Development of Air Cycle
System for Refrigeration
SHIGEMITSU KIKUCHI* 1

SEIICHI OKUDA* 1

HIROSHI IGAWA* 1

SHIGEKI MORII* 2

MASATO MITSUHASHI* 2

HIROTAKA HIGASHIMORI* 3

Mitsubishi Heavy Industries, Ltd. (MHI) has developed the most environment-friendly, easiest to use, and maintenance minimized system that has superior operating performance at - 50 oC or lower. It has been attributed tothree features. The First is the single shaft with an expansion turbines, compressor and a built-in motor. The Second
is the oil-free turbo machine with an active magnetic bearings. The Third is inverter controlling for revolution. MHI
has successfully made it possible not only to minimize power consumption but also maintenance. Besides, low pressure (maximum: 0. 1 MPa) also leads to easy to use.
MHI has developed the air cycle and presents technical details in this paper. The air cycle is realized Low
Life Cycle Cost (LCC) adopting single shaft turbo machine consisted of the expansion turbine, compressor andbuilt-in motor. Furthermore, the active magnetic bearing contributs to low LCC, by minimizing themaintenance. In addition, the system is also userfriendly working at maximum 0.1 MPa, compared withconventional vapor compressors.

1. Introduction
Due to concerns of the ozone hole, CFC and HCFC
had been banned and then, HFC has been developed as
the alternative. Recently, however, due to concerns ofglobal warming, natural refrigerants have been focusedand beginning to be available commercially. In thesesituations, practical researches of an air cycle are being
conducted. The conventional vapor compressors arebased on the utilization of the latent heat of the refrigerant. In contrast, an air cycle system is operated in
the only gaseous state, accordingly, the latent heat value
is not used. Consequently, it is at -55 oC or lower thatthe power consumption of air cycle is lower than them.
As natural refrigerants of these low temperature range,
ethane and ammonia are also available in the dual compressor, however, these have risk for toxicity andcombustibility. Air is non-toxic, non-flammable, therefore easiest to handle.
Compressor

CMotor(Drive source)

RadiatorF
CoolingfanE

Inletvalveo-80 CDefroster

Expansion
turbinePrimary heatexchanger

2. Air cycle system

Defrostingline

Warehouse,Freezer,etc.

-55oC

Recuperator
Outletvalveo-55 C

Fig. 1 System diagram of Air Cycle System
Schematic diagram showing the process for the air cycle developed by MHI.

*1 Yokohama Dockyard & Machinery Works*2 Yokohama Research & Development Center, Technical Headquarters*3 Nagasaki Research & Development Center, Technical Headquarters

2.1 Outline of the system
Fig. 1 shows the schematic diagram of the system,
while Table 1 shows the principal specifications. Theturbo machine in the principal portion consists of a compact single shaft that combined with the turbine that
produces low temperature air by expansion, the compressor that raises the pressure of the air to be introducedin the turbine, and the built-in motor that supplements
the drive force.
In addition to the turbo machine, the air cycle system
also consists of three parts. The first is the recuperator
to exchange refrigerated air from the warehouse toheated one after pressurized. The second is the heat exchanger for cooling air coming out of the compressor. The
third is the defroster for hygroscopic moisture in thesystem. If the temperature and the thermal load in thewarehouse are determined, the temperature and pressure in each part of the system should be uniquelydetermined. In addition, another line for defrosting is
set up in the system.
Table 1 shows the heat balance at -55oC in the warehouse. The maximum pressure of air in the system is0.1 MPa, The highest and lowest temperatures are+125oC, and -80oC respectively, and refrigeration capacity is 42 kW, and the overall system COP (Coefficient ofPerformance) is the value of 0.44.
Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 4 (Nov. 2005)

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Table 1 Specifications of Air Cycle
NoSpecification

ItemRemarks
o1Refrigeration capacity

2

Maximum output of motor

100 kW

3

Rated number of revolutions

21 000 rpm

4System COP

0.44 (Warehouse temperature: -55 C)

5

Pressure

Maximum 0.1 MPa (Gauge)

6Dimensions

W 2 300 x L 2 950 x H 3 400

Including soundproof panels

7

WeightApproximately 5.5 tons

Including soundproof panels

8Noise70 - 80 dB (A)42 kW (Warehouse temperature: -55 C)

oIncluding auxiliary

Pwr Spec
0.0001
mPeakAmplitude

Compressor

Upper bearing

020 690 RPM

4 000 RPM
103 RPM

Motor
0.0001800 Hz0 Hz
Pwr SpecmPeak
Amplitude

Lower bearing

020 690 RPM

Expansion turbine

4 000 RPM
103 RPM

Fig. 2 Photo of single-shaft oil less turbo machine
The compressor is located in the upper portion, while theexpansion turbine is located in the lower portion. The motorand magnetic bearing are set in between.

0 Hz

800 HzFig. 3 Shaft vibrations, raising the speed of turbo machine
Shaft vibration becomes small after 4 000 rpm of the critical range.

2.2 Outline of the turbo machine
As shown in Fig. 22, the turbo machine is made up ofcomponents with characteristics that include(1) Highly efficient impeller of turbine/compressor for
micro gas turbines, based on the supercharger technology;(2) Highly efficient, synchronous, built-in motor with permanent magnet(3) Minimum-loss, non-lubrication, non-contact and fully
active magnetic bearings used for turbo-molecularpumps.
The existent air cycles are necessary two-stage.
However, with these technologies integrated, MHI has
been able successfully to develop the vertical, highlyefficient turbo machine, which is the single shaft work-ing at 21 000 rpm in the maximum revolution of 100
kW motor.
Furthermore oil-free and, variable speed operations
with the inverter have been realized. Besides, the turbo
machine is designed to pass the critical speeds of therigid mode in lower speed and operate sufficiently under the critical speeds of the elastic mode. Then, thishas high reliability concerning vibration over variable
speed operation.
Fig. 3 shows the three-dimensional analysis result
indicated the actual measurement of the shaft vibrations,in the progress of raising the speed. As it is clear fromthe figure, the machine passed critical speed until reaching at 4 000 rpm, after that, was able to steadily operate
until reaching at the maximum revolution.
Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 4 (Nov. 2005)2

2.3 Features of the system
The features of the air cycle based mainly on the turbo
machine are summarized below. Fig. 4 shows the photoof the air cycle that was fabricated for outdoor-setting.
Using readily air as refrigerant and operating low
pressure allow the users easier to handle, especiallyfor refrigerant leak.
Complete oil-free system enables users to have no concerns of splashing it.
Inverter controlling enables users to minimum power
consumption, adjusting automatically to the designated temperature
Lower LCC at -50 oC or lower and lower power consumption at -60oC or lower
Variability of operation from -100 oC to -20oC due toutilize the only latent heat value of the refrigerant.
This enables to backup, in case of troubling the othersystem operating in tandem at the different temperature
Approximately same power consumption between direct and indirect with the secondary refrigerant system.
Table 2 showed comparison of the characteristics between air, ammonia and ethane.

3. Refrigeration performance.
.
.
.
.

3.1 Evaluation of the power consumption
Fig. 5 shows the comparison of the power consumption of the air cycle to two-stage R22
compressor at -30 oC, to dual NH 3/R23 compressor at-55 oC, and to liquid nitrogen at -80 oC respectively. It
turns out to be that power consumption of the air cycle
ends up approximately twice more at-30 o C, almost
equivalent at -55 oC, and further better one eighth lessthan at -80 oC.
3.2 Evaluation of LCC
As the case of Fig. 55, Fig. 6 shows the LCC the comparison of the air cycle to a two-stage R22 compressor
at -30 o C, to a dual NH 3 /R23 compressor at -55 o C, andto liquid nitrogen at -80 o C. LCC consists of initialcost and operation cost for 15 years, including powerand maintenance. LCC of the air cycle results in lessthan a dual NH 3 /R23 compressor at -55 o C and further better one forth less than at -80 o C.
It can be also seen from Fig. 6 that LCC of the air
cycle is less than conventional vapor compressors andliquid nitrogen at -50oC or lower.

.

Power consumption

Power consumption
comparison value

Air Cycle System

987
65
43
210

: Two-stage compressor (R 22): Dual compressor (NH3 /R23): Liquid nitrogen
Air refrigerant-30-55-80Application temperature (oC)Fig. 5 Comparison of power consumption
Power consumption of the air cycle ends up, almost equivalent at -55oC,
and further better one eighth less than at -80oC.

Fig. 4 Photo showing the air cyclePhoto shows the system fabricated with the soundproof covers
for outdoor.

Comparison of LCC regarded air cycle as 1
4.5
43.5

Air

Ammonia

EthaneToxicity

None

Present

None

Combustibility/flammability

None

Present

Present

Corrosiveness

LCC value

Table 2 Comparisons with Characteristic of Handling

32.5
2Air refrigerant

1.5

None

Present

10.5

None

0Low pressure High pressure High pressure
Working pressure
About 0.1 MPa About 1.5 MPa About 1.5 MPa
Total Risk

: Two-stage compressor (R 22): Dual compressor (NH3/R23): Liquid nitrogen

LowHigh-30-55-80oHolding temperature ( C)
Fig. 6 Results of evaluation of LCC comparison
The LCC of the air cycle result in less than the dual NH3/R23 compressor
at -55oC and further better, one forth less than liquid nitrogen at -80oC.

High

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 4 (Nov. 2005)

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4. Test results

5. Conclusion

To verify performance, controllability, automatic operating, durability, defrosting and other factors,operating tests were carried out aimed at the warehouse(3 600 mm long x 2 700 mm wide x 3 625 mm high) inFig.4.
The following were verified as the result of the tests.
Stable operations while avoiding surging, etc. until
reaching 21 000 rpm as the full load
Capability of outputting targeted refrigeration capacity at the designated temperature, while giving thethermal load into the warehouse
Controllability for various wave of thermal load in thewarehouse, based on smoothly controlling of the revolution
Capability of defrosting and draining frozen moisturerising in the warehouse
Capability of Automatically operating for starts &stops, for full & part loads and for defrosting
Capability of varying the temperature according
to demand, therefore, back upping if operating intandem
Capability of reducing the noise in the level of 70 dB(A) if installed soundproofing panels; refer to Fig. 4In addition, remote monitoring system has been developed prior to the field-testing.

MHI has developed the oil free, clean refrigeration
system in less LCC including the cost of initial , power
and maintenance at -50 oC or lower. Using the only
natural air is indicated both environmentally- and userfriendly. Performance and reliability of the system had
been verified through actual in-house testing. MHI plansfield-testing in actual working warehouse for tuna, withthe aim of enhancing its reliability and durability evenfurther.
Since 2003, this air cycle was nominated for ICETT(International Center for Environmental Technology
Transfer Association in Japan) Investment and received
subsidies because it was permitted to make huge contributions to prevent from global warming. MHI and theauthors would like to extend their deepest appreciation
to each of the persons that provided invaluable cooperation and support in this research.

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Shigemitsu Kikuchi

Seiichi Okuda

References(1) Gigiel, A. et al., Using Air for Cooling BRE Publication Draft
No. 202296 (2001)(2) Ibaraki et al., Development of Turbochargers in Mitsubishi
Heavy Industries, Ltd., Journal of the Gas Turbine Societyof Japan Vol. 33 No. 4 (2005) p. 48(3) Itai et al., Development of Active Magnetic Bearing Type
Turbomolecular Pumps for Wide Vacuum Range, Mitsubishi
Heavy Industries Technical Review Vol. 27 No. 2 (1990) p. 82(4) Katayama et al., Development of Totally Active MagneticBearings, Mitsubishi Heavy Industries Technical Review
Vol.26 No. 1 (1989) p. 12Hiroshi Igawa

Shigeki Morii

Masato Mitsuhashi

Hirotaka Higashimori

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 4 (Nov. 2005)

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