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
