Spraying Water on Dryers
Recently, a customer asked about spraying water on hot paper dryer heads. We informed the customer that spraying cold water on a hot dryer surface can be a risky way to clear broke from a dryer surface; the risks can be minimized if done with caution. The following post highlights some issues to be considered when spraying water on paper dryers.
Cast iron dryer heads are susceptible to developing hairline cracks from high thermal stress, particularly if the heads were not annealed during the manufacturing process. Any thermal gradient will create thermal stresses and the magnitude of the thermal stresses depends heavily on the dryer head temperature, the dryer head construction, and how the water is sprayed on the dryer head surfaces. There is no simple way to give an upper limit to the dryer head temperature that would be considered safe under all conditions.
However, it would generally be considered acceptable if the dryer heads were less than 100°C. The water is then only able to cool the heads by sensible heating (i.e., by the change in water temperature) and not by rapid evaporative cooling.
The dryer head temperature will normally be very close to the saturation steam temperature associated with the operating steam pressure, even closer if the dryer heads have heat shields. Consequently, if the operating dryer pressure had been low (near 0 gauge pressure) before stopping the machine, it should be okay to use water to clear out paper fibers and debris. If not, the dryer heads should be allowed to cool until they are below 100°C. In either case, the water should be kept warm (at least 50°C) to minimize the thermal stresses.
The heads could be hotter than 100°C and still be okay, but it would then be very important to avoid cooling small localized areas of the dryer head. This could be difficult to do when trying to clean out a large build-up of paper and debris that is stuck between the dryer head and the dryer framing or dryer head insulating shield. A pressure washer (high water pressure, low water volume) might be safer and more effective than a high-volume water hose, if the build-up is not too thick.
When cleaning the dryers with water, other precautions provided in the Kadant Johnson white paper, Spraying Water on Dryers, should be followed to ensure the safety of the operators and avoid damage to the dryers.