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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.  

 

 

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