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Steam Reheating
Posted by: Dan Ullfig (IP Logged)
Date: December 29, 2004 09:08PM
<HTML>Hi all:
I'm working out the details to a modern steam car engine, and I was wondering the following:
Has anyone considered reheating the steam on its way from the HP to the LP cylinder in a compound engine?
It would seem to me that you would get a greater economy of steam, because the total heat available for doing work would be greater. I understand powerplants use reheat between high and low pressure turbines.
Dan</HTML>
I'm working out the details to a modern steam car engine, and I was wondering the following:
Has anyone considered reheating the steam on its way from the HP to the LP cylinder in a compound engine?
It would seem to me that you would get a greater economy of steam, because the total heat available for doing work would be greater. I understand powerplants use reheat between high and low pressure turbines.
Dan</HTML>
Re: Steam Reheating
Posted by: Ben in Maine (IP Logged)
Date: December 29, 2004 09:33PM
<HTML>Marsh Bros patent'd this around turn of the century,,,Brockton Mass USA Cheers Ben</HTML>
Re: Steam Reheating
Posted by: Andy Patterson (IP Logged)
Date: December 29, 2004 10:11PM
<HTML>Hi Dan
I is a bit more complex. The main reasion for using reheat is to keep the HP inlet temperature down. With high supper heat at the HP inlet it wouldn't be neccessary to reheat be tween stages. On the other hand is the expansion in the LP cylander or in any stage for tha mater resultes in to wet of a steam quality then reheat is advised.
There are diminishing gains with increased supperheat. There also meterial temperture limits. There really is no simple answer. If you already have high enough supperheat comming in then additional heating may not result in much os a gain.
In a variable power application like an automobile were the power level is very dynamic the interstage volume can cause loss of efficiency. At a steady power output the interstage volumes are stabilized (not changing) and causing no efficiency loss. But when you are changing there content densities as throttling up and down they produce efficiency loss.
A reheater would be additional volume. It would take a very detailed analysys over the engines power or maybe a standard driving cycle to tell is it would be a gain or loss.
My compound design uses fixed interstage reviever pressures requiring seperate throttling control on each stage when in throttling mode. So far my analysis shows no need for interstage reheat.
Andy</HTML>
I is a bit more complex. The main reasion for using reheat is to keep the HP inlet temperature down. With high supper heat at the HP inlet it wouldn't be neccessary to reheat be tween stages. On the other hand is the expansion in the LP cylander or in any stage for tha mater resultes in to wet of a steam quality then reheat is advised.
There are diminishing gains with increased supperheat. There also meterial temperture limits. There really is no simple answer. If you already have high enough supperheat comming in then additional heating may not result in much os a gain.
In a variable power application like an automobile were the power level is very dynamic the interstage volume can cause loss of efficiency. At a steady power output the interstage volumes are stabilized (not changing) and causing no efficiency loss. But when you are changing there content densities as throttling up and down they produce efficiency loss.
A reheater would be additional volume. It would take a very detailed analysys over the engines power or maybe a standard driving cycle to tell is it would be a gain or loss.
My compound design uses fixed interstage reviever pressures requiring seperate throttling control on each stage when in throttling mode. So far my analysis shows no need for interstage reheat.
Andy</HTML>
Re: Steam Reheating
Posted by: George Nutz (IP Logged)
Date: December 30, 2004 11:47AM
<HTML>Dan,
A 4:1 cylinder ratio compound can decrease the steam rate quite a bit and approach the efficiency of a triple without using extremely high initial steam temperatures---have done a number of analysies on this over the years. It would be easy to do if the engine and boiler were next to each other as the reheater would be in essence the receiver volume, Doble wrote a bit on this.
Keep steaming, George</HTML>
A 4:1 cylinder ratio compound can decrease the steam rate quite a bit and approach the efficiency of a triple without using extremely high initial steam temperatures---have done a number of analysies on this over the years. It would be easy to do if the engine and boiler were next to each other as the reheater would be in essence the receiver volume, Doble wrote a bit on this.
Keep steaming, George</HTML>
Re: Steam Reheating
Posted by: Jim Crank (IP Logged)
Date: December 30, 2004 01:34PM
<HTML>Dan,
Buy the Doble notebook from SACA stores and read it.
Doble used this for the Sentinel-Stanley-Nordberg and Greyhound triple expansion bus engines. The main problem is that unless a second normalizer is used, the reheat temperature is not under any control and the LP cylinder oil is carbonized.
Standing at a stop with the fire on raises the reheat temperature beyond what the cylinder oil can stand. Abner finally abandoned the idea for the Paxton car and went back to a compound.
JC</HTML>
Buy the Doble notebook from SACA stores and read it.
Doble used this for the Sentinel-Stanley-Nordberg and Greyhound triple expansion bus engines. The main problem is that unless a second normalizer is used, the reheat temperature is not under any control and the LP cylinder oil is carbonized.
Standing at a stop with the fire on raises the reheat temperature beyond what the cylinder oil can stand. Abner finally abandoned the idea for the Paxton car and went back to a compound.
JC</HTML>
Re: Steam Reheating
Posted by: George Nutz (IP Logged)
Date: December 30, 2004 02:46PM
<HTML>Jim,
As you know about Abner's troubles with these things---and that he never buried his superheaters behind a few tube walls did give him problems. I think a wisely designed and protected reheater could be quite protected from wild temperature swings and have a longer time constant.
George</HTML>
As you know about Abner's troubles with these things---and that he never buried his superheaters behind a few tube walls did give him problems. I think a wisely designed and protected reheater could be quite protected from wild temperature swings and have a longer time constant.
George</HTML>
Re: Steam Reheating
Posted by: Dan Ullfig (IP Logged)
Date: December 30, 2004 03:47PM
<HTML>HI!
Well, I guess I havn't explained the whole plan, and I didn't want to give up too many details, but I had in mind having two steam units, instead of one, a lamont boiler putting out satturated steam, and a separate combination superheater-reheater, with its own burner. The superheater burner would be regulated by a thermostat.
Since the mass of steam going into and out of the HP cylinder is exactly the same, regardless of pressure, then if the coils in the superheater and reheater side are the same length, the temperature in both sections should remain proportional at all times. In other words, if the initial temperature in the HP cylinder is, say, 800 deg, then the reheater temperature can never be more than that.
As for the location of the reheater, it seems that as long as you are using intake valves in the LP cylinder ( and I don't see why not ) set to the same cuttoff as the HP, the volume of and location of the reheater should bear no influence on efficiency.
Dan</HTML>
Well, I guess I havn't explained the whole plan, and I didn't want to give up too many details, but I had in mind having two steam units, instead of one, a lamont boiler putting out satturated steam, and a separate combination superheater-reheater, with its own burner. The superheater burner would be regulated by a thermostat.
Since the mass of steam going into and out of the HP cylinder is exactly the same, regardless of pressure, then if the coils in the superheater and reheater side are the same length, the temperature in both sections should remain proportional at all times. In other words, if the initial temperature in the HP cylinder is, say, 800 deg, then the reheater temperature can never be more than that.
As for the location of the reheater, it seems that as long as you are using intake valves in the LP cylinder ( and I don't see why not ) set to the same cuttoff as the HP, the volume of and location of the reheater should bear no influence on efficiency.
Dan</HTML>
Re: Steam Reheating
Posted by: Dan Ullfig (IP Logged)
Date: December 30, 2004 03:48PM
<HTML>To clariffy, the superheater-reheater unit has two sets of coils, of the same length. One superheats the saturated steam on its way to the HP cylinder, the other coild reneats the steam on its way to the LP cylinder.
Dan</HTML>
Dan</HTML>
Re: Steam Reheating
Posted by: George Nutz (IP Logged)
Date: December 30, 2004 04:17PM
<HTML>Dan,
Not the way to go---the amount of heat added to the reheat cycle is much less generallly than the amount added to superheat. Also the superheater is generally in a hotter part of the boiler than the reheater---using two identical coils could lead to trouble---its all in the heat analysis of the boiler and how protected and safe one wants each section. Also the superheater can have a significant pressure drop thru it to protect it while pushing the same amount of steam thru an identical coil of a higher specific volume steam would have a large(lost) drop to the low pressure cylinder. They should be designed for each individual case. The volume of the "receiver" should have different relative volumes depending upon the engine being of the "E" type or a 90 degree "F" type compound. There are ideal receiver volumes.
George</HTML>
Not the way to go---the amount of heat added to the reheat cycle is much less generallly than the amount added to superheat. Also the superheater is generally in a hotter part of the boiler than the reheater---using two identical coils could lead to trouble---its all in the heat analysis of the boiler and how protected and safe one wants each section. Also the superheater can have a significant pressure drop thru it to protect it while pushing the same amount of steam thru an identical coil of a higher specific volume steam would have a large(lost) drop to the low pressure cylinder. They should be designed for each individual case. The volume of the "receiver" should have different relative volumes depending upon the engine being of the "E" type or a 90 degree "F" type compound. There are ideal receiver volumes.
George</HTML>
Re: Steam Reheating
Posted by: Rolly (IP Logged)
Date: December 30, 2004 05:15PM
<HTML>Dan
I built a super-heater coil for my friends Stanley 14-1/2 feet long of ½” 316L SS. When carrying the correct water level he can maintain 650F. The super-heater is located just above the burner. Depending how long he is stopped at a light the temperature has soared to 950F. When starting off from this temperature is only there for a few seconds. We haven’t noticed any damage to the valves as yet.
In my Derr boiler I have 60 feet of 3/8 SS for the super heater located two rows of boiler tubes above the fire. This gives a more stable temperature around the super-heater coil.
I maintain 650F as well when running but have seen it creep to 900 on occasion. This happens from low water in the boiler, the boiler is entirely different then a Stanley and requires a more constant water level.
This stuff is not easy to figure. You can have all the controls you can think off and still have wiled things happen.
In my boat I left the super-heater and economizer out, as I wanted to get the thing in the water after three years of building. The compound low-pressure cylinder was running about 12 percent wet. I added the super-heater to the boiler and economizer for the next season and this gave me 100 Degree super heat on the HP and by changing the cutoff to around 68 percent of piston travel changed my LP to 40 PSI. with about 20 Degrees super-heat. This changed the whole performance of the plant. I used a lot less water & fuel to maintain the same performance.
The boat is a lot difference then a car. Your not stopping and starting, you don’t have the pressure swings and temperature changes. Maintaining constant performance in an automobile is a whole different power plant.
Good luck with your endeavors, be prepared to spend more then you will ever earn.</HTML>
I built a super-heater coil for my friends Stanley 14-1/2 feet long of ½” 316L SS. When carrying the correct water level he can maintain 650F. The super-heater is located just above the burner. Depending how long he is stopped at a light the temperature has soared to 950F. When starting off from this temperature is only there for a few seconds. We haven’t noticed any damage to the valves as yet.
In my Derr boiler I have 60 feet of 3/8 SS for the super heater located two rows of boiler tubes above the fire. This gives a more stable temperature around the super-heater coil.
I maintain 650F as well when running but have seen it creep to 900 on occasion. This happens from low water in the boiler, the boiler is entirely different then a Stanley and requires a more constant water level.
This stuff is not easy to figure. You can have all the controls you can think off and still have wiled things happen.
In my boat I left the super-heater and economizer out, as I wanted to get the thing in the water after three years of building. The compound low-pressure cylinder was running about 12 percent wet. I added the super-heater to the boiler and economizer for the next season and this gave me 100 Degree super heat on the HP and by changing the cutoff to around 68 percent of piston travel changed my LP to 40 PSI. with about 20 Degrees super-heat. This changed the whole performance of the plant. I used a lot less water & fuel to maintain the same performance.
The boat is a lot difference then a car. Your not stopping and starting, you don’t have the pressure swings and temperature changes. Maintaining constant performance in an automobile is a whole different power plant.
Good luck with your endeavors, be prepared to spend more then you will ever earn.</HTML>
Re: Steam Reheating
Posted by: George Nutz (IP Logged)
Date: December 30, 2004 05:48PM
<HTML>Rolly,
Yup, you know who told your friend how long to make that Stanley superheater ;o) . It is a very good example of how bad the temperature swings can be on an unshielded superheater. Ideally about 50% of the heat energy transfered should be by convection and the other half radiation, neither the Stanley or Doble did this but Abner had his normaliser (later called by B&W a "attemperator". Thomas Derr buried his superheater as he took that into consideration and your new "Derr" boiler is much more stable on the superheat end of things. Congrates! It is probably much harder to design a really good variable load small boiler than an engine---just a huge amount of theoretical heat transfer work. Very tiring stuff doing it longhand. The advantage of the Lamont is that the superheater is always nestled amongst a constant temperature Lamont coil.
HAPPY NEW YEAR-George</HTML>
Yup, you know who told your friend how long to make that Stanley superheater ;o) . It is a very good example of how bad the temperature swings can be on an unshielded superheater. Ideally about 50% of the heat energy transfered should be by convection and the other half radiation, neither the Stanley or Doble did this but Abner had his normaliser (later called by B&W a "attemperator". Thomas Derr buried his superheater as he took that into consideration and your new "Derr" boiler is much more stable on the superheat end of things. Congrates! It is probably much harder to design a really good variable load small boiler than an engine---just a huge amount of theoretical heat transfer work. Very tiring stuff doing it longhand. The advantage of the Lamont is that the superheater is always nestled amongst a constant temperature Lamont coil.
HAPPY NEW YEAR-George</HTML>
Re: Steam Reheating
Posted by: Dan Ullfig (IP Logged)
Date: December 30, 2004 06:02PM
<HTML>no, see, I think you guys are still thinking in terms of one boiler, superheter coil built in, all powered by a single burner. When you have a steady state boiler, like in a marine application, you can design the whole thing so that the right proportion of heat is going into the different section, namely producing saturated steam by boiling the water, and then superheating it.
But when you are powering an engine with wild fluctuations in power output, the single boiler paradigm runs into trouble. After all, a lot of the discussions about steam cars center around how to accurately proportion the input of water into the boiler. If you pump too much, initially the pressure drops because the water is cold. Since the burner is now heating the cold water, there is less heat available to superheat the steam. Then, if you pumped too much water, the pressure will soon go way up. It becomes very difficult to maintain a steady state.
With two sepparate units, however, control becomes less problematic. The lamont produces saturated steam. So the lamont burner is tied only to the water temperature in the lamont. The second unit, completely detatched and independent of the lamont, is the superheater, with its own burner. The second burner is tied to the temperature of the steam, and is easy to control also. And with the superheater ang the reheater sharing the same burner ( entirely sepparate from the lamont burner ) you can precisely control the superheat of the steam.
Dan</HTML>
But when you are powering an engine with wild fluctuations in power output, the single boiler paradigm runs into trouble. After all, a lot of the discussions about steam cars center around how to accurately proportion the input of water into the boiler. If you pump too much, initially the pressure drops because the water is cold. Since the burner is now heating the cold water, there is less heat available to superheat the steam. Then, if you pumped too much water, the pressure will soon go way up. It becomes very difficult to maintain a steady state.
With two sepparate units, however, control becomes less problematic. The lamont produces saturated steam. So the lamont burner is tied only to the water temperature in the lamont. The second unit, completely detatched and independent of the lamont, is the superheater, with its own burner. The second burner is tied to the temperature of the steam, and is easy to control also. And with the superheater ang the reheater sharing the same burner ( entirely sepparate from the lamont burner ) you can precisely control the superheat of the steam.
Dan</HTML>
Re: Steam Reheating
Posted by: Rolly (IP Logged)
Date: December 30, 2004 06:09PM
<HTML>Yes George
It sure would be nice to see your Nutz-Teel Lamont under the hood of an automobile; to bad you guys didn’t have another 10 grand to blow. You Rod and Greg should pair up, that little S-10 might sprout wings.
Rolly</HTML>
It sure would be nice to see your Nutz-Teel Lamont under the hood of an automobile; to bad you guys didn’t have another 10 grand to blow. You Rod and Greg should pair up, that little S-10 might sprout wings.
Rolly</HTML>
Re: Steam Reheating
Posted by: Rolly (IP Logged)
Date: December 30, 2004 06:15PM
<HTML>Yes Dan you may be on to something. We have talked about the same thing. The plant will become very cumbersome. On paper things are a lot different. Let us know when you start building this thing. Might be interesting.
I through of re-heating my LP cylinder on the boat but pluming gets messy, Room in the boiler, Cost of fittings, Not worth the gain.
Have fun.
Happy New Year guys.
Rolly</HTML>
I through of re-heating my LP cylinder on the boat but pluming gets messy, Room in the boiler, Cost of fittings, Not worth the gain.
Have fun.
Happy New Year guys.
Rolly</HTML>
Re: Steam Reheating
Posted by: Andy Patterson (IP Logged)
Date: December 30, 2004 06:41PM
<HTML>Hi Dan
Though you have a seperate burner you still have different steam states. As Geroge says The heat transfer rates will be different with different steam properties. You will probably have varing steam states going to the reheat stage. Hard to say how much fluctuation will occure. It may be in a reasionable range.
Andy</HTML>
Though you have a seperate burner you still have different steam states. As Geroge says The heat transfer rates will be different with different steam properties. You will probably have varing steam states going to the reheat stage. Hard to say how much fluctuation will occure. It may be in a reasionable range.
Andy</HTML>
Re: Steam Reheating
Posted by: Dan Ullfig (IP Logged)
Date: December 30, 2004 07:03PM
<HTML>Andy:
right, you have different steam states, but since you can control each burner independently, you can control how much heat goes into boiling water, and how much into superheat.
Dan</HTML>
right, you have different steam states, but since you can control each burner independently, you can control how much heat goes into boiling water, and how much into superheat.
Dan</HTML>
Re: Steam Reheating
Posted by: Dan Ullfig (IP Logged)
Date: December 30, 2004 07:04PM
<HTML>Rolly:
Build?...........What do you mean build?.............. ;-)</HTML>
Build?...........What do you mean build?.............. ;-)</HTML>
Re: Steam Reheating
Posted by: Andy Patterson (IP Logged)
Date: December 30, 2004 07:18PM
<HTML>Dan you said you supperheater and reheater would be in the same unit. I was talking about the steam going into the supper heater and reheater having different states.
I am thinking that you will have a fairly steady state going into your supper heater. But with varing power the steam going to the reater will vary a bit.
Though you might be able to balance the two coils(reheater and superheater) by varing the evaporation boiler fire so vary the steam going to the superheater. Think that would be a bit tricky to do though.
Andy</HTML>
I am thinking that you will have a fairly steady state going into your supper heater. But with varing power the steam going to the reater will vary a bit.
Though you might be able to balance the two coils(reheater and superheater) by varing the evaporation boiler fire so vary the steam going to the superheater. Think that would be a bit tricky to do though.
Andy</HTML>
Re: Steam Reheating
Posted by: Dan Ullfig (IP Logged)
Date: December 30, 2004 07:31PM
<HTML>Andy:
I see what you mean, about the steam comming out of the HP having varying conditions depending on power. But I figure since by design the mass flow of steam through the superheater and the reheater are identical ( it's the same steam comming and going ) that one could proportion the length of both coils to pretty much get a steady state. At the very least, you should be able to proportion it so the reheater doesn't over heat the steam and carbonize the oil in the LP cylinder.
The point Rolly makes about the plumbing getting complicated is well taken, though. One would have to calculate (havn't done it) if the gains in either power or water economy warrant the extra complication.
Dan</HTML>
I see what you mean, about the steam comming out of the HP having varying conditions depending on power. But I figure since by design the mass flow of steam through the superheater and the reheater are identical ( it's the same steam comming and going ) that one could proportion the length of both coils to pretty much get a steady state. At the very least, you should be able to proportion it so the reheater doesn't over heat the steam and carbonize the oil in the LP cylinder.
The point Rolly makes about the plumbing getting complicated is well taken, though. One would have to calculate (havn't done it) if the gains in either power or water economy warrant the extra complication.
Dan</HTML>
Re: Steam Reheating
Posted by: Rolly (IP Logged)
Date: December 30, 2004 08:55PM
<HTML>What? another Andy, all talk and no show. You will make a nice pair. Get off the computer and in the shop. Mortgage your house, wife, kids, pension plane, and 401K. and build something. You will die and leave nothing accomplished.
Rolly</HTML>
Rolly</HTML>
Re: Steam Reheating
Posted by: Dan Ullfig (IP Logged)
Date: December 30, 2004 09:53PM
<HTML>Ok, now you got me curious. Have you built anything? do you have a website?
Dan</HTML>
Dan</HTML>
Re: Steam Reheating
Posted by: Rolly (IP Logged)
Date: December 30, 2004 10:04PM
<HTML>Yes
More then I ever should. My car stuff is not on it, I need to up date the site.
[ourworld.cs.com]
Rolly</HTML>
More then I ever should. My car stuff is not on it, I need to up date the site.
[ourworld.cs.com]
Rolly</HTML>
Re: Steam Reheating
Posted by: Dan Ullfig (IP Logged)
Date: December 30, 2004 10:26PM
<HTML>Cool!
how much for the boat?
Dan</HTML>
how much for the boat?
Dan</HTML>
Re: Steam Reheating
Posted by: Peter Heid (IP Logged)
Date: December 31, 2004 04:54PM
<HTML>Dan,
Don't forget 2 boilers can easily be less efficient than one, not to mention extra weight and cost. Of course the plumbing as has been said but also the controls and their interrelationship can be troublesome.
Peter Heid</HTML>
Don't forget 2 boilers can easily be less efficient than one, not to mention extra weight and cost. Of course the plumbing as has been said but also the controls and their interrelationship can be troublesome.
Peter Heid</HTML>
Re: Steam Reheating
Posted by: Dan Ullfig (IP Logged)
Date: December 31, 2004 09:11PM
<HTML>Ok, here's my plan:
first boiler, a lamont type, overpressurized so the water in it does not boil. The tank that a lamont would normally have is replaced by an accumulator, in other words a cylinder with a spring loaded piston. A switch cuts the pump in and out depending on the position of the piston. water level in the lamont is not a problem this way. No need to precisely meter the water. The burner is controlled by a thermostat. So at this point you have superheated water at a constant pressure and temperature.
Second boiler is a monotube type superheater. There is a metering valve between first and second, controlled by the pressure in the superheater. When the pressure drops, water is admited wich instantly flashes to steam. The burner is controlled by a thermostat.
By having two separate units, you also have more flexibility in locating the components. The lamont could be long and skinny, and be placed under the car were the exhaust would normaly be. The superheater can be short and fat, and placed close to the engine.
You could have a canister were the water flashes to steam, so that any mineral deposits are deposited there, easy to clean out. Because the water is circulated in the lamont, and not allowed to steam, no deposits should occur in the lamont.
Comments anyone?
Dan</HTML>
first boiler, a lamont type, overpressurized so the water in it does not boil. The tank that a lamont would normally have is replaced by an accumulator, in other words a cylinder with a spring loaded piston. A switch cuts the pump in and out depending on the position of the piston. water level in the lamont is not a problem this way. No need to precisely meter the water. The burner is controlled by a thermostat. So at this point you have superheated water at a constant pressure and temperature.
Second boiler is a monotube type superheater. There is a metering valve between first and second, controlled by the pressure in the superheater. When the pressure drops, water is admited wich instantly flashes to steam. The burner is controlled by a thermostat.
By having two separate units, you also have more flexibility in locating the components. The lamont could be long and skinny, and be placed under the car were the exhaust would normaly be. The superheater can be short and fat, and placed close to the engine.
You could have a canister were the water flashes to steam, so that any mineral deposits are deposited there, easy to clean out. Because the water is circulated in the lamont, and not allowed to steam, no deposits should occur in the lamont.
Comments anyone?
Dan</HTML>
Re: Steam Reheating
Posted by: David Nergaard (IP Logged)
Date: January 05, 2005 11:58AM
<HTML>In city driving, you may have a serious problem you have overlooked. After you close the throttle, the engine will not stop until all the steam trapped in the re-heater and its connecting plumbing has passed through the low pressure cylinder. This was a significant problem in manuevering tug boats with triple expansion engines and no re-heat, it will be much worse in driving a car with the added volume of a re-heater!</HTML>
Re: Steam Reheating
Posted by: Dan Ullfig (IP Logged)
Date: January 05, 2005 02:43PM
<HTML>David:
couldn't you have intake valves at the LP cylinder? that would solve that problem...
Dan</HTML>
couldn't you have intake valves at the LP cylinder? that would solve that problem...
Dan</HTML>
Re: Steam Reheating
Posted by: Ben in Maine (IP Logged)
Date: January 05, 2005 07:20PM
<HTML>AAAAAHH,,,In order,, #1 boiler #2 throttle #3 Hp cyl, #4 reciever[or not] #5 Lp cyl . Once the steam is past the throttle it will do work til it exits the engine,,,unless of course the load torque is greater than produced torque. One could reverse the link but this in traffic can be a bit edgy. A simpling valve may be a solution? Just a thought,,Ben</HTML>
Re: Steam Reheating
Posted by: George Nutz (IP Logged)
Date: January 05, 2005 09:11PM
<HTML> The reheater becomes the receiver and its volume of steam kept at an amount appropriate for a receiver---hard to do. I think the Doble reheater on his triple had a number of small diameter parallel tubes, this kept its internal volume down while providing a high heat transfer coefficient for the gas side of the equation----start running some heat transfer numbers guys!
George</HTML>
George</HTML>
Re: Steam Reheating
Posted by: Andy Patterson (IP Logged)
Date: January 06, 2005 03:38PM
<HTML>In My compound design I have throttles on each stage and keep fixed set pressure in the recievers. That is the only way I have come up with to get close to equal torque from each stage over any significant power range. No problem stoping. Each interstage reciever is also a water trap. Pressure in the recirevers is maintained by regulation. The regulators have a high hystrisis so as only to charge the recievers when the normal exhaust steam from previous stage is low. It a makeup steam to keep reciever pressure. I found that the stages need different amounts of steam for equal power at varing power outputs. So I designed in stage sizing so that lower pressure stages always use an equal amount or more steam then the previous. And the regulators provide the little bit of make up steam.</HTML>
Re: Steam Reheating
Posted by: Rolly (IP Logged)
Date: January 06, 2005 04:01PM
<HTML>Balancing a compound or triple has nothing to do with keeping a balanced torque or pressure on each cylinder or rod journal. It’s balancing the expansion in each cylinder at the steam temperature used. Study the valve setting and cylinder size of the older engines.
It is not necessary to have the same torque applied to each journal to have a smooth running engine.
Rolly
Snow 6” today.</HTML>
It is not necessary to have the same torque applied to each journal to have a smooth running engine.
Rolly
Snow 6” today.</HTML>
Re: Steam Reheating
Posted by: Andy Patterson (IP Logged)
Date: January 07, 2005 11:19PM
<HTML>Hi Rolly
I am looking at the torque profile at very low power. Also 6 DA cylanders in a 3 stage compound all evenly spaced at 60 degree throws. A power stroke starting every 30 degrees. A selfstarter with around 45 degrees of cutoff. Thats like 17% cutoff.
You really don't think unequal power from the stages will matter with direct drive at say 5 MPH.
With the make up steam I don't have to have a balanced steam use in each stage. My valving is set up to always have the expansion ratio in each stage. I maintain a fairly constant interstage pressure. The normal control at speed is to vary the clearance cutoff and compression maintaining a set end of expansion pressure just above the interstage pressure. Getting an almost constant efficiency in the power range of this control method. At very low speed I throttle the inlet pressures and maintaine the ending pressures through varing cutoff and clearance. At very low power I have the steam throtlled down close to the ending pressure. The end of expansion is maintained in most power situations except at very high power output at which time the cutoff is extended for more power. It's a very complex control. I have done lots of analysis over the constant expansion power range. I have MathCad work sheets that calculate the cycles. It takes several passes. First calculating 1000 points in the power range. Figuring the displacement of each stage to completely consume the steam from the previous. Finding the max relation so that a stage always takes more then the previous. Then recalculating with makeup steam calculated in.
Each stage is a compression cycle with m amount of inlet steam and r amount of residual (recycled steam). The sum of m and r in the HP stage is equal to 1 pound. m steam in, m steam out. m varies drom around 0.02 to around 0.97 with limitations I have set in clearance and cutoff. So power varation is a littler over 48 to 1. in the first stage. The second stage is then run with the exhaust properties of the first stage si inlet steam. Same thing I have residual and inlet mass for a 1000 points. Now m in the second stage must be greater or equal to the firstage. Forgot I figure a scaling factor in here so as to all second stage m's greater then or equal to the first strage. Then the make up part is figured out for all the cycles of the stage. The make steam is throttled down steam from the main inlet. Once all this down I have the mass output for each of 1000 power levels os the second stage. And it's property. I repeat again for the third stage. The worst case turns that the totle make up part(throtled down steam) is at most 15% of the admitted steam in the lower pressure stages.
The is a very simplistic aproach. My simulator actually simulates the mixing of makeup steam in the recievers. The results of the MathCad work sheets get me the first set of valve timmings I use in my simulations. I hand tune the valve timming and clearance with my simulator. Eventually a microprocessor will control the engine.
My simulator also gives me some idea of adjustment need for variations of the steam properties going into each stage.
I am shooting for an over all 30 to 1 power range in the cutoff control mode. Which is better then a 5 to 1 spead range. 15 to 75 MPH, 25 to 125, 12 to 60 or what ever. At the top of this power range each stage is at around 30% cutoff. The short cutoff gets down to around 1% withe aclearance of around 33% We down to around 42 RPM About 0.0079 seconds of inlet open time. Well above electroic valve open close time of 0.004 seconds. And in this system inlet open degrees increasses with RPM. It stayes close to 8 MS roght on up the end of the power control range and then increasses for more power. The 4 MS open close time of electric latched valves are a problem with a single stage. 3 stages gets to were min valve open time is workable with electric valves.
I tried letting the interstage pressures vary keeping the same mass flow through each stage. The first time I ran a simulation that way Inever could get to a stable situation. I don't have a way of seting initial states in the simulation I did with MathConex. Abd could only run for a short number of revoloutions. It is like a dynamic situation at sim start up. I did get some the longest runs in this mode as it is a simpler model. MathConex has memory leak problems and runs out of memory in short order. Any way the simulator showed up a problem with letting the mass flow self balance in dynamic situations. The actual power output fluctuates wild during the stablazition. The interstage reciever pressures flutuate. Now I can control some of that varing valve timing during transition but it takes a more complex control that has to monoter the interstager recievers and figure that in. I am trying to use a table driven control that takes throttle position and RPM as inputs. Needing only very minor adjustments from other parameters.
I may try again when I get my vissim simulation working. It's a bit easier to set initial conditions with vissim.
I do think a relative smother torque profile is need at low RPM with direct drive. I have no idea at what speed the vehicial inirtia will be enough to smother larger fluctation in the torque profile. How slow can you go in a Stanley at short cutoff before the ride gets jurky. How about up a hill.
As it turns out, it is always the third stages the needs the most make up steam. If I remember right it is at the lowest power that it needs the most steam. Were are not talking about very mush make steam steam to keep from over expansing in the last stage here. The higher enthalpy of the makeup steam also acts like a reheater being throttled down from the first stage. It adds quite a bit of heat.
Torque Profile is the varation of torque through a single rotation of the crank.
Andy</HTML>
I am looking at the torque profile at very low power. Also 6 DA cylanders in a 3 stage compound all evenly spaced at 60 degree throws. A power stroke starting every 30 degrees. A selfstarter with around 45 degrees of cutoff. Thats like 17% cutoff.
You really don't think unequal power from the stages will matter with direct drive at say 5 MPH.
With the make up steam I don't have to have a balanced steam use in each stage. My valving is set up to always have the expansion ratio in each stage. I maintain a fairly constant interstage pressure. The normal control at speed is to vary the clearance cutoff and compression maintaining a set end of expansion pressure just above the interstage pressure. Getting an almost constant efficiency in the power range of this control method. At very low speed I throttle the inlet pressures and maintaine the ending pressures through varing cutoff and clearance. At very low power I have the steam throtlled down close to the ending pressure. The end of expansion is maintained in most power situations except at very high power output at which time the cutoff is extended for more power. It's a very complex control. I have done lots of analysis over the constant expansion power range. I have MathCad work sheets that calculate the cycles. It takes several passes. First calculating 1000 points in the power range. Figuring the displacement of each stage to completely consume the steam from the previous. Finding the max relation so that a stage always takes more then the previous. Then recalculating with makeup steam calculated in.
Each stage is a compression cycle with m amount of inlet steam and r amount of residual (recycled steam). The sum of m and r in the HP stage is equal to 1 pound. m steam in, m steam out. m varies drom around 0.02 to around 0.97 with limitations I have set in clearance and cutoff. So power varation is a littler over 48 to 1. in the first stage. The second stage is then run with the exhaust properties of the first stage si inlet steam. Same thing I have residual and inlet mass for a 1000 points. Now m in the second stage must be greater or equal to the firstage. Forgot I figure a scaling factor in here so as to all second stage m's greater then or equal to the first strage. Then the make up part is figured out for all the cycles of the stage. The make steam is throttled down steam from the main inlet. Once all this down I have the mass output for each of 1000 power levels os the second stage. And it's property. I repeat again for the third stage. The worst case turns that the totle make up part(throtled down steam) is at most 15% of the admitted steam in the lower pressure stages.
The is a very simplistic aproach. My simulator actually simulates the mixing of makeup steam in the recievers. The results of the MathCad work sheets get me the first set of valve timmings I use in my simulations. I hand tune the valve timming and clearance with my simulator. Eventually a microprocessor will control the engine.
My simulator also gives me some idea of adjustment need for variations of the steam properties going into each stage.
I am shooting for an over all 30 to 1 power range in the cutoff control mode. Which is better then a 5 to 1 spead range. 15 to 75 MPH, 25 to 125, 12 to 60 or what ever. At the top of this power range each stage is at around 30% cutoff. The short cutoff gets down to around 1% withe aclearance of around 33% We down to around 42 RPM About 0.0079 seconds of inlet open time. Well above electroic valve open close time of 0.004 seconds. And in this system inlet open degrees increasses with RPM. It stayes close to 8 MS roght on up the end of the power control range and then increasses for more power. The 4 MS open close time of electric latched valves are a problem with a single stage. 3 stages gets to were min valve open time is workable with electric valves.
I tried letting the interstage pressures vary keeping the same mass flow through each stage. The first time I ran a simulation that way Inever could get to a stable situation. I don't have a way of seting initial states in the simulation I did with MathConex. Abd could only run for a short number of revoloutions. It is like a dynamic situation at sim start up. I did get some the longest runs in this mode as it is a simpler model. MathConex has memory leak problems and runs out of memory in short order. Any way the simulator showed up a problem with letting the mass flow self balance in dynamic situations. The actual power output fluctuates wild during the stablazition. The interstage reciever pressures flutuate. Now I can control some of that varing valve timing during transition but it takes a more complex control that has to monoter the interstager recievers and figure that in. I am trying to use a table driven control that takes throttle position and RPM as inputs. Needing only very minor adjustments from other parameters.
I may try again when I get my vissim simulation working. It's a bit easier to set initial conditions with vissim.
I do think a relative smother torque profile is need at low RPM with direct drive. I have no idea at what speed the vehicial inirtia will be enough to smother larger fluctation in the torque profile. How slow can you go in a Stanley at short cutoff before the ride gets jurky. How about up a hill.
As it turns out, it is always the third stages the needs the most make up steam. If I remember right it is at the lowest power that it needs the most steam. Were are not talking about very mush make steam steam to keep from over expansing in the last stage here. The higher enthalpy of the makeup steam also acts like a reheater being throttled down from the first stage. It adds quite a bit of heat.
Torque Profile is the varation of torque through a single rotation of the crank.
Andy</HTML>
Re: Steam Reheating
Posted by: Rolly (IP Logged)
Date: January 08, 2005 12:06AM
<HTML>Andy
You really need to spend some time running steam engines. They’re very forgiving. I’ve never been in a jerky steam car, or boat. They’re all very smooth.
In my Stanley I almost never run, not linked up. What your describing sounds like one complicated mess. You don’t need this in a steam engine.
In a compound or triple you want cylinder sized to accommodate the expansion at the temperature / pressure your working with. In a compound the low is usually is 4, 5, or 6 times the cylinder volume of the high, depending on temperature / pressure of expansion. You don’t want to expand the volume to the point of getting wet steam or water in the cylinder. Leads to cylinder blocks getting cracked. Even Nat Herreshoff gave up on five cylinder expanders for this very reason.
There were some compounds built for high speed running with one high and two lows. Three cylinder compounds. Most had the two lows 180 D to each other and 90 degrees to the high. A lot to triples were not at 120 Degrees on the journals. Some had the 90 degrees between the H and I and 135 degrees from the H to the L but in rotation it was H-L-I for the valves. A lot of compounds that look like there 180 degrees for the journals are not. They are 190 in rotation so as not to have a TDC on one cylinder.
May be I’m just stuck in the old world. (KISS)
Rolly</HTML>
You really need to spend some time running steam engines. They’re very forgiving. I’ve never been in a jerky steam car, or boat. They’re all very smooth.
In my Stanley I almost never run, not linked up. What your describing sounds like one complicated mess. You don’t need this in a steam engine.
In a compound or triple you want cylinder sized to accommodate the expansion at the temperature / pressure your working with. In a compound the low is usually is 4, 5, or 6 times the cylinder volume of the high, depending on temperature / pressure of expansion. You don’t want to expand the volume to the point of getting wet steam or water in the cylinder. Leads to cylinder blocks getting cracked. Even Nat Herreshoff gave up on five cylinder expanders for this very reason.
There were some compounds built for high speed running with one high and two lows. Three cylinder compounds. Most had the two lows 180 D to each other and 90 degrees to the high. A lot to triples were not at 120 Degrees on the journals. Some had the 90 degrees between the H and I and 135 degrees from the H to the L but in rotation it was H-L-I for the valves. A lot of compounds that look like there 180 degrees for the journals are not. They are 190 in rotation so as not to have a TDC on one cylinder.
May be I’m just stuck in the old world. (KISS)
Rolly</HTML>
Re: Steam Reheating
Posted by: Andy Patterson (IP Logged)
Date: January 08, 2005 05:25AM
<HTML>"In a compound or triple you want cylinder sized to accommodate the expansion at the temperature / pressure your working with."
That is great advice.
Do you have one (pressure, temperature, expansion) in mind that can give variable power output. I really just can't figure how to do that.
I don't think any body ever got a triple to work in a car.
Ok! Heres the thing. Figure a triple expansion: inlet 1400 PSIA and 750 F and last stage ending pressure at 20 PSIA exhaust at 15 PSIA 10 to 15 PSIA between end of expansion pressure and reciever pressure in first two stages and what kind of steam quality do you have at the end. Ideal Rankine Cycle comes out around 83% quality. A bit two wet. With compression and the makeup steam it never gets below 95% quality in a static analysis. The engine runs with the same end of expansion point pressures over a torque (sorry I said power before when I ment torque) range of better then 30 to 1.
You are right it is complicated. It is a simple mater to design for a specific power output. Variable power complicates the heck out of things. And very much more so with any kind of a compound.
Doing a normal compound and trying to control power with throttle and cutoff just doesn't work very well efficieny wise. You very easly wind up over expanding the steam. Over expansion kills efficiency quickly. And you also get into wet steam with over expansion. (over expansion: expanding below exhaust pressure).
I like complicated things. Thats why I am a programmer.
Andy</HTML>
That is great advice.
Do you have one (pressure, temperature, expansion) in mind that can give variable power output. I really just can't figure how to do that.
I don't think any body ever got a triple to work in a car.
Ok! Heres the thing. Figure a triple expansion: inlet 1400 PSIA and 750 F and last stage ending pressure at 20 PSIA exhaust at 15 PSIA 10 to 15 PSIA between end of expansion pressure and reciever pressure in first two stages and what kind of steam quality do you have at the end. Ideal Rankine Cycle comes out around 83% quality. A bit two wet. With compression and the makeup steam it never gets below 95% quality in a static analysis. The engine runs with the same end of expansion point pressures over a torque (sorry I said power before when I ment torque) range of better then 30 to 1.
You are right it is complicated. It is a simple mater to design for a specific power output. Variable power complicates the heck out of things. And very much more so with any kind of a compound.
Doing a normal compound and trying to control power with throttle and cutoff just doesn't work very well efficieny wise. You very easly wind up over expanding the steam. Over expansion kills efficiency quickly. And you also get into wet steam with over expansion. (over expansion: expanding below exhaust pressure).
I like complicated things. Thats why I am a programmer.
Andy</HTML>
Re: Steam Reheating
Posted by: Rolly (IP Logged)
Date: January 08, 2005 11:29AM
<HTML>I agree Andy I do not think its practical to use a triple as an auto engine. A compound as the Doble E makes since but not the F.
Rolly</HTML>
Rolly</HTML>
Re: Steam Reheating
Posted by: Andy Patterson (IP Logged)
Date: January 10, 2005 03:33PM
<HTML>Hi Rolly
Thinks for the updates on your Derr Boiler on the SACA froum. I guess it was B&H type boiler. Seams they are simular.
I wound up looking at multi stages because of the valve open close time. I am trying to make electrid valves work. But so far all the electrac valve designs I have found, Take a minum time of around 4 MS. The one design I liked was developed by AURA Systems. They no longer have any info on their site. Seams the have changed directions and into mobol power generation. [www.aurasystems.com]
The AURA design was an electromagnetic latched valve. You had the valve position held between two compressed springs. Electrico magnatics were located at each end of it's possable trevel. Initially a large curent is needed to pull the valve to one side. But once in contact the current can be reduced and you still have a great retaining force when the metel contects are together. A relese of the holding magnetic and the spring on that side being more compressed then the other forces the valve away toword the other magnetic. The other magnetic is energized with holding curent. Initia caries the valve close enough that it is latched. This system takes minum energy as you are not moveing the valve by the electric energy. Just holding it. Letting the springs do the work. A minum of energy is used pulling it to the contact point.
You don't get a lot of movement and are time limited.
It may be impratical or impossable to do what I am trying to do. But one never knows with out trying. The basic idea is that the most efficienct cycle (On Paper) fully expands the steam to exhaust pressure. That is not really pritical in a positive displacement engine. And in reality it seams the best results are just short of full expansion. There are other considerations. Like space requirements etc. But to get the over all best performance and efficiency every things must be balanced to the optimal. Power requirments make the engine displacement neccessarly large with high expansions. And must balanced with boiler size. High pressure doesn't mean a smaller engine if you utilize most all of it's expansive power.
Over expansion to below exhaust pressure really degreads efficiency rapidly. I a single expansion you can reduce expansion all you wont and avoid over expansion. You get better efficiency over it's power band as you are never over expanding. With any engine you have expansion from the boiler pressure down to the exhaust pressure. Part of which takes place in the engine. Hopefully. For efficiency we would like to see most of that expansion take place in the engine (isentropic). Some expansion will always take place outside the engine contrubuting nothing to the output work of the engine. With a normal compound you have a minum specific volume change set by the stage displacements. Operatoring between any two pressure temperature states also sets a specific volume. So a compound with it's it set in concreat minum volume changes dictates a minum operatoring pressure below which you will go into over expansion. So a compound has a minum power range below which efficiency dwops rapidly. In an automobile ie need a wide power range and a high expansion compound is simply eleminiting it's self with it's nerrow bower band limitation.
Andy</HTML>
Thinks for the updates on your Derr Boiler on the SACA froum. I guess it was B&H type boiler. Seams they are simular.
I wound up looking at multi stages because of the valve open close time. I am trying to make electrid valves work. But so far all the electrac valve designs I have found, Take a minum time of around 4 MS. The one design I liked was developed by AURA Systems. They no longer have any info on their site. Seams the have changed directions and into mobol power generation. [www.aurasystems.com]
The AURA design was an electromagnetic latched valve. You had the valve position held between two compressed springs. Electrico magnatics were located at each end of it's possable trevel. Initially a large curent is needed to pull the valve to one side. But once in contact the current can be reduced and you still have a great retaining force when the metel contects are together. A relese of the holding magnetic and the spring on that side being more compressed then the other forces the valve away toword the other magnetic. The other magnetic is energized with holding curent. Initia caries the valve close enough that it is latched. This system takes minum energy as you are not moveing the valve by the electric energy. Just holding it. Letting the springs do the work. A minum of energy is used pulling it to the contact point.
You don't get a lot of movement and are time limited.
It may be impratical or impossable to do what I am trying to do. But one never knows with out trying. The basic idea is that the most efficienct cycle (On Paper) fully expands the steam to exhaust pressure. That is not really pritical in a positive displacement engine. And in reality it seams the best results are just short of full expansion. There are other considerations. Like space requirements etc. But to get the over all best performance and efficiency every things must be balanced to the optimal. Power requirments make the engine displacement neccessarly large with high expansions. And must balanced with boiler size. High pressure doesn't mean a smaller engine if you utilize most all of it's expansive power.
Over expansion to below exhaust pressure really degreads efficiency rapidly. I a single expansion you can reduce expansion all you wont and avoid over expansion. You get better efficiency over it's power band as you are never over expanding. With any engine you have expansion from the boiler pressure down to the exhaust pressure. Part of which takes place in the engine. Hopefully. For efficiency we would like to see most of that expansion take place in the engine (isentropic). Some expansion will always take place outside the engine contrubuting nothing to the output work of the engine. With a normal compound you have a minum specific volume change set by the stage displacements. Operatoring between any two pressure temperature states also sets a specific volume. So a compound with it's it set in concreat minum volume changes dictates a minum operatoring pressure below which you will go into over expansion. So a compound has a minum power range below which efficiency dwops rapidly. In an automobile ie need a wide power range and a high expansion compound is simply eleminiting it's self with it's nerrow bower band limitation.
Andy</HTML>
Re: Steam Reheating
Posted by: Dan Ullfig (IP Logged)
Date: January 10, 2005 11:45PM
<HTML>Hi all:
by the way, i've been reading up on compounds, and the book I have says that the number one reason for using compounds is using high pressure steam. With high pressure, you would have to have impractically short opening of intake valve, to have any decent expansion. But the book says that "modern" steam plants (book was written in 1937) have done away with compounding, by superheating the steam. With superheating, you get a higher amount of energy crammed into the steam, without having to resort to high pressure and compounding.
So I'm starting to lean towards a simple expansion, lower pressure engine for an automobile.
Comments?
Dan</HTML>
by the way, i've been reading up on compounds, and the book I have says that the number one reason for using compounds is using high pressure steam. With high pressure, you would have to have impractically short opening of intake valve, to have any decent expansion. But the book says that "modern" steam plants (book was written in 1937) have done away with compounding, by superheating the steam. With superheating, you get a higher amount of energy crammed into the steam, without having to resort to high pressure and compounding.
So I'm starting to lean towards a simple expansion, lower pressure engine for an automobile.
Comments?
Dan</HTML>
Re: Steam Reheating
Posted by: Andy Patterson (IP Logged)
Date: January 11, 2005 09:40PM
<HTML>Hi Dan
First I have heard of the valve timming reasion in a book. Thats is what I have been saying all along. That to utilize that high pressure efficienctly you large expansion ratios and that means short valve times. How ever in an automobile a major resion for high pressure is engine size. By not utilizing the pressure for efficiency through expansion you get a lot more power in a smaller package.
The following table gives an idea of the benifit of supper heat and pressure. The table gives the theoritical efficiency of a fulexpansion cycle to 14.696 PSIA exhaust. In each row we are looking at two cycles of the same pressure. One operatoring on saturated steam and the other with 1000 F steam.
The first colume is pressure PSIA. Second colume is efficiency operatoring on saturated steam. Third column is the efficiency operatoring at with 1000F steam. The fourth and fifth columns are the rejected exhaust heat(specific enthalpy of the exhaust steam). At the higher pressures operatoring a full expansion on exhaust steam is not recomended as the quality is well below the acceptable level. Some ware between 200 and 250 PSIA we reach the 85% quality upon expansion to atmospheric pressure.
50 8.729% 12.446% 1084.53 1361.01
100 13.489% 18.087% 1047.00 1281.67
150 16.195% 20.971% 1024.67 1240.61
200 18.072% 22.844% 1008.52 1213.60
250 19.501% 24.200% 995.73 1193.76
300 20.649% 25.248% 985.07 1178.22
350 21.603% 26.091% 975.87 1165.52
400 22.418% 26.791% 967.73 1154.82
450 23.125% 27.386% 960.41 1145.56
500 23.749% 27.914% 953.73 1137.26
600 24.806% 28.819% 941.80 1122.74
700 25.675% 29.575% 931.27 1110.31
800 26.407% 30.222% 921.76 1099.39
900 27.034% 30.786% 912.98 1089.64
1000 27.578% 31.284% 904.77 1080.79
1100 28.056% 31.729% 896.99 1072.68
1200 28.477% 32.130% 889.56 1065.18
1300 28.852% 32.494% 882.41 1058.18
1400 29.187% 32.828% 875.48 1051.61
1500 29.486% 33.133% 868.72 1045.41
I think the problem with geting efficiency with higher temperature alone is the higher heat content of the exhaust. You will be paying for that efficiency with increased boiler size to get the higher heat into the steam and again with larger condense to remove it from the exhaust.
At 50 PSIA in the high temperature cycle, 77% of the super heat is going out the exhaust. At 1000 PSIA 56.3 % of the super heat is going out the exhaust. At 1500 PSIA you are down to 55.2%. At 3000 PSIA you get down to 53.7%
At higher pressure more of your super heat BTU's get converted into work.
Not that at 50 PSIA saturated steam you wind up with 1084.53 BTU/lb in the exhaust steam. While at 1000 PSIA and 1000 F (supper heated) steam at exhaust you only have 1080.79 BTU/lb
It not an easy task to get the most out of a steam engine. Most anything will work to some extent.
As you see at lower pressure you will probably need a larger boiler and condensor. If you utilize the expansion available in highe rpressure steam then the engine woild be a bit biger but condenser and boiler can be smaler. It is very hard to figure the optimal parameters. It's more try a bunch of things and see what works. Theory leads to soloutions but the best theoritical soloution may be un-implementable, impritical or to costly.
Andy</HTML>
First I have heard of the valve timming reasion in a book. Thats is what I have been saying all along. That to utilize that high pressure efficienctly you large expansion ratios and that means short valve times. How ever in an automobile a major resion for high pressure is engine size. By not utilizing the pressure for efficiency through expansion you get a lot more power in a smaller package.
The following table gives an idea of the benifit of supper heat and pressure. The table gives the theoritical efficiency of a fulexpansion cycle to 14.696 PSIA exhaust. In each row we are looking at two cycles of the same pressure. One operatoring on saturated steam and the other with 1000 F steam.
The first colume is pressure PSIA. Second colume is efficiency operatoring on saturated steam. Third column is the efficiency operatoring at with 1000F steam. The fourth and fifth columns are the rejected exhaust heat(specific enthalpy of the exhaust steam). At the higher pressures operatoring a full expansion on exhaust steam is not recomended as the quality is well below the acceptable level. Some ware between 200 and 250 PSIA we reach the 85% quality upon expansion to atmospheric pressure.
50 8.729% 12.446% 1084.53 1361.01
100 13.489% 18.087% 1047.00 1281.67
150 16.195% 20.971% 1024.67 1240.61
200 18.072% 22.844% 1008.52 1213.60
250 19.501% 24.200% 995.73 1193.76
300 20.649% 25.248% 985.07 1178.22
350 21.603% 26.091% 975.87 1165.52
400 22.418% 26.791% 967.73 1154.82
450 23.125% 27.386% 960.41 1145.56
500 23.749% 27.914% 953.73 1137.26
600 24.806% 28.819% 941.80 1122.74
700 25.675% 29.575% 931.27 1110.31
800 26.407% 30.222% 921.76 1099.39
900 27.034% 30.786% 912.98 1089.64
1000 27.578% 31.284% 904.77 1080.79
1100 28.056% 31.729% 896.99 1072.68
1200 28.477% 32.130% 889.56 1065.18
1300 28.852% 32.494% 882.41 1058.18
1400 29.187% 32.828% 875.48 1051.61
1500 29.486% 33.133% 868.72 1045.41
I think the problem with geting efficiency with higher temperature alone is the higher heat content of the exhaust. You will be paying for that efficiency with increased boiler size to get the higher heat into the steam and again with larger condense to remove it from the exhaust.
At 50 PSIA in the high temperature cycle, 77% of the super heat is going out the exhaust. At 1000 PSIA 56.3 % of the super heat is going out the exhaust. At 1500 PSIA you are down to 55.2%. At 3000 PSIA you get down to 53.7%
At higher pressure more of your super heat BTU's get converted into work.
Not that at 50 PSIA saturated steam you wind up with 1084.53 BTU/lb in the exhaust steam. While at 1000 PSIA and 1000 F (supper heated) steam at exhaust you only have 1080.79 BTU/lb
It not an easy task to get the most out of a steam engine. Most anything will work to some extent.
As you see at lower pressure you will probably need a larger boiler and condensor. If you utilize the expansion available in highe rpressure steam then the engine woild be a bit biger but condenser and boiler can be smaler. It is very hard to figure the optimal parameters. It's more try a bunch of things and see what works. Theory leads to soloutions but the best theoritical soloution may be un-implementable, impritical or to costly.
Andy</HTML>
Re: Steam Reheating
Posted by: David Nergaard (IP Logged)
Date: January 12, 2005 01:32PM
<HTML>Dan, was your 1937 book talking about locomotives? In locomotives, compounding was abandoned when superheating AND long stroke piston valves became common. The longer stroke valves meant they still had acceptable openings when run "linked up" for shorter cut off. I believe the main reasons for dropping compounding were that the engines required more skill in running and more maintainance than the simples, even when superheated. A superheated compound would have been quite efficient, IF the enginemen learned how to work them properly!</HTML>
Re: Steam Reheating
Posted by: Dan Ullfig (IP Logged)
Date: January 12, 2005 02:56PM
<HTML>David:
Actually, no the book pretty much sticks to stationary power plants, governed to a narrow rpm range.
Dan</HTML>
Actually, no the book pretty much sticks to stationary power plants, governed to a narrow rpm range.
Dan</HTML>
Re: Steam Reheating
Posted by: Robert Goldman (IP Logged)
Date: October 20, 2005 08:36PM
<HTML>Hi Andy
Your idea sounds intriguing but seems (to a previous lurker and possible steam entusiast) that : "The basic idea is that the most efficienct cycle (On Paper) fully expands the steam to exhaust pressure. That is not really pritical in a positive displacement engine." might only apply to a fixed stroke! I need a good software simulator to check but seems a variable (longggg) stroke, duplex compound with control of open/cutoff would improve effieciency. This was not possible before microprocessors and I don't see anyone doing it yet. Maybe computer types think steam is dead?
Bob
ElectricNut</HTML>
Your idea sounds intriguing but seems (to a previous lurker and possible steam entusiast) that : "The basic idea is that the most efficienct cycle (On Paper) fully expands the steam to exhaust pressure. That is not really pritical in a positive displacement engine." might only apply to a fixed stroke! I need a good software simulator to check but seems a variable (longggg) stroke, duplex compound with control of open/cutoff would improve effieciency. This was not possible before microprocessors and I don't see anyone doing it yet. Maybe computer types think steam is dead?
Bob
ElectricNut</HTML>
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