Local heat and mass transfer in slot nozzle driers




Holst Centre, Eindoven, The Netherlands.

Project description:

Organic electronic technologies such as Organic Photovoltaic (OPV) and Organic Light Emitting Diodes (OLED) are currently taken from lab to pilot scale at the Holst Centre (Holst Centre, 2009). The active layers are coated on foil and subsequently dried in a pilot scale slot nozzle drier.

Figure 1: Schematic three dimensional sketch and foto of a slot nozzle drier. The green lines show hypothetical flow patterns between two nozzles.

In contrast to conventional coatings, these functional layers are highly sensitive to drying. Their nanomorphology and thus opto-electric properties are developed during drying. Therefore it is important know the drying kinetic as well as its homogeneity within the drier.
Local variations of heat and mass transfer coefficients are caused by different fluid dynamic patterns above the film. In conventional slot nozzle driers one cause is the acceleration of the effluent air which leaves the drier at the sides (Figure 1). To measure these variations locally, a specialized sensor was developed and tested. The sensor uses a heating foil to induce temperature gradients over a Polystyrene (PS) sheet and into the drying air. The temperature difference over the PS gives the heat flux and the temperature difference to the drying air is used to calculate the heat transfer coefficient. Temperature is measured using thin wire thermocouples, thus allow for a small measurement area of few square millimeters.

Figure 2: Sketch of the novel sensor for point measurements of local heat transfer in slot nozzle driers.

Measurements of the heat transfer coefficient using the newly designed sensor show that variations by up to 180% can occur in technical driers. A maximum is reached at edge of the web, close to the next downstream nozzle. The effect on the drying kinetic is smaller because the web moves and causes an effective averaged transfer coefficient in web direction.

Figure 3: Test measurement of local heat transfer coefficient in a slot nozzle drier for organic electronic films on foil. The local heat transfer varies by up to 180% during highest volume flow. 12 point measurements were done from the center of the web to the edge (at -150 mm) and from one nozzle (at 0 mm) to the nozzle (at 150 mm). The position is indicated by the shaded area in Figure 1.

These first results emphasize the need to further investigate local heat and mass transfer for drying sensible processes. A new design incorporating a sensor array, an improved positioning system and faster sample rate is currently under construction.