3 replies to this topic

[rosa]

Hi friends
as you know there are preheaters at inlet of boilers.in the case of preheaters damage we put it out of service.I want to know what are bad sideffects on Boilers and also economizers?
would you please help me in this regards or introduce some helpful applicable document?

Kind regards,
rosa
cheers!!

[Art Montemayor]

Rosita:

The only effect that lack of BFW (Boiler Feed Water) pre-heating will have in a fired boiler, that I am aware of, is a decrease in steam generation capacity (if you maintain the same fuel firing rate). Of course, if you can't tolerate the capacity decrease and are forced to increase the fuel firing rate (assuming that your boiler design will allow you), then the cost of fired fuel per lb of steam generated is going to increase.

To understand the effect (& results) all you have to do is make a heat and material balance around the boiler. If you want to maintain the same steam generating capacity, but eliminate the heat input contributed by the BFW preheater, then you are going to have to burn more fuel to make up for the loss in the BFW preheat step. The BFW is a conditioning AND energy input step. The "ideal" BFW preheater would heat the BFW just below the BFW's boiling temperature. In other words, it would supply the SENSIBLE heat required to bring up the BFW to its boiling point. The boiler proper would then take care of only supplying the LATENT HEAT OF EVAPORAION. In real life this is usually not the case. The BFW preheater does not supply ALL of the sensible heat required. It supplies as much as can be economically and process justified - which is usually 75 - 85% of the sensible heat required. This step, of course, depends on the installation as well as the design of the boiler in question and the application that has been imposed on it.

What I understand you are doing now is simply over-firing the boiler to make up for the loss of the BFW preheater. As long as your boiler can handle the over-firing safely all you are doing is wasting fuel. BFW preheaters are usually relatively small in size and can be designed and fabricated locally - if you know what you are doing. The size, of course, depends on the size of the boiler in question. Perhaps you can design and fabricate a local BFW preheater as a replacement for the one that failed. Without any specific information such as basic data or sizing data, I don't think we can say much more than that.

I hope this information helps you out.

[muchlis nugroho]

Are we talking about package boiler or other kind of boiler?
I assume that there is preheater working together with economizer, is it correct? And the one that out of service is the preheater, the economizer is still working fine.

If the preheater is taking heat from process steram (heat source other than flue gas), then it will decrease the efficiency. Since the economizer will increase to compensate the "out of service" preheater duty by increse firing. And the heat from process stream will be wasted to cooler (cooling water or fin fan cooler).

There is no side effect other than decreasing efficiency if you still have economizer that works fine (if there is no bottleneck in economizer).

Rgds,
MNG

[Adriaan]

There is a VERY good reason for using a feedwater pre-heater for a boiler - prior to the Economizer - raising the feedwater temperature to a certain minimum.

The purpose is to prevent condensation in the boiler on the fired side (which can lead to damage to the piping etcetera) at the ECO. Where I work the problem is sulphuric compounds in the gasses, so a fairly high feedwater inlet temperature is required.

Though this indeed lowers the boiler THERMAL efficiency somewhat it can in fact increase the ECONOMIC efficiency of the boiler and the operational safety.