Lowering gas firing rate on dual fuel boiler

MJW

We have a 125HP RockMills Boiler with an Iron Fireman WhirlPower Burner, C240-GO-SMK. Single pipe steam. 70 unit residential building. Heat -timer control (antiquated, no indoor sensor).
We're an 'interruptible' gas customer, so almost exclusively Nat. gas except during cold snaps, when we're switched to #2 oil.
Last week we were on oil and I noticed that in addition to running much quieter (no nat. gas booster pump whining), it took the boiler about 15 minutes to reach cut-off pressure of 2.5p.s.i (I know, too high...working on it :-) ).
When we're on NG, it takes about 8 minutes to reach the same cut-off pressure of 2.5p.s.i..
This difference in time to cut-off pressure seems to indicate a higher firing rate for the NG versus #2 oil.
Got to say, the heating from the slower oil firing seems much preferred; less 'off-time' for the radiators in any given 60 minute cycle for similar amount of fuel. And slower build to pressure may mean quicker heating to far away radiators (lower pressure steam moving heat faster)?
Is it possible or desirable to reduce the firing rate for the NG? Can firing rate be reduced for NG while leaving oil firing rate the same?
In shoulder months when boiler is off for most the day, there is water hammer on first cycle. Might this be reduced by lowering the firing rate? Thinking that the condensate may have a better chance of draining out of the pipes (wet returns at the ends of the mains).

Thanks much!

nicholas bonham-carter

See if the burner has any staging features in it. It may be able to burn high or low (75%), on gas.
Get the pressure down, as it can force water out into the returns and up high enough to interfere with what ever means of main venting you have. Possibly a vaporstat will help you do this. A low pressure gauge (0-3 psi),will show you how efficient your venting is, and enable you to shorten the burn cycle.--NBC
PS I found some information which appears to show the gas burner at a lower rate than oil!:

http://www.njmechanical.com/ironfireman/Series_36_boiler_service_installmanuals.pdf

http://www.njmechanical.com/ironfireman/PAO-PAGO-AG-AO_servicemanual.pdf

MJW

Thanks much for the manuals. I'll try to dig in to them. But the little I did read suggests that once the burner is adjusted for oil/air mixture, that locks in mixture for gas/air as well.
Maybe the gas firing rate can be reduced through a pressure reduction or the butterfly valve, but that would seem to effect the fuel/air mixture.
So perhaps reducing the firing rate of NG independent of oil is not really possible. And oil rate seems fine in that it takes 15 minutes to reach cut-off pressure.
I'm not looking to shorten firing times, rather to lengthening firing time of NG, just at a lower rate (same fuel consumption, just stretched out).
It would be great to have the NG rate match the oil rate so that both took similar amount of time to reach cut-off pressure.

Thanks for the links.

EBEBRATT-Ed

The manuals are correct. You set the gas and oil flames with a combustion analyzer. Say 11% Co2 on oil with a zero-trace of smoke. Gas should be 8% Co2 with your ppm of co under 100.

Once the combustion is set youre all done. Gas can probably be burned with less air than oil which makes for more input on gas. Once the air is set for one fuel then you have to adjust the other fuel to match the combustion you need. The imput on two different fuels will not match exactly. Be glad your working on a #2 oil job. #6 oil and gas are even further apart.

Long live the C240 burners. They were WAY ahead of their time. They were out in the late 50s and will kick the hell out of most burners made today. Used to work on lots of them. They were a really rugged burner except for losing a blower wheel now and then.

MJW

You mean losing a blower wheel like the attached photo from a month ago? :-)
So I guess that firing rate is pretty fixed. You have to calibrate the mixture with the velocity of the air coming in? which is determined by the blower wheel rpm?
So, I'm gathering just not that tune-able. It does have high and low fire. But I guess we're up against the limits of the burner.

All in all, working well. I'll keep at trying to lower operating psi. But my only concern is that if operating psi gets below 16 ounces, the cycling will be super short. A bunch of cycling in any given 'on portion' of the 60 minute cycle. Is there a limit to how many periods are desirable in the sinusoidal wave of on/off cycles? Seems like a bit of wear and tear for the burner to be kicking on/off a bunch.

Thanks.