Biomedical Engineering Reference
In-Depth Information
The density of blood is slightly higher than that of water, being about 1062.1 kg/m
3
.If
the outlet and inlet pressures of a LVAD are, respectively, 120 mmHg and 20 mmHg, the
head rise provided by the pump is
(
120
−
20
)
×
133
.
32
1062
h
a
=
.
×
.
1
9
81
=
1
.
27 m
From this result, it can be seen that if a major artery is severed the heart would be capable of
pumping blood even higher than this because the full 120 mmHg head would be available
into the atmosphere.
8.7.1.3 Specific Speed
The specific speed determines the general shape of a centrifugal pump impeller. As the
specific speed increases, the ratio of the impeller outlet diameter to the inlet of the eye
diameter decreases. This ratio becomes unity for an axial-flow pump.
Radial flow impellers develop head through centrifugal force and are low-flow high-
head designs, whereas pumps with higher specific speeds develop head partly by centrifu-
gal force and partly by axial force. In the limit, an axial pump develops head using axial
forces only. Specific speed,
n
s
, can be determined using
n
e
√
Q
h
3
/
4
n
s
=
(8.13)
where
n
e
(rpm) is the rotation speed for maximum efficiency,
Q
(m
3
/s) is the flow rate,
and
h
(m) is the head delivered by the pump at the point of maximum efficiency.
As shown in Figure 8-67, high
n
s
pump impellers have inlet diameters that approach or
equal the outlet diameter and relatively large open flow passages. Low
n
s
pump impellers
have outlet diameters that are much larger than the inlet diameters and relatively narrow
flow passages.
By using different impeller and casing designs, it is possible to vary the pump charac-
teristics over a wide margin, as shown in Figure 8-68. A flat characteristic allows consider-
able variation in the flow rate with little change in the head, whereas a steep characteristic
gives a small variation in flow for a large change in head.
It can be seen that the axial-flow pumps have a much steeper head capacity curve than
centrifugal pumps, and instead of power at shutoff being a minimum, as it is for centrifugal
pumps, it is not only a maximum but is also much larger than the power required at the
point of maximum efficiency. This is a disadvantage both in starting and also running at
low capacity. This consideration is reflected in the narrow operating speed for axial pumps
used in LVADs.
FIGURE 8-67
Relationship
between the pump
configuration and
specific speed.
