Boiler economizer on watertube boiler: Good idea?

[mkn432]

A customer of mine is considering installing an exhaust economizer on an existing 4 MMBtu/hr forced draft watertube boiler to preheat the makeup water. I've been told that watertube boilers are not good candidates for economizers because the backpressure increases too much and damages the boiler. Is that the case? If so, are there any additional steps that could be taken to mitigate the excess pressure (e.g., installing a power venting system)?

 

[davefitz]

Bizarre! Nearly all watertube boilers have economizers. It might be neccesary to increase the boiler feed pump's head capability to account for the increase in pressure drop, but usually there is enough margin.

 

[GBENARD]

Hi,
To avoid excessive vaccuum in the boiler ducts, NFPA 85 requires an interlock between the pressure measurement in the combustion chamber and the ID/Fan, if I remember well.
Otherwise you just have to check that the casing design pressure is correct, as well on overpressure as under vaccuum.

 

[rmw]

Double Bizarre!!! Great technical term Dave.

MKN432, In addition to the provision for additional pressure loss in the feedwater circiut, some provisions for the FD fan might also have to be made. If it is a direct connected fan, it might have to be "tipped out", or have a wheel size change, limited, of course by the motor HP. If the FD fan is belt driven, as a lot of FD fans are, then it might just be a pulley size change, again limited only by the fan HP.

Since this is an addition, and you can specify anything you like, then you can limit the amount of extra back pressure on your boiler (and you are talking about only inches of water pressure addition) by carefully writing your economizer spec.

Go for it. It is incredulous that it hasn't already been done. Economizers are there for just that, economy.

Be careful not to go over board, and get the economizer outlet temperature too low. If your fuel has sulfur in it, you will have to keep the outlet above 300-350F to prevent condensation of the flue gasses in the flue and stack. If you know the exact sulfur percentage in the fuel, you can determine this more closely, but be careful if the boiler is not constant load.

Hope these few pointers help. One more pointer. Whoever that was that told you that about economizers, don't take any race horse tips from him/her.

rmw

 

[Rick360]

I beleive the ID fan would require a change if one is required. The FD fan flow, Sec duct and windbox pressure shouldn't change because the furnace pressure would control at its normal setpoint, therefore the FD fan would see no difference. Since furnace pressure would be the same the ID fan(s) would have lower(more neg) pressure because of the added flue gas pressure drop across the new economizer section.

Regards,

Rick

 

[rmw]

Rick 360,

First, the poster stipulated that it was a FD boiler, without the mention of an ID fan, so I assumed that there was none, which is common in watertubes of that size.

What you have said is true, but in light of the fact that if there is no ID fan, then there is nothing to "push" the air, and the products of combustion through the boiler than the FD fan, and that was exactly what the detractor to the concept of adding an economizer was referring to, is that the furnace pressure would have to increase by a small amount to accomodate the additional pressure drop of the economizer on the tail end.

rmw

 

[davefitz]

Often the small boilers that only use an FD fan will also provide a stack inlet damper and other measures to hold a balanced pressure at the boiler outlet/stack inlet. This damper counters the "stack effect" in which a tall stack will induce a negative pressure at the boiler outlet .A balanced pressure will prevent air inleakage from diluting the furncace gases.

Basically, if the stack is tall enough, the stack effect is sufficient to overcome the add'l gas side pressure drop thru the new economizer . If not , one may ned to increase the stack height. But also note that the use of the economizer will lower the stack gas temperature and thus lower the stack effect.