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Garlicdesign
Post subject: Technical question to our resident engineersPosted: January 17th, 2020, 8:56 pm
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Hi all!

Maybe it's a dumb question (nobody ever tried it in reality), but - would it be possible to create enough steam for the catapults of a CATOBAR carrier, late 80s/early 90s tech level, by heating water with the exhaust heat of a 120 kshp gas turbine plant? I've no idea how much steam you really need for continuous operation of two steam catapults.

Thanks in advance

Greetings
GD


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acelanceloet
Post subject: Re: Technical question to our resident engineersPosted: January 17th, 2020, 11:09 pm
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Quite an technical question indeed!
If we assume a gas turbine, say an LM2500, of about 20k shp of power, has an thermal efficiency of 30%. Then we can assume about 60% of the energy comes out as heat, of which we might be able to harvest another 20% or so? (maybe more but let's assume now for a quick estimation)

That means 8000 hp of heat could be gathered into steam from a single LM2500. For a full powerplant as you describe (6 of these) that means 48000 hp

Well, a C7 steam catapult can push 40000 pounds to 171 mph. A quick calculation leads me to believe that ballpark 20k shp is needed for a single launch. 40k shp would be needed for 2 catapults. That means by my balpark estimation, it should be possible to operate steam catapults from the steam generated by waste heat boilers on top of gas turbines, as the catapults would be recharged in seconds while being fired far less then that.

That said, all this is based on assumptions on the efficiency of these systems, you will have far less control then with a separate boiler and the first regenerative heat system on a navy ship operational is on the T45 destroyer IIRC, quite a few years after early 90's. It was proposed for the burkes, which might fit..... but never installed due to difficulties. So in other words, not impossible, but it would be high risk.

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Zen9
Post subject: Re: Technical question to our resident engineersPosted: January 18th, 2020, 12:10 am
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Garlicdesign wrote: *
Hi all!

Maybe it's a dumb question (nobody ever tried it in reality), but - would it be possible to create enough steam for the catapults of a CATOBAR carrier, late 80s/early 90s tech level, by heating water with the exhaust heat of a 120 kshp gas turbine plant? I've no idea how much steam you really need for continuous operation of two steam catapults.

Thanks in advance

Greetings
GD
Certainly the exhaust temperature of a GT is well above the temperature of steam catapult systems. So it's entirely feasible to heat water up to the desired figures.

Frankly it's even more feasible to pre-heat the water/steam with the waste heat, and then take the resultant steam up to the final pressure and heat by additional means.


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Garlicdesign
Post subject: Re: Technical question to our resident engineersPosted: January 18th, 2020, 11:56 am
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Hi and thanks for the prompt answer.

Doesn't sound too bad - it's for an AU carrier, so I'll just run with it. BTW, the earliest warship I've read about with a main gas turbine plant plus exhaust steam turbine is (according to Navypedia) the Soviet Slava-class missile cruiser built 1976-9, although in her case only 10% additional power is gained.

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GD


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Zen9
Post subject: Re: Technical question to our resident engineersPosted: January 18th, 2020, 2:00 pm
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Yes but that was for propulsive power. 10% isn't bad though.


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heuhen
Post subject: Re: Technical question to our resident engineersPosted: January 18th, 2020, 2:51 pm
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Steam

I wonder how much more effect you can get out of the system, by using fluid with low boiling point. But a liquid that boil fast, for that steam...


just me, rambling again!!!


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Hood
Post subject: Re: Technical question to our resident engineersPosted: January 19th, 2020, 10:43 am
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Some data on the British BS.4 catapult:
1,000lb of steam at 500psi was required for less than 2 seconds for each launch.
With a launch rate of every 30 seconds the ship's boilers needed to supply 120,000lb/hr. The steam receivers had2,385cuft capacity to act a slight buffer in supply.

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acelanceloet
Post subject: Re: Technical question to our resident engineersPosted: January 19th, 2020, 11:21 am
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Zen9 wrote: *
Certainly the exhaust temperature of a GT is well above the temperature of steam catapult systems. So it's entirely feasible to heat water up to the desired figures.

Frankly it's even more feasible to pre-heat the water/steam with the waste heat, and then take the resultant steam up to the final pressure and heat by additional means.
My oven in my kitchen can be set hotter then the temperature of steam catapult systems. Can I add an steam catapult to that now? (The answer is yes, but it would take quite a long time to gather enough steam, so long that the isolation of the steam tank would become quite important to not loose the energy faster then gathering it)

Sorry to say, but just looking at the temperature is a massive oversimplification, to the point of being useless.

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