Page 219 - Fluid Mechanics and Thermodynamics of Turbomachinery

200 Fluid Mechanics, Thermodynamics of Turbomachinery
Centrifugal compressors were the reasoned choice for refrigerating plants and
compression-type heat pumps used in district heating schemes described by Hess
(1985). These compressors with capacities ranging from below 1 MW up to nearly
30 MW were preferred because of their good economy, low maintenance and abso-
lute reliability. Dean (1973) quoted total-to-static efficiencies of 80 84 per cent for
small single-stage centrifugal compressors with pressure ratios of between 4 and 6.
Higher pressure ratios than this have been achieved in single stages, but at reduced
efficiency and a very limited airflow range (i.e. up to surge). For instance, Schorr
et al. (1971) designed and tested a single-stage centrifugal compressor which gave
a pressure ratio of 10 at an efficiency of 72 per cent but having an airflow range of
only 10 per cent at design speed.
Came (1978) described a design procedure and the subsequent testing of a 6.5
pressure ratio centrifugal compressor incorporating 30 deg back swept vanes, giving
an isentropic total-to-total efficiency for the impeller of over 85 per cent. The overall
total-to-total efficiency for the stage was 76.5 per cent and, with a stage pressure
ratio of 6.8 a surge margin of 15 per cent was realised. The use of back swept
vanes and the avoidance of high vane loading were factors believed to have given
a significant improvement in performance compared to an earlier unswept vane
design.
Palmer and Waterman (1995) gave some details of an advanced two-stage
centrifugal compressor used in a helicopter engine with a pressure ratio of 14, a
mass flow rate of 3.3 kg/s and an overall total-to-total efficiency of 80 per cent. Both
stages employed back swept vanes (approximately 47 deg) with a low aerodynamic
loading achieved by having a relatively large number of vanes (19 full vanes and
19 splitter vanes).
An interesting and novel compressor is the “axi-fuge”, a mixed flow design with a
high efficiency potential, described by Wiggins (1986) and giving on test a pressure
ratio of 6.5 at an isentropic efficiency (undefined) of 84 per cent. Essentially, the
machine has a typical short centrifugal compressor annulus but actually contains
six stages of rotor and stator blades similar to those of an axial compressor. The
axi-fuge is claimed to have the efficiency and pressure ratio of an axial compressor
of many stages but retains the compactness and structural simplicity of a centrifugal
compressor.

Some definitions

Most of the pressure-increasing turbomachines in use are of the radial-flow type
and vary from fans that produce pressure rises equivalent to a few millimetres of
water to pumps producing heads of many hundreds of metres of water. The term
pump is used when referring to machines that increase the pressure of a flowing
liquid. The term fan is used for machines imparting only a small increase in pressure
to a flowing gas. In this case the pressure rise is usually so small that the gas can
be considered as being incompressible. A compressor gives a substantial rise in
pressure to a flowing gas. For purposes of definition, the boundary between fans
and compressors is often taken as that where the density ratio across the machine is
1.05. Sometimes, but more rarely nowadays, the term blower is used instead of fan.