Saturated, Wet, and Superheated Steam in Paper Drying
Saturated, wet, and superheated steam are common in paper mill steam systems. Ensuring steam is in the right condition as it enters a dryer roll directly impacts heat transfer and equipment life. To optimize heat transfer and equipment life, Kadant Johnson recommends steam enters a dryer at saturation temperature. However, in most cases, steam that is superheated no more than 30° F (17° C) over its saturation temperature is acceptable. This guideline is based on extensive research at Kadant Johnson’s global research and development center on heat transfer and the impact of superheated steam on steam joint seal performance.
Saturated steam co-exists with water at the same pressure and temperature. At atmospheric pressure, this temperature is 212° F (100° C). Saturated steam contains a large amount of latent heat that is transferred to the surface of the dryer roll as the steam condenses into water. It is ideal for heat transfer, but saturated steam leaving the boiler can lose heat in the steam piping and arrive to the dryer as very wet steam.
Wet steam occurs when saturated steam and condensate water molecules are mixed. Often, heat loss in piping causes some of the saturated steam to condense and create a steam/water mix. The condensate droplets can cause pipe erosion and require drip traps. Additionally, the condensate no longer carries significant heat energy compared to saturated steam.
Superheated steam exists when the steam temperature is greater than where water and steam co-exist (saturation point). Because of this, superheated steam can lose some heat to the environment without condensing. It is commonly referred to as “dry steam.” It eliminates the need for drip traps and prevents pipe erosion from water droplets but requires larger piping for the increased volume of the superheated steam.
It will also reduce the life of rotary joint seals and dryer journals due to the higher temperatures and increased friction caused by dry steam. To minimize the impact on equipment life, superheated steam should be no more than 30° F (16° C) over its saturation point as it enters the dryer roll.
Optimizing steam systems to deliver either saturated steam or the right amount of superheated steam to the dryer requires extensive systems knowledge. Kadant Johnson offers nearly 90 years of process expertise designing and updating steam systems for paper mills while its Services group can install and maintain steam systems and dryer section components. Whether you are seeking to increase drying capacity or reduce steam energy consumption, the first place to start is with a Dryer Performance Evaluation.
(revised July 2019)