November 2013

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Yet Another Collection of Short Stories

Keepin’ them coming

Steven R Calabrese

Steven R. Calabrese
Control Engineering Corp.

Contributing Editor

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Why is that Ole VFD so Darn Big?

I remember when I first got into this business, and was schooled on what the acronym VFD stood for. The abbreviation stands for Variable Frequency Drive, although it’s sometimes referred to with other acronyms, such as VSD (Variable Speed Drive). Anyway, way back then these devices, which are used to vary the speed of a fan or pump motor, were real bones of contention. They were costly, they generated a lot of heat, and they were subject to failure. More than anything, they were HUGE!

If you ever run across any of these dinosaurs that are still functional and in operation, you will see what I mean. VFDs have come a long way in terms of all of the above attributes. Cost has come down considerably, with reliability going way up. And the size has shrunk to a fraction of what they were twenty-five years ago.

So getting back to question..why is that ole VFD so darn big? Well, given nothing to compare to, you might not be so quick to judge. But like anything else, advances in electronic technology have a tendency to shrink the equipment and devices that we’ve come to rely on. Take for instance your cell phone or your computer. If you’re old enough to remember when cell phones first came to market, you also remember how big and unreliable they were. Even TVs these days are a fraction of the size they were just a few short years ago. So to answer the question…I really don’t have the technical answer that you might expect, and more-so, I really don’t feel like using the space in this column to ramble on about that anyway. Just be thankful for the fact that they’re not that big anymore!

More Archaic Terms

Earlier in this series of short stories, I brought up some terms that, in my opinion, were outdated and probably should be retired, terms like Duty Cycle and Night Setback. Again, my opinion, however there may be some that feel there’s still some validity to using these terms. And that’s fine. Two more that I share here are used to describe pneumatic controls. And if you’re using them in reference to pneumatics, then you’d best be talking about existing controls that were installed decades ago! The two terms that I refer to are Receiver Controller and Reverse Acting. And believe it or not I still see these in written specifications (time to update those specs!).

In pneumatics, a receiver controller is a device that receives a pneumatic signal from a sensor, such as a temperature sensor, and controls an end-device such as a pneumatic control valve. So in this context, it’s basically a temperature controller. The setpoint is established locally at the device, and is functionally equivalent to a digital controller employed to do the same thing, electronically. Time to ditch the term when referring to temperature control in the modern era.

The other term, reverse acting, is an old throwback used to describe how an end device responds to a pneumatic control signal from a receiver controller. Given that, on an increase in temperature (from setpoint) as sensed by a pneumatic temperature sensor, the sensor in turn “tells” the receiver controller to send an increasing control air pressure to the end device in order to try and “get back to setpoint”. This is an example of “direct-acting”, another archaic term. Conversely, if an increase in temperature will affect a decrease in control air pressure, the process is called reverse-acting. The term has no relevance in modern digital controls, and should be retired once and for all!

Interlock…the Chicken or the Egg?

When we have two pieces of equipment that are to operate together, we often refer to these pieces of equipment as being “interlocked”. The classic example is that of an exhaust fan and a make-up air unit. It is sometimes said that an exhaust fan is interlocked to a make-up air unit. I note here that, in the basic definition of the term, make-up air units exist because exhaust fans exist. So a better utilization of the term is to say “make-up air unit is interlocked to exhaust fan.” It’s just more “technically correct”. It’s kind of like “Which came first, the chicken or the egg?” For interlocking scenarios, equipment that exists, because of the precedential existence of another piece of equipment, is the “interlocked” equipment. An exhaust fan exists in a generator room because the generator exists. The exhaust fan is interlocked to the generator. Likewise, a make-up air unit exists in a warehouse because an exhaust fan exists. The make-up air unit is interlocked to the exhaust fan.

RTZs…Do They Work?

I use the acronym RTZ (RoofTop Zoning) as the defining term for a system that takes a single zone, constant volume packaged rooftop unit and turns it into a multiple zone heating and air conditioning system. These are microprocessor-based systems, and are purpose-built, which isn’t to say that the basic functions of a “store-bought” RTZ can’t be implemented within a DDC system. An RTZ will minimally consist of a system controller, motorized zone dampers, and zone temperature sensors. Also typical for an RTZ are a supply air temperature sensor, a supply duct static pressure transmitter, and a motorized bypass damper.

The rooftop unit serves not one, but multiple zones, via motorized zone dampers. Each zone has a space sensor, and each zone gets to “cast a vote” as to whether it wants heating or cooling (or neither). The system controller tallies the votes, and puts the rooftop unit into the appropriate mode of operation. The zone dampers for those zones that are in need of the current mode (heating or cooling) will modulate in order to maintain desired space temperature setpoints, and the zone dampers for those zones that aren’t in need of the current mode will drive fully closed or to a “minimum position”. As setpoints are reached and zone dampers close off, the motorized bypass damper will modulate open to maintain airflow through the rooftop unit and to prevent excessive pressure buildup in the ductwork.

So…do they work? Sure they do, otherwise they wouldn’t sell them! Okay, now for the real story. RTZ systems are a decent, relatively inexpensive means of providing multiple zoning to a space served by a single rooftop unit. It works well when applied properly. This means that zones must be similar in load. Interior and perimeter zones should not be served by the same RTZ system. It also means that the setpoints to be maintained in the zones are close in value, relative to one another. It is not realistic to expect precision temperature control from an RTZ system. The number of zones that an RTZ can have is limited; the more zones a system has, the less of a chance to consistently and continually satisfy the individual zones. And while the concept of RTZ is solid, it is not a panacea for all zoning design challenges. Sometimes called a “poor man’s VAV system”, the customer should know up front what he is getting for his money. Of course the alternatives are more costly, and the customer may be just fine with it, even having the requisite comprehension of what it is. With that being the case, then it’s RTZ all the way!

More Terminologies (Unitary Heating Equipment / Terminal Units)

In a prior column I talked about some of the terminologies that we endear ourselves to in this chosen business of ours. That isn’t even including all of the acronyms that we’ve come to recognize over our careers. It’s like some secret code, or text message shorthand! Unit heaters alone have about a hundred or so acronyms associated with them, given the type of heater, their application, and what the design engineer feels like designating them on any given project. Read the subsequent short story on these, further on in this series.

Reliable ControlsThe point of this short, is to show how a couple of terms can encompass a wide array of similar equipment. Of the two that I’ve chosen, the first certainly has more of a story. Unitary Heating Equipment includes all manner of equipment that is manufactured to provide single-zone, unitary heating. The first word, “unitary” is used to categorize a piece of equipment as “unit level”, or more simply put, a small piece of stand-alone equipment built to serve a single space, or zone of temperature control. Meaning that it has a thermostat or space temperature sensor associated with it. If the equipment can provide cooling as well, then it need not be listed under this description!

Terminal Unit, in my experience and personal/professional opinion, is used to describe all manner of variable air volume (VAV) and fan-powered (FPB) boxes. This starts with the VAV cooling-only unit, the VAV with electric reheat, and the VAV with hot water reheat. And it ends with the parallel and series fan powered box with electric or hot water heat. And there’s a few in between, trust me.

I use these general terms a lot, because on any given project, there is typically a multitude of equipment that is similar in form and function, and it’s nice to be able to refer to these differing pieces of equipment, in conversation and in quick email dialogs, in a general sense. As long as the message gets through, then I suppose it’s all good!

Line Voltage Thermostat…?

Line voltage thermostats, or line stats for short, are an interesting breed of thermostat. They have a niche, albeit small, in the HVAC controls world. Their most common application is controlling a single-phase, fractional horsepower exhaust fan in a stand-alone (non DDC / BAS) fashion. Line voltage thermostats are typically capable of handling up to 8 amps, which is on the level of 1/3 HP at 120 volts. So for a small single-phase fan, the line voltage thermostat directly controls the fan. The typical application is a cooling application, whereby the thermostat makes on a rise in temperature above its setpoint, and completes the electrical circuit to the fan motor. When the temperature in the space served by the exhaust fan drops below the setpoint of the line voltage thermostat, the thermostat breaks the electrical connection to the fan motor, and the fan stops.

If the fan is three-phase and requires a motor starter, then the thermostat is wired to the starter. The thermostat chosen for this scenario is also most often a line voltage thermostat, since the starter’s control circuit is often 120 volts. Line voltage thermostats are typically constructed as single-pole-double-throw, “snap acting” controllers, capable of being used in either a heating or a cooling application (but not both!).

Tip of the Month: Beware the low-v thermostat carrying line voltage! Electrical codes and “good practice” prohibit the use of low voltage thermostats, those used for switching the heating/cooling/fan functions of a typical packaged rooftop unit or residential furnace system, in a line voltage application. Always exercise caution when working with any kind of electrical circuit. Just because that stat is designed for low-v applications, doesn’t mean that it’s being used that way!


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