Heisler Boiler Design I
Nelson Riedel, Nelson@NelsonsLocomotive.com
3/25/2006, last updated

Note: The following boiler design is for the aesthetics of a scale model.  No structural engineering had been done.  I gave this information to a competent boiler maker who will adjust the design as required to meet appropriate safety standards.

Update 2/26/10: The baffle originally shown on this page was not made as part of the boiler.  Instead, the baffle was made part of the fire pan.   This page was updated to reflect this change.

There is plenty of data on Heisler boilers in the reference sections so it would be a simple matter to create an exact scale boiler.  However, on my Shay, the limitation is the ability to generate steam so I want to make the Heisler capable of generating as much steam as possible.  The Heisler engine is about 50 % more powerful than the Shay engine so I want to be able to generate at least 50% more steam and preferably about twice as much steam.

Apparently the firebox size is the major factor determining the amount of steam generated.  Tests on the Shay revealed that there was a maximum amount of oil fuel that could be burned.  Additional fuel beyond this maximum causes the fire to shoot out any small opening like a blast furnace.  My guess is that maximum amount of fuel that can be consumed and hence the maximum steam that can be generated is proportional to the firebox volume.  

The boiler and hence the firebox width is limited by the 8.5" distance between the diamond frame members.  I'd like to use 1/4" cork insulation and have a thin jacket so the maximum width (OD) of the boiler is about 7.75"  Ken Schroeder  used 6" pipe (6.625" OD) for his Shay boiler.  Seven inch pipe (7.625" OD) could be used but doesn't appear to be readily available.  I was able to obtain a piece of 7.5" OD, 7" ID seamless steel tube.  The tube is more expensive than pipe but is also has more controlled dimensions (it's round) than the typical piece of pipe. 

With the tube diameter determined I then sketched out a number of different boiler configurations.   I ended up pretty much copying  Ken's Shay boiler design with the larger diameter and somewhat greater length.   Once the basic configuration was selected, it was adjusted to maximize the firebox size while staying pretty close to scale dimensions.  The back of the boiler is a bit over scale in length.  Most of the additional length is inside the cab.  The cab was also made a bit longer than scale. The deviation from scale is 10% or less except for the height at the back which is about 15% over scale.  Ken Schroeder's shay boiler is quite a bit over scale height and it  worked out well.   If one is concerned about the deviation from scale, the height at the rear can be cut by half inch to an inch with a corresponding reduction in the firebox height and possibly eliminating one row of tubes --- or spacing the tubes closer together.

The final firebox size is 8.125" H  X  5.5" W  X  10.75" L for a total volume of 480 cubic inches.   The Shay firebox is 7.25" H  X  4.5" W  X   8" L for a total volume of 261 cubic inches.  Hence, the Heisler firebox will be 184% of the Shay firebox.  The plan is to have both a superheater and a feed water heater that I hope will contribute at least 10% each which should get the capability to the 200% desired.

The same type 5/8" OD copper flue tubes that Ken used on the Shay boiler were selected.  Ken specified 21 tubes, each with length of 14.75" between the tube sheets for a total effective length of 310".  My Shay boiler has a slightly different configuration with a total of 25 tubes at 14.75" between the tube sheets for a total length of 369".  The Heisler boiler will have 31 tubes but 3 will be used for the superheater so it will have 28 effective tubes, each with length 20.75" between the tube sheets for a total effective length of  581".  This works out to 187% of Ken's design and  157% of my Shay boiler.  I suspect this is more than adequate, especially if I work on the burner design to minimize the total air flow through the firebox and tubes.

The later Heisler throttle design with a valve in the steam chamber was copied.  The output of the valve goes into a pipe that runs above the flue tubes, through the upper part of the front tube sheet into the smoke box where it connects to the superheater.  The valve is controlled by a rod that exits the rear of the steam chamber and runs along the top of the boiler into the cab, just like the prototype.

The smoke box on Ken's Shay boiler was merely a continuation of the boiler tube (6" pipe)  When the boiler was lagged and jacketed the boiler OD was 1/2" larger diameter than the smoke box diameter, similar to the prototype Shay.  The Heisler smoke box has a larger diameter than the boiler. When the lagging is added to the boiler the OD becomes the same as the smoke box and the jacket covers the rear edge of the smoke box and continues on back over the boiler lagging.  I followed this part of the prototype design by making the smoke box separate from the boiler.  The smoke box will be a length of 8" OD - 7.5" ID Tube.   The smoke box will be attached to the saddle which in turn will attach to the frame.  The front inch or so of the boiler will slide into the smoke box tube but not be fastened.  The rear of the boiler will set on brackets attached to the frame. 

The Shay steam turret is supplied via a 1/4" NPT coupling welded into the top rear of the boiler.   One problem I have with this arrangement  is getting water in the turret when going uphill.  My operating technique is to try to start up long hills with a full load of  water and full pressure.  I can stop water input and use all heat to generate steam on the up slope and then refill the boiler on the flat or the downhill side.    The up slope causes the water level in the rear to rise and, if the fireman added too much water, some goes into the steam turret.   For the Heisler,  I decided to supply the turret from the steam chamber via a 1/4" pipe along the top of the boiler.   The steam chamber is more near the center and hence not as effected by the change in slope and it is of course higher than the rest of the boiler. With this arrangement a shutoff valve can be inserted between the steam chamber and the turret --- a feature that several experienced live steamers have suggested.    

The drawing above gives the overall dimensions of the boiler plus smoke box. 

 The next drawing shows the side of the boiler (without the smoke box).  

Boiler Outside:  The outside surface of the boiler is made up of 13 pieces  Twelve of the pieces are welded together and one, the steam chamber top, screwed in place.  The next drawing identifies the 11of the pieces.  Note that two of pieces D and E are required bringing the total to 13 pieces. .


Pieces A, B and C are cut from a 32" length of 7.5" OD - 7.0" ID  seamless steel tube as shown in the following drawing.      



The following set of drawings show the other eight individual pieces.


Boiler Forward Side:  These pieces are cut from 1/4" steel plate.

Boiler Rear Side:  These pieces are cut from 1/4' steel plate.
Boiler Back:  The boiler back is cut from 1/4" steel plate.

Boiler Front:  The boiler front is cut from 1/4" steel plate.

Steam Chamber: The steam chamber is a piece of 3" schedule 80 pipe.  The holes for the coupling that are welded into the back of the chamber must be cut, drilled or bored such that the axis of the couplings are parallel to the boiler axis.  
Steam Chamber Top: The steam chamber top is cut from 1/4" steel plate.  Additional holes will be drilled and tapped for the two relief valves.

Front Tube Sheet: The front tube sheet is cut from 1/2" steel plate.  The same tube hole pattern is used in the rear tube sheet.  The tubes are 1/2" (5/8" OD) type K copper water pipe. The tubes are rolled or swaged  into the tube sheets.
Firebox Door Sleeve: The firebox door sleeve is a piece of 2" OD - 1.5" ID steel tube.  The tube is welded to the boiler back with ~1/4" sticking out. 

Firebox: The next drawing shows the firebox. 


Firebox Front: The firebox front is cut from 1/2' thick steel plate.  The flue tube hole pattern is identical to the pattern in the the front tube sheet.
Firebox Sides: The firebox sides are simple 1/4" thick steel rectangles.
Firebox Back: The firebox back is cut from 1/4" steel plate.  The firebox door sleeve slides through the hole and is welded in place.
Firebox Top: The firebox top (crown sheet)  is a simple 1/4" thick rectangle.
Girders: The girders are made from 1/2" X 2" steel bar. The boiler maker said he'd cut a couple notches in the bottom to improve water circulation. 

The drawings above show the boiler end views as well as a section through the firebox.  Note that the gap between the firebox and the boiler is 1/2".   There are 1/8" NPT clean out plugs at each end of the mud ring.  The fifth hole in the center of the front side is for the blowdown valve.

The drawing above shows a possible stay bolt configuration --- 3/8" bolts on 2" centers. 

This sketch shows the relationship of the frame, engine, boiler and cab. The 12.75" boiler height includes the 1/4" of lagging. Everything seems to fit together.  

The boiler maker has finished the boiler and this page has been updated to reflect the changes he made.       

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