HVAC Building Construction Question
HVAC Building Construction Question
(OP)
Hi,
I am an entry level engineer working in an engineering firm. Our 100% construction documents have been submitted and the building now is under construction. However, I found that some of my calculation in selecting some equipment was wrong, but a submittal of this equipment is now submitted for our review. May I know if I am going to admit that I did the calculation wrongly, and we actually need an equipment with a higher horsepower, will it involve some delay or addditional cost of construction? What is the normal way to deal with this kind of situation. Althougt we can ask them to resubmit with a new selection, but for sure the contractor is going to be mad at us, and complain with the owner also. What should I do?? I don't want to lose my job because of this.
Please adise...Thanks.
Tom
I am an entry level engineer working in an engineering firm. Our 100% construction documents have been submitted and the building now is under construction. However, I found that some of my calculation in selecting some equipment was wrong, but a submittal of this equipment is now submitted for our review. May I know if I am going to admit that I did the calculation wrongly, and we actually need an equipment with a higher horsepower, will it involve some delay or addditional cost of construction? What is the normal way to deal with this kind of situation. Althougt we can ask them to resubmit with a new selection, but for sure the contractor is going to be mad at us, and complain with the owner also. What should I do?? I don't want to lose my job because of this.
Please adise...Thanks.
Tom





RE: HVAC Building Construction Question
RE: HVAC Building Construction Question
You'll also find that most contractors are quite capable of making their own sets of mistakes. Especially on jobs where the lowest bid is the main focus.
If I was your boss, I'd want to know if you think you've made an error, so we could fix it. If every engineer that botched a calculation got fired, there wouldn't be very many left.
RE: HVAC Building Construction Question
Thanks,
Tom
RE: HVAC Building Construction Question
If you are pumping to a height and fluid return in pipe is by gravity you need not bother at all.Even the pressure drops which you have considered may even be less actually when you are operating system.
For example if you are pumping water to a height of 20ft and returning it by gravity by installing a buffer tank at the topmost point will reduce the static head requirement.Only you have to suuply the frictional loss due to velocity head.
RE: HVAC Building Construction Question
thanks,
RE: HVAC Building Construction Question
Are you using steam heat exchangers? If you are, that may be your salvation right there. Most steam equipment is so grossly oversized it almost defies desciption. You may very well be able to compensate for lower flow by running a higher delta-T on the loop water.
Why don't you post the system info? People here can check your numbers. You may not be in as much trouble as you think.
RE: HVAC Building Construction Question
RE: HVAC Building Construction Question
How much heat do you have deliver? GPM? What are the supply and return temps?
RE: HVAC Building Construction Question
RE: HVAC Building Construction Question
so far you have not commented anything about the static head requirement. Flow rate of hot water through AHU coil is varialbe flow type. If I am correct you are using hot water to maintain RH or for reheating of controlled air. The frictional losses including pressure drop across the equipment reduces by square as the flow reduces. As an estimate 70 ft seems to be quite ok if yours is not a high rise building. As I have just completed a project involving VFDs (for Air Conditioning system)I made a little bit good studying. If you can give me some details I can advise you on this system. Let me know.
RE: HVAC Building Construction Question
so far you have not commented anything about the static head requirement. Flow rate of hot water through AHU coil is varialbe flow type. If I am correct you are using hot water to maintain RH or for reheating of controlled air. The frictional losses including pressure drop across the equipment reduces by square as the flow reduces. As an estimate 70 ft seems to be quite ok if yours is not a high rise building. As I have just completed a project involving VFDs (for Air Conditioning system)I made a little bit good studying. If you can give me some details I can advise you on this system. Let me know.
In a closed loop (similar to yours)I still emphasize, you need not consider static head.
RE: HVAC Building Construction Question
RE: HVAC Building Construction Question
Fair enough. My background is more process industry related, where noise issues are less of a concern.
RE: HVAC Building Construction Question
RE: HVAC Building Construction Question
I forget the exact story; but Lee Iacoco as he was rising thru the ranks screwed up BIG TIME and cost GM MILLIONS of $$$$. His boss (I don't remember who) called him "on the carpet" and Lee was expecting to be fired!... and expressed as much. His boss in TOTAL PERPLEXITY asked him to the effect of: "Why would I fire you when I've just invested $30MILLION in your "education"??
So: Cheer up, Fess up, move on with more wisdom ... and if you do get "fired" ... you don't need that firm. Anyway ... the next firm will value your "experience"!!
RE: HVAC Building Construction Question
I don't find any problem in your system.
Ironically most of the times I found lot of pumping systems running without any trouble eventhough not designed to match the system for puping system is so flexible.
To justify my claims I would like to put few points.
1. When your head requirement is higher than what you have designed pump performance shifts towards left of the performance. i.e. head developing capacity increases and discharge capacity decreases. But then you have your standby pump.
Even though you may be running your pumps in low efficiency zone final power consumption will reduce.
2. As you rightly said with new piping frictional losses will get reduced.
3. At low flow conditions, once again I emphasize, frictional losses will reduce by great extent.(By square)
4. Apply proper logic for the operation of two pumps simultaneously at all load conditions and you will get best benifit of your system and credit to you.(If you are in US contact ITT Industries at least for knowledge sake, I bet you will be enlightened)
Further I don't know what your total experience is but one point worth suggesting you is as the bosses will have least interest in understanding technical matters there are lot of ways to juggle the things by putting them in hi-fi technical terms.
Still you have problem(rather psychological I say) then simply put a booster pump some where in the middle and raise the head.
Best of Luck
RE: HVAC Building Construction Question
You need more money, slight redesign, time. Lifes quest. Maybe you could stick a thrid pipe with one way valve/needle between the two parallel pumps. Add another "D" just before chiller tubes ( from my point of view the tubes need over 750 gpm to keep a required temp.) Buy 5 more larger pumps and stick in the new 5 "D". Tell your customer to get head, run all 3 per but after start switch to the one high gpm pump, save him money long range.
just wandering
rentapen
RE: HVAC Building Construction Question
RE: HVAC Building Construction Question
What a brotherly advise from Goorah. I like it, sensible-- experience is learning anyway but not much than learning is experience.
csr
RE: HVAC Building Construction Question
Good luck.