Monitoring Pressure to Confirm Flow

I don’t see it so much anymore, but there used to be an old “carryover” in some consulting engineers’ written specifications, that required the use of differential pressure switches to confirm proof of operation of pumps and fans. This “old school” method of confirming pump and fan status has all but been replaced by the tried and true current sensing switch. Using a pressure switch on a pump or fan gives a true confirmation of flow, be it water or air, and this was the argument that kept these pressure switches in the specs for so long. It was a direct means of confirming what we wanted confirmed: flow. When the current sensing switch manufacturers started hitting the consultants with the concept of proving flow by proving that the motor was conducting electricity, well, not all consultants were convinced that this would be adequate. The issue is/was, that you weren’t proving flow, you were only proving that the motor was operating. Not good enough.

Those were the days. Working for a controls contractor for the past decade, I remember submitting on using current sensing switches to confirm pump and fan status, only to have it bounced back by the engineer, demanding that we use pressure switches. More costly, harder to install, and harder to set up properly. All the while the current sensing switch manufacturers were touting their product as the “true” means of verifying flow. After all these years, it finally seems that most, if not all, of the consulting engineers have accepted current sensing as a legitimate means of confirming flow in pump and fan systems. And so the story goes…you have these died-in-the-wool engineers holding on to proven concepts that have stood the test of time, working alongside the younger chaps who are more open to new ideas. Eventually these old-school guys retire, and the specs get changed. Probably the right way of ushering in new concepts, albeit a cautious road, to say the least!

Any Way You Slice it…Still a Chiller

Chillers come in many different sizes, types, and configurations. Regardless of size or configuration, all chillers utilize the principles of refrigeration to perform their function. As such all require the basic components of the refrigeration cycle, namely, compressor(s), evaporator, and condenser. How these components are “brought together” to operate as one varies among chiller applications.

Air-cooled chillers exist outdoors, in their entirety! All components for the refrigeration cycle are integral to a single piece of equipment that is normally located either on grade or on a roof. The chiller consists of an air-cooled condenser, an evaporator, and a compressor or two (or more). The condenser is a finned coil and condenser fans. The evaporator is a “shell”  which houses a “tube bundle” of refrigeration lines. Chilled water is pumped through the shell, and rejects heat to the refrigerant flowing through the tube bundle. The compressors enable the whole refrigeration process, by pumping refrigerant from the evaporator to the condenser, and so forth. Controls for the packaged air cooled chiller are simple: turn it on and let it go! Well, almost that simple.

Split system chillers are air-cooled chillers in which part of the refrigeration cycle is outdoors, and part of it is indoors, with refrigeration piping running between the indoor and outdoor equipment. In order to explore the two unique configurations of split system chillers, we start with the packaged air-cooled chiller. For the first configuration, move the evaporator indoors. The condenser and compressors remain outdoors as part of the outdoor equipment, and refrigerant piping is run from the outdoor equipment, inside to the evaporator. This type of chiller is commonly referred to as an “air-cooled chiller with remote indoor evaporator”. The outdoor equipment, that which is referred to as the “chiller”, is really nothing more than an air-cooled condensing unit. The controls for this type of split system reside at the outdoor unit.

For the second configuration, now picture the compressors coming indoors. With the compressors and the evaporator indoors, all that’s left outside is the condenser. Even the control system comes indoors! A chiller configuration of this type is often referred to as a “condenserless chiller with remote air-cooled condenser. It is interesting to note that the equipment referred to as “the chiller” is that which holds the compressors and the control system.

A water-cooled chiller is a chiller that has all of its refrigeration components indoors, including the condenser. The major difference between air-cooled and water-cooled chillers is the type of condenser. Whereas with the air-cooled chiller, the condenser is located outdoors in the form of a finned coil and fans, with a water-cooled chiller, it’s indoors, an integral part of the chiller. The water-cooled condenser takes the form of a shell and tube bundle, closely resembling the evaporator. Condenser water piping connects the condenser to a cooling tower located outdoors, where the heat rejection takes place. Controls for the water-cooled chiller reside at the chiller, and are typically the most complex out of all of the different chiller configurations discussed.

So there you have it. The next time you’re asked to explain the differences between all of the different possible configurations of chillers, you’ll be well-prepared!

Face/bypass Damper Control

Face and bypass dampers are another method of “Taming the Beast” that is steam. In a past “short”, I discussed controlling a steam coil in a make-up air unit with two valves in lieu of one. This time around, I’ll use the same make-up air unit for the topic of this discussion, but instead of two modulating control valves, we’ll equip the unit with a single two-position control valve, and…face/bypass dampers!

The steam coil, in this case, does not occupy the entire cross-section of the make-up air unit. There is space above the coil, for “untempered” air to pass through. The area of the space above the coil is usually less than the face area of the coil. Each of these two sections (the coil and the open area above) is fitted with a damper. The dampers are linked together and operate in unison, in opposite directions, so that when the face damper (coil) is open, the bypass damper (open area) is closed, and vice versa. Of course the two dampers can assume any position within their entire range of motion, when fitted with a proportional damper actuator.

So when the unit is in operation, let’s say it operates to maintain a discharge air temperature setpoint. As for our example the make-up air unit brings in 100% outside air, and operates to maintain a discharge temperature setpoint of 70 degrees. So when the OA temperature is below 70, the two-position steam control valve is fully open. So steam flow through the coil is available for all outside air temperatures below 70 degrees. To maintain discharge air temperature setpoint, we must pass varying amounts of air through the steam coil, and mix it with air bypassed around the coil. We control the face/bypass dampers to meet this end. As the outside air temperature drops, more and more air must pass through the coil, with less and less being bypassed. A fairly precise method of control, at least when you compare it with trying to modulate a steam valve to maintain setpoint.

Face/bypass damper control is sometimes specified in conjunction with proportional steam control. While this may seem like overkill, it’s seen often enough, especially in colder climes. Typically the valve(s) will modulate to maintain discharge air temperature setpoint, with the face damper being fully open, for all OA temperatures above say, 40 degrees. At that point, the valve is forced fully open, and the face/bypass dampers take over. Now that’s truly the best effort to Tame the Beast!

Two-position vs. Modulating Control on Small Systems

There’s an ongoing trend to specify everything to have modulating control, down to the smallest unitary equipment such as hot water baseboard and unit heaters. To give a unit level piece of equipment modulating control, you need to equip it with a control valve that has a modulating actuator. Moreover, you need to control it via a controller, whether DDC-based or stand-alone electronic, that has an analog output.

[an error occurred while processing this directive] A point that I always used to argue, is that for equipment that consumes only 1 or 2 GPM, there is simply no need to pay the premium for proportional control. I use my own experience with my home’s heating system (central boiler with zone pumps feeding radiators throughout the house) to justify this argument. For years I have not been able to tell if and when the heaters are active, unless I go to the heater and physically feel it. Now, my sensitivity to temperature changes may not be that of my wife’s, but she’s never complained about the accuracy of our home’s temperature control either. I simply do not believe that one can feel the difference between two-position and modulating control on this small of a scale. Maybe I’m wrong, but at any rate, I never thought that the benefit of proportional control was worth the added cost, at this level.

Nowadays, my argument may be moot, for the cost difference between two-position and proportional control is decreasing, as small control valves with two-position actuators are not a whole lot (if at all in some cases) cheaper than those equipped with modulating actuators. And while it’s true that an analog output off a digital controller is more costly than a binary output, even this is becoming a non-factor in many applications, as unit level digital controllers with a single analog output can be competitively priced. Regardless, I’m holding on to my point of view on this…can’t beat two-position control for the sheer simplicity of it!

Tip of the Month: When in doubt, choose a proportional (modulating) actuator for a valve or damper actuator. Unless you have dozens and dozens of these to procure, you can’t go too wrong with a proportional actuator. You can always “two-position” a proportional actuator. But you can’t modulate a two-position actuator, no matter how hard you try!

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