Page 177 - Mechanical Engineer's Data Handbook

166 MECHANICAL ENGINEER'S DATA HANDBOOK

Some pumps have a ring of fixed (diffuser) vanes Head
into which the impeller discharges. These reduce the
velocity and convert a proportion of the kinetic energy Refemng to velocity triangles
into pressure energy. ( vw2 Vt 2 - vw I vt I )
Symbols used: Theoretical head Hth = 9
D, =mean inlet diameter of impeller
D, =outlet diameter of impeller It is usually assumed that V,, is zero, Le. there is no
b, =mean inlet width of impeller 'whirl' at inlet. The outlet whirl velocity V,,, is reduced
b, = outlet width of impeller by a whirl factor K to KVw,(K < I). Then:
t = vane thickness at outlet vwZ VtZqh
b1 =vane inlet angle Actual head H = 9
bz =vane outlet angle
N = impeller rotational speed where tfh= hydraulic efficiency. Or:
K = whirl coefficient Pressure rise p = pK Vw, VtZqh
Q = flow
H=hMd Flow Q= V,,A,= Vf,A,
Z=number of vanes ="D,b,Vf,tl"
p = fluid density
1 refers to impeller inlet
2 refers to impeller outlet
3 refers to diffuser outlet where qv = volumetric efficiency
P =power
Vt = tangential velocity
Vw = whirl velocity
V, =flow velocity
V, = velocity relative to vane
V= absolute velocity of fluid
qh = hydraulic efficiency
q, =volumetric efficiency
q,, = mechanical efficiency
qo = overall efficiency
a =diffuser inlet angle
d,=diffuser inlet width
d, =diffuser outlet width
b = diffuser breadth (constant)
a, =diffuser inlet area = bd, I
a, =diffuser outlet area = bd,
V, =diffuser outlet velocity
p = pressure rise in pump

Velocity relationships