Thread: natural gas usage estimate

1. Regular Guest
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Nov 2004
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I have a stack heater that has a burner set for:

250,000 btu/hr
natural gas supply pressure = 7"w.c.
manifold supply pressure = 3"w.c.
exhaust gas temperature = 900 C

I am trying to estimate/calculate the amount of natural gas being consumed CFM??? The air CFM is 62. The temperature controller measures the exhaust gas temperature and controls (open/close) the natural gas solenoid valve to maintain the exhaust gas temperature to around 900 C. Any idea how to estimate/calculate the natural gas usage?? other than buying a meter and read the values on a typical 24hrs. basis...

2. Professional Member
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I would reccomend contacting your gas utility and request the BTU heat content of the supplied fuel. I typicly us a value of 1000 BTU/Cubic Foot on natural gas. I have seen actual values as high as 1100. As far as figuring consumption that depends on outdoor conditions and space temp. There are software programs out there that can calculate btu requirements over time based on outdoor ambiant conditions. Maybee an hour meeter wired up on the gas valve? I hope this helps

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If there are 1000 btu per cubic foot of natural gas, and my burner setting is for a maximum off 250,000 btu/hr, then
Is This Correct...

250,000 btu/hr / 1000 btu/cubicft / 1 hr/60min

= 4.1 CFM or 250 CFH

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that would be correct. The gas company will give you a correction factor on delivery, although this isn't much of an issue here. -edit acutally I shouldn't say that because it is temperature compensated.

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You can also clock the gas meter with this unit only running,be it a 1/2 ft,1 ft,2 ft,5 ft or 10 ft dial.Clock how long it takes for one complete revolution and that will tell you exactly at what rate units firing at.Of course a reading over the max would be overfired and I think you want to be within 10%.
Your local gas utility company should have those cards available for you.There exists a hand held HVAC pocket calculator,I have one and it comes in very handy for clocking gas input.

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Test hand value 1/2--2/1 -5/1--10/1 Pick one-look on meter dial Times number of divisions meter test hand moves in one minute times - Most test hands have ten divisions so check to make sure.times btu value

Sample 1/2 X 82/10 X 60/1 = 246 X BTU value
1020 X 250,920 btuh

Here we use 1020 Check your gas bill it may show value.
With a input that big it maybe better to clock more than once to be sure you didn't go
+eyed. When we do clocking we normally go 2 minutes. Some meters are not as accurate in less than 2 minutes for some reason. The formula doesn't change. Your bill should also show what they charge per therm if you are billed in therms. We adjust to +- 5%
Hope I didn't make it clear as mud.

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