Abstract: The air conditioning system of a cigarette factory consists of three parts: air conditioning units, refrigeration, and heating chambers. The air conditioning control system controls the air conditioning units to regulate the temperature of production areas and office spaces, adjusting them to achieve the specified ranges. The refrigeration units provide the cold source for cooling and dehumidification, while the heating chambers provide the heat source and steam source for heating and humidification. This paper proposes an air conditioning automatic control system scheme based on the control requirements, implementing it from the equipment layer, control layer, and management layer, meeting the requirements of distributed control and centralized management in a cigarette factory's DCS control system.
Keywords: Cigarette factory air conditioning subsystem automatic control refrigeration station PLC
Design features and introduction of an air conditioning solution
Features of the Air Conditioning Control System in the Main Plant of a Cigarette Factory
Silk processing workshop
The air conditioning in the silk refining workshop uses a single-fan supply system (without return air ducts) to deliver hot air in winter and cool air in summer. This is determined by the production characteristics of the silk refining workshop. Therefore, the control of the air conditioning in the silk refining workshop is relatively simple. The temperature and humidity monitoring in the workshop is for reference only and is not used as the basis for control. Its air conditioning control is open-loop.
winding and packaging workshop and yarn storage room
The cigarette making and packaging workshop and the tobacco storage room have high requirements for temperature and humidity control, with the temperature varying within ±5% of the specified value. In areas with large heat load disturbances, achieving the required temperature and humidity control necessitates multiple effective measures. Careful consideration is needed in the overall design and process layout. If a dual-fan air conditioning system is used, the placement and layout of the supply and return air vents must ensure even distribution throughout the workshop. Multiple temperature and humidity monitoring points should be selected to ensure consistent temperature and humidity values throughout the workshop. In terms of air conditioning functionality, it should have humidification, dehumidification, heating, cooling, and variable airflow adjustment functions. Automatic control should automatically adjust the temperature and humidity of the supplied air, employing a combination of PID control and fuzzy control methods to ensure that the temperature and humidity in the tobacco storage and cigarette making workshops meet the process standards and specifications—a key aspect of air conditioning control in the main cigarette factory building.
auxiliary materials workshop
The heat load of the auxiliary material workshop (including the filter rod workshop) is relatively small. The air conditioning adopts a dual-fan system, and its temperature and humidity control requirements are relatively high. It is necessary to adopt a control scheme with adjustable heating, cooling, dehumidification, humidification and fresh air return ratio. In order to achieve the temperature and humidity control accuracy requirements, PID regulation enthalpy control and other means should be adopted.
Introduction to the Air Conditioning Control System and Control Equipment of the Main Plant of the Cigarette Factory
The air conditioning control systems in the main buildings of domestic cigarette factories generally employ a combination of automatic and manual control. The level of control precision and accuracy increases with the level of automation. Currently, most cigarette factory main building air conditioning control systems use a distributed control approach, with the distributed management layer implemented via PCs and network communication. The control equipment for the main building air conditioning systems in domestic cigarette factories primarily uses products from the following companies:
Control equipment from British company Zoling
This includes Zhuoling's IQ251 series DDC controllers, temperature, humidity, pressure, differential pressure transmitters, and hot and cold water regulating valves, etc. The distributed control equipment is available in the 945 system. Currently, cigarette factories in Beijing, Yuxi, Chongqing, Xuzhou, Huaiyin, and Zhanjiang use Zhuoling's control equipment.
Honil Control Equipment (USA)
This includes a complete set of Honeywell DDC controllers and supporting temperature, humidity, pressure, and differential pressure measuring transmitters, as well as hot and cold water regulating valves and distributed control system equipment. Currently, Shanghai Gaoyang Cigarette Factory is one of the domestic manufacturers using Honeywell control equipment.
Siemens LANDIS & GYR control equipment
Boaklan Gild S600 series DDC controllers and matching temperature, humidity, pressure, differential pressure transmitters, and hot and cold water regulating valves are used in the centralized control system, which employs the S600 series network and workstation software. Currently, this control equipment is used in cities such as Changsha, Kunming, Yuxi, Guangzhou, Shenzhen, Chuxiong, Xinzheng, Zhumadian, Xuzhou, and Guiyang in China.
Two-Air Conditioning Automatic System Scheme
1. Description of the controlled object
The cigarette factory has a total of 22 air conditioning units, including combined air conditioning units K-1~K-3 in the tobacco processing workshop; combined air conditioning units K-4 and K-5 for vacuum rehumidification and burley tobacco; combined air conditioning units K-6~K-9 in the cigarette making and packaging workshop; combined air conditioning units K10 and K11 in the filter rod forming workshop; combined air conditioning units K-12 and K-13 for tobacco storage and spinning; combined air conditioning unit K-14 for tobacco expansion; combined air conditioning unit K-15 for blending and flavoring; combined air conditioning unit K-16 for waste storage and transfer; and combined air conditioning units K17~K-22 for the general warehouse.
2. Structure and Composition of Air Conditioning Control System
The air conditioning control system consists of three parts: the equipment layer, the control layer, and the monitoring layer. These three parts are organically integrated through an equipment control network.
(1) Equipment layer
The air conditioning unit electrical control cabinet controls the start/stop of the air conditioning unit motor and the frequency conversion of the fan.
Temperature, humidity, and pressure detection devices.
l A device for regulating valves for cold water, hot water, air supply, and exhaust.
l A device for detecting the operating status of the unit.
(2) Control layer
The control layer consists of either a DDC (Direct Digital Control) or a PLC (Programmable Logic Controller). The DDC comprises a CPU and I/O modules. With a control program, it can perform PID regulation control of the air conditioner. This layer can connect to the equipment layer via I/O modules, or it can connect to the field control layer via a fieldbus (an advanced control connection method that eliminates most field cable connections, improving control accuracy and reliability). With a network communication module, the DDC can communicate with a monitoring PC.
(3) Monitoring layer
The monitoring layer consists of computers and monitoring software. It enables centralized control, detection, and monitoring of each air conditioning unit. The computers are connected to the energy management center's monitoring station and data acquisition server via Ethernet for data transmission.
3 System Functions
This system has the following functions:
Manual and automatic control functions: Manual and automatic switching can be achieved through hardware switch or monitoring computer operation according to the actual use and on-site maintenance operation requirements.
Automatic protection function: The air conditioning unit's electrical control cabinet is equipped with a motor overcurrent protection switch and a system power protection device, which automatically protects the passenger team's motor and DC power consumption.
Centralized monitoring function: This system can achieve centralized control of the entire system on the monitoring machine.
Remote setting and modification functions: Process parameters can be set and modified as needed on the monitoring unit. The controller program can also be modified and programmed.
Display functions: Real-time display of equipment operating conditions and process parameters; display of real-time bar graphs and loop control dynamic diagrams of process parameters; display of historical trend graphs of process parameters.
Data acquisition function: The monitoring computer can collect and store data on various parameters of the system, and transmit it to the data acquisition server in the energy management center via the network. This serves as the original basis for pipeline reports.
Query function: In order to enable the workstation to query information such as the production status and task completion status of the entire air conditioning system, and to set the query of process parameters of each section of the current air conditioning according to the actual situation.
l Print report function: The control system has shift reports, daily reports and contract reports. The data is saved every hour. The system can print out the detection parameters of each point for a month in the form of reports at regular intervals or at any time. It can also print the current temperature and humidity control curves.
Password protection function: The system has multi-level password protection. The use of multi-level passwords will provide an effective protection tool for managers at different levels, while preventing the system from being used by unauthorized personnel. The password is divided into six levels, and different levels of operators have different operating permissions.
Alarm function: The monitoring computer is equipped with temperature and humidity exceedance alarm and air conditioning system equipment abnormality alarm. When the temperature and humidity exceed the alarm set value (as required by actual requirements) and the equipment is in an abnormal state, the computer will issue a flashing alarm and an audible and visual alarm on the screen.
4. System Working Principle
(1) General principles of system operation
The general principle of air conditioning control is as follows: The actual signal detected by the indoor temperature and humidity sensor is compared with the given temperature and humidity values for analysis and judgment. When the actual signal sampled from the temperature and humidity sensor is equal to the given set signal, the chilled water valve for cooling capacity, the steam electric regulating valve for heating capacity, the steam electric regulating valve for humidification capacity, the fresh air valve, the return air valve, and the exhaust air valve are adjusted to their corresponding positions. When the sampled actual signal is greater than or less than the given signal, the controller processes the data and outputs a deviation control signal, automatically adjusting the electric valves or dampers to maintain the temperature and humidity indicators of the air conditioning system at a certain given value.
(2) Design principles of air conditioning system in cigarette factories
Different locations in a cigarette factory, such as the tobacco processing workshop, the cigarette making and packaging workshop, and the office building, have different temperature and humidity requirements. Based on the different temperature requirements of different locations and past engineering experience, the air conditioning systems of cigarette factories are divided into four categories:
L-type system (fresh air system)
The air handling process for the silk-making workshop air conditioning in winter and summer is shown in Figures 1 and 2.
The system is characterized by its use of fresh air throughout. Its control principle is based on the indoor temperature and humidity requirements. When the indoor heat and humidity load and outdoor weather conditions change, the system controls the surface cooler, heater, and electric valves for steam injection to ensure the required indoor temperature and humidity.
Its processing
The system features a constant fresh air ratio. Its control principle involves comparing the set indoor temperature and humidity values with those from indoor sensors. If they differ, such as if the measured temperature is 30°C, the controller will open the cooling valve to lower the room temperature setpoint (27°C). Even after reaching the setpoint, the cooling valve remains partially open to maintain the indoor temperature. If the indoor humidity is too high, for example, reaching 70%, the cooling valve will open, lowering the air supply temperature to zero, causing condensation and dehumidification (the controller will then control the heating valve to reheat the cooled air back to the room temperature setpoint). Driving and heating are also achieved through corresponding control valves.
Each differential pressure switch (sensor) detects the status of the fan and filter. If the fan stops working, its differential pressure switch will detect this status, and the controller will then divert and stop the flow. The filter differential pressure switch will detect whether the filter screen is clogged. If clogged, it will open the compressed air valve to clean or replace the filter screen.
Fresh air ratio of one-time return air system
The air handling process is the same as above, except that the ratio of fresh air to return air is changed, i.e., the position of point C after mixing changes. The system's characteristic is that, during transitional seasons, it can fully utilize outdoor fresh air for regulation (under available conditions, i.e., ensuring temperature and humidity control), effectively saving on cooling and heating. Based on the actual situation of the national cigarette factory, we propose two methods for comparison.
The fresh air volume is controlled based on the enthalpy value.
Based on local meteorological data (creating an enthalpy-frequency diagram), when outdoor meteorological parameters and indoor heat and humidity loads change, the PLC program performs comprehensive logical judgment on information such as the status of the actuators (heating, cooling, humidifying or dehumidifying capabilities), selects the most suitable air handling mode, and automatically switches from one operating condition to another to maximize the saving of heating and cooling.
The functions include: automatic adjustment of indoor relative humidity; in summer and transitional seasons, when the outdoor air humidity is high, the opening of the vacant cold water valve is adjusted; in winter, the opening of the electric valve of the steam humidifier is controlled, and the room temperature is automatically adjusted.
In winter, control the electric regulating valve for hot water or steam; in summer, control the electric regulating valve for cold water; during transitional seasons, control the fresh and return air valve.
Design of a three-cooling station
1. Description of the controlled object
The main function of a refrigeration system is to provide sufficient cooling capacity for the air conditioning system. This is achieved through the automatic control of the main equipment—the refrigeration unit—and its subordinate equipment (chilled water pumps, cooling water pumps, cooling towers, etc.) to meet the needs of the air conditioning system.
Four centrifugal chillers, model 19XR6565-467DJS, with a cooling capacity of 2813KW, a chilled water outlet temperature of 7℃, a cooling water inlet temperature of 32℃, a chilled water flow rate of 484M³/h, and a cooling water flow rate of 579M³/h.
One 30HXC-400A screw chiller with a cooling capacity of 1392KW, a chilled water outlet temperature of 7℃, a cooling water inlet temperature of 32℃, a chilled water flow rate of 241M3/h, and a cooling water flow rate of 284M3/h.
17 chilled water pumps (5 in use, 2 on standby).
17 cooling water pumps (5 in use, 2 on standby, one of which is in the warehouse as a backup).
15 cooling towers.
One floor-standing expansion tank.
The control parameters are as follows:
Start/stop control of equipment such as refrigeration units, water pumps, and cooling tower fans.
Control of the opening/closing of the cooling tower inlet valve, chilled water/cooling water (inlet/outlet of the chiller) valve, and chilled water supply branch valve.
The regulation and control of the balancing valve between the chilled water supply valve and the return water valve.
l Control of the opening/closing of the expansion tank water supply valve.
2. Composition of the refrigeration control system
The refrigeration control system consists of a PLC for dispensing medication, distributed I/O control cabinets, and various transmitters. The main PLC control cabinet can be equipped with an operator panel and audible/visual alarms. It is responsible for acquiring parameters for the entire system, and the measured parameters are transmitted to the air conditioning and refrigeration system monitoring and energy management center via Ethernet. The main PLC is connected to the distributed I/O via a PROFIBUS-DP bus. It enables group control and coordinated control of the refrigeration equipment through the PLC. Alarm signals can be triggered via audible/visual alarms, and alarm buttons are also provided.
The internal process parameters of the chiller are transmitted directly from the chiller's own controller to the air conditioning and refrigeration system monitoring unit or the main control PLC of the refrigeration system via a network. For equipment closely related to the chiller and requiring interlocking control (chilled water pumps, cooling water pumps, valves, etc.), distributed I/O devices are selected, and the main PLC directly controls the start and stop of these devices. Equipment relatively unrelated to the chiller and not requiring interlocking control (cooling tower fans, cooling water makeup valves, etc.) is controlled by the PLC around independent target values.
3. Refrigeration System Functions
There are two control methods: "local/remote control" and "manual/automatic control".
The main PLC cabinet is equipped with a "local/remote control" switch. When the switch is set to "local", the PLC controls all equipment within the system; when the switch is set to "remote control", the energy management monitoring computer can directly control the power equipment within the system.
Each individual device is equipped with a "manual/automatic" switch on the electrical control cabinet. When the switch is set to "automatic," the PLC program automatically controls the equipment; when the switch is set to "manual," the equipment is manually controlled by the operator. This ensures that the power equipment can continue to operate normally even if the PLC malfunctions.
For equipment with a power rating of 7.5KW or higher, a soft-start method is used. For equipment with a power rating of less than 7.5KW, a direct-start method is used. For equipment with a built-in soft starter, direct control via I/O contacts is employed.
The refrigeration unit has its own controller, which can be used to start/stop the refrigeration unit remotely or via network connection, in addition to powering it.
4. Refrigeration System Control Principles and Programming
Five refrigeration units and their associated equipment (seven chilled water pumps, six cooling water pumps, five cooling towers, and valves) are centrally controlled and interconnected. The expansion tank's water supply valve is individually controlled based on its liquid level. The cooling tower fans are individually controlled based on their outlet water temperature.
Group control of the chiller units can be achieved manually or automatically. Automatic control is executed by a PLC, which can automatically increase or decrease the number of operating chillers according to actual load (cooling capacity) requirements, balancing the output of the already running chillers. Specifically, the start/stop of the chiller units is determined by detecting the chilled water return temperature. That is, if the chilled water return temperature exceeds 12℃, it indicates a reduced load and excessive cooling capacity (low chiller efficiency), in which case one chiller should be removed, as shown in Figure 5. The start/stop of the chiller units requires the coordinated interlocking action of the chilled water pump, cooling water pump, and cooling tower, among other linked equipment.
Start-up sequence: Open the cooling tower water supply valve → Open the chilled water pump → Open the chiller chilled water and cooling water valves → Start the chiller.
Shutdown sequence: Stop the chiller → Turn off the chiller chilled water jet valve → Stop the chilled water and cooling water pumps → Turn off the cooling tower water supply valve.
Figure 5. Refrigeration unit group control process
The start/stop of the cooling tower fans is controlled by the main PLC, based on the cooling water supply temperature (i.e., the cooling tower return water temperature). According to the chiller's cooling water requirements (18-34℃), if the cooling water temperature exceeds 30℃, an additional fan is activated; if the cooling water supply temperature is less than 19℃, one fan is deactivated. The opening/closing of the cooling tower supply valves and the chiller cooling water valves is controlled by the main PLC, and is linked to the chiller's start/stop, as shown in Figure 6.
Figure 6 Flow diagram of cooling tower fan group
An electrically operated regulating valve is used between the distributor and the collector to regulate the bypass of chilled water. When the pressure difference between the two ends exceeds a set value, this valve opens, allowing chilled water to flow directly from the distributor to the collector, maintaining pressure balance between the two systems. The refrigerant itself has a vapor valve control, and the system is regulated by the input of vapor. The chiller provides start/stop signals for the chilled water pump and the cooling water pump, and receives linkage signals from the chilled water pump and the main chilled water pump. The chilled water and cooling water pumps are configured with 5 operating and 2 standby pumps. Each chiller unit corresponds to one chilled water pump and one cooling water pump. If a pump fails during operation, the switching of the backup pumps (chilled water and cooling water) is automatically completed by PLC control.
The water replenishment of the floor-mounted expansion tank is determined by the tank's liquid level. When the liquid level is below the lower limit, the water replenishment valve is opened to replenish water. When the liquid level is above the upper limit, the water replenishment valve is closed to stop water replenishment. The control process is shown in Figure 7.
IV. Conclusion
The air conditioning system in this tobacco factory can be controlled using a PLC. A monitoring computer is installed in the energy management center, communicating with the PLC via an industrial Ethernet network to achieve data acquisition and centralized control. This monitoring computer exchanges data with the energy management center through the local area network of the energy monitoring system. The refrigeration section of the air conditioning subsystem is controlled by a single PLC, connected to the monitoring computer in the energy management center via Ethernet, allowing for monitoring of the refrigeration section from the energy management center.
About the author:
Bi Jingbin, male, is a master's student. His main research area is power electronics and electric drives.
