Flow path of cooling water

[Grayson_Stewart]

Every now and then the question is posed concerning the flow direction of cooling water in a condensor, whether its a cooling coil or a leibig condensor. I ran across this while playing on the internt today. Hopefully this will explain to anyone reading over the posts why a cross flow path works more efficiently than having the cooling water travel in the same direction as the vapor.

http://www.srcoils.com/maximize.html" onclick="window.open(this.href);return false;" rel="nofollow

[Guest]

I have been looking at ways to improve cooling without water and have looked at this area a fair bit.

you are geting confused with 2 different cooling techniques.

the technique you are looking at is based on air cooling, in most cases were air cooloing is used you are trying to strip of the most ammount of heat possible in a closed circulated system such as refrigiration, airconditioners, car radiators etc..

in these instances you are better of blowing cool air were your output is and and have the air work its way to the hotter surfaces otherwise you would end up re heating the output with heated air from the input.

with the water cooling commonly used on this site the water is circulated thru the the cooling jacket by a small pump at a rate higher then it is posible for the heat to build up in the liquid flowing thru the condensor.

also the output temperature of the alcohol vapor on the reflux still is approx 82 deg C so a drop of only 5deg C is requierd to start the condensation process.

[Guest]

If my wife is asked, I've been wrong before ...but I'm pretty sure this is not one of those times.

A cooling coil is a heat exchanger, and heat is energy. The purpose of the the heat exchanger is exactly what the name implies...the exchange of energy from one plae to another. This is possible because the world wants to reach a state of equilibrium. An area of high heat will search out an area of lesser heat until the energy is evenly dispersed and can't disperse anymore.

the technique you are looking at is based on air cooling, in most cases were air cooloing is used you are trying to strip of the most ammount of heat possible in a closed circulated system such as refrigiration, airconditioners, car radiators etc..

in these instances you are better of blowing cool air were your output is and and have the air work its way to the hotter surfaces otherwise you would end up re heating the output with heated air from the input.

I agree with the use of cross flow paths that you describe here, but you do have some misleading information when you say "otherwise you would end up re heating the output with heated air from the input". The incoming cooling air that has recieved energy in the form of heat AND traveled along the same path as the material it recieved the energy from will not impart any energy back to the material. The cooling air will only accept energy until the system reaches equilibruim.

with the water cooling commonly used on this site the water is circulated thru the the cooling jacket by a small pump at a rate higher then it is posible for the heat to build up in the liquid flowing thru the condensor

If the liquid was flowing through the cooling jacket at a rate higher than is possible for energy to be transfered (read as: not possible), there would be no cooling effect at all. (I'm discounting the idea of pushing water through so fast that it has such friction as to actually generate surplus energy in the form of heat back to the material being cooled)

also the output temperature of the alcohol vapor on the reflux still is approx 82 deg C so a drop of only 5deg C is requierd to start the condensation process.

The condensation process is not dependent on the loss of temperature but rather the loss of energy. Vapor is going to be at the same temperature at which the liquid was the instant before it went to the vapor state. But its going to have alot more energy than it did in the liquid state. You have to impart a great deal more energy to advance from the liquid state to the vapor state yet still remain at the same temperature( on the order of 7 in alcohol I think) than required to raise the temperature 100 C in the liquid state alone. So technically you can remove enough energy from the vapor to condense the vapor back to liquid and still have the liquid at the same temperature as the vapor was at.

[Grayson_Stewart]

sorry that was me in the last post

[possum]

To guest ...the energy grayson is refering to in refrence to the vapor is called the latent heat of vaporization, and the value varies with the substance in question.After the latent heat of vaporization is overcome, the actual cooling of the liquid further warms the coolant water or air.

You may find the topic interesting to research, I think it is interesting myself.

[Grayson_Stewart]

MIT puts alot of their course work online now. You can go to their web site and see the instructor's notes which are used to teach a class. Most Instructor's have notes which are fairly self explanitory and easy to follow and any difficult concepts can be rooted out with just a little effort and research on the internet. Guess that leaves TN_Frank out.

Anyway, here is the link for a Thermodynamics class. The whole thing would be good to learn, but unless you have a potential bomb of a still all you really need to understand is constant pressure topics. http://web.mit.edu/16.unified/www/FALL/thermodynamics/" onclick="window.open(this.href);return false;" rel="nofollow

Although I never used it till I picked up this hobby, it was one of the more enjoyable classes I had in school.

[Uncle Fred]

Coolant flowing against the direction of the vapor makes sense to me. The water would be warmer where the hot vapor first enters the condenser. The vapor temperature would then decline as it flowed towards the colder part of the condenser.
Why this feels right I can't say. But it sure works well in my new head.

[golden pond]

I built a four way manifold out of PVC pipe with a cap on each end, then drilled a hole in each cap to increase the water pressure. I have two discharge valves below this. I can create a whirlpool effect in the top of my condenser with this setup. Sorry I do not know how to post a picture of this.Try this maybe http://usera.imagecave.com/vaquero/cond-top.jpg" onclick="window.open(this.href);return false;" rel="nofollow

[Chub]

I am trying to understand this so here goes. If both tails of the coil come out of the top of my condenser[offset reflux unit] I would take from this thread that I would want to hook my cooling water, feed/supply[cooler water]to the coil proper; and let it return thru the straight piece of copper pipe going thru the center of the coil. Is this correct? Thanks for the insite. Chub

[linw]

Technically, for a condenser that has vapour flowing right past it, counter flow is more efficient. But for a condenser that has to hit the vapour hard and good, put your entry water into the bottom of the coil where the vapour first meets it. But, probably both will work! (Wot I have been told by people I know have more knowledge than me on these things).

With some condensers e.g. the Still Spirits one, be careful to put the water in at the bottom and let it flow upwards because if you do it the other way round the water can exit faster than it is entering thus emptying the coil. Gets a bit steamy hot!!

[Harry]

Well, pictures being worth a thousand words...




If the two pipes both come out the top, the water will find its own level. Try it. Stand the coil on a tabletop and run water in; first one side, then empty, then in the other side. The level will always equalise, meaning that this type of coil will always be full with coolant, regardless of the flow direction.

As to efficiency, theory says counter-current is best, as the exiting heated coolant can actually become hotter than the vapour, thus removing more heat for a given flowrate.

However in practice, Mikes Nixon & McCaw did many tests on this and discovered that for this application, the opposite was true, i.e. co-current (same way) flow gave better results. I strongly suspect that is because for this application, no vapour is to go past the coil and exit. IOW, the setup is, the coolant enters and exits from the same side of the coil, so too the vapour/condensate (opposite end of the coil, of course).

Please note from my drawing that all talk of counter or co-current flow direction being right or wrong for this single-ended coil design is irrelevant. The vapour meets the coils and coolant at virtual rightangles in both cases. Therefore you should be thinking in terms of crossflow efficiency, for that is what's really happening. The only difference between this coil and the crossflow I designed, is the water friction, and therefore the pressure requirements to drive it.

Of course, as Lindsay inferred, with those poorly designed upright condensers where the inlets & outlets are at OPPOSITE ENDS of the coil, one should ALWAYS feed the coolant from the bottom. Do not try to defy gravity, or the condenser will be partially filled with air, and very inefficient.

Hope that clears up a few misconceptions.

[Ricky]

Man, i concider myself rather inteligent, not highly educated though; but this stuff is deep!

[Guest]

Harry - wondering about the "either way is the same due to the angle of coils/vapor".

The coil will be competely filled both ways, true, but one way the top of the coil will be colder and one way the bottom of the coil will be colder.

Won't this then be subject to the exact same co/counterflow physics as a liebeg? When cooled from the top, vapor/water temps cross, when fed from the bottom they approach the midpoint.

Could you clarify why you say the actual direction (cross-flow) of the water flow matters more than the overall temperature gradient of the coil?

~Rectifier

[TEC]

Of course, as Lindsay inferred, with those poorly designed upright condensers where the inlets & outlets are at OPPOSITE ENDS of the coil, one should ALWAYS feed the coolant from the bottom. Do not try to defy gravity, or the condenser will be partially filled with air, and very inefficient.

You can feed the water into the top of this kind of condenser, if the drain side is under water (no air can get in) and the tube size is the same from the coil to the outlet. All that is necessary is to have the flow high enough to push the air out at the start.

If no part of the flow circuit is exposed to the atmosphere, it is considered a closed circuit and once air is purged it cannot re-enter. Unless of course you reach the boiling point of the cooling liquid.

[Guest]

Harry - wondering about the "either way is the same due to the angle of coils/vapor".

The coil will be competely filled both ways, true, but one way the top of the coil will be colder and one way the bottom of the coil will be colder.

True

Won't this then be subject to the exact same co/counterflow physics as a liebeg? When cooled from the top, vapor/water temps cross, when fed from the bottom they approach the midpoint.

True. And that's why I suspect that Mssrs Nixon & McCaw got the results they did. Look at it this way. In a 'normal' co-current condenser setup, the fluid being cooled normally goes all the way through the condenser to the other side, and the coolant travels in the same direction. The exiting coolant can never be hotter than the condensate, in your words "they approach the midpoint". But in our application, where the condensate travels back out of the condenser shell downwards, the vapour/condensate never travels all the way through the condenser to the other side (unless you haven't set the watercoolant -flow correctly). It falls back down as condensate, or cooled liquid, meeting rising hot vapours AND freshly-injected COLD water at the bottom of the condenser, thus the overall operation is a combination of co-current, counter-current, crossflow AND multi-pass action, and the two fluid products of the condenser (the condensate and the coolant) leave the condenser at opposite ends.

Could you clarify why you say the actual direction (cross-flow) of the water flow matters more than the overall temperature gradient of the coil?

~Rectifier

Good question. Simple answer, condenser size vs vapour volume processed. More complicated answer...

The items that are involved in a condenser, whether they be liquid or vapour, are all referred to as "fluids". You must understand this terminology to understand how condensers work.

Crossflow exists when one fluid flows perpendicular to the second fluid; that is, one fluid flows through tubes and the second fluid passes around the tubes at 90° angle. Cross flow heat exchangers are usually found in applications where one of the fluids changes state (2-phase flow) as in our application, where the vapour changes state to liquid. An example in industry is a steam system's condenser, in which the steam exiting the turbine enters the condenser shell side, and the cool water flowing in the tubes absorbs the heat from the steam, condensing it into water.

Large volumes of vapour may be condensed using this type of heat exchanger flow. Compare the overall size of our coil with a liebig, or a worm-tub that can handle the same volume output. Big difference in sizes, yes?

Actually, most large heat exchangers (condensers) are not purely parallel flow, counter flow, or cross flow; they are usually a combination of the two or all three types of heat exchangers. This is due to the fact that actual heat exchangers are more complex than the simplified types we use for our application. The reason for the combination of the various types is to maximize the efficiency of the heat exchanger within the restrictions placed on the design. That is, size, cost, weight, required efficiency, type of fluids, operating pressures, and temperatures, all help determine the complexity of a specific heat exchanger.

One method that combines the characteristics of two or more heat exchangers and improves the performance of a heat exchanger is to have the two fluids pass each other several times within a single heat exchanger. When a heat exchanger's fluids pass each other more than once, a heat exchanger is called a multi-pass heat exchanger. If the fluids pass each other only once, the heat exchanger is called a single-pass heat exchanger.

Commonly, the multi-pass heat exchanger reverses the flow in the tubes by use of one or more sets of "U" bends in the tubes. The "U" bends allow the fluid to flow back and forth across the length of the heat exchanger. A second method to achieve multiple passes is to insert baffles on the shell side of the heat exchanger. These direct the shell side fluid back and forth across the tubes to achieve the multi-pass effect. In our spiral-wound condenser coils, the coolant is the fluid passed back-and-forth across the vapours' path by virtue of the turns in the coils.


HTH

[Harry]

As you may have guessed, that post was mine. Bloody login didn't work again. :-/

[Grayson_Stewart]

My original post about cross flow was actually intended for Leibig condensors and only generally as far at single wound coils are concerned. All this is really interesting and gives a mind some exercise....I on the other hand have what everyone refers to as a double helix coil so I don't even try to figure out what I've got going on in the cooling shell. Vapor moving that fast is going to be turbulent flow, then combined with a change of direction in the cooling shell as vapor is contracted back to liquid, and the fact that cooling water travels down the inner coils and back up the outer coils Too much for my brain to contemplate.

I just take the civil engineer aproach which is to lean heavily on the fact that all systems are an aproximation of the the actual workings and therefor a simple algebraic will suffice for the more difficult differential Equation type aproach

[linw]

I just take the civil engineer aproach which is to lean heavily on the fact that all systems are an aproximation of the the actual workings and therefor a simple algebraic will suffice for the more difficult differential Equation type aproach

There's even a simpler approach. If it works and is safe, stop worrying!!

[rectifier]

Great reply, Harry! Thanks, I knew I wasn't considering something, and it turned out to be the fact that the condensate doesn't pass the coil...

ahh, simple things can make stuff very complex