Understanding the emergence of micro-channel and micro-groove coils
Other than the changes in refrigerants, probably the most important recent developments in the field of HVAC have to do with emerging coil technologies.
Why should this be of interest to you? If your company sells an equipment line, you will probably become keenly aware of the effects and the differences each type of design brings to the table.
As just about everyone knows, the prices of metals have been extremely volatile in recent years. And to keep new equipment prices within a range that common people can still afford, a lot of effort is being put into finding ways to reduce the amount of copper and aluminum used in the coils, as well as the coil sizes.
When it comes to aluminum, the new technology that has become popular for coils is the all-aluminum “micro-channel” design. And for copper, the new “micro-groove” construction technology reduces the coil tube size and wall thickness.
As many likely already know, micro-channel coils look very much like automobile radiators because they are being promoted by automobile-radiator manufacturers such as Delphi.
Micro-channel coils have headers on each side that parallel-connect to thin aluminum “micro-channel” tubes, which are separated zigzag radiator fins. The advantage of using this method is not only that it requires no copper (other than at its external connection lines), but these coils also are:
- Extremely efficient (so they can be smaller and lighter);
- Less expensive to manufacture; and
- The lack of a copper-aluminum bond (where aluminum fins meet copper tubing) means that there is less potential for electrolysis and a slower reduction in the efficiency as the coils age.
However, the aluminum coils have disadvantages because the price of aluminum also is rising. And for technicians, brazing holes in leaking aluminum coils requires a steady hand and cautious use of a torch. In addition, service techs should never use “brightening” solutions to clean them. In fact, even soap and water isn’t recommended because aluminum is sensitive to any form of corrosive materials.
I’ve watched the progression of micro-channel coil designs over the past few years and have noticed some interesting changes. At first, the coils were only designed for use as condensers on straight cooling systems, not as evaporators because of the problems involved in trying to properly drain condensed moisture. About two years ago, that problem was solved and they started offering evaporator coils. However, micro-channel coils still couldn’t be applied for heat pumps because of problems in trying to drain condensation in the defrost cycle (this was solved during the past year).
The premise behind “micro-groove” coil construction is to continue with the standard copper-tube and aluminum fin design we all are so familiar with, but do so with smaller internally enhanced tubes that have thinner walls. This reduces the amount of copper, as well as the cost, size and weight of the coils.
How is it possible to reduce the tubing wall thickness without blowing up the coils? According to the folks at the International Copper Association, the smaller the tube diameter, the stronger its copper wall becomes. Also, putting the tube under pressure hardens it, so the standard pressure charts don’t really reflect actual tube pressure limits.
An advantage of micro-groove copper coils over aluminum micro-channel coils for equipment manufacturers is that this method solves the internal “political” problems companies face when they try to shut down their coil-manufacturing departments.
Although micro-groove coils are much harder to manufacture and assemble (requiring heavy investment in new tooling), they still can be built on site, while manufacturing micro-channel coils is best done by the few companies that have the required expertise. Thus, most micro-channel coils usually are outsourced.
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