Heat treatment furnaces, car type and car and ball type

              HEAT-TREATMENT FURNACES
CAR TYPE and CAR-AND-BALL TYPE
N heat-treating large and irregularly
shaped forgings, steel castings, shafts,
or miscellaneous material packed in
pots or boxes, the problem of charging
and discharging, of providing supports to permit
circulation of heat under and around each unit of
the charge and to prevent distortion, are often the
determinative factors in selecting furnaces for this
class of work.
Furnaces of the car-type, Fig. 1, or the car-and-ball
type, Fig. 2, are frequently used to meet such
conditions. Although each has a movable support
for the charge, there is quite a difference in the
design of furnace and car and in the method of
applying and utilizing the heat.
SELECTION OF FURNACE
Selection of the type of furnace and determina­tion
of size, number and arrangement of chambers
and working openings, fuel, etc., must be governed
by the nature and quantity of material to be heated,
the accuracy required in the heat-treatment opera­tion,
and the plant conditions influencing both
the method of heating and of handling.
DIFFERENCE IN DESIGN
In the car furnace the car itself forms the
hearth of the heating chamber, the top alone being
exposed to the heat. In the car-and-ball furnace
the car rests upon the furnace hearth and is entirely
exposed to the heat.
The car furnace has well defined limitations in
heating, but has certain structural advantages that
facilitate the handling of large, heavy or irregularly
Fig. 1. CAR FURNACE
Fig. 2. CAR-AND-BALL FURNACE
shaped pieces which cannot be as conveniently
handled in other furnaces of better design from the
heating standpoint.
The car-and-ball furnace, while free from most
heating limitations, has certain structural disadvan­tages
which limit the size of the individual car that
can be successfully employed.
Car furnaces are generally fired from above
the hearth. The upper part of the car, which
forms the hearth of the heating chamber, is made
of refractory material. The lower part is generally
made of structural steel and castings. The car is
provided with standard car wheels and roller bear­ings
and moves upon steel rails. The metal struc­ture
is protected from destructive action of the
heat by sand or water seals provided at the sides
and end of the car. Provision must be made for
the circulation of air under the car, which naturally
acts to cool the hearth and carry off heat.
Car furnaces have been designed with combus­tion
chambers in the body of the car; and for
the passage of heat from the chamber through
flues in the refractory section of the car. Such
construction, intended to overcome the disadvan-
Copyrijhted 1920
W. S. ROCKWELL C
November, 1920 No. 222