Clocking a gas meter
Have a project with a 12.5ton heat/cool RTU (natural gas 2psig inlet pressure). Unit is running fine, 4900 cfm air delivery. Having an issue with not heating the space fast enough so we verified the unit capacity by clocking the gas meter. We are using/burning 200,000 btuh when the unit is rated at 250,000 btuh (input).
Inlet gas pressure is fine, manifold GP is at 3.5"wc. Called gas company to verify btu content of ft3 gas and that is ok @ 1000. Verified burner orfices and those too are OK.
What next? is possibly the gas meter giving us a false reading? is it somethign with the unit. I am sure we can raise the manifold pressure but that is not according to the manufactures data. Any suggestions. ???
Here's what little I know about it and a few questions. Whats the temperature rise? Whats the OSA temp? Whats design temp? If it is very cold weather you may not be getting enough gas simply due to demand and temperature. I doubt you have a demand/temp compensated meter. I doubt the firing rate is your problem. Ken
200mbh should equate to about a 38 degree temp rise, are you sure you have the right gas pressure ? If you have the right gas pressure and orifaces you are getting the gas, does the unit call for 3.5 inches ?
You do need to know what your temp rise because something aint right, clocking the meter is inaccurate at best, if you have 3.5" and the unit calls for 3.5 " and you have the right orifices, you are getting the gas flow, I would look at the temp rise thats what tells the story, just measure the air in and out, you are working with heat so you only need to know the dry bulb temps, right now you are saying you have a 38 degree rise when you should have a 47 degree rise, soemthing tells me you either have more outside air than you think or the unit is moving more air than you think it is, like 6100 cfm instead of 4900,
Last edited by PrestonPierce; 02-05-2007 at 07:56 PM.
What was the correction factor used to determine the btu useage?
Did you confirm the meter inlet pressure at 2 psi?
Did you use a u tube manometer and not a guage? I have had many bad guages.
I would not set the gas pressure above 3.5"
Is it a new unit? Has it ever heat the room fast enough? Maybe it is undersized? Is your economizer set-up properly? What is your heat rise?
Believe none of what you hear and half of what you see.
is there anything else on the meter? 200k may be what you are using but maybe not all of it is going to this unit. i am sure that you checked it, but i thought that i would put that out there. the unit is rated at 250k but at what gas btu content? i have seen some oems use 1060, if you have 1000, then your actual btuh input would be about 236k. not a lot different but something else to think about. what about elevation? units are typically derated for higher elevations.
if you are certain of the cfm, i would check the temp rise like preston said.
you might also double check the orifices...for example: a size 43 orifice (0.0890") that is dropped to a size 45 orifice (0.0820") @ 3.5" W.C. due to fouling, corrosion, etc would cut the gas flow by 16%.
maybe it is a whole bunch of little things. start with one thing and eliminate them one at a time.
Have you ever noticed that geese walk around like they are drunk and trying not to get a public intox ticket?
Do not increase manifold gas pressure from manufacturers specs, check your meter clocking results again and verify them accurate. Size and count orfices again, seems like that's where your problem is.
The problem is that it is a 2 PSIG system. There is typically a factor to multiply when dealing with a system over 4 ounces. Download this book: http://www.questargas.com/brochures/59060_401.pdf and you can find out more starting on page 78 (80 PDF). Page 86 gives a table for the multiplier. YOUR AREA may be different but it will possibly steer you in the right direction.
Generally manufacturers will allow you to go up to 4" WC and as low as 3" WC (sometimes lower) to adjust the burners for the proper BTU.
Thanks, Sadlier. I've looked and asked around for info of this type and always got a blank stare as a response. The reading will keep me busy for the next couple of days.
Originally Posted by sadlier
As for the pressure adj. thats what I've heard also, this is to compsate for the varation in gas BTU content, makes sense. Just recheck with the temp rise method to check that, the adj., didn't exceed the the btu rating of the unit.
So you didn't ask, what you though was a stupid question. Now how, are you going to tell the boss about your stupid screw up?