Condensate in a dryer cylinder has three stages of behavior that are speed dependent. At slow speeds, the condensate forms a pond at the bottom of the cylinder. As the dryer speed increases, this pond moves in the direction of dryer rotation and widens. As the speed is further increased, the trailing edge of the puddle extends over the horizontal centerline and condensate tumbles (“cascades”) back to the bottom of the dryer cylinder. The third stage of condensate behavior occurs as speed is increased, and the condensate forms a rimming layer on the inner surface of the dryer cylinder.
These three stages of condensate behavior are shown in Figure 1. The three stages are normally referred to as “ponding”, “cascading”, and “rimming”.
Rimming condensate presents a resistance to the flow of heat from steam to the sheet, a resistance that increases with cylinder speed. At speeds over 600 mpm, this resistance can exceed that of the thermal resistance of the dryer shell. The condensate film coefficient (hc) for a close-clearance rotary syphon (operating with a constant pressure and condensing rate) is shown in Figure 2 for various speeds.
Turbulator bars can be placed in the cylinder to create turbulence in the rimming condensate layer, even at high dryer speeds. If the spacing of the bars is chosen correctly, a resonant wave is created in the condensate film, between each pair of bars. This increase in turbulence significantly reduces the resistance to heat transfer from the steam, through the condensate film, to the dryer shell.
Figure 3 shows that heat transfer is optimum when the condensate film is the proper thickness to be resonant. At thickness either larger or smaller than this optimum, heat transfer diminishes. The optimum syphon clearance to achieve the required film thickness is a function of condensing load, speed, bar spacing, bar height, syphon shoe design and should be calculated by the equipment supplier. Kadant Johnson offers this detailed calculation with every installation of Turbulator® bars to ensure optimal performance and heat transfer in the dryer cans or rolls.