# A gas consisting of 20% CO 2 and 80% oxygen and nitrogen leaves a lime kiln at 2000 F and enters a..

A gas consisting of 20% CO2 and
80% oxygen and nitrogen leaves a lime kiln at 2000F and enters a
square duct measuring 6 in. by 6 in. in cross section. The specific heat of the
gas is 0:28 Btu/lbm F, and it is to be
cooled to 1000F in the
duct, whose inside surface is maintained at 800F, and whose walls
have an emissivity of 0.9. The mass velocity of the kiln gas is 0:4 lbm/ft2s and the convective
heat-transfer coefficient between the gas and duct walls is 1:5 Btu/h ft2
F.

a.
»

A gas consisting of 20% CO2 and
80% oxygen and nitrogen leaves a lime kiln at 2000F and enters a
square duct measuring 6 in. by 6 in. in cross section. The specific heat of the
gas is 0:28 Btu/lbm F, and it is to be
cooled to 1000F in the
duct, whose inside surface is maintained at 800F, and whose walls
have an emissivity of 0.9. The mass velocity of the kiln gas is 0:4 lbm/ft2s and the convective
heat-transfer coefficient between the gas and duct walls is 1:5 Btu/h ft2
F.

a. Determine the required length of duct to
cool the gas to 1000F.

b. Determine the ratio of radiant energy
transfer to that by convection.

c. At what temperature would the gas leave
the duct if the length of the duct were twice the value determined in part (a)?

(Courtesy of the American Institute of
Chemical Engineers.)

Hint. As the response of the gas to
emission and absorption of radiant energy differs, an approximation for the
radiant energy exchange between the enclosure and gas contained within an
arbitrary control volume is given by

»

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