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Steam Stationary Engines, Traction Engines Antique steam engines, their boilers, pumps, gauges, whistles and other related things that make them run.

Steam Stationary Engines, Traction Engines

Mollier Charts


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  #1  
Old 10-10-2009, 02:11:30 PM
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Default Mollier Charts

Does anyone have a Mollier Chart, and know how to use it? I don't have one, but wish I did! Below is an example what can be done with one:


Advantage of Low Back Pressure (High Vacuum)

200 lb. per sq. in. pressure (saturated steam), the heat content, or enthalpy, is1198 Btu/lb. (from the Mollier Charts)

Atmospheric exhaust pressure ( 14.7 lb. per sq. in.), heat content, or enthalpy, is 1009 Btu/lb. (from the Mollier Charts)

The difference - 189 Btu/lb. - represents work done by the steam.

Now assume that condensing apparatus is added, so that the back pressure can be reduced below atmospheric pressure. Say that: 2 lb. per sq. in. absolute (26" Hg. vacuum).

The work done by the steam now is 1198 - 897 = 301 Btu/lb. Therefore, the addition of condensing apparatus produced a gain of 301 - 189 = 112 Btu/lb. of steam. The theoretical steam consumption for the two conditions is:

2544 / 189 = 13.46 lb./HP.hr., non-condensing.
2544 / 301 = 8.45 lb./HP/hr., condensing.

This represents a gain in economy or reduction in water rate of: [1 - (8.45 / 13.46)] x 100 = 37.3%

In other words, to get the same power from the engine it takes 37.3% less steam at 2 lb. back pressure than at 14.7 lb. back pressure. (Bear in mind, the 2 lb. back pressure is absolute pressure, that is, 26" Hg. vacuum, and the 14.7 lb. back pressure is atmospheric pressure.)

Of course, all of this gain is not realized, since power must be expended to drive circulating pumps, condensate pumps, and air pumps, but the example is basically correct, and indicates the importance of maintaining a low back pressure.

I wonder what "water rates" could be calculated off of these old reciprocating steam engines?

Gary K
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Old 10-10-2009, 05:14:56 PM
fransborghouts fransborghouts is offline
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Default Re: Mollier Charts

Gary,

Mollier charts should be available in every good technical bookshop. But believe me working with them is everything except easy. I remember that I hated it wenn I was still at school to work and calculate with them !!!

I sailed 8 years as a chief-engineer on steamships with condensors. Basicly there are three big advantages over the non-condensing systems: With condensing U (as stated) lower the back-pressure to a vacuum, with other words the piston will develop a higher hp output by doing so and so explains the saving of steam and thus water !! (this is a very simple explanation) We normale tried to keep a vacuum from almost 70 % (-0.7 bar) The second advantage is the fact that the steam is sucked out of the cilinder wenn the exhaust valves opens and for that reason the valve opening time can be shorter. And the last (and for ships maybe the biggest advantage) by condensing the steam back to water it can (after cleaning and bringing it back on temp.) be used as boiler feedwater again. I only sailed with compound and triple expansion engines and specialy on the compound with the rotating valves it was of great advantage. (the triple was a slide valve engine) So I hope somebody will try to explain to U how to work with it, and that u will be succesfull

From my side I never expected any engineer onboard of the vessels where I was in command to know this kind of things.
Basicly for me it was more important that they had a feeling for the engine and a sound and healthy mind and the ability to learn in praxis.

Sorry for my English mistakes ........ stupid Dutch

Frans
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Old 10-10-2009, 08:56:42 PM
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Default Re: Mollier Charts

Gary,
For practical purposes, you can do just as well with the tabular form. I think you will find, on closer examination, that 1.5% wet exhaust ( 98.5% dryness ) has an enthalpy of 94 BTU/Lb @ 2 PSIA. The work done is 1198 - 94 = 1104 BTU/lb. That will turn your propeller shaft and make it worth running circulation and condensate pumps and air ejectors !

Also, don't forget that some condensate is formed and taken out of the steam path at each low point whenever at saturated conditions. For a proper and thorough engineroom calorimetric, the drip leg condensate must be measured. Don't forget to account for 15 % flash vapor.

I never worked with recips on the job, only turbines, but the principles are the same.

good luck,
Jim
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Last edited by Jim Conte; 10-10-2009 at 09:13:11 PM.
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Old 10-10-2009, 09:43:30 PM
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Default Re: Mollier Charts

Hi Frans:

Thanks for responding to my posting, as I enjoyed reading it. I'm an ex-sailor too, as I worked on the Great Lakes Steamships for 6 years. I never went any higher than Oiler-Watertender, as I didn't want to spend my life out there. I worked with two different reciprocating triple-expansion main propulsion steam engines while employed out there. Nothing humungous (huge), as one was 1,800 h.p., and the other 2,000 h.p.

The triple expansion engines usually ran between 14" and 15" Hg. vacuum on the jet condensers, otherwise the L.P. cylinder's connecting rod bearings would knock, due to low compression. The steam turbines ran between 28" and 29" Hg. vacuum.

I liked the steam turbines the best, as diesels were too loud, with lots of maintainence. I hated maintenance on those triple expansion engines, as everything was so huge. The turbines were more user-friendly, and could usually run 5 years with very little maintenance. (Every 5 years they were torn down for inspection.)

Anyway, I thought the Mollier Charts would be interesting if a person wanted to calculate the theoretical water rates of these old steam traction engines . . . also, steam turbines with various exhaust pressures!

By the way, your English is very good . . . you should see what some of these North Americans write . . . me included, and we have no excuse! Thank God for dictionaries, as I use mine all the time!

Quote:
Gary,
For practical purposes, you can do just as well with the tabular form. I think you will find, on closer examination, that 1.5% wet exhaust ( 98.5% dryness ) has an enthalpy of 94 BTU/Lb @ 2 PSIA. The work done is 1198 - 94 = 1104 BTU/lb. That will turn your propeller shaft and make it worth running circulation and condensate pumps and air ejectors !

Jim Conte:
Looking on my steam charts, I can find the proper enthalpy for steam pressure above atmospheric, but I get higher numbers from atmospheric and under, and the water rates are too high? I found a Mollier chart on a website, but couldn't make out the numbers.

I'll have to take a closer look at my charts, and see if I can find what's missing!

Thanks Jim, I really appreciate it!

Gary K
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