A thesis submitted to the Faculty of Engineering, University of the
Witwatersrand, Johat.nesburq, in fulfilment of the requirements for the
degree of Doctor of Philosophy
Johannesburg, 1998 / This thesis contributes to accurate, practicable techniques of ascertaining
and assessing site performance of large refrigerating machines chilling
water 'for cooling deep South African mines. It applies to all vapourcompression
machines cooling fluids in steady, continuous processes.
To assess whether a water chilling machine is performing satisfactorily,
both its actual performance, and the corresponding normal or optimal
performance of which it is capable, must be ascertained. Both
requirements r esent difficulties on site. in particular, the traditional "heat
balance" method of verifying the apparent performance obtained from
measurements in the water circuits does not prove that such performance
is accurate. The calibration of typical site instrumentation is not assured,
so an "acceptable" heat imbalance may conceal large but similar errors -
which thus also balance out - in the apparent constituents of the heat
balance.
Three methods of independently ascertaining actual performance, so
verifying apparent performance, are presented, The first is an enhanced
method, applicable to custom-built machines as well as conventional
ones, of ascertaining the efficiency of the actual refrigerating process from
measurements in the refrigerant circuit. This detects errors concealed in
an "acceptable" heat balance. Where some refrigerant-circuit
measurements are unavailable, an inexact version of this method still
indicates the relative likelihood of the apparent performance being
acceptably accurate. The third method, where these two are inadequate,
is ascertaining actual performance using available measurements and
fundamental machine modelling.
Such modelling is also the most versatile method of predicting
corresponding normal or optimal performance. A computer program
simulating complete mine water chilling installations is used here. Actual
performance can then be meaningfully assessed and appropriate
remedial action justified, as shown in seven case studies. An outcorr.e for
conventional water chillinq machines with a centrifugal compressor is that
keeping heat exchangers clean may prejudice efficiency under part-duties
lf a machine has been designed for optimum efficiency at full duty. An
alternative control philosophy of maximising the machine load may then
yield better performance.
If these techniques are included ill an automated system of fault
diagnosis, they will be of most use to burdened mine staff, who are
generally not refrigeration experts.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/20441 |
| Date | 08 June 2016 |
| Creators | Bailey-McEwan, Michael |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf, application/pdf |
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