Condenser Cooling water flow

I’im building the interface systems for my triple expansion engine which includes a stand alone cooling water tank/tower for the condenser. The condenser is a two compartment water tube unit meaning the water flows from a pump from the entry end then flows back through the second tube bank before returning to the tank.

The steam enters the top of the condenser body and condensate is pumped out of the bottom.

Question is: which way does the cooling water flow? Should it enter the top, hot section and flow Down or should it get pumped into the bottom and flow up?

i wish I had paid more attention during a previous life when I installed Allen steam turbines fitted with under slung condensers!

Tony

I would go for water in the bottom

H

At a guess water in the bottom makes sense to me.

I suggest the other way round. To maximise power by creating a strong vacuum early the condenser should mix coldest water with hottest steam in order to remove heat as fast as possible. So steam and water should both enter at the same end. In practice with a small condenser it may not make much difference...

Dave

In a full size condenser I would suggest the layout would be as Harry and Jon mentioned, to reduce the thermal shock on the various tubes and plates. It would extend the life of these components significantly. However in model sizes, condenser tubes and plates and joints will be stronger per unit of cooling area so it may not matter at all. Just my $0.02 worth.

Hello Tony,

On the compound marine condensing engine I'm currently building the drawing shows the circulating pump connected to the condenser lower flange position and shows the cooling water 'outlet' at a top positioned flange at the other end. The author of the article states that the engine is based on full size practice so can only assume this is correct - for this engine.

However, in the newly arrived Reeds 'Yacht Trawler and Launch Engines' book it shows just one surface condenser and it is shown with the sea water inlet at one end and the circulating pump - extracting - at the other, both of which enter and leave the condenser at the bottom.

I think Jeffs point about thermal shock has a resonance and with the condensate lying in the bottom of the condenser possibly the reason behind a low inlet position.

Not a defined answer but perhaps helpful in some measure

Regards - Tug

I found about this out as a 10 year old with a love of chemistry! The local chemist gave me a glass condenser, which was duly set up to make distilled water with the cold initially going in the top. Result, steam not distilled water flowed out the bottom. Very disappointed I cursed (sulk), then had a think, switched the water input to the bottom n bingo it worked. By the way I set my wooden retort  stand on fire, I used a mammon  burner for heating 

easy reasoning.  With  the cold water in at the top, it hit the hot steam at the top, it also got hot, therefore cannot cool at the bottom. If. It goes in at the bottom, it will rise being forced to the top getting hotter all the time, but will always be less than the steam. So the bottom where the steam is coolest, get the biggest temp drop with the cold water input. At the top, the cooling water although hot by now is still lower than the steam input

so in at the bottom, out at the top  very logical when you think about it 

Edited By Zan on 23/02/2021 22:33:17

edit.  Dont you hate auto type

Edited By Zan on 23/02/2021 22:34:38

ISTR from when I studied marine engineering at post-trade night school that the general rule of thumb for heat exchangers, including condensers, is for best thermal efficiency the two fluids flow in opposite directions. So steam comes in the top, cooling water enters through the bottom.

This maximises temperature differential between the two fluids thoughout the pass. The rate of heat transfer is proportional to temperature differential so more heat is extracted without making the condenser bigger this way.

That way, your entering coldest water is hitting the coolest steam so there is still a good temperature differential to promote heat flow. Then as the water becomes warmer as it makes its way to the upper passes in the exchanger, it is in contact with the hottest entering steam and again the temperature differential is still large so heat transfer is rapid.

If you feed the cooling water in the top, the fresh hot steam will rapidly heat it up and by the time the water hits the lower passes through the tubes it will be lukewarm, trying to extract heat from also "lukewarm" steam so not much heat transfer will take place due to the small temperature differential. And you will get a result like Zan's above. (Ah those were the days when 10 year old boys played unsupervised with bunsen burners and bought hydrochlorice acid and the components to make gunpowder and the like from the local chemist shop unquestioned!)

The textbook experiment to demonstrate the relation between temperature differential and heat transfer rate can be useful in the workshop.

Question: If you make a cup of tea, without putting the milk in, but need to take a cut on the lathe for five minutes before drinking it, will your tea end up hottest if you put the milk in straight away or five minutes later after taking your cut?

Answer: Your tea will be hottest if you put the milk in immediately on making it and then let it then sit for five minutes. In that five minutes, the lower temperature of the tea/milk mixture means less heat is lost to the atmosphere as it sits there. If you left the hotter, undiluted tea sit there for five minutes the rate of heat transfer to the atmosphere would be higher so more heat lost. Then you put the milk in after five minutes and the final temperature is lower than the other option. I think you can see this demonstrated on YouTube if you search for Professor Julius Sumner Miller.

I'm not saying any of the above is wrong, in fact I think it's right, but it's a condenser, so the steam isn't cooling down it's condensing. That is assuming it's saturated or wet when it comes in.

the purpose of the condenser is to change steam at its saturation temperature to water at the same or slightly below temperature. You don't want to overcool it, as the next thing you would do in a real steam plant is pump it back into the boiler and reheat it

True. They will keep exhaust steam temp as low as possible but i would expect a few degrees of superheat due to the vacuum in the condenser. So talking a fairly small drop in actual temp of the steam vs a massive amount of latent heat to be transferred with no drop in temp. So i think i would still feed from the bottom. Plus that is where you want the condensate to be so the coldest water will do that best.

Greetings, condensers need to operate with the cooling side flooded. With coolant fed in at the top, it might run straight through, "without touching the sides" with limited cooling effect.

The tube bundles often had supports that acted as baffles, causing swirling of the coolant increasing cooling efficiency.

The two flows was always always contra to each other.

Regards

Brian

The contra flow is definitely the ideal.

On the subject of trying to keep the condensate as hot as possible, if you want a good vacuum this is a mistake. Where you have water (condensate) and steam in contact, the lowest pressure that you can attain is determined by the temperature of the water. If you try to lower the pressure further, the water will just boil off, and you can happily boil water off at room temperature with a good vacuum pump. So what you do is use the coldest cooling water you can find, and the biggest condenser you can manage, and draw off the condensate as close as possible to the cooling water temperature. then you pump the condensate, now at atmospheric pressure or higher, through an exhaust steam feedwater heater to bring it back up to near boiling to go into the hot well. The hot water will absorb less air than colder water, as your goldfish would like you to know. I haven't made an exhaust steam feedwater heater yet, so my hot well is running colder than is ideal but when all is well I can get a good vacuum.

You will see discussions in steam boat circles with guys claiming that they can get really hot condensate out of a condenser while also getting a good vacuum. This is not possible and if you consult a set of steam tables you can see what vacuum is actually attainable for any nominated condensate temperature. At 100 degrees C the best you can expect is 14.7 psi absolute, or 0 inches of mercury

Then of course you only need a pinhole leak somewhere in your system, as I seem to have with "Dancer" at the moment, and all that wonderful vacuum will get out.

John

That sucks.

Oww. Sorry.

Yes there is quite a bit more to it than meets the eye. Are those guys using an air pump on their models?

In retrospect i find it amazing that when I took marine engineering classes in the late 1970s, the textbooks (Reeds) were full of triple expansion engines, Scotch boilers and Weirs feed pumps and other 1930s technology. Thats what the government exams were still based on to get a ME ticket. Ships by then had been diesel engines or steam turbines with watertube boilers and rotary pumps etc for 40 years or so.

It was like living in a museum. But interesting thermodynamics on all that old stuff . And not a single calorie of heat was wasted. As an old time chief engineer told me, the warmest overboard discharge came from the urinal.

Yes hopper. I did make gunpowder!

no Duncan the condenser is there to provide a vacuum to give a greater pressure difference, and to reduce the  sink temperature of the engine as much as possible

Regarding condensate temperature, thermal efficiency is all about temperature difference, the hotter the hot source (T1) and cold sink (T2) difference the greater the efficiency so a cold condensate means a bigger temp difference and therefore More efficiency.

Maximum theoretical efficiency = (T1-T2)\ T1. %. Measured in degrees Kelvin

if one considered only the engine efficiency itself and take the boiler heat source out of the equation, steam tables show that a tremendous increase in pressure is needed to increase temperature, and of course that brings lubrication problems associated with reciprocating engines. Remember all early steam engines were powered by the vacuum in the condenser, not the steam pressure

Stumpf in his treatise of the “Unaflow Engine” had a simple maxim “ Keep the hot end hot and the cold end cold” amazingly, his cylinders were cooled in the centre where the exhaust is, and that caused expansion problems resulting in the cylinder needing to be bored barrel shape. so the smaller dia hot ends expand more and result in a parallel bore. This was achieved by reversing the process, the normally water cooled centre jacket was fed with steam while the hot end steam jacket was water cooled thus reversing what is found when the engine is running. His other important aspect by the way was to keep piston end clearance very very small , just a few thou carefully calculated in relation to the overall expansion of cylinder and piston systems.

one of the many reasons aircraft fly so high is because of the air temp being lower results in a lower cold sink. This also enables greater cooling of the turbine blades which run an a higher temperature fluid than their melting point

Edited By Zan on 24/02/2021 10:41:11

Edited By Zan on 24/02/2021 10:44:09

I would put cold water in at the bottom too, and exit at the top. The general convention is that the flows go against each other, as stated above.

Chris

Had to dig out my old 1949 boiler attendant and engine drivers handbook. It shows cold water going in the bottom and says condensate temp in the bottom hot well should be no more than 120 degrees F. Doesnt state what the vacuum is though.

But it goes on to mention a Weirs regenerative condenser that uses incoming steam to then reheat the condensate so it is only one degree colder than the incoming steam, thus preserving as much heat as possible for the next cycle a la Duncan's scenario. It too has the cooling water entering at the bottom and steam entering at the top.

Edited By Hopper on 24/02/2021 11:16:59

Edited By Hopper on 24/02/2021 11:18:04

Just spent 1/2hr reading my 1942 Marine Engineering Book. No mention of thermal shock or concern about directions of flow. It does emphasise the need for:

• input water to be cold enough and in sufficient volume to remove all the steam's latent heat
• steam to be minimally constrained on entry. (No bottlenecks!)
• sufficient space between tubes for the steam to flow efficiently around them. Point made that too many designs concentrate on increasing cooling surface area and spoil the condenser; they perform better after tubes are removed, or 'laned' to improve flow
• efficient removal of air and other gases entrapped with the steam. This gets the most emphasis.

Various designs discussed:

• A thing very like a steam injector, which condenses the steam by forcing a water jet through and a succession of nozzles. Simple, small and does away with the need for a separate air pump. Water in and out at the ends, steam in the middle.
• A pear-shaped container with steam at the top, water in the middle, and water out at the bottom going to a 'wet-air' pump.
• The only tube condenser is a rectanglar box with steam fed in though a large aperture at the top, where it passes over a few hundred water filled tubes. Condensate exits at the bottom again to a wet-air pump. The water enters the tube complex at one side, flows through them to the other side, and then back again to exit under the input. Not crystal clear but this seems to have cold water at the top and warm water at the bottom.

The common feature is steam entering at the top, or middle.

Unfortunately the book doesn't describe a condenser with vertical tubes. This may be because the book covers maritime practice, where the form and layout of machinery is influenced by nautical considerations. Land condensers might well be different.

Dave

Getting the condensate colder than the saturation temperature corresponding to the vacuum won't increase efficiency. If exhausting to say 1 psi (abs) the saturation temp is 102 deg F. This is 28" mercury, a pretty good vacuum. It would take a very good condenser to only just condense all the steam and not overcool the condensate, so the condensate will be a bit cooler, but cooling more than necessary won't help. What governs the level of vacuum? Not sure, but I suspect it is the air pump. Getting a bit out of my depth, but I think a means of removing oil fouling from the outside of the tubes is important.

Good point about making sure the water side is flooded

Edited By ChrisH on 24/02/2021 13:10:53

Edited By ChrisH on 24/02/2021 13:11:59

Chris, Just had another look at this drawing - there is no indication of any internal plates to act as you say. I agree though that that would be the likely effect if the incoming cooling water did not fill the inlet chamber fully

I do note now though that the outflow has a tube that rises to the top of the outlet chamber end which would be drawing water from the top of that end chamber.

I have little knowledge of condensing per se other than the models made - the jet condenser on the corliss and now this one. Despite the claim this is made to full size specifications there are no baffle plates shown.

I'll leave this to those who do have and learn something from them

Tony has had some good input that's for sure

Regards - Tug