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Table 4. Refrigerant charges for optimal cooling capacity and COP at 4.5 turns expansion valve opening
Refrigerant charge (g) at 4.5 turns expansion valve opening
2400 RPM

3600 RPM

4500 RPM

Optimal cooling capacity

780 g

780g

800 g

Optimal COP

740 g

740 g

740 g

Table 5 shows the most relevant thermodynamic parameters at the optimum operating point of each
compressor speed. It can be seen that the compressor power, cooling capacity and mass flow rate all increase
significantly with the compressor speed, while the system COP is only slightly reduced. It can also be noted
that the cooling capacity increases more than the mass flow rate due to the drop of the suction pressure with
increasing compressor speed (Chen and Gu, 2004). Although it might be possible to obtain an even higher
COP for each compressor speed with a distinct combination of restriction and charge, a single pair that can
be used with any compressor speed offers a good compromise between performance and flexibility.
Table 5. Thermodynamic parameters at the optimum operating point
PARAMETER

2400 RPM

3600 RPM

4500 RPM

COP (-)

1.33

1.28 (-3.8%)

1.23 (-7.5%)

Compressor power (W)

248

431 (+73.8%)

588 (+137.1%)

Cooling capacity (W)

354

551 (+55.6%)

724 (+104.5%)

Suction pressure (bar)

35.2

29.2 (-17.4%)

26.1 (-25.8%)

Intermediate pressure (bar)

59.7

54.6 (-8.5%)

48.4 (-18.9%)

Discharge pressure (bar)

86.2

91.0 (+5.6%)

96.8 (+12.3%)

o

o

Discharge temperature ( C)

52.5

67.0 (+14.5 C)

79.1 (+26.6oC)

Evaporator inlet temperature (oC)

0.7

-6.1 (-6.8oC)

-10.1 (-10.8oC)

Evaporator superheating (oC)

1.9

7.6 (+5.7oC)

10.9 (+9.0oC)

Mass flow rate (kg/h)

15.0

17.9 (+19.3%)

20.8 (+38.5%)

Vapor quality at evaporator inlet (-)

0.52

0.49 (-5.8%)

0.48 (-7.7%)

4. CONCLUSIONS
The effect of the compressor speed on the thermodynamic performance of CO2-based refrigerating systems
was studied herein. To this end the optimum refrigerant charge for each pair of compressor speed and
expansion restriction was experimentally found. It was shown that the discharge pressure varies almost
linearly with the refrigerant charge, giving room for a performance optimization process based only on the
amount of refrigerant contained in the system.
It was found that a single pair of expansion restriction (4.5 turns) and refrigerant charge (740g) provides the
maximum COP, independently of the compressor speed. These finding, which is in line with those reported
by Montagner (2013), is of utmost importance for variable capacity CO2-based refrigeration systems, since
it indicates that a properly sized fixed restriction expansion device, such a capillary tube, can be used to
meter the refrigerant flow. The drawback is a penalty in cooling capacity since the suction pressure increases
at lower speeds.

ACKNOWLEDGEMENTS
This study was carried out at the POLO facilities under National Grant No. 573581/2008-8 (National
Institute of Science and Technology in Refrigeration and Thermophysics) funded by the Brazilian
11th IIR Gustav Lorentzen Conference on Natural Refrigerants, Hangzhou, China, 2014
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