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Development of Air Cycle
System for Refrigeration
Mitsubishi Heavy Industries, Ltd. (MHI) has developed the most environment-friendly, easiest to use, and maintenance
minimized system that has superior operating performance at -50oC or lower. It has been attributed to
three 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.
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 of
global warming, natural refrigerants have been focused
and beginning to be available commercially. In these
situations, practical researches of an air cycle are being
conducted. The conventional vapor compressors are
based 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 -55oC or lower that
the 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 and
combustibility. Air is non-toxic, non-flammable, therefore
easiest to handle.
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 and
built-in motor. Furthermore, the active magnetic bearing
contributs to low LCC, by minimizing the
maintenance. In addition, the system is also userfriendly
working at maximum 0.1 MPa, compared with
conventional vapor compressors.
2. Air cycle system
2.1 Outline of the system
Fig. 1 shows the schematic diagram of the system,
while Table 1 shows the principal specifications. The
turbo 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 introduced
in 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 to
heated 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 the
system. If the temperature and the thermal load in the
warehouse are determined, the temperature and pressure
in each part of the system should be uniquely
determined. 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 is
0.1 MPa, The highest and lowest temperatures are
+125oC, and -80oC respectively, and refrigeration capacity
is 42 kW, and the overall system COP (Coefficient of
Performance) is the value of 0.44.
-55oC
-55oC
-80oC
C
F
E
Fig. 1 System diagram of Air Cycle System
Schematic diagram showing the process for the air cycle developed by MHI.
Compressor
Motor
(Drive source)
Radiator Inlet
valve
Defroster
Expansion
turbine
Primary heat
exchanger
Recuperator
Defrosting
line
Warehouse,
Freezer,
etc.
Outlet
valve
Cooling
fan
SHIGEMITSU KIKUCHI*1
HIROSHI IGAWA*1
MASATO MITSUHASHI*2
SEIICHI OKUDA*1
SHIGEKI MORII*2
HIROTAKA HIGASHIMORI*3
*1 Yokohama Dockyard & Machinery Works
*2 Yokohama Research & Development Center, Technical Headquarters
*3 Nagasaki Research & Development Center, Technical Headquarters
2
Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 42 No. 4 (Nov. 2005)
2.2 Outline of the turbo machine
As shown in Fig. 2, the turbo machine is made up of
components 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-molecular
pumps.
The existent air cycles are necessary two-stage.
However, with these technologies integrated, MHI has
been able successfully to develop the vertical, highly
efficient turbo machine, which is the single shaft working
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 the
rigid mode in lower speed and operate sufficiently under
the critical speeds of the elastic mode. Then, this
has 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 from
the 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.
Including auxiliary
Including soundproof panels
Including soundproof panels
No
1
2
3
4
5
6
7
8
42kW (Warehouse temperature: -55oC)
100kW
21 000 rpm
0.44 (Warehouse temperature: -55oC)
W2 300 x L 2 950 x H 3 400
70 - 80 dB (A)
Table 1 Specifications of Air Cycle
Item Specification Remarks
System COP
Refrigeration capacity
Maximum output of motor
Rated number of revolutions
Pressure
Dimensions
Weight
Noise
Maximum 0.1 MPa (Gauge)
Approximately 5.5 tons
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