Calculating gpm from a Pressure Diff reading?

[jordi29]

Hello all,
First time poster here.
I have a CW AHU (55Tons Nominal) and I am trying to figure out how many actual tons of cooling I am currently getting out of it. For this, I need to know the gpm's going through the coil. Since I don't have a flow meter, I believe there is a way to calculate gpm, if I can read a pressure drop (diff) from a specific location on the unit's CWS and CWR lines.

Can anyone offer any assiatance on this? I am basically trying to figure out if the unit is starving for air or starving for water. Here are the current conditions on the unit:
- CWS Temp: 42'F
- CWR Temp: 56'F
- Supply Air Temp: 55.5'F
- Return Air Temp: 70.5'F
- CW Valve Open: 100%
- Supply Air Setpoint: 55'F (this is min value allowed)

Thanks in advance!
jordi29.

 

[hepa99]

can you get air flow rate from your main duct?

you can compute for the actual cooling load capacity (not 55ton) using your air side information (SA temp, RA temp or mixing temp, air volume at main duct) then get the GPM using your water side info and computed cooling capacity from air side information.

 

[MintJulep]

You've got a half-degree approach temperature.

More air isn't going to improve things.

Not enough water.

The coil manufacturer should be able to get you a flow vs. pressure curve for the coil.

 

[hepa99]

mintjulep,
no need to call the coil manufacturer to get coil water flow rate.

if he/she is engineer, he can compute for coil water flow rate.

.002

 

[MintJulep]

Hepa,

You're correct that you can do a heat balance.

However, the water side of the coils may be fouled, or there might be a chunk of debris obstructing flow.

Heat balance won't tell you that. Manufacturer's curves will.

 

[hepa99]

jordi29 question is finding out actual tonnage and water flow rate.

 

[MintJulep]

But is that the right question to ask?

 

[hepa99]

let us ask the thread starter then......

 

[jordi29]

Thanks y'all.

My ultimate goal is getting the actual tonnage being delivered by this unit. However, I believe there is a formula where, if I have the gpm, I can get the tonnage. Can't remember the formula, it was something like:
BTU = (gpm)(500)(dT), where dT is the Delta of the water.
Not sure if this formula is correct.

Eaasy with the replies fellows, I just recently became an EIT... ;-) But I love this new-found forum!!

Thanks again.

 

[MintJulep]

Well, then hepa99 is correct. You can calculate the delivered capacity from the air side data as well. You'll need cfm, but that is relatively easy to get from a pitot traverse.

As you note, you can also do this from water side data.

If you can't get the pressure vs. flow characteristic from the coil manufacturer perhaps there is some other device - a valve, orifice plate or whatever in the circuit with a know pressure vs. flow characteristic that you could use.

Or you could rent a non-contact flow meter.

http://www.instrumentsdirect.com/f-...eTaxiNaN0Te34Pa38Ta38Nbhn0?sc=1&category=-101

Or hire a local T&B contractor who probably has one.

 

[hepa99]

jordi29,

Q=(p/V) x Cp x AT

Q=cooling load capacity
p=density of water
V=water flow rate
Cp=Specific heat of water
AT=difference of entering-leaving water temp.

try to familiarize with this because you will need it when your dealing with HVAC.

if you can give me the total air flow rate (cfm) of ahu, i can help you to get actual tonnage & water flow rate.

 

[jordi29]

heppa99,
Thanks for your help!
The cfm ratings from the manufacturer are 28,000 - 30,000 cfm. Though in-house we usually assume 30,000cfm.

I will check to see if I can get my hands on a non-contact flow meter... I think that can come in real handy at various sites we have where we don't have flow meters for individual AHUs. Any info on their accuracy (+/-%)?

Thanks again to all!

 

[DRWeig]

Jordi,

Ultrasonic time-transit flowmeters and magnetic types are among the most accurate flow devices out there.

±1.0% at higher flow rates is typical.

Good on ya,

Goober Dave

 

[hepa99]

jordie,
if you want to get the ACTUAL tonnage, you need to get the ACTUAL cfm too not using the manufacturing cfm rating.

get the size of the duct and get the air velocity so that you can get the air flow rate thru the main duct.

 

[mauricestoker]

For the system you have, there's a 14*F split for the chilled water, and supply air is within a half degree. Seems like the system is running pretty well, in the absence of flow data. The return air seems to be reasonable, not knowing internal load and OA temp/RH. How much outside air you are using is a key factor. You can save a lot of money if you don't use outside air, but I would not recomend it (an IAQ no-no).

If you want to see what tonnage is being delivered, you'll need both hydronic and air side. That is no different than any other heat exhanger-calculating one side only doesn't do much good.

If you don't have a flow meter, then a low tech approach might be to get a 55-gallon drum, stop watch, and hose to fit up to the coil drain. That won't provide velocity, but the dP for supply and return will indicate whether you have plating/clog at the coil. An IR thermometer would probably tell you as much (or contact thermometer).

For the air side, you'll need to have OA flow and temperature (and preferably RH) as well as return. Space or return RH would also help. Hard to overlook condensate when considering a cooling coil. Is it humid there? Does much condensate go through the air gap? With that, you can use the equations from the ASHRAE Fundamentals Chapter 6. Even easier, go to the Trane web site and use the mixed air psychometrics tool. No need to memorize equations if the program is free on line.

 

[BM4BRANES]

You can easily get pd readings at a coil's circut setter, and then calculate a fairly accurate flow rate with the manufatcurer's info, they also make a handy wheel calculator that should be availible thru your local B&G distributor. From there, you can determine your water side tonnage

Go to :

http://www.bellgossett.com/literature/files/22.pdf:

 

[kstum]

Another method, if your circuit setter is covered with insulation and you don't have the setter curves is to use the coil itself, assuming you have P/T plugs on each side of the coil. It's not quite as accurate, but could be adequate. Take the submitted cut sheet (or if nothing else, the equipment schedule on the drawings) and look at the coil water pressure drop and the coincident gpm. Then use the following equation, which is a slight modification of the classic pump laws (the pump laws are for straight pipe, so the exponent has been modified to better fit coils and chiller bundles).

gpm = scheduled gpm x (coil pressure drop / scheduled pressure drop)^0.53

 

[jhayvee]

i am not an engineer but my job is a sales man of filters for AHU's, i encountered many queries regarding air change. can anyone help me out how to calculate the air change and why is the design of AHU is different when you get the actual reading? what are the factors to consider in the differences?

 

[engair]

jordi, I didnt see where anyone said it, but your formula you wrote earlier is correct. BTU=gpm x 500 x deltaTwater. As a manufacturer of AHU's we use it all the time to check the engineers gpm vs. BTU's