The Graebener stack press supports the technical testing of electrolysis stacks at the end of their production process. The pre-assembled stack, which consists of a large number of plates, is first moved into the machine and then compressed to a defined height until a certain pressure is reached within the stack. This pressure must then be maintained unchanged for several hours. During this time, the stack undergoes all the necessary technical tests conducted by the customer with the assistance of further procedures. These tests are essential to confirm its full operability. Once the tests are completed, the stack is finally assembled using tie rods or tension straps, and the finished stack is removed.
The stack press for electrolyzers has a force of 800 tons and can handle stacks with a maximum height of approximately 3 meters, a maximum diameter of 1.60 meter, and a weight of up to 12 tons per stack.
To guarantee assembly work during operation with maximum safety, a hydraulic drive was deliberately avoided. Instead, the stack is compressed inside the press by means of six servomotor-driven spindle units that operate synchronously, enabling particularly uniform compression of the stack. Each spindle unit can exert a maximum pressing force of 150 tons.
The primary challenge was to compensate for production-related stacking tolerances. Stacking tolerances are inevitable, but when the stack is compressed, they cause transverse forces that can damage the stack. To eliminate these issues and establish an even pressure distribution within the stack, these transverse forces must be continuously compensated for during the compression process. This is achieved through the use of short-stroke cylinders located under the six spindle lifting elements, enabling an even distribution of pressure.
The company had previously developed a similar press, but it was designed for fuel cell stacks. However, since plates for alkaline electrolyzers, and consequently their stacks, are typically significantly larger, the design of the current system proved to be much more complex, according to Graebener.