Heisler Boiler Design II
Smoke Box Fittings + Throttle + Superheater
Nelson Riedel, Nelson@NelsonsLocomotive.com
12/15/2008, last updated

Update 12/27/2010: Several changes were made to this page to reflect changed made during the construction.  The changes of significance are noted. 

Throttle: The sketch below shows the overall throttle layout.   This is very similar to the arrangement in the late Heisler prototype where the throttle valve is located in the steam dome and the output steam pipe from the throttle is routed through the top of the boiler to the smoke box.  The throttle valve for the Schroeder Shay is also located in the steam dome.  The valve has been in my Shay for 6 years with no trouble.   The design shown here will allow all the throttle plumbing to be removed for maintenance.  However, the smoke box will probably have to be slid off the front of the boiler to access the throttle and some of the superheater fittings in the smoke box. 

The throttle valve is a simple 3/8" full port brass valve.   The throttle valve, nipple and the elbow are assembled outside the boiler and lowered into the steam chamber.  The long 3/8" pipe is inserted through the fitting in the front tube sheet and screwed into the elbow below the throttle valve.  The long 3/8" pipe is ~ 0.675" OD and ~0.475" ID.   A 3/4" OD sleeve with 3/4" hex on the end is soldered to the outside of the 3/8" pipe to enlarge the OD to match with the 3/4" compression fitting in the front tube sheet.  A short length of 1/2" OD tube is soldered to the inside of the pipe with about 1/2" extending beyond the end of the hex.  This tube matches with the 1/2" tube compression elbow.    The 3/4 hex can be used to grasp the pipe to screw it into the elbow below the throttle valve.  The throttle linkage will be connected after the throttle valve is installed.  

The drawing at the right shows the  throttle valve and associated hardware in more detail.  

The throttle rod is 3/16" diameter stainless steel.  The plan was to put a packing gland for the throttle rod on the outside of the steam chamber.   However, it was  observed that the rod must bend vertically about 0.3" as the throttle lever is rotated.  There is a concern that this amount of flexing might cause the packing to leak.   An alternative is to run the throttle rod through a length of 1/4" pipe with the gland at the end of the pipe.  The pipe gives the rod room to flex.

The throttle is held in position by the pipe that runs through the forward tube sheet and the bracket above the valve.  The set screw is tightened against the pipe running out the top of the valve after everything is positioned correctly.   

Note:  After the throttle valve was positioned in the model it was decided to rotate the throttle bracket 90 degrees and secure it to the left side of the steam dome instead of the back as shown on the drawings.    The fabrication of the bracket should be deferred until the throttle is installed so that it can be made to match the exact position of the valve. 

The throttle lever shown here is cut from 1/8" thick stainless steel.   The slot is cut such that the valve is off when the lever is pushed toward the front.  The bend in the lever is to align the top of the lever with the hook in the throttle rod.   
The throttle bracket is made of mild steel.  A length of 5/16" square steel is welded to the side of the seam chamber above the coupling for the throttle rod.  The bracket is secured to the 5/16" rod with a pair of 10-32 stainless steel screws.    The pipe out the top of the throttle valve is secured to the bracket with the 10-32 set screw. 

Note: As mentioned above, the bracket was secured to the left side of the steam dome rather then the back.  It was secured to a length of 1/2" angle welded to the side of the steam dome.  The bracket was made of brass.   


Superheater:  The late Heislers had superheaters so I want to have a superheater in the model.  The  following types of superheaters were considered:

  •  Coil the steam line around the inside of the smoke box to get some additional heating. The advice from several experienced live steamers was that it's not worth the effort.    

  • A second option is to run several small diameter superheater tubes down some of the boiler tubes like the prototype.  Ken Schroeder said he has had success with this technique.  He pointed out that the elbow on the end of the tube would have to be welded, not silver soldered.  That sounded like a lot of work.  However, the major objection to this scheme is that the tubes will likely be plugged with soot from my oil fire and quickly become useless.   Ken burns propane which doesn't soot the tubes.

  • The third option is to loop the steam lines through the firebox.   Ken Schroeder mentioned that he did this on one of his locomotive and it was effective.   This is the option selected.       

The sketch above shows the superheater tubes that carry the steam from the smoke box to the inside of the fire box and then back to the smoke box.   The middle tube is the input from the throttle.   The outside tubes are the output, one feeding each cylinder.    The 1/2" OD stainless steel tubes slide inside the 5/8" OD copper boiler tubes.   The NPT fittings will be assembled outside the fire box using a high temperature (1200 degree F) thread sealer.   The fittings will then be placed in the fire box, the tubes positioned in the compression fittings and the fittings then tightened.   The compression fittings are used to make it easy to assemble the superheater, not for ease of disassembly.   It is unlikely that the tubes can be removed from the compression fittings without damage.   If a superheater tube has to be removed from the boiler tube, the end of the super heater tube with the compression sleeve and nut will probably have to be cut off.  Hopefully the superheater will need to be assembled only once.   The tubes and fittings will probably glow red when there throttle is closed.   It will be necessary to put a fixture in the fire box to support the fittings.   

The sketch on the right shows the connections to the superheater in the smoke box.  The 3/8" brass pipe from the throttle valve is in the upper middle.  All the other pipes are 1/2" OD tubes with compression fittings.   One requirement is that a brush can be used on all flue tubes without removing any of the plumbing.  The top row of boiler tubes are partially behind the compression nuts but it is possible to get the brush and a thin rod past the nuts.   

The original plan was to have a 45  degree bend in the output tubes as shown in the drawing.   However, once the the boiler and smoke box were put together it was decided that it would be too difficult to make the output pipes as shown --- things are just too tight.  Instead, the superheater output tubes were run to the front of the smoke box where Us made from a pair of compression fittings connected to tubes that return to the rear of the smoke box and exit through the sides via more compression fittings.  See the Boiler Construction II page for more information.     

This shows the original design of the superheater output connection.  As mentioned above, the design was changed to run the   superheater tubes to the front of the smoke box and then loop back via Us made of pairs of compression fittings.  This extra length and additional fittings provides additional freedom to adjust things as necessary so that everything can be connected together. 
The photo at right shows the MRSR91 right side steam supply line where it leaves the smoke box toward the cylinder. (It took me a while to realize that the flat plate is the cover for the smoke stack when the locomotive is not in use.)  The elbow has a plate on the bottom which attaches to the smoke box with 4 bolts.  I plan to use a standard compression elbow with a similar plate described below.  The small pipe off the top of the elbow is combined with a similar pipe from the left side at a tee (under the cover plate behind the big elbow).  The third side of the tee connects to the rear side of the snifter valve which is a simple check valve.   The other end of the snifter valve is combined with the pump exhaust at the tee to the right of the valve.      
The drawing shows a design for the plate which will be between the steam supply elbow at the top of the smoke box and a mating compression fitting on the inside of the smoke box.  The material is a piece of 1/8" thick CFS bar stock that has been bent to match the curvature of the smoke box.   The hole for the elbow is threaded  3/8"-18 NPT to match the threads on the compression fitting.   A 1/2" tube compression to 3/8" NPT fitting on the inside of the smoke box is screwed onto the end of the male threads of the outside fitting .    The plate will be attached to the smoke box with four 6-32 screws.  A 1/4"-40 nipple will be screwed into the top of the compression elbow for the pipe to the snifter valve.        

Blower, Exhaust Nozzle & Petticoat:   

The MRSR91 petticoat is shown in photo at right.  This general design was successfully tried on my Shay.  See More Power for a description of this and other modifications made to the Shay to generate more steam.  

The relationship of the blower, exhaust and petticoat are shown in the drawing below. 

The top of the nozzle and blower ring is at the mid point of the lowest flues.  That middle tube can be cleaned by passing the brush over the nozzle.    


The plan is to fabricate the petticoat from a 1.25" black pipe nipple and a 2" to 1.25" reducing coupling.   Note that the mounting bracket for the petticoat was changed to attach to the smoke box front.   See the Boiler Construction III page.

The drawing on the right shows the cross section of the nozzle, blower ring and flange.  The nozzle and blower ring are copied from Ken Schroeder's Shay design.  The blower ring, nozzle and flange are all silver soldered together.   The flange is secured to the smoke box saddle with two screws that run through the flange and the smoke box.    Note: The O-Ring was later deemed unnecessary.
This drawing shows the details of the blower ring which will be machined from 1.5" diameter brass or bronze bar.    Note: The blower line was reduced from 1/4" to 3/16".   A compression fitting will be positioned in the line near the ring to permit disassembly.    

The blower steam line will exit the bottom of the smoke box to the front of the saddle run under the boiler to the blower pressure regulator in or below the cab.  

The nozzle shown on the right is machined from 1" OD brass or bronze.  The blower ring slides down on the exhaust nozzle and is silver soldered in place.   
The exhaust flange is made from a piece of the tube used for the smoke box.  The exhaust nozzle is silvered soldered to the flange to complete the assembly.

Feed Water Heater: The last thing that will be added to the inside of the smoke box is a feed water heater consisting of several loops of 1/4" OD copper tube.  The feed water will flow  from the axel pump on the front truck through a check valve into the smoke box, through the feed water heater and through the tube sheet into the boiler.  The feed water heater is essentially an extension of the boiler.   The exact shape of the feed water heater coil, it's position in the smoke box as well as the exact position of the water tubes is deferred until the other parts that go in the smoke box have been fabricated and installed.    

This a good point to terminate this page.    The design of the smokestack, the smoke box front & door and the remaining details of the smoke box design will be described in  subsequent design pages.  

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