Flue gas condensing, economisers, & chimney lining

Keith W.

Hi: I mostly lurk, but I'd really appreciate people's thoughts: Complicated situation, simplified:

2 gas boilers, ~100 HP each, 25+ years boilers and old brick chimneys > 50' tall. The idea is to add a Flue Gas Condenser (some would say an Economiser: in this case Sidel Systems brand), which in principle is going to reduce stack temp below dew point when in full-fire operation.

I believe the chimney requires lining, others don't. It's a fairly big expense, of course, so if there were any way to avoid it that would be great.

In addition to chimney condensation issues, there are concerns about draft: some would say an inducer fan is required, though others suggest a pressure switch at the fire box would be adequate to protect from inadequate draft... All near-boiler/FGC piping is in stainless.

Thanks for any help thinking about these issues.

Keith

Keith W.

Should have added:
The Sidel units come with a pressure switch at the unit -- the installation is suggested with trapped or inverted venting, that is, the exhaust gases go down to the unit mounted on the floor, and then back up, when the unit is in-line (dampers control this).

Steamhead

Don't run this setup into an unlined brick chimney. There will still be some moisture in the flue gases and that will deteriorate the mortar. Stainless steel liner is the way to go.

And the reduced flue gas temperature may not allow the chimney to draft properly, in which case you'd need a draft inducer whose moving parts add to future servicing costs.

It's not just a simple matter of adding an economizer.

Jamie Hall

I second Steamhead's comment. The flue gas leaving the economiser will be at 100% relative humidity; it's the nature of the beast. As it cools further -- which it will -- going up the chimney, it will condense, and the condensate is a mean acid. In my humble opinion, a liner is mandatory.

The draft inducer might not be required -- but I wouldn't bet on that.

Keith W.

Thanks for your thoughts. Has anyone seen a pressure switch installed in the boiler firebox or breeching to address inadequate draft, maybe in other circumstances? I've seen them in small (residential) condensing boilers, but of course these are non-condensing commercial boilers.

Tim McElwain

I would check with local fire and code officials as to whether they will allow such modifications under the commercial boiler codes.

My own experience with economizers over my 50 years in the business has been nothing but trouble.

I am assuming this system has had a combustion analysis done and proper adjustments made to insure adequate efficiency.

Some test numbers would help to see if we can help in some other way.

By the way the cost for lining the chimney may far out weigh any efficiency gained.

What kind of gas burner is used on this unit?

Larry_52

How is return condensate to the boiler controlled?

To run an economizer your feedwater flow should be constant and analog control, preferably not digital. On/off boiler water level control will induce a quench effect of the economizer pipes and possibly put the water into a H2O saturation zone once it reaches the boiler. It is quite a water level nightmare to be pumping water that flashes the second it hits the boiler or filling a piping system that already boiled, full of water (depends where fdw reg is). There are ways to prevent these issues which require economizer recirc from boiler water back to econ (pumped with econ above boiler water level, natural circulation below boiler water level) with stop checks on inlet of economizer. This protects the economizer from low demand feedwater and boiling of the economizer. I prefer analog feedwater regulation with a feedwater pump, placing regulator after economizer. This configuration prevents economizer boiling by raising the pressure of the economizer but does not protect flashing after the regulator. Placing the reg before the economizer allows natural convection recirc with economizer placed below boiler water level.

Water flow for economizers relates to Dan's alanog to landing gear of airplanes with steam system startup and shutdowns.

Keith W.

Thanks again. I don't know about the burner offhand, sorry. WRT to feedwater, these feed both boiler return & DHW to a fairly large building, so they are expected to bring flue gas temps down below 100F. In any case, presumably that means there's no concern about flashing, but I hear your point. Also, the boilers are all HW low pressure, no steam.

K