AutomatedBuildings.com Column - Pressure Transmitters

Ever hear the old adage “Locate static pressure transmitter 2/3 down the length of the main trunkline.”? Or how about “Install differential pressure transmitter at the end of the furthest run.”? These seemingly arbitrary little “rules of thumb” are written over and over again, in specs, on control drawings, in sequences of operation, etc. But where did they come from, and what exactly are the governing principles behind them? In this article we’ll explore those issues, and hopefully answer those questions. But first, a little background on just exactly what we’re talking about here.

On the airside, static pressure transmitters are implemented in VAV air handling systems, those that operate to maintain a constant supply duct static pressure. As VAV boxes connected to the main trunkline open and close, the static pressure in the trunk has a tendency to vary. The trunk is kept pressurized by the supply fan’s variable frequency drive (VFD), which has the means to slow down and speed up the supply fan as required to maintain pressure. The pressure transmitter in this case measures duct pressure with respect to the pressure outside of the duct. The pressure signal is transmitted to the air handler’s main controller, and processed against pressure setpoint. The resulting signal is fed to the variable frequency drive, which in turn varies the speed of the fan to precisely and continuously maintain duct static pressure setpoint (we hope!).

On the waterside, differential pressure transmitters are utilized in variable flow pumping systems, whereby the pump speed is controlled (via VFD) to maintain a suitable differential pressure across the supply and return mains. Unitary equipment fitted with two-way control valves connect to the mains (hot and/or chilled water); the control valves modulate as a function of their individual unitary temperature control process. As this occurs, the pressure in the mains tends to fluctuate. A differential pressure transmitter installed across the supply and return mains monitors the pressure, and transmits it back to the pumping system’s main controller. The pressure signal is compared with the operating setpoint, and the calculation results in a control signal that feeds the pump’s variable frequency drive. The VFD in response varies the pump speed in order to achieve and consistently maintain differential pressure setpoint.

Properly sizing a pressure transmitter for an application shouldn’t be an arduous task. We’re not re-inventing the wheel here, and there’s plenty of practical and theoretical data to call upon when faced with the undertaking of selecting the appropriate range for the given application. Manufacturers typically offer a family of transmitters with differing ranges, so that makes things a little tricky right form the get-go. You may ask why these manufacturers just don’t make a single, range selectable or auto-ranging device, a “one size fits all”, so to speak. Well, I’m sure they’re all working on it, but until that time comes, we’re forced to choose from their offering, and hope that our selection fits the application.

For the airside application, the selection is actually pretty straightforward. There is a widely acknowledged rule of thumb calling for the “design” supply air static pressure setpoint to be in the area of 1.5” W.C. (inches of water column), or more commonly heard as “an inch and a half of static”. So select a pressure transmitter with a range greater than this. The trick is to be conservative with your selection, allowing for pressures greater than the “design” setpoint, but not so conservative that your selection ends up being the highest range that the manufacturer offers. Why not? Well, it has to do with signal resolution and precision. Just as you wouldn’t want to use a yardstick to measure the thickness of a penny, you don’t want to use a large ranged pressure monitoring device to measure a pressure whose practical range falls within a fraction of the device. Better to “rightsize” the device rather than oversize it. Your commissioning crew will appreciate it, at the very least.

For the waterside application, the selection of the pressure transmitter requires a little more insight. The “system setpoint” is generally not given in these applications, and is something that is typically determined after the system is installed, upon balancing the system. But the transmitter needs to be purchased and installed long before the balancer hits the jobsite. For a typical small to mid-sized HVAC piping system, required system design pressure is generally in the range of 10 to 20 PSI. That’s a starting point. A good practice is to select the pressure transmitter in accordance with the pumps selected for the system. For instance, if the pumps are selected to provide 40 “feet of head”, this value can be divided by 2.31 (PSI = ft. hd. / 2.31), yielding a result of 17.3 PSI. A controller with a 0 to 20 PSI range will suffice for the application. Location will play a part in selecting the range as well. If you’re locating the transmitter right there at the discharge of the pumps, then you would size for system design pressure. If you’re required to locate the device further downstream in the system (more on that in a minute), then you may be able to get away with the next smallest available range. Again, a transmitter with an oversized range will tend to not have the accuracy or the resolution that a “right-sized” transmitter will, so be careful with your selection, and put some thought into it instead of arbitrarily selecting a controller sized for total system pressure.

Placement rules for pressure transmitters are backed by a combination of both theoretical data and traditionally accepted practices. In the end, your choice for locating the transmitter will prove whether or not you’ve made the right decision. Not to worry. There’s no magic here. Just some things to know and some guidelines to follow.

For the airside, static pressure placement depends on the geometry of the duct system. For systems that have straight duct runouts, the static pressure transmitter should be installed in the main trunk, at least two-thirds of the way down the line, if not further. This assures that the most remote VAV boxes on the system get the pressure that they need, when the system is maintained at setpoint. For looped duct systems, placement of the transmitter is more arbitrary, assuming that the static pressure within the loop is relatively uniform and consistent throughout the loop. Put some distance between the transmitter and the discharge of the supply fan, get it in a straight section of duct away from elbows and transitions, and you should be good to go. For larger systems (both looped and unlooped), multiple transmitters may be required whose locations can be chosen by reviewing the mechanical plans and determining proper sensing points. The signals of these transmitters can be averaged, and the main controller can perform an averaging calculation in order to determine the proper course of action on the supply fan.

For the waterside, theory dictates that the differential pressure transmitter should be located at the end of the furthest piping run. This ensures both optimum energy savings and adequate flow for all loads in such a system. By way of example, consider that, in order to guarantee design flow at the “extremity” (furthermost load in the piping system), the differential pressure at that point in the system needs to be maintained at 5 PSI. Now consider under worst case conditions (all loads in the system calling for, and incurring, full design flow), the differential pressure right off the discharge of the system pump is found to be 10 PSI. Reasoning would have it that, if we maintained 10 PSI at the discharge of the pump, then we should be able to satisfy all loads on the system under all conditions. This is true, and therefore we can install the differential pressure transmitter right there, establish the setpoint as 10 PSI, and be done with it. However it is also true that, as loads are satisfied and flow is reduced, there is less friction in the pipes to overcome. Thus under part-load conditions (some loads calling and others satisfied), to guarantee 5 PSI (and hence design flow) at the extremity, a differential pressure of somewhat less than 10 PSI is all that is needed at the discharge of the system pump. By locating the differential pressure transmitter at the pump discharge and setting the setpoint at 10 PSI, for virtually any operating condition other than worst case, the system is using excess energy and potential savings opportunities are lost. On the other hand, by locating the differential pressure transmitter at the extremity, we can directly control to the needs of the extremity. Assuming that if we control to the needs of the extremity, all prior loads on the system will be guaranteed their design flow rates, then the system is optimized, that is, from an energy usage standpoint.

Steve Calabrese earned his BSEE degree in 1990 from the University of Illinois at Chicago (UIC). He has since spent much of his professional career working for a mechanical contracting company, in various roles including mechanical systems design, control systems design, project management, and department management. Currently employed by a large Chicagoland controls company, Steve couples his broad mechanical knowledge and experience with a strong background in the area of electricity and electronics. His control systems expertise includes electrical and electronic stand-alone controls, as well as microprocessor-based direct digital controls (DDC) and networked Building Automation Systems (BAS). You can visit his website at www.pcs-engineering.com.

In 2003 Steve’s book, Practical Controls: A Guide To Mechanical Systems, was published. Geared toward the HVAC professional, the book details practical methods of controls and defines the role of HVAC controls in an easy-to-understand format. Steve brings his mechanical and controls contracting experience to this writing, and offers practical approaches to control systems issues.

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