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Back to Basics – Part Three
A collection of short stories
So we arrive at the third and final installment of this series on the basics of control. If you remember way back in June, I indicated that part three would get into some miscellaneous material, of which at that time had not had the chance to figure out. I wasn’t kidding! So with that said, I present a small collection of “shorts”, developed to give insight to some basic principles and at the same time be fun to read (ok…not that fun, my wife is currently into the third book of a popular trilogy, and I can only imagine how much more “fun” that three-part series is to read than this one!).
(To read Part 1 and/or Part 2 of this series.)
Ma’am, Yer Thermostat Ain’t Workin’
A residential service technician calls on a woman who’s complaining about her heating system. Upon his arrival, the woman explains to the tech that her thermostat doesn’t seem to be working. “It’s 60 degrees in my parlor and the thermostat is set for 70, so I turned it up to 80, and nothing happened!”
Turning your thermostat up when the temperature in your space is more than 2 or 3 degrees below your setpoint will likely do no good, as it is not the thermostat that is your problem. Regardless, human nature tells us to “crank that bad boy up!”. No, in reality, the problem lies with the equipment being controlled. The thermostat is probably just fine, and doing its job like it’s supposed to, calling for heat when the temperature in the space is below setpoint, by the amount of the “differential”. In other words, it the heat turns off at setpoint, the temperature in the space has to fall through the differential, which is typically set for 2 or 3 degrees. Once it has fallen to this value, the heat kicks back in, again till the space temp reaches setpoint. And the cycle continues. The moral of the story is, if your space temperature is more than a few degrees from setpoint, you have a problem with your equipment, not with your stat!
Window Shaker Lessons
Expanding on the concept of differential, I’d like to share my own personal education on the subject. When I was younger, we had a through-the-wall air conditioner in our living room. With the television set off, I could hear the thing periodically kicking on and off (the “thing” being the compressor). Never gave much thought to it. Years later I’m lying in bed in a motel room, same thing going on. The AC unit’s fan would run continuously, and the air conditioning would turn on and off, seemingly in a very quick cycle (like, every 30 seconds). What I learned years later is that there needs to be a temperature differential between the AC kicking on, and turning off. For instance, with a 2-degree differential, the temperature reaches setpoint, the compressor shuts off (and rests), the temperature begins to rise, and the compressor kicks back on when the temperature rises through the differential. With no differential, the compressor would literally turn on and off, on and off, and on and off, in an effort to maintain a very precise temperature setpoint within the space. And probably break down in the process! With differential incorporated, control is much more stable, though at the expense of compromised comfort control, however likely unnoticeable by the average person. Funny though, now that I know the concept, I’m still fascinated every time I stay at a hotel, and the unit in the room is seemingly operating with little to no differential…or does it just seem that way as I lie awake in bed desperately trying to fall asleep!
Analogy of a Controller
Remember back in last month’s column, when we were talking about controlling a light bulb? If we think about this particular process of control, we can pinpoint some of the components that make up the process. The “controlled variable” was the level of light produced by the bulb. The “sensor”, that which measured the level of light, was your eyes. And the controller? You! As the controller, you were able to receive data from the sensor (your eyes), process it, establish a preference, or “setpoint”, and act upon the controlled variable in an effort to bring it closer to your liking, or to your setpoint. Next time you turn on a fan to get some relief from the heat, and then turn it off a few hours later when you are cool and satisfied, think about how that relates (hint…you are the controller…!).
Diary of a Control Valve
Normally open, normally closed, mixing, diverting…ever wonder what exactly all those terms really mean? Specifically as they apply to control valves, as they tend to have other connotations when associated with other processes.
Anyway, the terms mixing and diverting refer to three-way valves and how the water flows though the valve body. Mixing means “two in and one out”. Meaning that all of the water flows out of one port at all times, with the water entering the other two ports in various percentages (depending on the type of control and state of the control loop at any given time). Diverting means “one in and two out”, kinda the opposite of mixing, where all of the water flows “in” to one port at all times. The terms have nothing to do with how much mixing or diverting is taking place at any given time; only how the water is entering and leaving the valve body as described herein.
The terms “normally open” and “normally closed” only apply when a control valve is equipped with a “spring-return” actuator. The terms refer to the state of a (two-way) control valve when the actuator is unpowered and the spring-return mechanism forces the valve body to be in its “normal” state. A normally open control valve would be one that allows full flow through its valve body when the spring-return actuator is unpowered. And a normally closed valve is…you get the picture.
Cruise Control Analogy
I used to like to use this when discussing the concept of proportional control with newbies in my industry. The intent of cruise control is to automatically maintain a fixed speed, without having to use the accelerator pedal. The first step is to establish setpoint by getting up to the desired speed and then pressing a button to “lock in” the setpoint. Once setpoint is established, the accelerator automatically positions itself to try and maintain the “speed setpoint”. The accelerator can assume varying positions to accomplish this. If a hill is encountered, the accelerator will increase to compensate for the added “load” on the automobile. Conversely, when traveling downhill, the accelerator will “lighten up”, as there is less pedal required in this situation. The speed of the automobile is being proportionally controlled; the accelerator is “modulated” in an effort to maintain a fixed, constant speed. Final word of advice: setpoint = speed limit!
Tip of the Month: Did you know??? Referring back to the snippet on control valves, and the terms “normally open” and “normally closed”…if you were referring to a set of electrical contacts, as would be part of a relay, these terms take on a different meaning. A relay’s “coil” can be energized or de-energized. A normally open set of contacts would be open when the relay coil is de-energized, and would thus not allow electrical flow (current) to pass through the contacts. Were the coil to be energized, the normally open contacts would “change state” and close, thereby allowing electrical current to flow through the contacts. Pretty much the opposite of how these terms (normally open, normally closed) are used in describing control valves. Causes some confusion with entry level controls designers, hopefully this clears it up!
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