Steam coils can be challenging in terms of proper design and application, especially when heating outside air below 32°F (0°C). Corrosion issues in steam coils can involve both chemical and mechanical factors.
For instance, during cold winter months, the possibility exists for preheat steam coils that are exposed to outside air to freeze and burst or experience severe thermal shock, leading to failure. Figure 1 shows an example of typical failure on a steam coil resulting from the coil freezing.
These issues can be avoided if preheat coils are designed, piped and trapped correctly. To ensure that your preheat steam coils perform successfully in freezing conditions, be sure your system design addresses the following 10 points.
1. Select the Proper Steam Coil Type
For heating outside air in below-freezing temperatures, the coil design should be a basic steam type (single-tube, single-pass) with tubes oriented vertically or horizontally.
Do not use steam-distributing type coils or multiple-pass basic steam coils. Steam-distributing, tube-in-tube coils have inner distributing tubes that allow the entire face of a steam-distributing coil to heat more evenly. The proper application of steam-distributing coils is for reheating air and using a modulating steam-supply valve for air temperature and capacity control. When exposed to freezing air temperatures, the steam-distributing design is more likely to freeze up before a single-pass, basic steam coil.
2. Control Air Temperature
To allow the coil to reach operating temperature slowly and minimize thermal shock, the steam should be supplied to the coil for 15 min before the fans are turned on. The steam control must be either on or off, and control valves should never be oversized — bigger is not better in this situation. Modulating steam supply valves are not recommended for preheat systems because reducing the steam flow too much with cold air being supplied to the coil may result in the desired leaving air temperature to be met while also causing the coil to freeze.
To control air temperature, use air bypass dampers. Design for coil-face velocities from 200 to 1000 ft/min. Higher face velocities may result in burst tubes due to coil freezeup. Design coil ductwork to distribute air evenly across the face of the coil, avoiding high velocity streams of low temperature air to hit the coil face.
3. Use Steam Traps Effectively
Trap each coil independently. Locate the steam trap a minimum of 14" below the return/outlet connection of the coil when on/off (nonmodulating) steam supply valves are used. Use only traps such as the inverted bucket- or float-type, which drain continuously.
Thermodynamic disk-type traps operate intermittently (alternately fill and dump condensate). They should not be used on any steam-heating coil as the primary steam trap. The steam trap also should be sized according to the manufacturer’s recommendations. Upstream of the coil, be sure to install drip traps to allow condensate drainage from steam-supply lines.
4. Prepare for Condensate Removal
Coils must be properly pitched toward the condensate return and installed in a way that promotes condensate drainage. This will aid in preventing destructive thermal shock, freezing and the buildup of corrosive elements within the tubes. Coils must be installed so tubes are pitched at least 0.25" per foot of run toward condensate return.
Also, from the coil outlet to the steam trap, the piping should be the same size as the coil outlet connection.
5. Perform Water Treatment
Steam can be corrosive in coils when it is improperly treated. Boiler water treatment should be done on a periodic schedule based upon local conditions to remove dissolved oxygen and carbon dioxide. All drip traps, dirt pockets, steam traps, vacuum breakers, air vents, strainers and valves should be flushed and inspected regularly.
6. Size Steam Lines Appropriately
Proper steam piping and coil design are critical for good coil operation and long coil life. All piping must be self-supporting and flexible enough to allow for thermal expansion and contraction. The use of flexible connections or swing joints is recommended.
Steam coils can experience erosion corrosion from excessive tube-side velocity. For a given steam flowrate in pounds per hour (lbm/h), steam velocity decreases as pressure increases. This is due to the increase in steam density as pressure increases. Care must be taken to size coil connections and headers for the minimum anticipated operating steam pressure — and never less than 5 psig. This approach will ensure that the coil is designed to handle the maximum velocity condition. Steam coil connections should be designed to prevent velocities from exceeding 6,000 ft/min. This not only minimizes the chances for erosion to occur, it also reduces steam noise.
7. Avoid Thermal Shock
Thermal shock occurs when condensate is allowed to remain in a steam coil after shutdown or during idle periods. The residual steam becomes sub-cooled (cools to a temperature lower than steam temperature). If live steam then is introduced into the coil and contacts the sub-cooled condensate, the live steam will violently re-condense into the condensate. Tiny bubbles of the live steam are injected into the condensate at the steam-condensate interface, where they collapse with tremendous destructive force.
This process is similar to cavitation in pumps, where local pressures in the pump body are allowed to fall below the vapor pressure of the water being pumped. In the case of thermal shock, erosion corrosion occurs where the collapsing steam bubbles contact the tube surface. The erosion corrosion creates severe localized pitting, similar to what is shown in figure 2.
Thermal shock is prevented by properly designing steam piping to thoroughly eliminate all condensate from steam coils both during operation and after shutdown.
8. Plan for Pumping Condensate
When condensate must be lifted into overhead or pressurized return mains, a vented condensate receiver must be installed with a properly sized steam pressure pump or some other means of pumping the condensate from the receiver into the return main.
9. Install a Vacuum Breaker
A vacuum breaker must be installed in the steam piping prior to the coil to prevent retention of condensate during shutdown. Also, a vacuum breaker should be installed on the downstream side of the coil when steam pressure is to be modulated. If you use check valves as vacuum breakers, they should be 15-degree swing checks.
10. Add a Thermostatic Air Vent
Providing venting of noncondensable gases individually on each coil ensures maximum heat transfer and minimum internal corrosion. Venting can be accomplished with an independent thermostatic vent or by using a combination float-and-thermostatic steam trap.
Steam coils provide an effective means of air heating. By taking some key features into account when specifying a coil, your steam coil will deliver effective heat transfer.