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 Post subject: Jet condenser help needed
PostPosted: Mon Mar 20, 2017 9:23 am 
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Just Starting Out
Just Starting Out

Joined: Sun Mar 19, 2017 3:26 pm
Posts: 4
Boat Name: SND No4
I am responsible for a canal (fresh water) based steam boat with a condenser. It's a jet condenser using canal water as the feed water and the air pump expels it through a pipe through the hull above the water line. The problem is that it doesn't seem to be working. The vacuum gauge shows about 15-20" vacuum being created to begin with but this decays over about 5-10 minutes to zero, from then on the needle jumps but only to about 2". At this point the water inlet pipe to the condenser begins to heat up which I was told was not to be allowed to happen and the engine should be stopped and the condenser allowed to cool. There is a crack in the condenser. Is this the cause? As far as I am aware the crack has been there for a long time, possibly to her working days. Is it ok to allow the condenser to stop working and to overheat? The engine is running at pretty much no load so power loss isn't an issue. The condenser is cast iron.


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 Post subject: Re: Jet condenser help needed
PostPosted: Mon Mar 20, 2017 9:28 am 
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Full Steam Ahead
Full Steam Ahead

Joined: Sat Aug 13, 2011 6:41 pm
Posts: 554
Location: Middle Earth
Boat Name: B.N.Y.S.
I'm no expert, but logic says to me that the crack is indeed the problem in that it opens as it warms up, destroying your vacuum, once the vac is gone the condenser ceases to work and starts to overheat.

Others (more knowledgable) on here will no doubt chime in shortly.

Welcome to the Forum.

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 Post subject: Re: Jet condenser help needed
PostPosted: Mon Mar 20, 2017 11:13 am 
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Warming the Engine
Warming the Engine

Joined: Mon Dec 14, 2015 2:12 pm
Posts: 86
Location: Very eastern England
Boat Name: Platypus, Shelduck
I wonder if there ARE any experts on jet condensers theses days? I wouldn't bet either way, and while I know quite a lot about vacuum systems, jet condensers are certainly outside my competence.

If the water supply pipe to the jet is getting hot, it says that not much, if any, water is flowing through it. It isn't mentioned as to whether that water is pumped, or sucked in under the vacuum produced by the air pump. Assuming the latter, then a crack in the condenser casing would, as Mike says, drop the vacuum and do as is mentioned. There are other possibilities though: if the water inlet jet is partially blocked, it could be that the condenser gets enough canal water into it to apparently work when it's cool, but not enough to keep it that way. The other main possibility is the air pump: it's quite possible that it isn't pumping as well as it should.

Given that the condenser jet and pipework have perhaps had canal water flowing through them for years, I would dismantle them and check them out first, obviously giving them a good clean.

After that the air pump needs checking out next. It will do the main engine no harm to exhaust to atmosphere, given some temporary pipework so that the engine room doesn't get filled with steam. If this is arranged, it should then be possible to shut off both the steam inlet and the jet water inlet to the condenser, and see if a reasonable vacuum can be achieved. If it can't, then either the air pump is excessively "tired", or air leaks into the condenser are too big. Don't go with just my opinion, but I think that a condenser should never get all that hot, and it may be possible to seal cracks using an ordinary silicone sealant. I obviously don't know what seals and valves are employed within the air pump, but I will hazard a guess that dating from 1920 that rubber O-rings etc. are unlikely, and it may rely on close fitting metal pistons and valves, or just possibly oiled leather or other packing. Unless the air pump has had a major overhaul fairly recently, it's probably a good idea to give it one. Vacuum pumps CAN be extremely long lived, but pumping a mixture of air, condensate and canal water doesn't sound good for life to me, and I would guess that an overhaul ought to be considered annually, whether or not it has been used much.

Perhaps it would be better to disconnect the air pump inlet, and put a blanking fitting over it with a vacuum gauge before checking the condenser chamber for leaks. If it doesn't pull a good vacuum with only that connected, it really does need an overhaul.


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 Post subject: Re: Jet condenser help needed
PostPosted: Mon Mar 20, 2017 1:04 pm 
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Just Starting Out
Just Starting Out

Joined: Sun Mar 19, 2017 3:26 pm
Posts: 4
Boat Name: SND No4
One of my university lecturers used to say that in order to become an expert in something you just needed half an hours dedicated study. All I've done is confuse myself! Most info I can find on the web assumes the use of a hot well, and also that the condenser is vertical. Mine is horizontal, about 4" deep, 2' long and from 8" at one end to 18" wide at the other end.

The air pump and auxillaries sit fully on top of the condenser as well, making access to it impossible. Also the only way to check the water inlet is in dry dock as it's about four foot under water. Fortunately she should be dry docked at the end of the year.

I had been considering just bypassing the condenser and allowing the engine to exhaust straight over the side, providing it wouldn't harm the engine.

I shall go back to banging my head against brick walls!


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 Post subject: Re: Jet condenser help needed
PostPosted: Tue Mar 21, 2017 12:37 am 
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Full Steam Ahead
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Joined: Fri Nov 20, 2009 5:34 am
Posts: 1326
Location: Phila PA USA
Boat Name: Margaret S.
A jet condenser requires the wet air pump to remove the condensate, the air that leaks in, as well as the cooling water flow. Cooling water flow is on the order of 20 to 30 times more than the engine exhaust steam flow. With a surface condenser, (or a keel cooler) the pump duty is considerably less.

The vacuum is formed due to the cooling water sprayed into the condensing chamber, usually with several spray nozzles about 1/4 inch diameter. Free discharge of the several nozzles is required to get steam condensing surface on the cool water streams. With adequate vacuum, cooling water is forced into the condensing chamber by river and atmospheric pressure.

Possible issues to check:

1. Air leakage into the condenser, from outright cracks in the condensing vessel to poor gaskets and poor packings.
2. Spray nozzles properly functioning, sometimes partially clogged with river debris, this can let a decent vacuum form when there is very small steam flow, but not enough cooling water for higher steam flows.
3. Proper seals and packings on the wet air pump.


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