Chapter 1 Overview
With the increasing demands of modern factories for cooling, heating and ventilation, how to effectively control and manage air conditioning and ventilation equipment, how to control the rational use of energy while ensuring normal operation of equipment, and how to meet the requirements of production processes for air conditioning systems have become important issues in modern factory design. Integrating the concept of building control systems into the modern factory HVAC integrated control system, using advanced computer control technology, management software and energy-saving programs, enables the scientific control and management of various electromechanical equipment, effectively solving the problems of energy saving, saving maintenance and management workload and operating costs [5] . Most building control systems use dedicated DDC controllers, which are more suitable for controlling dedicated equipment such as air conditioners and air valves. With the development of control systems and network communication technology, various control systems in factories have gradually merged into a whole. PLC controllers have slowly entered the control field of factory air conditioning. PLC controllers are suitable for most industrial control fields. Now many factory air conditioning systems use PLC controllers. Factory air conditioning systems are becoming more and more complex and have more and more functions. PLC controllers have good versatility and can easily expand various functions. Moreover, their communication functions are also good.
Based on the process requirements of the air conditioning and exhaust systems, the entire control system is divided into 8 distributed field control stations and 1 main control station. The hardware system uses Siemens S7-300 series controllers as data acquisition and control units, leveraging the real-time characteristics of the PRFIBUS industrial fieldbus to achieve integrated control of the plant's air conditioning units and exhaust fan units. This control system has been put into operation at the truck painting plant, operating well and passing acceptance testing. It has achieved the expected control objectives, received high praise from the manufacturer, and brought considerable economic benefits to the user.
Chapter Two: Air Conditioning System Process Flow and Control Requirements
The fundamental premise of air conditioning and ventilation system design in painting workshops is the effective removal and treatment of exhaust gases; otherwise, the system's effectiveness will be directly affected. Therefore, this project integrates exhaust gas treatment with air conditioning and ventilation, applying a zoning concept to maintain a sealed, slightly positive pressure environment in the workshop, effectively isolating high-cleanliness areas from pollution and heat sources to create conditions for normal production. The paint shop requires whole-room ventilation, which, in addition to improving the working environment, must also meet the cleanliness requirements of the workshop. Due to the large workshop area, and considering the layout of the painting process zones and air quality requirements, several ventilation zones were specifically divided. Efforts were focused on coordinating the supply air volume at workstations with the supply and exhaust air of the painting process equipment and the overall ventilation volume of the workshop, maintaining a slightly positive pressure throughout the workshop, with airflow organized from clean areas to less clean areas. Air curtains were installed in the main passageways leading to the outside to isolate external dust, ensuring indoor cleanliness and the quality of the painted products.
The factory building is a three-story, fully enclosed, cast-in-place reinforced concrete frame structure. The workshop is 60m wide and 300m long, with a building area of approximately 54,000m². The first floor houses living quarters, offices, auxiliary departments, and shared power facilities; the second floor contains a paint drying area; and the third floor houses the air conditioning room and the substation that supplies power to it. The factory's ventilation system consists of 5 combined air conditioning units for large-space air supply, 19 roof exhaust fans, 1 exhaust fan box, and 3 area supply and exhaust fans, responsible for air conditioning in the approximately 100,000 cubic meter space of the factory.
Of the five air supply air conditioning units, four have an air supply capacity of 175,000 cubic meters per hour each, and one has an air supply capacity of 11,000 cubic meters per hour. There are 20 rooftop fans and 3 area exhaust fans distributed throughout the factory.
2.1 Introduction to the composition and functions of the air conditioning system
This system uses centralized air conditioning units, totaling five (S-1, S-2, S-3, S-4, and S-5), distributed longitudinally along the factory building. High-temperature water is supplied to the air conditioning system by the factory. Units S-1 and S-3 each include two 75kW fans, one 15kW spray pump, and one 5kW circulating pump. The fans and spray pump use variable frequency speed control. Units S-2 and S-4 each include two 75kW fans, one 15kW spray pump, and one 5kW circulating pump. The two fans use star/delta starting and operate at industrial frequency, while the spray pump uses variable frequency speed control. Unit S-5 includes three 15kW fans (two in operation and one on standby), one 5kW spray pump, and one 5kW circulating pump. The fans use direct starting and operate at industrial frequency. All circulating pumps use direct starting and operate at industrial frequency.
A simplified plan view of the air conditioning unit is shown in Figure 1:
Figure 1. Schematic diagram of the air conditioning unit
The air conditioning unit uses a constant air volume (CAV) all-air handling system. In summer, the temperature in Northeast China is relatively cool, so it operates with 100% fresh air. In winter, the air temperature in Northeast China can drop to -30°C, so high-temperature, high-pressure hot water above 120°C is supplied directly from the factory's centralized heating system and delivered to the air conditioning unit's heating coils. Outdoor fresh air and indoor return air mix at the front end of the unit, pass through a primary filter, and then exchange heat through the heating coils. Because the outdoor fresh air temperature is too low in winter, the air conditioning system uses two-stage heating. After heat exchange, the air passes through a medium-efficiency filter and is delivered to the work area where it is needed. A portion of the used indoor air is then exhausted through the exhaust fan system (exhaust air must be maintained at positive pressure in the work area according to process requirements), while the remaining air is mixed with fresh air and returned to the air conditioning system. This utilizes indoor return air to reduce energy consumption and achieve good energy-saving results.
In air handling, a humidifier is used to humidify the air during air supply to meet humidity requirements. During humidification, a spray pump is activated, spraying water outwards through spray nozzles. The spray pump cannot start if the water level in the spray tank is low; it will only start when the water level in the spray tank is increased above the low level. The spray pump must not be allowed to run dry for extended periods.
When starting the air conditioner, the fresh air damper must be opened first, and the return air damper adjusted to open at an appropriate ratio. Simultaneously, the circulation pump should be started to allow hot water to enter the heater and preheat the heating coils. Then, two 75kW fans should be started to draw in fresh air. The heater has an inlet valve to control the inflow of hot water and a return valve to control the return of hot water to the piping system. Temperature control of the air conditioning system primarily controls the return valve; the inlet valve is generally not controlled.
Differential pressure switches are installed at the primary and secondary filters. When the filter is blocked, the pressure difference across the filter increases. When the pressure difference increases to 200 Pa, the differential pressure switch will activate, and an alarm should be triggered to notify the operator to replace the filter cloth.
A wind speed sensor is installed in the middle of the air conditioner to detect the wind speed and record the air volume, ensuring that the system's air supply meets process requirements. It can also detect whether there is any blockage in the air duct.
Heaters are expensive, and the temperature in Northeast China is low. If the hot water supply is insufficient, the heating coil inside the heater can easily freeze and crack in a short time, causing serious equipment accidents. Generally, the heater is equipped with an anti-freeze protection switch inside. When the measured air temperature after heat exchange is less than 5°C, the electrical circuit is immediately cut off, the fan stops, the fresh air damper is closed, and the return water valve is fully opened.
Energy-saving measures:
Energy saving in air conditioning units has many requirements. Maintaining the optimal ratio of return air to fresh air is an essential measure for energy conservation. Since the air conditioning units in painting plants have relatively large capacities and high energy consumption, adopting necessary energy-saving measures can reduce daily operating costs.
The following energy-saving measures are adopted for the air conditioning system of the painting plant:
- Adjustment of the ratio of return air to fresh air
Analog air volume valve actuators are installed at the fresh air inlet and return air outlet. In winter, the ratio of fresh air to return air is adjusted appropriately according to the temperature difference between indoors and outdoors to fully improve energy-saving effect.
- Summer operation mode adopts all-fresh air mode
- Minimum fresh air volume control
When there is a significant difference between the outdoor and indoor temperatures, we adjust the ratio of fresh air to return air in the system by reducing the fresh air valve and minimizing the fresh air volume. When the difference between the outdoor and indoor temperatures is small, we adjust the ratio of fresh air to return air in the system by reducing the return air valve and minimizing the return air volume.
2.2 Composition of the exhaust system and introduction of the functions of each part
The ventilation system consists of 19 rooftop exhaust fans, 1 exhaust fan housing, and 3 zone exhaust fans. These devices are divided into 4 groups and controlled by 4 control cabinets AC1-AC4. AC1 includes 8 units: P163.73KW, P173.73KW, P183.73KW, P193.73KW, P201.49KW, S65.59KW, S75.59KW, and S85.59KW; AC2 includes 7 units: P15.59KW, P23.73KW, P33.73KW, P112.24KW, P135.59KW, P145.59KW, and P155.59KW; AC3 includes one 90KW blower unit; and AC4 includes 7 units: P45.59KW, P55.59KW, P62.24KW, P75.59KW, P85.59KW, P95.59KW, and P105.59KW.
The ventilation system's fan control is relatively simple. Each fan only has start/stop control and overcurrent detection. Additionally, a differential pressure switch is installed at the exhaust fan to detect blockages and protect the fan in a timely manner. Fan control can be performed manually or in groups automatically, depending on the ventilation system's requirements.
2.3 Specific control requirements of this control system
2.3.1 Overall System Monitoring Requirements
Each modular air conditioner is equipped with one local control cabinet and four roof fan and district air supply fan control cabinets. One of these control cabinets serves as the main control cabinet, which can uniformly monitor all air conditioning units and exhaust fan units.
The local control cabinet has a manual/automatic switch, enabling local and remote control. It displays various parameters such as temperature, humidity, airflow speed, and fan status. The main control cabinet can monitor the status of 5 air conditioning units and 23 exhaust fans, and can also control various devices in groups. The main control cabinet has a local/remote switch; when switched to remote, the painting plant's main control room gains control and monitors the air conditioning fans from the host computer. The main control cabinet also has a winter/summer mode switch; in summer mode, high-temperature water supply is stopped, and the heating valve does not participate in regulation.
2.3.2 Control requirements for S-1 to S-4 combined air conditioning units
The main detection and control measures include the following:
- Primary and secondary filter clogging alarm detection
Two differential pressure switches are installed at the two filter screens. When the differential pressure on both sides of the filter screen reaches a certain value, it indicates that the filter screen is blocked by dust and an alarm needs to be triggered in time so that maintenance personnel can replace it.
- Hot water supply and return water temperature detection alarm
Understand the operating conditions of the unit's heat transfer medium. If the return water temperature is too low, an alarm should be triggered in time to prevent the heater from freezing and cracking.
- Outdoor fresh air temperature detection
Because the winter temperatures in the eastern region are very low, the temperature measurement range of the fresh air temperature sensor is -50 to +50℃.
- Air conditioning supply air temperature detection
The supply air temperature is detected using a PT100 standard resistance temperature detector, which is an important parameter for temperature control.
- Air supply speed detection
The air volume can be estimated based on the wind speed in the duct. Since all units supply air to a large space, the fixed frequency units can maintain a constant air volume without necessarily adjusting the air volume through dampers. The total air volume of the space is adjusted by the variable frequency units.
- Temperature and humidity detection in the air supply area
- Indoor and outdoor pressure difference detection in air supply area
- Air conditioner fan inverter current and operating status detection
- Air conditioner fan fault detection
Fans S-1 and S-3 use variable frequency speed control, and the fault point is taken from the frequency converter; fans S-2 and S-4 operate at power frequency, and the fault point is taken from the thermal relay.
- Sprinkler pump frequency converter current and operating status detection
The spray pump needs to be interlocked with the fan so that the spray pump stops automatically when the fan stops.
- Sprinkler pump frequency converter fault detection
- Hot water circulation pump status monitoring
- Hot water circulation pump fault detection
- Low temperature antifreeze protection and alarm
Winter temperatures in Northeast China can reach -20 to -30°C. The outdoor fresh air temperature is very low and can easily freeze the heater. Therefore, antifreeze protection is very important. When the air temperature after the heating coil is lower than the alarm temperature value, the system will issue an antifreeze alarm signal, cut off the electrical circuit, shut down the air conditioner, and the control system will automatically shut down the fan, close the fresh air damper, open the hot water valve to 100%, start the hot water circulation pump, and stop the spray pump.
- Fire signal linkage
Fire hydrants are installed at the air outlets of air conditioners. When the temperature is too high, the fire hydrants will activate, requiring the air conditioner to stop running, the heating water valve to be closed, the fresh air valve to be closed, and the sprinkler pump and circulation pump to be stopped.
- The fresh air damper and supply air damper actuators are interlocked.
To prevent overpressure in the air conditioning unit, the fresh air damper and the supply air damper need to be interlocked to prevent an accident where the fresh air damper opens while the supply air damper closes during air conditioning operation.
- Fresh air damper and air conditioning fan interlock
The air conditioning fan can only run after the fresh air damper is opened. The fresh air damper must be closed when the air conditioning fan stops running or malfunctions to prevent the heater from freezing.
- Hot water circulating pump interlock
The circulating pump and the air conditioning fan are interlocked. In winter mode, the circulating pump starts 20 seconds before the air conditioning fan starts and stops running, at which point the circulating pump immediately stops running.
- Hot water regulating valve control
By adjusting the hot water valve, the flow rate of high-temperature and high-pressure hot water into the air conditioner heater is controlled, thereby achieving control of the air supply temperature, with a control error of ±1℃.
- Variable frequency control of humidifying spray pump
The relative humidity in the air supply area is controlled at 55% ± 5%. The humidification tank has upper and lower limit float switches. The water level in the tank must be within a certain range. If it is too high or too low, the spray pump cannot operate. Generally, water needs to be drained every half hour.
- Fan frequency conversion control
Based on the air pressure difference between indoors and outdoors in the air supply area, the frequency of the fan is controlled to adjust the air supply volume, thereby achieving the purpose of controlling the indoor slightly positive pressure.
2.3.3 Control Requirements for Unit S-5
In addition to performing the same parameter detection and control as units S-1 to S-4, unit S-5 will issue an alarm signal and automatically activate the standby fan when a fan malfunctions, with 2 of the 3 fans in operation and 1 in standby.
2.3.4 Exhaust System Control Requirements
- Fan start/stop control
- The control cabinet of the exhaust system must have a manual/automatic switch, and the system must be able to realize local manual control of the fan and remote group control on the main station touch screen.
- The S6 fan is interlocked with the cleanroom; the S6 fan can only start if the cleanroom's start-up permission signal is detected.
- P12 is interlocked with the forced cooling chamber; P12 can only be activated when a permission signal from the forced cooling chamber is detected.
- The P1-P6, P11-P20, and S1-S4 air conditioning units are interlocked, meaning that when the main station automatically starts/stops the S1-S4 fans, these fans are automatically started/stopped. The P7-P9 fans are interlocked with the S5 air conditioning unit.
- Micro-positive pressure control
When using micro-positive pressure control, if the indoor pressure does not reach the set value, some fans need to be stopped to ensure the positive pressure requirement. In this case, P6, P10, P11, P13, P14, and P18 can be stopped in sequence. When the indoor positive pressure is restored, the above fans can be started in reverse order.
Chapter 3 System Overall Design
To construct a control system, various hardware components must first be equipped according to the control requirements. Then, these components are connected via a network, forming the framework of the control system. Further software programming and networking then create a system capable of fulfilling specific functional requirements. Currently, distributed measurement and control systems, using a PC as the host computer, in conjunction with field touchscreens, operator screens, and multiple PLC control units, are widely used in many production automation fields. This system leverages the high cost-effectiveness, strong control and communication capabilities, flexible operation, and good anti-interference ability of programmable logic controllers (PLCs), touchscreens, and operator screens to build front-end control units suitable for distributed field control. It also utilizes the rich hardware and software resources, powerful management and control functions, and user-friendly interface of PCs.
This project comprises nine electrical control cabinets. Five modular air conditioners each correspond to one control cabinet (S1-S5), while the remaining exhaust fan systems are controlled in groups of four cabinets (AC1-AC4) according to optimized group control principles. AC1 also serves as the master control cabinet, providing group control for all ventilation equipment. We selected a SIEMENS S7-313CPLC as the field control unit, i.e., the control substation, to connect field monitoring instrument signals and control/acquisition signals to each remote substation. An S7-315PLC was selected as the master control station. The master control station and remote substations are connected via a fieldbus. Since the entire ESENMAN coating line's control systems and the coating plant's main control room use industrial Ethernet for network communication, the ventilation system, as a substation system in the main control room, also has its master control station connected to the coating plant's central control room via industrial Ethernet. This arrangement is chosen for compatibility and because industrial Ethernet offers its own advantages. The entire automated monitoring system constitutes a SCADA (Supervisory Control and Data Acquisition) system, which performs data acquisition, processing, monitoring, and control of field equipment. The control system structure diagram is shown in Figure 2.
Figure 2 Overall structure diagram of air conditioning control system
Chapter 4 Application of Network Communication Technology in Air Conditioning Systems
4.1 MPI Network
In the five air conditioning units S1-S5, communication is required between OP3 and the controller of each unit. In the main station AC1, communication is required between the touch screen TP270 and the controller of the main station. OP3 and TP270, along with the controllers they communicate with, are all located in the same control cabinet. The amount of data exchanged between them is small, therefore the MPI network protocol is used.
To achieve synchronization, the controller CPU needs to be configured using the NetPro software in STEP7, and the operation panel needs to be configured using the Protools software. The specific settings are as follows:
1. Communication between OP3 and S7-313C in Unit S1
Figure 3 Communication hardware configuration of OP3 and S7-313C
First, establish an MPI network in NetPro software. Set the MPI address of OP3 to 1 and the MPI address of PLC1's CPU to 2. Then, select a network communication rate of 187.5 kbit/s for OP3 and PLC1, as shown in Figure 3 above.
Figure 4 MPI Configuration
In the Protools software, you need to select the previously established MPI (1) network, write the MPI addresses of OP3 and PLC1, select the communication rate, and also write the slot number and guide rail number of the PLC1 station where the CPU connected to OP3 is located, as shown in Figure 4 above.
2. The configuration of the connection between OP3 and S7-313C of other units and the MPI network connection between TP270 and S7-315-2DP of the master station is similar to that above, and will not be repeated here.
4.2 Ethernet
1. Click the SIMATICManager icon on the desktop to enter the STEP 7 application, open the previously created project, enter the hardware configuration window, and select CP343-1 in the left window as shown in Figure 5:
Figure 5 Ethernet configuration in hardware configuration
Double-click this icon to bring up the Ethernet module properties window. Click the Properties button above to enter the Ethernet interface configuration window, as shown in Figure 6 below:
Figure 6 Ethernet Properties
Click the "new" button in the image to create a new Ethernet network, as shown above. Configure the IP address and subnet mask for 343-1. Since the factory network does not use routing, select "donotuserouter" in the gateway field on the right, then confirm and exit. Compile and save the hardware configuration, and then download the hardware configuration to the 300 control station.
2. In the control panel of the supervisory control computer in the central control room, select the following option: setpg/pcinterface. A pop-up window will appear as shown in Figure 7:
Figure 7 Communication driver configuration
Select TCP/IP (auto) -> sis900-base interface, then confirm and exit. This completes the Ethernet configuration, and we can now communicate with the 300 control station via Ethernet from the host computer.
Chapter Five Conclusion
The system studied in this project is a relatively complete measurement and control system with a wide range of applications. It integrates modern information technologies such as communication, network, PLC, computer, intelligent detection and automation to monitor air conditioning units and exhaust fans in real time, control temperature and humidity, and realize a comprehensive monitoring system that integrates on-site distributed control, central centralized control and upper-level supervision control.
