Temperature and humidity control of spray booth in painting workshop
Text / Zhen Zhen
I. Introduction
The temperature and humidity control of the air-conditioning room of the paint shop in the paint shop is different from the general air-conditioning control system. The temperature and humidity requirements in the paint booth of the paint shop are relatively high, and the requirements are consistent and stable. However, due to the cross-coupling between temperature and humidity in the paint booth, temperature control will cause humidity changes, humidity control will also cause temperature changes; and the air intake of the spray booth air conditioning is directly collected in natural wind, inlet air temperature and humidity It is greatly affected by changes in the outside weather and is not easy to control. Therefore, the temperature and humidity control of the painting booth in the paint shop is a nonlinear, lagging, time-varying and complex process. It is obviously inappropriate to use such traditional instrument control. This paper provides a method for temperature and humidity control of spray booths based on the experience of controlling the temperature and humidity of spray booth air conditioning in multiple paint shop projects.
Second, the system design
The whole air conditioning temperature and humidity control system is mainly divided into three control sections, which are the table cooling section, the heating section and the spray section. The amount of cold water entering the cold section of the watch and the amount of hot water in the heating section are controlled by the cold and hot water valves on their respective pipes, and the amount of water spray entering the spray section is realized by the spray pump. Its hardware is shown below:
Figure 1 system hardware design schematic
Third, the system control principle:
Before introducing the principles of system control, we first introduce a concept of depreciation.
The devaluation is a unit of energy, which represents the combined amount of moisture and temperature contained in a certain volume of air. It is a comprehensive measure of the temperature and humidity of the air. The value of the enthalpy is only the temperature (t) and humidity. (k) Decide that the formula is as follows:
(1) Open temperature calculation:
T=273.15+t
(2) Calculation of saturated air pressure:
(3) Calculate the saturated air pressure and steam partial pressure
Pq=k/100*Ps
(4) Calculate the moisture content
d=622* Pq/(101000- Pq)
(5) Calculating the threshold
h=1.01t+0.001d(2501+1.84t)
In other words, as long as the current temperature and humidity are known, the corresponding threshold can be calculated by the above formula.
The controlled parameter of the temperature and humidity control system is the temperature and humidity in the paint booth. The temperature and humidity signals are detected by a temperature and humidity sensor installed in the paint booth. The temperature and humidity sensor inputs the detected temperature and humidity signal into the PLC, whereby the current value of the temperature and humidity under the current condition can be calculated by the conversion relationship between the current value and the temperature and humidity value in the PLC program. In the program, input the KP, KI, and KD parameters to the PID function module, and input the temperature and humidity setting values. The PID function module in the program outputs the control percentage according to the input values, and then outputs the analog output on the PLC. The module converts the control percentage into 4~20mA current signals to control the corresponding actuators. The output of the PID is divided into heating, cooling and humidifying signals. Each PID function module separately controls the respective hot water regulating valve, the cold water regulating valve opening degree and the frequency of the humidifying pump frequency converter, thereby realizing the control of the temperature and humidity in the painting room. .
Then we may wish to define that the PID function module whose set value and current value are temperature is the temperature function module; the PID function module whose set value and current value is humidity is the humidity function module; the set value and the current value are depreciated The PID function module is a depreciation function module. The performance of the function module in the PLC program is shown in Figure 2.
Figure 2 PID control block
Which functional modules are used to control the opening degree or frequency of the cold water regulating valve, the hot water regulating valve and the spray pump inverter respectively. In the past debugging process, we often use the depreciation function module to control the opening degree of the cold water regulating valve to dehumidify, use the temperature function module to control the opening of the hot water regulating valve to control the temperature, and use the humidity function module to control the spraying. The pump frequency is used to adjust the spray volume to control the humidity. This control scheme has passed the test of actual production on site, which proves to be feasible, with high control accuracy and stability, and can cope with various complicated and varied climates.
Solve the problem of which function module is which actuator, the next thing to study is how to control these components in the program to cope with the complicated and varied weather throughout the year. Under these different seasons, the difference of temperature and humidity of the outside world Very large, such as hot summer, the external temperature and humidity are relatively high, under such conditions need to cool down and dehumidification; and in the cold winter, the temperature is low, the climate is dry and the humidity is small, then heating and humidification are needed; but in the weather comparison In the complicated and varied spring and autumn, heating, cooling, dehumidification and humidification should be carried out simultaneously in one day to ensure the high temperature and high stability of the temperature and humidity in the spray booth. In order to solve this problem, we have set three operating modes for the air conditioner, namely the summer mode, the winter mode and the spring and autumn mode. The switching of these three modes is artificially performed by a three-position selector knob mounted on the control cabinet, and in which season the knob is switched to which mode.
Summer mode: High temperature and high humidity are the climatic characteristics of this season. In this mode, the enthalpy function module is used to control the opening of the cold water valve to control the enthalpy of the air to the set threshold. For example, when the temperature and humidity requirements of the spray booth are 25 ° C and the humidity is 60%, the required enthalpy value is 10.041758, that is, the set value of the enthalpy value is 10.041758. The first step is to The dehumidification of the cold section is used to control the enthalpy of the air to this value. In the second step, the temperature function module is used to control the opening degree of the hot water regulating valve. After the air passes through the cold section of the first section, the temperature value will be lower than the set value. At this time, heating is required to control the temperature to the setting. value. In these two steps, the temperature and humidity control of the summer mode is completed. When the enthalpy and temperature are both controlled to the set value, the humidity is naturally controlled to the desired set value.
Figure 3 Control flow chart of summer mode
Winter mode: winter temperature and humidity are relatively low, need to be heated and humidified, in this case we also control in two steps: the first step is to use the temperature function module hot water to adjust the valve opening, the temperature is controlled to the set value; The second step is to control the inverter of the spray pump with the humidity function module to control the humidity to the set value. What needs special explanation here is that the output range of the inverter corresponding to the PLC output to it 4~20mA current signal should be 10~50HZ, because when the output of the inverter is lower than 10HZ, the amount of water sprayed by the spray pump is too small, Water mist cannot be formed in the air-conditioning spray section, and the air is not humidified. The control flow chart is shown in Figure 4.
Figure 4 Control flow chart of winter mode
Spring and Autumn Mode: This is a season where the climate is relatively complicated and the temperature and humidity are difficult to control. During the same day, the temperature and humidity change greatly. Sometimes it needs dehumidification, and sometimes it needs to be humidified. Sometimes it needs to heat up, and sometimes it needs to cool down. . In this mode, the best way is to open cold water, hot water, and spray together, so that these three forces can be mixed and adjusted in the air conditioner to achieve balance. In this mode, the enthalpy function module is used to control the opening of the cold water regulating valve, the temperature function module is used to control the opening of the hot water regulating valve, and the humidity function module is used to control the output frequency of the spray pump frequency converter. The control flow chart is shown in Figure 5.
Figure 5 Spring flow control flow chart
The long-term operation of the control system in the actual production process shows that the control system has small overshoot, stable control performance, strong anti-interference ability, and meets the requirements of users. The temperature control error is ±1 degree, and the relative humidity can be controlled. Within 3% of the company, it is more important to achieve stable operation from start-up to temperature and humidity in a short period of time, and achieve satisfactory results, and the operation is relatively simple, achieving the goal of high efficiency and low cost in the spray booth operation.
Figure 6 temperature and humidity temperature rise curve
Fourth, the conclusion
The temperature and humidity control in the painting workshop of the paint shop adopts the above control scheme, which achieves the purpose of simple operation, high efficiency and low cost, and lays a foundation for reliable and low-cost operation of the project.
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