Boiler Venting Issue

[poncho802]

I have a project installed as follows.
(4) 400 BHP steam boilers in boiler room common vented. (3) boilers will only be on line at a time. The stack extends 15 floors on the roof into a plenum with draft inducers. The initial stack read .75" of available draft. We installed barometrics at each boiler to bring down the available draft to .18". with (3) boilers at 1/3 power the system drafts with the barometrics balanced. When one boiler goes to full fire the barometric at that boiler dumps into the room. None of the fans opperate. The stack is still showing a .4" SP with boilers running. Any suggests on how to stop the boilers from dumping and lowering the static pressure?

[beenthere]

Should be set up with all 3 boilers fired at 100%.

Why don't the draft inducers work.

[klrogers]

Do you have enough make up air going into the room? for 4 x 400
HP boilers you need a very large intake opening.

[poncho802]

The draft inducers are combustion air fans on the boilers not natural draft induction cones.

With (3) at full fire one boiler will dump though the barometric.

Combustion air comes from motorized dampers the room is under a negative compaired to the outdoors but under a positive compaired to the rest of the building.

[klrogers]

The draft inducers are combustion air fans on the boilers not natural draft induction cones.

With (3) at full fire one boiler will dump though the barometric.

Combustion air comes from motorized dampers the room is under a negative compaired to the outdoors but under a positive compaired to the rest of the building.

Negative pressure compared to outside is a problem, the flue gas finds it easier to come into the room rather than go up the stack.

Kevin

[poncho802]

The negative in the room is from the stack drawing through the barometrics. We have been able to balance the barometrics to (3) boilers operating at mid fire. When one boiler ramps to high fire the barometric closes and starts to spill flue gas.

[BarTech]

Add some more combustion air to get your boiler room out of a negative pressure. It is probably overpowering the stack. Is stack sized correctly?


Pay me now or pay me later

[HeatXfer]

How long have these boilers been operating? Is this a new problem on an established set-up? Or is it a new installation that has never been fully operational?

Some clarification needed:

You said the Draft inducers were on the roof and then you said they were combustion air fans on each boiler. Which are they: Combustion blowers at the boilers, or inducer fans on the stacks at the roof?

If there are inducer fans, are they fixed or variable speed?

Does each boiler stack run directly to the roof, or does each boiler stack plug into a manifold with a single common stack to the roof?

[poncho802]

HeatXfer

This is a new installation and the problem was established on start up. The boilers are manufactured by Vapor Power with a combustion air fan installed on the boilers. Each boiler is common vented into a single stack. The stack is run up 15 floors to the roof into a plenum with 5 utility set variable speed fans to induce draft.

The available draft in the chimney stack as originally designed and installed caused flame roll out on the boilers. Barometric dampers where installed in order to alleviate the negative pressure in the flue. The boilers when at high fire where spilling flue gases back into the boiler room. A balancing damper was added to the stack in order to balance the pressures to vent this system properly.

To date we have balancing dampers at each boiler outlet, barometrics at each boiler outlet and a balancing damper in the stack at the third floor.

We have tried balancing the system with all dampers without resolve. Boilers one and two we can balance fairly well. When we move to boiler three we can not balance it at all with out spilling flue gas into the boiler room.

[HeatXfer]

Total stack flow with all three boilers at full fire would be approx 11,000cfm. You said when all 3 were at 1/3 power the system drafts fine. Thats should be the equavalent draft of one boiler at full fire, approx 3200cfm.

You said the fans weren't running. From the outside it sounds like you simply need to run one or two of the draft inducer fans on the roof at a minimum to keep the stack no higher than .05"wc. Don't let the fans speed up until the stack pressure climbs over .05"wc.

It sounds like the fan ramp speeds are too low and there is a big lag before they overcome the high positive in the stack. You'd have to start an inducer at the same time the boiler is in pre-start to temporarily bring the stack into a negative until the boiler fires. This should be enough to prevent roll-out. Once the draft is established, the system is then allowed to modulate the common stack pressure to the desired set point.

The inducer fans should be running all the time, and the barometrics might have to be fixed to a single position to prevent excessive dilution of the stack gasses. As those gasses cool, it's harder to pull them to the roof 180' away.

How are the inducer fans sequenced? Do the fans all modulate up & down at the same rate, or is each one staged and then ramps up as needed?

The third floor damper should start out open until you're able to see if the inducers will stabilize the stack pressure by themselves.

Like I said, I'm on the outside looking in, but your stack pressures are consistently too high.

I hope that helps