High-Performance Direct-Driven
Transport Refrigeration Units
Using 3D Scroll Compressor
MASAKAZU KAI*1

KAZUMI OSADA*1

YASUSHI WATANABE*2

TAKASHI TANAKA*1		

KOUSHI TARUMA*1

The transportation industry has undergone many changes in recent years. The strict control of truck exhaust emissions,increase in fuel costs, and increase in efforts to prevent global warming have all had an impact on transportation. Mitsubishi
Heavy Industries, Ltd. (MHI) has responded to these environmental changes by developing the S-series of direct-driven transport
refrigeration units. These units have the advantages of reduced compressor size and increased refrigeration efficiency.

refrigeration units using the new type of 3D scroll compressor
in this report.

1. Introduction
The transportation industry produces about 20% of theCO 2 emissions in our country, and we must do everything
possible to reduce them. The need to enhance the efficiencyof transport refrigeration units has increased sharply becauseof skyrocketing fuel prices. At the same time, the additionalemission control equipment required for compliance withtruck exhaust emission control measures is competing for thephysical space required to mount compressors for refrigerationon vehicles. A reduction in compressor size is an importantconsideration in the design of new vehicles to comply with thepost new long-term regulations for exhaust gases.
To achieve a substantial reduction in energy consumption
and a simultaneous decrease in physical size, MHI has
developed a special compressor for transport refrigeration
applications that uses three-dimensional compression scroll
technology (3D scroll). MHI developed this technology
through its experience with air-conditioning compressors. We
describe the characteristics of S-series direct-driven transport

2. Characteristics of the S-series
2.1 Open 3D scroll compressor
A reciprocating compressor, which has been used indirect-driven refrigeration units for a long time, decreases in
efficiency as the rotation speed increases. This is illustrated
in Figs. 1 and 2. For this reason, it is extremely difficult toreduce the size and improve the efficiency of a reciprocating
compressor at the same time.
A scroll compressor, on the other hand, has a highvolumetric efficiency and maintains its high efficiency evenwhen the rotation speed is high. However, a conventional scroll
compressor is not easily mounted on vehicles because when
the high-pressure ratio is designed according to the operating
point of the refrigeration unit, the number of turns in the scroll
increases and the barrel diameter expands.
We addressed this technical problem in our S-series units.
We succeeded in obtaining an appropriate design compression

3D scroll compressor
Adiabatic compression efficiency

3D scroll compressor

Volumetric efficiency

Rated point

Our conventional
reciprocating compressor

01000

2000

3000

4000

0Compressor rotation speed (min-1)

Rated point

Our conventional
reciprocating compressor

1000

2000

3000

4000

Compressor rotation speed (min-1)

Fig. 1 Volumetric efficiency characteristics

Fig. 2 Adiabatic compression efficiency characteristics

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 2 (Jun. 2008)*1 Air-Conditioning & Refrigeration Systems Headquarters*2 Nagoya Research & Development Center, Technical Headquarters

23

ratio suitable for refrigeration units without expanding
their barrel diameter by adopting a 3D scroll with an axial
compression mechanism. As a result, we achieved substantial
reductions of 60% in the compressor volume and 50% in thecompressor weight compared to our conventional reciprocating
compressors with equivalent capacities. In addition, thecompression efficiency at the rated rotation speed improved byabout 15%.
2.2 Improvement of the availability factor due to the
expanded operation range

Refrigeration unit body
Cooling van body
Cabin controller

Hightemperatureenvironment

For a direct- dr iven ref r igerat ion unit in which t hecompressor is driven by the vehicle engine, the rotation speedof the compressor is determined by the rotation speed of thevehicle engine and cannot be controlled by the unit itself. A
scroll compressor delivers a high volumetric efficiency for theentire rotation speed range, and therefore, the change in theoperating pressure due to variations in rotation speed is larger
than that of a reciprocating compressor. In the S-series, apressure transducer is mounted on the high- and low-pressure
sides of the unit, and the operating pressure is controlled ona two-dimensional map, i.e., high pressure and low pressure.
T his means that the allowable operating range for thecompressor will be used to the maximum extent possible,
and the frequency of protective stopping for the unit due totransitional engine speed variations will be minimized.
2.3 Reduction of electric power consumption
A direct-driven refrigeration unit operates on electric power
supplied by the vehicle power source. Because the power
generation efficiency of the alternator is about 50% at theengine’s medium speed range, the reduction of electric power
consumption of the refrigeration unit is highly effective forengine load reduction, and thus it reduces the fuel consumptionof the vehicle. In the S-series, we focused our attention on theair blower system, which consumes at least 80% of the electricpower. We reduced the eddy loss by enlarging the condenser
fan diameter and adopting an impeller with serrations (Fig. 3).
We also improved the efficiency by eliminating brushes fromthe condenser fan motor, and improved the air flow inside
the container. These measures reduced the electric power
consumption by a maximum of 23% compared with ourconventional unit. At the same time, we substantially reduced
the blowing air noise of the condenser.

Compressor

Heat loss

Refrigerant piping

Pressureloss

Fig. 4 Vehicle installations

2.4 Reduction of heat loss and pressure loss
A compressor mounted on the vehicle engine and arefrigeration unit mounted on the wall of a cooler van bodyare connected by refrigerant piping through the engine
compartment, which is a high-temperature environment,
as shown in Fig. 4. For this reason, the low-pressure andlow-temperature gas refrigerant, which evaporates in theevaporator, absorbs a large amount of heat until it is sucked
into the compressor. This reduces the operating efficiency.
In addition, there is generally little room in the engine
compartment to allow for the passage of thick refrigerant
piping. This often means that smaller diameter refrigerant
piping is used, which increases the pressure loss and greatly
reduces the operating efficiency. In the S -series, a large
internal heat exchanger is installed as standard equipment, andthe amount of heat that is thermally exchanged uselessly inthe high temperature engine compartment is effectively usedby the heat exchanger to increase the refrigeration capacityof the unit itself, and thus increase the efficiency. In addition,
the increase in supercooling by the internal heat exchanger
reduces the amount of refrigerant required to obtain thespecified refrigeration capacity, and it also has a large effect onpressure loss reductions.

Conventional unit

New unit

Serrations
Flow direction

AA
Cross-section A-A

Separately installed parts

Fig. 3 Reduction of eddy losses using an impeller with serrations

Integration inside the unit

Fig. 5 Integration of separate components in a unit

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 2 (Jun. 2008)24

2.5 Streamlining of installation work
Direct-driven refrigeration units are installed on vehicles
at the reefer box manufacturer or installation company. In theS-series, the components such as the unit controller that wereinstalled separately in the past are now integrated in a unit, asshown in Fig. 5. This reduces the installation time and cost,
while increasing the system quality after installation.
2.6 Enhancement of reliability
For transport refrigeration units where warming of thecompressor by a heater is not possible while the operation issuspended, migration of the refrigerant into the compressor
is liable to occur. The start of liquid compression and poor
lubrication caused by refrigerant flushing are major causesof compressor failures. In the S-series, measures to prevent
migration of the refrigerant are introduced in which valvesprovided on the suction and discharge sides of the compressor
close when the unit is stopped. The compressor is also
equipped with a multi-relief port, which greatly reduces thestress that develops when liquid compression occurs.

Table 1 Annual fuel consumption

42040
35 30
25 20
15 10

Fuel consumption

365 L267 L

4. Conclusion
By using 3D scroll compression technology, we havesimultaneously achieved a substantial improvement inefficiency and a reduction in compressor size, which are both
excellent marketing features. We currently have products forsmall and medium refrigerated vehicles in our lineup, and areworking on products for large vehicles and multi-temperatureunits. To help reduce CO 2 emissions in the transportationindustry, we will make every effort to distribute this new seriesof direct-driven refrigeration units which provide a substantial
improvement in energy efficiency.

Vehicle speed (km/h)

Appearance frequency (%)

0 Idling

6

New S-series unit

Table 1 shows the fuel consumption of the units bycomparing the data with and without the refrigeration unit foran assumed annual operating time of 2000 hours. The newS-series unit reduced the amount of fuel consumed by 27%
compared to our conventional unit.

We installed our conventional refrigeration unit and thelatest S-series unit with an equivalent refrigeration capacity on
the same cooling van one after the other to compare the annual
fuel consumption. We calculated the annual fuel consumptionof the vehicle using the data measured by operating the vanunder the JE05 mode and the operating conditions frequency
allocation shown in Fig. 6, with the vehicle speed and airtemperature as parameters based on air temperature data fromTokyo, Nagoya, and Osaka.

8Conventional unit

A reduction of 27%

3. Energy-saving properties when used on an actual vehicle

10ItemMasakazu KaiKazumi Osada

Takashi Tanaka

Koushi Taruma

Yasushi Watanabe80

5 0Outside air temperature (°C)

Fig. 6 Cooling van operating conditions frequency distribution

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 2 (Jun. 2008)25