Numerical determination of the allowable operating pressure of pillow plates by means of the finite element method

Alexander Zibart, Eugeny Kenig, Chair of Fluid Process Engineering, University of Paderborn, Paderborn/Germany; Bernhard Spang, BUCO Wärmeaustauscher International GmbH, Geesthacht/Germany

Pillow plates represent an innovative type of heat transfer equipment and are characterized by a high degree of flexibility in terms of application and production. Due to the fact that the production is usually carried out by means of CNC-controlled laser welding equipment and a downstream hydroforming process, geometric adjustments are very easy to implement and do not require the production of expensive forming tools. The high flexibility and the associated complexity of the cushion plate geometry make a mathematical verification of the permissible operating pressure impossible. Therefore, an experimental verification according to [1] is usually carried out, in which the permissible operating pressure results as a function of the bursting pressure. However, the determination of this pressure requires time-consuming and cost-intensive bursting tests. In this work, therefore, it was examined whether the bursting pressure can be predicted non-destructively by means of structural-mechanical finite-element simulations, which represent a numerical replica of the experimental bursting tests. As a basis for validation, test records of burst tests performed for two strongly differing cushion plate geometries were evaluated.

In [2] it was further shown that the thermal resistance of cushion plate heat exchangers can be reduced by up to 25% by using aluminum as the material of the cushion plates instead of stainless steel. Therefore, based on the methods derived in this work, the feasibility was tested. For this purpose, a parameter study was carried out in which the spot weld pattern and the plate thickness were varied. The material selected was the aluminum alloy EN AW 5083, which is widely used in process engineering and is characterized by good weldability and formability combined with high strength. The investigations focused on the attainable bursting pressures and the maximum expandability of the cushion plates that can be realized in the hydroforming process.

[1] VD TÜV, AD 2000 Merkblatt S5, Beuth Verlag GmbH, 2009

[2] A. Zibart, E.Y.. Kenig, Numerical investigation of conjugate heat transfer in a pillow-plate heat exchanger, Int. J. Heat Mass Transf. 165 (2021), 120567.

Ice maker for operation with single-phase coolants

In contrast to a BUCO ice-maker with evaporating refrigerant (with or without temperature glide), here the periodic freezing and defrosting takes place with liquid coolants (brine or single-phase mixtures). The brines MEG (monoethylene glycol) and MPG (monopropylene glycol) with concentrations of 35% or higher are often used. A smaller refrigeration system can be used to cool the coolant down to a sufficient low temperature. In this way, the ever stricter regulations worldwide with regard to the energy efficiency of refrigeration systems and the use of refrigerants can be implemented to the satisfaction of our customers. Even with a coolant flow temperature of -10°C, excellent results can be achieved in terms of ice volume per time unit.

Compared to evaporating refrigerants, the use of single-phase coolants results in an essential thermodynamic difference: The temperature of the single-phase coolant during the flow through the pillow plate changes by several Kelvin, since the energy transferred from the freezing falling film does not lead to the evaporation of a refrigerant at constant temperature. Since the build-up of the ice layer during freezing depends mainly on the local temperature of the single-phase coolant and the internal heat transfer, there are slight differences in ice thickness across the heat exchange plate. The task of BUCO's engineers here is to find the optimum compromise between the volume flow of the coolant, the temperatures and the pressure loss in the pillow plate system.

In the defrosting process, the selection of the correct temperature of the warm single-phase coolant is crucial. If the temperature is too low, defrosting becomes uneven or can be greatly delayed. If the temperature is too high, the ice will increasingly flake off instead of sliding off the pillow plate. Too high temperatures of the warm coolant are on the one hand not energy efficient and on the other hand not necessary, because the higher heat transfer coefficient of the liquid coolant heats up the wall faster and thus forms a liquid film between the ice and the heat pillow plate faster than with hot gas defrosting.

In our in-house test facility various test series with different coolant mixtures, temperatures, flow rates and flow arrangements were carried out over several years in order to be able to offer customers an optimal unit for their applications using specially developed calculation models and correlations.