Double-chimney Castle

[India Four Two]

I grabbed this image from a video of Clun Castle. I had never before seen the small extra hole in the chimney. I tried ChatGPT but beyond finding out that it was related to the double-blastpipe, I couldn't find a satisfactory explanation.

Can someone explain how it works?

 

[Ashley Hill]

Isn’t that something to do with the air system? Not too au fait with steam.

 

[hexagon789]

View attachment 181555

I grabbed this image from a video of Clun Castle. I had never before seen the small extra hole in the chimney. I tried ChatGPT but beyond finding out that it was related to the double-blastpipe, I couldn't find a satisfactory explanation.

Can someone explain how it works?

Isn’t that something to do with the air system? Not too au fait with steam.

I thought it was called the "exhaust port".

A Google search to see if my memory was correct didn't find confirmation of what it was for, but I did find one reference on RMWeb calling it the "exhaust".

 

[Ashley Hill]

I just remember seeing air braked steam locos sat in platforms with what I believe is a steam operated compressor exhausting through the chimney.

 

[david_g]

Given the plume of steam from it is it the vacuum ejector exhaust?

 

[Bevan Price]

It is a device to try and improve steam flow and the exhaust system of steam locos. In some circumstances it can improve performance and/or reduce fuel consumption. Castles only started to receive double chimneys in the 1950s/ And they are not always successful. Some early LMSR Ivatt 4MT 3-6-0s were built with double chimneys, but were lated converted to single chimneys.

 

[midland1]

It is a device to try and improve steam flow and the exhaust system of steam locos. In some circumstances it can improve performance and/or reduce fuel consumption. Castles only started to receive double chimneys in the 1950s/ And they are not always successful. Some early LMSR Ivatt 4MT 3-6-0s were built with double chimneys, but were lated converted to single chimneys.

That Ivatt 4MT 3-6-0 should something to look at

 

[Gloster]

In Great Western Saints and Sinners (George Allen & Unwin, 1971) W A Tuplin comments on double chimneys ‘their effect was discernible in the attainment of higher top speeds in easy running conditions rather than in higher power when pulling hard’. H Holcroft in An Outline of Great Western Locomotive Practice 1837-1947 (Ian Allan, 1971 paperback) refers to improved performance of 25-100%, but doesn’t indicate what sort of performance and it is unclear if other factors may have also helped the improvement.

 

[Taunton]

It's the vacuum ejector exhaust port, same on the double chimney Kings. The ejector is back by the cab, the steam exhaust from it runs together with the right hand boiler handrail, visible in the photo, which is why the right hand appears larger than the left side, and into the smokebox. It commonly had wisps of steam when stationary as in the photograph, and when fully opened to blow the brakes off before departure was responsible for the "rude noise" (as in 'raspberry') that ex-GWR locos make.

GWR locos did not have two ejectors, large (blow the brakes off) and small (maintain vacuum when running), they just had one, effectively the large one, and a vacuum pump attached to the left hand crosshead to maintain vacuum continuously when running. If we are speaking about noises this is what made the "tiff" noise once per revolution, rapid staccato when coasting at speed, slow and drawn out when backing on to a train.

 

[India Four Two]

It's the vacuum ejector exhaust port, same on the double chimney Kings. The ejector is back by the cab, the steam exhaust from it runs together with the right hand boiler handrail, visible in the photo, which is why the right hand appears larger than the left side, and into the smokebox.

If that's the case, where is the exhaust port on single-chimney Castles and Kings?

 

[david_g]

There will be a pipe from the vacuum ejector which feeds into the single chimney, you can just see the vacuum ejector pipe feeding into the left hand side of the petticoat pipe in the photo below. This is on the heavily Swindonised boiler/smokebox on GWR 822, aka The Earl, Welshpool & Llanfair Light Railway No. 1.
On a larger boiler I imagine the petticoat pipe would be much shorter so the ejector exhaust will just point up the bottom of the pipe from below.

 

[Taunton]

If that's the case, where is the exhaust port on single-chimney Castles and Kings?

It is down just inside the chimney. There will have been a design reason why conventional single chimneys had it inside, mixing with the overall exhaust, whereas for double chimneys it was found necessary to pipe it separately right out to the atmosphere, possibly to avoid asymmetric amounts of steam coming from the two exhausts when the ejector was working. You would no doubt need a Swindon 1950s (when the double chimneys were designed) draughtsman (in both senses) to explain why.