Components of Hydronic Systems

Transcription

1 Valve and Actuator Manual 977 Hydronic System Basics Section Engineering Bulletin H111 Issue Date 0789 Components of Hydronic Systems The performance of a hydronic system depends upon many factors. Because of the large number of components which comprise a hydronic system there are an almost infinite number of possible system configurations. This report will discuss design intent, benefits/disadvantages and control considerations for only the most common configurations. The main emphasis of this report will be proper control of flow not optimizing primary equipment performance. To understand how a system composed of several components will function it is essential to have at least a limited understanding of each component. A hydronic system is generally composed of pumps, primary equipment (boilers, chillers), piping, fittings, coils and control valves. Pumps The relationship between the flow and pressure developed by a pump is graphically represented by a pump curve. This curve shows all the operating points of a constant speed pump as its discharge is throttled between zero and full flow. Figure 1 shows a pump curve for a centrifugal pump Johnson Controls, Inc. 1 Code No. LIT-351H111

2 When the speed of a pump is changed, the size and shape of the pump curve also changes. Therefore, a pump curve is only valid for a single speed. Figure 2 depicts how a reduction in speed changes the pump curve. When the pump speed is changed, both the flow and developed pressure will vary in accordance with the pump affinity laws. The pump affinity laws which relate changes in pump speed to flow, developed pressure across the pump and brake horsepower for a pump are shown below. 2 H111 Engineering Bulletin

3 Water Chilling Units (Chillers) Piping and Fittings The various chiller manufacturers have established recommended minimum and maximum water velocity limits for the evaporator tube bundles. Generally speaking, the chilled water velocity should not drop below 4 fps or the potential for freeze damage will increase to an unacceptable level. Conversely, the water velocity should not increase above 10 fps or premature tube failures are likely to occur because of water side erosion. These chilled water velocity limitations should be used as guidelines only. When in doubt contact the manufacturer. It is best from a safety standpoint if the chilled water is maintained at a constant flow rate so that problems with flow switches and control instability are minimized. From the control perspective, varying flows would increase the potential of control instability due to unpredictable changes in the chiller operating characteristics. To compensate, the chilled water temperature controller gain would have to be continually readjusted. The piping and fittings in a chilled water distribution system create a resistance to flow which is a function of the water velocity. For the remainder of this report this resistance to flow will be referred to as the pipe-friction factor. The maximum value of the pipe-friction factor occurs at the design (maximum) flow rate and is related to the square of the water velocity. To illustrate consider a 1 in. pipe: Design Velocity = 2.85 fps Corresponding Pressure Drop = 4 ft H2O / 100 ft of Pipe Note: If the type and size of the pipe, as well as the flow rate/velocity through the pipe are known, pipe-friction factors can be obtained from the ASHRAE Fundamentals Book. If Velocity is dropped 25% to 2.1 fps, then: New Pipe-Friction Factor = (100% - 25%) 2 X 4 = 2.25 ft H2O / 100 ft of pipe The actual pressure drop across any section of pipe (in ft H2O) can then be determined by multiplying the pipe-friction factor by the length of pipe (in feet), divided by 100. Pressure Drop = Pipe-Friction Factor X Length of Pipe 100 Various tables are available to relate the pressure drop of the different types of fittings into equivalent feet of straight pipe. The fittings can then be treated as lengths of straight pipe. Ideally the distribution system piping should be sized so that at design flow the pipe-friction factor will not exceed 4 ft H2O/ 100 ft pipe. This is discussed in detail in Engineering Report #H112, titled Dynamics Of Hydronic Systems. H111 Engineering Bulletin 3

4 Coils Coil Gain A graphical representation of how the capacity of a coil varies with water flow rate is shown in Figure 3. This curve assumes a fixed water supply temperature, entering air condition, coil face velocity and coil construction. Notice how quickly the capacity increases as the flow is increased from zero to 25% of design flow. Coil Gain is the incremental change in coil capacity produced by an incremental change in flow through the coil. If a coil characteristic curve was fitted into equation form, the gain could be directly calculated for any flow by taking the derivative of this equation. 4 H111 Engineering Bulletin

5 Control Valves Valve Flow Characteristics The shape of the valve plug determines how the flow through a valve will change with stem lift (see Figure 4). This relationship of flow to valve stem position is commonly called the valve flow characteristic. Valve plugs are normally shaped to provide one of three types of flow characteristics. They are: quick opening, linear or equal percentage. A graph of these three different valve flow characteristics is show in Figure 5. H111 Engineering Bulletin 5

6 A Quick opening valve plug produces a large increase in flow for a small initial change in stem position. Conversely when the valve is at a high percentage of maximum stroke there are small increases in flow for large changes in valve stem position. Quick opening type plugs are normally utilized in two position applications only. The flow through a Linear valve plug varies directly with the position of the valve stem. This type of valve plug is normally utilized in process control applications. They can be useful where it is desirable to control mass flow rates into and out of a process. In a Equal Percentage valve plug, for equal increments of valve travel, the change in flow rate with respect to valve stem travel may be expressed as a constant percent of the total flow rate at the time of change. This type of valve plug has the most widely utilized characteristic. It is useful for the control of coils and heat exchangers found in hydronic systems. Valve Gain Valve gain is the incremental change in flow rate produced by an incremental change in stem position. The gain at any point in the stroke of a valve is equal to the slope of the valve characteristic curve at that point. The valve gain for any lift can be directly calculated by taking the derivative of an equation which defines the flow characteristic. A valve should be selected so that its characteristic, when combined with the coil characteristic, allows the combined lift versus capacity relationship to be as linear as possible (see Figure 6). 6 H111 Engineering Bulletin

7 When a Equal Percentage valve is utilized with a coil, equal increments in valve stem position will hopefully match equal increments in heat transfer into/out of the coil. Unfortunately most coil characteristic curves are different. For any coil its actual coil characteristic depends on the coil construction, face velocity, entering air conditions, and water supply temperature. Figure 7 depicts how variations in these conditions affect the coil characteristic of a standard chilled water coil. Considering the large number of possible coil characteristic curves, it is almost impossible to exactly match the coil and valve flow characteristics together. Therefore, an exactly linear combined characteristic is seldom achieved. However if a limited number of valve sizing recommendations are followed, it is possible to achieve a workable match between the coil and valve characteristics. H111 Engineering Bulletin 7

8 Valve Authority The Equal Percentage flow characteristic curve shown in Figure 5 and Figure 6 is valid only when the control valve is operating at a constant pressure drop. This pressure drop must be equal to the full pressure developed by the pump. Obviously, in a real distribution system the piping, fittings, and coil also have a pressure drop. The amount of actual deviation from the shape of the Equal Percentage curve shown in Figure 5 and Figure 6 is determined by a property called valve authority. By definition valve authority is the ratio of the pressure drop through the control valve to the total system pressure drop at design flow. The Equal Percentage valve characteristic shown in Figure 5 and Figure 6 would relate to a valve with 100% authority. The control valve in the system shown in Figure 8 is a more realistic example having an authority of 33%. 8 H111 Engineering Bulletin If the system utilizes more than one control valve (see Figure 9), the branch pressure drop between the supply and return headers at design flow will be inserted into the denominator of the valve authority equation. The system depicted in Figure 9, has the exact same control valve and cooling coil installed in both Branches #1 and #2. Notice the flow rate and pressure drop in Branch #1 is larger than that in Branch #2. This occurs because each branch has a different differential pressure resulting from the friction losses in the distribution piping. As a result the differential pressure across Branch #1 is larger than the differential pressure across Branch #2. Since Branch #1 has a higher pressure drop more water will pass through Branch #1 than through Branch #2 when the identical control valves are wide open. How does this affect the authority of the control valves located in these branches? It actually has no effect whatsoever. The pressure drops across each branch and valve increase at the same ratio. As a result the authority is the same in each branch. Conclusion: valve authority is dependent upon the initial sizing of the control valve and it does not change with pressure shifts in the distribution system.

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10 Figure 10 shows how the shape of a Equal Percentage valve characteristic is affected by different valve authorities. Notice that as the authority increases we come closer to achieving the ideal Equal Percentage valve characteristic shown in Figure 5. It is also interesting to note that there are diminishing gains in achieving the ideal Equal Percentage valve characteristic by increasing valve authority after 50% valve authority is obtained. Therefore from a practical standpoint, a control valve with an authority of 50% is an acceptable compromise for most applications. To insure good control, valves with relatively high authorities must be selected (30% minimum). Unfortunately, many Contact Specifications require the control valves to be sized for a 5 psig (or less) pressure drop at full flow regardless of the branch pressure drop. Often this is not a high enough percentage of the total branch pressure drop to achieve good valve authorities. Ideally the control valve pressure drop should be at least 50% of the total branch pressure drop. Unfortunately, increasing the valve design pressure drop to help controllability, with all other things held constant, will require additional pumping power. The distribution system shown in Figure 11 is identical to the system shown in Figure 9, except the control valves were selected for a larger pressure drop. 10 H111 Engineering Bulletin

11 You probably noticed the pump in Figure 11 develops more pressure than its counterpart shown in Figure 9. The installation of the smaller control valves shown in Figure 11 would require an additional 13% of pump brake horsepower over the system shown in Figure 9. There is an obvious tradeoff between achieving good control valve authority and minimizing pump brake horsepower requirements. See the calculation below: Pump Brake Horsepower (BHP) Required = (gpm) (Head) (3960) (efficiency) % increase = Pump BHP (new) = 68 = 1.13 Pump BHP (old) 60 or a 13% increase (if pump efficiency is assumed constant) One possible solution to the above dilemma is to size the distribution piping so that it has a reasonably low pipe-friction factor (i.e., 4 ft drop / 100 ft straight pipe or less). It might then be possible to utilize a smaller valve with better authority without requiring an excessive amount of pumping power. Unfortunately the piping installation costs will rise. Valve Rangeability In the previous discussion, valve authority was shown to be unaffected by pressure changes in the distribution system (the shape of the valve Equal Percentage characteristic curve is not affected). However, there are other properties which must also be considered when determining how well a valve will function in a hydronic system. Valve rangeability and the actual distribution system dynamics can have a significant impact on how well a valve will function in a particular system. All valves have some amount of uncontrollable flow which occurs when the valve plug is first lifted from the seat. It can be thought of as a small opening around the plug which opens up when the valve plug begins to lift off of its seat. The size of this opening reflects the machining tolerances between the seat. The size of this opening reflects the machining tolerances between the valve plug and the machined orifice through which it moves. The flow rate around the plug is uncontrollable since the open area between the plug and orifice cannot be made any smaller unless it is totally closed by seating the valve plug. The magnitude of this phenomena is a function of valve rangeability. Rangeability is the ratio of maximum to minimum controllable flow rates for a given valve. Valves with high rangeabilities will have the benefit of lower uncontrollable flow rates for a given size valve plug.! CAUTION: Not all valve manufacturers define rangeability in the same way. Typically, due to manufacturing tolerances, valve rangeabilities for Equal Percentage valves do not exceed 75. If you do encounter a valve with a stated rangeability greater than 75, you should find out how its rangeability was calculated. H111 Engineering Bulletin 11

12 Remembering the coil characteristic curves in Figures 5 and 6, the goal is to minimize the amount of uncontrollable flow. A high percentage of the heat transfer from a coil occurs at very small percentages of its design flow rate! There are two obvious ways to minimize the uncontrollable flow through the valve. First, a valve with a smaller minimum opening should be found which still has the capability of passing design flow at the other extreme of its stroke. This would be analogous to selecting a valve with better rangeability. Unfortunately, the precision of ordinary machining processes limit valve rangeabilities. The second solution to the problem of minimizing the uncontrollable flow through the opening would be to try to reduce the differential pressure across it. Obviously, more water will flow through an opening of a given size when the differential pressure across it is increased. In this case the differential pressure across the valve depends upon the design of the distribution system. This is covered in more detail in Engineering Report #H112. It is desirable to hold the variation in the branch pressure drop to a minimum regardless of the building load, otherwise the amount of uncontrollable flow can become unacceptable. This problem occurs naturally in direct return distribution systems since the differential pressure across the branches will vary with location and load. The magnitude of the problem is directly related to the friction losses in the piping and the method of pressure control. Figure 12 shows how an increase in the pressure differential across Branches #1 and #2 of the system shown in Figure 9 affects the valve characteristic. As discussed previously, the valve authority is unaffected. However, increasing the branch differential pressures will increase the amount of uncontrollable flow which will, in turn, render a portion of the valve travel useless. Figure 12 illustrates how the higher branch differential pressure can cause the valve uncontrollable flow to increase by 25% and the amount of effective valve travel to decrease by nearly 20%. The magnitude of these problems is related to both the amount of the increase in branch differential pressure and the valve rangeability. Valve Cavitation From the proceeding discussion it should be well understood that selecting valves with large design pressure drops will enhance control. However, there is a pressure drop limit which should not be exceeded or cavitation inside the valve will occur. Cavitation occurs when the local velocity of the water in a valve is so high that the water vaporizes. As it continues to move through the valve the velocity drops and the vapor bubbles collapse causing very large pressure changes on the wall of the valve. 12 H111 Engineering Bulletin

13 Cavitation can destroy a valve. The maximum allowable pressure drop is a function of the pressure on the inlet of the valve, the vapor pressure of the water and the construction of the valve. A conservative estimate is shown in Equation 1 below. Equation 1: Maximum Allowable Pressure Drop = 0.5 (Pi - Pv) Where: Pi = Inlet Pressure, in psia Pv = Water Vapor Pressure, in psia H111 Engineering Bulletin 13

14 To help prevent cavitation in chilled water systems, two-way control valves should be installed in the coil supply piping to take advantage of the slightly higher inlet pressures. This logic may not be applicable in hot water systems, however, because the difference in the water vapor pressure across the coil may be higher than the static pressure drop through the coil. In summary, to obtain an adequate Equal Percentage valve characteristic, a control valve should be selected with a pressure drop that roughly equals the sum of the pressure drops of the coil and branch piping. If this pressure drop exceeds the maximum allowable pressure drop (see Equation 1), then the valve size should be increased until its pressure drop does not exceed this maximum for any operating condition. Actuator Sizing Considerations In the case of pneumatic valves and actuators, the actuator will modulate a normally closed valve from fully closed to fully open as the air pressure pushing against the diaphragm varies across the actuator s spring range. Consider a two-way, normally closed valve with a nominal 9-13 psi spring range. If the control air pressure is 9 psi or less, the valve will be closed. If the control air pressure is 13 psi or more, the valve will be fully open. Between 9 and 13 psi, the valve will be proportionally modulated. These nominal spring ranges are valid only when the pressure difference across the valve body is 0 psi. In reality this does not occur. Typically, charts are published in the valve product data sheets which show the actual spring range for different valve sizes with different differential pressures across them. This information must be considered when sequencing two or more valves from one output to prevent valve overlap. 14 H111 Engineering Bulletin

15 The amount of actuator force required to modulate a control valve is a function of the friction between the valve stem and packing and the differential pressure across the valve plug. The friction force inside the valve is relatively constant, and therefore, does not vary with the installation. The force imposed on the valve plug is related to the available system pressures and is, therefore, installation dependent. In the case of two-way control valves, the actuator force required to modulate the valve will vary in direct proportion to the differential pressure across the valve inlet and outlet and the size of the valve plug. Depending on the installation it is possible for the differential pressure across the valve to be as high as the shutoff head of the distribution pump. It is also important to understand that the actual spring range depends on the differential pressure across the valve not the magnitude of the system pressure. The magnitude of the system pressure is related to both the pressure developed by the pump, as well as the height of the building. In the case of a high rise building, the static pressure attributed to the height of the building may be 300 psi at a valve located in a lower floor, but this will have no effect on actual spring range of the actuator. The static pressure attributed to the building height is the same in both the chilled water supply and return risers with a net effect of canceling themselves out across the valve plug. The pressure developed by the pump, however, must be counteracted by the valve actuator. Therefore, the actual spring range is independent of the building height, but dependent upon the pump differential pressure. Generally when three-way valves are installed in a hydronic system, a mixing valve is utilized in a bypass application (see Figures 13 and 14). H111 Engineering Bulletin 15

16 If a three-way valve is piped as shown in either Figure 13 or 14, the actual spring range of the valve will depend upon the valve plug size and the difference in pressure across the two valve inlets. Consequently, the actuator must only overcome a force related to the pressure drop across the heat transfer device. The remainder of the forces created by the static pressure in the system, as well as the pressure developed by the pump, will cancel each other across the valve plugs. By adding the pressure drop across the heat exchanger (coil) to a small (less than 2 psi) pressure drop to compensate for friction between the packing and the stem, the curves in the product data sheets can be used to determine the actual spring range of the actuator. To reiterate, neither the height of a building nor the pressure developed by the pump have an effect on the actual spring range of a three-way valve installed as shown in Figure 13 or 14. The difference between the actual and nominal actuator spring ranges will usually be less than 2 psi. Therefore, actuator spring range shift and shutoff problems in three-way valves are normally negligible and generally do not present a problem. Controls Group 507 E. Michigan Street P.O. Box 423 Milwaukee, WI Printed in U.S.A. 16 H111 Engineering Bulletin