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How Safety Devices (Should?) Affect System Components
Simple, albeit important, rules of thumb regarding equipment fail safe modes
Steven R. Calabrese
In the June 07 column, I talked about air handling unit safety devices, what they are, how they function, and how they should be wired. Typical safeties found on an air handler include (but are not limited to): low limit temperature controllers, high and low limit static pressure controllers, smoke detectors, and fan vibration switches. In this column I’ll talk about what happens to the various air handler components upon a trip of a safety device. Specifically we’re talking about the “moving parts”: pumps, fans, control valves, and motorized dampers. Let’s work in reverse order of that list and start with the dampers first.
A typical air handler is equipped with at least two dampers: one for the outside air and one for the return air. If the unit has an exhaust/return fan, then there will also be an exhaust air damper. These dampers are fitted with spring-return, proportionally operated damper actuators. When power is interrupted to the damper motors, they drive their respective dampers to a “fail safe” position, by means of a wound-up spring integral to the damper motor. Common practice dictates that the outside air damper and exhaust air damper spring shut, while the return air damper springs open. This is to ensure that upon a failure (either a safety device tripping or a general loss of electrical power to the unit), the unit will not allow any outside air to migrate into it and pass through it. This is especially crucial in colder climates during the winter months, to where the outside air temperatures drop below freezing. Cold air migrating through an air handling unit and into the spaces served is a undesirable enough situation. With air handling units that are equipped with hot and/or chilled water coils, the situation becomes that much more critical, seeing as subfreezing air temperatures passing through a water coil (if the water is not rigorously flowing through the coil) can and will freeze the coil, resulting in a ruptured coil. Remember that water expands when it freezes, and the resulting expansion will burst open the coil and cause a whole mess of other problems, as you can imagine.
Face and bypass dampers are oftentimes utilized in air handlers equipped with steam heating coils, although their application is certainly not limited to steam heating. Anyway, the arrangement is such that the steam coil does not occupy the entire cross-section of the make up air unit. There is space up above the coil, for untempered air to pass through. Each of these two sections (the coil and the open area above) is fitted with a damper. The dampers are linked together and work in unison, in opposite directions, so that when the face damper (coil) is open, the bypass damper (open area) is closed, and vise versa. Temperature control is implemented by modulating a single spring-return damper motor to allow varying amounts of heated and untempered air to mix, downstream of the assembly. It is generally accepted that the fail safe positions of these dampers is the face damper closed and the bypass damper open. Oh yeah, and spring open the coil’s steam control valve as well!
|Tip of the Month:
The subject matter contained herein should be regarded as a “rules of the road” when it comes to determining fail safe modes for the general components of an air handling unit. However there are always reasons to do things other ways, so don’t blindly go with the rule of thumb for any given application. Treat every project as unique, and always understand the why’s before implementing the how’s!
Which brings us to hydronic control valves. Consider if you will an air handling unit with both a hot and a chilled water coil. Common practice dictates that these control valves be fitted with spring return actuation. Traditional wisdom states that the hot water valve springs open to the coil and the chilled water valve springs closed to the coil, upon failure. Understanding the fail safe position of the hot water valve is simple; if a safety device such as the low limit temperature controller trips, we would want full flow of hot water through the coil, as this can only help the situation. To understand the traditional fail safe position of a chilled water valve requires a little more insight. Some engineers might specify this valve to spring open upon a trip of a safety device, the thought being that moving water is harder to freeze than standing water. However most will still abide by the conventional wisdom that dictates this valve to spring shut. We’ll leave it at that, and allow you to contemplate further as to why this may be the case.
For air handling unit supply and return fans, the fail safe mode for these components is OFF. That’s a given. How we go about ensuring that the fans shut down upon a failure is a subject of continual debate. For the constant volume fan, the fan motor will be controlled by a starter. The starter will typically be equipped with what is called a HAND-OFF-AUTO (H-O-A) switch, as this is how these starters are generally specified. With the switch in the OFF position, the fan motor will be positively off, with no chance of the fan starting. With the switch in the HAND position, the fan motor will be energized and the fan will run. In the AUTO position, the fan starter will be under automatic control, as interlocked to some external, remote command, perhaps from the building automation system, and the fan will start and stop as via this interlock.
The ongoing debate pertains to how the safety circuiting affects the fan starter, when equipped with an H-O-A switch. Certainly when in the AUTO position, a trip of any safety device must shut down the fan. But what about the HAND position? Should the safety circuit be incorporated into the HAND mode as well? What are the implications either way?
On one side of the argument, you have the opinion that the fan should shut down in the event of a trip of a safety device, regardless of the position of the H-O-A switch. This means incorporating the safety circuiting into the HAND position as well as the AUTO position. On the other side of that argument is the end-user’s opinion that he aught to be able to override any given piece of equipment by throwing its respective H-O-A switch into the HAND position. After all, isn’t that what it’s there for? I’ll let you be your own judge as to which method is philosophically correct. In most cases however, the prevailing opinion is that of the consulting engineer, that the safety circuiting should be incorporated into both sides of the H-O-A switch.
For fans equipped with variable frequency drives (VFDs), which is the case for most VAV air handlers, the situation is perhaps even more critical. In this scenario, there will be an input for remote start/stop of the drive, and a switch or mode button that allows the user to choose between drive and bypass operation. In bypass mode, the VFD will bypass the drive electronics entirely and operate the fan motor at full speed, with no regard as to what’s happening out in the air system. If the safety circuiting does not affect the VFD in both the drive and bypass modes, then the risk exists that someone could go to the VFD, manually put the drive in the bypass mode, and cause some serious damage. Consider the following scenario: a VAV air handler shuts down due to its high limit duct static pressure controller tripping out. The maintenance personnel goes to the VFD and throws the supply fan into bypass mode. If the safeties aren’t incorporated into the bypass mode of the VFD, then the unit will again tend to over pressurize the ductwork, but instead of shutting down, could very likely blow the ductwork apart at the seams!
The last items to discuss are pumps. Not the base mounted system pumps that you find in a mechanical room. What we’re talking about here are coil circulating pumps. It is important to note that not all coils are equipped with circ pumps, but when they are, it is essential to understand the piping and valving configuration in order to determine the desired failure modes of these pumps. Oftentimes the piping arrangement will allow for the pump to provide full flow through the coil, regardless of the valve’s position. This is made possible by way of a bypass line thrown in to the piping configuration. In these cases, the pump’s fail safe mode is ON. Meaning that, upon a trip of a safety device, the pump will remain on, or if off, will turn on to flow water through the coil. Again, make sure to study the piping setup, in order to determine the circ pump’s required failure mode.
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