The Flow Of Gases In Furnaces

DESIGN OF OPEN-HEARTH FURNACES

235

LEAKAGE THROUGH VALVES AND DAMPERS

Leaky valves and dampers are another source of trouble
when -waste-heat boilers are installed. The simple butterfly
valve is the oldest form of reversing valve. When carefully made
and new these valves are tight, but they do not remain in that
condition very long when exposed to hot gases. In modern prac-
tice the butterfly valve is rarely used, except for reversing the air,
the gas being reversed by valves better designed for the prevention
of leakage.

There are a number of valves on the market which have proved
more or less successful in operation. Many of these valves have
water seals, which prevent leakage as long as the water supply is
maintained and the pressure differential between the flues or the
flue and the air is less than the seal. All water-seal valves lose
their seal during the reversal period, and while this period, when
the sealing lip is lifted above the water surface, may be only a
fraction of a minute, a certain amount of loss occurs which cannot
be prevented. All water-seal valves add perceptibly to the
moisture in the hot gases
which pass through thorn.

Fig. 150 shows an ar-
rangement of valves and
flues which has boon used in
the United States. It is
rather costly, involving the
installation and upkeep of
eight valves and two dam-
pers. A method of reversal
which experience has shown
to be satisfactory-with this
valve system is as follows: assuming that the air and gas
are entering the furnace through the checkers K and L and
passing out through the checkers / and .7, the sequence of opera-
tion is:

1. Steam is cut off from the producers;

2. Air-stack valve F is closed;

3. Gas-inlet valve A is closed;

4. Air-inlet valve Cy Is closed;

Gas K
Air L

Chimney
Damper
Fi<«. 156.—An Arrangement of Flues and
Valves Used in the United States.
Refer to sequence of valve operation.