Differences Between Standard Steam Coils and Distributing Steam Coils

Steam coils are a critical component in many HVAC systems, particularly for heating applications in a wide range of commercial and industrial environments. There are two common types of steam coils: standard “straight-through” steam coils and distributing steam coils. While both transfer heat from steam to air, they perform this function in distinctly different ways, and are selected for different reasons, which are discussed below.

In all steam coils, steam enters the coil and travels through a series of tubes which are connected to metal fins. Air then travels across the hot tubes and fins, and becomes heated before exiting the other side of the coil. Heat is transferred to the air stream causing the steam to condense into water, which drains from the coil.

At Marlo, we manufacture both standard straight-through steam coils and distributing steam coils in a wide range of sizes and configurations. We custom design and build every steam coil to meet the unique needs of its specific application, using many different types of materials – including copper, aluminum, steel, stainless steel, copper/nickel and others.

Straight-Through Steam Coils

Standard steam coils feature a straight-through steam path. Steam enters the coil at the supply header and travels straight through one or two rows of tubes to the other end of the coil, heating the tubes and fins along the way.

The steam then condenses into water and drains out the return header at the opposite end of the coil. Cool air enters one side of the coil and is heated as it passes through the hot tubes and fins, exiting the coil on the other side at the designated air temperature. Straight-through steam coils can be used in either vertical or horizontal applications.

Straight-through steam coils are typically used in environments where the operating conditions are stable, the temperature of the air entering the coil is above freezing, and steam flow and pressure may be modulated. Advantages of standard steam coils include:

• Cost-effectiveness: The simpler design makes standard steam coils less expensive and easier to maintain.
• High efficiency in non-freezing conditions: Standard steam coils work efficiently in moderate climates and in applications where freezing temperatures are not a concern.
• Simpler installation: Fewer components make these coils quicker and easier to install.

Distributing Steam Coils

Distributing steam coils feature inner and outer steam tubes, with smaller-diameter tubes located inside the larger tubes, which distribute the steam across the coil. The inner tubes have small, backward facing nozzles that “spray” the steam against the inside walls of the outer tubes. This distributes the heat more uniformly, resulting in more consistent air temperatures leaving the coil. Unlike in standard steam coils, the steam condenses into water and drains on the same end of the coil as the steam enters.

The tube-in-tube design not only provides a more uniform distribution of steam – which eliminates cold spots – but also better condensate drainage. With no cold spots in the coil and no water trapped in the tubes, a distributing steam coil virtually eliminates any possibility of the coil freezing. As a result, this type of steam coil is often referred to as a “freeze proof coil” – making it ideal for applications where temperatures fall near or below 32°F. (A more accurate term would actually be “freeze resistant coil”, as all coils can freeze in certain conditions at extreme cold temperatures.)

Distributing steam coils are ideal for applications with low or variable steam pressure, cold or freezing air temperatures, or situations where freeze protection is critical, such as in fresh air intakes and outdoor air handlers in cold climates. Advantages of distributing steam coils include: 

• Freeze protection: By evenly distributing steam and minimizing condensate pooling, these coils prevent the coil from freezing in cold temperatures.
• Improved performance in low pressure conditions: The distributing design ensures steam is utilized effectively, even at low pressures.
• Uniform heating: Distributing steam coils provide consistent heat transfer across the coil surface, which is critical for environments requiring precise temperature control.

Whichever type you need, Marlo has you covered

Whether your application involves high or low freeze risk, and high or low steam pressure, Marlo can provide a standard straight-through steam coil or a distributing steam coil that meets your needs. We can also help you determine which type of steam coil is right for your particular application.

In addition, either type of steam coil can be AHRI certified, assuring that it will deliver the performance determined by our MarloMetrics coil selection software. All AHRI certified steam coils are tested for performance over a wide range of operational conditions by an independent third-party laboratory.

As with all Marlo coils, our steam coils come with a wide range of quality features not provided by other coil manufacturers, assuring you of long coil life, in addition to outstanding performance. And of course, our steam coils are backed by our 3-year warranty – the best in the industry!