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Screw compressors are rotating machines
that transform the mechanical work performed by electric
motors, turbines or IC engines into potential energy
of a gas, vapor or multi-phase mixture by reducing
its volume and thereby raising its pressure.
 The
essential elements of such machines are a pair of
meshing helical lobed rotors contained in a casing.
Together, these form a series of working chambers,
as shown in Figs. 1 and 2 by means of views from
opposite ends and sides of the machine. The dark
shaded portions show the enclosed region where the
rotors are surrounded by the casing and compression
takes place, while the light-shaded areas show the
Fig. 1: View of Screw Compressor
Main Parts
regions where rotors which
are exposed to external pressure. The large light-shaded
area corresponds to the low-pressure port. The small
light  shaded
region between shaft ends B and D corresponds to
the high-pressure port. Admission of the gas to be
compressed occurs through the low -pressure port
which is formed by opening the casing surrounding
the top and front face of the rotors. Exposure of
the space between the rotor lobes to the suction
port, as their front ends pass across it, allows
the gas to fill the passages formed between them
and the
Fig. 2: View of Compressor
Rotors
casing. Further rotation then leads
to a cut off of the port and progressive reduction
in the trapped volume in each passage, until the rear
ends of the passages between the rotors are exposed
to the high pressure discharge port. The fluid then
flows out through this at approximately constant pressure.
The main users of compressed gases
supplied by screw compressors are the building industry,
food, process and pharmaceutical industries and, in
some instances, the metallurgical industry. They are
also used for pneumatic transport. Screw compressors
are capable of operation over a wide range of operating
pressures and flow rates with high efficiencies.
Despite their wide usage, the complexity
of their internal geometry and the non-steady nature
of the processes within them has meant that, up till
recently, only approximate analytical methods were
available to predict their performance. Thus, although
it is known that their elements are distorted both
by the heavy loads imposed by pressure forces and through
temperature changes within them, no methods were available
to predict the magnitude of these distortions accurately,
nor how they affect the overall performance of the
machine.
Computational Continuum Mechanics
(CCM) may be used if the accurate estimation of velocity,
pressure, temperature and concentration fields, as
well as stress and deformation within a screw compressor
is required. To enable these to be computed, a rack-generation
procedure has been developed to produce the rotor profiles
and an analytical transfinite interpolation method
to obtain a 3-D numerical mesh. Moreover, adaptive
meshing, orthogonalization and smoothing are employed
to generate a numerical grid that takes advantage of
the innovative techniques used in recent finite volume
numerical method solvers. They were required to overcome
problems associated with i) rotor domains which stretch
and slide relative to each other and along the housing
ii) robust calculations in domains with significantly
different geometry ranges, iii) a grid moving technique
with a constant number of vertices. These features
have been used together to develop an independent stand-alone
CAD-CCM interface program to generate a numerical grid
of the screw compressor.
Modifications are also implemented
in the calculation procedure to improve solutions in
complex domains with strong pressure gradients. Typical
results arising from its use when linked to Comet,
CD-adapco’s alternative solver based on the continuum
mechanics approach, are shown for an oil-flooded screw
compressor. Examples are given to demonstrate the scope
of the method for accurate calculation of processes
within these machines.
Some changes within the solver
were made to increase the speed of calculation through
user functions. These include a novel method to maintain
constant pressures at the inlet and outlet ports and
consideration of two-phase flow resulting from oil
injection in the working chamber.
In summary, the pre-processing code
and calculating method have been tested on a commercial
CFD solver to obtain flow simulations and integral
parameter calculations. The results of calculations
on an oil injected screw compressor are compared with
experimental results. Good agreement was obtained between
them.
As a result
of these developments, full 3-D calculations can
now be performed to quantify the interaction between
the compressor structure and compressor fluid flow.
The effects of a change in working clearances are
compared for different compressor applications and
presented through distortion diagrams and flow-power
charts.
For more information contact: a.kovacevic@city.ac.uk
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