The Steamboating Forum

What is meant by "triple reheat"? Is this actually triple expansion? I understand triple expansion but not triple reheat. I was stated the boat would make 40knots. Really? Why? How is it so powerful? My 1914 Navy k runs better with the starting valve open. Maybe i don't have near enough pressure (90psi) to effectively "re-expand".

The engines used superheaters to re-heat the exhaust steam between expansions. I.e: HP-reheat-1stIP-reheat-2nd IP- reheat- LP

Every time the steam expands it drops in temperature, thus loosing efficiency. By re-heating between expansions it made the plant more thermally efficient, thus extracting every last ounce of power they could out of the engines.

She could have been faster if they had let them test at the designed pressure of 440psi and hadn't pulled the safeties 1/3 Of the way into the trial. Were you able to view the article?

Your 90 psi is pretty low for your navy k to truly take full advantage of the efficiency gains that compounding provides. I'd say 150psi is a much more realistic number.

Thanks, I found the article and I am quite amazed. Being a mechanical engineer, I "thought" i knew a thing or two. But, this engine operation confounds me.
What in the world could allow the heat transfer to the expanding steam at such a high rate as it seems it would need be between piston strokes. What speed (rpm) were these engines? Perhaps they were much slower than i imagine they were.

They didn't just heat up the outside of the receiver (the connecting pipe between two cylinders). I'm sure there was a fairly sophisticated heat exchanger with at least superheated steam on the high temp side if not being an actual superheater exposed to the fire.

In the some later compounds the intermediate stage(s) were turbines with power taken to a separate shaft. Maybe even tied in to the shaft powered by the reciprocating engine. I'm not an expert on this stuff at all. Others here will have the straight dope.

The ARROW had quadruple expansion engines, with steam reheated three times. Reheat was accomplished with superheater tubing in the fired boiler, and hot flue gas reheated the steam between each expansion/cylinder.

I have worked in the utility steam power industry for over 40 years, and 99% of the big (over 150,000 horsepower) generating stations are "single reheat". Main steam issues from the boiler well superheated, passes thru the high pressure turbine, then returns to the boiler for reheating, then passes thru the intermediate pressure turbine, and finally thru the low pressure turbine. The end of expansion usually involves wet steam, around 10% wetness fraction.

In all those years I have only worked on one "double reheat" unit, (NYC Hudson station, about 1,000,000 horsepower) and was very surprised to discover the ARROW with "tri[ple reheat. It does make sense tho, because on the turbine plants each reheat stage brings the steam up to about 1000F, far too high for a reciprocating steam engine. I am sure the reheat stages on the ARROW brought the steam only to around 600F, so more frequent trips back to the boiler were made for the life of the reciprocating engine.

I think the ARROW ran at several hundred RPM while being pushed for speed, probably 500 - 600 RPM. Note that the steam exhausting from a cylinder did not need to immediately enter the next cylinder, instead it was fed all the way back to the boiler, passing thru a large number of reheat tubes inside the boiler casing,then back to the next expansion cylinder. The reheat steam flow had plenty if time to get reheated, this process is independent of the engine's RPM.

"it was fed all the way back to the boiler, passing thru a large number of reheat tubes inside the boiler casing,then back to the next expansion cylinder."

I didn't have this concept at all. I thought it was time dependent between pulses.
As i see it this system is capable of a higher pressure in the secondary without having a higher pressure in the primary. But, the additional plumbing to accomplish it would be very complex.

Why would this system convert more energy from the boiler than simply having a higher primary pressure?

Only so much heat you can put into a reciprocating engine without getting in to serious ring and cylinder wall problems. That's one of the lesser reasons that turbines took over.

So glad that my little launch is running on steam that would qualify as a muggy day in Houston.

One way of using reheat that is relatively simple is to use a superheater in the boiler, but first use the superheated steam to warm the steam leaving the high pressure cylinder. Another approach is to steam jacket the high pressure cylinder, but if you want to avoid lubrication issues, the reheat method is ideal. I'd have to run the numbers, but it would seem ideal to have sufficient superheat to allow the low pressure steam to be reheated to a mild superheat, and the high pressure steam cooled to a similar point. This would minimize the effects of condensation, and keep steam conditions mild enough that lubrication would not be necessary.

- Bart

Time to bring up the Temperature - Entropy Diagram for Steam. The steam properties, and thus how steam behaves during expansion and energy extraction from the steam is unique to this substance, and was figured out very well at the close of the 19th century. It will take some time to dig into this from an analysis standpoint.

fredrosse wrote:Time to bring up the Temperature - Entropy Diagram for Steam. The steam properties, and thus how steam behaves during expansion and energy extraction from the steam is unique to this substance, and was figured out very well at the close of the 19th century. It will take some time to dig into this from an analysis standpoint.

The tricky bit is estimating steam conditions after expansion in the high pressure cylinder. One could assume adiabatic expansion, but that's likely to over-estimate steam quality significantly, esp. after passing through a slide or piston valve.

A set of steam tables in a spreadsheet would help here, since the calculation is rather iterative.

- Bart.