Why Are Fin Designs On HVAC Replacement Coils Important?

Replacement HVAC Coils

At first glance, fin designs on HVAC replacement coils seem about as exciting as watching grass grow. “Why would I ever care about fin designs 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 replacement 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.

RELATED POSTS

Repair or Replacement Coils?

Top 5 Reasons HVAC Coils Prematurely Fail

Are You Asking Your Coil Supplier These Questions?

 


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

For those that work with HVAC installations on a regular basis, you have run across the problem of needing to install new chilled 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.

RELATED POSTS

Top 5 Reasons HVAC Coils Prematurely Fail

Coils and Counter-flow: 5 Common Questions

Chilled Water Coil Circuiting Made Easy


Chilled Water, DX (Evaporator) Coils & Moisture Carryover

Moisture carryover is present on DX (Evaporator) Coils or Chilled Water coils where dehumidification happens.  Many people do not think it’s a problem…until you have moisture running down ductwork or spewing all over the inside of an air handler. If you’ve ever experienced that then you probably know all of these rules regarding moisture carryover.

  • 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. Chilled Water Coil

    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 (Evaporator) Coils 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 is here to help you avoid situations like the one described in this post, and we would love for the chance to work with you!

 

RELATED POSTS

10 Things You Need to Know About Chilled Water Coils

Top 10 Chilled Water Coil Facts

Chilled Water Coil Circuiting Made Easy


Top 10 Tips For Measuring HVAC Coils

1. When measuring HVAC coils, performance has very little to do with accurately measuring for replacement coils. Fitting the coil in the existing space with the least amount of labor has everything to do with measuring a coil.  If you duplicate the coil in almost every respect, the performance will match and take care of itself.  New is always more efficient than old.

2.  If you’re ever in doubt about a dimension, smaller is always better than bigger. You can always “safe off” around any coil as long as you can fit it in the space.  If a coil is too big, it makes a really ugly coffee table in your shop.  Too big is the enemy of measuring coils.

Chilled Water Coil

3.  The fin height and fin length are not the determining factors in measuring a coil. The overall casing dimensions are the most important, and you work backwards to determine fin dimensions.

4.  The depth of any coil is the total casing depth in the direction of airflow. The height is the number of tubes high in any row.  Depth is a function of rows deep and height is a function of tubes in a row.

5.  Overall length (OAL) is not the fin length and it’s not the casing length. It is the length from the return bends to include the headers that are inside the unit.  Again, it is necessary to work backwards to get the other dimensions once you know this critical dimension.

6.  Circuiting is the number of tubes connected to the supply header. Generally, you just want to count the number of tubes connected to the header and that will tell you whether it’s full, half, or even a double circuit.  It does not matter how the return bends are configured.  Your goal is to count the number of supply tubes and all performance is based on that.

7.  Fins are measured in fins per inch. Hold a tape measure up to the coils and count the number of fins in one inch.  If you can’t get in to take the measurement, a safe rule of thumb is 10-12 fins/inch.  That will work on almost every coil.  The exception to that rule is a condenser coil.  14-16 fins/inch on a condenser coil is usually pretty safe.

8.  Connection locations are difficult only if you are using the existing piping in the system (which are welded). Copper piping is brazed and can be changed easily.  If a system is old and the piping is being replaced as well as the coil, the connection location is not a major deal.  It’s very easy to match up!

9.  With replacement coils, the concept of “left hand vs. right hand” doesn’t actually exist. Connections are “top left-bottom right” or vice versa.  Ideally, all coils should be counter-flow which means that the water and air flow in opposite directions.  The air hits row one first and the water is piped into row eight first.  However, there are lots of installations that are piped backwards, and they work just fine.  Just match them up, and the coil’s performance will be equal to the old coil.

10.  Connections are not measured from the top of the header! They are measured from the top of the casing to the centerline of the connection.  Or the bottom of the casing to the centerline.  You need a point of reference, and the header height can be anything just as long as it doesn’t stick above or below the casing height.

 

All of the above “suggestions” or “secrets” are in no particular order.  They are just things that you should know to ensure that you are selecting the correct replacement coil. While most seem like common sense, your best bet is to talk with the sales team at Capital Coil & Air, who can walk your through the entire process and help you to fill out coil drawings when trying to measure the dimensions.

RELATED POSTS

Replacement HVAC Coils: 10 Common Ordering Mistakes

The Smart Approach to Buying HVAC Coils (and Saving Money)

Top 10 Chilled Water Coil Facts


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.  

Related Posts

Coils and Counter-flow: 5 Common Questions

Chilled Water Coils & Moisture Carryover

Top 10 Things You Need to Know About Chilled Water Coils


Light-Duty Commercial Belt Drive Air Handler Units

There are lots of jobs that require large custom built Air Handler Units, as well as jobs that require Central Station Air Handler Units that are semi-custom built units. There are also plenty of jobs that Belt Driverequire neither of the above products. This application only requires that a light-duty standard unit be built that will match the duty and requirements of the job.

 Examples

  • Strip Malls/Shopping Centers
  • Dormitories and other college campus buildings
  • Military installations
  • Office areas in factories or other industrial settings
  • Condominiums

These examples do not need large heavy-duty units that are expensive and custom built. Instead, most of these units require standard light-duty construction that can provide 800-12,000 CFM. These units come with various options and choices, and are well-built with the highest quality components available. However, the units are not designed for overkill.

Capital Coil manufactures “Belt Drive” units that operate from 800 CFM (2 Tons) – 12,000 CFM (30 Tons). They are available in both horizontal and vertical airflow styles. Included are blowers, coils, drives, filters as well as additional accessories if required. Additionally, they are available in Chilled Water Coils, DX Coils, Hot Water Coils, and electric heat if needed.

So, why would you want to use these units in lieu of a heavy-duty air handler? Cost, cost, and cost again are the primary reasons. In addition, the light-duty units weigh less, they are more economical to install, and long-term maintenance is much easier. The overall cost of a Custom-Built Unit or a Semi-Custom Station Unit is dramatically higher and not needed in most installations. Please visit Capital Coil’s Belt Drive Product Page for additional descriptions of our units.

Shipping Schedules

  • Standard lead time: (6) weeks
  • (4) week quick-ship: 30% adder
  • (2) day quick-ship: 40% adder

Capital Coil is 100% transparent on all prices and pricing options. Each option is clearly listed on all quotes to help make your buying experience that much easier.

The Air Handler industry is pretty easy to understand when boiled down. If your job allows you to stay within “Production” type units that are built to normal standards without any customizations, you can save a lot on your price. The proper units for your project are still of the highest-grade quality, but they are much more of a fit for commercial, light-duty jobs.

Capital Coil & Air looks forward to working with you on all of your future Air Handler projects. You’ll be glad you did.

RELATED TOPICS


OEM Replacement Coils: Repair or Replace???

When considering OEM replacement coils, there are multiple reasons why coils can fail prematurely. Sometimes, OEM Coils simply freeze and can never be repaired. Other times, the coil was selected incorrectly, which in turn, made the coil significantly underperform. Many times, there is substantial corrosion or something else in the system that causes the coil to fail. However, most coils, when selected correctly, and in systems that are properly maintained, can last anywhere from 10-30 years!  10-30 years is also a pretty wide range, and there are many variables in how long you can expect a coil to perform. Factors, such as ongoing maintenance, air quality, and water/steam quality all have an effect on a coil’s lifespan.

OEM Replacement Coils

Reasons Why Coils Fail Of Old Age

  • While the coil’s tubes are considered the primary surface, 70% of all coil performance is performed by the finned area on a coil, which is known as the secondary surface. The fin/tube bond is easily the most important manufacturing feature in any coil. Without the bond between the tubes and fins, the coil could never properly function. Like all things however, over time the fin/tube bond becomes less efficient with constant expansion and contraction. While the construction of the coil, as well as the fin collars, does not allow the fins on the coil to move, that fin/tube bond naturally weakens a coil’s life over time after installation. Because of this, it is not a stretch to say that a coil is easily 30% less efficient after (20) years.
  • Cleaning coils often pushes dirt to the center of the coil, and this occurs even more so on wet cooling coils. Just remember that coils can become great air filters if not properly maintained. The BTU output of any coil is in direct proportion to the amount of air going through the coil. If you decrease the CFM by 20%, you are also decrease the BTU’s by 20%!
  • Cleaning agents often corrode aluminum fins. Since every square inch of fin surface matters in performance, corrosion of the fin surface is always detrimental to the coil’s performance.
  • Many times, there are coil leaks simply because of old age. No coils are immune to erosion. You might find the brazing in the tubes, as well as the brazing in the header/tube connections failing over time. Steam can be both erosive and corrosive under higher pressures. Water travels through the coil at 2 – 5 ft/second, so erosion is an enormous part of coil failure, regardless of how well-maintained. Erosion is always there, whether you realize it or not.
  • Water/steam treatment and the corrosive effects of bad steam/water can all be causes of coil failure…which then necessitates the need for a reliable manufacturer for OEM replacement coils.

So What Is The Solution?

Some coils can last 5 years, and some coils can last 30 years. As you have read, there are numerous factors that contribute to a coil’s life. In the end, there will most likely have been multiple attempts to repair that coil to make it last as long as possible. The depressing news is that most of these “Band-Aid” attempts do not work well. The most likely outcome is that you are buying a new coil anyway, so why waste the time and money on a temporary solution?

Coil failure is a “pressure event”, which is a fancy way of saying that a coil is leaking. We’ve listed some of the most common repair methods that you are likely to come across:

  • Drop leaking tubes from the circuit: Keep in mind however that every dropped tube reduces the coil’s performance by triple the surface area of the tube that is dropped. Again, while ok in the short-term, this is simply another “Band-Aid” fix. Over time, your energy costs will rise exponentially, and you will probably end up buying a new coil anyway.
  • Braze over the existing braze: As mentioned above, erosion has caused the original braze to fail, so all that you are really doing is pushing the pressure to another braze, which will then begin to fail as well.
  • High Pressure Cleaning: This method bends the fins, further restricts the airflow, and pushes dirt more to the center of the coil, which can never be adequately cleaned.

The real reason why coils need to be replaced rather than repaired is due to energy costs. If your coil is not operating near desired levels, you’ll need to increase the energy to make it work at its peak performance. Energy increases might be slight at first, but they are guaranteed to continue to rise over time. For example:

  • Somebody adjusts the fan drive for higher speeds, higher CFM’s and higher BTU’s.
  • Someone adjusts the boiler; the water and steam temperatures are higher.
  • Someone adjusts the chiller (1) degree higher for colder water to the chilled water coil.

Whichever method is used, performance begins to suffer and adjustments to the system occur. These adjustments cost energy efficiency and ultimately, money!

If you have ever experienced repairing a coil, then you know it is labor intensive and typically will not work as a permanent solution. With very few exceptions, repairs should be seen as nothing more than temporary until you’re able to replace that coil!

Capital Coil & Air has seen every “repair” method used, as well its inevitable outcome, so instead of putting yourself through that, call Capital Coil and allow us to be your coil replacement experts.

RELATED POSTS

Is Your Quick-ship Shut Down When Needed Most??

Four Things That You Need When Buying Replacement Coils

Replacement HVAC Coils: 10 Common Ordering Mistakes

Top 5 Reasons HVAC Coils Prematurely Fail