Selection and calculation of differential pressure regulating valve for central air-conditioning chilled water system

Selection and calculation of differential pressure regulating valve for central air-conditioning chilled water system

In order to ensure that the flow of the chiller in the central air-conditioning chilled water system is basically constant; the operating conditions of the chilled water pump are stable, the general method is: the load side is designed as a variable flow to control the water flow of the terminal equipment, that is, the electric two-way valve is used as the end The regulating device of the equipment controls the flow of chilled water into the terminal equipment. The pressure difference bypass control device is set on the cold source side to ensure that the chilled water flow of the cold source part remains constant. However, in actual work, because designers often ignore the importance of the selection and calculation of the regulating valve, the design process is generally simple A bypass valve is installed on the chiller and the user side. The determination of the bypass pipe diameter and the selection of the bypass regulating valve have not been calculated in detail. In this way, the flow stability of the chiller in actual operation is often better than the design. Large gap, the bypass device generally cannot achieve the expected effect, which brings unnecessary troubles to the operation management. This article makes a brief analysis on the selection and calculation method of the differential pressure regulating valve combined with the actual project.

　　 1. The working principle of the differential pressure regulating device

The differential pressure regulating device is composed of a differential pressure controller, an electric actuator, a regulating valve, a pressure measuring tube, and a bypass pipe. Its working principle is that the pressure differential controller uses The difference is detected. According to the comparison between the result and the set pressure difference, the output control signal is controlled by the electric actuator to change the opening of the regulating valve by controlling the stroke or angle of the valve stem, thereby controlling the bypass pipe between the water supply pipe and the return pipe The chilled water flow rate ensures that the pressure difference of the system is constant at the set pressure difference value. When the system operating pressure difference is higher than the set pressure difference, the pressure difference controller outputs a signal to open or increase the opening of the electric control valve, and the water volume of the bypass pipeline increases, so that the system pressure difference tends to the set value; When the pressure difference is lower than the set pressure difference, the opening of the electric control valve is reduced, and the bypass flow is reduced, so that the system pressure difference is maintained at the set value.

　　2. Factors that should be considered when selecting a regulating valve

The caliber of the regulating valve is one of the most important factors in the selection calculation. If the regulating valve selection is too small, it may not provide enough flow when z* is large. If it is too large, it may often be in a small opening state. If the opening is too small, it will cause frequent oscillation and excessive wear of the valve plug, and the system is unstable and increases the engineering cost.

　　The regulating valve obtained by calculation should be adjusted in the opening range of 10%-90%, and the use of less than 10% should be avoided.

In addition, the valve capacity PV (that is, the ratio of the pressure difference on the valve to the total pressure difference of the pipe section when the control valve is fully opened) should be considered when installing the control valve. From the analysis of the pressure drop of the control valve, the adjustment must be combined when selecting the control valve. The front and back piping of the valve, when the PV value is less than 0.3, the flow characteristic curve of the linear flow characteristic control valve will seriously deviate from the ideal flow characteristic, which is similar to the quick opening characteristic, and is not suitable for valve adjustment.

　　3, the selection calculation of the regulating valve

The size of the regulating valve is determined by its flow capacity. When the regulating valve is fully opened, the pressure drop at both ends of the valve is 105Pa and the fluid density is 1g/cm3, the number of cubic meters of fluid flowing through the regulating valve per hour . There are usually two ways to express the flow capacity of an imported control valve, where kv=c, and cv means that when the control valve is fully opened, 60oF of clean water flows, and the pressure at both ends of the valve drops to 1b/in2 every minute. The flow rate through the valve, cv=1.167kv. 　　 Differential pressure bypass adjustment device is as follows:

　　 (1) Determine the pressure difference value of the regulating valve (⊿P)

As shown in the figure above, the pressure difference value acting on the regulating valve is the pressure difference value between E and F. Since the resistance of the CD bypass pipeline and the DUC pipeline passing through the end user is equivalent, the pressure difference between EF The value should be equal to the DUC pipe pressure difference (referring to the end user z * unfavorable loop pressure difference) minus the pressure difference between the CE pipe section and the FD pipe section.

　　 (2) Calculate the z*large and z*small flow rates that need to be bypassed by the regulating valve

For a single-unit air conditioner system, the flow required for the minimum load can be determined according to the z*low load actually used by the end user, thereby determining the maximum bypass flow. The formula is: G=(Q-Qmin)*3.6/CP* ⊿T (1)

In the formula, G is the flow unit (m3/h), Q is the cooling capacity of the water chiller (kW), Qmin air conditioning system z*small load (kW), CP is the specific heat of water, CP=4.187kJ/kg. Celsius, ⊿T is the temperature difference between the supply and return of chilled water, generally 5 degrees Celsius

　　According to the actual adjustable ratio RS=10(PV)1/2(2), the bypass z*small flow rate of the regulating valve can be calculated

　　 (3) Calculate the required flow capacity C of the differential pressure regulating valve

　　C=316G*(⊿P/ρ)-1/2(3)

In the formula of 　　, ρ is the density and the unit is (g/cm3), G is the flow, the unit is (m3/h), and ⊿P is the pressure difference between the two ends of the regulating valve, the unit is (Pa). According to the calculated C value, select the regulating valve so that its flow capacity is greater than and closest to the calculated value.

　　 (4) Checking calculation of the opening and adjustable ratio of the regulating valve. According to the C value of the selected regulating valve, calculate whether the adjustable ratio of the regulating valve meets the requirements when the regulating valve is at the minimum opening degree and the maximum opening degree. According to the calculated adjustable ratio, the maximum flow rate and the minimum flow rate and the regulating valve are at the minimum opening The flow rate under the maximum opening degree and the maximum opening degree are compared, and the calculation is repeated until it is qualified.

　　4. Examples of control valve selection

　　 An office building has 12 floors, with a construction area of ​​approximately 11,000 square meters and a floor height of 3.6 meters. It uses a York screw chiller with a cooling capacity of 1122kW.

　　 (1) Determination of pressure difference

According to hydraulic calculation, the total resistance loss H of the system under the minimum load (the bypass pipe is at the maximum load) is about 235kPa. A pressure difference bypass control device is installed at the main pipe of the chilled water supply and return of the system, and the bypass pipe is on the cold source side The resistance loss of the water pipeline is 80kPa, and the resistance loss of the most unfavorable loop at the end is 155KPa.

　　(2) Water volume calculation of the control valve

After calculation, the load of the air-conditioning system is the minimum load when its minimum branch circuit is circulating, which is about 35% of the total load. Using the formula (1) G=(Q-Qmin)*3.6/CP*⊿T, it is calculated The maximum flow rate required by bypass is 125.4m3/h, and the most unfavorable loop pressure difference is 155KPa.

　　 (3) Calculation of circulation capacity

　　According to formula (2) C=316G*(⊿P/ρ)-1/2, C=100.6

　　 (4) Selection of control valve

The following table is the technical parameter table of the ZDLN electronic electric straight-through two-seat control valve of Shanghai Hengxing Pump & Valve Manufacturing Co., Ltd., calculated by formula (2), C=100.6, the inherent flow characteristics of the control valve are linear and equal percentage characteristics , Choose the C value close to z* according to the equal percentage characteristics, and get the pipe diameter of DN80 and the C value of 110, which meets the selection requirements.

　　 (5) Checking calculation of opening and adjustable ratio of control valve

The total length of the bypass pipe section is 6m. When C=110, ⊿P=129.8KPa is obtained from the formula (4)⊿P=ρ(316G/C)2. When the bypass pipe adopts the same pipe diameter as the regulating valve, when The large water volume of the bypass pipeline z* is 125.4m3/h. After hydraulic calculation, the total loss along the way is 42.8KPa, the total partial loss is 23KPa, and the pressure difference between the two ends of the regulating valve is 129.8-42.8-23=64KPa<129.8KPa. Capacity PV=64/129.8=0.49. At this time, the flow characteristic curve of the regulating valve is equal percentage characteristic. At this time, the actual maximum bypass water volume processed is 88.1m3/h<125.4m3/h, and its flow rate is only the maximum bypass required by the system. For 70% of the flow rate, the actual adjustable ratio Rs=7 can be obtained from formula (2), that is, the actual z*small flow rate is 88.1/7=12.6m3/h, and the maximum flow rate and the minimum flow rate obviously cannot meet the actual requirements. Therefore, DN80 is not suitable for the diameter of the bypass pipe.

According to the above calculation method, continue the trial calculation. When the DN125 bypass pipe is selected, the pressure difference between the two ends of the regulating valve is calculated to be 123.2KPa, PV=0.95, and the maximum bypass water volume processed at this time is 122.1m3/h, which is relatively open The relative flow rate is 90% and the relative flow rate is 97.3%. The actual adjustable ratio Rs=9.7 can be obtained from the formula (2), that is, the minimum bypass water volume is 122.1/9.7=12.6m3/h and the regulating valve works at 10% opening. Compared with 12.21m3/h, the flow rate is very close. At this time, the flow characteristic of the regulating valve is close to the ideal flow characteristic curve, which can meet the needs of the system.

5 Conclusion

　　 Through the above analysis, the following conclusions can be drawn:

　　 (1) The determination of the flow capacity C of the regulating valve is a crucial step in the selection of the regulating valve. Only when the flow capacity C is calculated correctly can the regulating valve meet the process requirements.

　　(2) The valve capacity PV of the regulating valve is also one of the important indicators for selecting the regulating valve. In principle, the Pv value should be as large as possible.

　　(3) The actual adjustable ratio Rs of the regulating valve is one of the parameters that determines whether the regulating valve can meet the technological requirements. The actual adjustable ratio is often much smaller than the ideal adjustable ratio, but when selecting a regulating valve, the actual adjustable ratio should be as close to the maximum as possible.

　　 (4) The maximum flow and minimum flow that the regulating valve can pass are the key links in the selection calculation. These two values ​​should be determined by the actual adjustable ratio and process requirements.

　　(5) It can be seen from engineering examples that the calculation of the diameter of the bypass pipe is also very important. If the same diameter as the regulating valve is selected without calculation, the process requirements cannot be met.

　　 Through the above five points, it can be seen that the differential pressure regulating valve occupies a relatively important position in the regulation and control of the central air-conditioning chilled water system. Only after careful calculation can the selected differential pressure regulating valve meet the process requirements.