At the end of this article is a typical enthalpy table for saturated steam. You can use this type of chart to determine what the temperature is inside a dryer can at a given pressure.
Saturated steam is steam that is in equilibrium with heated water at the same pressure (it has not been heated past the boiling point for that pressure). Reducing saturated steam temperature while maintaining pressure will cause the steam to condense, even though the temperature may be well above the 212 °F boiling point at 1 atmospheric pressure.
Note that this is for saturated steam only. Superheated steam first has to be cooled to the saturated temperature before it can partake in the condensing process. This is why too much superheated steam in a dryer can actually reduce heat transfer to the sheet.
The mechanism to cool superheated steam is by convection, not a very fast way to bring the steam temperature down. There are instances of machines having to max out their steam pressures even at slow production rates and still not dry the sheet, only to find out that the steam temperatures were well above their saturation temperatures.
Steam should be at its saturation temperature for maximum heat transfer. Of course, the higher the pressure of the saturated steam, the higher its temperature will be, and the corresponding drying rate.
The can surface temperature is approximately 40 °F lower than the inside steam temperature if all is working properly. It should be noted however, that the enthalpy of condensation (the state change from vapor to liquid) is actually slightly lower as the pressure increases. This is why early dryer cans are efficient at increasing sheet temperatures because they use low pressure steam with high enthalpies of condensation.
Saturated steam enthalpy table
|Gauge pressure (psig),||Temperature (°F)||Specific Volume Saturated Vapor (ft3/lb)||Enthalpy|
|Saturated Liquid (Btu/lb)||Evaporated (Btu/lb)||Saturated Vapor (Btu/lb)|
|23 (inches vacuum)||134||145||102||1017||1119|
|20 (inches vacuum)||162||73.9||129||1001||1130|
|15 (inches vacuum)||179||51.3||147||990||1137|
|10 (inches vacuum)||192||39.4||160||982||1142|
|5 (inches vacuum)||203||31.8||171||976||1147|
For more information on maintaining and optimizing your steam and condensate systems, contact your Valmet representative.