How to Make Sense of Heating Coils

Various winter storms have already ripped through multiple parts of the country necessitating the obvious need for working heating coils!!  With conditions such as these, emergencies with your HVAC systems are almost inevitable. THIS is precisely when you need your coil supplier to have the speed and flexibility to be able to tackle whatever emergencies arise.Steam Coil

Of all the types of HVAC coils available, heating coils are probably the easiest to select and understand. Capital Coil has listed the three most common types of heating coils that you are likely to come across.

Hot Water Coils

When looking at a water coil, the first thing to remember is that it is in effect “dumb”. In other words, water coils do not know the temperature of the water moving through it. While standard HVAC water coils are mostly constructed in the same design, the number of rows contained within the coil is a key differentiator in determining if the coil is hot water or chilled water. 99% of all hot water coils are (1) or (2) rows based on performance requirements. The “Delta T” between the entering air temperature and the hot water temperature is very large. As a result, you only need a coil with (1) or (2) rows to do the job. Face velocities across the coil can be anywhere from 600 to 1,200 FPM (feet/minute), while water velocities are usually 2-4 FPS (feet/second). As a result, you will always end up with a (1) or (2) row coil with somewhere between 8-14 fins/inch.

Booster Coils

Hot water booster coils are just another variation of the standard HVAC hot water coil.  They are always (1) or (2) rows as well, but the casing is designed for duct work installation. This basically means there is usually a 1” or 1.5” casing on all four sides of the coil. You select the proper size of the coil to make sure the air velocity across the coil is correct and simply transition the duct work to make the casing size around the coil.  Generally speaking, the size of the coil face is larger than the duct size, so you must transition the duct to make it larger. Just remember, a booster coil is nothing more than a standard hot water coil that is used for duct work installation.

There can also be a casing variation called “slip & drive”, where the coil is installed in the duct work. The coil does not have the standard casing on all four sides like a flanged coil. Capital Coil’s selection program allows you to pick either option.

Steam Coils

The real secret to any steam coil installation is to know that steam coils are much more of a product of the entire system than any other coil; meaning that other coil types are much more independent of the system.  Steam coils require numerous considerations, such as traps, pitched casing, piping flexibility, and vacuum breakers.  Steam coils are also built according to steam pressure through the tubes and headers.  2 – 50 psig is considered “low pressure” steam for coils, while 50-100 psig is considered “medium pressure”. Anything over 100 psig is considered “high pressure” steam.  An important fact to remember is that each of these categories requires a separate kind of construction with different brazing and different materials.  Additionally, almost all steam coils are (1) or (2) rows deep. Steam coils are known either as “standard” steam or “steam distributing”, but the differences are very easy to understand.  Outside air temperatures below 40˚F require that you have a “non-freeze” steam distributing coil.

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How Should Steam Coils Be Designed??

Steam coils

Of all of  the various types of coils, steam coils operate in the most complicated ways. They are, in effect, a product of the system and controls around the coil. If not installed correctly, steam coils simply won’t work properly.

Overview:

The object of any steam coil is to have steam enter the coil as steam and exit as condensate. In a perfect scenario, the BTU load on the coil turns steam into condensate just before it’s ready to exit the coil. Under real world conditions however, condensate usually begins to form inside the tubes almost immediately. Especially when dealing with low-pressure systems, you have to find a way to evacuate the condensate from the steam coil.

Coil Pitch: 

A good coil manufacturer will internally pitch the steam coil within the coil casing to force the condensate toward the outlet connection. This pitch is usually 1/8 “ per lineal foot of coil.

Coil Length: 

If you require steam to travel 144” and make multiple passes through the coil, then, simply put, your system will not work properly. Condensate forms too early, and it cannot escape the coil. Because of this, coils cannot be too long. A better strategy is to break one long coil into two smaller coils side by side, while feeding from both sides.

Tube Diameter:

Steam Distributing coils often have to be 1  1/8 ” diameter tubes. If the BTU load on a coil is really large, then as a result, you will generate many more Lbs./hour of condensate. If the tube diameter is too small, then the condensate, which needs to evacuate, has no place to go.

Traps:

Traps are required on steam coil systems. The traps should be “float & thermostatic” type traps and be located 18 “ below the condensate connection on the steam coil. Without this, the condensate just sits in the system without any place to go.

Vacuum Breakers: 

Vacuum Breakers are often installed in coil systems to remove any excess condensate that may remain within the coil.

Insulated Piping:

There is no such thing as a “Condensate” Heating coil, built as a steam coil. IT DOESN’T WORK.  However, and this happens an astounding amount of times, due to the long distances the steam has to travel from the boiler to the coil, many times, the steam will enter the coil as condensate due to the piping not being insulated.

Anything that makes condensate lay in a coil is harmful to both the steam coil and the system. You will get a “water hammer” when the system is turned on and the incoming steam just blasts against the condensate. Worse than the loud and annoying sound that produces is the fact that it just destroys the steam coil. The brazing was never designed for “water hammer”.  Also, the coils do not heat properly. Have you ever seen a long coil and run your hand down its length only to feel that the entering steam end of the coil is hot but the far end is cold? More times than not, this means that condensate is laying in the coil and not allowing the steam to properly travel the length of the coil.

Steam Coils require a real expertise to design & build. We at Capital Coil have a long history in solving coil problems and building steam coils so that they work correctly the first time. Give us a call for your next job – you’ll be pleasantly surprised!

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Different Types of Steam Coils

There are two types of steam coils:  standard steam coils, which are used in most reheat applications, and steam distributing coils, which are used in applications where the entering air temperature is below 40 F degrees.  Many times, this type of coil is also known as a “non-freeze” coil, but that name is misleading because in reality, there is no such thing as “non-freeze”. 

Standard Steam

Standard steam coils operate a lot like hot water coils, but the construction is very different even if the coils appear to be constructed the same.  The supply and return connections are often on the same end like a hot water coil.  But, steam is very different than hot water, and the coil must be built for and circuited for steam.  Keep in mind that steam is always more erosive than hot water.  The brazing and tube wall thickness must account for steam. ALWAYS remember that even low pressure steam is more erosive than hot water, and a steam coil needs to be built accordingly.

Steam Distributing (Non-Freeze)

Steam distributing coils are a completely different type of coil because they are constructed as a tube within a tube. Every place that you see an outside tube or header, there is an inside tube and header that you can’t see. The steam on the inner tube keeps the condensate in the outer tube from freezing.  The purpose of the Steam Coiloriginal coil design was to distribute the steam evenly along the length of the coil and to eliminate any dead spots on the coil.  A byproduct of this coil was also found.  The coils didn’t freeze nearly as easily as the standard steam coil, so the coils became known as “non-freeze”, which as mentioned, is not completely accurate.  Any coil can freeze under the right conditions, but, this design is what needs to be used when the entering air is under 40F degrees!!! 

Steam Coil Design

Steam coil designs can be very tricky.  Steam coils are totally a function of the system and installation, while other coils operate more independently of the system.  There needs to be correctly designed traps, and they need to be installed in the correct place and depth in the system.  Often, vacuum breakers are also needed in the system.  The piping must also be installed correctly to make sure the steam is entering the coil and not the condensate.  Even with all of those factors, you’ll need a correctly designed steam coil that matches the steam pressure, length of the coil, and the entering air temperature.  Coils can freeze easily.  Coils can be too long in length and the steam cannot travel the length of the coil and distribute evenly.  Condensate can easily be trapped somewhere in the coil, and the result is water hammer. 

Capital Coil & Air has years of experience designing steam coils, and is here to answer any questions and help to design the right coil for your project! 

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