Vacuum Pumping and Stirling Cycle: Calculation and Efficiency Study

Vacuum Pumping with a Vacuum Pump Rated at SP = 1 m3/min

1. Many thin lm processing such as deposition and etching is done in vacuum chambers. These methods are used for making integrated circuits to solar cells. Consider the act of evacuating a chamber with a vacuum pump. Vacuum pumping rates are usually given in volumetric pumping speeds, SP , whose units are typically volume/unit time, e.g., 1 m3/min. In other words, the number of molecules in 1 m3 at the chamber pressure and temperature are removed in one minute. A 0.1 m3 chamber is initially at atmospheric pressure (1 bar) and 298 K. The gas may be considered to be an ideal diatomic gas. Calculate and plot the temperature and pressure in the chamber as a function of time with a pump rated at SP = 1 m3/min if the pump down process may be considered to be (a) adiabatic (b) isothermal. 2. There are many dierent types of power cycle other than the Carnot cycle. One such cycle, the Stirling cycle, consists of the following steps: a) An isothermal compression; b) A constant volume heating; c) An isothermal expansion; d) A constant volume cooling. (a) Draw a reversible Stirling cycle on a P-V diagram and on a T-S diagram. (b) Derive an expression for the eciency of a reversible Stirling cycle with an ideal gas (with a temperature-independent heat capacity) as the working uid. The eciency is dened by