To improve the efficiency of a steam boiler system which generates 600 psia stea

Question

To improve the efficiency of a steam boiler system which generates 600 psia steam for refinery process use, the boiler feed water (BFW) is to be preheated to 400 degrees F from 100 degrees F before it enters the boiler. This preheat will be accomplished by installing a heat transfer coil in the boiler flue gas stack, recovering heat energy that would otherwise be exhausted to the atmosphere. This BFW is a "compressed liquid water." How much heat energy, experssed as MBTU/hr., is required to effect this BFW preheat at 75 gal./min. BFW rate? This value is needed to design the heat transfer coil.

Please show work so I'll understand the answer! Thanks!

Explanation / Answer

It is given that, BFW is a compressed liquid water, hence its specific heat capacity would be soleley a function of temperature only.

BFW is preheated from 100F (311K) to 400F(476K).

average temperature = (311 + 476)/2 = 393.5 K

specific heat capcity of water 400K = 1.9 KJ/Kg-K ( available on google)....this value could be used for the further calculation, as specific heat cpacity doesn't vary much with temperature.

now,

flow rate given = 75 gal/min = 75*3.8 L/min = (75*3.8L)*(1kg/L)/min ....(density of water= 1Kg/L)

=285 Kg/min....

therefore heat energy required to preheat the BFW from 100F (311K) to 400F(476K) = m*(specific heat capcity) * temperature difference = (285 Kg/min)*(1.9 KJ/Kg-K)*(476-311)K = 285*1.9*165 KJ/min = 89347.5 KJ/min = 89347.5* 0.948 BTU/min =84701.43 BTU/min = 84701.43 * 60 *10^-6 MBTU/hr = 5.082 MBTU/hr

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