I. Introduction
Our company is a pharmaceutical company primarily producing hepatitis B vaccines. A cleanroom environment is provided in our production workshop by a central air conditioning system, ensuring that the temperature, humidity, and pressure differential in each room meet the requirements of national GMP standards. Due to seasonal and diurnal variations, the airflow demand in each room of the production workshop varies significantly. However, the airflow and water flow of the pumps and fans are manually adjusted using dampers and throttle valves. When the demand for airflow and water flow decreases, the opening of the dampers and valves decreases; when the demand increases, the opening of the dampers and valves increases. While this adjustment method is simple, easy to implement, and has become the norm, it comes at the cost of increased pipeline losses and significant energy consumption on the dampers and valves. Furthermore, during normal operation, most dampers and valves are open at 50%-60%, indicating that the designed capacity of the existing central air conditioning pumps and fans is much higher than the actual needs, resulting in a serious "oversized engine pulling a small cart" phenomenon and a substantial waste of electricity. In recent years, with the rapid development of power, electronics, and computer technologies, variable frequency speed control technology has become increasingly mature. Therefore, we carried out energy-saving renovations by installing 19 frequency converters on the central air conditioning water pumps and fans in the company. Furthermore, due to the dispersed nature of the water pumps and fans, in order to reduce the workload of on-duty personnel during inspections and to facilitate timely monitoring of the operating status and fault detection of the water pumps and fans, we added a frequency converter monitoring system to the central monitoring room through the communication application between the PLC and the frequency converter via a human-machine interface. This allows on-duty personnel to directly set frequency values and start/stop each frequency converter on the human-machine interface, and to monitor the actual operating current, voltage, and frequency of the water pump and fan motors in real time, with alarm functions also available.
II. Central Air Conditioning Water Pump and Fan Variable Frequency Retrofit Solution
1. Equipment condition before renovation
(1) Status of air conditioning equipment in the Gene Department
① The refrigeration units are Hitachi units, three in total. ② Chilled water pumps: 11kW, 4 units, 2-pole full-pressure start, 30m head, outlet water temperature 6℃, return water temperature 10℃, outlet water pressure 0.35MPa, rated current of each motor 21.8A, normal operating current 16.6A. Generally, two pumps are in operation and two are on standby. ③ Cooling pumps: 15kW, 4 units, 2-pole full-pressure start, 30m head, outlet water temperature 32.5℃, return water temperature 28.2℃, outlet water pressure 0.38MPa, rated current of each motor 29.9A, normal operating current 18.0A. Generally, two pumps are in operation and two are on standby.
(2) Air conditioning equipment status in the air conditioning room on the second floor
① The refrigeration units are Hitachi units, two in total. ② Chilled water pumps: 15kW, 2-pole full-pressure start (3 units), head 30m, outlet water temperature 6.1℃, return water temperature 9.8℃, outlet water pressure 0.36MPa, rated current of each motor 29.9A, normal operating current 21A. Generally, one pump is operated with two as backup. ③ Cooling pumps: 15kW, 2-pole full-pressure start (3 units), head 30m, outlet water temperature 31.8℃, return water temperature 27.7℃, outlet water pressure 0.41MPa, rated current of each motor 29.9A, normal operating current 20.6A. Generally, one pump is operated with two as backup.
(3) Status of air conditioning equipment in the packaging air conditioning room
① The refrigeration units are Hitachi units, two in total. ② Chilled water pumps: 15kW, 2-pole full-pressure start (3 units), head 30m, outlet water temperature 5.8℃, return water temperature 9.3℃, outlet water pressure 0.38MPa, rated current of each motor 29.9A, normal operating current 20.2A. Generally, two units are operated with one unit on standby. ③ Cooling pumps: 15kW, 2-pole full-pressure start (3 units), head 30m, outlet water temperature 31.6℃, return water temperature 27.3℃, outlet water pressure 0.40MPa, rated current of each motor 29.9A, normal operating current 21.2A. Generally, two units are operated with one unit on standby.
(4) The company has a total of 13 air conditioning fan units. ① 7 air conditioning fan units in the Gene Department, including 3 22KW fan motors, 2 11KW fan motors, and 1 15KW and 18.5KW fan motors. ② 3 air conditioning fan units on the old second floor, including 2 15KW fan motors and 1 11KW fan motor. ③ 3 air conditioning fan units in the Quality Inspection Department, including 2 11KW fan motors and 1 7.5KW fan motor.
2. Water pump frequency conversion retrofit plan
Since the temperature difference between the inlet and outlet water of the chilled water pump and the cooling water pump is less than 5℃, this indicates that the chilled water flow rate and cooling water flow rate have a margin. Furthermore, the normal operating current of the motors is less than the rated current (5-12A), clearly indicating a "powered motor for a small load" phenomenon. Therefore, we implemented a one-to-three drive system for each of the chilled water and cooling water systems in the gene division, using one Delta VFD-P11KW inverter and one Delta VFD-P15KW inverter respectively (as shown in Figure 1). As needed, PLC1 controls the three chilled water pumps and three cooling water pumps to operate alternately (but at any given time, one inverter can only drive one pump motor), enabling flexible, convenient, timely, and appropriate automatic control of the chilled water and cooling water flow rates to meet the needs of the production process. Similarly, one Delta VFD-P15KW inverter was used to implement a one-to-three drive system for each of the chilled water and cooling water systems in the air conditioning room and packaging room on the second floor, with the control method being the same as that for the chilled water and cooling water systems in the gene division. The following explanation uses the chilled water system of the gene division as an example:
(1) Closed-loop control
The chilled water system in the gene division employs a fully closed-loop automatic temperature difference control. A single 11kW frequency converter is used to control three chilled water pumps. The specific method is as follows: First, all air valves in the central air conditioning water pump system are fully opened. While ensuring the required chilled water volume and pressure for the chiller units, the minimum operating frequency for one chilled water pump frequency converter is determined (35Hz during commissioning), set as the lower limit frequency, and locked. Two temperature sensors collect the outlet and return water temperatures on the main chilled water pipeline, transmitting the temperature difference signal to the temperature difference controller. The PID controller converts the temperature difference into an analog signal, feeding it back to the frequency converter. When the temperature difference is less than or equal to the set value of 5℃, the chilled water flow rate can be appropriately reduced. At this time, the frequency converter VVVF2 operates at a lower frequency, and the motor speed decreases. When the temperature difference is greater than the set value of 5℃, the frequency converter VVVF2 operates at a higher frequency, the motor speed increases, and the water flow rate increases. The number of chilled water pumps in operation and their adjustment are controlled by PLC1. This ensures that the appropriate flow rate is provided according to the real-time needs of the system, preventing energy waste.
(2) Open-loop control
Set the selector switch on the control panel to the open loop position and rotate the potentiometer clockwise to change the speed of the chilled water pump motor.
(3) Switching between power frequency and frequency conversion
In automatic operation, if the frequency converter fails, PLC1 will control another standby water pump motor to start operating at the mains frequency, and simultaneously issue an audible and visual alarm to alert on-duty personnel to promptly detect and handle the fault. Alternatively, the manual/automatic selector switch on the control cabinet panel can be switched to the manual position, and the corresponding start button can be pressed to start the corresponding water pump motor.
Figure 1. Schematic diagram of central air conditioning water pump frequency conversion retrofit.
3. Fan frequency conversion retrofit plan
Since all fans in the air handling units are operating at full capacity and under normal load, the airflow is regulated by the cold air outlet valves during temperature control. If the temperature inside the production workshop is too high, the valves are opened wider to increase the cold air volume and lower the temperature. If the temperature is too low, some valves are closed to reduce the cold air volume and maintain the thermal balance. Therefore, the airflow supplied to the production workshop is adjustable and variable. Especially during the night shift, there are few people and little activity, resulting in a light system load and significantly reduced cooling requirements. Only a small amount of cold air is needed to maintain the positive pressure and cooling capacity of the production workshop. Therefore, all 13 fans underwent frequency conversion energy-saving retrofitting, using frequency converters to regulate the airflow.
Figure 2 shows the schematic diagram of the central air conditioning fan frequency conversion retrofit. Based on the original power frequency control, seven frequency conversion control cabinets are added, using 13 Delta VFD-P series frequency converters to drive 13 fan motors. Frequency conversion and power frequency can be switched between each other. When operating in power frequency mode, the original operating procedure remains unchanged. When operating in frequency conversion mode, the frequency converters automatically output different frequencies at different times. Specifically, the 13 frequency converters are controlled by a time-controlled switch program. From 7:30 AM to 11:00 PM, Monday to Friday, the frequency converters are set to operate at 45Hz. From 11:00 PM to 7:30 AM the following day, Monday to Friday, and on Saturdays and Sundays, the frequency converters are set to operate at 35Hz (the operating frequency can be set as needed) to change the fan speed. Simultaneously, the 13 frequency converters communicate with the human-machine interface and PLC in the central monitoring room, enabling remote human-machine monitoring.
Figure 2. Schematic diagram of central air conditioning fan frequency conversion modification.
III. Energy-saving retrofit effect of frequency converter for central air conditioning water pumps and fans
To visually demonstrate the energy-saving effect of the frequency conversion retrofit, we conducted the following tests: Taking the No. 1 Hitachi unit cooling water pump 14# (15KW) and the K4 air handling unit 4# (22KW) as examples, we installed electricity meters on their respective main circuits. We first operated them at the power frequency for one week, recording the meter readings daily. Then, we operated them at the frequency for one week, repeating the same process. The data are shown in Tables 1 and 2.
Table 1: Energy Saving Data Statistics for Cooling Water Pump of Unit #1 Hitachi
1. Data Analysis in Table 1: During mains frequency operation, the pump load does not vary significantly, with daily power consumption around 298 kWh. During variable frequency operation, the daily power consumption varies considerably due to the greater influence of ambient temperature, but it is clear that the daily power consumption during variable frequency operation is significantly lower than that during mains frequency operation. Calculating based on the total weekly power consumption, the mains frequency consumption is 2580 - 891 = 1689 kWh, while the variable frequency consumption is 5248 - 4121 = 1127 kWh. Therefore, the power saving rate of the cooling water pump for Hitachi Unit #1 is: (1689 - 1127) / 1689 = 33%.
2. Data Analysis in Table 2: Since the daily load of the fan does not change significantly, its power consumption is relatively stable. It can be seen that the daily power consumption is approximately 350 kWh during mains frequency operation and approximately 220 kWh during variable frequency operation. Based on 350 kWh and 220 kWh, the power saving rate of the K4 fan motor is: (350-220)/350 = 37%.
The above calculations show that the average energy saving rate after the frequency conversion modification of water pumps and fans is 35%, and the energy saving effect will be even better in actual use.
Table 2: Energy Saving Data Statistics for K4 Air Handling Unit
IV. Central Air Conditioning Water Pump and Fan Variable Frequency Monitoring System
1. System Hardware Components
Figure 3 shows the hardware structure diagram of the central air conditioning water pump and fan frequency converter monitoring system. It consists of four subsystems: the company's tap water constant pressure pump, the chilled water pump on the second floor of the packaging department, the water pump and fan in the air conditioning room on the second floor of the quality inspection department, and the water pump and fan in the gene department. It remotely monitors 19 frequency converters distributed across different departments. The details of each component are as follows: ① The frequency converter used is the Delta VFD-P series. This series of frequency converters features high reliability, low noise, high energy efficiency, comprehensive protection functions, and a built-in powerful RS-485 serial communication interface. The RS-485 serial communication protocol is also publicly available to users. ② The PLC, as the control unit, is the core of the entire system. A Delta DVP24ES01R is selected. The program is written using its communication instructions, downloaded to the PLC, and then connected to the RS-485 serial communication interface of the frequency converter to achieve real-time communication. ③ The human-machine interface uses a Hitech PWS-3760, a 10.4-inch color screen. It is a new generation of high-tech programmable terminal, an interactive workstation designed specifically for PLCs. It has the capability to connect and monitor PLCs of various brands, making it suitable for use in harsh industrial environments and a replacement for ordinary or industrial control computers. Its main features include: large screen capacity (up to 255 screens), simple screen layout; use of ADP3 full Chinese operating software, compatible with WINDOWS95/WINDOWS98 environments, rich macro instruction set, and simple programming; good interactivity, strong anti-interference capability, and high communication reliability; high degree of automation, simple and convenient operation, low failure rate, long lifespan, and minimal maintenance. Its main functions include: designers can edit various screens as needed, displaying real-time equipment status or system operation instructions; touch buttons on the human-machine interface can generate corresponding switch signals or input values and characters for data exchange with the PLC, thereby generating corresponding actions to control equipment operation; multiple screens can be overlaid or switched, displaying text, numbers, graphics, strings, alarm information, action flows, statistical data, historical records, trend charts, and simple reports. ④ RS-485 serial communication method: RS-485 adopts a balanced transmit and receive method, which has the advantages of long transmission distance, strong anti-interference ability and multi-station capability.
Figure 3 Hardware structure diagram of the frequency converter monitoring system
2. Human-computer interface design
All screens of the HMI in this system are designed using the ADP3 fully Chinese software. They include a main screen, parameter settings, operation settings, parameter display, status information, alarm information, and help screens. After being compiled correctly by the ADP3 software, the HMI can be downloaded to a personal computer and used immediately. The HMI and PLC are connected via an RS232 communication cable in a master-slave configuration. The PLC reads and writes data to the status control area and notification area of the HMI to achieve information exchange between the two. The PLC reads data from the HMI status notification area to obtain the current screen number, and forces a screen switch by writing data to the HMI status control area. One of the parameter display screens is shown in Figure 4.
Figure 4: Monitoring screen of the No. 1 central air conditioning fan and water pump in the Gene Department
The user needs to monitor the voltage, current, and frequency of 19 water pumps and fans. Therefore, three sets of numerical display areas are set up to display the voltage, current, and frequency values respectively, which is achieved using the numerical display function of the components. After the system starts, the 19 frequency converters periodically report their operating status to the PLC. After processing by the PLC, the data is sent to the human-machine interface (HMI), allowing the HMI to display these three sets of values in real time. The format, number of bits, and precision of the values are set in the numerical display attribute box according to the actual situation.
3. System Control Methods
This system requires remote monitoring of 19 frequency converters distributed across different departments at considerable distances. It should allow for automatic/manual setting, modification, and writing of frequency values and start/stop of each frequency converter via a human-machine interface in the central monitoring room. It should also monitor the actual operating voltage, current, and frequency of the central air conditioning water pump fan motors in real time and include audible and visual alarms. The specific control method involves using a DVP-PLC, a PWS-3760 human-machine interface, and 19 VFD-P series frequency converters to form a real-time communication network via RS-485 serial communication (as shown in Figure 3). The communication parameters for the 19 frequency converters are set on-site, such as RS-485 communication commands, communication addresses (1-19), baud rate (9600), and communication data format. The system PLC program is designed, and its flowchart is shown in Figure 5. The manual control requires functions such as real-time setting, modification, and writing of frequency values, as well as starting and stopping individual frequency inverters. Automatic control employs two time-period control, allowing for the setting of two time-period values and their corresponding frequency values at any time. Currently, time-period one is set to 7:30, corresponding to frequency one at 45Hz, and time-period two is set to 23:00, corresponding to frequency two at 35Hz. The program design references the VFD-P frequency inverter communication protocol, using RS-485 communication commands between the PLC and the frequency inverters to achieve remote system monitoring. Reports can also be printed via a printer.
Figure 5 System Program Flowchart
V. Conclusion
The use of AC variable frequency drives (VFDs) for energy-saving retrofitting of water pumps and fans in central air conditioning systems not only simplifies operation and saves electricity, reducing production costs, but also significantly improves operating conditions and reduces maintenance requirements for pumps, fans, and valves. This VFD retrofit project and monitoring system have been running continuously for over two years since its commissioning in May 2002. The system operates reliably and stably, with accurate and timely communication data, standardizing equipment management and improving work efficiency. The online adjustments required are for time-period and frequency setpoints. A human-machine interface (HMI) is used for interaction, offering a simple, intuitive, and easy-to-operate interface. A PLC serves as the central processing unit. The combined use of the VFD monitoring system enables remote monitoring, manual real-time frequency conversion, and automatic time-period frequency conversion. This approach has yielded excellent results in practical use and is worthy of wider application in other industries.
