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hammurabi
05-09-2006, 03:02 PM
hey
best wishes for all u
the Cu price is so high so I have to optimize tube amount.

can u give me some suggestion about the arrangement of tubes in condensor/evaporator ?
can u introduce some basic thoery about the arrangement of tubes .

by the way, condensor/evaporator is two passes type.the refrigerant is in the shell.

thanks at advance

US Iceman
05-09-2006, 03:51 PM
The tube count is based on the tube OD, the tube pitch, and the clearances required around the tube layouts and the shell ID. TEMA has some specific requirements for this I believe.

Here is a link to their website:
http://www.tema.org/

The TEMA standards provide the construction details for shell & tube heat exchangers for tolerances, baffles, tube supports, head construction, etc. A lot of details to absorb.

The TEMA standards are typically driven by customers who dictate what they want. These are higher quality exchangers.

This is the only source of information I am aware of that offers a great amount of construction details in such a small package.

Almost every shell & tube heat exchanger I have seen is built with triangular pitch tubes. The other option is a square pitch. These are exactly as they sound.

A triangular pitch is: the tubes are arranged in a triangular fashion, just like a pryamid.

A square pitch is: four tubes aligned in a box, 90 degrees apart.

Peter_1
05-09-2006, 06:02 PM
Even if the price skyrockets, you can't just simple reduce number of pipes or the center distance.

I was some months ago in Italy at at coil manufacturer and they explained me there that there's a maximum and a minimum distance between the tubes.

If they're too close, then there's not enough aluminum to transfer the 'cold' from the tubes to the aluminum of the fins and there's also negative interaction between the tubes.
There's also not enough aluminum to transfer the 'passing heat' to the cold refrigerant inside the tubes.

If they're too far, then there's too much aluminum between it where you will have cold and warm zones in the fins, so un unequal spread of the fins. So too much aluminum is used for an optimal effect.

The heart distance you mostly see now is based on very extensive field tests, so I shouldn't try to re-invent the hot water again. ;)

US Iceman
05-09-2006, 07:54 PM
Even if the price skyrockets, you can't just simple reduce number of pipes or the center distance.


You raise a good point Peter. I sort of bypassed this issue.

The heat transfer surface should have been optimized initially. If he can use enhanced surface tubes in a S&T exchanger he might be able to decrease the tube count and then reduce the shell size.

However, he also needs to make sure the enhanced tubes are suitable for their intended service too.

He cannot decrease the heat transfer surface to just save money without loosing something in the process.:o

Andy P
05-09-2006, 10:39 PM
If he can use enhanced surface tubes in a S&T exchanger he might be able to decrease the tube count and then reduce the shell size.

Good point, but enhanced tubes are much more expensive than plain - by a factor of 4 is my memory of them. You would think that as the price of copper goes up the manufacturers of enhanced tubes would narrow the price differential because the labour/machinery cost of working the tube doesn't increase just because the raw material is more expensive. I bet they don't!

Cheers
Andy P

US Iceman
06-09-2006, 01:15 AM
Andy, you are expanding on a theme I find quite interesting... Cost savings.

I don't have a feel for the tube costs anymore, but your factor of 4 does not surprise me. I do remember some of these tubes could be costly.

So by comparing the enhanced tubes vs. the prime tubes the cost differential is a loosing battle.

On the other hand, depending on manufacturing costs the overall differential may be much less.

If we are considering fabricated heads, the smaller heads may cost less.

The smaller diameter shell may be less expensive by:
1) A smaller quantity of holes in the tube sheet X 2
2) smaller shell & less welding time
3) less freight

Even if we consider the "total" cost differential I seem to remember it is still an uphill battle, but the differential may be more acceptable.

Especially if we consider the refrigerant charge for a flooded chiller.



You would think that as the price of copper goes up the manufacturers of enhanced tubes would narrow the price differential because the labour/machinery cost of working the tube doesn't increase just because the raw material is more expensive.


That would be a very good question to ask them Andy. I wonder if they charge a fixed cost or a percentage of material costs for the tube processing?

autt
06-09-2006, 04:59 PM
I have built some model to calculate these things, the model package hereafter includes several kinds of tube-fin and tube-shell heat exchangers, people can click the image to see the input and output data sheets to know more about how it works.
http://www.flycarpet.net/en/Ref/RefrigerationIndex.html
Althrough every single model can be optimized physically, I think it is hard to optimize a single heat exchanger. The fin pitch, center distance, tube diameter, enhanced tube and fin surface, rows depth etc can bet set as optimized parameter, but the result usually increase the flow resistance, cause pump and fan size, weight, and electric cost increased.
People can set a constraint to limit the pressure drop, but how big should they select? Best way is to add these influence, connect a whole system model for optimization. When done this, I think people can get a reasonable optimized HX.
Besides, Cu can be replaced with Aluminum or steel to reduce cost, depends on a good technic systems.

Josip
06-09-2006, 09:49 PM
Hi,


I think it is hard to optimize a single heat exchanger. The fin pitch, center distance, tube diameter, enhanced tube and fin surface, rows depth etc can bet set as optimized parameter, but the result usually increase the flow resistance, cause pump and fan size, weight, and electric cost increased.

People can set a constraint to limit the pressure drop, but how big should they select? Best way is to add these influence, connect a whole system model for optimization. When done this, I think people can get a reasonable optimized HX.
Besides, Cu can be replaced with Aluminum or steel to reduce cost, depends on a good technic systems.

Agree with all said. It is possible to compare the same size evaporators (kW) made by different manufacturers to see the differences. I think there is not too much space for some big optimization.

This is my personal view (maybe I'm not right) but I belive each manufacturer has a team of people researching how to reduce amount of material needed to manufacture an evaporator/condenser.

Can we do that better in our backyards, hardly, :rolleyes: but it is possible to make a trial...

Best regards, Josip :)

autt
07-09-2006, 06:47 AM
Yes Josip you are right, My expression is not so good because of language, except the space for optimizing, I means that people should be careful when they want to optimize a real HX, best way is to put it into whole system for optimization, sometimes a bigger and more cost HX is more economical than a small one, what the customer bears is the whole equip and running cost.

Simply for HX, there are many work for people to do the optimization, for a high yield product, even a little optim will be important.

I don't know why that the sufface enhanced material cost so much, it was said that just coil some metal thread out of the tube can remarkably enhance heat transfer without additional pressure drop, also some micro fin inner tube have same effect, maybe this have little cost? if true it will be a good news.

Thanks and regards,
autt

hammurabi
08-09-2006, 09:55 AM
sorry for coming late.
the evaporator and condensor has two passes. So each pass has the same tube count,am i right?
I doubt of this.

Expect for your view

US Iceman
08-09-2006, 02:17 PM
If you keep the number of tubes equal for each pass, then the velocity will be somewhat uniform. This is what you would want to provide if you expect to have uniform film coefficients in the tubes.

You originally stated you wanted to optimize the use of the tubes, since copper was expensive. Now we are discussing something very basic...?

What is your experience with shell & tube design procedures?

TXiceman
09-09-2006, 09:31 PM
There are several factors that have to be considered in the optimization of a heat exchnager. Are we talking about a DX fed unit cooler or cooling coil? Or are we discussing shell and tube liquid chillers?

On DX feed coils, you have to watch tube velocity, number of circuits fed and also tube diamater and pressure drop. There are many papers on two phase and flashing flow inside a tube. After reading several you will determine this is a very complex item to consider. There are corrolations to cover most areas of use, but they are very limited and at the most will get you into a design area. The best way is emperical data ...test results.

On a shell and tube design, you face the same challenges on a DX feed design. If it is a flooded design, you need to evaluate if the design is better with the fluid in the tubes or in the shell. If you are running the refrigerant in the tubes, is the right answer DX or thermosiphon feed.

Once you go flooded shell side, you can do conventional flooded or do you look at a sprayed refrigerant design.

I have been in this field since 1970 and my mechanical engineering degree was specilaized in aplied heat transfer and thermodynamics. With the advent of computers, it is much easier to look at various designs to see which is the most economical to build. However you do have to know the limits of the particular design program you are utilizing.

I wish I could find a program that would easily let me make the most economical design in just a couple of minutes. But you have to have a lot of seat of the pants feel to get started on the right path.

Ken

US Iceman
10-09-2006, 01:05 AM
Ken,

One program that seems to be used a fair amount is the B-Jac heat exchanger program. The last version of it I used was pretty good for fast analysis.

autt
10-09-2006, 08:25 AM
I also use a fair amount for passes in calculation, because it is easy for programming, or there will have many problems such as iteration, segment consideration etc, but using different amounts will be better than calculation.

autt

TXiceman
11-09-2006, 04:26 AM
If you are looking at a DX chiller or a DX coil, there are several manufacturers out ther with pretty good selection software andmost have a design mode. For DX chillers check out www.ketemalp.com. On DX coild, see if you can get a copy of the Super Radiator program. Another old DOS program is the C Plus (Coils plus)program. If you try to use the RAE software, be very careful, it may or may not be right.

You might also contact the folks at ARI.

ken

Peter_1
11-09-2006, 06:42 AM
On DX coild, see if you can get a copy of the Super Radiator program. Another old DOS program is the C Plus (Coils plus)program

You accidentaly don't have a copy of it or :( a link to their website.

Josip
11-09-2006, 07:53 PM
Hi, Autt :)


Yes Josip you are right, My expression is not so good because of language, except the space for optimizing, I means that people should be careful when they want to optimize a real HX, best way is to put it into whole system for optimization, sometimes a bigger and more cost HX is more economical than a small one, what the customer bears is the whole equip and running cost.

Simply for HX, there are many work for people to do the optimization, for a high yield product, even a little optim will be important.

I don't know why that the sufface enhanced material cost so much, it was said that just coil some metal thread out of the tube can remarkably enhance heat transfer without additional pressure drop, also some micro fin inner tube have same effect, maybe this have little cost? if true it will be a good news.

Thanks and regards,
autt


Agree with you Autt, I am not against optimization but it is much better to optimize complete system, as you said, instead of only one unit.

For me it is the same having a team with only one good player it is hard to win ;)

Best regards, Josip :)

TXiceman
12-09-2006, 02:13 AM
You should be able to find them at http://www.srcoils.com/

Ken