Food-grade twin screw pump
Multinational company producing progressive cavity pumps
Design of a new leading product for the food industry to be added to the product catalogue
The industrialisation process has begun
A multinational company specialised in the production of progressive cavity pumps intended to introduce a leading product for the food industry into its catalogue. The choice fell on a particular type of volumetric pump, the twin screw pump, which allows the pumping of food fluids even with high viscosity (e.g. chocolate, honey, yoghurt), minimizing the alterations of the organoleptic properties and respecting the highest hygienic standards.
The company had already made a prototype of a twin screw pump, but the tested product had not met the requirements regarding efficiency, functionality and production costs. Given the high number of design parameters involved, and their high degree of interaction, a systematic approach was required to estimate the costs and performance characteristics of the pump in advance.
In fact, a single parameter can have a significant impact on design: a change in the hydraulic part can affect the mechanical part and vice versa. The existence of external design constraints, such as the obligatory choice of standard industrial components, should not be overlooked.
Of particular importance is the estimate of the shaft deflection, which depends both on purely hydraulic factors (operating pressure, speed of rotation) and structural factors (shaft section and length, bearing positioning, etc.) and can directly affect efficiency and costs. Lower deflections, in fact, make it possible to reduce the distance between rotor and stator and consequently also the hydraulic recirculation losses; however, lower deflections can be obtained only by increasing the bearings or shaft diameter, choices that normally lead to an increase in costs.
Since we previously had developed software for the automatic design of various types of pumps, we were able to quickly implement a specific module for twin screw pumps, able to carry out:
• the design of the pump hydraulics with the estimation of the characteristic flow-prevalence curve, the estimation of the efficiency as a function size, the pitch of the screws and the tolerances between rotor and stator;
• the design of the gearbox, including gear sizing, the choice of the bearings and their lubrication system;
• the prediction of friction losses related to the mechanical seal.
Having a method able to quickly identify pros and cons of different configurations offered us the opportunity of selecting the optimal design for the target audience.
Once we had chosen the basic configuration, we took care of the detailed design of the pump, in order to:
• simplify as much as possible the assembly, disassembly and shaft-gear coupling;
• prepare the mechanical seal housing for flexible use of both single and double seals.
• design the internal ducts and all static seal housings in compliance with both European (EHEDG) and American (3-A) hygiene regulations.
At the end of the detailed design, we built the prototype of the twin screw pump. Then we defined the experimental test campaign: bench tests made it possible to trace the characteristic head-flow curves and to detect the energy consumption of the various sub-components (hydraulics, mechanical seal, transmission). We used fluids at different viscosity levels (different mixtures of water and food-gel).
After the tests, we verified that the twin screw pump complied with the design requirements with a wide margin. Minimal changes were made to the lubrication system to increase oil circulation in the bearings.
The industrialisation process has now begun. Further activities are planned shortly:
• the development of larger twin-screw pumps, facilitated by a fast design method, which has already been successfully tested;
• the search for design solutions that use innovative materials to achieve a reduction in weight and production costs.