Compressor Design I
Nelson Riedel, Nelson@NelsonsLocomotive.com
9/27/2005, last updated
The plan is to use a steam powered air compressor to
supply air to the brake system. Two different types of
compressors were made by Westinghouse. The first produced
was a single stage pump consisting of a single steam piston driving a
single compressor piston. The cross compound compressor with
a double set of pistons was the later design. The cross compound
steam cylinders are connected such that the exhaust from the smaller
high pressure cylinder drives the larger low pressure cylinder.
The compressor is a two stage design where the piston of the large low
pressure (input) air cylinder is driven by the small high pressure steam
piston. The output of the low pressure air cylinder feeds into the smaller
second stage high pressure air cylinder which is powered by the large
low pressure steam piston. Both the single stage and the
cross compound designs were
available is various sizes.
The 8 1/2" cross compound is the compressor
on MRSR91. This page describes the operation and the pump and
design decisions such as exact piston sizes and the lubrication scheme.
A primary source of information for this page was a Westinghouse
Instruction Pamphlet which can be downloaded from
|Cass 6: Cass 6 uses the single stage compressor shown on
the right. This is the same compressor that was modeled as a
water pump for the Shay. See
Air Compressor/Water Pump Part I and subsequent parts.
|MRSR91: MRSR91 uses the 8 1/2" cross compound compressor
shown on the right. This pump is identical to the first pump
described in that Westinghouse Instruction Pamphlet.
Note that the cover and probably some lagging have been removed from
the steam cylinders.
|Mount: This photo of a smaller late Heisler frame shows
the air compressor mount that is cast as part of the right side
|8 1/2" Cross Compound Compressor: This and the
subsequent drawings are from the Westinghouse Instruction Pamphlet.
The steam cylinders are at the top and the air compressor cylinders
are on the bottom. The high pressure steam and low pressure
air cylinders are on the right.
The unit on the upper right is a pneumatically driven mechanical
lubricator. A couple cylinders on the Heisler mechanical lubricator
will be dedicated to the compressor; one feeding the steam cylinders and
the other feeding the compressor cylinders.
The canister on the lower right is the air input filter.
|This drawing shows the back of the compressor. Some
of the subsequent drawings suggest that the two steam cylinders
don't quite meet in the center. The same is suggested for the
two air cylinders. This sketch shows that the side by side
cylinders are joined in the center.
The compressor is made up of six basic subunits (excluding the
lubricator). The top unit is the Upper Steam Head which also
contains the two steam valves. The next unit down is the Steam
Cylinders. The middle unit is the Center Piece which
includes the lower steam head and the upper air head. The next
unit is the Air Cylinders. The bottom unit is the Lower Air
Head. The Air Filter unit is off to the side as shown in the
|Reversing Valve: This cross section drawing is intended
to highlight the reversing valve which is driven by the high
pressure steam piston in the upper right cylinder. The reversing
valve is a piston type valve. The valve shown has four pair of
rings. Only two pair of rings are required for the reversing
function. The extra two pair of rings are used for a
cushioning function for the main valve. This cushioning
will be left out of our model.
The reversing valve is driven by the valve stem. The stem
inside a hollow part of the high pressure steam piston rod.
The reversing valve piston is pushed to it's upper position when the
high pressure steam piston is at it's upper most position. The
reversing valve piston is pulled down to it's lower most position when the
high pressure piston is at it's lower most position. In other
words, the valve only moves when the high pressure steam piston is
near it's two extreme positions. The valve piston having
much less motion than the steam piston is referred to as to
motion" which is typical of reciprocating steam devices.
|Main Valve: This side view of the compressor shows a
cross section of the main valve. The main valve has a total of
five pair of rings. (The rings are fitted in pairs with each
pair forming one sealing unit.) Four pairs of the rings
are the same size while the fifth pair enclose a part of the
valve piston (on the right end) with twice the cross sectional area
of each of the the four smaller pairs.
The left end of the valve cylinder is connected to the exhaust
which is at atmospheric pressure. The left side of the the larger
section of the right end of the piston is at the input steam
pressure. This pressure exerts a force to the left on
the small part of the valve piston and twice the force to the right
on the larger piston. The pressure in the right (larger) end
of the valve cylinder is controlled by the reversing valve.
When the reversing valve opens the right end to the exhaust, the
force to the right on the large part of the piston will overcome the
force to the left on the small part of the piston and the valve
piston will move to the right. When the reversing valve
supplies the input steam pressure to the right end of the cylinder,
steam pressure on the two sides of the large part of the piston will
be balance while the pressure on the small part of the piston will
cause the valve piston to move to the left.
|The conceptual drawing (Plate 1) on the right has the main valve
rotated so that the valve position can be related to the piston
motion. The drawing depicts the situation just after the
high pressure steam piston starts the down stroke.
The valve piston is in the left position. Steam is supplied
to valve chamber A' and though passage c to the upper end of the
high pressure steam cylinder forcing the piston down.
Steam from the bottom of the high pressure cylinder travels to valve
chamber F and then down to the bottom of the low pressure steam
chamber forcing the low pressure piston up. The top of
the low pressure cylinder is vented via valve chamber D to the
On the compressor side, outside air enters the top part of the
the low pressure side via a check valve. Air from the bottom
of the low pressure cylinder goes via a check valve to the bottom of
the high pressure air cylinder. Compressed air exits the top
of the high pressure air cylinder via a check valve to the air
discharge on the left side of the compressor.
|Plate 2 on the right shows the situation after the high
pressure steam piston has reached the bottom of the cylinder and has
pulled the reversing valve piston down. The right side
of the main valve cylinder has been switched from input steam pressure
to exhaust pressure and the main valve piston has moved to the right
end of the valve cylinder.
Steam will enter the bottom of the high pressure cylinder via
valve chamber A causing the high pressure steam piston to move
up. Exhaust from the top of the high pressure cylinder is
pushed into the top of the low pressure steam cylinder via valve
chamber D causing the low pressure steam piston to move down.
Exhaust from the bottom of the low pressure steam cylinder exits via
valve chamber F to the steam exhaust port.
Air enters the bottom of the low pressure air cylinder via a
check valve. Air in the upper end of the low pressure
air cylinder is forced to the upper end of the high pressure air
cylinder via a check valve.. Air from the lower end of
the high pressure air cylinder is forced into the air discharge port
via a check valve.
The general data for two versions of the 8 1/2"
compressor are listed in the the Westinghouse Instruction Pamphlet.
The one listed below is designed to run at the lower working pressure of
160 psi. The initial model design used 1.75" ID for the low
pressure air cylinder matching the low pressure steam cylinder.
There was a problem with the operation once the output pressure reached
about 50 psi. This problem went away after the low pressure air
cylinder diameter was reduced to 1.5" as shown in the chart. The
design air pressure was calculated to be about 90 psi which was
|High Pressure Steam Cylinder ID
|Low Pressure Steam Cylinder ID
|High Pressure Air Cylinder ID
|Low Pressure Air Cylinder ID
|Design Steam Pressure
|Design Air Pressure
|Design Strokes per min
The prototype compressor is equipped with a switch
that turns the steam input on and off as necessary to maintain the
pressure within preset limits. The initial plan for the model
was to let the compressor run until it stalls. This is the simple
solution and will cause no operational problems if there are no leaks.
When the air pressure drops the compressor should start up and build the
pressure back to the stall pressure. After building a model using
the "run till stall" operation I decided to make a pressure activated
switch to cycle the compressor. The design of the switch is
described in Part V.
The details of the model compressor design are in the
subsequent three design WebPages.