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The construction of a Steam Distributing Coil is entirely different than that of a Standard Steam. Everywhere that you see an outside tube or header, there is an inside tube and header that you can't see. All steam is distributed through these inside tubes and headers and slowly released to the outside tubes as the steam turns to condensate. The condensate then flows back down the outside tubes in the same direction that the entering steam comes from. The idea is that all the steam in the inside tubes keep the condensate in the outside tubes from freezing when air passes across the coil at less than 32ºF.

The idea behind the Steam Distributing coil was to keep an even flow of steam across the whole length of the coil, so that there were no uneven heating spots. On long coils, it's very difficult to get steam to the far end of the coil, hence the name "Steam Distributing". Soon, however, manufacturers determined that these coils don't freeze easily and they became known as "non-freeze". This really is a slight misnomer, because under the exactly correct conditions, even non-freeze coils can freeze. They'll tell you that it's possible to freeze any coil. There is no such thing as a "Non-freeze" steam coil. In most commercial applications, however, Steam Distributing coils are used successfully to handle all outside air preheat applications where the entering air is 40ºF or below.

The proper nomenclature to use when describing a steam coil is "Steam Distributing". For this type of coil, you have to to insert an inner tube down the entire length of the outer tube. Steam is distributed down the inner tube evenly. There are holes in the inner tube that are spaced approximately 9" to 12" apart. As the steam turns to condensate (as it travels down the inner tube), the condensate is released through these holes to the outer tube.

Whatever steam doesn't turn to condensate is eventually dumped out the end of the open inner tube to the outer tube. The outer tube is "capped" with no return bends, so that the condensate under pressure has no place to go but back down the outer tube toward the manifold where the steam originally came from. The steam traveling down the inner tube kept the condensate traveling the opposite direction in the outer tube from freezing. Today it's an accepted industry practice to install these coils in applications where entering air temperatures are 40°F or below. Keep in mind however, that Non-freeze coils don't freeze easily, but it's possible to freeze them under the right conditions, and it happens all the time.

Most Steam Distributing coils have a 5/8" outer tube with a 3/8" inner tube. This leaves a space between the tube that is 1/4", but really this space is 1/8" on each side of the tube. From this 1/8" you must subtract the thickness of the outer tube. As you can see, there is very little space between the outer and inner tube to pass condensate back through the outer tube. Whenever you are using a lot of outside air across a Non-freeze coil, the temperature rise and the amount of B.T.U.'s becomes larger. As a result, you need more lbs. per hour of steam and you get more lbs. per hour of condensate that have to pass through the coil. If you want a high air temperature rise or you have a low entering air, then you want to use a 1" Non-freeze steam coil. If you have mostly recirculated air or need only a 30ºF or 40ºF air temperature rise, then a 5/8" Non-freeze coil is usually sufficient.

Standard steam coils might also be termed “u-bend” steam coils. For the most part they look like a hot water coil in that they have two headers, one for the steam supply and one for the condensate return. Frequently the connection on the supply header will be halfway up the header whereas on a hot water coil the supply will usually be at the bottom of the header. The condensate connection must be at the very bottom of the return header in order to not trap steam inside the header. Assuming that the supply and condensate connections are both on the same side, the end of the coil opposite the headers will have return bends just like a water coil. For all your Carrier steam distributing coil needs, browse our site at HVACPLUS. We provide a wide selection off all Carrier products as well as products from many other reputable manufacturers in the HVAC industry.


The principle of a coil is that it moves steam through a tube of heat-conducting material, such as copper, which quickly cools the gas and reduces it to liquid form. The problem with standard coils is that the gas and liquid are not always distributed evenly, resulting in hot or cold spots. In applications below 40 degrees Fahrenheit, standard coils are susceptible to freezing. Distributing coils are sometimes referred to as "non-freeze," but they can be frozen given the right circumstances. The risk, however, is lower.

Steam distributing coils were developed to solve the problem of uneven distribution of the gas and liquid, and a serendipitous result was that the coils were much less likely to freeze. In a distributing coil, a perforated inner tube carries the steam in one direction. This steam escapes through the holes, as it travels, into an outer tube. There the steam condenses and flows back in the opposite direction where it can be removed from the system as condensate through a header. A second header is present where the steam enters the coil. The pressure of the moving steam keeps the system in circulation, and the heat in the inner tube heats the condensate enough to reduce the chances of freezing.

Coils are a standard feature of certain types of heat exchangers, and have wide applications in HVAC including refrigeration and air conditioning. Industrial autoclaves use the same technology.

Carrier steam distributing coil products include 1" outer tubes with a 5/8" inner tube and 5/8" outer tube with a 3/8" inner tube. The most commonly used is 5/8" outer tube, however the effective space between each side of the inner and outer tubes is 1/8". This limited size can be too restrictive for the free passage of condensate. This type of coil should be used where there is a small temperature rise, such as in recirculated air.

One-inch distributing coils have an actual outside diamter of 1 1/8" with a 5/8" inner tube. This leaves a clearance that is double that of the smaller coil. This type of coil is best in applications where there is a high temperature rise or low entering air pressure.

Cost is often the determining factor, since the large diameter coil is more expensive. However, if the cheaper, smaller material is installed inappropriately, the coil can become blocked and need repair or replacement with the correct size.

All Carrier products are high-quality; the company is a recognized leader in the HVAC field. Carrier steam distribution coils are no exception. Proper installation of these units will provide superior performance for years to come.