HVAC Controls

[sparky666]

I'm an industrial electrician by trade and am looking into taking over HVAC control via a PLC. Can anyone suggest any books or websites? Thanks in advance.

[osiyo]

Originally posted by sparky666
I'm an industrial electrician by trade and am looking into taking over HVAC control via a PLC. Can anyone suggest any books or websites? Thanks in advance.

Hi Sparky.

Certainly a PLC could be used to control an air handler, condensing unit, chiller, boiler, etc. And they have been used for this purpose before and currently ... in a limited fashion.

By limited I mean that PLC's have not often been used for complete control of a whole airhandler (for instance) and all it's various functions and modes. ie Warmup, occupied, unoccupied, nightsetback, economizer control, smart enthalpy copare and mode selection, heat recovery control, PID loop controls, etc. Same goes with other HVAC equipment.

More often, PLC's have been used for a single specialized function control or simple on/off control of a unit.

In the first case, above, an example would be a PLC one manufacturer I am familiar with who set up and included a small PLC which came included with their energy recovery unit. The PLC did one thing. It monitored entering and leaving conditions at the heat wheel, and the heat wheel's speed; and modulated that speed.

An example of the second case, above, is a factory I am familiar with. Where the in-house controls guys simply broke the various unit control circuits and installed their relay which was controlled by a PLC. So they could essentially just turn a unit on or off based upon a schedule in the PLC or by operator override of the schedule. But otherwise, the various HVAC gear when it was running utilized it's own built in controls. Most commonly, to achieve this one just finds the output of the unit's control voltage transformer and interrupt it with a relay. In the case of this factory, the in-house guys also added some basic instrumentation (sensors) to the units so they could see some basic info. ie Current draw on main motors, discharge air temp, space temp, that sort of thing. They kept it minimal, just enough so they could see at a glance if the unit was running and maintaining setpoints.

BUT ... once sarted the units utilized their normal, as installed, controls.

The reason for this is that while they understood digital controls, ie PLCs, they also knew they didn't know much about the technology and science of modern heating and air conditioning. Which can be every bit as involved and technical as industrial process control of a factory production machine, chemical distillation tower, etc. It's a technology and science guys who specialize in it spend years learning. Try to directly control a HVAC condensing unit without knowing and understanding it, and if you're lucky ... you'll just not be as cool as you'd like to be. If unlucky ... you'll be paying a HVAC guy to rebuild or replace the unit. Try to control water levels and firing rates of a steam boiler without some very technical knowledge specific to such things ... and yah could be ducking as things go BOOOOOOMMMM ... or at the very least the boiler will need replacing after it performs a meltdown.

Just injecting a few words of caution here. Being a controls guy does NOT mean you also know anything about the what's and why's and how's of what all those gadgets on you're existing HVAC systems are doing. Some don't even look like much, or that they're very important but they are. And some can fool you by their appearance. May look like it's just an on-off thingie when in reality it's a bit more complicated than that.

So most often, when I've seen industrial controls guys put their HVAC systems under control of their PLC(s), they've done mostly as I described in my example above.

Certainly a PLC could be programmed to perform far more of the control functions than simply turning something on or off and monitoring a few points. But the sequence of operations and multiple modes of modern air handler control, for instance, can be quite convoluted and complex. Not only in implementing the sequences, but doing it in a manner that achieves the desired result without stressing or damaging the equipment.

But I know of no canned routines or libraries of canned subroutines for the various HVAC functions for PLCs. And creating such from scratch can be a pain and time consuming. Not to mention risky if one tries to control too much with the PLC.

In any event, even if you should just elect to implement a simple start/stop (enable/disable) control scheme you might be interested in learning more about the control theory and the what, why, and how as applied to HVAC control and energy management of buildings. A good start is here:

Honeywell Engineering Manual of Automatic Control for Commercial Buildings

Can be downloaded from here:
http://customer.honeywell.com/Honeyw...D=77-E1100.pdf

It's a sizeable manual and a long read. But introduces the basic concepts of system control. DOES NOT cover details of individual operational and component control in any depth.

ie There are reasons that short cycling a refrigerant compressor is bad. Hot gas bypass can be your friend or your enemy. Delay-on-make, or delay-on break, or both? The benefits and penalties of a humidity control cycle. So on and so forth. There is a LOT of details not covered by this manual. And the details would take a whole library of manuals to explain.

But it's a start. And enough for you to implement a SIMPLE control scheme while gaining an understanding of some of the basics of what some of those built-in controls and components are doing and why.

Good luck. Have fun.

[mandrake]

In my opinion who gets sold a plc to do control in havc is getting screwed, reason is how many hvac guys know how to program one up? let alone support it when the origional installer has long gone.

as it is, we got asked to pull out some plc's out of 3 chillers, these sat above the bmcs system, anyway the owner thought it was a risk having these things onsite because they apear to want to get rid of the guy who done the work 10 years ago.... hence there goes the expertise in these things

anyway, i suggested they go to trane in this example, and get them to put a new controller in the chiller. we would replace any other plc doing anything else onsite with a hvac brand name controller that can be supported forever...at least while the tecnology is current anyway

NOW everything on site can be looked after by experienced HVAC people.


also got a job with a telemacenique plc that does the control on a pac and its the time clock...we cant do anything with it...so when they ask us to change setpoint or time program we have to either rip it out or get the origional installer...fat chance...we maintain it and have to get some other guy out there just to change the time shedule

i can think of a few other examples as well... we usually rip the crap out because its unservicable....


PLC's belong in factories, in machinery, manufacturing plants etc .....NOT in HVAC control





[Edited by mandrake on 05-18-2006 at 06:30 AM]

[sparky666]

Thanks for the reply and great link osiyo.

Sorry I didnt elaborate on what I was trying to do. I wouldnt think of touching the internal controls of HVAC as I know nothing about them.

All I am looking into is getting rid of the various programmable thermostats and controlling the units via a PLC via a SCADA package.

Cheers

P.S. anyone suggest where to find a good stainless steel duct RTD?


[Edited by sparky666 on 05-18-2006 at 06:58 AM]

[sysint]

What type of equipment?

There are many Application Specific Controllers that will handle the task very inexpensively and are very easy to setup with drop down menus. You won't lose your distributed control of the units.

Then you can have your OPC server or an inexpensive web server get all the information for you. More useful than just supply and space temperature. So, if using a package like Wonderware you aren't reinventing the wheel.

If it's a more complicated piece of equipment it's likely you can get various protocol interface to it without slapping sensors over the top.

[sparky666]

"What type of equipment?"

I dont even know yet, It is a small 1 floor commercial building. I'm coming up with a proposal to a potential costomer to control other processes and the HVAC control is just an added bonus. So the PLC with extra I/O will already be there.

[sysint]

ASC's are cheap, reliable and designed to do what you want in a distributed control environment.

If you do it the way you are suggesting you may (will) have issues you don't want to deal with.

I could have four monkeys and a couple of squirrels available, but I wouldn't want them running my HVAC either. Or, my ASC's running a process control scheme.....

[crab master]

"P.S. anyone suggest where to find a good stainless steel duct RTD?"

http://www.veris.com/product.asp?idM...&idProduct=137

Good luck with your endeavor - HVAC control is much more complicated than most people realize. I've had engineers write programs that would have killed most equipment in a short period of time if I followed their schemes. Sysint has an excellent point and most ASC's if they've been around any period of time have been proven and won't allow you to do things that shouldn't be done.

[rad74ss]

You may want to pick a controller and try to go to school on it.

As an example a Johnson Controls DX-9100. You will have to learn the capabilities and limitations of the control, i.e. how much memory, how much I/O and what accessories can you use with it (like displays).

I have been programming HVAC controls under the watchful eye of a 25 year veteran in controls and the absolute key to making them work is making sure the guy in the field can get to the adjustment points but not the critical points. You will want to give them setpoint, propband, etc., but not the timing controls on a sequencer. In that example the timing controls limit number of starts, minimum on/off time and should not be adjusted in the field. They are set to protect that model of compressor.

Plain English displays are worth their weight in gold as well. Pressing the K button five times is much harder than going to the display and keying down to 'Setpoint' and adjusting it there.

You also have to make sure that the controller is not fighting mechanical controls. You will have problems if you have a mechanical low pressure switch and are also trying to pumpdown on suction pressure. If someone adjusts the mechanical valve you can cause the control to do unforseen things.

An example of a standard unit where I work is to display temperatures, suction and discharge pressure, oil pressure, all the inputs and outputs, failures, etc. We also control minimum on/off, time delay between stages, pumpdown, positive start, one time pumpdown, fan staging, loading/unloading, automatic standby pump changeover, compressor run time and number of starts, and many other items depending on the application. It all depends on how well you know the system and how the components inter-react with each other and what components work with your controller. There are also things like scaling inputs and outputs, connecting to Building Management Systems with LON/BACNet/N2/N2open/ModBus, that it would take years to explain.

You may want to pick a controller and find someone who works with that product to mentor you. I started PLC's when I was 30 and I would have been dead in the water without the guy who is teaching me. You can control HVAC with PLC's if you know exactly how the machine works and exactly what components are necessary for the controller to make it operate.

[codewriter]

9 times out of 10 the guy in the office should not be "locking" any setting out, the final commissioning/startup guy should be doing that. I dont know of many "office desk programmers" who can know exactly how things will perform in the real world.

[rad74ss]

The "desk programmer" should lock out things that are not supposed to be adjustable in the field, especially if the programming team has decades of experience in the HVAC field. On complicated chillers many things are intricately tied together. Unless the field technician or operator understands how everything is tied together, manipulating certain points can damage or destroy the equipment. If a compressor is designed for no more than six starts per hour then that point should not be field adjustable.

In many cases the equipment operator does not know how to program a PLC therefore they do not need to have access to the PID or AI/AO scaling. They need adjustment parameters like setpoints, propbands, and cut in/cut out settings. If they have problems tuning the unit with those then that is what service support is for.

Personally I have worked my way up from electrician, through electrical engineering, and into programming. I can design and build the equipment and I know how to tune a loop. That is why I make it easy for operators to get to the necessary points, and why I prevent them from getting to sensitive points without factory assistance.

I agree that some programmers don't know how to make a unit run, but some operators don't know how to make a PLC run the unit correctly either. If I knew the capabilities of everyone who was ever going to lay hands on the unit then I could determine their level of knowledge and open points accordingly. Until that time I have to protect my equipment and be ready to offer service advice when necessary.

[sysint]

I get the feeling it's simply SA temp and space temp overlaying what's already there. Problem with that is that it's not distributed, and with that cost associated with it you probably could have ASC's and get better information.

[codewriter]

Not to many "desk programmers" have decades of experience in the HVAC field. Speaking from my area atleast.

Absolutly the field guy needs access to PID, the guy sitting behind a desk is not going to be able to perfectly tune a loop for a peice of equipment that is not even installed yet. What one may think is going to happen on paper is not always what is fact in the real situation.