Cost of Superheat, is it justifiable?

Can anyone help my analysis? (forgive terminology errors) Let's say we have water @ 500 f 1200 psia. Now we release it until we hit 425 f 325 psia saturated steam (not sure how to predict quality at this point). Now let's expand it 2.5 times. This expansion will suck down the temperature until the steam is a 100% vapor and then temperature will depend on thermal losses, right? Now let's say we gained a little heat by compression. That should help aid the point at which 100% vapor is reached, ? So further expansion will takes us further away from saturation if we aren't loosing a lot of heat elsewhere, correct?

So my question is, can I get away from saturated steam without superheat? Why spend the $$?

Put aside all the math and calculations.
If you have ever run your car with bad gas and found you couldn’t get the power or RPM
Or if you ever went from regular gas at the drag strip and then used some nitro, and experienced the difference. That’s why you use superheat.

When you run any kind of receipting steam plant you always want superheat, 200F over saturated temperature is the magic number up to 750F; unless you are sure you have oil; that will handle higher temperature. And I don’t know of any.

If you go back to your calculations and look at the volume of expansion over 200F from saturated for superheat it is just not worth the cost or gain and the size of the superheater to build into the boiler.
Rolly

Attached photo of 60-foot 316L superheater I used in my Derr boiler.
It maintained 750F constant.

I was kind of lacking in my last post.
The real advantage comes as can you save as much as 20-30% of the water you need to pump through the boiler, depending on the engine, weather it is single expansion, compounded or condensing.
That’s means you save engine power from not pumping the extra water, you save fuel as you don’t have to boil the extra water and you save on the condensing system, both in size and any pumping of the extra water.
Rolly

But why says the dumb one? Isn"t Harry's engine hanging out in saturated/water lube area? I want to use oil, be not saturated, but quite a few reasons to try and stay below 500 F. But I need 320 psi...sigh (added, I also don't care if I drop to 120 psi at the 2.5 expansion, guess the question is will I loose the 83 deg and not get unsaturated)

Looks like you snuck in a why while I wasn't looking. All my numbers say water requirements and condenser capacity won't be a problem at all. Am I wrong thinking I won't be saturated in my first statement?



Edited 3 time(s). Last edit at 11/21/2009 12:10PM by kdc2.

Another note, it looks impossible to predict fluid density without some degree of superheat with any accuracy.

kdc2,

I'm not sure what you are trying to achieve with this exercise.
Steam at 325psi and 500 deg F is superheated and will run a suitably sized engine. You need to spend more money on generator tubes and fuel if you don't have a superheater (as Rolly indicates).

If you want to really save money, don't mess with steam in the first place, unless you have a free fuel source and all the time in the world to build the system to take advantage of it.

Graeme

Quote
Rolly
Put aside all the math and calculations.

I have to agree with Rolly (and Graeme) here, and put my two cents worth in also. Here's some observations from an old steam car nut:

1. Given a choice between more superheat or more pressure--superheat wins out every time. I could run my car on 50psi and 600F with more power than 600psi saturated. Of course, it ran best with both design pressure and temp.

2, Most "good", "efficient" steam engines extract a lot of heat from the working fluid; this means that without adequate superheat the steam will fall below the saturated line.

3. An interesting thing to do is to run an engine on compressed air; the engine gets so cold that moisture will condense out of the surrounding atmosphere on the outside of the engine, and there's a potential for frost to form. IMO this is a perfect vision of the conversion of heat to mechanical work.

4. On my car, the exhaust fans (steam powered) would really roar when the steam was still acquiring its superheat on first drive-off; once warmed up it was obvious how much less water/steam was being used to produce a given level of power. I would estimate the reduction to be as much as 4-5x, or more. I'm sure this varies with the engine type and expansion ratio.

Experiencing the effects of superheat first-hand will make you an instant believer!!

Bill

But why says the dumb one?
Kdc2
What are you an Ignoramus , where in my post did I call anyone dumb. I have enough to fill your mouth a thousand times.
You asked a question and I gave you an honest answer, based on more then fifty years of building engines boilers and steam plants. Don’t put words in my post.
Rolly

HUH,,,,,,,,,,Saturated is a [balence ] ,,,state,,,,,,on one side,,,Water,,,on the other side ,,Superheat,,,,BALENCE in the middle,,,,If there is no superheat,,,there will inevitably be condensation on expansion,,[and wet condensation will cool the cylender faster than dry steam][just a side],,Now remember how much larger that cu ins of water was back before it condensed,,,think ,,Black Hole,,,haha,,,Refer to Keenan and Keyes,,[Prof,Frederick G,Keyes,,Thermodynamic properties of Steam[1936?]] Dr Keyes loved cars too,,,Ben

I presume you will not add heat to the expansion cylender [area],or between stages,,,,,#####,,,To answer your initial question,,,Without superheat,what you have is Saturated,,,,HUH,,,,,whadddimiss,,,not sure,,,Ben

Before I read the replies, I called myself dumb because I don't know this stuff very well yet and need to ask possible dumb questions as I do need to get it right the first time. Sorry to imply anything else Rolly. Need to come back in an hour.

I believe the only reason to run saturated in this [these] centuary is to keep temp/press down for the nuc plants,,and that is DEFINATELY not my area of ,,,,er,,concern,,,Superheat is only way to go,,,,Stanley found this to be true in 1901,,,,Ben

kdc2 asked, So my question is, can I get away from saturated steam without superheat? Why spend the $$?
First question; No, by definition if it is not wet or saturated steam it is superheated without regard for how it is arrived at.(some turbine powerplants expand water from the boiler to superheated steam at the entrance to the turbine) I think the question you wanted to ask is "Can I expand higher pressure saturated steam instead of adding a superheater." yes...but the superheater is less expensive. Higher pressure boilers are more expensive to build. Higher pressure boilers are heavier. Higher pressure boilers are more expensive to maintain. Expanding steam to a superheated state is difficult to control. On the other hand, superheaters are simple,(see Rolly's picture) inexpensive, and can be added to any boiler. D. Smith

Appreciate all the replies, thanks!

Whew. I think I was trying to portray a purposeful dedication to superheat via a heat system added to increase heat via heat exchanger techologies vs gaining an unsaturated state by use of compression gains of heat though its simple function and an expansion curve that would not let it return to or stay saturated. I have seen through calculations that certainly enough compression can be designed into the system, at a loss of net hp to yield superheat conditions. It doesn't seem plausable to go that route..too much hp loss due to an early compression in the (otherwise) power stroke.

Again, I really want to appologize to Rolly for any indication that dumb was not self implied. I respect all your inputs. I have vast experiences in many fields, but steam is a new thing for me. I think there is a modern place for the return of steam power. (added: to transportation, I know that steam is used in ablout 80% of the worlds power generation)



Edited 1 time(s). Last edit at 11/21/2009 09:26PM by kdc2.

You all have convinced me. Added a superheater. Due to all the other complications it is a separate dedicated device. Good though, will lend itself to variability and data collection until it can be packaged properly into the main exchanger. Thanks for the input!! Keith

Rolly, My old books say you should not go below 85% saturated at end of expansion. That may be because it takes some time to condense. During expansion into the saturated region you would have a sub-cooled vapor for a short period of time. The IAPWS-95 scientific steam property formulations ar good for calculating the properties of sub-cooled vapors. I however, have not been able to find any info on figuring how long the sub-cooled vapor takes to chang to a saturated mixture.

On the other hand when debating the merits of a non-expanding engine. I found super-heat makes a drastic difference to its efficiency. The max being about 9%. More then double of what could be obtained on saturated steam. Most piston engines operating at substantially above full expansion would be getting a good percentage of their power non-expanding. I.E.

NonExpansiveWork = (Pi - Px)*Vi + (Pe - Px)*(Ve - Vi)
ExpansiveWork = Ui - Ue - Pe*(Ve - Vi)

I am not discounting anyone's experience. But if we are going to advance steam technology it is very important to have theory correctly explain and agree with observed experience.

In this case, at high RPM, full expansion to a sub cooled vapor state might be different then observed experience of partial expansion engines.

I would like input on this case. How long does it take a sub-cooled vapor to condense. What RPM would condensation of a sub-cooled vapor be avoided.

Andy

November 22, 2009 In agreement with Ben and having had this explained to me several times by Dave Nergaard, the reason for superheat is because you do not want any condensation at any time in the cylinder because of the great volume lost when steam turns back into water. This is one of the many truisms about steam that old time experienced people know and that takes a while for new people to catch onto. Tom Kimmel

Hi All,

Uknow its kind of interesting to note, after reading the replies, that I was thinking about Jerrys recent discussions about a zero-clearance zero-compression engines.

My first reaction to Andys reply was "ooooop" expansion all steam engines have expansion in the working cylinder, then I pondered for a moment stevenson link motion, it was designed to do two things.

1 replace a ball-valve type of throttle,
2 and allow the enginee to conserve steam in the boiler under low load conditions, such as coasting down an incline.

Its true, variable admission cuf-off can infact, do these things.

Then I looked again at Andy suggestions for clasifications, and thought about Jerrys...

So lets say we could classify engines this way.

A. non-expanding engine

B. partial-expanding engine

C. full-expanding engine

Im now wondering if ZC ZC could intersect such definitions, or at the very least, a mode of operation?


Jeremy

Jeremy

A donkey pump is an example of a non-expanding engine.

A while back we had a fellow advocating a non-expanding engine design operating at 40 PSI . Of course the efficiency would suck. And that was my position. But in making my point I wonted to find the max efficiency a non-expanding engine could obtain. I don't remember exactly the specific conditions. I think it was around 480 PSIA with around 200F super heat. That agrees with what Rolly said about super heat of 200F. The max efficiency peaked at around 200F over the pressure range I analyzed.

Andy



Edited 1 time(s). Last edit at 11/22/2009 11:30PM by steamerandy.

Most piston engines operating at substantially above full expansion would be getting a good percentage of their power non-expanding.
Andy

True Andy
Most of the engines out there use eccentrics for their valve movement. The geometry of an eccentric is not the best for expansion. The percentage of expansion in one cylinder is small. On the other hand if you compound into a second cylinder with eccentric operated valves you can use most of the whole second cylinder for expansion.
Rolly

Jeremy:

"So lets say we could classify engines this way.

A. non-expanding engine

B. partial-expanding engine

C. full-expanding engine

Im now wondering if ZC ZC could intersect such definitions, or at the very least, a mode of operation?"

The three types of Rankine cycles are described in older themodynamics books. Not my idea.

A. The non-expansion cycle would apply to a donkey pump that admits full stroke.

B, The partial-expanding cycle would apply to a normal piston engine

C. The full expansion cycle generally applies to a turbine. But coule apply to a piston engine.

Note, thoes are text book Rankine cycle not engines. They do not include any clerance or compression so they are ZCZC.

I do not believe it is pritical to have zero clearance.

Andy



Edited 1 time(s). Last edit at 11/27/2009 12:16PM by steamerandy.

Now that you all convinced me that superheat is needed, can anyone help with a general conceptual?

If we flash 40 lb/m of water @ 1200 psi/500f into a superheater peaking at 320 psi, 1) would there be better quality steam at higher/lower pressure? 2) how much heat is lost initially that has to be made up? My parameters say I should target 320 psi w/only about 50 f superheat for the system as a whole at superheat discharge. I'm a bit lacking in heat loss data. Built lots of industrial refrigeration systems and I'm quite concerned that this release will be a pretty effective cooling system.

It takes nearly the same amount of BTU's to make saturated steam at any pressure. The BTU's you put into the liquid to reach the saturation point increases with pressure. While the heat of vaporization decreases.

Going from 1200 PSI 500F to 320 PSI would result in about 25% quality saturated mix. You would need another 700 BTU/lb to get to 100% saturated steam. Then at that point you would begin super heating the steam.

Andy

Thanks for the reply. So does that mean I need 1200 f supercritical to have 100% saturated @ 320 psi before superheat? Having a tough time grasping this.

Kdc2
Where are you getting this Supercritical stuff
Supercritical steam starts at 3200 psi. around 705 F Not the same as saturated steam.
A fired pressure vessel at 320 PSI is 423F saturated steam. Look at your steam tables. You want to take this steam and heat it 200 F higher in a superheater coil to 623 F for a good performing running engine. You also want to inject steam cylinder oil into any reciprocating piston engine.
Rolly

By forgetting that adding 700 btu/lb looses the definition of raising each lb by 1 deg through phase change. You'd be happy with my oil induction system. Still looking for the magic wand that solves the putting so many btu's in to only have to take them back out. Like in photo shop how you can filter out highs and lows with a magic wand.

Hi KDC2,

I don't have the numbers handy, but generally a good amount of superheat means you put fewer extra BTUs into the steam than the extra BTUs which you get out of the steam at the engine ... in the form of horsepower. Not the real numbers, but like, for 100 btus extra put into superheat (on top of the heat used to heat and evaporate the water), you get 200 btus worth of extra horsepower out of the engine, relative to running on saturated steam. The main reason is, superheat eliminates condensation in the engine cylinders, which is very wasteful. Superheat also increases the volume of steam available to do work, for a given amount of "water boiling" heat. Superheated steam flows easier too, for better engine "breathing" and performance. I may be overlooking some other advantages.

It should be noted that there was a serious and pretty complicated controversy over superheated steam engines, including among competent professional engineers, approximately a century ago. Even Stumpf promoted the Unaflow engine as an alternative to superheat. Abner Doble designed his unaflow Doble-Detroit steam car to avoid superheat, and at the time (circa 1916-1918) claimed that this was a big advantage. The Stanley Brothers, on the other hand, were big believers in superheat from Day One. Considering that superheat is generally considered a more advanced feature today, this kinda turns the usual/consensus "who had advanced ideas and who didn't" rankings upside down.

The main issues in the superheat debate seem to have been the cost and reliability of higher-temperature lubricants and materials. With today's oils and materials, even today's cheap & easily-worked stuff, the anti-superheat position is a harder sell. Notice that I weaseled out of saying that "the debate is over". LOL

Today there are some hobby steam folks who advise against superheat for reasons of safety and ease of system setup, in the hands of amateur hobbyists. Personally, I don't advise much of anything, other than careful thinking, and reading as much about steam power as possible. I will say that I believe in the value of superheat.

Peter

Thanks for inputs. Have to agree with all your points on justifications..especially the flow. Chunks of water must be like walking on jupiter with ankle weights. Still, if there was a way to get it in and out..less work on boiler and condenser..where's that magic wand?

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File Name	File Size		Posted by	Date
Derr SS super heater.JPG	104.1 KB	open | download	Rolly	11/21/2009	Read message