urface to grate surface being 50:1; the
flues being 100 feet long and containing two right-angle turns; the
stack to be able to handle overloads of 50 per cent; and the rated horse
power of the boilers based on 10 square feet of heating surface per
horse power.

The atmospheric temperature may be assumed as 60 degrees Fahrenheit and
the flue temperatures at the maximum overload as 550 degrees Fahrenheit.
The grate surface equals 400 square feet.

2000 x 34-1/2
The total coal burned at rating = ------------- = 8624 pounds.
8

The coal per square foot of grate surface per hour at rating =

8624
---- = 22 pounds.
400

For 50 per cent overload the combustion rate will be approximately 60
per cent greater than this or 1.60 x 22 = 35 pounds per square foot of
grate surface per hour. The furnace draft required for the combustion
rate, from the curve, Fig. 34, is 0.6 inch. The loss in the boiler will
be 0.4 inch, in the flue 0.1 inch, and in the turns 2 x 0.05 = 0.1 inch.
The available draft required at the base of the stack is, therefore,

_Inches_
Boiler 0.4
Furnace 0.6
Flues 0.1
Turns 0.1
---
Total 1.2

Since the available draft is 80 per cent of the theoretical draft, this
draft due to the height required is 1.2 / .8 = 1.5 inch.

The chimney constant for temperatures of 60 degrees Fahrenheit and 550
degrees Fahrenheit is .0071 and from formula (30),

1.5
H = ----- = 211 feet.
.0071

Its diameter from curve in Fig. 33 is 96 inches if unlined, and 102
inches inside if lined with masonry. The cross sectional area of the
flue should be approximately 70 square feet at the point where the total
amount of gas is to be handled, tapering to the boiler farthest from the
stack to a size which will depend upon the size of the boiler units
used.

Correction in Stack Sizes for Altitudes--It has ordinarily been assumed
that a stack height for altitude will be increased inversely as the
ratio of the barometric pressure at the altitude to that at sea level,
and that the stack diameter will increase inversely as the two-fifths
power of this ratio. Such a relation has been based on the assumption of
constant draft measured in inches of water at the base of the stack for
a given rate of operation of the boilers, regardless of altitude.

If the assumption be made that boilers, flues and furnace remain the
same,