Page 211 - Chemical process engineering design and economics

194 Chapter 5

at low pressures and then decreases at high pressures. For the liquid-ring vac-
uum pump, the curve increases at low pressures, resembling the ejector, and
then flattens out at high pressures, resembling a positive-displacement pump.
Isenrropic efficiencies for some vacuum pumps are plotted against the suction
pressure, as shown in Figure 5.4.
To size a vacuum pump requires calculating the volumetric flow rate and
factional pressure loss in the vacuum system. The volumetric flow rate consists
of condensable and noncondensable gases. The noncondensable gases originate
from the material being processed and from air leaking into the system. Assum-
ing that reasonable care is taken when sealing a vacuum system, Ryans and
Croll [3] have devised a procedure for estimating acceptable leakage rates
through various pump seals, valves, and sight glasses. To estimate the flow rate
of condensable gases, it is assumed that the noncondensable gases are saturated
with the condensable vapors. Once the total flow rate of gases and the required
pressure are known, the vacuum pump power can be calculated according to a
method used for compressors described later.

40 -

20 -

100 150 200 250. 300

Pressure, torr

Figure 5.3 Characteristic curves of vacuum pumps. From Ref. 3 with
permission.

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