Abstract

This paper describes an investigation of the influence of the electric power input and the molecular weight of gases on the speed and critical backing pressure (CBP) of an oil diffusion pump. The experiments were done using an Edwards Diffstak 100⧹300 diffusion pump. The speed and CBP of the pump have been measured at different electric power inputs using nitrogen and a lighter gas helium. The comparison of the experimental results obtained for the two gases reveals that in estimating the speed ratio the use of the square root of the molecular weights of the two gases is not valid. Even, the equation suggested by incorporating the Ho coefficient of the two gases is found to be in disagreement with the experimental results obtained. In the present study a modified equation to find out the speed ratio of the two gases is suggested which is found to be in good agreement with the experimental results. An attempt has also been made to correlate the CBP with the speed of the pump suggesting a relationship in which the speed of the pump at a particular electric power input is found to be proportional to the logarithm of the critical backing pressure.

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