2271 Valve and Damper Selection

Selection criteria usually includes a minimum pressure drop for proper authority and best linearity over the process.

22.7.1.1 Valve Sizing

The typical sizing procedure for a hydronic control valve is 5 psig wide open pressure drop. The following explains the reason for this. The underlying principle for reasonable control of a heat transfer coil is for the control

Figure 22.8 Control Valve Characteristics

valve wide open pressure drop to be at least as high as the coil it controls. So, if an air handler coil is sized for a 5 psig pressure drop at full flow, then the control valve at 5 psig full flow pressure drop would be appropriate. It is common practice to select HVAC water coils at 10 ft. w.c. or so wide open pressure drop, which equates to 10/2.31 = 4.3 psig. Thus the 5-psi valve pressure drop convention is a reflection of the coil sizing convention. An extension of this logic would be that if the coil were selected at 1 psig wide open pressure drop, then the control valve sizing criteria could also be reduced. This is in fact the case, although seldom done in practice since coils selected at extremely low pressure drop have other, new issues, namely reduced velocity and laminar flow heat transfer degradation at reduced flows.

Editorial Comment: The practice of adding system resistance to achieve good control is counterintuitive and definitely an opportunity for improvement in the industry, because adding circuit resistance to any fluid handling system increases the system energy requirements.

22.7.1.2 Leak-By and Close Off

When specifying control actuators, specifications and close attention are important to achieve reliable close-off performance. This is true for both valves and dampers, especially those with marginal actuator close-off ratings, excessive system pressure, large damper sections, metal-

seated valves, undercut butterfly valves, and actuators without a positive seating mechanism to impart a residual tight seating force. Spring-return pneumatic actuators are forgiving in this sense because they inherently provide residual seating force. Some electronic actuators have mechanisms to provide ample seating force, but others rely on simple travel adjustments that define the open and closed positions—these are very undesirable since the opportunity for internal leak by, with subsequent heat/cool overlap, is high. Ball valves can be used for modulating service if they have characterized seats, and are inherently tight seating.

Actuators that are only marginally strong enough to close off against system flow can rob the system of efficiency over time as system pressures change, valve stems bind, damper axles stiffen, etc. Determining system needs and requiring close off ratings well in excess of this (e.g. at least 50% more) is good practice for sustainable operation.

22.7.1.3 Damper Sizing

Just like valves, down-sizing dampers will improve control at the expense of raising system pressure. In practice, dampers are often left duct-sized, even though resulting control is poor. There are several reasons for this:

• Dampers are relatively cheap, compared to the transition costs for a duct fabricator.

• Dampers are often large, and transitions take up space that may not be available.

• At outside air intakes, a down-sized damper can cause rain or snow entrainment.

For air flows control by dampers, other than HVAC, proper sizing will yield more linear control and is recommended where practical. For control purposes, the opposed blade damper is normally used, since its aperture size and overall resistance varies the most directly with travel. Conversely, "parallel blade" or round dampers are highly non-linear in nature and hard to control unless drastically down-sized, or used for two position control only.

22.7.1.4 Other Damper Considerations

Large damper sections are often problematic. For cost reasons, there is often a desire to use fewer, larger actuators and link the dampers together so an adjacent actuator is driven by another damper, not an actuator. In practice, this can easily result in one end of a long section substantially open even as the actuator-end is closed. This is due to the fact that the damper blades and axles will twist and stretch. Methods to prevent this undesirable condition include multiple actuators or jack-shafting.

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