Impinging jets of various configurations are commonly used in numerous industrial drying operations involving rapid drying of materials in the form of continuous sheets (e.g., tissue paper, photographic film, coated paper, nonwovens, and textiles) or relatively large, thin sheets (e.g., veneer, lumber, and carpets), or even beds of coarse granules (e.g., cat or dog food).
In this chapter, we will not examine the last-mentioned application, which is a novel operation in which hot jets are directed normally onto thin beds of pellets transported on a slow-moving conveyor.
The jets pseudofluidize the bed to ensure good gas–solid contact needed for effective drying.
Since impingement yields very high heat or mass transfer rates, it is a popular system for convective drying when rapid drying or small equipment is desired.
High production capacities are attained at the expense of increased capital and operating expenses because of the more complex fabrication and increased air-handling requirements.
Impinging jet drying is recommended only if a major fraction of the moisture to be removed is unbound.
If the drying rate is internal diffusion controlled, the high heat transfer rates of the impingement system can often result in product degradation if the product is heat-sensitive. For rapid drying of very thin sheets (e.g., tissue), high-velocity impingement of hot air jets is very effective.
On the other hand, for drying of heavier grades of paper and textiles, for example, impingement drying is effective only to remove surface moisture.
Another important industrial application of impingement drying is in the printing, packaging, and converting industry, in which printing techniques are used to deposit a thin film of coating onto a moving substrate (Arun S. Mujumdar, “Impingement drying”).
The flow of a submerged impinging jet passes through several distinct regions, as shown in Fig. 1. The jet emerges from a nozzle or opening with a velocity and temperature profile and turbulence characteristics dependent upon the upstream flow.
For a pipe-shaped nozzle, also called a tube nozzle or cylindrical nozzle, the flow develops into the parabolic velocity profile common to pipe flow plus a moderate amount of turbulence developed upstream.
In contrast, a flow delivered by application of differential pressure across a thin, flat orifice will create an initial flow with a fairly flat velocity profile, less turbulence, and a downstream flow contraction (vena contracta).
Typical jet nozzles designs use either a round jet with an axisymmetric flow profile or a slot jet, a long, thin jet with a two-dimensional flow profile.
After it exits the nozzle, the emerging jet may pass through a region where it is sufficiently far from the impingement surface to behave as a free submerged jet.
Here, the velocity gradients in the jet create a shearing at the edges of the jet which transfers momentum laterally outward, pulling additional fluid along with the jet and raising the jet mass flow, as shown in Fig. 2.
In the process, the jet loses energy and the velocity profile is widened in spatial extent and decreased in magnitude along the sides of the jet.
Flow interior to the progressively widening shearing layer remains unaffected by this momentum transfer and forms a core region with a higher total pressure, though it may experience a drop in velocity and pressure decay resulting from velocity gradients present at the nozzle exit.
(N. Zuckerman and N. Lior: “Jet Impingement Heat Transfer: Physics, Correlations, and Numerical Modeling”)
In paper production drying process jet impingement have a primary importance, in order to obtain a good product, saving as much energy as possible.
Most of dewatering in the paper machine occurs mechanically in the wire and press sections. Only the water that is not possibile to remove mechanically evaporates in the dryer section.
Web consistency or dry content after the wire section is typically 16-23%, 40%-50% after the press section, and 91%-95% after the dryer section.
This corresponds to removal in the wire setion of 100-170 kg water/kg produced paper, 2-4 kg water/kg produced paper in the press section, and 1.0-1.5 kg water/kg produced paper in the dryer section.
Although the dryer section in only responsible for a small fraction of total dewatering, it is the major energy consumer. (Pertti Heikkila, Jouni Paltakari : “Paper drying”)
Mevas is constatly researching about paper drying physics, in order to develop solutions to guarantee energy saving to customers.
Currently our R&D office is working on the optimization of jet fluid-dynamics for tissue paper drying process.
Following photo shows just an example of the results of our studies in term of Nusselt number related to yankee hood wrap angle.