HVAC Building Construction Question

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

 

[ChasBean1]

Very interesting dilemma! How far under were you in size? Is it a fan system? Can an undersize account for system diversity? Is it a VAV system? I have many questions, but if the questions have already been reviewed and you're sure the submitted equipment won't work, come clean now and say you made a mistake! Don't wait until it's too late and then blame the balancer! Hopefully the contractor hasn't yet ordered the equipment without the engineer's submittal approval anyway. Ask a more senior person on your staff (or your architect, if you work under one for this project) about Errors and Omissions. I don't know if it will apply in your circumstance, however. Good luck. FESS UP! With a name like Christ, you gotta be a pretty honest guy! Feel free to re-post any additional sizing details/info. -Chas

 

[TBP]

I doubt that a rookie would be given a critical aspect of a given project. If he was, then I'd drop the blame on management of the engineering firm. However, that doesn't help YOU very much.

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.

 

ChasBean1 and TBP, Thanks for your reply. Actually it is a pump system (close loop), already considered diversity, and have variable frequency drives. When I was calculating the pressure drop of the pipes, I only considered the distance of pipes from pump discharge up to the furthest or longest point, but didn't consider the return loop. So right now the pump head is short of half of the pressure drop (about 30 ft at least). So I would say this would be a pretty serious problems since the HP and propeller size have to be bumped up quite a lot. And the worst thing is that, there were 10 pumps like this. Please help me on this.... I really really appreciate that.

Thanks,
Tom

 

[quark]

Tom Christ,

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.

 

Quark, I actually also did think about this. But how about for example, this is a preheat hot water system, the hot water was suction from the heat exchanger and discharge to about 70ft height building, then return to the heat exchanger? When I pump the water up, I didn't consider the pressure drop due to heigth differences, so will the returning by gravity have any compensation on this? Please advise.

thanks,

 

[TBP]

Elevation changes in a closed loop are a net zero. All you need to consider is the equivalent length of piping. The best description I've heard is a ferris wheel. The ride operator balances the people on the wheel, so the weight going up is pretty much the same as the weight going down. Closed loop liquid piping is exactly the same.

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.

 

For the preheat hot water system, right now I have about 1500 ft equivalent length, with the maximum pressure drop of 4ft/100ft (but of course most of them won¡¦t be as high). And the one way (about half the length) pressure drop that I calculated was approximately 30ft.PD. Then I included 12ft PD for the AHU heating coil, 10ft PD for the air separator, 5ft. PD for the convertor (steam to water), and apply 10% safety factor. The pump that I selected right now had the head of 70ft, but I suspect that it need about 90 to 100ft. Please advise.

 

[TBP]

The quickie calculation for req'd circ head is the longest piping run times 0.06, so 1,500 X 0.06 = 90 ft of head. So you need a pump with that kind of head just to cover the piping losses. Then there's the coils and heat exchanger.

How much heat do you have deliver? GPM? What are the supply and return temps?

 

[butelja]

The statement that height changes can be ignored in a closed loop is valid ONLY IF YOU CAN FILL THE LOOP. When starting with an empty pipe, your pump must have enough head to push the fluid "over the hill". Once the pipe is filled and the stystem reaches steady state, then the elevation change has no effect. The pump with 70 ft TDH trying to pump 70 ft up has no extra margin whatsoever.

 

[quark]

Tom Christ,
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.

 

[quark]

Tom Christ,
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.

 

[TBP]

butelja - the closed loop system fill arrangement is typically off the domestic water system, through a PRV. Loops like this MUST be full to deliver the heating and/or cooling. Once the loop is filled, and the dissolved air removed from the water, it should require very little, if any, make-up water. All the circulators need to do is just move the water around the loop. Also, using high-head pumps in service like this can actually erode the loop piping, not to mention generating noise complaints from people in offices and apartments.

 

[butelja]

TBP,

Fair enough. My background is more process industry related, where noise issues are less of a concern.

 

Thanks for the comments from you guys. I really appreciate that. Actually, let me make it clear. The total flow rate needed for my system (preheat hot water, used in preheat coil of AHU only) is approximately 780gpm. I have two pumps with 75% redundancy each in parallel, so I selected each pump with about 590gpm. I have VFD in these pumps. I found that the pressure drop for preheat coil in my longest run is about 5ft pressure drop, and I have about 1700ft run (including equivalent lengths) with about 3 to 4 ft pd per 100ft. I assume that the static head requirement that quark mentioned is the highest point that I wish to reach, and it is about 70ft height building. And actually right now I am calculating it based on old pipes, but actually this building is a new one, so this will further reduce the pressure drop (I think). I would say that most of the time it won't reach the full load, so would it help? and since the pumps are in parallel, if there are not operating in full load, can they support higher head(I doubt it, unless it's in series connection). Please advise. Thanks a million.

 

[Goorah]

I've been in HVAC 26 years and what I love about it is that I still learn things everyday!... keeps life interesting. I also make honest mistakes ... everyday. As someone else on this forum said: "If engineers (especially rookies) were fired for making mistakes, there'd be few "experienced" engineers around!" THAT'S WHAT "experience" is! Each "mistake" is part of your education. So what do you do?: Fess up, get back on the horse and go out and make some more "mistakes"... that much the wiser.

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"!!

 

[quark]

Tom Christ,
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

 

[Rentapen]

Tom,
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

 

Thanks for you opinion since I posted the question. I really appreciate that. I think right now most of the pumps should be okie, just one set of pump might be a little low head capacity. Hope that I can find a way out sooner. Thanks.

 

[csr]

Hi--

What a brotherly advise from Goorah. I like it, sensible-- experience is learning anyway but not much than learning is experience.
csr