Condenser or atmospheric exhaust / a hybrid system?

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Anne from Little Britan
Anne from Little Britan
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Condenser or atmospheric exhaust / a hybrid system?

Post by csonics » Wed Nov 18, 2009 5:12 pm

Posted on behalf of Doug B:

Doug B
Just Starting Out


Joined: 09 Apr 2008
Posts: 8
Location: Issaquah WA , USA
Posted: Mon Sep 29, 2008 8:12 pm Post subject: Condenser or atmospheric exhaust / a hybrid system?
I've steamed about 50 hours this summer with my little 19' fantail, all in fresh water. However, we have occasional meets here in the Pacific NW in the Sound.

I like the sound and look of steam trailing out of the stack, so would like to run atmospheric exhaust most of the time.

But is it possible to build a hybrid system, that is, switching from condenser to stack exhaust and back? Possibly shifting the vacuum pump in and out of gear and additional valving?

I'm running a 20 square foot VFT boiler to power my 2 1/2 X 3 3/4 X 2 fore and aft compound engine and have about 12 gallons capacity of make-up water on board. Steaming a long time in the salt water would be a problem without the condenser.

Any thoughts out there?

Doug B
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Edward
Lighting the Boiler


Joined: 01 Oct 2007
Posts: 39
Location: Ambleside, Cumbria, UK
Posted: Thu Oct 02, 2008 10:35 am Post subject:
Dear Doug B ,

There s no reason why you shouldn't have a hybrid sysem unless the space available isn't sufficient for the additional pipework that will be required . I know of one UK steam launch which is a hybrid and it goes well on both systems

There will be a loss of power/efficiency when puffing rather than condensing as there will be no vacuum on the exhaust side of the LP cylinder. This will lead to an imbalance in the power transmitted to the crankshaft between the HP and LP cylinders but providing the bearings are adequate this shouldn't be a problem .

You say you like the sound of the exhaust on puffers but , depending on the pressure and RPM at which you normally run , you may be surprised at the noise level that an unsilenced exhaust can produce . A simple silencer will reduce this , all it needs to be is a a cylinder between the engine and the stack ; if you lubricate your steam/cylinders a silencer with the exhaust steam introduced at an angle/radially will give a cyclonic flow through the silencer and will reduce the oil fall-out on passengers/environment . If you lubricate you'll need a condensate drain from the silencer to a sump, if not to anywhere you like .

Puffing introduces a virtuous circle : the higher the RPM the larger the amount of exhaust steam , the larger the amount of exhaust steam the greater the draught created through the stack (and therefore the boiler) and the greater the draught the more steam the boiler will produce to meet the demand of higher RPM . This extra draught will be enhanced if the exhaust is introduced into a venturi in the stack. It can be complicated to get the exact size of this venturi and the position of the exhaust nozzle correct , there are just too many variables but most of the standard reference books have examples and a bit of experimentation and a common sense interpretation are unlikely to go far wrong .

A hybrid is definitely feasible but if your boat goes well at the moment why change things ? If you're running on good fresh water you needn't carry make-up water : just dip a bucket over the side . If however you like the idea and the slightly greater flexibility that a hybrid can give there should be no particular problems in incorporating it into your present system .

Yours Sincerely , Edward
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Doug B
Just Starting Out


Joined: 09 Apr 2008
Posts: 8
Location: Issaquah WA , USA
Posted: Thu Oct 02, 2008 5:13 pm Post subject:
Thanks for your fast reply, Edward.

The sound is pretty quiet so a silencer won't be necessary. As I cruise around the Lake, the folks can hear me coming. It's OK. Also space is not a problem.

I know that the engine would perform better with LP vacuum. I'm aware that some folks inject a smal amount of water into the vacuum pump in order to keep a prime. What if I run the vacuum pump all the time and tickle a little water into it? Could I not route that exhaust up the stack (a little water and lots of steam)? A side effect would be wetter exhaust (which I now have pointed at 45 degrees as it just clears the top of the stack). It seems to generally be blown over the side, so getting folks wet isn't a problem. The other down side is a greater drain on the make-up water tank, but as you say, there's the Lake...I've used that a lot this summer.

My main problem with the hybrid system was how to slip the vacuum pump in and out of gear. This way I could run it all the time. My plan is to drive both the feedwater and vacuum pumps off a common # 41 chain sprocket on the crankshaft.

Hey, thanks for your great ideas. Please feel free to comment on my project.

Keep Steamin',

Doug B
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Edward
Lighting the Boiler


Joined: 01 Oct 2007
Posts: 39
Location: Ambleside, Cumbria, UK
Posted: Sun Oct 12, 2008 9:30 am Post subject:
Dear Doug B ,

I'm not sure that I can be of much help as the launch I was referring to has a twin high engine and relied on exhaust pressure to push condensate into the hotwell : so no air/vacuum pump .

My gut feeling is that you would be better off if you disconnect the air pump when puffing . It then won't need any wetness to keep it "lubricated". If I've understood you correctly an option you're considering is to allow just enough exhaust into the condenser to give this wetness to the pump to prevent it from seizing ; I can't see why this shouldn't work but I think you might then have some of the disadvantages of condensing with none of the advantages .

You will have lost the extra power that a vacuum on the LP can give but will still be using some power to drive the air pump ( admittedly not much since it will be doing very little work.) As most of our fairly small launches are not over powered to start with this might be a consideration.

A gearbox would be an expensive and rather power-hungry option . Something like the de-railer gears used on pedal bicycles might suit but I doubt that many of them would be strong enough , though some of the competition mountain bikes are very robust.

Would it be possible to have either an extra sprocket for the chain drive to the air pump and simply remove the chain when puffing or key this sprocket so that by sliding it along the shaft it would disengage from the key/shaft/drive ? Another posible option avoiding the use of two sprockets might be to have two different length chains : the longer to drive both the feed and air pump , the shorter to drive just the feed pump. Then depending on the conditions you are anticipating you fit the required chain .
This solution is perhaps the simplest and cheapest but obviously disconnecting the air pump when switching from condensing to puffing by changing chains , though pretty straightforward , would not be instantaeous .

I don't think a sliding sprocket with a simple dog-clutch would be very difficult to arrange , but it depends on the length of the shaft available for the"slide" and the ability of the chain to acept a certain amount of lateral distortion (it wouldn't be taking any load or even be moving whilst distorted)

So to a non engineer it would seem that there are several possible options , I think it depends how quickly/easily you wish to switch from condensing to puffing and how many colateral changes to your present set up you are able/prepared to make .

Sorry I can't be more precise with my comments/obsevations.

Yours Sincerely , Edward
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