Boiler Flue Oversizing Issues
Boiler Flue Oversizing Issues
(OP)
I have an existing external steel flue feed from a natural gas boiler plant. The boiler plant is going to be reduced in capacity by 50%. I found by calculating the theoretical draft and flue pressure loss per the ASHRAE Systems and Equipment Handbook that the theoretical draft is sufficient. My concern is that reduction in boiler plant capacity will cause condensation that could damage the flue over time. How can I calculate when condensation will occur and if it is an issue? The flue is 140' tall and my client does not want to add a flue liner.





RE: Boiler Flue Oversizing Issues
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RE: Boiler Flue Oversizing Issues
PS: I have no direct experience with NG fired boilers though I have seen many installations.
Regards,
RE: Boiler Flue Oversizing Issues
I have experienced condensation problems with NG-fired hot water boilers (fire-tube type; 190F) that spend a lot of time at a low firing rate. The biggest concern is not the stack, but the boiler itself -- if that condensation can run down into the boiler, then you will have rusting of tubes, tubesheets, inner doors, etc.
If you determine that you have condensation, insulating the stack might be one answer. Another fix (although not the most energy efficient one) is to keep the stack gas hotter with a bypass damper inside the boiler. On a 4-pass boiler, the damper would direct some of the hotter second-pass gas into the fourth-pass outlet to raise the temperature of the gas leaving the boiler. An actuator modulates the damper according to stack temperature. No Green Building awards here, but it does work.
---KenRad
RE: Boiler Flue Oversizing Issues
As long as you don't reset the boiler temp you should be fine. I typically design at 200F HWS temp and 40-60 deg drop in the coils. One pump for the boiler and another for the system ....... primary/secondary.
What size boilers you talking about?