Question for MR. KING: N&W Ashcroft Duplex Gauge

NW Mailing List nw-mailing-list at
Wed Mar 16 11:24:12 EDT 2016


The duplex steam gauge was used only on the Mallet compound locomotives on 
N&W where a receiver pipe was used to feed high pressure exhaust steam to 
the low pressure cylinders.  On the four-cylinder 1200s and two-cylinder 
600s, there was a steam chest pressure gauge to indicate the high pressure 
steam entering the cylinders and a gauge to indicate the once expanded 
exhaust steam pressure at the nozzle under the stack.

The late Frank Collins, the road foreman on the first excursion program, 
explained to me that an engineer determined the best cutoff by looking at 
the back pressure at the exhaust nozzle on single expansion engines while on 
the Mallet compound this was determined as Mr. Phillips described.

N&W was the only user of Mallet compound in main line service after 1930 to 
the end of steam.  The explanation and use of the duplex gauge described why 
the Mallet compound was able to use its steam more efficiently with greater 
power needed for heavy trains in slower mountain work.  The 2100s did all of 
this having a boiler the size of the Class J's.

Bud Jeffries

-----Original Message----- 
From: NW Mailing List
Sent: Wednesday, March 16, 2016 9:54 AM
To: N&W Mailing List
Subject: Re: Question for MR. KING: N&W Ashcroft Duplex Gauge

Recently I posted photographs of an N&W duplex Ashcroft steam gauge from an 
N&W compound engine, asking for an interpretation.

Mr. Al Phillips of Chattanooga has furnished the answer. He is a retired 
marine boiler engineer who now volunteers at the Tennessee Valley Railroad 
Museum and has participated in the boiler work for restorations of Engines 
4449, 765, 610, 630 and others.

Here is Mr. Phillips' interpretation of the gauge:

"The black hand shows steam chest pressure to the high pressure (rear) 
cylinders, and its indication would be utilised by the engineer the same as 
it would be on a conventional two-cylinder engine. The red hand shows 
receiver pressure (High Pressure exhaust = Low Pressure inlet,) and the 
engineer could alter his cutoff accordingly. It would also show L.P. inlet 
pressure when the engine is 'simpled' while exerting the maximum starting 
tractive effort."

With this explanation, the utility of the gauge becomes transparent. Thank 
you, Mr. Phillips!

-- abram burnett

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