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