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Steam Coil Circuiting and Construction Options
Standard Steam coils are designed differently than coils using single phase fluids and refrigerants. The combination of high steam temperatures, chemicals to control lime and scale on boiler surfaces, dissolved air in the steam, and the velocity of the steam through tube openings, creates a corrosive fluid that thins the tubing material and corrodes brazed/welded joints. This mix becomes more destructive if a steam coil is cycled on then off, allowing potentially corrosive condensate to sit on internal coil surfaces. Cycling also work hardens the tube material as it is flexed from thermal expansion then contraction. Eventually the material succumbs to this corrosion, erosion and embrittlement.
To help ensure a steam coil achieves an acceptable service life under these adverse conditions at a reasonable cost, our coils
w Are built with thicker than normal materials
w Have a more resilient alloy as part of the brazed or welded joints
w Have Finned tubes can move within the coil casing, minimizing thermal induced stresses at critical points
w Are designed to hold the maximum amount of steam in the coil tubes for peak performance and displace/force out damaging steam condensate that has less heat value/lb than steam.
To get the most steam possible into the coil tubes, all the tubes are connected to the steam supply header. To quickly remove the less efficient and damaging condensate, all the tubes are connected to the condensate return header. This creates a once-through flow path in the coil with steam entering at one end and leaving as condensate at the opposite end. If the coil is properly sized for the operating conditions the steam condensate accumulates only in the last several inches of the tube. Some steam coils are designed similar to water coils, having return bends at one coil end to place steam supply and condensate return connections on the same end of the coil. This may be required for a coil mounting and/or piping requirement. But doing so degrades thermal efficiency and accumulates more condensate in the longer tubes. Also coil freeze up can occur if the steam pressure is low or modulated, the coils finned length is long, and sub-freezing air enters the coil side with tubes containing substantial amounts of condensate. The tube opening in the steam supply header will limit how much steam can enter that tube. If the volume of steam that can pass through the tube opening is less than what the tube could actually condense, then the remaining tube heat transfer surface is wasted on sub-cooling lower heat value condensate than the higher heat value and efficient steam. The best way to ensure the amount of steam, that can enter a tube, comes close to matching that tubes' ability to condense it, is to feed all the coil tubes, keep the finned tube lengths appropriately short and select the best fins per inch and tube OD combination. If same end pipe connections are critical, then a Steam Distributing coil may be more appropriate. Its unique (tube inside a tube) design allows all the coil tubes to be connected to the steam header, minimizes the potential for condensate freeze-up, and places the condensate connection on the same end of the coil. It is recommended that steam coil tubes be pitched to drain toward the condensate header with an approximate drop rate of 1/8" per foot of finned tube length
The characteristics of steam distributing coils make them less likely to freeze than standard steam coils. Steam distributing coils are typically used in pre-heat low temperature applications where the entering air temperature is below 40ºF. Steam distributing coils are constructed with a “tube within a tube” and also known as non-freeze steam coils, tube-within-a-tube, and IDT (inner distributing tube).
Our Standard Steam and Steam Distributing Coils are made to ARI 410 Standards. Certification Acceptable coils are to have ARI Standard 410 certification and bear the ARI symbol. Non-certified coils or coils outside ARI's rating range will be considered if the manufacturer is a current member of the ARI air-cooling and air-heating coils certification program and the coils have been rated in accordance with ARI Standard 410.