الفهرس 
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Abstract
It is well known that the majority of refrigeration and air
conditioning systems encounter part loads that don’t match their designed
full capacity for the most of the running hours. Running compressors at
these part loads can harm them, because the circulated refrigerant exceeds
the existing load which causes refrigerant liquid flood to the compressor,
which causes mechanical damage to the compressor due to the
incompressibility of liquid refrigerant. So, designing of a refrigeration
system should include a method for capacity control. Many methods are
available but most of them can match part loads with negligible energy
saving. Compressor speed modulation is the most precise method that
combine both, large amount energy saving and close load matching at the
same time .
A test rig was designed and constructed to study the performance
of a simple refrigeration system using variable speed scroll compressor.
The test rig consists of a refrigeration system provided with a 4 HP scroll
compressor which uses R-404A as a refrigerant. This system is water
cooled and serves a glycol solution chiller equipped with electric heaters
as an artificial load. The compressor speed was changed using a variable
frequency drive which can vary the supplied compressor’s frequency
from 30 Hz to 60 Hz at different operating conditions. The chilled glycol
solution flow rate was changed from 20 liter/min to 50 liter/min and the
condensing pressure was changed from 200 psig to 300 psig at different
cooling loads The refrigerant flow rate, the condenser water flow rate, the inlet
and the outlet temperatures of the chilled glycol solution, the inlet and the
outlet temperatures of the condenser water, the refrigerant pressures and
temperatures of the refrigeration cycle were measured and recorded. Also,
the input power to the compressor, the water pumps, and the electric
heaters were measured and recorded. All these data were used to calculate
the system coefficient of performance and the second law of
thermodynamics efficiency at different operating conditions. Then, the
results were analyzed to determine the optimal operation.
The present study shows that the system coefficient of
performance increases by decreasing the compressor speed from its
nominal speed and on the contrary, it causes the second law efficiency to
decrease. The decrease of the chilled glycol solution flow rate increases
the coefficient of performance of the system while the second law
efficiency decreases. The decrease of the condensing pressure increases
the coefficient of performance of the system and increases the second law
efficiency. The increase of the cooling load increases the coefficient of
performance of the system, but decreases the second law efficiency. The
optimum coefficients of performance were determined at each
combination of the different parameters and curves of optimal operation
were obtained.