Question for MR. KING: N&W Ashcroft Duplex Gauge
NW Mailing List
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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.
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
With this explanation, the utility of the gauge becomes transparent. Thank
you, Mr. Phillips!
-- abram burnett
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