A three-stage pump variable flow automatic control method

Abstract

The present application discloses a three-stage pump variable flow automatic control method, and the combined unit cascade control module is as follows: the main circuit uses the supply air temperature set value as the control parameter and the return air temperature as the controlled parameter, detects the return air temperature feedback value in real time, performs PID calculation based on the deviation between the return air temperature feedback value and its set value, and obtains the unit supply air temperature set value under the current working condition; the secondary circuit uses the water valve opening as the control parameter and the supply air temperature as the controlled parameter, performs PID calculation on the deviation between the supply air temperature set value and the detected supply air temperature feedback value, and adjusts the regulating valve opening on the return water pipe to ensure that the return air temperature meets the set value requirement. The present application not only realizes the automatic adjustment of the frequency of water pump operation and the automatic increase and decrease of the number of water pumps in operation, but also ensures the load demand of the equipment end of the most unfavorable loop, while also taking into account the energy-saving operation of the waterway transportation system, so that the system loop impedance is minimized and the efficiency is optimized.

Description

Technical Field

[0001] The present application belongs to the technical field of heating, ventilation and air conditioning and building automation control, and specifically relates to a three-stage pump variable flow automatic control method, which can be applied to central air-conditioning systems. Background Art

[0002] Variable flow water systems with three-stage pumps at the end of central air conditioning systems are one of the most effective energy-saving measures, and their research and application are increasingly gaining attention. However, in actual operation, these pumps often fail to achieve the desired energy-saving results by varying their frequency according to design requirements, or rely solely on manual frequency setting based on field staff's operational experience. This is primarily due to the ineffectiveness of their automatic control mechanisms. Currently, the most commonly used control method for variable flow water systems with three-stage pumps at the end of central air conditioning systems is worst-case differential pressure control. This method monitors the pressure differential before and after the air conditioning equipment in the worst-case loop of the air conditioning water system. Once this pressure differential meets the design requirements, other branches can operate normally. However, this control method can easily result in a "high flow rate with low temperature differential" scenario, increasing system energy consumption. Temperature differential control is also a common control scheme for three-stage pump variable flow water systems. Temperature sensors are installed on the supply and return pipes, and the measured temperature differential is used as a reference to adjust the pump's operating frequency. When the temperature difference is used as the control signal for the water pump frequency conversion, due to the long water circulation loop, the actual adjustment process is usually dynamic, and the temperature adjustment cycle is often relatively long. At the same time, the temperature difference set value is difficult to determine and cannot accurately match the terminal load demand. Summary of the Invention

[0003] The purpose of this application is to provide a variable flow automatic control method for a three-stage pump to solve the problem in the prior art that the three-stage pump cannot meet the terminal load demand and "large flow and small temperature difference" in a timely manner. The control method of this application is applicable to the variable flow water system of the three-stage pump at the end of any central air-conditioning system.

[0004] The purpose of this application is achieved through the following technical solutions:

[0005] A three-stage pump variable flow automatic control method includes a combined unit cascade control module and a three-stage pump variable flow cascade control module;

[0006] The automatic control method of the combined unit cascade control module is as follows: the main loop uses the supply air temperature set value as the control parameter and the return air temperature as the controlled parameter. The return air temperature feedback value is detected in real time. PID calculation is performed based on the deviation between the return air temperature feedback value and the return air temperature set value to obtain the unit's supply air temperature set value under the current operating conditions. The secondary loop uses the water valve opening as the control parameter and the supply air temperature as the controlled parameter. PID calculation is performed based on the deviation between the supply air temperature set value and the detected supply air temperature feedback value. The opening of the regulating valve on the return water pipe is adjusted to ensure that the return air temperature meets the set value requirement.

[0007] The automatic control method of the three-stage pump variable flow cascade control module is as follows: the main loop uses the supply and return water temperature difference set value as the control parameter, the water valve opening as the controlled parameter, and detects the water valve opening feedback value in real time. PID calculation is performed based on the deviation between the water valve opening feedback value and the water valve opening set value to calculate the supply and return water temperature difference set value under the current working conditions; the secondary loop uses the water pump frequency as the control parameter, and the supply and return water temperature difference as the controlled parameter. PID calculation is performed on the deviation between the supply and return water temperature difference set value and the detected supply and return water temperature difference feedback value to adjust the water pump frequency to ensure that the water supply flow meets the terminal load demand.

[0008] Furthermore, the control valve adopts an equal percentage control valve. When the pressure difference before and after the control valve remains unchanged, the flow characteristics of the control valve conform to the ideal flow characteristics. The ideal flow characteristics of the equal percentage control valve are expressed by the mathematical expression:

[0009] ;

[0010] Among them: G i -Flow rate through the regulating valve at a certain opening, m 3 / h;G iMAX -Flow through the regulating valve in fully open state, m 3 / h; l-valve opening at a certain valve position; l MAX -The opening of the valve in the fully open state; R-the ideal adjustable ratio of the regulating valve.

[0011] Furthermore, in a chilled water cycle, when the terminal load is fixed, the temperature difference between the supply water temperature and the return water temperature reflects the chilled water flow required by the terminal under the current load. The process is expressed by the following formula:

[0012] ;

[0013] Where: Q is heat, in joules (J); c is specific heat, in joules / kilogram•degrees Celsius (J / kg•℃); - Fluid density, kg / m3 (kg / m 3 ); t1-return water temperature, degrees Celsius (℃); t2-supply water temperature, degrees Celsius (℃).

[0014] Furthermore, in order to ensure that the water valve opening feedback value can truly reflect the chilled water flow required by the system, the water valve opening feedback values of all units in the system are weighted averaged, that is, The weighted value of the valve opening of the unit is used for conversion to calculate the converted value of the valve opening of the system. The process is equivalently described by the following formula:

[0015] ;

[0016] in: -Weighted weight; Q i - Rated cooling capacity of a single unit, Watt (W); Q 总 -Total system cooling capacity, watts (W);

[0017] ;

[0018] in: -Conversion value of system valve opening; - Weighted value of valve position of a single unit; -Valve opening of a single unit.

[0019] Furthermore, in the unit pipe network system, the relationship between the relative values of pump efficiency, flow rate and speed is expressed by the following expression:

[0020] ;

[0021] Where: η-water pump efficiency; Q-relative value of water flow; n-relative value of water pump speed; C1, C2-constants.

[0022] Furthermore, in order to keep the pipe network impedance to a minimum and the pump efficiency to a maximum while meeting the pipe network flow demand, the local resistance of the pipe network is reduced by opening the terminal regulating valve, the pipe network characteristic curve is adjusted, the pump efficiency is increased by reducing the pump speed, and the pump characteristic curve is adjusted, thereby effectively reducing the pump energy consumption.

[0023] Furthermore, the water valve opening setting value is set to 90% to ensure the minimum impedance of the pipeline and the highest efficiency of the water pump; the supply and return water temperature difference under the maximum load and minimum load of the system is 、 As the upper and lower limits of the water valve PID controller, determined based on historical data 、 The initial value will be updated in real time according to the actual data in the later operation stage.

[0024] Furthermore, considering the impact of the most unfavorable loop, the opening of each water valve on site is detected. If the system is in a stable state, that is, the water valve opening of most units is near the set value, but there are still a small number of units with water valves opening at 100% fully open, it is considered that the loop where the unit is located is the most unfavorable loop; at this time, the water valve opening feedback value of the unit is used to replace the system valve opening conversion value as the water valve opening feedback value to ensure that energy saving effect is achieved while meeting the load demand of the most unfavorable loop.

[0025] Furthermore, when the water valve opening feedback of the most unfavorable loop unit is less than 90%, the water valve opening feedback value is automatically adjusted to the system valve opening conversion value and compared with the water valve opening set value.

[0026] Furthermore, when the frequency of a single water pump reaches the maximum but the supply and return water temperature difference still does not meet the demand, a water pump is loaded at this time; considering the sudden change in flow when adding or removing the water pump, the operating frequency of the current water pump is reduced to the minimum frequency, and the loaded water pump is also increased to the minimum frequency to ensure a smooth flow change; the two water pumps are operated at the same frequency as a whole; otherwise, the water pump is unloaded.

[0027] This application adopts the cascade control of the terminal unit valve position and the supply and return water temperature difference, which accelerates the adjustment speed and accuracy of the temperature difference control while ensuring the flow demand of the most unfavorable terminal. This solution can find the optimal energy-saving working condition that meets the current load according to the system load, and minimize the occurrence of "large flow and small temperature difference" situation. Its embodiment is implemented as follows:

[0028] The variable flow automatic control method of the three-stage pump of the central air-conditioning system is divided into two control modules: the combined unit cascade control module (combined air-conditioning unit cascade control module) adopts the supply air temperature-return air temperature cascade control scheme, compares the return air temperature feedback value with the set value, and calculates the supply air temperature set value based on the deviation between the two using the PID (proportional-integral-differential algorithm). The deviation between the calculated supply air temperature set value and the supply air temperature feedback value is used to adjust the opening of the unit's electric control valve using the PID (proportional-integral-differential algorithm) to ensure that the ambient temperature in the target area is maintained near the return air temperature set value. The combined unit cascade control module ensures that the opening of the unit's electric regulating valve can meet the load requirements of the adjustment area. On this basis, the three-stage pump variable flow cascade control module detects the corresponding valve openings of all air-conditioning units in the current system in real time, calculates the weighted average value of the water valve opening in the area, compares the weighted average value with the set value, and uses the PID (proportional-integral-differential algorithm) to calculate the supply and return water temperature difference set value of the area based on the deviation between the two. It also detects the current supply and return water temperature difference feedback value of the system in real time, and uses the PID (proportional-integral-differential algorithm) to calculate the current area three-stage pump frequency set value based on the deviation between the two, so as to achieve the purpose of real-time matching of the three-stage pump flow according to the terminal load changes.

[0029] The beneficial effects of the present application are as follows: the logic not only realizes the automatic adjustment of the frequency of the water pump operation and the automatic increase and decrease of the number of water pumps in operation, but also ensures the load demand of the equipment end of the most unfavorable loop, while also taking into account the energy-saving operation of the water transportation system, so that the system loop impedance is minimized and the efficiency is optimized.

[0030] The aforementioned main solution and its various further options can be freely combined to form multiple solutions, all of which are solutions that can be adopted and protected by this application. Furthermore, in this application, (non-conflicting options) can also be freely combined with each other and with other options. After understanding the solution of this application, those skilled in the art will understand that there are many possible combinations based on existing technology and common knowledge, all of which are technical solutions to be protected by this application, and these are not exhaustive here. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a flow chart of the cascade control module of the combined unit of this application.

[0032] Figure 2 This is the process diagram of the three-stage pump variable flow cascade control module of this application Figure 1 .

[0033] Figure 3 This is the QH curve diagram of the pipeline network in this application.

[0034] Figure 4 This is the process diagram of the three-stage pump variable flow cascade control module of this application Figure 2 .

[0035] Figure 5 It is a parallel diagram of the combined air conditioner of this application. DETAILED DESCRIPTION

[0036] The following non-limiting examples illustrate the present application.

[0037] Example 1

[0038] refer to Figures 1 to 5 As shown, a three-stage pump variable flow automatic control method includes a combined unit cascade control module and a three-stage pump variable flow cascade control module.

[0039] refer to Figure 1As shown in the figure, the automatic control method of the combined unit cascade control module is as follows: the main loop uses the supply air temperature setpoint as the control parameter and the return air temperature as the controlled parameter. The return air temperature feedback value is detected in real time. The PID (Proportional-Integral-Differential) algorithm is used to calculate the deviation between the return air temperature feedback value and the return air temperature setpoint to obtain the unit's supply air temperature setpoint under the current operating conditions. The secondary loop uses the water valve opening as the control parameter and the supply air temperature as the controlled parameter. The PID (Proportional-Integral-Differential) algorithm is used to calculate the deviation between the supply air temperature setpoint and the detected supply air temperature feedback value. The PID algorithm is used to adjust the opening of the regulating valve on the return water pipe to ensure that the return air temperature meets the setpoint requirement, that is, the terminal load demand.

[0040] The regulating valve adopts an equal percentage regulating valve (one of the most commonly used valve types in terminal air conditioning units). When the pressure difference before and after the regulating valve remains unchanged, the flow characteristics of the regulating valve conform to the ideal flow characteristics. The ideal flow characteristics of the equal percentage regulating valve can be expressed by the mathematical expression:

[0041] ;

[0042] Among them: G i -Flow rate through the regulating valve at a certain opening, m 3 / h;G iMAX -Flow through the regulating valve in fully open state, m 3 / h; l-valve opening at a certain valve position; l MAX -The opening of the valve in the fully open state; R-the ideal adjustable ratio of the regulating valve, take 30.

[0043] In a chilled water cycle, when the terminal load is fixed, the temperature difference between the supply water temperature and the return water temperature reflects the chilled water flow required by the terminal under the current load. The process is expressed by the following formula:

[0044] ;

[0045] Where: Q is heat, in joules (J); c is specific heat, in joules / kilogram•degrees Celsius (J / kg•℃); - Fluid density, kg / m3 (kg / m 3 ); t1-return water temperature, degrees Celsius (℃); t2-supply water temperature, degrees Celsius (℃).

[0046] When circulating in the same closed cold water system, the specific heat capacity c and fluid density The flow rate required by the system is inversely proportional to the temperature difference between the supply and return water. The greater the temperature difference, the smaller the required flow rate; the smaller the temperature difference, the greater the required flow rate.

[0047] The most energy-efficient air conditioning system is one in which the cooling capacity provided by the system exactly matches the cooling capacity required by the terminal load. As can be seen from the above formula, cooling capacity is primarily determined by the chilled water flow rate and the supply / return water temperature difference. The air conditioning unit's regulating valve position is positively correlated with the flow rate, meaning that cooling capacity depends on the unit's regulating valve position and the supply / return water temperature difference. Therefore, a valve position-supply / return water temperature difference cascade control module was designed. This module uses the air conditioning unit's valve position and the supply / return water temperature difference to determine the terminal system's flow rate and supply / return water temperature requirements, thus fully ensuring the terminal load requirements.

[0048] refer to Figure 2 and Figure 4 As shown in the figure, the automatic control method of the three-stage pump variable flow cascade control module is as follows: the main loop uses the supply and return water temperature difference set value as the control parameter and the water valve opening as the controlled parameter. The water valve opening feedback value is detected in real time. The PID (proportional-integral-differential algorithm) calculation is performed based on the deviation between the water valve opening feedback value and the water valve opening set value to calculate the supply and return water temperature difference set value under the current operating conditions. The secondary loop uses the water pump frequency as the control parameter and the supply and return water temperature difference as the controlled parameter. The PID (proportional-integral-differential algorithm) calculation is performed based on the deviation between the supply and return water temperature difference set value and the detected supply and return water temperature difference feedback value. The pump frequency is adjusted to ensure that the water supply flow meets the terminal load requirements, thereby ensuring that the water valve opening meets the set value requirements.

[0049] In order to ensure that the water valve opening feedback value can truly reflect the chilled water flow required by the system, the water valve opening feedback values of all units in the system are weighted averaged, that is, The weighted value of the valve opening of the unit is used for conversion to calculate the converted value of the valve opening of the system. The process is equivalently described by the following formula:

[0050] ;

[0051] in: -Weighted weight; Q i - Rated cooling capacity of a single unit, Watt (W); Q 总 -Total system cooling capacity, watts (W);

[0052] ;

[0053] in: -Conversion value of system valve opening; - Weighted value of valve position of a single unit; -Valve opening of a single unit.

[0054] In the central air conditioning unit pipe network system, the relationship between the relative values of water pump efficiency, flow rate and speed is expressed by the following expression:

[0055] ;

[0056] Where: η is the efficiency of the water pump; Q is the relative value of the water flow rate; n is the relative value of the water pump speed; C1, C2 are constants, and C1-C2=1.

[0057] The above expression shows that, given a constant chilled water flow rate, the lower the pump speed, the higher the efficiency. Considering that the resistance loss caused by the regulating valve is a significant component of the resistance loss in the piping system, given a constant chilled water flow rate, the lower the system piping impedance, the lower the required pump frequency, the lower the pump speed, and the higher the pump efficiency.

[0058] refer to Figure 3 As shown in the figure, before adjustment, the pump operating point is at point B. To minimize network impedance and maximize pump efficiency while still meeting the network flow requirements, the network characteristic curve is adjusted by opening the terminal control valve as much as possible to reduce local network resistance. Pump efficiency is increased by reducing the pump speed, adjusting the pump characteristic curve. This shifts the pump operating point from point B to point C, effectively reducing pump energy consumption.

[0059] To ensure that the water pump operates in a high-efficiency range and the system operates in an energy-saving state of "small flow and large temperature difference", the maximum opening of the system valve is maintained as much as possible to reduce the pipeline impedance while meeting the load demand during operation. The water valve opening setting value is set to 90% to ensure the minimum pipeline impedance and the highest efficiency of the water pump. The temperature difference between the supply and return water under the maximum and minimum loads of the system is 、 As the upper and lower limits of the water valve PID controller, determined based on historical data 、 Initial value, and updated in real time according to actual data in the later operation stage.

[0060] Considering the impact of the most unfavorable loop, the water valve openings of each unit are tested on-site. If the system is stable, meaning that the water valve openings of most units are near the set value, but a small number of units still have water valves at 100% open, the loop where this unit is located is considered the most unfavorable loop. In this case, the water valve opening feedback value of this unit replaces the converted value of the system valve opening as the water valve opening feedback value to ensure energy saving while meeting the load requirements of the most unfavorable loop.

[0061] When the water valve opening feedback of the most unfavorable loop unit is less than 90%, the water valve opening feedback value is automatically adjusted to the system valve opening conversion value and compared with the water valve opening set value.

[0062] When the frequency of a single pump reaches its maximum, but the supply and return water temperature difference still doesn't meet demand, a second pump is added. To account for sudden changes in flow rate when adding or removing pumps, the current pump's operating frequency is reduced to its minimum frequency, while the added pump is also increased to its minimum frequency to ensure a smooth flow rate change. The two pumps are operated as a group at the same frequency, using the frequency generated by the above logic as their common frequency setting. Otherwise, the pumps are unloaded.

[0063] refer to Figure 5 As shown, the control method of this application is based on the terminal structure of the air conditioning system. The terminal is composed of modular air conditioning units connected in parallel. Each modular air conditioning unit is equipped with an electric regulating valve on the return pipe. The water supply to the modular air conditioning unit coil is adjusted by adjusting the opening of the electric regulating valve. The cooling and heating station provides chilled water to the terminal. The three-stage pump group pressurizes the chilled water provided by the cooling and heating station and delivers it to the modular air conditioning unit to provide cooling capacity.

[0064] The logic of the present application not only realizes the automatic adjustment of the frequency of water pump operation and the automatic increase and decrease of the number of water pumps in operation, but also ensures the load demand of the equipment end of the most unfavorable loop. At the same time, it also takes into account the energy-saving operation of the water transportation system, so that the system loop impedance is minimized and the efficiency is optimized.

[0065] The aforementioned basic examples and their further selected examples can be freely combined to form multiple embodiments, all of which are embodiments that can be adopted and claimed for protection in this application. In this application, each selected example can be arbitrarily combined with any other basic examples and selected examples.

[0066] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principles of the present application should be included in the scope of protection of the present application.

Claims

1. A three-stage pump variable flow automatic control method, characterized by: Including combined unit cascade control module and three-stage pump variable flow cascade control module; The automatic control method of the combined unit cascade control module is as follows: the main loop uses the supply air temperature set value as the control parameter and the return air temperature as the controlled parameter. The return air temperature feedback value is detected in real time. PID calculation is performed based on the deviation between the return air temperature feedback value and the return air temperature set value to obtain the unit's supply air temperature set value under the current operating conditions. The secondary loop uses the water valve opening as the control parameter and the supply air temperature as the controlled parameter. The deviation between the supply air temperature set value and the detected supply air temperature feedback value is calculated through PID calculation to adjust the opening of the regulating valve on the return water pipe. The automatic control mode of the three-stage pump variable flow cascade control module is as follows: the main loop uses the supply and return water temperature difference set value as the control parameter and the water valve opening as the controlled parameter. The water valve opening feedback value is detected in real time. The PID calculation is performed based on the deviation between the water valve opening feedback value and the water valve opening set value to calculate the supply and return water temperature difference set value under the current working conditions; the secondary loop uses the water pump frequency as the control parameter and the supply and return water temperature difference as the controlled parameter. The deviation between the supply and return water temperature difference set value and the detected supply and return water temperature difference feedback value is calculated through PID calculation to adjust the water pump frequency; Take the weighted average of the water valve opening feedback values of all units in the system, that is, The weighted value of the valve opening of the unit is used for conversion to calculate the converted value of the valve opening of the system, which can be equivalently described by the following formula: ； in: -Weighted weight; Q i - Rated cooling capacity of a single unit, Watt; Q 总 -Total system cooling capacity, watts; ； in: -Conversion value of system valve opening; - Weighted value of valve position of a single unit; -Valve opening of a single unit; Check the opening of each water valve on site. If the system is in a stable state, but some units still have water valves at 100% open, the loop where the unit is located is considered to be the most unfavorable loop. In this case, the water valve opening feedback value of the unit is used to replace the system valve opening conversion value as the water valve opening feedback value; When the water valve opening feedback of the most unfavorable loop unit is less than 90%, the water valve opening feedback value is automatically adjusted to the system valve opening conversion value, and the system valve opening conversion value is compared and calculated with the water valve opening set value.

2. The three-stage pump variable flow automatic control method according to claim 1 is characterized in that: The regulating valve adopts an equal percentage regulating valve. When the pressure difference before and after the regulating valve remains unchanged, the flow characteristics conform to the ideal flow characteristics. The ideal flow characteristics of the equal percentage regulating valve are expressed by the mathematical expression: ； Among them: G i -Flow rate through the regulating valve at a certain opening, m 3 / h;G iMAX -Flow through the regulating valve in fully open state, m 3 / h; l-valve opening at a certain valve position; l MAX -The opening of the valve in the fully open state; R-the ideal adjustable ratio of the regulating valve.

3. The three-stage pump variable flow automatic control method according to claim 1 or 2, characterized in that: In a chilled water cycle, when the terminal load is fixed, the temperature difference between the supply water temperature and the return water temperature reflects the chilled water flow required by the terminal under the current load, which is expressed by the following formula: ； Where: Q-heat, joule; c-specific heat capacity, joule / kg•degrees Celsius; -Fluid density, kg / m3; G i -Flow rate through the regulating valve at a certain opening, m 3 / h; t1-return water temperature, degrees Celsius; t2-supply water temperature, degrees Celsius.

4. The three-stage pump variable flow automatic control method according to claim 1 is characterized in that: In the unit pipe network system, the relationship between the relative values of pump efficiency, flow rate and speed is expressed by the following expression: ； Where: η-water pump efficiency; Q-relative value of water flow; n-relative value of water pump speed; C1, C2-constants.

5. The three-stage pump variable flow automatic control method according to claim 1 or 4, characterized in that: Under the premise of meeting the flow demand of the pipeline network, the water pump maintains the minimum pipeline impedance and the highest pump efficiency. By opening the terminal regulating valve to reduce the local resistance of the pipeline network, the pipeline network characteristic curve is adjusted. By reducing the water pump speed to increase the water pump efficiency, the water pump characteristic curve is adjusted to reduce the water pump energy consumption.

6. The three-stage pump variable flow automatic control method according to claim 1 is characterized in that: Set the water valve opening to 90% to ensure minimum pipe impedance and maximum pump efficiency; The supply and return water temperature difference under the maximum load and minimum load of the system 、 As the upper and lower limits of the water valve PID controller, determined based on historical data 、 The initial value will be updated in real time according to the actual data in the later operation stage.

7. The three-stage pump variable flow automatic control method according to claim 1 is characterized in that: When the frequency of a single water pump reaches the maximum but the supply and return water temperature difference still does not meet the demand, a water pump is loaded at this time; the current water pump operating frequency is reduced to the minimum frequency, and the loaded water pump is also increased to the minimum frequency; the two water pumps are operated as a whole at the same frequency; otherwise, the water pump is unloaded.

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