Frozen Steam Coils: How Do You Prevent This?

Regardless if you have steam coils or steam distributing (non-freeze) coil, you can freeze ANY coil.  When freezes happen, everyone immediately looks to the steam coil as the cause.  When in fact, there are numerous reasons that must be looked at well before the coil.

Freezes generally happen in older systems, however if your new system is not maintained properly or correctly installed, your steam coil can and will freeze.  For instance, you’d be surprised at how many times dampers are left open, controls fail, freezestats don’t work, etc.Steam Coils

In a Standard Steam or Steam Distributing Coil, a freeze-up can occur when condensate freezes within the tubes of the steam coil.  The two most common reasons for freezing steam coils are the steam trap and the vacuum breaker.  The function of steam trap is to remove the condensate as soon as it forms.  Condensate usually collects in the lowest part of the coil.  If your steam trap isn’t installed properly, that condensate will lay in the coil and it will inevitably freeze as soon as it sees outside air.  The vacuum breaker also helps clear the condensate, minimizes water hammer, and helps with uneven temperatures. This must be installed on the control valve and always above the steam trap.

Unfortunately, there are no ways to determine exactly where your steam coil will freeze.  And a common misnomer is that the condensate turns to ice and the expansion is what causes the tubes of the coil to pop.  In reality, it’s the pressure that builds up between freeze points.

Here’s couple tips in your coil design that can help prevent your standard steam and steam distributing coils from freezing:

  • Standard steam coils should NEVER see any outside air below 40 degrees.  If it does, steam distributing is the only way to go!
  • 5/8” OD Steam distributing coils over 72” long are recommended to have a dual supply
  • 1” OD Steam distributing coils over 120” long are recommended to have a dual supply
  • Make sure your steam coil is pitched if possible.  This slopes the condensate to the return connection making it easier to remove the condensate

Give Capital Coil & Air a try on your next project. Our engineering, pricing and service is the best in the industry!

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Need A Replacement Heat Exchanger/Tube Bundle?

To begin with the obvious, shell & tube heat exchangers consist of a tube bundle surrounded and protected by a steel shell. While the shell can seemingly last forever, the tube bundle inside most certainly cannot. The tube bundle is not attached inside the shell, which means that the bundle can be removed if needed to be replaced. In other words, you can simply replace the tube bundle without having to replace the entire heat exchanger. A good comparison can be found with coils inside Air Handling Units (AHU’s). Due to erosion, corrosion or both, coils tend to fail before the entire AHU and subsequently need to be replaced. If everything is done correctly, the new coil should simply “slide” into the place of the old coil. The same is true of tube bundles in that ideally you want to be able to remove the old bundle, and slide in a replacement with little to no extra work. Capital Coil & Air is here to help you throughout the entire process.

Replacement Tube Bundle

The easiest route to ensure that the correct duplicate is built is to first try to cross-reference the model #. While you may not have that information at your fingertips, there is a very good chance that Capital Coil has already built that specific bundle, and/or has the ability to cross-reference whatever model that you are looking to replace. Simply send us a picture of the tag with the model & serial number, and we will do the rest.

Regarding tube bundles, the most common type of replacement bundle can be found in “U” tube type exchangers. These types of bundles are typically water/water or steam/water exchangers.

Steam/Water Bundles

When looking to duplicate a tube bundle, you HAVE to know the bundle’s correct diameter, as well as the diameter of the tubes in order to duplicate the tube area. Additionally, you must duplicate both the tube length and the flange on the bundle to match it up correctly to the shell.

Steam/Water bundles are rather basic in that water is in the tubes, with steam filling up the rest of the shell. Matching performance is almost never the problem, but matching up the dimensions is PARAMOUNT!!  

Water/Water Bundles

Water/water bundles really only differ from steam/water bundles in that there is now water on the shell side as well. Steam automatically fills the shellside, but water does need to be redirected back & forth across the bundle by a series of baffles that operate on GPM & water velocity. The baffles are also an integral part of the bundle, so if duplicated correctly along with the other physical dimensions, performance will not be an issue.  Once again, physical dimensions are PARAMOUNT!

How Can We Go Wrong?

The surest and most common mistake is not correctly matching up the bundle tubesheet to the rest of the exchanger. The tube sheets must fit between the head and the flange on the exchanger, and the bolt-hole pattern must also be an exact match. The bolts are inserted through all three parts simultaneously, and the pattern should match up with all three parts. Sometimes other tube sheets fit between the head and flange, but are also smaller in diameter. The bolts simply bypass the tube sheet and go around it. There needs to be a concerted effort to ensure that the tube sheet dimensions are correct and not too big. Additionally, the gaskets need to be sized and replaced correctly. 

Capital Coil & Air has an established track record of building tube bundles efficiently and quickly. And, in all probability, we already have information on whatever unit you need to get replaced. Like coils, replacing tube bundles usually has a level of emergency attached to it, so we offer multiple quick-ship options if needed. Please give us the opportunity to work with you on your next project, and let us help to take a lot of the guesswork out of your next project.

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Repair or Replace Your HVAC Coils?

Regarding replacement hvac coils, there are multiple reasons why coils can fail prematurely. Sometimes, they 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 on-going maintenance, air quality, and water/steam quality all have an effect on a coil’s lifespan.


Steam 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.

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.

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