HEAD PRESSURE OF WATER AT 60° F.
DRAFT IN NECKED-DOWN FIRED HEATER STACK FIG. 1
Draft = 0.45 in. H20
ft (Equation 11). Having already established a volume of 32,000 cu ft for the
giant sphere, the balloon would be able
to lift 740 lb (Equation 12).
A tall stack can develop a very strong
sucking force. Let’s say the stack’s draft
is 2 in. H2O (meaning pressure outside the stack is 2 in. H2O higher than
pressure inside the stack). Let’s also say
there is a metal plate measuring 2 ft ×
2 ft to be pulled off the stack’s bottom.
How much force, in pounds, would one
have to use to pull off the plate?
First, we calculate the area of the
plate as 4 sq ft (Equation 13), or—since
we know there are 144 sq in. in 1 sq
ft—576 sq in. (Equation 14).
Over the surface of this plate, there
is a force of atmospheric pressure equal
to 2 in. H2O pushing on the plate,
which we can calculate as 1,152 cu in.
Since 1 cu in. H2O weighs 0.58 oz,
we can determine the force required
to pull off the plate would be 664 oz
(Equation 16), or 42 lb (Equation 17).
The author encountered this problem in 1980 at Amoco Oil Co.’s Texas
City, Tex., refinery where, while wearing a Scott Air-Pak, he had to pull off
a manway cover on a reactor that was
150 ft high and full of hot flue gas from
burning sulfur residue.
Stack height effect
Draft in a stack is not caused by combustion. Also, as the molecular weight
of the flue gas is rather close to air, its
effect is neglected. And while wind
blowing across the top of a stack occasionally contributes to draft, its effect
is generally ignored when calculating
draft since it’s not always windy.
You may have seen on certain stacks
that the top end of the stack is necked-down (Fig. 1). This is done to slightly
increase the draft. It’s called the Venturi effect, which will be discussed later.
Major factors contributing to draft
in a stack include:
• Height of the stack.
• Temperature of the flue gas.