Pillow Plate Falling Film Evaporators (PPFFE)

Here is our part of our series about Pillow Plates Heat exchangers. The current article will focus on examining the Pillow Plate Falling Film Evaporators (PPFFEs), their operating principle and applications. To keep the readers up to date, a brief review will be made regarding some essential concepts of the pillow plate heat exchangers.

As discussed in the previous articles, pillow plates heat exchangers (PPHE) present an alternative design to many types of heat exchangers. PPHE is an innovative heat exchanger where the name pillow refers to its characteristic wavy plates. The plates are made by welding a spot pattern on the plates and sealing the edges. After that, they pass through a hydroforming process, where a fluid at high pressure is inserted between the plates making them expand which gives them the wavy aspect.

Falling liquid films are present in several industries, like HVAC, pharmaceutical, chemical, power plants, etc. In falling film evaporators (FFE) the evaporating liquid flows down a heat transfer surface, driven by gravity. Usually, FFEs provide low-pressure drops and offer very low operating pressures, enabling the treatment of specifically thermally sensitive products such as the evaporation and concentration of milk prior to spray drying, the concentration of fruit juices or solvent separation of pytiextracts. (Scholl, 2018).

Figure 1 shows a schematic arrangement of a PPFFE in a single pass. The heat transfer area must be fully covered by a stable liquid film at all operating conditions, including and specially at the lower exit of the evaporator. If this condition is not observed, a recirculation operation is required. This need can be verified by investigating the exit concentrate flow.

Figure 1: Schematic representation of a pillow plate falling film evaporator in single pass (Scholl, 2018).

Most frequently in standard shell-and-tube FFE, the heat transfer surface is in vertical orientation with the evaporating liquid flowing down at the inner side of the tubes (School and Rinner, 2006 apud Scholl, 2018). As in the same way of the shell-and-tube FFE, the PPFFE are constructed in vertical orientation however, the liquid stream to be partially evaporated is flowing down on the external surface of the pillow plates. In this way, there is liquid and vapor concurrently flowing to the lower exit of the evaporator. Furthermore, the plates are set as rectangular stacks inside a cylindrical shell as can be observed in Figure 1, allowing the vapor to go out the stack at the sides, having as consequence a lower pressure drop compared to shell-and-tube FFE,  where the vapor is confined along the full length of the evaporator tubes.

A distributor device is fixed above each pillow plate enabling all plates to be supplied by the fluid entering at PPFFE heads. The welding spots of the pillow plates induce turbulence in the flow within the plate, enhancing heat transfer.

The falling films are also used for other processes. Piper et al. (2015) mentions that falling films are also typical for condensation processes. Condensation is commonly carried out in shell-and-tube and plate condensers; on the other hand, pillow-plate condensers (PPC) represent a promising alternative to conventional equipment and offer a significant energy saving potential  (Piper et al., 2015).

The list below presents several items that must be considered to calculate the life cycle cost an entire equipment.

• The initial cost
• Bid evaluation and inspection costs
• Delivery and installation costs
• Energy cost
• Maintenance cost
• Cost of lost production due to poor efficiency of low-cost fans, heat exchangers and other components.

It seems the PPHEs are also a good choice from a financial point of view and might have a positive impact in the life cost of an equipment. Since PPHE have a high heat transfer coefficient, the costs of lost production due to their potential inefficiency may be lower. Moreover, their cost of delivery, installation and maintenance are low. To understand more about the advantages of using the PPHE check out our previous article.

Milena Vilar França
Dsc degree in Mechanical Engineering
Engineering Dept., Unilab Srl

References

• Scholl S. (2018) Pillow Plate Heat Exchangers as Falling Film Evaporator or Thermosiphon Reboiler. In: Bart HJ., Scholl S. (eds) Innovative Heat Exchangers. Springer, Cham. https://doi.org/10.1007/978-3-319-71641-1_9
• Piper, C. Wecker, A. Olenberg, J.M. Tran, E.Y. Kenig (2015) An experimental analysis of the topology and dynamics of a falling liquid film over the wavy surface of a vertical pillow plate, Chemical Engineering Science 130 (2015) pp. 129–134