Boiler Flue Oversizing Issues

[belz]

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.

 

[imok2]

Go here, this may be of some help for you:

http://www.blesi-evans.com/dew point calculation.pdf

 

[quark]

Condensation of flue gas is severe if SO₂ is present in the flue gas. As there is no sulphur content in natural gas, condensation may not be severe. You can drop down the flue temperature to as low as 55⁰C as far as my knowledge is concerned.

PS: I have no direct experience with NG fired boilers though I have seen many installations.

Regards,

 

[KenRad]

Is this a double wall stack? Steam or hot water boiler? What operating temperature/pressure? 140 feet is a long run - you'll need to determine how much heat loss the stack gas will experience on its way up. If it's enough to reduce the temperature below about 55C, you'll have condensation. Worse case would be when the boiler is operating at low-fire, and the stack gas leaves the boiler at a lower temperature and volume.

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

 

[cme]

Since you have a 140' tall stack, wou will experience problems with the combustion system (flame) because you will have too much available draft. I suggest you install a draft control system (one per boiler) as manufactured by Hays Cleveland or Preferred Utilities. I just did a system for 3-400hp HPS firetube boilers connected to an 150' concrete old coal chimney. The boilers that were replaced were converted coal boilers.

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?