Top 10 Chilled Water Coil Facts

Every Chilled Water Coil selection is about balance. Your coil selection balances the rows/fins versus the cost of the coil pressure drops/performance. Trying to cut corners on your initial selection may save you money upfront, but you will inevitably pay it back down the line through added energy costs. This is a truism for every manufactured coil.

  1. Fins cost less money than rows/tubes. A good cost-cutting tool when selecting a coil is to choose 14 fins/inch. This will turn your (8) row coil into a (6) row coil, which will dramatically lower your costs. If you choose to go this route, one thing to keep in mind is that 14 fins/inch will be semi-inconvenient to any maintenance crew tasked with cleaning the coil. Don’t expect a Christmas card from them that year.
  2. That raises the question of whether or not you can even clean a deep (6) or (8) row coil? In short, you can, but it is not easy. Chilled water coils are especially difficult to clean because they are almost always wet. Due to this fact, they typically attract dirt and additional particles that other coils do not. Generally, when cleaning a coil, most of the dirt get pushed to the middle, and for that reason, 14 fins/inch may not be the best idea after all.
    Chilled Water Coils
  3. Did you know that fins do approximately 70% of the heat transfer in a chilled water coil, while the tubes are only responsible for the remaining 30%? This is precisely why the fin/tube bond is so important. Without a perfectly crafted fin/tube bond, coils become inefficient very quickly. You pay for that inefficiency through increased energy costs.
  4. How long does a coil last? At what age can I expect my coil to fail? Unfortunately, there is no single answer to either question. Everything is dependent on a combination of maintenance, duty, and numerous other factors. If your initial selection was correctly chosen, and proper maintenance was kept, 15-20 years is a good timeframe.
  5. You may have a situation where your coil is 20 years old, and everything appears to be operating in good condition. There are no leaks and all looks ok. However, over that length of time, what you don’t see is that the fins have thinned and are no longer bonded to the tubes, and the coil is dirty in places that you cannot see. Again, while the coil may look to be running in top form, it’s probably only running at 60% capacity. Most likely, the tubes have also thinned over time, so when the next deep freeze occurs, you can guess the likely outcome.
  6. You really need to replace the coil, but have been told to make do with the current coil? To make up for the lack of efficiency, you might try to “jury-rig” your system. One method is to change the drive on the fans to deliver more CFM. This increases the air pressure drop, which in turn increases motor brake horsepower. Another option to help increase the coil’s efficiency is to lower the temperature of the chilled water from the chiller. We tend to mess with the system and apply temporary Band-Aids, when replacing the coil is the only guaranteed long-term solution.
  7. If you want to spend money wisely on a chilled water coil, simply make the tubes thicker. The tube thickness for a 5/8” tube coil is .020” thick, so increase the tube thickness to .025”. The same applies for a ½” tube coil, with a tube thickness of .016”. Increase it to .020”. By doing this, you get the added bonus of making your return bends thicker, which also helps to extend the life of the coil.
  8. Not quite sure about circuiting on a chilled water coil? You are going to have a hard time making an accurate selection unless you understand how to circuit a coil. Circuiting is really nothing more than selecting the number of tubes that you want to feed, and how many passes the water makes through the coil – depending on your GPM. Circuiting is one of the most important factors in ensuring that your coil is running at peak-performance.
  9. Curious about the balance between cost, size, materials, and maintenance? Every chilled water coil needs to be maintained for its entire life-span. If you’ve made your selection, and something seems off about the coils, chances are mistakes were made during the selection process. Some indicators include the coil being too big for the space allowed, or incurring out of control energy costs. What is the point of saving $500 on a chilled water coil if you have to spend $5,000 in maintenance over its life-span?

As coil replacement experts, we run into this issue every day. Our goal is to work with you to ensure your selections are correct the first time. The person in charge of budgets will be grateful to you over time. Capital Coil & Air welcomes the opportunity to work with you on your next coil project! We want to be your coil replacement specialists.

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Chilled Water, DX (Expansion) Coils & Moisture Carryover

Moisture carryover is present on DX or Chilled Water coils where dehumidification happens.  Many people do not think it’s a problem. That is, until you have moisture running down duct work or spewing all over the inside of an air handler.  After you’ve experienced that, you probably learned all of these rules regarding moisture carryover.Chilled Water Coils

  • Capital Coil typically does not build chilled water or DX coils over 50”. For applications that call for a 96” high coil, we will build (2) 48” high coils and stack them with an intermediate drain pain. We do this for a couple reasons. First, the shipping & handling is far easier and there’s less chance for damage before the coil even gets to the job-site. Second, the drain pan in the bottom of the unit for a 96” high coil would be enormous. And, it would be practically “raining” off the top of a coil 96” high.
  • Air velocity for chilled water or DX coils should never be higher than 550 feet/min. Anything higher and you are asking for complications. You’d be surprised how many manufacturers won’t tell you that to keep you out of trouble.
  • Entering air temperatures of 80/67 of return air in the Northeast carry far less moisture than an outside 95/78 entering air temperature in Florida. Outside air always has more moisture. Your location plays a part as well. The drain pans will absolutely have be sized differently. Florida’s will be much larger in size.
  • Fin design is irrelevant when it comes to moisture carryover. Whether you have copper corrugated fins, or aluminum flat fins, plate fins or even the old fashioned spiral fins, none of it has any effect on moisture carryover.
  • Lastly, be careful when installing a new chilled water or DX coil in a system. Many end users like to increase the airflow on older coils because those old coils can act like filters, the fins are covered in dirt/dust and you’re not getting the same airflow through the coil. This dirt on the coil also semi-prevents moisture carryover. When that brand new chilled water coil is installed, the airflow might be higher than that 550 ft/minute and that, of course, will cause moisture carryover problems. 

 

Please give us a call with any questions about your coil, your system or its design. Capital Coil’s engineering is unparalleled in the industry!

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What Is Meant By A “Bank” Of Water Coils?

For those that work with HVAC installations on a regular basis, you have run across the problem of needing to install new water coils in very tight, confined areas. The coil is too big to fit in the Chilled Water Coilselevator, and/or the HVAC room is so small that you are likely to damage the coil simply by moving it. As a solution to this challenge, chilled water coils are often installed in “banks” of coils. You are most likely to see this configuration in Air Handler Units, as well as “built-up” systems. Due to face velocity limitations across the coil, you will need larger coils in order to meet your required face area. With this in mind, there are a few specific reasons why you want to avoid having a single, large coil in one of your units.  Starting with the obvious: larger coils are much more difficult to transfer and install. This is especially true for older buildings, where the rooms were essentially built around the HVAC system.

As you’ve probably experienced, some of these areas can barely fit a single person, so installation – if even possible – is a logistical nightmare. Also, the larger the coil, the easier it is to damage during transport to the jobsite. To avoid these issues, simply break down the single, larger coil into smaller coils. When piped together, those smaller coils are stacked into “banks” of coils in the system. If installed correctly, this “bank” should have the same performance as the larger, single coil.

Casing

There are many different casing options available, but “stackable” flanges are required for heavy chilled water coils that are “banked”. The flanges are often inverted inward and down to give added strength to the casing, which is needed due to the fact that another coil of equal weight will be stacked on top of it. When ordering coils in a “bank” configuration, be sure to let the manufacturer know that they will be “stacked”.

Many engineers also use stainless steel casings on chilled water coils. While more expensive than traditional galvanized steel, stainless steel protects against excessively wet coils and/or corrosive elements in the airstream. Keep in mind that the majority of coils fail because of old age and its casing, as opposed to failure with the coil’s core. With that in mind, doesn’t it make sense to select heavy-duty stainless steel casings that are more durable and meant for stackable installations?

Drain Pans & Water Carryover

Water Coils

All chilled water coils must be sized so that the face velocity across the coil does not exceed 550 ft/minute. Water on the outside of the coil is carried away from the coil’s leaving air side in an arc, while water in the highest point of the coil is carried further down the unit or ductwork. “Stackable” coils often require intermediate drain pans under each coil to catch the excess water carryover. Each coil in a bank requires its own drain pan, as a single, large pan under the bottom coil is not enough.

Circuiting/GPM

If all of the coils in a “bank” are of equal size and handling the same CFM, then the GPM of each coil will also be the same.

Always feed the bottom connection on the supply header on the leaving air side of the coil. This ensures counter air and water flow. This also prevents the coil from short circuiting because the header fills first and circuits all of the tubes equally.

Designing Banks Of Coils

Almost all coil “banks” perform more efficiently if you design something more square in shape, as opposed to long and/or high. In a “bank” of coils, you may find that one coil has points of 300 ft/minute, with other points at 800 ft/minute. Scenarios such as this will cause water-carryover! You generally want to be as close to 550 ft/minute as possible in order to allow equal airflow distribution across the face area of the coil.

Anytime you are designing and/or building coils, work closely with the manufacturer as an added resource to ensure that you are getting the ideal solution for your HVAC system. Capital Coil & Air works on similar jobs such as these daily, and we welcome the opportunity to work with you in whatever capacity is needed.

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