ENGINEER S NOTEBOOK This article was published in ASHRAE Journal, April 2013. Copyright 2013 ASHRAE. Posted at www.ashrae.org. This article may not be copied and/or distributed electronically or in paper form without permission of ASHRAE. For more information about ASHRAE Journal, visit www.ashrae.org. 100 Graphic Unavailable Full Flow at Constant Pressure Drop (%) 90 80 70 60 50 40 30 20 10 Quick Opening Linear Equal Percentage Figure 1 (left): Traditional coil pack. Figure 2 (right): Control valve flow characteristics. 0 0 10 20 30 40 50 60 70 80 90 100 Full Valve Stem Travel (%) Control Valves & Coil Packs By Mikhail Nudelman, P.E., Member ASHRAE All coil packs on the market have two devices on the return line that regulate flow: a control valve and a manual or automatic balancing valve. This article proves that only one valve is necessary to regulate and balance flow at the coil packs and as a result, this arrangement will reduce material, installation, and operation costs. A coil pack is a customizable coil solution to meet the needs of the end user that can be provided on the jobsite as a pre-connected unit for faster and easier installation. A traditional coil includes (Figure 1): On supply line: combination ball valve/strainer with pressure test plug and blowdown/drain valve; On return side: Union with manual air vent and pressure test plug; control valve; automatic flow-control (limiting) valve or manual balance/metering valve with pressure test plugs and union end. Return Side Control valve. The control valve is used to regulate flow as demands warrant. It regulates a flowing fluid, such as water, to compensate for the load disturbance and keep the regulated process variable (as close as possible to the desired setpoint). The pressure drop across the control valve varies between a maximum, when it is controlling, and a minimum, when the valve is near full open. Typical control valve flow characteristics are shown in ASHRAE Handbook HVAC Systems and Equipment, Chapter 47, Figure 16 (shown here as Figure 2). Selecting the correct control valve size for a given application requires knowledge of process conditions that the valve will actually undergo in service. Using the principle of conservation of energy, Daniel Bernoulli found that when a liquid flows through an orifice, the square of the fluid velocity is directly proportional to the specific gravity of the fluid. The greater the pressure differential, the higher the velocity. The greater the density, the lower the velocity. A basic liquid sizing equation is shown in ASHRAE Handbook HVAC Systems and Equipment, Chapter 47, Equation 3: Q= C p v Where Dp = pressure drop, psi Q = volumetric flow, gpm C v = flow coefficient, gpm at Dp = 1 psi 48 ASHRAE Journal ashrae.org April 2013
C v is numerically equal to the number of U.S. gallons (liters) of water at 60 F (20 C) that will flow through the valve in one minute when the pressure differential across the valve is 1 lb/in. 2 (1 kpa). C v varies with size and style of valve, which provides an index for comparing liquid capacities of different valves under a standard set of conditions. Knowing the control valve C v (from valve submittals) and the design flow, it is easy to calculate the required pressure drop across the control valve and verify it with a water meter. Therefore, it is possible to set design flow using control valve C v. Automatic Flow-Control Valve. Automatic flow-control valves regulate the flow of fluid, such as water, to a design value. When the differential pressure across it is varied, the automatic flow control usually has a spring-loaded cartridge that dynamically absorbs pressure fluctuations, resulting from changing flow conditions, valve modulation, etc. These fluctuations in pressure, within a given pressure differential control range, do not change the flow through the valve. A typical performance curve for the valve is shown in ASHRAE Handbook HVAC Systems and Equipment, Chapter 47, Figure 22 (shown here as Figure 3). The flow curve is divided into three ranges of differential pressure: the start-up range, the control range, and the above-control range. At the startup range of differential pressure, the spring-loaded cartridge is fully open and cannot control the flow. At the above-control range of differential pressure, the spring-loaded cartridge is at maximum closed position and cannot control the flow. Only in the control range of differential pressure is the spring-loaded cartridge controlling constant flow to within ±5%. Manual-Balancing Valve. Manual balancing valves allow the measurement of differential pressures that can be used to calculate and set a determined flow. By measuring the pressure drop across measuring ports at a particular hand-wheel setting, the water flow for the valve size can be easily read from the appropriate pressure drop graph or flow-balancing wheel. All balancing valves include a memory feature with a locking setting. Conclusions and Recommendations Consider the joint operation of the control valve, which keeps regulated process variable, as shown in Figure 2, and the automatic flow-control valve is controlling constant flow, as shown in Figure 3. When the control valve is fully open, the automatic flow-control (limiting) 160 Start-Up Range 140 120 Control Range Above-Control Range Rated Flow (%) 100 80 60 Fixed Port ± Accuracy Band 40 20 Variable Port 0 r s r v Differential Pressure Figure 3: Automatic flow-limiting valve curve. www.info.hotims.com/44631-14 April 2013 ASHRAE Journal 49
www.info.hotims.com/44631-17 valve s differential pressure springloaded cartridge controls constant flow to within ±5% if differential pressure across the valve is in control range. The control valve regulates the flowing fluid to compensate for the load disturbance and keeps the regulated process variable (as close as possible to the desired setpoint). At the same time, the automatic flow-control (limiting) valve s differential pressure spring-loaded cartridge is adjusting to control constant flow. If the differential pressure across the automatic flow-control (limiting) valve is in control range, the cartridge is opening when the control valve is closing and then staying in a fully open position at differential pressure below control range (i.e., the automatic flowcontrol [limiting] valve is opposing the control valve to keep regulated the process of the desired setpoint). In other words, operating together, the automatic flow-control (limiting) valve is providing cartridge setting flow at differential pressure across the valve in control range with the control valve in the open position. If the control valve is decreasing flow, the automatic flow-control (limiting) valve still is trying to provide cartridge setting flow at differential pressure across the valve in control range, resulting in the overflowing of the component. Considering that about 10% of operation time, the control valve will be fully open and the automatic flow-control (limiting) valve will regulate the flow to design value, and that about 90% of operation time, the control valve manipulates a flowing fluid and the automatic flow-control (limiting) valve is opposing the control valve, there is no sound reason to install a control valve and an automatic flow-control (limiting) valve together in a coil pack. Automatic flow-control (limiting) valves should be used in constant volume systems or in variable systems only for constant flow components. Consider the joint operation of a control valve and a manual balancing valve in a traditional coil pack. We have two devices that regulate flow: a control valve and a manual metered balancing 50 ASHRAE Journal April 2013
ENGINEER S NOTEBOOK valve/orifice, commonly referred to as a circuit setter. To decrease material and installation costs and needless resistance in the system, thus less head, kw, etc., installation on the return line of a coil pack should consist of only the control valve, ball valve with pressure test plugs, memory stop and union end on the return line of the coil pack (Figure 4). The required pressure drop (flow) on the control valves must be set by adjusting ball valves with memory stop. The balance/metering valve would be omitted. Installation of balancing valves would only be on main pipes, branches and on components without control valves. Mikhail Nudelman, P.E., is a mechanical engineer at Aero Testing & Balancing Systems, Inc. Chart Unavailable Figure 4: Coil pack with memory stop ball valve. www.info.hotims.com/44631-60 April 2013 ASHRAE Journal 51
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