Control Requirements for the Variable Frequency Speed Regulation System of the Primary Chilled Water Pump

01 / The variable primary flow control system of air conditioning water (VPF: Variable-Primary-Flow, also known as: the variable frequency speed regulation control system of the primary chilled water pump) is equipped with variable frequency speed regulation chilled water pumps, which can adjust the flow rate of chilled water. Although variable water volume control is applied on the load side, the primary side control with variable frequency speed regulation is different from the traditional primary pump system with fixed rotation speed, and its control requirements are much higher than the traditional method. This article discusses this solution in combination with the variable primary flow control engineering scheme of a large building.
The chilled water system of a large building discussed here has a total of 3 centrifugal chillers with equal capacity, and the capacity of a single unit is 500 refrigeration tons. The control scheme of the air conditioning chilled water system with variable primary flow is closely related to the selection and layout of the chiller equipment. In order to achieve the predetermined control target, certain requirements should be put forward for the technical elements of the relevant equipment. In a word, it is to optimize the system design scheme. This is the responsibility that architects and designers of building automation control systems should undertake.
02 / The monitoring and control scheme of the chiller
According to the equipment configuration of the air conditioning chilled water system of this building, its monitoring and control system can be divided into several aspects, which are described as follows:
1. Monitoring and control of the working conditions of all equipment
1) General monitoring contents of chilled water pumps, cooling water pumps and cooling tower fans: Monitor the operation status, fault status and manual/automatic status; Accumulate the operation time and count the number of operations; The BAS can start and stop these equipment.
2) Variable frequency speed regulation control of chilled water pumps: Monitor the pressure difference at the most unfavorable load point between the main supply and return water pipes; Use the above pressure difference as a process variable to carry out variable frequency speed regulation control of the chilled water pumps.
3) Control of cooling tower fans: Monitor the temperatures of the main supply and return water pipes of the cooling tower; Take the set value of the cooling tower supply water temperature as the target to carry out variable frequency speed regulation control of the cooling tower fans.
4) Monitoring contents of the electric butterfly valve: Corresponding interlock switch control.
5) The control system should also monitor the temperatures of the main supply and return chilled water pipes; The flow rate of the main supply chilled water pipe;
6) Set up a data communication interface for the chiller: Through this interface, the following technological parameters of the chiller are transmitted (or received) between the BAS and the chiller:
Operation status of the main unit
Fault alarm status of the main unit (which can represent multiple fault information in a coded manner)
Absolute value or percentage of the load level of the main unit, or the current or percentage of the current of the main unit
Current set value of the supply water temperature
Measured values of the inlet and outlet water temperatures of the evaporator
Measured values of the inlet and outlet water temperatures of the condenser
Measured value of the (chilled) water flow rate of the evaporator
Measured value of the pressure in the refrigerant pipeline of the evaporator
Measured value of the (cooling) water flow rate of the condenser
Operation time of this run and accumulated operation time
Accumulated number of starts
Temperature and supply level of the lubricating oil
Receive the reset value of the chilled water supply temperature given by the BAS
Receive the start and stop control commands issued by the BAS
It is an optimal solution for the control system to obtain the corresponding key working condition parameters through communication. However, which data can be exchanged between the BAS and the main chiller unit depends on the brand, model and specification of the chiller. Therefore, it is necessary to have a further understanding of the controller of the main chiller unit and confirm the signals that can be transmitted. The parameters listed above are the parameters required for the BAS to complete the control, and it does not mean that any main chiller unit will provide these technological parameters. Therefore, from the perspective of the requirements of system composition and system integration, the monitoring points of the BAS must supplement and complete the parameters obtained through data communication.
Especially because this system is a variable primary flow control, the monitoring of the water flow rate in the evaporator pipeline is essential and requires high attention. Similarly, for the parameters that can be obtained through communication, the BAS does not need to be equipped with monitoring elements to avoid the repeated setting of monitoring points and causing waste.
2. Interlock control of the start and stop of the chiller
The interlock control of the start and stop of the main unit and the related equipment is an important condition for ensuring the normal and safe operation of the equipment.
1) Interlock start sequence: Cooling tower fan → Electric butterfly valve of the cooling tower → Electric butterfly valve of the condenser of the chiller → Cooling water pump → Signal indication of the condenser water flow switch (as the return signal of the interlock condition) → Electric butterfly valve of the evaporator of the chiller → Chilled water pump → Signal indication of the chilled water flow switch (as the return signal of the interlock condition) → Main chiller unit
2) Interlock stop sequence: Chiller → (delay for a few minutes, determine the delay time according to the equipment requirements and working conditions) → Chilled water pump → Signal indication of the evaporator water flow switch (as the return signal of the interlock condition) → Electric butterfly valve of the evaporator of the chiller → Cooling water pump → Signal indication of the condenser water flow switch (as the return signal of the interlock condition) → Electric butterfly valve of the condenser of the chiller → Electric butterfly valve of the cooling tower → Cooling tower fan
Aspects that need special attention in dynamic start and stop control: The full stroke time for opening or closing the electric butterfly valve is approximately more than 30 seconds. During the start and stop switching process, the phenomenon of water backflow caused by the shared main pipe should be avoided as much as possible. This system uses variable frequency speed regulation water pumps. If the opening degree of the electric valve, the water volume of the water pump and the start of the chiller can be properly coordinated and controlled, this problem may be better overcome. The time interval of this synchronous control and interlock control needs to be determined according to a certain commissioning method during the commissioning. Before achieving this refined control, this phenomenon of water backflow within a limited time period is generally tolerable.
3. Variable primary water volume control of the chiller
In order to save energy, the number of operating chiller units is controlled according to the total load level of the cold source system, with the aim of making the number of operating chillers meet the needs of the system load and making the started chillers operate in the working condition area with the highest efficiency as much as possible. The following briefly introduces this control scheme in combination with the variable frequency speed regulation control of the chilled water pumps.
The optimization strategy of the operation number control is as follows: Firstly, the supply amount (total flow rate) of the chilled water of the control system should always be maintained at a level that just meets the requirements of the system load, and it is ensured that the pressure difference at the most unfavorable load point between the main supply and return pipes of the system is stable at a certain preset value, which needs to be completed by high-quality variable frequency speed regulation control of the chilled water pumps; Secondly, according to the difference between the system load and the current supply capacity of the chillers, determine the timing of the next start or stop. The principle is to choose the working mode that can meet the load demand and is closest to the system demand value.

The key to this control is to find the best start and stop switching points during the commissioning; In addition, in the program of the operation number control, the chillers should be around the optimal working efficiency point. The working efficiency distribution curve of the chiller is provided by the supplier.
Configuring chillers with different capacities in the system can reduce the steps of the change in the system load capacity in the number control, make the operation capacity closer to the system demand, and have a better energy-saving effect. However, the control requirements are much more complex than those of the parallel units with equal capacity.
Now, after using all variable frequency speed regulation chilled water pumps, the system's water supply flow rate (cooling capacity) can be continuously adjusted according to the load demand. Therefore, there is no need to configure parallel units with different specifications and capacities.
In the traditional primary pump system, variable flow control is only implemented on the load side, while the flow rate on the primary side of the cold source is constant (the chilled water pump runs at a fixed speed). The flow difference and system pressure between the two are balanced by the differential pressure bypass control. During normal operation, there will inevitably be bypass water flow in the bypass pipe. That is to say, a part of the work done by the chilled water pump running at full speed is consumed in the bypass circulation. At the same time, the relatively high pressure between the main supply and return pipes generated by the pump head causes a large amount of water flow kinetic energy to be consumed in overcoming the pipeline resistance.

The most valuable function of the variable primary flow (variable frequency speed regulation of the chilled water pump) control system of the air conditioning chilled water system is to avoid most of the unnecessary bypass water flow and maintain a lower system pressure, thus greatly reducing the power consumption of the pump.
General method of variable primary flow control: Take the design differential pressure at the most unfavorable condition point between the main supply and return pipes of the system (generally, it should be selected at the far end of the main supply and return pipes) as the control preset value, take the measured value of the differential pressure at this point as the process variable for PID adjustment, and take the variable frequency speed regulation chilled water pump as the actuator of the control system to adjust and control the chilled water supply. The control objective is to make the process variable approach the preset value; when the system load flow rate of the chilled water is greater than the minimum allowable flow value of the evaporator of one chiller, the flow bypass control valve is closed; when only one unit is in operation and the system load flow rate of the chilled water is lower than the minimum allowable flow value of the evaporator of one chiller, it is necessary to start the bypass flow control between the water collector and the water divider. The control objective is to make the water flow rate of the evaporator of this running chiller greater than or equal to the minimum allowable flow value. This bypass valve is different from the traditional differential pressure bypass control valve and can be called a low-flow bypass control valve.
Ensuring the minimum water flow rate of the chiller evaporator is a very important technical measure. Otherwise, it may disrupt the normal working state of the chiller and even damage the chiller. The bypass control of the variable primary flow control is actually set up specifically for this protection.

For the working mode of the variable primary flow system, this bypass valve does not need to be very large, and its flow capacity only needs to ensure the minimum water flow rate of the evaporator of the large-unit chiller. However, currently, when designing a variable primary flow system, it is often necessary to retain the possibility of working in the traditional mode (all chilled water pumps run at a constant speed). Considering this point, this flow bypass valve may also perform the function of traditional differential pressure bypass control, so its diameter should meet such requirements. For this reason, the bypass valve in this scheme does not choose a smaller flow capacity. This design configuration is a necessary measure to enhance the fault tolerance of the chilled water system and improve the reliability of system operation.
The control logic should ensure: At any time, ensure the minimum water flow rate of the evaporator of the running chiller (this minimum water flow rate is specified in the chiller's instruction manual); synchronous variable frequency speed regulation control of parallel operating units is the most commonly used control method; the chilled water pumps of the chiller units should preferably be started in a soft-start mode to avoid a large current impact on the pump motor; when the chilled water adjustment and control are switched from one unit to another, a smooth and seamless transition should be ensured.
4. Control of Cooling Water and Cooling Tower
The basic requirement for the control of cooling water is to ensure that the temperature of the cooling water treated by the cooling tower (i.e., the temperature of the cooling water entering the condenser of the chiller, which can also be called the supply water temperature of the cooling tower) meets the requirements of the chiller. This temperature requirement is provided in the chiller's instruction manual.
Control the number of operating cooling tower fans according to the supply water temperature of the cooling water. When the supply water temperature of the cooling water is higher than the required value, increase the air volume (or increase the number of operating cooling tower fans), and vice versa, to reduce the energy consumption.
For a cooling tower with multiple fans (or multiple speeds), if the supply water temperature of the cooling water still cannot meet the process requirements after all fans are fully opened (or running at the highest speed), another cooling tower should be started through the BAS program to enhance the cooling effect.
Therefore, the necessary conditions for the control of the cooling tower are: monitoring the supply water temperature of the cooling tower; controlling the cooling tower fans and the corresponding electric butterfly valves.
The variable frequency speed regulation control of the cooling water also aims to save energy. When the system's demand for chilled water decreases, the demand for cooling water will also decrease accordingly. When this load change is reflected in the cooling water, it is manifested as the outlet water temperature of the condenser being lower than the rated value and the inlet water temperature of the cooling tower being lower.

At this time, the number of operating fans of the cooling tower can be reduced, and at the same time, the operating level of the cooling water pump can be lowered to save the power consumption of the cooling water pump.
There is no direct logical relationship between the variable frequency speed regulation of the cooling water and that of the chilled water in terms of control, only an indirect influence. However, similar to the minimum water flow rate limit of the evaporator, the condenser also has a minimum water flow rate limit. Therefore, when adjusting the speed of the cooling pump, there is also a requirement for setting a minimum value limit.
03 / Technical Measures to Improve System Fault Tolerance and Reliability
The air conditioning chilled water control system includes the cold source side and the load side, involves many devices, and has complex control. Whether the air conditioning device operates normally directly affects the comprehensive functions of the building's equipment and is of great significance. The control scheme adopts variable primary flow, which has stricter technical requirements. Therefore, the design of the control system should include technical measures to improve the system's fault tolerance and reliability.
1) In addition to the main working mode, a backup working mode is also considered. The main working mode of the system takes variable primary flow control as the design objective, and at the same time, it retains the possibility of working in the traditional mode (as a backup mode). When all equipment is operating normally, the closed-loop control of the variable frequency speed regulation chilled water pump is put into operation, and the system operates under variable primary flow control; in case the closed-loop control of the variable frequency speed regulation cannot be put into operation, the system can also be easily switched to the traditional working mode, that is, the chilled water pump runs at a constant speed, and the differential pressure bypass control is put into operation at the same time. The system can still operate normally, but the energy-saving effect of the latter is worse than that of the former.
To achieve this backup operation capability, it is only necessary to select the specifications of the low-flow bypass valve that can also be used for differential pressure bypass control. Set the programs for these two control modes in the corresponding controller and enable the operator to select the mode. Since there is not much difference in equipment configuration between the two working modes, the configuration and selection of the control components can meet the requirements of these two working conditions.
2) The system operation can be partially independent of the communication between the BAS and the chiller. As introduced before, the supply and return water temperatures of the cooling water (i.e., the inlet and outlet temperatures of the condenser) can be obtained through the data communication of the chiller. On the surface, it seems that there is no need to measure the temperatures of the main supply and return pipes of the cooling water. In fact, this is not the case. Adding the measurement of the temperatures of the main supply and return pipes of the cooling water can make the monitoring and control of the cooling water system independent of the data communication between the BAS and the chiller. The BAS can still control the supply water temperature of the cooling water as usual, greatly improving the fault tolerance of the system operation. The cost is not high, but the flexibility and reliability brought by it can play a role in the long term. When the system does not rely on the communication between the BAS and the chiller, the system can only operate in the backup mode, and the start and stop control of the chiller units is manually operated.