PDF Book on Triple and Quadruple Expansion engines

A special section just for steam engines and boilers, as without these you may as well fit a sail.
User avatar
Lopez Mike
Full Steam Ahead
Full Steam Ahead
Posts: 1903
Joined: Wed Dec 07, 2011 6:41 am
Boat Name: S.L. Spiffy
Location: Lopez Island, Washington State, USA

Re: PDF Book on Triple and Quadruple Expansion engines

Post by Lopez Mike » Sat Jan 07, 2017 2:41 am

My favorite is when they are moving around under no load. Almost totally silent. As dangerous as a hybrid car in a parking lot!
If you think you are too small to make a difference, try sleeping with a mosquito.
Dalai Lama
wsmcycle
Full Steam Ahead
Full Steam Ahead
Posts: 218
Joined: Sun Mar 13, 2011 5:43 pm
Boat Name: FEARLESS,l'il steamy
Location: Fort Smith Arkansas USA

Re: PDF Book on Triple and Quadruple Expansion engines

Post by wsmcycle » Mon Jan 09, 2017 6:35 pm

In the multi expansion and reheat engine, the steam is returned to the flue after the primary exhaust right? You raise the pressure of the steam exhausting from the primary by running it back through the flue, If you raise the temperature of the steam, you raise the pressure. Now you have a higher pressure at the discharge of the primary. The drop in pressure ACROSS the primary is reduced and therefore, the power of the stroke. How does this work out positively?
LIGHT THE FIRE!!
User avatar
Lopez Mike
Full Steam Ahead
Full Steam Ahead
Posts: 1903
Joined: Wed Dec 07, 2011 6:41 am
Boat Name: S.L. Spiffy
Location: Lopez Island, Washington State, USA

Re: PDF Book on Triple and Quadruple Expansion engines

Post by Lopez Mike » Mon Jan 09, 2017 9:01 pm

Ah, were it that it was that simple:
https://en.wikipedia.org/wiki/Superheated_steam
If you think you are too small to make a difference, try sleeping with a mosquito.
Dalai Lama
Oilking
Full Steam Ahead
Full Steam Ahead
Posts: 186
Joined: Sun Sep 02, 2012 2:39 pm
Boat Name: No Boat Yet
Location: Cathlamet, WA

Re: PDF Book on Triple and Quadruple Expansion engines

Post by Oilking » Mon Jan 09, 2017 11:03 pm

If you had a compound with 100psi inlet(338 Deg F)and an hp exhausting at 50psi(296 Deg F)a reheat of 42 deg F would only raise the pressure 3psi if the whole receiver increased by that amount. The actual average temp would be lower and the system is not static so I would expect that any increase in pressure due to thermal expansion to be 1.5 psi or less. Other factors such as valve timing and the friction of additional piping is likely to have a greater influence on the receiver back pressure than the increase in temperature.

P1 V1/T1 = P2 V2/T2
P=Atmospherers
V=1 and is unit less since it remains constant and cancels out in the equation
T=Degrees Kelvin

I realize that this is part of the "Ideal" gas law and can differ some from real world conditions.

Welcome to have someone prove me wrong.

Dave
User avatar
fredrosse
Full Steam Ahead
Full Steam Ahead
Posts: 1906
Joined: Fri Nov 20, 2009 5:34 am
Boat Name: Margaret S.
Location: Phila PA USA
Contact:

Re: PDF Book on Triple and Quadruple Expansion engines

Post by fredrosse » Tue Jan 10, 2017 7:26 am

In a compound engine with or without reheat, there will never be a condition where the inlet to the LP cylinder will be at a higher pressure than the HP cylinder. Fluid (steam) always flows from a higher pressure to a lower pressure.

While it is theoretically possible to imagine a case where the hp cylinder exhausts to a receiver, then HP exhaust valve closes, then HP exhaust is reheated (thus increasing the initial receiver pressure), then LP admission begins, then some increase in inlet pressure to the LP is theoretically possible. For this to work, the reheating process would need to be very rapidly applied to the receiver (during the short time between HP exhaust closing, and the shortly following LP admission opening), and this is not a practical possibility for real engines. In real engines with reheat, the exhaust steam from the HP must travel through a long path of heat transfer tubes, this trip requires the time frame of several engine strokes, so an average condition results in the reheated steam entering the LP admission with consideragly less pressure than the HP exhaust pressure. I have never seen an indicator diagram where any part of the LP diagram has a pressure higher than the HP exhaust pressure.
User avatar
Lopez Mike
Full Steam Ahead
Full Steam Ahead
Posts: 1903
Joined: Wed Dec 07, 2011 6:41 am
Boat Name: S.L. Spiffy
Location: Lopez Island, Washington State, USA

Re: PDF Book on Triple and Quadruple Expansion engines

Post by Lopez Mike » Tue Jan 10, 2017 7:42 am

Let's say that the boiler pressure gauge reads a certain amount. And that between the boiler and the throttle is a superheater.

Now the steam would surely flow backwards if the superheater raised the pressure. If anything there would be a little drop due to flow losses. However much I accept that fact, I cannot say that I understand it. I have always dealt with situations where pressure and temperature were very much tied to each other. As in my boiler. At a certain pressure there is a certain temperature. But with superheated steam the situation seems to be different.

Is there a way, without an alarming excursion into the enthalpy/entropy tables, that you can explain what the heck is going on? All these years and I haven't bothered to understand it. Typical electronics type.
If you think you are too small to make a difference, try sleeping with a mosquito.
Dalai Lama
User avatar
DetroiTug
Full Steam Ahead
Full Steam Ahead
Posts: 1863
Joined: Fri Nov 27, 2009 5:56 pm
Boat Name: Iron Chief
Location: Northwest Detroit

Re: PDF Book on Triple and Quadruple Expansion engines

Post by DetroiTug » Tue Jan 10, 2017 3:29 pm

As I understand it: The level of benefit gained from superheating is dependent on the temperature of the steam prior to superheating. Most of us at our low pressures under 200 gain significantly due to the steam at that temperature/pressure being considered wet as there is still moisture that has not expanded to steam (saturated). I read at around 1100 degrees F there is not much more expansion to be had and the benefits above that are minimal. Doble designed for 1200F max, and that concurs with other things I've read.

If the pressure in the steam line is raised it surely raises boiler pressure/negates loss, probably not very noticeably as the boiler is typically a much larger vessel. It's sort of like my vaporizing burner on the car which is essentially a superheater, manifold air pressure may be 50 psi, but boiling the liquid fuel surely raises the pressure, what happens to it? The delivery nozzle orifice acts as a regulator by simply releasing more gas at higher pressure, thereby finding a balance in pressure. In much the same way the throttle on our engines do when superheating.

Thermal loss as I see it is the biggest performance robbing element in a steam plant - (and injecting feedwater at lower temperatures than the boiler water). By raising the temperature of the steam prior to the temperature loss, the pressure is delivered at the engine closer to the same pressure as it left the boiler. So I see the benefit of superheating not so much to raise pressure, but temperature to offset thermal loss and pressure loss.

-Ron
wsmcycle
Full Steam Ahead
Full Steam Ahead
Posts: 218
Joined: Sun Mar 13, 2011 5:43 pm
Boat Name: FEARLESS,l'il steamy
Location: Fort Smith Arkansas USA

Re: PDF Book on Triple and Quadruple Expansion engines

Post by wsmcycle » Wed Jan 11, 2017 12:02 am

Mikes reduction in the complexity and simplification of the (perceived) dilemma is a much better question. Can this be explained ( a feel for it) without the use of Black magic? The simpleton thinking (mine) would say that the pressure can't be different anywhere in the boiler (perhaps some slight variation for lack of homogeneity of the steam quality . Especially, It would seem the pressure if any thing would be lower at the discharge point (exhausting) point.
LIGHT THE FIRE!!
User avatar
fredrosse
Full Steam Ahead
Full Steam Ahead
Posts: 1906
Joined: Fri Nov 20, 2009 5:34 am
Boat Name: Margaret S.
Location: Phila PA USA
Contact:

Re: PDF Book on Triple and Quadruple Expansion engines

Post by fredrosse » Wed Jan 11, 2017 3:50 am

To get a better understanding of these processes, first read the FAQ section of this forum, under the topic “Wet, Dry and Super heated steam”. Understanding that saturated steam, where liquid water and gaseous steam can coexist, with a definite pressure/temperature relationship is only one condition of steam.

Superheated steam, where the pressure/temperature relationship no longer bounds the conditions is another condition entirely. Superheated steam and liquid water cannot exist in equilibrium with each other, and superheated steam can possibly exist at any temperature above the saturation temperature. The saturation temperature is fixed by the pressure, a definite fixed relationship, as found in the steam table publications, but this relationship only holds true for SATURATED STEAM.

wsmcycle
“If you raise the temperature of the steam, you raise the pressure.”

(this is true if the steam is saturated steam. If the steam is superheated, then raising the temperature may not necessarily raise the pressure)

Lopez Mike
“Now the steam would surely flow backwards if the superheater raised the pressure.”

(that is correct, in a flowing condition here, the steam always flows from a higher pressure to a lower pressure)

“If anything there would be a little drop due to flow losses.”

(exactly correct, when steam flows, the pressure is always lower downstream, and for many flow conditions there is far more than “a little pressure drop” due to flow friction)

“However much I accept that fact, I cannot say that I understand it. I have always dealt with situations where pressure and temperature were very much tied to each other. As in my boiler. At a certain pressure there is a certain temperature.”

(Yes, this is an established fact for SATURATED STEAM, the pressure/temperature relationship is fixed)

“But with superheated steam the situation seems to be different.”

(Yes, with superheated steam the temperature can be anything above the saturation temperature. At one atmosphere pressure, the steam can exist at any temperature above 212F or 100C, all the way up to several thousand degrees F or C, this is superheated steam)

Oilking
“…….hp exhausting at 50psi(296 Deg F)a reheat of 42 deg F would only raise the pressure 3psi if the whole receiver increased by that amount.

P1 V1/T1 = P2 V2/T2
P=Atmospherers
V=1 and is unit less since it remains constant and cancels out in the equation
T=Degrees Kelvin”

(The condition Oilking describes here is where the HP exhaust steam at 50psi is bottled up in a container of constant volume, and for this condition raising the temperature would indeed increase the pressure. However this is NOT a flowing condition where the HP exhaust flows into the LP cylinder. If there is flow, the pressure always reduces as flow proceeds)


DetroiTug
“I read at around 1100 degrees F there is not much more expansion to be had and the benefits above that are minimal. Doble designed for 1200F max, and that concurs with other things I've read. “

(Actually there is plenty more expansion available at temperatures far above the 1100F-1200F range, but steam plant designs do not enter these very high temperature realms because there are no practical metals that are strong enough to economically contain the steam at higher temperatures. Typical modern steam engines at power plants (steam turbines) have the limitation of 1100F-1200F purely based on the economics of not having metals available to endure higher temperatures. For reciprocating steam engines the maximum allowable temperatures are considerably lower, due to lubrication difficulties of rubbing surfaces at high temperatures.)
User avatar
fredrosse
Full Steam Ahead
Full Steam Ahead
Posts: 1906
Joined: Fri Nov 20, 2009 5:34 am
Boat Name: Margaret S.
Location: Phila PA USA
Contact:

Re: PDF Book on Triple and Quadruple Expansion engines

Post by fredrosse » Wed Jan 11, 2017 4:11 am

DetroitTug ".......It's sort of like my vaporizing burner on the car which is essentially a superheater, manifold air pressure may be 50 psi, but boiling the liquid fuel surely raises the pressure, what happens to it? "

The vaporizing burner is typically a boiler of the liquid fuel, followed by superheating of the gaseous fuel that is generated after boiling the fuel in the vaporizing coil. With steady conditions, boiling the fuel will not increase the fuel pressure above the 50 psi feed pressure to the vaporizing coils.

There can often be surging and transient pressure excursions of the fuel in the vaporizing coils, but overall the fuel feed pressure from the fuel supply will be the highest pressure throughout the vaporizing burner circuit. The surging occurs when liquid fuel flows into contact with very high temperature metal in the vaporizer coil, and this liquid flashes to a large quantity of fuel vapor almost instantly, causing a temporary pressure excursion, which is quickly relieved by the fuel outlet jet flow. If there were any steady mechanism where the fuel being boiled in the vaporizing coil could increase the coil pressure, then fuel would no longer flow into the vaporizer coil, and no more firing could exist.

For the processes here, or in our steam boilers and superheaters, and steam lines right into our steam engine cylinders, flow, on average, is ALWAYS occurring with lower and lower pressures in the downstream path.
Post Reply