Chilled Water Coil Circuiting Made Easy

Chilled Water Coil

Circuiting a chilled water coil 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 a chilled water coil is ultimately up to the performance of that coil. 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.  Check out Capital Coil’s Chilled Water Coils product page.

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Top 5 Reasons Commercial HVAC Coils Prematurely Fail

  • Commercial HVAC CoilsCoil Plugging: If you are not changing filters and/or your commercial HVAC coils are not properly cleaned in a timely manner, your coil will actually begin to act as a filter. When dirt builds up on the coil, that blockage prevents heat transfer and can cause an approximate 20% to 40% drop in performance. Dirt adds to the coil resistance and can be a primary cause for your coil to fail prematurely.
  • Vibration: When your HVAC coils are installed near a moving piece of equipment, vibration can occur and cause leaks. You can tell if vibration is the main cause if leaks are near the tube sheet and look like they are slicing through the tube. If/when that happens, the coils should be isolated from the rest of the system to prevent vibration from causing damage. One way to combat this is by oversizing the tubesheet holes, but many manufacturers will not do this. Condenser Coils are usually the most common victims of vibration.
  • Corrosive Environment: This applies to both the air in the environment and inside the tubes. For instance, if there is a corrosive element in the air, it will eat away at the copper tubes; whether you have 0.020” wall or 0.049” wall. This is very common in coastal areas where there may be salt in the air. To keep the costs down from going to a stainless steel or cupro-nickel coil, we usually suggest coating the HVAC coils. Coatings are almost always within your budget, and its application will only add about a week to the overall lead time. Steam condensate and untreated water can cause corrosion within the tubes of HVAC coils as well. If you have a steam coil that has failed before the one year warranty, there’s a great chance that corrosive agents are in the steam, and it’s eating away at the copper tubes.
  • Freeze-Ups: Most people think that when HVAC coils freeze, the water or condensate laying in the coil freezes into ice and it expands causing the tubes to bulge and eventually spring leaks. What really happens is that the coil will freeze in multiple areas simultaneously, and it’s the pressure between these areas that cause the tubes to swell and eventually burst. These are very easy to spot as the leaks will run the length of the tube rather than around the tube.  ALSO be very careful when considering “freeze-proof” coils!  If you remove 5-6 inches from the fin length to make the “freeze-proof” application fit, your coil’s performance will suffer considerably. 
  • System Design: You would be amazed to learn how many HVAC coils were never designed properly for their systems. If there is a design problem, replacing the coil will only waste time and money; while you have done nothing other than duplicate the previous problem. A little known fact in the replacement market is that a high percentage of all our projects are because the coils were built incorrectly or were never designed correctly in the first place. In some cases, owners attempt to improve the coil’s performance by adding additional rows. Most however do this without taking into account the air pressure drop or fluid pressure drop that comes with it.

When dealing with an HVAC coil manufacturer, try to 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 and can help you diagnose whatever problem that you are experiencing correctly the first time. We look forward to working with you on your next project!

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Why Is Fin Design On HVAC Coils Important?

HVAC Coils

At first glance, fin designs on HVAC coils seem about as exciting as watching grass grow. “Why would I ever care about fin design on any coil” was probably your initial response to our question. Nevertheless, we would not dedicate a newsletter to this subject if fins were not important.

One of the primary reasons fins are so important is that you want to keep your coil as clean and maintained as possible. In order to properly maintain your coil, you need to have an understanding as to how HVAC coils are constructed. While fins do not look like much, they are MUCH more complicated than what you can observe at the entering or leaving airside of the coil.

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, (which we will discuss below).

  1. Fins are known as secondary surface, while tubes are referred to as primary surface. While this may seem counter-intuitive, the secondary surface is responsible for twice the amount of heat transfer as the primary surface.
  2. There are special dies (see picture) that stamp out aluminum or copper fins with the correct thickness, height, and depth to make the coil the correct size. For example, a coil might be 36” (height) x 96” (length) x (8) rows deep x 8 fins/inch.
    1. Fin Height: 36”
    2. Fin Depth: 12”, (8) rows deep
    3. # of fins in the coil: 768 (8 fins x 96”)
  3. Each fin has 192 holes stamped in the fin for 5/8” OD tubes (8 Rows x 24 Tubes), and each fin is identical.
  4. Each hole has extruded metal, which is more commonly referred to as the fin collar. The collars are sized to self-space the fins and allow for later expansion of the tube into the fin collar. This practice is also known as “bonding” and is essential to having your coil run efficiently/correctly.
  5. Each fin is rippled at the entering and leaving edge of the fin to help create air turbulence.
  6. Each fin is corrugated in the direction of airflow to allow for greater air turbulence. This is important to remember because turbulence creates heat transfer.

So again, what is the point of understanding the importance of fins in HVAC coils? While coils can be built with flat fins for various reasons, the vast majority of coils are built with enhanced fins. Enhanced fins help to ensure that the airflow is not running straight through the coil.

Regardless of fin type, keep in mind that HVAC coils can and will act as great “filters”. The tubes are staggered and not in-line; while the fins are designed to help break up the airflow and not facilitate an easy, straight-through air path. Dirt and/or other particles in the air get caught easily, which again, is why coils can act as great filters. Additionally, coils with more rows will usually get dirtier than coils with less rows. Lastly, chilled water or DX coils are typically wet coils, which results in them catching virtually everything in the air.

The amount of BTU’s through any coil is in direct proportion to the amount of air through the coil. For example, if you are only getting 90% of the design air through the coil, then you are only getting 90% of the BTU’s.

Coils require good filtration and periodic maintenance. If not done correctly, you’ll pay the price of higher energy costs on an inefficient coil.

By now, you have hopefully come to the realization that HVAC coils are much more complicated than they appear, and that fins are an integral part of the coil as a whole. Again, while admittedly not the most exciting topic, understanding the role and importance of fins in HVAC coils cannot be overstated. 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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