Why are HVAC Coils Copper Tube and Aluminum Fin

HVAC CoilsIt’s really not a coincidence why HVAC coils use copper tubes and aluminum fins. Copper is great for heat transfer, and aluminum – while still very effective -is simply not as good. The first goal of any HVAC coil is to cool or heat. Heat transfer is always the first consideration. Cost is the second. Copper works well for the tubes, but would be prohibitive for the fins. You would need a compelling reason for the fins to be copper, and sometimes there are reasons to do just that. However, the vast majority of HVAC coils that you see are built with copper tubes and aluminum fins. That combination offers the most effective heat transfer at the most efficient cost. 

To begin, fins are responsible for a surprising 65% – 70% of the heat transfer on any coil, while tubes are responsible for the remaining 30% – 35%. Additionally, in order for your coil to work at optimum performance, you need to have a terrific fin/tube bond. Fins are known as secondary surface, while tubes are referred to as primary surface. While this may seem counterintuitive, the secondary surface is responsible for twice the amount of heat transfer as the primary surface.

The tubes are expanded into the fins, and for that reason, the fins become secondary. As mentioned above, the fins are responsible for 65% – 70% of all heat transfer that takes place in the HVAC coil.  When you think about it logically, it really makes sense. At 8 fins/inch or 10 fins/inch, and with fins that run the height and depth of the coil, there is much more fin surface than tube surface. However, it also points out how good the fin/tube bond must be in the expansion process. Without that bond, the fins cannot perform their job.

Understanding the role and importance of the materials used in HVAC coils cannot be overstated. There is a distinct reason why the vast majority of coils are constructed using these materials. While coils can be built with other tube materials, such as steel, 304/316 stainless steel, 90/10 cupro-nickel, as well as various different fin materials, none of these are as efficient or economical as copper/aluminum.

Capital Coil & Air is here to help you with any and all coil selections, and we look forward to working with you on your next project.

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Chilled Water Coils – Circuiting Made Easy

Chilled Water Coil

Circuiting chilled water coils is one of life’s great challenges in the coil business. You’re bound to run across folks with years of experience in the industry that can not effectively explain this concept. While not the most exciting of subjects, the necessity of circuiting chilled water coils can not be overstated. Capital Coil & Air has attempted to simplify the idea of circuiting as much as possible.

For starters, circuiting chilled water coils is ultimately up to the performance of those coils. Circuiting is really a balancing act of tube velocity and pressure drop. In other words, think of a coil as a matrix. Each coil has a specific number of rows, and a specific number of tubes within each row. For example, a chilled water coil might be 36 inch fin height and 8 rows deep. The coil has 24 tubes in each row, and multiplied by 8 rows, there is a total of 192 tubes within the coil. While you can try to feed any number of tubes, there are only a few combinations that will work.

    • Feeding 1 tube – you will be making 192 passes through the coil, which will essentially require a pump the size of your car to make that process work.
    • Feeding 2 tubes – equates to 96 passes, and your pressure drop will still be enormous.
    • Feeding 3 tubes – 64 passes, which is still too many.
    • Feeding 4 tubes – See above.
    • Feeding 5 tubes – Impossible as 5 does not divide evenly into 192 (passes).
    • Feeding 6 tubes – Still constitutes far too many passes, which again leads to additional pressure drop.
    • Feeding 7 tubes – Same rule for feeding 5 tubes.
    • Feeding 8 tubes –  Same rule for feeding 6 tubes.
    • Feeding 24 tubes – This feed consists of 8 passes, which is in the ballpark, and with a pressure drop you can live with.
    • Feeding 32 tubes – 32 tubes will see 6 passes. You might see a slight decrease in performance, but it’s off-set by a continuously better pressure drop.
    • Feeding 48 tubes – The magic combination, as 4 passes typically elicits the best performance and pressure drop simultaneously.

 

Rule #1: The number of tubes that you feed must divide evenly into the number of tubes in the chilled water coil.

Rule #2: The chilled water coil must give you an even number of passes so that the connections end up on the same end.

Rule #3: Based on the number of passes, you must be able to live with the resulting pressure drop. Acceptable tube velocity with water is between 2 and 6 ft. per second.

You’re bound to run into different terminologies depending on the manufacturer. More times than not, the different verbiage confuses more than it clarifies. However, understanding the basic tenets of chilled water coil circuiting will remove much of the perceived difficulty.  

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Tips on Hand Designation & “Counter-flow”

Are your chilled water coils right hand or left hand?  Are you looking into the face of the coil with the air hitting you in the back of the head?  What exactly is counter-flow and why is it important?  Are you completely confused by why right hand vs. left hand even exists?  Most manufacturers probably do not know or understand the technical reasons themselves.

First, let’s figure out what coils even need a hand determination.  Chilled Water Coils, Direct Expansion (Evaporator) Coils, and Condenser Coils are the only coils that need this figured on almost every job.  Hot Water Coils, Booster Coils, and Steam Coils rarely need this determination!  The reason for this is when the coils are only 1 or 2 rows deep, they can be flipped over.  When a chilled water coil is 3+ rows deep, hand determination is much more important because it needs to be counter-flow.  With most suppliers determining hand designation with the air hitting you in the back of the head….do you want the connections on the right or left?

Chilled Water CoilsYou’ve probably heard the term “counter-flow” countless times, but here’s the simplest explanation.  For peak performance, you want the air and the fluid traveling in opposite directions through the coil.  Is it the end of the world if your coils are not counter-flow?  The short answer is no, but you will lose anywhere from 12-15% of the output.  So if your coils are piped incorrectly, don’t expect to get the full performance.  Steam and hot water coils are 1 or 2 rows deep, so again, counter-flow is pretty much irrelevant.  However, it can make a BIG difference with any chilled water or direct expansion coils (3-12) rows deep.

We also get asked many times “what is the proper way to pipe coils?”  Put simply, steam coils should always be fed on the highest connection and the return on the lowest connection.  Water coils should always be fed on the lowest connection and returned on the top connection to ensure that all of the tubes are are fed the same volume of fluid. 

Hand designation and counter-flow are two pretty simple concepts when they are properly explained.  When dealing with a HVAC coil manufacturer, partner up with one who will walk you through the engineering and explain it along the way.  Capital Coil & Air has well over a decade of experience in handling pretty much any scenario that you may come across, so we want to be your coil resource for any and all projects. Please give us a try on your next job!

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You should never have to worry about performance on replacement coils…Well, almost never!