Boiler Types
Live steam models utilize many different varieties of boilers ranging from
the simple pot to the locomotive type. Each boiler type can give excellent
performance so long as it is operated within its design envelope. Copper
is the best material for small boilers. Brass should never be used for a
boiler barrel, but is satisfactory for fittings.
Major boiler types are discussed as follows:
Pot Type
The pot boiler, show in Figure 2, is the simplest type and consists of a
cylindrical copper tube with stayed end plates. The fire, which is
typically from an alcohol burner, is applied to the external surface of the
boiler. Its steaming ability can be significantly increased by the
addition of a smoke tube and a stainless steel shield which encloses the burner
and the lower portion of the boiler. Thus configured, the pot boiler can
be a god steam generator in moderate temperatures and mild winds.
Tank
locomotives are good candidates for pot boilers since the tanks hide the fire
shield. A suction fan and blower are not needed for firing since there is
no necessity for a forced draft. Pot boilers have large water capacity and
simple to steam. However, pot boilers tend to discolor since the burner
flame is in direct contact with the outer barrel surface. Either a wick
or vaporizing type burner can be used.
Water Tube Type (Smithies)
The water tube boiler was developed in the United Kingdom at the turn of the
century by Mr. Fred Smithies. It consist of a copper inner barrel with
stayed end plates and water tubes at the barrel's bottom, which extends from the
front to the rear. This assembly slips into an outer casing which is
shaped to resemble a locomotive boiler and firebox. There is an optimum
sizing and arrangement for the water tubes, which depends on the boiler's
dimensions.
Typically, the
tubes should be greater than 5 mm in diameter and should be space so as to allow
plenty of room for a flame path between them. Increasing the number of
tubes may not increase the boiler's ability to generate steam. If the
flame path is restricted, the boiler cannot perform well since the draft cannot
draw the flame forward toward the smoke box and increase flame temperature.
The inner surface of the outer casing is insulated with a ceramic sheet.
It is important that the insulation does not obstruct the flame path near the
throat plate.
Properly designed Smithies
boilers perform well in adverse weather conditions. If the boiler should run
dry, the lack of draft will automatically extinguish the fire. A Smithies
boiler is simple to build and operate, but it does require a suction fan and a
blower since it is a forced draft system. The biggest disadvantage of the
Smithies boiler is the limited water capacity of its inner barrel as compared to
other boiler types with similar external dimensions.
Vertical Type
The vertical boiler is a simple type which consists of a firebox at the
bottom and a copper barrel with a smoke tube. It typically is used to
drive stationary engines and boats. Firing is accomplished by alcohol or
solid fuel pellets. More sophisticated versions of the vertical boiler
contain many small tubes and are sometimes fired by coal or charcoal.
(Fig. 3)
Center Flue Type
The center flue
boiler, show in Figure 4, has a large water capacity and a low center of gravity
which makes it ideal for model boats. The center flue is surrounded by
water and sometimes has several cross tubes to improve circulations. This
type of boiler is usually fired by a gas burner, because the flame is completely
enclosed by the center flue. Therefore, the probability of an accidental fire is
reduced. It is necessary to maintain the proper water level in this type
of boiler to avoid damaging the center flue. It offers good performance
capabilities in adverse weather conditions.
Smoke Tube Type
The smoke tube
boiler, show in Figure 5, consists of a copper barrel with stayed end plates and
several curved smoke tubes extending from the front tube sheet to the rear
bottom surface of the barrel. Its chief advantage is a greater water
capacity than a comparable Smithies boiler, while retaining good performance
capability in adverse weather conditions. The sizing and number of tubes
is a function of the area of the smoke box tube plate. It is more
expensive to build than the Smithies type and requires a suction fan and blower,
since it is a forced draft system.
Type C
The type C boiler, shown in Figurate 6, was developed by J. T. Van Riemsdijk.
It consists of a copper barrel with stayed end plates and several tubes running
the length of the boiler. A stainless steel fire box with a duct is
attached to the rear of the barrel. A multi-wick burner provides the fire
which is drawn backwards by the draft, up through the duct and into the tubes.
The inner surface of the firebox and duct is lined with ceramic sheet. The
outer surface of the duct can also be covered with ceramic sheet to reduce cab
temperatures.
The type C boiler has a good water
capacity. It is easy to build and very strong. It requires a suction
fan and blower since it is forced draft system. It has excellent
performance capabilities in adverse weather conditions. Its name from the
fact that it is the follows two other small scale boilers developed by Van
Riemsdijk, the type A (center flue) and type B (smoke tube).
Mixed Type (Smoke/Water Tubes)
The illustration above shows the Mixed Type of boiler developed by Dr.
Seiichi Atonable. It has water tubes filled in the firebox of a
locomotive type boiler in such as a way that the tubes acts a stays. This
configuration also increases the heating area and provides better water circulation
than a plan locomotive type boiler. The Aster JNR D 51 and JNR 9600
utilize this type of boiler.
Locomotive Type
The Locomotive
type boiler, shown in Figure 8, consists of a copper barrel and an outer and
inner firebox attached to the rear of the barrel. Perfectly round pressure
vessels carry pressure loads by developing hoop tension stresses in the wall of
the boiler. In this case, there is no tendency for the walls to bend.
If a pressure vessel is not perfectly round, the walls will bend because the
walls will bend attempting to form a perfect circle as it pressurizes. The
results in a combined stress field with high stress levels. Since the
firebox is not circular, it is necessary to provide additional structural
members to strengthen non circular surfaces. This is the reason for stay
bolts and structural beams in the firebox assembly.
Thermal stresses also occur as a result of thermal expansion, therefore a
boiler must be designed to carry both thermal and pressure loads. In a
Belpaire type firebox, the outer wrapper and crown sheet may be stiffened by
beams or girder stayed to each other. Both side plates of the outer
firebox are cross stayed since they are flat. Flat surfaces on the back
head and smoke box tube plate can be stiffened or stayed. Sometimes a
combination of both is used resulting in a very strong boiler. If two or more
stays are used, once can be hollow and used to route the blower line to the
smoke box. Tubes extend from the front tube plate in the smoke box to the
rear tube plate, which is located at the front of the fire box.
This type of
boiler can be fired by coal if grates are installed. Liquid fuel burners
can also be employed. The locomotive type boiler is the most difficult to
build but is very efficient and allows the realism of a coal fire. It
requires a suction fan and a blower, since it is a forced draft system.
Its water capacity may be less than a comparable type C boiler. The water
level must be carefully maintained so that the crown sheet is always covered,
otherwise the boiler will be damaged.
Saddle Type
The saddle type boiler, as shown in Figure 9, is used for the Aster
Glaskasten. In order to provide a reasonable running period, it was
necessary to have a boiler with a minimum water capacity of 100 cc. Since
space was limited, a unique boiler shape was required. The saddle type
boiler consists of a large diameter copper tube with end plates and a small
cross tube. It uses features of the type C boiler as well as the Pot Type
and Cross Water Tube Type. The inclined cross tube provides both extra
water capacity and additional heating surface. They improve water
circulation and also act as stays. The arc is filled a stainless steel
firebox guiding the flame flow, so that the maximum heating area is
realized.
