Critical backing pressure diffusion

At the high pressure end of the S vs pMet curve, (here labelled, overload region also known as the forepressure break-down region), S declines from iSmax until a critical pressure is reached in the backing line (critical backing pressure or forevacuum tolerance) which, if exceeded, causes the pumping action of the diffusion pump to cease. 

A diffusion pump has a pumping speed of 1000Ls-1 at pm < 10 3mbar, falling to 400 L s 1 at 10"2 mbar. The critical backing pressure for the pump is 4 x 10 1 mbar. 

The maximum permissible backing pressure (the critical backing pressure, pcrit) for a diffusion pump (pumping speed = Seff Dp) is 2 x 10 l mbar. If the pump is used only at inlet pressures (/ in) of 10 3mbar or below, calculate the speed of the backing pump (Sback) in terms of Seff DP. 

In Example 3.12, the diffusion pump is to be operated for 1 h with an inlet pressure of 10 5mbar with the backing pump switched off (and the backing valve closed) to eliminate the effect of vibration from the backing pump on the system. Calculate the volume of the backing line to achieve this if the critical backing pressure of the DP is 0.6 mbar. 

The lower velocity in the throat does not affect the jet s performance, as long as the velocity remains above the speed of sound. If the velocity in the throat falls below the speed of sound, we say that the jet has been forced out of critical flow. The sonic pressure boost is lost. As soon as the sonic boost is lost, the pressure in the vacuum tower suddenly increases. This partly suppresses vapor flow from the vacuum tower. The reduced vapor flow slightly unloads condenser 1 and jet 2 shown in Fig. 16.2. This briefly draws down the discharge pressure from jet 1. The pressure in the diffuser throat declines. The diffuser throat velocity increases back to, or above, sonic velocity. Critical flow is restored, and so is the sonic boost. The compression ratio of the jet is restored, and the vacuum tower pressure is pulled down. This sucks more vapor out of the vacuum tower, and increases the loads on condenser 1 and... 

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