Economizer Design 3e2r22

Economizer Design -

Assume bunker fuel oil- LHW = 17,500 Btu/lb Assume that the client has specified a boiler. Efficiency (ηb) of 86% at 12% CO2 in the stack gas

1. Calculate the mass flow rate of fuel, m f a. m f (LHV) ηb = m s (hsho-hin) b. LHV = lower heating value (calculated or from literature) c. m s = mass flow rate of steam (same as for feed water in Project 1conservation of mass). d. hsho = enthalpy of superheater outlet. e. hin = enthalpy at economizer entrance. Assume that the outlet economizer temperature is 100 oF less than the saturation temperature (to prevent steam in the economizer). 2. Calculate the mass flow rate of stack gas. Note: 12% CO2 corresponds to 19.5 lbgas/lbfuel. 3. Draw a diagram of the economizer, label points, write heat exchanger equations, calculate. ↑ T4=410oF Feedwater out T2 = Tsat-100oF

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←Feedwater in T1=150oF m gas ↑ T3

% stack loss = (100- ηb). Enter curves (pg. 25) to get stack temperature, T4. q = m s (h2-h1) q = m g (h4-h3) Solve for h3 and get T3 from curves: “Sensible heat of Flue Gas.” q=UAFD Calculate overall heat transfer coefficient; log mean temperature difference; look up F; calculate require heat transfer surface; select a tube size; calculate number of tubes. First calculate ηo, then find U and total required area, A. next, calculate the fin area and base area (A*) and divide by the spacing to get area per foot. Divide this into A to get total length of tube with fins. Suggested Layout for Economizer

Cross Section A-A

By: Dr. Dennis Ballou