Modulation Techniques for Compressors â Part II
Modulation Techniques for Compressors â Part II
Modulation Techniques for Compressors â Part II
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<strong>Modulation</strong> <strong>Techniques</strong> <strong>for</strong> <strong>Compressors</strong> – <strong>Part</strong> <strong>II</strong><br />
Per<strong>for</strong>mance and Efficiency Behavior of<br />
Screw <strong>Compressors</strong> – Comparison between Slider<br />
Unloading System and Frequency Inverter Operation<br />
Hermann Renz – Director Application Engineering<br />
Bitzer Kuehlmaschinenbau GmbH – Sindelfingen, Germany<br />
Mod<strong>Techniques</strong> <strong>II</strong>_ASHRAE01.2003 – 1
<strong>Modulation</strong> <strong>Techniques</strong> <strong>for</strong> <strong>Compressors</strong> – <strong>Part</strong> <strong>II</strong><br />
– Agenda –<br />
Introduction<br />
Compressor Design Features<br />
Comparison of <strong>Modulation</strong> Systems<br />
Slide Valve <strong>for</strong> Infinite or Step Unloading<br />
Variable Speed Drive (VSD) with<br />
Frequency Inverter<br />
Test Conditions<br />
Full and <strong>Part</strong> Load Behaviour with<br />
Slide Valve vs. VSD<br />
Summary<br />
Mod<strong>Techniques</strong> <strong>II</strong>_ASHRAE01.2003 – 2
Compressor Design Features<br />
Twin Screw Type CSH6561-60Y<br />
Semi-Hermetic, Suction Gas Cooled<br />
Displacement 7244 CFH (3500 RPM)<br />
Motor RLA 108 Amps<br />
Double-walled, pressure compensated<br />
rotor housing<br />
Integral Oil Separator and<br />
Oil Management<br />
Slider Unloading System<br />
Mod<strong>Techniques</strong> <strong>II</strong>_ASHRAE01.2003 – 3
Comparison of <strong>Modulation</strong> Systems<br />
– Slide Valve <strong>for</strong> Infinite or Step Unloading –<br />
Suction Gas<br />
CR1<br />
CR2<br />
CR3<br />
CR4<br />
Sliding<br />
ECO-Port<br />
Discharge Gas<br />
Oil Pressure<br />
Mod<strong>Techniques</strong> <strong>II</strong>_ASHRAE01.2003 – 4
Compressor Housing & Position of Slide Valve<br />
Position of<br />
Slide Valve<br />
Mod<strong>Techniques</strong> <strong>II</strong>_ASHRAE01.2003 – 5
Screw Rotors with Slide Valve<br />
Slide Valve<br />
Mod<strong>Techniques</strong> <strong>II</strong>_ASHRAE01.2003 – 6
Comparison of <strong>Modulation</strong> Systems<br />
– VSD with Frequency Inverter –<br />
Components <strong>for</strong> Per<strong>for</strong>mance Tests<br />
Frequency Inverter KIMO Type 75FEP<br />
max. Operating Current 105 Amps<br />
Screw Compressor CSH6561-60Y<br />
Frequency Range 25 .. 80 Hz<br />
L1<br />
L2<br />
L3<br />
Rectifier<br />
Transition<br />
Circuit<br />
Inverter<br />
Motor<br />
3~<br />
Control Circuit<br />
Control Voltage<br />
Mod<strong>Techniques</strong> <strong>II</strong>_ASHRAE01.2003 – 7
Basic Test Conditions<br />
– Slider Unloading System –<br />
Power supply with fixed frequency (60 Hz)<br />
Capacity modulation by axial movement of<br />
unloading slider<br />
Vi-Control at part load conditions<br />
by adaptation of discharge port<br />
Capacity steps during test: 100 – 75 – 50 – 25%<br />
75 – 50 – 25% are nominal values<br />
real capacity steps depend on SST and SDT<br />
Mod<strong>Techniques</strong> <strong>II</strong>_ASHRAE01.2003 – 8
Vi-Control at <strong>Part</strong> Load Conditions<br />
– with Slider Unloading System –<br />
3,5<br />
Compression Ratio Vi [%]<br />
3,0<br />
2,5<br />
2,0<br />
1,5<br />
Vi Adaptation<br />
with<br />
Slide Valve<br />
1,0<br />
20 40 60 80 100<br />
Cooling Capacity [%]<br />
Mod<strong>Techniques</strong> <strong>II</strong>_ASHRAE01.2003 – 9
Basic Test Conditions<br />
– VSD with Frequency Inverter –<br />
Capacity control by VSD with frequency modulation<br />
Compressor slide valve at 100% position<br />
Ratio Voltage / Frequency U / f constant<br />
torque remains constant with speed change<br />
Capacity steps during test<br />
100% <strong>for</strong> comparison with “direct power supply 60 Hz”<br />
Nominal 75 – 50 – 25% steps<br />
by speed adaptation to reach identical cooling capacities<br />
as with slider control<br />
Trans-synchronous speed range with U / f constant<br />
voltage lift <strong>for</strong> test above power supply (trans<strong>for</strong>mer)<br />
– real systems motor: e.g. 460V at max. frequency<br />
Mod<strong>Techniques</strong> <strong>II</strong>_ASHRAE01.2003 – 10
VSD with Frequency Inverter – Power & Torque<br />
with Winding Layout 230V-3-60 Hz<br />
200<br />
Power Supply to Inverter 460V-3-60 Hz<br />
[%]<br />
180<br />
160<br />
140<br />
120<br />
100<br />
Torque<br />
80<br />
60<br />
40<br />
20<br />
Power<br />
Torque w/o<br />
voltage lift<br />
0<br />
0 20 40 60 80 100 120<br />
[Hz]<br />
0<br />
230 [Volt] 460<br />
2<br />
Mod<strong>Techniques</strong> <strong>II</strong>_ASHRAE01.2003 – 11
Power Consumption of CSH Screw<br />
– VSD vs. Direct Power Supply –<br />
Full Load Conditions 60 Hz<br />
105<br />
Relative Power with Inverter [ % ]<br />
104<br />
103<br />
102<br />
R407C<br />
SDT 52°C (125°F)<br />
SDT 40°C (105°F)<br />
SDT 32°C (90°F)<br />
101<br />
-10 -5 0 35°F<br />
5 10<br />
SST [°C]<br />
Mod<strong>Techniques</strong> <strong>II</strong>_ASHRAE01.2003 – 12
Per<strong>for</strong>mance Behaviour of CSH Screw<br />
– Slider Unloading vs. VSD –<br />
110<br />
Fixed Operating Conditions<br />
100<br />
90<br />
80<br />
R407C<br />
Power Input [% ]<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
0 10 20 30 40 50 60 70 80 90 100<br />
Cooling Capacity [%]<br />
proportional<br />
Slider<br />
+2/40°C (35/105°F)<br />
VSD<br />
+2/40°C (35/105°F)<br />
Slider<br />
+2/52°C (35/125°F)<br />
VSD<br />
+2/52°C (35/125°F)<br />
SST / SDT based on<br />
dew points<br />
Mod<strong>Techniques</strong> <strong>II</strong>_ASHRAE01.2003 – 13
Per<strong>for</strong>mance Behaviour of CSH Screw<br />
– Slider Unloading vs. VSD –<br />
110<br />
SST / SDT Varying vs. Load – LWT 6.7°C (44°F) / EDB 35°C (95°F)<br />
100<br />
90<br />
80<br />
R407C<br />
Power Input [%]<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
0 10 20 30 40 50 60 70 80 90 100<br />
Cooling Capacity [%]<br />
proportional -<br />
fixed conditions<br />
Slider -- SST / SDT<br />
varying vs. Load<br />
VSD -- SST / SDT<br />
varying vs. Load<br />
100% Load:<br />
SST 2°C (35°F)<br />
SDT 52°C (125°F)<br />
- based on dew points<br />
Mod<strong>Techniques</strong> <strong>II</strong>_ASHRAE01.2003 – 14
Per<strong>for</strong>mance Behaviour of CSH Screw<br />
– Slider Unloading vs. VSD 80 Hz –<br />
Power Input [%]<br />
140<br />
130<br />
120<br />
110<br />
100<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
Fixed Speed Compr. with Larger Displacement vs. VSD 80 Hz<br />
R407C<br />
CSH65 VSD<br />
60 Hz<br />
VSD 80 Hz<br />
CSH65 VSD max. 80 Hz<br />
+2/40°C (35/105°F)<br />
CSH75 / 60 Hz<br />
+2/40°C (35/105°F)<br />
CSH65 VSD max. 80 Hz<br />
+2/52°C (35/125°F)<br />
CSH75 / 60 Hz<br />
+2/52°C (35/125°F)<br />
SST / SDT based on<br />
dew points<br />
CSH75<br />
60 Hz<br />
20 40 60 80 100 120 CAP [%] 140<br />
20 40 60 Relative CAP [%] 100<br />
Mod<strong>Techniques</strong> <strong>II</strong>_ASHRAE01.2003 – 15
<strong>Modulation</strong> <strong>Techniques</strong> <strong>for</strong> <strong>Compressors</strong> – <strong>Part</strong> <strong>II</strong><br />
– Summary (1) –<br />
CSH Screw <strong>Compressors</strong> with slider modulation<br />
Allow <strong>for</strong> infinite and step-wise capacity control<br />
Show favourable part load efficiencies<br />
Cost effective modulation technique<br />
VSD from 60 Hz to lower frequencies<br />
Show higher energy demands than slider control<br />
inverter and motor efficiency losses<br />
reduced rotor tip speed – resulting in<br />
increased internal leak losses during compression<br />
Inverter cost much higher than slider arrangement<br />
Conclusion<br />
No convincing solution<br />
Mod<strong>Techniques</strong> <strong>II</strong>_ASHRAE01.2003 – 16
<strong>Modulation</strong> <strong>Techniques</strong> <strong>for</strong> <strong>Compressors</strong> – <strong>Part</strong> <strong>II</strong><br />
– Summary (2) –<br />
VSD <strong>for</strong> trans-synchronous speed ranges<br />
in comparison to a larger displacement fix speed compressor<br />
Pro’s<br />
Wider modulation range than with slider system<br />
Smaller compressor, lower weight, simpler design (no slider)<br />
Lower compressor cost, potential <strong>for</strong> high reliability<br />
Soft starting, low inrush current<br />
No need <strong>for</strong> Power Factor correction<br />
Mod<strong>Techniques</strong> <strong>II</strong>_ASHRAE01.2003 – 17
<strong>Modulation</strong> <strong>Techniques</strong> <strong>for</strong> <strong>Compressors</strong> – <strong>Part</strong> <strong>II</strong><br />
– Summary (3) –<br />
Con’s<br />
Efficiency penalty against compressor with larger displacement<br />
increased tip speed but higher throttling losses<br />
Very high inverter cost<br />
exceeds (in the lower capacity range) compressor<br />
cost savings by far<br />
Bearing life is in direct counter-proportion to speed<br />
Increased sound levels with high speed<br />
Conclusion<br />
Both modulation techniques offer specific benefits – the deciding<br />
factors <strong>for</strong> a final solution are there<strong>for</strong>e dependant on:<br />
the entire system configuration, reliability, energy efficiency,<br />
investment and maintenance cost<br />
Mod<strong>Techniques</strong> <strong>II</strong>_ASHRAE01.2003 – 18
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