Question

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 1

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.