Cross Compound Compressor Design I
Nelson Riedel, Nelson@NelsonsLocomotive.com
9/27/2005, last updated
01/12/2007

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 www.grandscales.com.   

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 bolster-saddle bracket  

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 previous drawing.  

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 end is 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 having "lost 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 exhaust 

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 actually achieved.  

Parameter

Prototype

1.6" Scale Model
Design
High Pressure Steam  Cylinder ID 8.5" 1.13" 1.125"
Low Pressure Steam Cylinder ID 14" 1.87" 1.75"
High Pressure Air Cylinder ID 8.25" 1.10 1.125"
Low Pressure Air Cylinder ID 13.125 1.77 1.5"
Design Steam Pressure 160 psi 100 psi 100 psi
Design Air Pressure 140 psi ? 90psi
Design Strokes per min 131 ? ?
Weight 1475 lbs ? ?

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.

    

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