Through five engineering examples, the possibility of using wind turbine and variable speed pump 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
A simple system that controls the circulation of the flow, the pump P powers the circulation of water through the valve V, the pipe and the user U. 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 by half, one way is to close the valve to a small extent and shift the equivalent resistance curve of the pipe to the left. At this time, 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. (See next page)
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.
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
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.
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. (See next page)
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.
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.
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.
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