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AAMAIK
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Homework Statement
A gas-fired tube heater has the following data:
Dimensions of the combustion chamber: 15 * 20 * 40 ft
Tube outside diameter: 5 in
Temperature of the tube=800° F
Center to center spacing: 8 in
Number of tubes (arranged in a single row of which 6 tubes are shield tubes) : 90
Tube length: 40 ft
Preheated air enters at 400℉ and fuel gas enters at 60℉.The fuel gas consists of methane and ethane and has the empirical formula C40Hn. The rate of heat liberated by the fuel is 91200000 Btu/h. Thirty percent excess air is used. The desired radiant heat flux is 12,000 Btu h.ft2℉ of circumferential tube area. The net heating value of methane is 21496 Btu/lb and that of ethane is 20550 Btu/lb. Calculate:
(i) The average flue gas temperature in the firebox.
(ii) The effect of reducing the excess air to 10% on the fuel requirements.
(iii) The effect of introducing combustion air at 60℉ on the fuel requirements.
(iv) The refractory average temperature
Given n=159 and the specific heat of flue gas, air, and fuel = 0.24 Btu/lbmoF
Homework Equations
The Attempt at a Solution
iv) I estimated the total area of the chamber by arranging 90 tubes of the radiant zone along the perimeter of the heater section. Given the tube separations, I computed the necessary dimensions. Then I calculated the area of the equivalent plane which will be the sum of the area of the radiant tube planes and the shield tube plane, the equivalent plane efficiency from the graph of the equivalent plane efficiency vs distance between centers/ tube diameter ratio graph, view factor, and equivalent hemispherical beam length. How can I proceed further after determining the emissivity of the flue gas?
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