculates the time that the solenoid valve needs to be opened in an on-off cycle according to the actual situation of the equipment, and controls the amount of refrigerant entering the evaporator through the switch of the solenoid valve, so as to achieve a balanced state at constant temperature. In this process, the heater does not need to be turned on, and the constant temperature is completely maintained by the refrigeration system itself.

d) Electrical control part

Electrical control includes temperature and humidity control system and protection system.

The controller of temperature and humidity control system can realize program and fixed value. The controller is a color LCD touch screen, which can realize man-machine dialogue, with friendly interface and convenient operation. The controller compares the temperature and humidity value in the box measured by the temperature sensor with the temperature and humidity value set by the user, and performs PID operation according to a certain regulation law to calculate the size of the control quantity to be output and determine the working state of each output point. The on-off and humidification volume of the solenoid valve are controlled through the i/o input output port to make the temperature and humidity in the box change as required.

The protection system includes power leakage circuit breaker, controller over temperature protection and over temperature protector protection, fan overheating protection, compressor over pressure protection and compressor overheating protection. When the fault occurs, sound and light alarms will be sent out, and the corresponding power supply will be cut off to ensure the safety of personnel, equipment, samples and the environment.

The temperature and humidity data recorded in the controller can be transferred to the U disk through the USB function, and the data can be read and edited on the PC through the special software. The standard RS485 interface is also provided, and the user can remotely monitor the device through the PC.

3.Conclusion

The cold control technology can accurately output and achieve the effect of temperature control through the opening and closing of the solenoid valve. The whole process of cooling and maintaining the temperature below the normal temperature does not need the participation of the heater, which is completely different from the widely used thermal control method at present. Moreover, its saving effect is obvious, and the energy saving is more than 30%.