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bintoito
23-06-2014, 07:54 PM
Hey guys,

I know that this isn't exactly the right place but I guess that this is a thermal eng problem as any other (in the negative range of temperatures this forum's used to...)

So I've been asked to estimate the annual savings from installing an economizer (Waste heat exchanger) to pre-heat feedwater to a steam boiler;

mass flow of flue gas: 1419.8 Kg/h;
Specific heat (aprox) 1.05 kJ/KgC;
Temperature: 355.6C

Now, assuming that I have an heat exchanger with sufficient area to cool the gases to 180C, the "recoverable" energy/power would be given by:

Q= (1419.8*1.05*355.6-180))/3600 =72.7 kW (h basis).

Now assuming that the economizer (HX) would loose 10% to the surrondings, I would have 90% of 72.7 kW of heat "FULLY AVAILABLE" to the seconde fluid or am I mistaken?

I'm asking this because at the office, the boss is insisting that I have only about 50% of these 72.7 kW....
I suppose he's quoting this due to economizer's efficiency being quoted in literature as being of about 50 to 70% efficient, but I bet that this efficiency regards to what I could recover if I would cool the gases down to the ref. temperature (assuming it to be 20C) indeed the so-called efficiency would be in this ball park), but as I figure it, the only efficiency factor I would have to acount for in the way I calculated would be these 5 - 10% losses to the surrondings (ofcourse doing the energy balance only to the HX)

I'm I dead wrong and indeed from all those flue gases I can only recover what would be 25% of their initial energy content (referenced to 20C)?!?

Please help ASAP!