March 2011

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Controls Devices (Part 2 of 3)
Rules of thumb to follow for TC installation & design

Steven R Calabrese
Steven R. Calabrese
Control Engineering Corp.

Contributing Editor

In this the second part of a three-part series on controls devices, (Part 1) we continue the discussion on Sensors & Transmitters, picking up where we left off last month. To reiterate the purpose of this three-part series, it is to present a “quick reference” guide to temperature controls devices, broken down in this fashion:

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This installment finishes the first topic, and covers the second topic as well, leaving the last two for next month’s column.

Sensors & Transmitters (continued)

Duct Pressure Transmitters

Differential pressure transmitters, those that have both a high and a low pressure port, that are used for duct pressure sensing, will have the high side ported into the duct and have the low side left “unported”, such that what we’re measuring is duct “static” pressure. Conventional wisdom dictates that the duct pressure should be monitored “2/3 down the trunkline”, which is a rather general statement. The reality of it is, it really depends on the geometry of the ductwork system. So while the rule of thumb holds fairly true for straight duct runouts, for looped systems the location is more arbitrary. And for larger systems, multiple sensing points may be specified, requiring an independent pressure transmitter for each sensing point. The pressure range selected for this application is generally in the 0-2 inches W.C. range, although certain applications could warrant a larger range.

Location of the actual transmitter, at least on paper, is somewhat arbitrary. The transmitter can be located back at the control panel for the air handler (where the thing gets wired to), and the high port tubing can be run from that location to where it needs to penetrate the duct. Whether it needs to run as poly tubing or hard copper is a concern. Just as often, the transmitter will be mounted right at the duct penetration location, with the signal and control power wires running back to the air handler control panel. In this case, the concern is whether or not the cabling needs to be in conduit.

Pipe Pressure Transmitters

Differential pressure transmitters that are constructed for monitoring pressures in piping systems will have both ports “hardpiped” into the pipes, typically the high side ported into the supply line and the low side ported into the return line. Conventional wisdom dictates that the pressure across the mains should be measured “at the end of the furthest run”. The reality is that other factors come into play here. The best you can do is strike a balance between what’s practical and what’s “theoretically correct”. The pressure range selected for this application is typically in the 0-20 psi range (or greater). Knowing the pump “head pressure” can help in choosing the most appropriate range for the given application.

As for location of the transmitter, it really needs to be right there where it gets ported into the mains. It can be brought to a location to where it's more easily serviced (within reason), and it should be equipped with a three-valve bypass assembly, for maintenance and replacement purposes.

Airflow Measuring Stations

ASHRAE Standard 62.1 and the potential for attaining LEED credits have driven the requirement for duct-mounted airflow measuring stations. The types of measuring devices run the gamut, from the simple pitot tube and pressure transmitter, to state-of-the-art technologies like thermal dispersion.

For small systems with relaxed specs for airflow monitoring, the pitot tube / pressure transmitter may suffice. Larger ducts typically require a “multiple probe” approach. When there’s not much clearance in terms of upstream and downstream straight duct distance, fan inlet probes might fit the bill (pair of probes per inlet). And when you require nothing but the best (and most costly), there are the manufactured airflow measuring systems that utilize the latest technologies. For a more in-depth read on AFMS, have a look at my August 2009 column written on this topic.

Flow & BTU Meters

Hot and chilled water flow meters are devices that typically insert into the pipe or are fit in as a “spool piece”. There are minimum upstream and downstream distance requirements for the location of these devices, that are dictated by the manufacturer and must be heeded in order to ensure accurate flow measurement. Generally, there needs to be a certain amount of straight pipe before and after the flow meter, typically more upstream than downstream. Either way, it’s important to take this into consideration when finding a spot for the device.

Add a pair of temperature sensors, one each in the supply and return mains, and you can monitor BTU consumption. You can perform the calculation within the Building Automation System (BAS), or you can purchase a complete BTU meter, which consists of a main panel that accepts the signals from the flowmeter and the temperature sensors. In going this route, you have two choices to get the flow and BTU information into the BAS. You can receive this information via discreet output signals from the BTU meter, or you can receive the data via a standard communication protocol, provided the BTU meter manufacturer offers this option.

CO Transmitters

Carbon Monoxide (CO) sensing is an important function in areas that will tend to have a CO buildup, such as enclosed parking structures and mechanical (boiler) rooms. Of course excessive CO can be lethal, so there are some very important guidelines that must be adhered to when designing/installing/maintaining a CO sensing system. I present here some very general information, as I will be dedicating an entire column to this topic in the near future.

CO sensors should be mounted 3-5 feet above the floor, and need to be initially calibrated or at least checked out for proper functionality. A typical boiler room may require one or two sensing points, whereas a parking garage will require many sensing points. Each sensor can cover a radius of area, which is sensor-dependent, although you typically see ranges in the neighborhood of 50’. Lastly, it’s imperative that, in a wireless CO sensing system, the batteries be checked and replaced on a periodic basis.

[an error occurred while processing this directive]Switches & Two-state Devices

Status for Fans & Pumps

Traditionally, fan status was monitored with either a differential pressure switch across the fan or a “sail switch” downstream of the fan. And pump status was monitored with a differential pressure switch across the pump or a “paddle switch” downstream of the pump. Nowadays it’s more common to take status of the motor itself, using a current sensing switch. And while this method does not directly confirm air or water flow, it’s reliable, simple to install, and easy to set up. This notwithstanding, those traditional methods still have their place (just don’t ask me where!).

Filter Status

In a typical air handling unit, there’ll be at least one filter section, the larger units having multiple sections, or banks. Monitoring the pressure drop across a filter bank can help determine when the filters need to be changed. Dirty filters incur more of a pressure drop than clean filters, just a matter of where you set that pressure switch across the filter bank to trip at. Using a two-state differential pressure switch to convey filter status is inexpensive, albeit perhaps a little tricky to functionally implement. Set the switch at a high initial value, visually monitor the filter bank until the filters are “dirty”, and then adjust the setpoint of the switch until it changes state. Leave it at that setting and go through another “maintenance cycle”, this time waiting for it to trip and noting the condition of the filters. Filthy beyond belief? Reset the switch’s setpoint accordingly. Still kinda clean? Okay, you get the picture!  

Tip of the Month: Use a differential pressure transmitter to monitor filter status. This is more costly than using a two-state switch, yet more intuitive in practice. The transmitter will provide a variable reading that will increase as the filter bank “dirts up”. For VAV air handling units, where the air volume through the filter bank is always fluctuating, and thus so is the pressure drop across it, establish a CFM value (or VFD speed value) at which a “snapshot reading” of the pressure drop will be taken and registered. Update the graphic with each new snapshot. Build logic into the sequence so as to “force” this CFM value perhaps on a weekly basis, in order to ensure that the graphic displaying the filter status is kept current (cool!).


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