equivalent coil from ss to cooper 4

[pardal]

How it can be calculated the equivalent lenght of a coil used on a chiller.
This are the facts.
Chiller capacity 35000 BTU or 10Kw at 20 °C
The evaporator is a stainless tube coil
tube diam 19.05 mm 3/4" 1.25 mm thick
Tube lenght 18.5 m 60 '

I intend to change it to cooper tube 12 mm diameter 1 mm thick.
Thanks in advance




Pardal

 

[quark]

I caution you to check the material compatibility first. For example, copper can't take care of ammonia. More data viz., internal and external flow rates, type of HX, evaporating temperature of the refrigerant and temp. difference of the cooled fluid etc. required for a better analysis. In the absense of these data, I estimate that you can easily cut down the length by atleast 30%(based on thermal conductivities of copper and SS and considering the decrease in internal convective coefficient).

Regards,

 

[pardal]

Hi Quark.

It will be a change to the same fluid , tap water

The ss coil it's damaged so I need to replace it with a cooper coil.I can not made a ss coil.
I will add some antifreeze and a cooper corrosion inhibitor.
The chiller is set now to 12 °C water leaving temperature.
I understand about the best thermal conductive of cooper .
Could you explain me, why the internal convective will decrease??
It has to be because the smaller tube diameter?










Pardal

 

[Montemayor]

pardal:

No one can give any design data results when you haven't furnished any essential basic data:

1) What kind of coil? Spiral? Helical? Horizontal? Vertical?

2) What fluid is it handling? If a refrigerant, identify it and state what type of refrigeration: direct expansion (so-called "DX") or submerged cooling. This is very, very important. If direct expansion, you're dealing with 2-phase flow (which leads up to the question, how did you come to select the smaller diameter?); if submerged cooling, then you dealing with another totally different type of heat exchange - one more dependent on a heat flux limit.

3) State clearly what you mean by "Chiller capacity 35000 BTU or 10Kw at 20 °C". What, exactly, is at 20 oC? The refrigerant of the cooled product? In other words, what are the design parameters of the cooling taking place? What is the configuration, the temperature, the pressure, and the phases (liquid or gaseous) of the fluids involved.

4) What's the reason for the Material of Construction change to copper? Has existing the coil worked before under the refrigeration load you state? Or is this an academic proposal?

5) No matter what you call it, this is a heat exchanger; and as such, it requires the obvious minimum data to design or rate one: all the flows, the temperatures, the pressures, the phases, and the physical properties.

Without the above minimal data, I can't expect to estimate whether the new, copper replacement coil will be the same length or shorter. Hell, depending on what you are doing and how you employ it, it might be longer!



Art Montemayor
Spring, TX

 

[pardal]

Hi Montemayor:
All my prior facts was , as I think, the fast way to calc by comparation.So as I had a chiller with the up specification , I was trying to copy it.

As I can see you can help me with the end task to do .

I need to calculate the such heat exchanger to the following data.
Chilled water to be used on Air conditioning fan coils.
Coil as a compression spring (helical), cilinder shape.
Position : vertical.
Two coils in paralell one inside the other.
Coil diameter 225 mm and 300 mm .
Direct expansion F 22 , at 58 to 65 psig , just above freezing point.
The condensing unit could deliver about 18.000 Cal/ hour , or 6TR.
The pipe size, because it is what I have on the shelf.
The water will be on a tank at atmospheric pressure .
The water flow : 18000 Cal/hour / 5°C = 3600 Ltr /hour.
Leaving water temp : 6 to 8 °C
Incoming water temp : 11 to 13 °C
PIPE on the shelf: 12 mm diameter , 1 mm thick
Hope you could help me.
Or maybe you could route me wher to learn about this facts.

















Pardal

 

[ClydeMule]

All the equations you may try to use are hugely dependent on so many variables, that it is difficult to come up with an accurate length. Plus you are limited by the fact that you can only get so much coil to fit inside the chiller tank.

It also sounds like a DIY repair project on an existing machine, so just make a coil out of copper with identical to the stainless one and rock and roll. ON a 3 ton it is not that big a deal. If you have a TEV on the system trying to keep a constant superheat, the valve will adjust to the conditions. Again, on a 3 ton, the change in capacity from SS to copper is probably well within the range of the TEV.


SInce the coil is damaged, just make sure your refrigeration system is dry. If the coil leaked and the system ran with a jumpered low pressure switch, you could have a bunch of water in the system. You know have a boat anchor.

Since you sound pretty hands on, why do you just buy a nominal 3 ton brazed plate? May mean a piping change, but probably minor, and if you have room you can't just shove the BP in the tank.

Hey, is this on a Filtrine?

Good luck!

Clyde

 

[Montemayor]

Clyde/Pardal:

Good, common sense response Clyde. Your answer preceded the one I was following up with – but it stated the same as yours so I deleted it.

This 3-ton, low level refrigeration application is so small and low-cost designed (DX is the cheapest first cost type of refrigeration there is) that it doesn’t merit further thought beyond just simply copying the original SS coil. By the way, how could a tougher and thicker coil get damaged and why is it being replaced with a softer and thinner metal??? Just a question, but I think (as you) that I know why. Pardal has a low-cost, tight budget and can’t afford anything else. Under these circumstances the trade off is to copy the original coil and stop wasting time on theory and academics. But be aware that you’ve now introduced a softer and thinner metal; if your first coil resulted damaged in operation, then beware of the replacement undergoing the same – or worse.

The essence of this response is to advise young engineers (& those wannabes) that dominating the basics of mechanical refrigeration is not just taking equipment apart and putting it together. You must appreciate the difference in obtaining the essential, ultimate heat transfer. Direct expansion is fine, when you need to go low budget and compact. But, like in all engineering, there is the ominous TRADEOFF. By circumventing a submerged coil with latent heat basis for design, you now have an apparatus that depends on 2-phase fluid flow and all the misery and unknowns that it is famous for – both in heat transfer and pressure drop. That’s why DX units are not calculated on the design board from a practical point of view. They are estimated, tested under actual conditions and, after verification and modifications, they are mass-produced. They work, but with empirical science and field experience. To try to design a DX unit using solely analytical methods is futile and a mistake. If you don’t believe it, try it and see how the equations start to breakdown under the 2-phase stress and application.

Additionally, I would stress that the convective effects of turbulent, 2-phase flow (which is DX) have a dominant effect on the results as compared to the conductive effect exhibited by either the Stainless Steel or Copper tube. Therefore, for this small application, forget about conductive effects and concentrate on the convective results by using the same SS coil dimensions for the copper tubing. An don't try to get "cute" by using parallel coils. The accurate distribution of the 2-phase mixture is critical and will confuse the results.

The above is the reason why I insisted on hearing "the rest of the story" - which was left out of the original query.


Art Montemayor
Spring, TX

 

[ClydeMule]

Art,

As always, you have backed up your theoretical understanding with pratical advice. I have actually printed some of your replies and put them in my mad scientist notebook that I keep.


Pardal,

I wanted to correct my horrible grammar and typo.

When referring to the brazed plate idea, I meant if you HAVE room you CAN just shove it in the tank.

Clyde

 

[pardal]

Quark, Art and Clyde:
Thanks for you answer .
Forget about the "damaged ss coil"
One self is slave of his words , and king of his silence.
I shall admit my guilt action.
Some time ago I read a post asking about the proper length of a coil to be used as a direct expansion in the same condition of my post.
What was my answer : do try and error and after it state it as the way to do.
I get a lot of answer telling me that this was not the "professional" way to do it .
Now I need to make the such Heat exchanger.
So I try to ask to get the "professional" way to do .
As you explain me, the try and error seem to be the best way to do it.
Budget is the main fact , as PLATE BRAZED are out of my reach.
I had about 30 year repairing air conditioning equipment, but now I need to build inexpensives chillers.
I'm not in USA , I'm from Argentine.
Space is not a problem , we have no regulation , except to give the client what it can afford.

I had made small chillers , about 10000 BTU just modifiyng a window air cond apparatus , just disassembling it, take out the front fan and it's envelope , cuting the shaft in this side, after it I made a small ss tank to put in the evaporator with all it's fins.
The fan and its envelope give space to suit a small pump inside the original chassis.
I made a new blind front , and I had a new chiller to be used on small load demand.
In this case I need to make a 18.000 Calorie chiller, but with out the original evaporator as I will use a commercial condensing unit.
To make my developing procces I will use a 4500 Calorie AC
just to scale it.
After it I will let you know all my experience.
I ask for your indulgence, to be so inrespetuose.
Hope you forgive me.
Pardal
























Pardal

 

[Montemayor]

pardal:

I'm glad we were able to help you out. It was difficult to establish how we could best help without much basic data, but you helped yourself too by sharing some of the details that we needed to know. As you can see, once all the facts are in, most of the guys on the forum can really be of help in resolving your problem. But first they need all the facts and data.

Entiendo tu problema y tus inquietudes ya que tambien he tenido que laborar con el mismo problema que te confronta: el de no tener capital o material para resolver el problema localmente. Por algunos anos trabaje en Peru bajo esas mismas condiciones y alli fue cuando aprendi usar el ingenio como se debe hacer y aprovechar todos los recursos disponibles localmente para ahorrar lo maximo en un minimo de tiempo. Alli aprendi a fabricar serpentines de cobre y acero oxidable para construir mis propios enfriadores, evitando importaciones y fugas de divisas. Te deseo todo exito en tus esfuerzos y que te vaya todo bien.

Art Montemayor
Spring, TX

 

[pardal]

Hola Art , gracias por tus concejos.
Could you giveme some fatcts about what do you didi in such circunstance , at Perú, just the initial kick .
Maybe the best diameter or if parallel coils are better to use.
As comparing with standard air conditioning system you can see , 3/8 finned pipe , and the most splited branchs of evaporator.
Could it be applied to the coil in water system?
Thank in advance


Pardal

 

[Montemayor]

Pardal:

I designed and locally built two classes of applications for spiral (Archimedes Spiral) heat exchangers in Peru. From what I know, most are still working 39 years later. The applications were as follows:

1) Compressor intercoolers and aftercoolers cooling gases such as CO2, air, Oxygen, Nitrogen, and Hydrogen. All gases were saturated with water vapor and were at pressures from 50 psig up to 3,000 psig. Initial gas temperatures were as high as 350 oF and cooling water was used as the coolant fluid.

2) Gas condenser. I used the same basic design to condense CO2 at 1,200 psig with cooling water; I also condensed the CO2 at 250 psig with R-22 (DuPont’s Freon 22).

The reasons I employed my design and fabrication instead of buying existing, proven equipment were:

1) I originally went there to salvage an almost bankrupt operation that suffered from badly engineered and operated industrial gas plants; when I got there the operating and maintenance budget was barely enough to keep going. I needed relief from expensive hard currency required for importation of equipment, so I had to design and fabricate locally to deal in local currency.
2) The pulsating characteristics of reciprocating compressors can be damaging to any heat exchanger and I needed a design that was flexible enough to withstand this feature.
3) The characteristics and hazards of dealing with pressurized, hot gases demanded that I allow for easy and leak-proof tubular expansion, while protecting from leakage with 100% full welding (or soldering).
4) With little or no budget monies, I had to resort to cheap, locally available, and versatile materials of construction. I selected copper tubing (1/4” and 3/8” OD) with sufficient wall thickness to withstand the 3,000 psig. I don’t remember the exact thickness, but I would guess at 1.5 mm. One of the advantages of using small diameter tubing is that it can safely deal with very high pressures using small wall thicknesses.
5) However, selecting a rather “soft” metal as my tube material meant I had to resort to soldering – as opposed to welding. I therefore designed for high Silver content soldering. This worked very well, as will be explained later.
6) A helical coil design is wasteful of space and labor – as well as cumbersome to handle and install; a much better solution on the basic design was an Archimedes Spiral (un “caracol”) which, more importantly, allows for manifolding and making the basic design one that can easily be expanded to larger and larger capacities. This is difficult to explain in writing and really needs to be sketched out in a drawing – as all engineering descriptions should. However, this forum doesn’t allow for this.
7) The basic spiral was made by winding the copper coil around a fixed, 1” flange on a flat, metal shop table. The resulting Archimedes Spiral was repeated with other coils in a similar manner. Since the tubing diameter is small, it keeps its round shape pretty well, without deforming and doesn’t require sand or other fillers to prevent deformation.
8) Two, stainless steel pipes (approximate length = 18 inches) were used as headers – one inlet and one outlet. The size I remember picking as “standard” was 3” schedule 40 in my case. The diameter depends on your applications. It seems that stainless steel takes very well to silver soldering – as does copper – and this gave the design the ability to work as desired.
9) Both SS headers were blinded in one end with butt-welding caps and drilled with staggered holes for the multiple coils to be inserted and soldered to them. One header was outside of the coils, while the other was in the innermost center of the coils. The coils are installed as physically close enough as is possible. This is to further spot-solder each coil to the next to maximize the proximity between them and stimulate little (or no) leakage between them and, in that way, obtain almost pure counter-current heat exchanger flow. I believe I was able to achieve this from the excellent results I obtained later.
10) The objective is to insert the tube bundle into a short, cylindrical steel shell that has one end blinded with a flat, circular plate. The other end has a flange that matches the bundles flat steel cover which is inserted over the headers and welded to them. A standard pipe flange is then attached to each of the two headers.
11) Depending on your configuration, you will pipe cooling water (when used as a cooler) to the shell in accordance of how you want to achieve counter-current flow around the spiral. Since I had a lot of compressor intercoolers that condensed water moisture, I piped up the hot gas entrance into the central header and the outer header was located outside the spirals and at the bottom of the shell. This orientation gas an excellent heat transfer and afforded easy and simple liquid water drainage from the flow, into a vapor-liquid separator.

This basic design can also be used as a refrigeration condenser or an evaporator – whichever you want or need. However, I found that the best configuration for refrigeration purposes was to use the refrigerant in the tubes when condensing it. When you want to use it as an evaporator, then (as in your case) you employ the shell-side as the (low-pressure, evaporating) refrigerant side while the tubes are used for the water being cooled. This works very efficiently. You don’t have to design for counter-current flow when you use the apparatus as a refrigeration evaporator. You expand your high-pressure refrigerant liquid straight into the shellside. The coils function as a pure, submerged evaporator and are used in the “flat position” –i.e., the headers are vertically up. Again, this is difficult to describe without a sketch. In the case of the evaporator application, you of course must allow for a reasonable vapor space on top of the boiling refrigerant liquid which covers all the coils.

Do not use any finned tubes to effect a heat transfer between liquids. Fins are primarily employed to combat the very low heat transfer coefficients found in the gas side. Liquids, fortunately, do not suffer this bad characteristic – especially if you can employ turbulent flow and maximize the convective currents resulting from the same. You will find that there is an extra “bonus” in using spirals. I found this out 25 years later after my field experiences: HTRI has published papers on the extraordinary heat transfer effect obtained from using spirals due to the eddy currents stimulated by that configuration. I never knew of this theory or knowledge (as no books up to that time even mentioned it) but I certainly noticed it when I saw my exchangers produce excellent results that no one expected or could explain away.

A lot of what I learned in my early years came from Graham Heliflow heat exchangers. Graham has a website and you can download some of their technical literature for free. I highly recommend you do this if you want to pursue this idea of designing and fabricating economical and efficient heat exchangers locally in Argentina. I know it can be done – it only requires determination, need, and know-how. The know-how is obtained with diligent study of heat transfer and its domination as well as with practice.

I’m afraid of abusing the space allowed in this thread with more details, so I’ll close and wish you well in your endeavor. I hope I’ve been of service and help. Good Luck!


Art Montemayor
Spring, TX

 

[ClydeMule]

Pardal,

Is it tough to follow Art with a post that can add any more, but I will do my best.

I used to work as an engineer for a chiller manufacturer, now I have my own company and build them myself. Not that it means I know anything, but that is how I try to make a buck.

You mentioned that how others suggested that not doing calculations was not "professional." You are in a different position, and carry a different definition of "professional."

The manufacturer that I worked for used bare tube coils, and only recently switched to brazed plates for some applications. The bare tube coils are built like a condenser or evaporator, but withiout the aluminum fins, and the number of rows deep is roughly equivalent to the number or rows tall. The header plates are copper as well. The thing kind of looks like a big duct heater.

Anyway, these were designed and continually improved by my grandfather (company founder) and the heatcraft sales engineer he worked with for 20 years. Basically they came up with a length/ton and extrapolated from there.

The number used was around 50'/ton, assuming 1/2" copper tubing. Sometime next week I will try to dig up the drawings I have of the evaporators and give you some more concrete numbers.

In addition, I have an old old article from a refrigeration magazine that gives similar recommendation for refrigeration evaporators. These numbers are based on R-12, but I think it would be easily enough to come up with a "fudge factor" for R-22 or R-134a. Or any refrigerant for that matter. But I will have to dig that up later this week as well.

One of the reasons I started to used brazed plates is that the manufacturers have done all the dirty work of the sizing for you. They are not as forgiving as dropping a coil in a tank, but you can ususally come up with a accurate solution.

I think it is interesting to note, that with all of the high level mathematics, thermal design software, and the number of staff engineers carried by a given heat exchanger manufacturer, all of that is backed up with testing, testing and more testing. I have found that most of the time when the manufacturers sizing software gets an update, it is usually because testing data has shown an "adjustment" in the sizing algorithm is necessary.

Almost all that I do is sit around and build and design chillers. Lemme know if I can offer any advice. Since you have had 30 years of repairing HVAC equipment, you know how to braze, evacuate, wire, troubleshoot, etc. That is more than half the battle.

Good Luck!!

Clyde

 

[pardal]

Art and Clyde:
Thanks a lot for your invaluable help.
Please feel free to send me any sketch or drawing to my e-mail.

devitg@ciudad.com.ar

They will be wellcome.

With the best regards.

Gabriel(aka)Pardal

Pardal