Sequence of Operations
Does anyone know a good resource for sequence of operations (AHUs, Pumps, VAVs, Central Chiller Plant with Primary/Variable Secondary distribution). I know there are endless possibilities for controlling equipement, but I want to standardize as much as possible and I would like to see what other people are doing to compare with what I have.
I'm new to this forum and pretty excited the wealth of knowledge I see here. I will try to contribute as much as possible, but the only controls system I am really well versed in is INET which is now a dinosaur. I am learning AX now and hopefully I will be able to contribute more in the future. Thanks, all.
Honeywell Gray Manual has a wealth of information related to controls, and can be found via Google as a free download. It's called Honeywell Engineering Manual of Automatic Control, or something like that.
You could always post your sequence and then get input on that.
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Thanks, those are all great resources. I think the Honeywell book will keep me busy for a while. Unfortunately, the Honeywell book knows better than to give a specification for a central plant with dissimilar chiller configuration, which is what I was looking for. It does tell you what to take into consideration though and that will help.
Here somthing they wont teach ya in a book....Back Up everything! and have a plan "B" - keep it simple, know the preformance of the (chillers/pumps/towers/etc) Eq, also design Alot of HOA on everything! including Valves!!!
Sorry seen to many over design - fancy dancy controllers, front end's, bla bla bla...and what happens when AND IT WILL crap out!
Or a palnt running at a delta 5 maybe - because Mr X programend A pump to run with B chiller...I could go on all day...common sense sometimes is better then A BOOK or the latest greatist software (wonderware)
Have fun in HVAC Controls
You should always keep in mind two of the comments that “phxurs” made. Keep It Simple and use Common Sense. Unfortunately a lot of design engineers today don’t believe in either of those concepts.
Oh yeah and above all...... Have fun with it.
Chuckle, I take it you've had some bad experiences?
Originally Posted by phxurs
Yes, everything and anything takes a dump sooner or later. But proper design, installation, commissioning, and MAINTENANCE can drastically reduce the problems and increase the MTBF.
Actually, most times I respond to a call back, the issue turns out to be mechanical, electrical, or the user changed something he or she should not have.
That said, I have seen some horrible DDC installations.
I'd agree, quite readily, that some get too ambitious and complicated for their own good. To no good purpose.
KISS is a good principle to live by.
And one should keep in mind reliability, usability, and maintainability of a system. Not to mention end costs.
i.e. One can design and install a system that will save N% of a total energy bill. Fairly simply and easily with minimal components (and thus points of possible failure) at a cost of X dollars.
But generally, if one were to decide that one wants to save N + 10% more, or N2%; one finds that you just about has to double the complexity of the circuitry and programming. Meaning you've also just about doubled the possibility of a failure occurring. And not infrequently the cost of the install has gone to 2X dollars.
Try for N2 + 10% more, same thing. Drastic increase in possible failure occurrence, drastic increase in program complexity (and thus the understanding of it), and drastic increase in installation costs.
At some point, it simply isn't worth it ... in many cases. Payback on investment goes from a reasonable 3 to 5 years, for instance, to 10 or even more years. And even that can be iffy when considering the costs of increased system maintenance costs and decreased reliability.
Sometimes the money is better spent, and overall reliability improved by employing KISS methods on the first item(s) and being happy with the original N% savings. Then spending future budget funds on the same savings goal and KISS principles on additional plant equipment. Or, perhaps, just replacing some of the aged stuff that is really inefficient with something newer.
Or spending some money on REAL mechanical maintenance, as is fixing what yah got so it works right.
I've seen some really weird and hodge-podge, cludged together control programs and SOO put together in an attempt to compensate for the inadequacies of a system that suffered mostly from poor mechanical maintenance/repair. Or in an attempt to make a piece of equipment perform beyond what it was designed to be able to do.
That said, I'll agree that a good system design should include adequate ability to cope with foul-ups and failures.
One problem we've experienced in the past is customers unwilling to pay the extra for additional HOA abilities, software manual overrides, etc. Keeping in mind, each such point of control is a cost we incur, in labor or materials, and we're not big on giving stuff away for free on low bidder gets the job projects.
Sometimes there isn't much yah can do. Customer isn't gonna spend one extra buck for such things. Often enough the person controlling the purse strings hasn't a clue how stuff really works, problems and issues faced by the owner's maintenance staff, etc. They're just interested in job getting done for lowest possible price.
We've customers like that. Not a lot, but some. And in after project completion training sessions I've taken it upon myself to show maintenance staff how they might accomplish such things on their own. If they're willing and motivated.
<Shrug> Some are, some aren't. Some are knowledgeable and good at what they do; others are basically filter changers and oilers/greasers of bearings.
Fortunately the majority of our larger and more savvy customers, if not initially smart about these things, have become increasingly so. They specify HOA on major, important equipment. Bypasses on VFDs. Manual control mode enable buttons on front end screens. Etc.
Chuckle, on a recent project, a new building, I even had the head of HVAC maintenance for a large organization we do DDC controls work for, an organization with many buildings and sites, approach me. He'd been looking at the spec'd SOO's for some equipment in the new building. And had come to the conclusion that the engineering firm the organization had hired to do the HVAC design, had gotten carried away. He asked me to simplify some of the sequences, flat eliminate some things, and in other cases asked for me to include control points in some programs that'd allow him to manually turn off some features, or override some setpoints that engineering firm had wanted to be fixed and not alterable.
And I did those things. For acceptance testing purposes, all operated as designers spec'd. Necessary for us to get paid without a lot of argument and debate. It's just that from the front end, head of HVAC maintenance had additional control points so he could turn off or alter certain things. (Requires his password to access) Things he knew from experience to be problematical and/or more trouble than they were worth.
Some of those things that he didn't know to be issues, I told him about. He and I have an understanding. I had put everything through their paces repeatedly during early testing. Had noted a number of issues with designers' original ideas of SOOs, and certain setpoints and deadbands that were too tight causing problems like excessive cycling, pushing equipment too close to limits of their abilities and thus causing trouble conditions and warnings.
i.e. IAW their specs my system was to, under certain conditions, to run down the firing rate of the boilers way too low. Trying to milk an extra couple percentage points of efficiency out of em. Manufacturer said they COULD be run at such firing rate. But strongly suggested that it not be often nor regularly done, nor done for long. Manufacturer's new installation commissioning guy, a man I've known for some time and who is very good at what he does, told me personally that running em down as low as the specs called for drastically reduced MTBF. And he suggested a different number as a low fire point.
So I included "buttons" on the front end screen that allowed customer's HVAC maintenance head to alter that low point setting to what the manufacturer's rep suggested, and to alter the sequence of how the standby boiler came online to supplement the lead boiler when needed. Program in the controller for this was suitably modified to allow this. Head of HVAC maintenance also did not like auto-switchover of lead and backup on pumps or boilers, etc. Which was called for in the spec'd SOO. So I included the ability for him to turn off that automatic action, and instead have the system just alert him that a switchover based on time was called for, but it wouldn't actually do it until he clicked on the "Switch" button. That way there is no switch until he is ready, watching, and has guys somewhere on the job and available to go to that building if there is a problem.
Etc and so forth. A good man, a lot of experience. And while we have our differences of opinion, they're HIS buildings that he's responsible for and he knows what problems he's had in the past and is likely to face in the future.
The cost of the extras he wants? We bite the bullet on. But not too badly. As his organization does not demand the lowest dollar bid. They give us credit for past satisfactory performance, etc.
Just some thoughts. Probably not even worth the time to read.