Use variable speed pump and variable speed fan instead of regulating valve with water valve

Abstract: Through the five engineering examples, the possibility of using wind-driven and variable-speed pumps instead of regulating the damper valve to realize the regulation of the wind and water system in the heating and air-conditioning system is discussed. Analysis shows that doing so can save operating energy, while improving the quality of the system regulation, the initial investment of the system will generally not increase. Pump and fan energy consumption accounted for about the total energy consumption of heating and air conditioning system 40? About 1/3 of these energy consumption is consumed by various regulating valves, but such a large price does not bring about good regulation effects, but instead leads to many problems in the system. The use of variable speed blowers and variable speed pumps as an adjustment means saves on energy consumption and eliminates many of the difficulties in adjustment. Keywords: frequency conversion water system wind system variable speed pump variable speed fan 1 Introduction In the HVAC project, the use of a large number of wind valve water system to adjust the volume of air flow to meet the required conditions. Their adjustment principle is to increase the system's resistance to consume excess pump or fan pressure head, to achieve the purpose of reducing the flow. Therefore, the regulatory role of these regulating valve is to consume fan or pump at the expense of running energy. At present, HVAC project more and more use of automatic control system. To achieve self-control, many damper valves also use electric actuators. The current good quality electric valve price of thousands or even tens of thousands. Electric damper also need a few thousand dollars. Electric damper valve costs often account for more than 40% of the total cost of the control system. Can change the composition of the system, reducing the use of these energy-consuming, expensive valves, using other means to achieve the regulation of the flow? Fan pumps and dampers are one-to-one correspondence between the two types of flow control equipment. Fan pumps provide fluid power while damper valves consume excess fluid. Therefore, if the fan pump instead of air valve, not to install additional valves at the energy surplus, but in insufficient energy to install additional pumps or fans, by adjusting the fan pump speed, the system can also be achieved through the flow regulation. At this point due to the reduction of the control valve, which reduces the energy consumed by the valve, thus reducing energy consumption. At the same time, the current variable speed fan, pump prices and the same flow of electric damper, water valve prices are close to, or even lower, so the initial investment will not increase. From this point of view, this article gives a few examples of replacing the valve with a pump, and then further discuss the significance of this program on the HVAC project and issues to be noticed in order to arouse the discussion of everyone. 2, case analysis 2.1 Simple system flow control Figure 1 shows a simple system for controlling the circulation of a flow. The pump P supplies power to circulate water through valve V, pipe and user U. Fig. Figure 2 shows when the valve is fully open, the pump speed n = n0 when the system operating point. At this point, the flow rate G0, pump efficiency η0, that is the highest point of efficiency. To reduce the flow in half, one way is to close the valve to make the equivalent resistance curve of the pipe shift to the left, as shown in Figure 2. At this point the efficiency of the pump is reduced to η1 and the pressure rises to p1. Due to the pressure increase, the efficiency is reduced, so the pump consumption is reduced by only 20% to 30% even though the flow rate is reduced to half. At this time, the pipe network other than the valve consumes only the pressure drop p0 / 4 because of its resistance characteristics. The remaining part of 3 (p0 + (p1-p0)) / 4 are consumed in the valve, which consumes 80% of the pump consumption, which is why the control valve consumes most of the energy consumption of the pump basis. In addition, the instability caused by the pump working point offset, the valve clearance small after the large and noise caused by pressure drop, have a negative impact on the system. If the same, but the pump speed down to 50%, Figure 2 at the same time given the working conditions at this time, then the resistance characteristics of the pipe network unchanged, pump down the curve, the pump efficiency will still Is η0 and pressure p2 is p0 / 4. In this way, after reducing the flow pump consumption is only 1/8 of the original, with a very significant energy saving effect. At the same time, due to the pump operating point and valve position have not changed, so the system work stable, and there will be no throttling noise. Figure 1 Simple water circulation system Figure 2 transfer The whole valve or adjust the speed to change the flow This simple example shows: (1) When the regulating valve is adjusted, it will consume most of the fluid power of the branch where it is located. And due to changes in the pipe network resistance characteristics, the pipe network in the work of mechanical shift point, in most cases this will lead to reduced efficiency. (2) When using variable speed mode to adjust the flow rate, pump or fan power consumption can be proportional to the third power flow changes. And due to the same system of resistance characteristics, pump or fan operating point unchanged, so the same efficiency, pumps, fans and systems can work stably. (3) to adjust the pump or fan speed to adjust the flow rate should be the best way to adjust. 2.2 hot water network Figure 3 shows a simple water heating system. When the user requests the same pressure head when the hydraulic pressure chart shown in Figure 4 solid line. Figure 3 heating water system diagram Figure 4 heating water pressure chart The figure below the dashed line for the user to consume the capital pressure head, while the above part of the dotted line is consumed by the valve. If the system is designed properly and the pump is properly selected, the residual pressure at the furthest end user is exactly the pressure head it needs. The valve V5 is fully open and consumes little energy. In this case, if the flow of each user is equal and the distance between them is equal, the friction on the main pipe is the same and ignores the resistance when the valve is fully open. For n users, the energy consumed by the valve V1 and the total energy consumed by the outdoor pipe network The percentage EV1 is: EV1 = (1 / n) × ((n-1) / n) The percentage of energy consumed by the kth valve to the total energy consumed by the user 's external network EV1 = (1 / n) × ((n-1) / n) Total energy consumed by the first n-1 valves is: When the number of hot users is enough, (n-1) / (2n) is equal to about 50%, which means that about half of the energy consumed by the external network is consumed by the control valves of each branch. On the average user's side, the lift actually needed is only 20% to 30% of the circulation pump's lift, that is, the external network consumes 70% to 80%. Therefore, the total pump from 35% to 40% of the energy consumed by the regulating valve. Sometimes for safety's sake, the pump head should be selected larger, and then through the valve V0 Figure 3 the excess part of the consumption. As a result, more than half of the pump consumption of the regulating valve in the general hot water heating network is consumed. If the switch to Figure 5 way to connect the hot water pipe network, users installed at the user back to the booster pump, instead of regulating valve to reduce the head of the main circulating pump, so that it will only assume the heat source and part of the main pipe pressure drop, the user Of the pressure drop and the other part of the main pipe pressure drop by the user within the booster pump to provide, then the hydraulic diagram shown in Figure 6. Figure 5 Pump instead of the valve hot water heating system Figure 6 pump generation valve program hydraulic map At this time no control valve, so there is no loss of control valve pump consumption, the user at all back pressure pump head should be carefully selected. If you choose too large, then use the valve to reduce the same energy consumption. However, if the variable speed pump is installed, the flow rate required by each user can be adjusted so that the pump is no longer consumed by the regulating valve, which reduces the operating power consumption by more than 50% even though many pumps are installed. In this case, if the flow rate required by each user changes frequently, the total flow of the whole system also changes within a relatively large range. The total circulating pump can also be controlled by a variable frequency pump, and according to the pressure difference between the supply and return water in the middle of the trunk , 6 midpoint A) to control its speed, so that the point pressure is maintained at zero, the system has a very good regulatory performance and energy saving. Analysis shows that when using the conventional pipe network as shown in Figure 3, if for some reason half of the users shut down and no water supply is needed, the water volume of the non-connected users will increase and the maximum flow rate will increase by more than 50% The pipe network uses the method shown in FIG. 5 and controls the rotational speed of the main circulating pump in the above manner, the water volume of the non-closed users increases by not more than 8% in the same situation, and the hydraulic stability of the system is greatly improved. Further analysis in this regard, see [1], this program is ready to start construction in Hangzhou Thermal Power Plant hot and cold cogeneration network, the user for the absorption chiller, domestic hot water heat exchanger, winter is For building heating and domestic hot water. The analysis shows that for this system with a wide range of load changes, this method saves 62% of the pump power consumption and improves the system's hydraulic stability than the conventional method. While also reducing the pressure range of the entire system, which can reduce the pressure pipe network requirements, director of pipe network life. The cost of installing the speed control pump at each user can basically be supplemented by the electric control valve saved by each user and the electricity storage capacity savings saved, so that the total investment may not increase or even decrease. 2.3 air conditioning water system To reduce pump power consumption, easy to adjust the system, many systems use two-stage pump, as shown in Figure 7. Pump P1 can start and stop according to the required number of refrigerators. Its lift only overcomes the resistance of evaporator and the pressure drop of some pipes in freezing station. Pump P2 overcomes the pressure drop of dry pipe and cold water users. In order to save energy, P2 sometimes also use variable speed pump, according to user requirements to regulate the pump speed, the regulation is to maintain the most remote users at the return water pressure is rated as the pressure head. It is pointed out in [2] that after P2 adopts variable speed pump, its energy consumption is not "proportional to the cubic of the flow rate" as advertised by the manufacturer. Assuming that the maximum pressure drop required by the user of the cold water and the pressure drop at the maximum flow rate of the mains each account for 50%, for example, both be 5 m, the speed of the pump group P2 should be controlled according to the constant pressure drop of 5 m at the extreme end. Assuming that the flow rate required by each user is 50% of the maximum flow rate, the control valves of the users themselves are all turned off one by one. At this time, the pressure drop at the end is still 5 m and the flow of the main pipe is reduced by one bucket, so the pressure drop becomes 1.25 m, The pressure drop required by the pump set P2 is reduced from the original 10m to 6.25m while the flow rate is reduced to half. However, the operating point of the pump shifts to the left and the efficiency is reduced, so the pump consumption is about 45% of the maximum flow rate, not according to The third law predicted by 12.5%. Caused by this phenomenon is due to the phenomenon is due to the small user control valve, consume the excess energy. See Figure 8. Figure 7 air conditioning two pump system Figure 8 air-conditioning two-stage pump system when the flow rate down changes In addition, if the main pipe pressure drop accounted for half of the P2 head, as in the previous case analysis, due to the different users, this part of the pump consumption is also consumed by the user's control valve. And air-conditioning system in order to improve its regulatory performance, but also hope that both sides of the control valve pressure account for more than half of the pressure head of the branch. In this way, it is estimated on average that even with a variable speed pump, more than 60% of the energy in pump group P2 is consumed by each regulating valve. Figure 9 is in accordance with the idea of ​​the previous case, the control valve to variable frequency pump, pump P2 to cancel the new program. Fig. 10a is the hydraulic diagram of the refrigerator operating according to this scheme when the amount of water required by the refrigerator is greater than the amount of water required by the user; and Fig. 10b is the hydraulic diagram of the water volume required by the user when it is greater than the amount of water on the side of the refrigerator. In this way will no longer need to adjust the valve, can be seen from Figure 10, for most of the branch, the back and forth between the main pipe is a negative pressure, when an air conditioner pump stops, the flow will automatically become zero. Change the user's pump speed, flow can be well adjusted. Since no control valve is installed, there is no longer any loss of control valve. When the flow is reduced by half, the pump user's work point will be slightly offset, but the energy consumption can still be reduced by more than 80%. When the average system operating flow is 70% of the maximum flow rate, it can be calculated that the total power consumption of each user pump is less than 35% of the power consumption of the pump P2 compared with the method using the variable speed pump P2. Figure 9 Frequency converter instead of regulating valve secondary pump system Figure 10a P1 flow is greater than the user when the total flow of hydraulic pressure Figure 10b Library P2 flow is less than the total flow of users when the pressure chart Analyze the stability of this system again. When for some reason, some users turn off, some users tune down, reducing the total flow of 50%, the main pipe pressure drop, the user does not change the speed of the pump flow maximum increase of about 10% to 20%, and the pump The shape of the performance curve. At this time as long as the corresponding reduction in speed, you can maintain the original flow. In this way, each of the small variable-frequency pump instead of a large pump, due to the total power reduction of 20% to 30%, so the price will not increase. After adopting the new program, but also save the electric control valve of each air conditioner, so the initial investment will be reduced. 2.4 air treatment room Figure 11 is a common air volume of fresh air treatment room. In order to make full use of fresh air, hope can be adjusted by the three air valve to change the new return air ratio, to achieve the required air supply parameters. Because the air is exhausted from the room by the exhaust fan to be discharged to the atmosphere via the damper A, the pressure must be positive at point a and the fresh air outside passes through the damper B to enter the air treatment chamber. Therefore, the point b must be negative Pressure The pressure difference between a and b will be equal to the sum of the pressure drop of fresh air valve B and fresh air duct and the pressure drop of air exhaust valve A and exhaust air duct. If the new duct fresh air duct and exhaust duct exhaust pressure drop of 100Pa respectively, then the pressure drop between a, b 200Pa. Such a large pressure difference can only be adjusted by a damper C difficult. Therefore, such a system is actually hard to truly realize the new return air ratio continuously adjustable. Due to the uncoordinated position of the three air valves, it is difficult to keep the pressure drop between the two points a and b unchanged. This also causes the actual total air volume to change with the adjustment of the three air valves. When the fresh air is running, Air volume is sometimes less than 10% of the minimum fresh air. Figure 11 Conventional variable fresh air volume air handling room According to the idea of ​​the fan instead of air valve, air treatment room can be designed as shown in Figure 12. The original back to the fan can change speed exhaust fan. Fan F1 to overcome the air handling room equipment and send, return air duct resistance. Point a is negative pressure, the value of the return air duct pressure drop. Adjusting the speed of fresh air valve B or exhaust fan F2 individually can change the pressure at point a to change the indoor positive pressure; meanwhile adjusting the speed of F2 and the opening of valve A, the new return air ratio can be accurately adjusted, Affect the total air volume. The one-way guide vane in the figure is used to prevent the fresh air from entering into the air mix with the return air, and part of it is drained away by the exhaust fan to ensure that the air at a is flowing to the air b without air backflow. When the air conditioning room fresh air and exhaust duct longer or smaller cross section, greater resistance, this method can also be completely resolved Figure 11, the way the amount of fresh air is difficult to increase. As long as the pressure drop of the fresh air duct is less than the pressure drop of the return air duct, continuous and effective adjustment can be achieved from the minimum fresh air to the fresh air. Figure 12 with the exhaust fan instead of air-conditioning treatment program The disadvantage of this approach is that when the resistance of the return air duct is greater, the negative pressure at point a decreases, so that the exhaust fan F2 needs to have a larger pressure head to overcome the point even in the case of a small amount of fresh air a negative pressure. This variable speed fan with inconsistent characteristics. For this reason, it is also possible to adopt the mode shown in FIG. 13 and use four blowers to perform the functions of blowing air, returning air, exhausting air and sending fresh air respectively. At this time, the pressure at the point a in the diagram is the same as the atmospheric pressure, the blower overcomes the resistance of the blower duct and the air handling room equipment, the blower F2 overcomes the resistance of the blower duct, and the blower fan F3 and the blower fan F4 can be a motor Connect synchronous variable speed fan. Change its speed to change the fresh air and return air ratio. The new return air ratio at this time is strictly proportional to the speed of F3 and F4. Room pressure can be located in the return air duct or air duct on the air valve to adjust, adjust the total amount of delivery will remain unchanged. The new return air ratio is entirely determined by the F3, F4 speed, their changes on the total air volume and pressure in the room without any impact. Figure 13 Install fan on both fresh air and air ducts Although the four turbines are installed in the mode of Fig. 13, the actual operation energy consumption is less than that of the conventional system of Fig. 11 and the double fan system of Fig. 12. This is due to the fact that it no longer has the throttling energy consumption of the mix valve C in the conventional system and the fresh air valve in the Fig. 12 system. Compared with the conventional system, in the fresh air, the sum of the pressures of the fans F4 and F1 in FIG. 13 is the same as the blower pressure of the conventional system. In this way, the motor power of the four blowers in FIG. 13 should be the same as the motor power consumption of the two blowers of the conventional air flow system of the same air volume, while the power consumption of the F3 and F4 of FIG. For the system shown in FIG. 12, if the fresh air duct and the exhaust air duct have the same resistance as in FIG. 13, the maximum fresh air will have the same energy consumption, and when the minimum fresh air, the exhaust fan of FIG. a negative pressure near the area to the outdoor row, still need to provide a higher pressure head, making the work point offset, the efficiency decreased, while the mid-point a of Figure 13 is always zero pressure, F3, F4 work point unchanged, the minimum fresh air hour Lower energy consumption. 2.5 VAV air conditioning system (VAV) Adopt VAV air conditioning system, install the VAV end device in each room air outlet, adjust the opening degree of the air valve in the end device according to the room temperature to change the room air volume, so as to meet the different environmental requirements of each room and adapt to each room Heat load changes. Figure 14 is a schematic diagram of this VAV system. Figure 14 VAV system schematic In order to make the total air volume can also be changed accordingly, the total exhaust fan also uses inverter fan, and adjust the speed according to the pressure within the end of the air duct to ensure that each VAV device has sufficient capital pressure head. Some engineers and researchers think [3] that in doing so, when only a part of the air volume is required at the very end, the VAV device needs to be turned off and the fan speed will not decrease, causing each VAV device to turn off and the fan speed will not As a result, the VAV units are shut down and a significant portion of the fan energy is consumed by the damper. It is recommended that the reference pressure measurement point be moved forward to 1/3 of the total length from the end. In this case, it is sometimes not possible to ensure a sufficient pressure head at the end when the air volume of the large air volume is hoped for in each room. And, as discussed earlier, the purpose of the damper is to regulate the air flow so that it has good conditioning characteristics and requires that its pressure drop be a large proportion of the total pressure drop across the branch. This will inevitably increase the energy consumed by the damper, while also increasing the pressure due to the pressure drop on both sides of the damper. This point no matter how to change the control method is also difficult to completely improve. In accordance with the idea of ​​replacing the damper valve in the fan, fan instead of air valve in the end device, adjust the fan speed to adjust the air flow, we can completely solve this problem. Figure 15 VAV end device with fan Figure 15 is a schematic diagram of the system in this manner, in which the dual fan system is shown as being convenient for analysis. In fact, the multi-fan system in the above example can be used for air treatment. The main blower still uses a variable speed blower, but it only takes the pressure drop across the air handling chamber and a portion of the air duct. The speed can be controlled based on the static pressure in the main blower duct near the first blower (point Q) The pressure is atmospheric and the pressure drop from this point to each room is borne by the fan in the end unit. When the blower in a room stops, the check valve will close automatically due to the negative pressure in the air supply duct. After the fan is turned on, the air supply in the room increases with the increase of the fan speed, and changes in a manner close to the linearity. By eliminating the energy consumption of the regulating damper in each VAV unit, it is possible to calculate that the total energy consumption of each fan is reduced by more than 10% from that of the original blower. The price of such low-power frequency converters and fans is basically the same as that of the electric air valve, and the regulation performance of the system is greatly improved. 3, conclusion This article through the HVAC project five examples illustrate the use of variable speed fan, pump to replace part of the regulation with the air valve, the valve method and its advantages. It can be seen from the above simple analysis that adopting this method can not only achieve a larger energy-saving effect but also improve the control and adjustment effect, and the initial investment increase will not be too large. So it should be a new way worth noting and considering. The role of the control valve is to increase the resistance to consume the excess part of the head, to achieve the role of regulating the flow, the pressure regulator valve to account for the greater the proportion of the total pressure loss, the better the regulatory performance. In this way, to obtain the regulation of performance and energy consumption become a pair of irreconcilable contradictions. The use of a variable speed fan or a variable speed pump as a regulator reduces energy by reducing the amount of energy being put into the system and it is possible to achieve good adjustment quality through reasonable system structure design without increasing energy consumption while achieving energy saving and adjustment Good performance results. Engineering design must consider the margin, to ensure that the system can still meet the required parameters when the actual situation changes. In actual operation, in order to eliminate these surplus, but also by the valve to adjust, resulting in waste. The use of variable speed pump or variable speed fan design also left a margin, but not by running the valve but by reducing the speed to eliminate these surplus. After the speed is reduced, the power consumption is reduced, so these margins are not consumed, avoiding waste. When the fan or pump is selected in the normal mode, the lift must be selected according to the most unfavorable branch, determined by the branch with the furthest branch or the branch with the largest pressure head required by the user. This branch of traffic may only be a small part of the total system traffic. For this small part of the flow selection of large head, other flow had to pump or fan to achieve the same head, and then consumed by the valve, both to create regulatory difficulties, but also a waste of energy. Variable speed fan or pump instead of the control valve, the required lift of each branch by the branch of the fan and pump alone to solve, not unified increase and then were consumed, but where the need for how much local plus. This naturally get a good regulatory effect and energy saving effect. The energy consumed by pumps and fans accounts for a large part of the energy consumed by HVAC. The energy consumption of heating is 15% ~ 25% of the energy consumed by the pump (based on price) and the energy consumption of air-conditioners is 35% ~ 45% Fan pump energy consumption. In this part of the fan pump energy consumption, more than 40% is a variety of regulating valve consumption, using the method discussed in this article, it is possible to save a large part of the energy consumed by the control valve, so this method has important energy savings significance. In this way, due to the required system flow conditions, flow distribution methods are the same, so the duct, water pipe design can be unchanged, but the system pressure distribution has greatly changed, in the choice of these local branches of the speed Fans, variable speed pumps, to be carefully calculated to make it a pressure-flow curve and system requirements to match. This ensures that, under different operating conditions, the operating points of these pumps or fans only change to the extent permitted. In the user branch has increased so much power equipment, will reduce system reliability, increase maintenance workload? It is now easier to find high-quality fan pumps for long-term trouble-free operation. Some sub-file variable speed pumps, by changing the winding resistance to change the speed, low prices, high reliability. On the contrary, the current electric valve, water valve reliability is lower than the fans, pumps, HVAC system is one of the highest failure rate of equipment. Therefore, the valve to pump or fan, the reliability should be increased. In some key locations, spare fans and spare pumps can also be installed, which can be switched in time when faults occur. 4, reference 1, Jiang Yi. Hot and cold water supply network user backwater back pressure pump scheme. District heating .1996, (2). 2, Thomas Hartman. Direct digital controls for HAVC. Chapter 1: The challenges and benefits of high performance systems. ISBN 0-07-026977-7. 3, Frank Mayhew. Two-thirds of the way down the duct-it is wrong !. ASHRAE Seminar 14, San Diego, 26 June, 1995.