Calculating Chimney Draught and Height

Chimney height and Draught head

(i) h_w = 353[(1/T_a) - {(1/T_g)(m_a + 1)/m_a}] mm of water.

(ii) h_w = P/(ρ_a - ρ_g)

Assuming that the same draught is produced by gas column of height H₁

H₁ = H[{(T_g/T_a) - 1}x {m_a/(m_a + 1)}]

Here

T_a = absolute temperature of atmosphere

T_g = Average absolute temperature of chimney gas

h_w = equivalent mm of water head

m_a = mass of air

H = height of chimney

Condition for Maximum Discharge through a Chimney

H_1,max = H

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Example

A chimney is 28 m high and the temperature of hot gases inside the chimney is 320°C. The temperature of outside air is 23°C and furnace is supplied with 15 kg of coal burnt. Calculate the following: (i) Draught in millimetre of water (ii) Draught head in metres of hot gases

Solution

(i) h_w = 353[(1/T_a) - {(1/T_g)(m_a + 1)/m_a}]

= 353 x 28 [(1/298) - {(1/593)(15 + 1)/15}]

= 15.61 mm of water

(ii) H₁ = H[{(T_g/T_a) - 1}x {m_a/(m_a + 1)}]

= 28[{(593/296) - 1} x {15/(15 + 1)}]

= 26.34 m

Example

Determine the height of a chimney to produce a static draught of 22 mm of water if the mean flue gas temperature in a chimney is 290°C and ambient temperature in boiler house is 20°. The gas constant for air is 29.26 kgm/kgK and for chimney flue gas is 26.2 kgfm/kgK. Assume barometer reading as 760 mm of mercury.

Solution

Density of air at 290 K

ρ_a = p/RT = 1.033 x 10⁴/29.26 x 290 = 1.217 kg/m³

Density of flue gas at 563 K

ρ_g = 1.033 x 10⁴/26.2 x 563 = 0.7 kg/m³

Static draught

p = H(ρ_a - ρ_g)

22 = H(1.217 - 0.7)

H = 42.55 m will be the height of the chimney

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