Purdue University Purdue e-pubs International Compressor Engineering Conference School of Mechanical Engineering 2012 Development of DC Inverter Scroll Compressor used for Marine Container Refrigeration Unit Tomomi Yokoyama tomomi.yokoyama@daikin.co.jp Katsumi Kato Nobuhiro Nojima Keiji Yoshimura Hiroshi Kitaura Follow this and additional works at: http://docs.lib.purdue.edu/icec Yokoyama, Tomomi; Kato, Katsumi; Nojima, Nobuhiro; Yoshimura, Keiji; and Kitaura, Hiroshi, "Development of DC Inverter Scroll Compressor used for Marine Container Refrigeration Unit" (2012). International Compressor Engineering Conference. Paper 2215. http://docs.lib.purdue.edu/icec/2215 This document has been made available through Purdue e-pubs, a service of the Purdue University Libraries. Please contact epubs@purdue.edu for additional information. Complete proceedings may be acquired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at https://engineering.purdue.edu/ Herrick/Events/orderlit.html
1365, Page 1 Development of DC inverter Scroll Compressor used for Marine Container Refrigeration Unit Tomomi YOKOYAMA*, Katsumi KATO, Nobuhiro NOJIMA Keiji YOSHIMURA, Hiroshi KITAURA Daikin Industries, Ltd. Compressor Development and Engineering Group 3-12 chikko-shinmachi, Nishiku, Sakai, Osaka, Japan Tel: +81-72-241-1261, Fax: +81-72-245-3740 E-mail: tomomi.yokoyama@daikin.co.jp * Corresponding Author ABSTRACT In recent years, energy saving of a container ship is becoming one of the most important issues from the point of environmental protection of the global marine transportation business. To solve this problem, we have developed a new gas injection inverter scroll compressor for marine container refrigeration unit. The new compressor is based on a conventional scroll compressor, which has been used in various industrial air conditioners for a long time, with some greatly improved technologies. For instance, the efficiency of the compressor is increased by optimizing the gas injection mechanism. On top of this, the efficiency at low speed operation is increased by using interior permanent magnet synchronous motor. As a result, it was possible to achieve higher refrigeration capacity without increasing the size of the new compressor. The energy consumption has decreased by 45% compared to conventional models. This paper describes details about the improvements of the compressor including the gas injection mechanism and the improvements in its performance at low speed operation. 1. INTRODUCTION Environment protection is becoming a more important issue in the maritime industry across the world. In July 2011, CO2 emissions controls have been introduced in the MARPOL Convention. Meanwhile recent increase in oil price is urging container operators to further energy saving throughout the vessel. These efforts to reduce CO2 emissions and operating cost have now been accompanying efforts to reduce energy consumption of marine container refrigeration unit. We have succeeded in meeting these needs by modifying our highly reputed, commercial use models of DC inverter scroll compressors for air conditioning applications, as shown below. 1) Increasing the number of injection port, enlarging the diameter of the injection port and horizontal holes leading to the ports to increase the injection circulation whereby the system s refrigerating capacity can be increased. 2) Using a concentrated-winding reluctance DC motor to reduce energy consumption during energy saving modes which account for the largest proportions of the total operating hours. These modifications could reduce energy consumption by maximum of 45% while reducing also the size and weight of the compressor compared to those for existing marine refrigeration unit. This paper describes our efforts to achieve these performance improvements, weight and size down of marine transportation container refrigeration unit.
1365, Page 2 2. STRUCTURE OF COMPRESSOR Figure 1(a) shows a sectional view of a conventional scroll compressor for marine refrigerating containers. Figure 1(b) shows the newly developed compressor. Table.1 shows specifications of these compressors. The conventional compressor has a low-pressure dome that contains the motor. The new compressor has a dome comprising two portions separated by the main bearing frame: one is a low-pressure compartment containing the compressing mechanism, and the other one is a high-pressure compartment containing the motor. The new compressor has a system to cool the compressing mechanism using the refrigerant that enters in the compressor, and cool the motor using the refrigerant gas that delivers from the compressor, which leads to higher volumetric efficiency. The suction volume of the new compressor is 70cc, which is more than 60% less compared with the suction volume of the conventional compressor 188cc. The maximum rotating speed of the new compressor is 130rps and its fixed scroll has a port that is used for injecting refrigerant gas into the compressor. High pressure side Low pressure side Suction pipe Fixed scroll Injection pipe Orbiting scroll Discharge pipe Main bearing Crank shaft Motor (Rotor) Motor (Stator) Sub bearing Oil pick up Low pressure side High pressure side (a) Conventional compressor for marine container (b) Developed compressor for marine container Figure 1: Sectional view of Compressor
Table 1: Specifications of compressor for marine container Conventional compressor (with gas injection) Developed compressor (with gas injection) Refrigerant R134a R134a Pressure Structure Low Pressure shell High / Low Pressure shell Cylinder Volume 188 cc 70 cc 1365, Page 3 Power Supply [Rotating speed] Motor type AC(50/60Hz) Induction motor Distributed winding DC Inverter [20 ~ 130rps] Reluctance DC-motor Concentrated winding 3. DESCRIPTION OF THE DEVELOPMENT Some major items which are applied in this newly developed compressor are described below. 3.1 Larger Energy Saving by Improving Refrigerating Capacity and Performance 3.1.1 Improve the injection port A refrigerating transport container is required to be cooled to the desired temperature before (or after) it is loaded. This preliminary cooling operation is called a pull down operation. This pull down operation is required to be completed down to the desired temperature as quickly as possible, for which a larger refrigerating capacity is required. This is one of the distinctive requirements for refrigerating transport containers. To increase refrigerating capacity, it is necessary to increase the suction volume of the compressor. However this may lead to a larger size and/or weight, which refrigerating transport containers cannot admit for space limitation. Our approach to addressing these conflicting problems was to modify an air conditioner DC inverter scroll compressor comprising a gas injection system into a machine with larger injection circulation so that the refrigerating capacity can be increased. Figure 2 shows illustrations of the scroll and injection port of a conventional DC inverter scroll compressor for air conditioning applications (a) and newly developed compressor (b). Figure 2 shows the position of the orbiting scroll,(colored area), immediately after the compression space is loaded with gas.
1365, Page 4 The conventional DC inverter scroll compressor (a) has one injection port whose size is φ4 while the new compressor (b) has four injection ports whose size has been increased to φ5 so that the injection circulation into the compression space could be increased. Note that the horizontal hole to the injection port was enlarged to an inside diameter of φ7.4 from φ6, when the number of the ports was increased. Suction refrigerant Horizontal holeφ6 Injection refrigerant Injection port (φ4 1) Injection port (φ5 4) Horizontal hole φ7.4 Injection refrigerant Suction refrigerant Orbiting Scroll Colored compression space (a) Conventional compressor for Air conditioner (b) Developed compressor for Marine container Figure 2 Injection Port and Compression space (at the end of suction process about colored compression space) Figure 3 shows the position of the orbiting scroll at which the injection port opening leading to the compression space (colored part) becomes the largest. The conventional compressor (a) has one opening (φ4) leading to the compression space while the new compressor (b) has two full openings (φ5 2) and two partial openings to the compression space. (a) Conventional compressor for Air conditioner (b) Developed compressor for Marine container Figure 3 Injection Port and Compression space (at the maximum opening square shaped by injection port and colored compression space )
1365, Page 5 Figure 4 shows, for both the conventional and new compressors, how the sectional area of injection port opening to the compression space changes with the crankshaft rotation angle (deg). The maximum opening area could be increased by 240% compared to the conventional compressor, and the duration, in terms of crank angle range, in which the injection ports are kept open to the compression space could be increased by 1.5 times. Developed (φ5 4) Injection Port Square [cm2] 0.5 0.4 0.3 0.2 0.1 +240% Conventional (φ4 1) 1.5 Developed (φ5 4) Injection port Square 0 0 90 180 270 360 Crankshaft rotation angle [deg] Figure 4 Relationship between the crank shaft rotation angle and the opening square of injection port Colored compression space Figure 5 shows actual measurements of injection circulation of both the conventional DC inverter scroll compressor for air conditioning applications and new compressor for marine container. The operating conditions for these measurements shown in Table 2 represent a pull down operation that needs the compressor to act at the maximum refrigerating capacity. This pull down operation is one of the chilled condition. The new compressor increased injection circulation by 3.5 times as much as the conventional one, which led to increase in refrigerating capacity by 1.33 times and also to making the compressor size smaller. 140% 120% 80% 60% 40% 20% 0% Total refrigerating capacity ratio (suction + injection) Refrigerant mass flow ratio of suction Refrigerant mass flow ratio of injection 1.33 13% (a) Conventional 3.5 133% (b) Developed 46% Figure 5 Increasing ratio of total refrigerating capacity and refrigerant mass flow (suction, injection) Table 2: Test conditions Operation mode Pull down (chilled) Ambient / Box temp [deg] 38/ 2 Condensing temp [deg] 63 Evaporating temp [deg] -10 Rotating speed [rps] 130 subcool = 5deg, superheat = 8deg
1365, Page 6 3.1.2 Use a concentrated-winding, reluctance DC motor The conventional scroll compressor for the marine container operates at the constant speed using induction motor. In case of induction motor, the compressor starts and stops its operation to adjust the refrigerating capacity, which is needed to keep the temperature inside the container after its temperature goes down to the desired temperature. Unfortunately such operations tend to be the negative factor from the point of the energy saving. To achieve energy saving, the new compressor controls its rotating speed using the inverter to adjust the refrigerating capacity, not by starting and stopping its operation. Especially for energy saving of the marine container unit, it is prerequisite to improve not just operating efficiency at the pull down operation, but also to improve operating efficiency at the low-middle speed (20-70rps) during the long temperature maintenance operation. To this end, the new compressor uses a concentrated-winding DC reluctance motor. To increase motor efficiency, the motor loss (copper loss and iron loss) must be reduced. To improve the motor efficiency especially at low speed, copper loss must be reduced. To reduce copper loss, we chose a concentrated-winding DC reluctance motor that has been widely used for home air conditioners. The motor can reduce the size of the coil end parts, which leads to reduction in copper loss. Figure 6 shows pictures and major dimensions of a distributed-winding AC motor for conventional scroll compressor for refrigerating container and concentrated-winding reluctance DC motor for the new DC inverter scroll compressor. The former has two poles (88 mm in laminated thickness) while the concentrated-winding stator has six poles and nine slots (90 mm in laminated thickness). The laminated height being unchanged, the outside diameter and total height could be significantly reduced, and the weight could be down to 1/3. 15.2kg 5.0kg 88 185 90 130 φ190 φ145 (a) AC Induction motor (Distributed Winding stator) (b) Reluctance DC-motor (Concentrated Winding stator) Figure 6 The Dimension and Weight of motor stators Motor efficiency on pull down operation Figure 7 shows comparison in motor torque and efficiency on Table 3 pull down(chilled) test condition. On that test condition, the developed compressor is operated at maximum speed of 130rps and its efficiency is 4.5% higher than the conventional one, which proves that it is effective for energy saving on the pull down condition as well.
1365, Page 7 105% 130rps 2 Table 3 Test conditions Motor efficiency ratio 104% 103% 102% Developed (Reluctance DC motor) 4.5 Operation mode Pull down (chilled) Ambient / Box temp[deg] 38 / 2 No. 1 2 Compressor( ) C D 101% 60Hz Conventional (Induction motor) 1 Rotating speed[rps] (AC power frequency) (60) 130 Motor efficiency ratio 104.5% 99% 0.2 0.4 0.6 0.8 1.0 1.2 Motor Torque ratio Figure 7 Relationship between the motor torque and the motor efficiency ratio C: Conventional, D: Developed Motor efficiency on energy saving condition Table 4 shows the condition for the conventional and developed compressor on typical energy saving condition (chilled). The stopping ratio on Table.4 is the actual stopping ratio when the compressor of the marine container unit is operated on start and stop condition. The developed compressor is operated with the low rotating speed (30rps). So that the efficiency of motor for the developed compressor must be high. Figure 8 shows comparison in motor torque and its efficiency for the conventional and the developed compressor on the operation condition of Table 4. The motor efficiency for the developed compressor is 1.9% higher than the one for the conventional compressor. And also its energy saving is higher on the energy saving condition. Besides on the condition of Table 4, the stopping ratio for the developed compressor is lower. It is the effect of using the inverter for the developed compressor. Table 4 Test condition Operation mode Energy saving Chilled Ambient / Box temp [deg] 25 / -1 No. 3 4 Compressor ( ) C D Motor torque ratio [-] 1.0 0.46 Rotating speed [rps] (AC power frequency) (60) 30 Compressor stopping ratio [%] 52 15 Motor efficiency ratio [%] 100 101.9 C: Conventional, D: Developed Motor efficiency ratio 103% 102% 101% 99% 1.9 4 30rps Developed (Reluctance DC motor) 60Hz Conventional (Induction motor) 0.2 0.4 0.6 0.8 1.0 1.2 Motor Torque ratio Figure 8 Relationship between the motor torque and the motor efficiency ratio 3
1365, Page 8 Figure 9 shows the comparison for input consumption ratio and stopping ratio of conventional and developed compressor on the Table.4 condition. The stopping ratio of the developed compressor is dramatically decreased. So that its input consumption is 66% decreased. 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Input consumption ratio 52% Conventional 3 compressor stop ratio 66% 37% 34% 15% Developed 4 Figure 9 Relationship between Input consumption ratio and compressor stop ratio at Energy saving operation (chilled) 3.1.3 Result of energy saving efforts The developed compressor is operated at the low speed using the inverter and the energy saving becomes higher by decreasing the stopping ratio. Also using the concentrated-winding reluctance DC motor prevents the motor efficiency from going down, which works out on the energy saving. The Figure 10 shows that the hourly input power consumption of the developed marine unit [LX10F] goes down to 55%, compared with the one of the conventional marine unit [LXE10E:~E series].(when the ratio of energy saving operation is 60% on chilled condition and 40% on frozen condition) Input Power consumption ratio 90% 80% 70% 60% 50% 45% 55% 40% Conventional Developed Figure 10 Input power consumption ratio of the developed marine unit
1365, Page 9 3.2 Size and Weight Figure 11 shows outline views of the conventional and the developed compressor for marine container. Figure 12 shows how much the size and weight could be reduced. The outside diameter of the casing was reduced to 85%. The volume could be reduced to 45%, and the weight could be reduced to 60%. The new, smaller compressor is appropriate for transport refrigeration unit. Weight :71kg (a) Conventional compressor Weight :43kg (b) Developed compressor Figure 11 Outline view and weight of compressor 110% Size and Mass ratio 536 547 φ200 φ170 15% 90% 80% 70% 60% 50% 102% 85% 55% 40% 60% 40% 30% 45% Height Diameter Volume Weight Figure 12 Size and Weight ratio of the developed to conventional compressor
1365, Page 10 4. CONCLUSIONS The following conclusions are made to a new DC inverter scroll compressor that can meet requirements for reducing energy consumption while reducing its size and weight. (1) The refrigerating capacity of new compressor is increased 1.33 times as much as conventional DC inverter scroll compressor by enlarging the diameter of injection port, increasing the number of injection ports and enlarging the diameter of the horizontal holes leading to the ports. (2) The motor efficiency is increased to 4.5% at chilled pull down condition and 1.9% at chilled energy saving condition by using concentrated winding reluctance DC motor. (3) The compressor input power consumption is decreased by 66% at chilled energy saving condition by optimizing the start and stop operation which is done by inverter. As a result, the hourly input power consumption of the developed marine unit [LX10F] is decreased by 45% compared to conventional marine unit [LXE10E:~E series].( when the ratio of energy saving operation is 60% on chilled condition and 40% on frozen condition) (4) Finally, the overall size and weight of the compressor is reduced by 55% and 40% respectively. REFERENCES Yoshida, Y. et al. 2002, Development of Scroll Compressors with the High Performance used for Marine Container Refrigeration Unit, International Compressor Conference at Purdue, C18-6