Steam per cubic ft of natural gas

[bethw]

We have firetube boilers here and crunching the numbers, I arrive at 10lbs steam produced per ft3 of gas. Is this number in the ballpark based on other peoples experience? I'm using this to emphisize effeciency numbers on each boiler. We are running in the low 80's.

 

[DRWeig]

Hi bethw,

At 80 PSIG, I get 900 BTU/lb, so 9000 btu total for only one ft3 of natural gas -- which is about 1030 btu/ft3. Are you sure one of your numbers isn't off by a decimal place?

I could believe 10 lbs for 10 cubic feet (just shy of 90% overall efficiency)...

Somebody catch me if I've goofed!

Good on ya,

Goober Dave

 

[bethw]

Our boiler pressure is about 90lbs, being firetubes, its saturated of course. Using the gas meter outside, the one we're billed on, and the steam totals from our individual boilers, I come up with these numbers. Everything seems to jive. I'll check again though, the steam numbers may be the culprit if it's off by a factor of 10.

 

[TBP]

Low 80s is a pretty good, real world, boiler eff.

How are you metering the steam? Do you have pressure compensation on the steam meter?

 

[rmw]

Especially for a fire tube boiler.

rmw

 

[bethw]

The steam flow sensors are Spirax Sarco coupled with Foxboro readouts. I have extensive documentation on thier calibration, serial numbers correction factors. We have three identical and each reads very close to what the other reads under identical boiler conditions. They are temp and pressure corrected, there is a temp probe next to each flow element. These boilers are still new, and effeciencies of 82-3 are common when on natural gas. 85 when on #2 oil. Here's an example of the numbers I got this morning: Steam produced 24 hrs: 618,018lbs. this number is very close to our hourly output X24. Gas used: 59,496 now this is where we get confused. If this is in CuFt, then it works out to 10.4 lbs/ft3. If this is 594,960 it's 1.04 lbs/ft3. Are we using 59,496 or 594,960 ft3 of gas in 24 hrs? I think we have a zeros problem.

 

[DRWeig]

Hi Beth,

I agree that there's a zero problem. The 1030 BTU per cubic foot that I quoted earlier is a standard that's been around for a long time...

A quick check would be how much you pay for that gas per cubic foot -- if your rate is about $5.00 per million BTU (kinda average), that translates to $0.00515 per cubic foot. If you have access to the bill, see if it works out with the 59,000 number or the 590,000 number...

Good on ya,

Goober Dave

 

[zdas04]

My steam tables say that you need to add 886 BTU/lbm to change water at 105 psia to steam. Natural gas is somewhere around 1,000 BTU/ft^3. If you made 618,018 lbm of steam then 59 MCF/day just ain't reasonable. I've seen a bunch of gas bills that reported numbers in hundreds of MBTU's. Your problem has to be a slipped decimal. Bottom line is that if you can boil 10 lbm of water with one ft^3 of natural gas then your gas is magical.

David

 

[bethw]

Thanks to all. Yes it did begin to sound like it was magical. I can easily add the zero and get a much more realistic number in the 1.04ish range. Beth

 

[Compositepro]

Ah! The infamous Murphy fudge factor reveals it usefulness again.

It sounds like you are reading the gas meter yourself. Make sure you know what the counter on it is actually counting. It might be tens or hundreds of cubic feet. It often is not clear from the meter.

 

[rmw]

Some meters have the last digit as the dial reading, not the numbers on the counter. Did you take that into account? That would account for a missed digit.

rmw

 

[danw2]