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High or low pressure?

Posted by Amish man 
High or low pressure?
March 04, 2015 03:07AM
Form dry formula of the working potential, between high and low pressure, High pressure seems to win out easily.

At 40 lbs gauge pressure, heat at 287 degrees, it puts out 7.79 cubic feet pressurized steam, and it takes 1,176 BTU for lbs water.
This calculates out to 44,870 ft lbs per sqr inch in, or 1.36 hp for .026 BTU per ft lbs

At 40o lbs gauge pressure, heat at 224 degrees, it puts out 1.16 cubic feet pressurized steam, and it takes 1,205 BTU for lbs water.
This calculates out to 66,816 ft lbs per sqr inch in, or 2.02 hp, for .018 BTU per ft lbs

But this not application, this is dry math.
Yes high pressures, no doubt, looks like the best choice, but is it?

Is it safe to put high pressure in public automobiles, where every part of it, will be broken with wear & tear, by bad repairs, and by crashes?
If in worst case scenario, no, then you got to use low pressure don't you?

Does it take more time to heat high pressure engine, than that of it;s lower counter part?
I would think so, as you got heat to higher temperature, even if it is just a few seconds?
For a long time, diesels, took half min to minute to heat the heads, and many people would not drive one because of that reason.

I a high pressure boiler, going to be as efficient as low pressure boiler, transferring heat to the steam?
The material is thicker, to handle higher pressures, this makes it less efficient does it not?

The engines, big topic here, I showed both are near same size, as high pressure runs slower and lower runs faster.
But will not higher pressure engine have more forces, 10 times more, this compound in mechanical?
From bearing thrust forces, rotational forces, and friction forces being greater at high pressure?

And is not the high pressure engine not more complicated to design for?
Often needing to be compounded with mid and low pressure cylinders, to capture lost steam pressure?
Does this not make it bigger in the end, more complicated, and more costly?

How about recapturing the steam heat, does not high pressure have problem here, because as it expands it has little heat?
While the low pressure retains, most of its heat, and can heat water coming in?

So looking at considerations, is it worth extra cost, complications, and safety concerns, to use higher pressure steam, to try to gain % of efficiency that is on paper?
I mean has any been able make small cheap high pressure reciprocating engine, with so many all striving for that one goal?
The compounding technical and engineering problems of high pressure, has only been solved in huge power plants with huge budgets.
It needs qualified engineers!
So how do expect to get all that equipment, into little engine bay, make it safe so a layperson can fiddle with it?
Re: High or low pressure?
March 04, 2015 06:00AM
You're so wrong it is embarrassing.

First off, no one with experience, math skills or ANY mechanical aptitude believes a car can run on those pressures. Spending a page to argue in favor of the idea simply highlights ones ignorance.

Second, as previously noted, pressure is not of itself necessarily dangerous. Potential danger comes from creating a situation where large amounts of stored energy can be released suddenly in an uncontrolled fashion. All the boilers discussed in this Forum are unable to release dangerous amounts of energy that rapidly. As has been stated and understood by those listening to anyone else.

Suffice it to say this entire topic is idiotic as there is only one side to the "argument".


Knock it off or else.



Edited 2 time(s). Last edit at 03/04/2015 06:03AM by frustrated.
Re: High or low pressure?
March 04, 2015 09:33AM
Hello Amish Man

For the sake of newbies entering this forum and reading these posts I feel some correction and explanation is required.

Amish Man, take a little of your time and ask yourself why people with experience do not agree with your opinions.
The broad statements made in your post are not substantiated, yes they are your opinion but you are using arguments reminiscent of fellow Scotsman James Watt's attempt to bury his competitors who proposed high pressure steam.

1) There is an understandable train of thought that low pressure steam of 800psi has some greater advantages than 1500psi or greater. However to suggest a vacuum or 40psi as an efficient operating pressure goes against practical experience, never mind the mathematics. To quote the history books; " it was not until after Richard Trevithick had developed the use of high-pressure steam, around 1800, that mobile steam engines became a practical proposition". High pressure steam wasn't developed just on the merit of a couple of calculations, it was the practicality of reducing the size and weight of the components to make a useable vehicle. Quote. "James Watt, worried by Richard Trevithick's experiments in high-pressure engines, tried to have the British Parliament pass an act forbidding high pressure on the grounds that the public would be endangered by high-pressure engines exploding." Sound familiar?

2) I agree with Ken on the safety aspect of pressure, the greater concern is the stored energy. Have you experienced a monotube failure? I have and at 1500psi and it is no worse than a fast puncture because there is little stored energy. In comparison most of us will be aware of the tragedy of the steam tractor explosion in Ohio when 5 were killed and many injured. The working pressure of this type tractor would be only 160 psi BUT the stored energy is enormous.

3) Length of time to heat high pressure steam at start up? Taking the monotube system as a basis, to reduce pressure drop through the circuit the diameter of tubing in a low pressure system is greater than that of a similar power high pressure. I would say both systems have disadvantages which balance themselves out. However this is an opinion and requires to be substantiated by calculations then real world testing.

4) Diesel engines are more efficient than petrol therefore there is less waste heat to warm up the cooling system. Modern design of cooling systems from the start of the 21st century means this is no longer a problem, maybe it was more of a problem way back in the 1980's.

5) Yes the tubing may be thicker in a high pressure tube but the low pressure will require larger diameter tubes. Plenty on the forum about the problem of heat transfer from the tube to the steam and the relationship of tube diameter. Large diameters are not always good. Thick wall tube can retain heat and buffer any thermal changes. Go read the forum for practical experience and some theory.

6) The engine design dictates how large or fast that it revolves, whether it is compounded or how complex it is going to be. I am afraid some of your statements sound as if they are based on early 20th century material about ships or locomotives. Steam cars are different by a long chalk.

7) Working with high pressure common rail diesels I have seen a recent spate of accidents caused by amateurs working with last century mentality. Be aware of the hazards and learn to take appropriate precautions is not beyond the average layman, steam or diesel.

I have used this forum to educate myself about steam. Early on I made the rooky mistake of assuming steam worked like hydraulics but there is a world of difference. Today there is a lot more available online for free to learn from. The SACA Steam Automobile magazines are a wealth of information, the Internet Archive and Google Books hold many volumes of books and of course there is youtube.

Brian
Re: High or low pressure?
March 04, 2015 11:49AM
An IC engine works at about 8:1 compression rate. Ambient air being 14.5 lbs/Sq in, that means peak pressure is around 120 lbs/Sq in. For a steam engine to even approach the size of an IC engine, you would need at least about 150-160 lbs/Sq in steam, right about were steam locomotives operate. Steam locomotives ran fine at that pressure with 200 year old technology. Where do you come up with 40 lbs? Lower than the first experimental locomotive? Is this some kind of joke? You must be trolling us. After repeated well thought out explanations, you keep coming back with your 40 pounds! Please!!
Re: High or low pressure?
March 04, 2015 03:08PM
Hi Brian,
You stated the case very well and accurately. Trying to reason with this fool is like arguing with your cat, a total waste of time. One would only hope our forum moderators would simply block him from ever posting.
Jim
Re: High or low pressure?
March 04, 2015 03:56PM
Just ignore him - eventually he will get fed up with the lack of response.
Mike
Re: High or low pressure?
March 04, 2015 04:49PM
Hi Folks

I was prepared to ignore him but felt some response was needed for the benefit of newbies visiting and to set them on the right path, I have no illusion that I can persuade Amish Man away from his misconception.
I would say that I am not a great believer in censorship, only if they are truly a troll rather than misinformed or overly enthusiastic.

Brian
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