Why Steam Coils Freeze, and How to Prevent It

Steam heating coils are widely used in commercial and industrial HVAC systems and manufacturing processes, because they provide fast, efficient heat transfer. Yet despite their effectiveness, steam coils have a downside in climates where temperatures can fall below or near freezing. In these environments, steam coils can freeze – often leading to ruptured tubes, damaging leaks, downtime, and expensive repairs.

Understanding why steam coils freeze and implementing proven design, installation, and operational strategies can dramatically reduce the risk of freeze damage and extend coil life.

How a Steam Coil Can Freeze – Even When Steam Is Hot

It seems counterintuitive that a steam coil – carrying high-temperature steam – could freeze. In reality, freezing occurs because condensate can get trapped inside the coil and then be exposed to subfreezing air.

As a quick review of how a steam coil works, steam enters the coil, gives up its heat, and condenses into water. If that condensate cannot drain properly, it remains inside the tubes. When airflow temperatures drop below freezing, the trapped water/condensate expands as it freezes, causing the tubes to also expand, and possibly rupture.

Common Causes of Steam Coil Freezing

1. Poor Condensate Drainage

The most frequent cause of steam coil freezing is inadequate drainage. Issues include:

• Improper coil pitch
• Undersized or incorrectly installed steam traps
• Plugged or failed traps

If condensate backs up into the coil as temperatures fall, freezing becomes likely.

2. Inadequate Steam Pressure or Control

Low steam pressure or poorly sized control valves can prevent steam from fully filling the coil. This creates uneven heating, leaving portions of the coil cold enough to freeze – especially during partial-load conditions.

3. Improper Coil Selection or Circuiting

Not all steam coils are designed for freeze-prone environments, and are therefore susceptible to freeze-up. When you purchase a steam coil from Marlo, we will offer options to minimize this risk, depending on your individual application. Read on to learn more about our solutions.

The High Cost of Steam Coil Freezing

The potential risks related to frozen steam coils cannot be underestimated. A frozen steam coil can result in:

• Burst tubes and leaking steam
• System shutdowns during peak heating demand
• Costly downtime in process heating environments
• Water damage inside air handlers or facilities
• Potential safety risks
• Expensive emergency coil replacements

Proven Strategies to Prevent Steam Coil Freezing

1. Proper Coil Design for Cold Conditions

Needless to say, the most important thing you can do to minimize the chance your steam coil will freeze is to make sure it’s designed and built specifically for the environment where it will operate – which is exactly what you’ll get with a custom coil manufacturer like Marlo. We’ll make sure you get a coil design that matches the demands of your environment, with tube sizes, wall thicknesses, materials and circuiting optimized for your application.

2. Ensure Correct Pitch and Drainage

Steam coils must be properly pitched in the direction of the condensate header, to promote the removal of condensate and not allow it to build up and freeze. The coil must also be installed correctly, as even minor errors can prevent appropriate coil drainage.

3. Use Proper Steam Traps and Piping Design

Pairing the coil with a correctly sized and located steam trap is also essential. Steam traps must match the coil load and operating pressure, be installed at the correct elevation, and be maintained and tested regularly. In certain conditions, and in low-pressure systems, condensate pumps may be required to ensure reliable drainage.

Two Marlo Coil Alternatives That Virtually Eliminate Steam Coil Freeze Risk

At Marlo, we offer two proven alternatives to traditional standard steam coils. These solutions can significantly reduce the risk of freezing – even in very cold climates.

Distributing Steam Coils

Distributing steam coils are engineered to deliver steam more evenly across the entire coil face. These steam coils feature a tube-in-tube design, with smaller-diameter tubes located inside the larger tubes. The smaller, 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, eliminating cold spots that contribute to freezing, while also resulting in more consistent air temperatures leaving the coil.

In addition, our distributing steam coil design results in better condensate drainage, removing the condensate on the same end of the coil as the steam enters. With no cold spots and no water trapped in the coil, there is almost no possibility of the coil freezing. That’s why distributing steam coils are often referred to as a “freeze proof coils”. We recommend these steam coils for any applications where temperatures can fall near or below 32°F.

The Marlo Stratomizer®

The Marlo Stratomizer – our enhanced version of a dampered face and bypass coil, also referred to as an integral face and bypass (IFB) coil – takes freeze protection to the next level. A Stratomizer consists of multiple steam coils (or hot water coils) and multiple damper blades that allow some air to pass through the coils to be heated, and some air to bypass the coils. The damper blades are infinitely variable between fully open and fully closed, resulting in optimal air mixing and precise temperature control.

Our unique design offers several important advantages:

• Maximum steam pressure (or hot water flow) is maintained at all times – not modulated –minimizing the chance of freezing
• Preheating cold incoming air increases energy efficiency, and also allows fresh air to be brought into a building, even in very cold climates
• The multiple damper blades allow precise downstream temperature control

We custom build each Stratomizer to meet the unique needs of each individual application. So we can provide a replacement for any existing integral face and bypass (IFB) coil. Stratomizers can also be added to air handling units (AHUs) to preheat incoming air and make the AHU more efficient. For more information, read our blog post on how a Marlo Stratomizer works.

Let Us Help Provide the Solution you Need

Traditional steam coils can be vulnerable to freezing in demanding environments. But fortunately, they are not the only options. Our distributing steam coils and Stratomizer provide engineered solutions that reduce freeze risk, improve reliability, and extend coil life in colder climates.

If steam coil freezing has been a problem for you, or if your application simply cannot tolerate the risk of a coil failure, the experts at Marlo can help identify the right steam heating solution for your system. Contact us today to discuss your needs or reach out to your local Marlo manufacturer’s representative, which you can find using our rep locator.