Chapter 4 - The geometrical problem in the positive displacement scroll machines

Abstract

For positive displacement machines (PDM) scroll-type PDMs pose some inherent advantages over reciprocating, screw and rotary vane. Scroll-type PDMs are typically more compact and reliable than other PDMs because of their low part count, simple dynamics and natural valving built into the scroll geometry. However, while they excel in dynamic simplicity, they lag in geometric complexity. In scroll-type PDMs, the mating scrolls together include significantly more complex machined surfaces than other PDMs. For this reason, understanding the geometric and design equations that govern the scroll involute geometry is crucial for understanding these machines. Additionally, depending on the number of scroll wraps, the compression (or expansion) volume can take up to 3 revolutions to move between suction and discharge. This results in a relatively slow compression process, exposing the machine to significant leakage if not designed correctly. Like the geometry, leakage inside the scroll is complex. Even the simplest scroll machines can have up to 5 closed volumes occurring simultaneously, all interacting through leakage and heat transfer. The following chapter summarizes the equations necessary in producing scroll geometry, leakage area, and leakage rate.