# Thread: How to Calculate BTU's on CHW System

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## How to Calculate BTU's on CHW System

Whats a good way to calculate BTU'S on a chilled water system.

2. Originally Posted by stopher9
Whats a good way to calculate BTU'S on a chilled water system.
Tons= (GPM*delta T)/24

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## BTU/hr Calculation

You will need the mass flow rate and temperature differential.
The formula is Q(BTU/hr) = (Mdot)(Cp)(delta-T).
Mdot can be solved for if you gave a GPM value by assuming that the treated chilled water weighs about 8.33 lbm/gal. Converting gallons to mass and coverting the time to hours yields a constant of 499. The formula would then reduce to:

Q(Btu/hr) = (GPM)(499)(delta-T)

Also as chillerfreak pointed out:

Q(tons) = [(GPM)(delta-T)]/24

or

Q(tons) = Q(BTU/hr)/12000

Units analysis:

Q(BTU/hr) = (Mdot)(Cp)(delta-T)
BTU/hr = {(Gal/min)(60min/hr)(8.33lbm/gal)}{1 BTU/(lbm-degF)}{degF}

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## Or, if you're not really good in math,

get one of these;

http://www.onicon.com/System10.shtml

I have 3 tied into my ems system and bill the 3 users for their BTU's

5. Originally Posted by jdd67
You will need the mass flow rate and temperature differential.
The formula is Q(BTU/hr) = (Mdot)(Cp)(delta-T).
Mdot can be solved for if you gave a GPM value by assuming that the treated chilled water weighs about 8.33 lbm/gal. Converting gallons to mass and coverting the time to hours yields a constant of 499. The formula would then reduce to:

Q(Btu/hr) = (GPM)(499)(delta-T)

Also as chillerfreak pointed out:

Q(tons) = [(GPM)(delta-T)]/24

or

Q(tons) = Q(BTU/hr)/12000

Units analysis:

Q(BTU/hr) = (Mdot)(Cp)(delta-T)
BTU/hr = {(Gal/min)(60min/hr)(8.33lbm/gal)}{1 BTU/(lbm-degF)}{degF}
JDD67....I would have gone on past the 8th. grade if I knew I was going to have to know all this.

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## Cqalculation

I have a chilled water system that has the following information:

285 GPM
Supply Temp - 45 Deg F
Return Temp - 48 Deg F

What is my BTU's per hour for this system??

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BTU/HR = 285 x 499 x 3 = 426,645

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JDD67-

Wouldn't it be more correct to say Qdot(BTU/hr) = (Mdot)(Cp)(delta-T)?

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The formula is fairly simple BTU/Hr = FlowRate(in USGPM)*500(Weight of Water Constant) * Delta T.

The accuracy of the data is another thing.

1. You need a highly accurate flow meter, installed with the correct number of up stream & down stream straight runs of pipe. Personal experience, I like Mag meters because of the low up/downstream pipe runs and they have a much better turn down ratio than most other meters.

2. Matched(meaning calibrated and read the same value at the same time in the same medium) temperature probes. This is very important because it can make a large difference in the data. Just for an example if you use standard nickel 1k ohm transducers -50 Deg F to 250 Deg F range +/- 0.1 % accuracy.....the range of error is +/- 3 Deg F. So if you have two sensors for your CHWS & R that are at the opposite ends of the +/- you could have a 6 Deg F error!!

2. Matched(meaning calibrated and read the same value at the same time in the same medium) temperature probes. This is very important because it can make a large difference in the data. Just for an example if you use standard nickel 1k ohm transducers -50 Deg F to 250 Deg F range +/- 0.1 % accuracy.....the range of error is +/- 3 Deg F. So if you have two sensors for your CHWS & R that are at the opposite ends of the +/- you could have a 6 Deg F error!!
Good point.
When I commission a CHW or HW system, I always make sure to run the pump(s) for a while (longer the better) to see if my inlet and outlet sensors are close to each other. More than a half degree difference and I'm looking for the cause.

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You dont have to have a flow meter to determine GPM. What you must have is the factory startup sheets that show design GPM and FTWG. Here is the equation if you have these variables.

GPM= dGPMx[ square root ( mFTWG/dFTWG)]

dGPM is design GPM
mFTWG is measured feet of head
dFTWG is design feet of head

To find the feet of head, measure preasure drop across cooler barrel in PSI and convert to feet of head.

example: 4PSI to feet of head is 4x2.31 or 8.4 feet of head. Multiply pressure drop by 2.31 to get feet of head.

Just for an example if you use standard nickel 1k ohm transducers -50 Deg F to 250 Deg F range +/- 0.1 % accuracy.....the range of error is +/- 3 Deg F. So if you have two sensors for your CHWS & R that are at the opposite ends of the +/- you could have a 6 Deg F error!!
If the accuracy is 0.1 % wouldn't the range of error be +/- 0.3 Deg F .... instead of 3 Deg F ?????

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Here is a low cost BTU meter non intrusive. Uses clamp on flow sensors. No hot tapping
or draining the system to pick up flow:

http://www.shenitech.com/STUF-300R1B.htm

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