Dynamics of Hydronic Systems

Transcription

1 Valve and Actuator Manual 977 Hydronic System Basics Section Engineering Bulletin H112 Issue Date 0989 Dynamics of Hydronic Systems A hydronic system can be configured in many different ways. This report will discuss the following types: 1. Reverse Return 2. Direct Return 3. Primary/Secondary The variant configurations of the hydronic systems noted above will be analyzed as well as the relative advantages and disadvantages of each. Part-load characteristics will also be considered. Any of the three types of hydronic systems could be utilized in either a heating or cooling system. To simplify the remainder of this report only the case of a cooling system will be considered. Most of the concepts covered however, would be applicable for either heating or cooling systems. It is strongly recommended that the concepts discussed in Engineering Reports H111 and H110 be understood before preceding with this report. Engineering Report H111 includes a detailed discussion of the performance of control valves. Engineering Report H110 discusses the relationship between fluid flow and pressure drop as it pertains to typical hydronic systems. Accurate, stable control is dependent upon two factors. The first is the ability of the controller and control valve to compensate for process nonlinearites. A coil is a good example of a nonlinear process. If the capacity of a coil is plotted for each flow rate through the coil, the result is a logarithmic relationship. Secondly, the physical limitations of the control valve must be considered. As discussed in Engineering Report H111, all control valves have some amount of uncontrollable flow when the valve plug is initially lifted form its seat. The magnitude of this uncontrollable flow is related to the valve rangeability and the pressure differential across the valve. Rangeability values are limited by machining process tolerances. Therefore, it becomes important to minimize the variation in pressure differential across the valve to minimize the uncontrollable flow through it Johnson Controls, Inc. 1 Code No. LIT-351H112

2 Even small amounts of uncontrollable flow can compromise the ability of a hydronic system to be properly controlled. This is the result of the extremely high coil gain at low flow rates. It is important to properly size the control valves. It is also necessary for the consultant to design a hydronic system which is able to maintain a relatively constant branch differential pressure, Pb, regardless of building load. This report discusses how well the standard types of distribution systems are able to maintain a constant Pb. Reverse Return Distribution Systems In a reverse return system the pressure drops in the distribution piping between the circulating pump and each air handling unit are equal. This is true because the system is designed so that the length of the water circuit is the same for each air handling unit regardless of its location with respect to the circulating pump. As a result the Pb for every branch is equal regardless of the magnitude of the building cooling load. If the air handling unit cooling coils are all selected for the same pressure drop this type of system is self-balancing. Unfortunately the installed cost of a reverse return piping system is higher than that of the other types of hydronic systems. This is due primarily to the cost of the additional chilled water return piping (See Figure 1). 2 H112 Engineering Bulletin

3 Reverse Return System with 3-way Valves When 3-way valves are installed in a reverse return system, each valve can be optimally selected based upon the same criteria. Changes in the magnitude of the cooling load will have little effect on the chilled water flow rate. As a result, the value of Pb for every branch will remain constant regardless of changes in the magnitude of the building cooling load. In the case of the reverse return system shown in Figure 1 the Pb for every branch will always be 50ft H2O. Since Pb is always 50ft H2O, every control valve can be optimally selected, based upon the above criterion. If a valve authority of 50% is desired, all of the control valves can be selected for a 25ft H2O pressure drop at design flow. Since the branch pressure drop does not deviate from 50ft H2O, there should be no increase in uncontrollable flow nor will there be any loss in effective valve travel as the building cooling load decreases. As with any system which utilizes 3-way valves, the flow rate through the chiller(s) can always be maintained at a constant level without the requirement for a large central plant bypass with its associated controls. Because the 3-way control valves shown in Figure 1 are mixing valves piped in a bypass configuration, problems associated with control valve actuator spring range shift or the inability to shut off flow through the cooling coil will be minimized. The disadvantages of utilizing 3-way valves are increased installation and operating costs; specifically, a more complex (costly) coil piping arrangement, a high continuous building flow requirement and a lower overall chiller coefficient of performance (C.O.P). When 3-way control valves are installed, the building demand for chilled water does not vary with diversity in the building thermal load. Remember, the flow rate through each branch does not change with the cooling load requirements of the space served by the air handling unit. Only the chilled water flow rate through the cooling coil changes with the load. Consequently, chilled water pumps must often be operated without their associated chiller to insure each branch will have chilled water available to meet the space cooling load. Otherwise the branches nearest the chilled water pumps will consume all of the available chilled water. In the case of a chilled water system the operation of extra chilled water pumps not only increases pumping horsepower requirements but also reduces the overall chiller plant C.O.P. The lower overall chiller plant C.O.P. is the result of mixing relatively warm water passing through a nonoperating chiller with exceptionally cold water leaving a operating chiller. This is required to meet both the building flow and chilled water supply temperature requirements. The C.O.P. of the operating chiller decreases since it is forced to provide a lower chilled water supply temperature for a given building thermal load. This problem is illustrated in Example 1. H112 Engineering Bulletin 3

4 Example 1 Refer to Figure 1. This distribution system was designed for a total cooling capacity of 400 tons. There are four air handling units (AHU), each with a design capacity of 100 tons. Each AHU requires 150 gpm of chilled water at a 16 F rise (45-61 F) when fully loaded. Assume that the AHU is Branch #4 serves a computer facility and has a constant cooling load of 100 tons regardless of outdoor conditions. Also assume the AHUs in branches #1 through #3 serve exterior zones in the building. Finally, assume it is a temperate spring day, such that the cooling load on the AHUs located in branches #1 through #3 is 20 tons per AHU. The total building cooling load is 160 tons. It would seem, since each chiller has a capacity of 200 tons only one chiller will need to operate. Unfortunately things aren t that simple. Because this distribution system has 3-way control valves, both chilled water pumps must operate to maintain flow in all branches. The cooling coil in branch #4 is fully loaded. Therefore 150 gpm is passing through this coil and is being heated from 45 F to 61 F. Meanwhile in branches #1 though #3 only 30 gpm of the chilled water actually passes through the cooling coil. The other 120 gpm of 45 F chilled water bypasses the coil. These two streams then mix resulting in a branch chilled water return temperature of 48.2 F. The temperature of the chilled water returning to the central plant will be 51.4 F after the water returning from all four branches is mixed. Here is the problem. A single operating chiller must produce 38.6 F chilled water if 45 F water is to be supplied to the building. This extremely low chilled water temperature requirement is dictated by the fact an equal volume of 51.4 F water is simply passing through the nonoperating chiller and the AHU in branch #4 requires 45 F to meet its cooling load. When a chiller designed to produce 45 F chilled water is forced to produce lower temperature chilled water its C.O.P. drops in a nonlinear manner. For each additional degree drop in chilled water supply temperature increasing amounts of compressor power is required. In this case it is unlikely that the one operating chiller could even provide 38.6 F chilled water. In this extreme case both chillers would be required to operate even though the actual cooling load is less than the capacity of one chiller. This is very inefficient. 4 H112 Engineering Bulletin

5 Reverse Return Systems With 2-way Valves When 2-way valves are installed in a reverse return system the Pb for every branch will the same regardless of load, but the magnitude of the Pb will vary with changes in the building cooling load. This occurs because the pipe-friction factor changes with the variable chilled water flow rate (See Figure 2). A reduction in cooling load will cause one or more of the 2-way control valves to reduce the system chilled water flow rate. The hydronic system depicted in Figure 2 does not have a central plant bypass. Without a bypass the pressure developed by the chilled water pump(s) will increase with the reduction in the cooling load. The pump s operating point will ride up its pump curve as the chilled water flow rate decreases. This increase in pressure developed by the pump, as well as the reduced pipefriction factor, will increase the differential pressure across the branches. This is undesirable. The numbers shown in Figure 2 without parenthesis represent the system pressures at maximum design cooling load. The numbers shown in parenthesis represent system pressures when each branch is at (1/2) of its design load. Notice how each Pb increased from 50ft to 120ft H2O. H112 Engineering Bulletin 5

6 If the distribution system had a central plant bypass, the magnitude of the Pb could have been held constant (See Figure 3). The control valve located in the central plant bypass will be controlled by a differential pressure controller. The high and low reference sensing lines for the controller will be connected across one of the branches in the distribution system. If the sensed differential pressure deviates from the controller set-point, the control valve in the bypass piping will modulate as required to maintain the desired Pb. Since each branch has the same Pb in a reverse return system, common sense would dictate that the sensing location for the differential pressure controller would be across the branch closest to the bypass. This would reduce the length of the controller sensing and/or output lines, therefore reducing installation costs. This system has all of the advantages of a reverse return system with 3-way valves, but does not have its disadvantages. 6 H112 Engineering Bulletin

7 As an alternative to using a central plant bypass to maintain a constant Pb, a variable frequency drive could be installed on the chilled water pump motor. The differential pressure controller would then vary the speed of the chilled water pump to maintain the desired Pb. The main problem with using a variable speed drive in lieu of a central plant bypass is that the flow through the chiller(s) will now vary with changes in the cooling load. This potential problem must be carefully considered, and it would be wise to consult the chiller manufacturer to obtain acceptable guidelines. The major advantage in using a variable speed drive in lieu of a central plant bypass is the potential for energy savings. The variable speed drive will save energy in two areas. First, any energy losses which normally occur within the bypass valve will be avoided. In addition, the reduction in the chilled water flow rate will proportionally decrease the chilled water pump brake-horsepower requirement. Remember, in a system with a central plant bypass the flow rate to the building is directly proportional to the cooling load, but the flow rate through the chilled water pumps actually increases with reductions in the cooling load. In a system utilizing variable speed drives the flow rate through the pumps will vary directly with the cooling load. Keep in mind however, that in a properly sequenced multiple chiller central plant the flow rate through the bypass line will be less than the water flow rate associated with only one chiller. In other words, the excess amount of chilled water produced in the central plant above and beyond the demand required by the building would be less than the amount of water provided by one chilled water pump. This assumes there is one chilled water pump for each chiller. Therefore, the savings provided by using a variable speed drive compared to a central plant bypass is not a function of the total plant chilled water pumping capacity. Instead the potential savings is a function of the capacity of only one, not all, of the chilled water pumps. If the central plant has a large number (greater than four) of chillers and chilled water pumps, variable speed drives will save very little energy over a properly operated constant speed system utilizing a bypass valve. Direct Return Distribution Systems In a direct return distribution system the length of the distribution piping between the chilled water pump and each air handling unit will vary (See Figure 4). As result the Pb for every branch will be different. The closer an air handling unit is located to the chilled water pump, the larger the Pb. The magnitude of the Pb will also be affected by the chilled water flow rate. Remember the pipe friction-factor changes with the square of the flow rate through the pipe. H112 Engineering Bulletin 7

8 The following four items can be utilized to minimize the variation in Pb: 1. Properly sizing the distribution piping can help to maintain a constant Pb. This piping should be sized for small (less than 4 ft head loss per 100 ft of pipe) friction-factors. The change in pressure drop of the distribution piping, as flow is varied from maximum to minimum, then should not exceed the design pressure drop across a properly sized control valve. This is true for all but the largest systems. Some central campus systems, and the like, would require booster pumps in remote locations. 2. Utilize a central plant bypass or variable speed pump. The Pb across one of the branches can then be maintained at a constant level. This will prevent unwanted changes in the pressure developed by the chilled water pump from being shifted out to the branches. 3. Utilize control valves with large design pressure drops. 4. In some cases, utilize balancing valves. 8 H112 Engineering Bulletin

9 Direct Return System with 3-way Valves Direct Return Systems with 2-way Valves When 3-way valves are installed in a direct return hydronic system the magnitude of the Pb for each branch will be constant. This is true because the chilled water flow rate is always constant and is not affected by the load. However the magnitude of the Pb will be different for each branch. This occurs because the length of water circuit between the branch and chilled water pump is different for each branch (See Figure 4). The control valves for each branch can, therefore, be optimally sized for any cooling load condition. Unfortunately each branch will have a different sizing criterion since the Pb is different for each branch. In comparison, the 3-way valves in the reverse return system can all be sized for the same pressure drop. As discussed earlier, the 3-way valves will insure a constant flow rate through the chiller(s). Problems with actuator spring range shift, and cooling coil shutoff will also be minimal. Unfortunately, the same disadvantages are still apparent. The installed cost of a system with 3-way valves is higher due to more complex coil piping arrangements. Also as discussed previously the chiller plant C.O.P. will be lower when compared to a system with 2-way valves. This results from the high continuous building demand for chilled water dictating operation of chilled water pumps without their associated chiller. When 2-way valves are installed in a direct return system, the Pb will be dependent upon the location of the branch relative to the chilled water pump and the magnitude of the cooling load (See Figure 5). The numbers in parenthesis indicate the system pressures when each branch is at (1/2) of its design cooling load. The numbers without parenthesis indicate the system pressures when the system is at maximum design cooling load. If the system does not have a central plant bypass the value of Pb will be extremely dynamic. These extreme changes in differential pressure occur for three reasons: the chilled water pump will ride up its curve, the pipefriction factor in the distribution piping will change with load, and the length of the water circuit is different for each branch. This system is as far from optimal as any can get. H112 Engineering Bulletin 9

10 If the system does have a central plant bypass, the pressure fluctuations across each branch are reduced (See Figure 6). The numbers in parenthesis indicate the system pressures when each branch is at (1/2) of design cooling load. Adding the central plant bypass keeps changes in the pressure developed by the chilled water pump from being shifted out to the branches. However, since the pipe-friction factor in the distribution piping will change with load, the value of the Pb will be affected. Whenever the load in the building decreases and the bypass valve opens, the flow rate in the central plant will increase. This occurs because the pressure drop across the bypass will decrease as it is opened. In turn the pressure developed by the pump will decrease. As the pump operating point moves further out on its pump curve, its flow rate will increase. This increased flow rate will, in turn, increase the pipe-friction factor of the central plant piping. Eventually a balance point between the increased central plant piping friction losses and the decreased pressure across the bypass will be found. This is graphically shown in Figures 7, 8 and 9. Notice how the slope (pipe-friction factor) of the line representing 1/2 design load changes within the central plant portion of the graph. 10 H112 Engineering Bulletin

11 Assuming the branches are uniformly spaced in a distribution system, the best differential pressure sensing location for the bypass controller is in the middle of the system. Figure 7 relates the change in Pb with location as a function of cooling load. The graph illustrates the possible variation in Pb as the building load changes from no load to maximum design load. The slope of each of the different load lines is equal to the negative of the respective piping friction factors. The area underneath the lowest load line represents the pressure available for the branch piping, coil and control valve. Depending on the location of the branch in the system the lowest load line could be either the design or no load line. In this case the control valves in the first half of the branches which are closest to the pump should be selected for a higher design pressure drop than those in the second half of the system. H112 Engineering Bulletin 11

12 By sensing differential pressure in the middle of the distribution system the magnitude of the maximum variation in branch differential pressure is minimized when compared to other sensing locations. Compare the maximum variation of Pb in the building portion of Figure 7 to that shown in Figures 8 or 9. The bypass control scheme shown in Figure 8 will provide good pressure control at the end of the system. Unfortunately the variation in Pb at the beginning of the system may be excessive. The bypass control scheme shown in Figure 9 can provide good pressure control at the beginning of the system. Unfortunately the variation in Pb at the end of the system may be excessive. In spite of some inherent problems, direct return hydronic systems with 2- way control valves are very popular. This is likely due to the fact it has a lower initial cost relative to other types of hydronic systems and it is a very energy efficient pumping system. Lastly, a direct return system incorporating either a central plant bypass or variable speed pumping can provide acceptable pressure control when distribution piping is properly sized. Proper distribution pipe sizing is critical to insure trouble free control. If the distribution piping is undersized the Pb can vary significantly with the load. 12 H112 Engineering Bulletin

13 As a rule, the potential change in Pb must be less than the design pressure drop of the control valve, or problems can occur. These problems can manifest themselves in the following ways: unacceptable uncontrollable valve flow, problems with valve shutoff and spring range shift, valve cavitation, and overflowing marginally sized cooling coils. Primary/ Secondary Distribution Systems A primary secondary distribution system utilizes two sets of pumps. The first set is used to pump water through the chillers. These pumps are called the primary pumps. The second set of pumps are used to pump water through the building. These pumps are called the secondary pumps. The primary and secondary pumps are hydraulically isolated from one another. This isolation is accomplished by installing a bypass line between the primary and secondary pumping systems as shown in Figure 10. H112 Engineering Bulletin 13

14 The bypass line is also often called a decoupler line since it isolates or decouples the primary and secondary systems. The direction of flow in the decoupler piping can be in either direction. The direction of flow will depend upon the amount of chilled water produced in the central plant (primary flow) and the amount consumed by the building (secondary flow). If the primary loop produces a greater volume of chilled water than the secondary system consumes, flow in the decoupler will be from supply to return. If then secondary system consumes a greater volume of water than the primary system produces the water in the decoupler will flow from return to supply. Normally flow within the decoupler should be from supply to return. Otherwise the chilled water supply temperature in the secondary loop will rise to unacceptable levels. The hydraulic isolation allows both the primary and secondary pumping systems to function as if the other was not present. The primary pumps are, therefore, able to maintain a constant flow rate through the chiller regardless of the building load. The secondary system can be configured as either a direct or reverse return distribution system. The same advantages and disadvantages discussed earlier will still apply. However, in either case only two-way control valves should be installed. The chilled water flow rate in the secondary system will then vary in relation to the building load. This is desirable for two reasons. 14 H112 Engineering Bulletin

15 First, a variable flow secondary system will require less pumping energy than other types of systems. The secondary flow rate, in a primary secondary system, can be reduced to extremely low flow rates. The minimum secondary flow rate is not fixed by flow limitations imposed on the system by the chillers. If a variable speed secondary chilled water pumps is installed, the amount of pumping power required will be minimized. In a variable speed pumping system, the pumping power requirement will vary with the cube of the flow rate as dictated by the pump affinity laws. The sensing location for the differential pressure transmitter should be near the middle of the secondary system. Once again, this location is the best compromise for minimizing the variation in the magnitude of the Pb. Secondly, if both the magnitude and direction of the flow rate in the decoupler are known, this information can be utilized in various building automation schemes. Particularly for chiller sequencing. Chiller sequencing is discussed in Engineering Report H324. Decoupler Line Sizing Considerations It is important for the decoupler line which separates the primary and secondary systems to be properly sized. It must be sized so that its pressure drop at full flow is kept very small (i.e., less than 1 psig). Normally full flow is about 115% of the flow rate associated with the largest chiller. Flow rates in excess of this value would indicate an operating chiller should be stopped. The decoupler should be a straight length of pipe with no restrictions. It is very important for the pressure drop in the decoupler to be kept very small to provide the hydraulic isolation between the primary and secondary systems. If the size of the decoupler is too small, pressure changes in the secondary system will be transmitted into the primary system. This interaction can cause stability problems and undesired flow variations through the chillers. Occasionally, a primary secondary distribution system with a check valve installed in the decoupler line is encountered (See Figure 11). H112 Engineering Bulletin 15

16 The purpose of this check valve is to prevent the building chilled water return from flowing directly back to the suction of the secondary pumps. The apparent logic behind this scheme is that at certain low load conditions it may be possible to operate the system without a primary chilled water pump. This is the result of a lower pressure drop in the distribution piping at low chilled water flow rates. This scheme is not recommended! With this check valve installed it is possible to overload the motor on the primary chilled water pump. This can occur when both primary and secondary chilled water pumps are operating at a time when the central plant is not providing enough chilled water to meet the building load. In this case both pumps are in series. Therefore, if the secondary chilled water pump is sized for a larger flow rate than the primary pump (common), the primary pump can overload. It is also possible in the above example to pump an excessive amount of chilled water through a chiller causing maintenance problems due to water side erosion in the evaporator tube bundles. The length of the decoupler line is also an important consideration. The decoupler line should be at least 10 pipe diameters long. If the decoupler line length is too short cross circulation problems can occur. When the decoupler length to diameter ratio is too small (less than 10) the decoupler can act as though it is a tank rather than a pipe. Cross circulation problems induced by turbulence at the fittings can then occur. This is very undesirable because it is possible, even if the primary system is producing a sufficient quantity of chilled water, for the chilled water return to mix with the chilled water supply. It will then be difficult to maintain the proper chilled water supply temperature to the building (See Figure 12). If it is necessary to measure flow in the decoupler line, it should be at least 30 pipe diameters long. This recommendation of 30 pipe diameters considers the amount of straight pipe necessary to measure flow accurately. This is important since flow measurements are often utilized for chiller sequencing schemes. 16 H112 Engineering Bulletin

17 Note: Do not use square law type flow meters to measure the flow rate in a decoupler line. Square law type flow meters include annubars, orifice plates, flow nozzles, and venturi tubes. The output signal cannot be accurately measured with these flow meters at turndowns greater than 3:1. A bidirectional turbine flow meter should be utilized because it can indicate both direction and amount of flow accurately at much lower flow rates. Turbine meters with turndown ratios of 10:1 are typical, but are available with ratios up to 30:1. Remember water must be able to flow both ways through the decoupler line. H112 Engineering Bulletin 17

18 18 H112 Engineering Bulletin Notes

19 Notes H112 Engineering Bulletin 19

20 Notes Controls Group 507 E. Michigan Street P.O. Box 423 Milwaukee, WI Printed in U.S.A. 20 H112 Engineering Bulletin