Saturated steam enthalpy table, steam mechanism, optimizing tips

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.

Steam tips

• Supply and return lines from low pressure sections must be sized large enough to handle the high volumes involved. Lines that are too small will not allow the cans to be vented properly.
• You can check the approximate temperature of a dryer can using an infrared temperature gun on the cylinder heads. If the temperature is above that of saturated steam at the running pressure, there is a problem with the steam temperature controls.
• Check the desuperheater for proper operation – the valve should not be wide open and condensate supply pressure needs to be sufficiently high. Desuperheater condensate supply pumps tend to be high pressure pumps subject to wear. Monitor condensate pressure periodically to provide baseline data.

Saturated steam enthalpy table

For more information on maintaining and optimizing your steam and condensate systems, contact your Valmet representative.