The generic terms “industrial “and “heavy duty” are used to describe an HVAC coil that has an arrangement and/or construction that creates quality heat transfer when adverse temperature, pressure, or a corrosive agent are present. Standard HVAC coils are constructed of copper tubes, aluminum or copper fins, and galvanized steel. The normal temperature for these materials is 250 degrees F and 200 PSIG pressure.
Enter Carbon Steel, Stainless Steel, and 90/10 Cupro-Nickel. 70/30 Cupro-Nickel and aluminum are commonly used alternative materials that can be applied if the 3 items mentioned above are present. However, these materials do not transfer heat as well and are usually more expensive.
The additional expense of these alternative materials occurs in two ways: the price per square feet for 90/10 Cupro-Nickel can be anywhere up to double and the additional expense for stainless steel can be up to 3 to 4 times. This is based on using more material, since it doesn’t transfer heat as well and has a higher material price. This can be minimized if the temperature is not over 250 degrees air or gas. Then you can use aluminum fins and not Cupro-Nickel, carbon steel or stainless steel. With aluminum fins, the tube material cost is only 10% to 15% compared to copper tube and aluminum fins. If the fin material also needs to be changed, this can sometimes double the rows in the direction of air flow; this is where the price goes through the roof.
Remember: The fin material change may also be predicated on a corrosive agent being present in the air flow, and aluminum can’t be used in that type of an environment.
The following are the pressure and temperature limitations of these materials*:
|Material||Medium||Pressure PSIG||Temp. F**|
|90/10 Cupro-Nickel tubes||Steam||230||250|
|Carbon Steel (.065”)||Steam||400||500|
|304 Stainless Steel (.049”)||Steam||400||250|
*These limits are based on normal headers and can be increased with heavier duty formed headers.
**Maximum surface temperature may be lowered by the selection of fin material.
The category of “heavy-duty industrial coils” includes removable header and removable plug coils that deal with untreated water; such as, river or lake water or any processed water that will need to have the inside of the coil cleaned at scheduled intervals. Cleaning of the inside of a multi-row coil is a time consuming and arduous task. Removable header coils have a fabricated bolted-gasketed header and can be at 1) the supply side of the coil only, 2) the return bend end only, 3) or at both ends. The downside of this arrangement is that the gasket needs to be replaced almost every time the headers are removed.
The alternative to this arrangement is the placement of “H” style removeable plugs versus the typical rounded return bends that you find on most coils. At the end of each long H is a removable plug that exposes the tube, and these can be on just one of the sides or both ends and usually predicated on access to the ends of the coil. The cost for removable plugs might be slightly less, but the hours of labor to clean the inside of a coil is longer.
Corrosion is a big reason why alternative materials need to be used on a given application. Many of these are process versus normal HVAC applications and a lot of thought and review is required before selecting the arrangement and materials that will give you quality heat transfer and more longevity.
There are so many types of corrosion that we only list them here in general. Corrosion fatigue and stress corrosion cracking are very prominent when steam is used and especially at high steam pressures. Galvanic corrosion, which includes salt laden atmosphere problems, is widespread in HVAC and process applications. Crevice corrosion usually starts as localized pitting of the tubes and comes from oxygen, carbon dioxide, sulfur, chlorine, and ammonia in both fluid and steam coils.
Atmosphere plays a big role in the selection of materials in a coil. Almost always, moisture and the corrosive contaminant must be present to create problems. There are process byproducts, industrial and marine atmospheres, and byproducts from steam treatment that can all cause premature failures.
Coil external coatings have reduced the need for using high priced materials if the corrosive problems are in an application below 250 degrees F temperature. The newer coatings are baked and dipped and provide a coating, a surface bond that usually lasts the lifetime of the coil. Do not get involved with spray coatings that almost never are applied to the inner core of the coil and especially where the expanded tube meets the fins. Most coatings are corrosion resistant, but total coverage is most important to receive longevity. Understand that if only 90% of a coil is coated and a corrosive agent is present, the corrosive agent will attack the 10% that is exposed, and you will need to replace the coil. In other words, more expensive baked and dipped coatings are far superior to the less expensive spray coatings.
Many times, the symptoms of failed heavy-duty industrial finned tube coils can provide the keys to the reason(s) for the failure and how a better design will provide additional longevity in the future.
USA Coil & Air can provide you with the knowledge, products, and engineering service to assist you with your specific applications. If needed, we can even have samples of older coils sent to a lab for a metallurgical analysis, like a DNA of the sampled surface. We know how to analyze the situation, create the best designs, and solve our customers’ problems. We’re the best in the business, take advantage of our 4 decades of knowledge and experience.