1.

Which is the net pressure equation used to find chimney height?(a) P = H (Wa-Wg)(b) P = H (Wa+Wg)(c) P = H (Wg-Wa)(d) P = H (\(\frac{W_a}{W_g}\))I had been asked this question in examination.The origin of the question is Draught System in chapter Steam Generators & Accessories of Energy Engineering

Answer»

The correct answer is (a) P = H (Wa-WG)

Explanation: The pressure acting on the grate from chimney SIDE,

P1 = Pa + Wg H

Pressure acting on the grate from atmospheric side

P2 = Pa + Wa H

Where, Pa = Atm pressure

Wa = weight density of air

Wg = Weight density of hot gases

 The net pressure acting on the combustion chamber due to the pressure exerted by gas column and air column is given by

P = P2– P1(as Wa> Wg )

P = (pa + Wa H) – (Pa+ Wa H)

P = H (Wa– Wg )

This pressure difference is known as static draught and is responsible for CAUSING the flow of air through the chimney.


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