Gases (assume hot air) from a fire in an industrial furnace escape up the chimne

Question

Gases (assume hot air) from a fire in an industrial furnace escape up the chimney at temperatures above 1000oF. Some of the energy wasted in these gases can be put to good use if they are put through a heat exchanger (constant pressure, continuous flow process) to heat cold water. If the chimney gases go though such a heat exchanger and the gas temperature is reduced to 600oF,

a) how much heat is recovered in the water (Btu/lbgas)?

b) how much wd does the chimney gas perform in the heat exchanger?

c) If 2000 lb/hr of chimney gas is created in the furnace, how many Btu/hr is recovered?

d) If the flow rate of the water in the heat exchanger is 4000 lb/hr, how much of a water temperature rise is there through the heat exchanger?

Explanation / Answer

Heat Loss by gas = Cp*dT Btu/lb

Cp of air = 0.24 Btu/lb.F

a)Heat loss by gas(h) = 0.24*(1000-600) = 96 Btu/lb

Heat loss by gas = Heat recoveres in water = 96 Btu/lb

b)Work performed by gas in the heat exchanger is ZERO

becuase work in control Volume is integral of (v*dP).here Pressure(P) is constant

c)REcovered Heat(H) = m*h = 2000*96 =192,000 Btu/hr

d)Mass flow rate of water(m1) = 4000 lb/hr

m1*Cp of water*dT = 192,000

4000*1*dT = 192,000

dT = 48 F

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