This was posted to the Stockton Locomotive Works Facebook page yesterday:

"SLW is now in possession of a 1906 Porter built former compressed air locomotive! Gifted to us by a friend, this locomotive is a dandy size for any number of circumstances! Stay tuned for more developments surrounding this diminutive member of the fleet!"

https://www.facebook.com/Stocktonlocomotiveworks for more photos!

Do you notice that the cylinders have cooling fins?

Seems to me that those fins would be more accurately described as "warming fins". ;>)

Keep in mind that the compressed air would be expanding in those cylinders, so without added heat it would likely get quite cold in there.

Compressed air locomotives generally run at very high pressures and compressed air tends to generate heat, lots of it. It has always been my understanding that the fins are indeed to cool the cylinders.

Compressing a gas is an exothermic action (generates heat), hence the cooling lines that follow the output of a locomotive air compressor, and on the other hand allowing it to expand and depresserize will produce an endothermic (cooling) action, especially when flowing from extremely high pressure to low. These compressed air locos ran at a minimum of a few hundred psi. An innert gas regulator for a welder (taking ~2500psi gas to only a handful of psi's) when turned high enough on a humid day will quickly become cold to the touch, with water condensing on it and in my experience even forming icicles sometimes. I'd wager the fins were part of helping keep the cylinders warm as the depresserizing gas cooled them down.

-Sam

An interesting page on various air locomotives here:
http://www.douglas-self.com/MUSEUM/LOCO ... irloco.htm

There are references to various methods to deal with the problem of the "cold air" resulting from the air expansion within the cylinders. In the case of 'double expansion' locomotives, often there is a "reheater" between the high and low pressure cylinders. In some cases the reheater is just an array of piping exposed to ambient air. In some air locomotives the reheater is actually a coal-fired heater!!

This particular locomotive is quite interesting:

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Boy that heater would negate the usual reason why a plant got a compressed air locomotive. As they tended to be used in combustion sensitive areas, i.e. near explosive compounds, explosive gasses, combustible dusts (coal, wood, fibers, silos, milled grains, etc...).

So, which Porter is this?