H i g h e s t - E f f i c i e n cy i n t h e
World, Variable Speed Drive
Turbo Chiller NART-I Series
WATARU SEKI* 1
KAZUMA TAITO* 1

KENJI UEDA* 1
YOSHINORI SHIRAKATA* 1
JUN KOGA* 2

The NART-I series turbo chiller, developed and marketed by Mitsubishi Heavy Industries, Ltd. (MHI), utilizes as abase a highly efficient turbo chiller that uses the refrigerant HFC134a, which does not affect the ozone layer. An inverter
has also been introduced in the drive source of the NART-I series turbo chiller to improve aerodynamic performance, anda newly designed optimum control capable of coping with variable speed is incorporated that has made it possible torealize remarkable savings in energy. The inverter also facilitates variable speed control of the NART series turbo
chiller, thereby helping to make it highly efficient. The turbo chiller has achieved a COP of 6.1 by high performance
aerodynamic characteristics, low mechanical loss, and a highly efficient heat exchanger. In addition, advanced calculation control has been incorporated in the NART series to realize optimum acceleration/deceleration and load control
based on the aerodynamic characteristics of the compressor. As a result, a remarkable increase in performance could beobtained for NART series turbo chiller during periods of partial load and when cooling water temperature is low. Thesefeatures have made it possible for the series to attain the highest level of efficiency in the world, achieving a COP as highas 17.8 max. (chilled water outlet temperature of 7oC). Trial-calculations of the annual energy saving effect of thismachine under general plant load patterns have shown remarkable reductions in electric power consumption of as highas 45 % compared with the level of power consumption of a fixed speed machine.
come a matter of great social concern at present.
As shown in Fig. 1
1, a turbo chiller using HFC134a was
first put on the market in 1994. Since then, subsequent
successors of the system have been developed in fairly quick
succession. For example, the ARS series was developed in
1998, and the NART series, which had the world's highest
level of efficiency with a COP of as high as 6.1 (at a chilledwater outlet temperature of 7oC), was developed in the year
2000. At present, an increase in efficiency of 20 % or morecompared with conventional MHI systems has beenachieved. In 2003, MHI developed and marketed the AART
series with a COP of 6.4, which was a turbo chiller series
with the world's highest efficiency.
The NART-I series reported on this time was developedin 2002 and put in the market on January 2003. The aimof this new series has been to increase operating efficiency
to even higher levels to meet the needs of users who desire

1. Introduction
Turbo chillers are heat source machines used for various purposes such as air conditioning of buildings, heatstorage, district heating and cooling in heat supply business, air conditioning in plants, and process cooling inchemical plants. To address problems associated with thedestruction of the ozone layer, efforts have been under way
in recent years to completely eliminate the use of CFCs
and to abolish the use of HCFCs in the future as refrigerants. It has thus become necessary to increase the efficiencyof turbo chillers using HFC134a.
As a result, since 1998, MHI has used a specific refrigerant HFC134a with an ozone depletion potential (ODP)of zero for its turbo chillers to protect the global environment, and has aimed at increasing the performance ofequipment for ever greater energy savings, which has be-

Problem withozone depletion

7
6

Establishment oftechnological base

Problem with
global warming

Oil shock

COP

5
4
3
2
1

Adoption of
HFC

COP calculation conditions (JIS),
Chilled water outlet: 7oC,
Cooling water outlet: 37oC
CFC11
CFC12

0
1960 19651970

1975 1980

Change in COP of
high-performance chiller

1985

HCFC123
HFC134a1990

1995

2000 2005

2010

Fig. 1 Change in turbo chiller performance
Change in refrigerant and improvement in turbo chiller performance are shown.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.41 No.2 (Apr. 2004)*1 Air-Conditioning & Refrigeration Systems Headquarters*2 Takasago Research & Development Center, Technical Headquarters1

to operate the system at high efficiencies throughout theyear and who seek to reduce power consumption so as toreduce air conditioning costs throughout the year.

to peel off from the leading edge to produce a region ofreverse flow. On the other hand, the impeller without an
inducer produces a slight reverse flow on the downstreamside of the leading edge. As can be seen in the lower portion of Fig. 2, the efficiency on the small flow rate is higher
in the impeller without a diffuser than in the other system. Accordingly, an impeller without a diffuser was
adopted in the design.
It is well known that the surge limit increases to the
low airflow rate side from the design point toward the lower
speed region. In this case, the stable region was increased
up to the region of minimum airflow rate at 50 to 70 %,with the result that region of high efficiency operation could
be increased remarkably. From the above examinations, it
was found that a compressor efficiency of 80 % or higher
could be maintained stably, even in a low Mach number
region.
On the other hand, since the rotational speed range isincreased under variable speed operation, adequate caremust be taken into account in the design of both the shaft
resonance and blade resonance. It could be confirmed byanalysis and actual measurement of the characteristic frequency concerned that resonance is sufficiently avoided ina wide rotational speed range.

2. Details of technologies for realizing increased efficiency
In general, the relationship between ambient air temperature and power consumption can be summarized asfollows: decrease in outside air temperature ---> decreasein cooling water temperature ---> decrease in condensing
temperature and pressure ---> decrease in compression ratio of compressor ---> decrease in power. In short, power
consumption drops as the temperature of the cooling water becomes lower. Since turbo chillers use a centrifugal
compressor, unlike positive-displacement types of systems(reciprocating type, screw type, etc.), their compression
ratio and power are determined by the rotational speed of
the compressor. Hence, notable energy savings can be realized throughout the year by optimizing the control of therotational speed of the compressor in accordance with variations in load.
In NART-I series, MHI newly designed an advanced
microcomputer panel capable of high-speed calculations,adopted an inverter for the motor power, and has mounted
a rotational speed controller. As a result, MHI could solve
the technical problems of suitable speed control with theupgraded series. The details of the new technologies are
explained for each element below.
2.1 Increase in aerodynamic performance
In NART series, the design of the profile of the impeller
forming the moving blade and the side shape of the stationary flow passage were optimized with the assistance ofa three-dimensional CFD and based on a database of theresults of aerodynamic verification tests. The design also
contributes to greater optimization through three-dimensional accurate cutting machining operation of the impeller
by an advanced five-axis NC machine. The applicable rotational speed of a fixed speed compressor comes to within
85 to 115 % of the design point, whereas it comes to within
50 to 115 % for the variable speed compressor. Namely,
the off-design region is increased remarkably. During thedevelopment phase of the NART series, efficiency is increased at the design point with the assistance of a CFD.
This has made it possible to develop an impeller with excellent performance characteristics, even in a rotational
speed range wider than the region of the fixed speed compressor. Through these improvements, a high level of
performance could be achieved in the NART-I series in awide rotational speed range.
Fig. 2 shows the results of the examination of the performance obtained using the CFD when the inlet guidevane, called an inducer, shown in the figure is both present
and absent. When the impeller with an inducer is applied
to the variable speed chiller under operating conditions of
low flow rate and low rotational speed, it causes gas flow

Impeller with inducer

Impeller without inducer

Y

Y
X

X

Mach number distribution
near shroud, velocity vector,
Impeller without inducer
Impeller with inducer
0.5
0.5
0.0

0.0
Y

Y

X

X
Z

Z

Expanded view of leading-edge
0.5

0.5

0.0
Y

0.0

X

Y
X

Z

Z

Efficiency/design
point efficiency

Region of reverse flow

Region of reverse flow

: Impeller without inducer: Impeller with inducer
Mo=0.45
Mo=1.13
Mo=0.7
Flow rate

Fig. 2 Analysis of impeller flow by CFD
Effect of inducer on impeller flow is studied. The six
figures show velocity-vector distribution at Mo=0.45.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.41 No.2 (Apr. 2004)

2

Cooling water inlet temperature (oC)35

Table 1 Communication data from turbo chiller

Inlet vane + Hot gas bypass + Speed control

Type of data

Quantity, item, etc.

30

32 items
Inlet vane
control +Speed control

25
20

Analog data
Speed control

Temperature, pressure, etc.
128 items

Digital data

Indication of state, trouble, etc.
15

48 items
Cumulative data

10
0

20

40

60

80

100

Operating time, number of starts, etc.

120

Refrigerating capacity (%)
Fig. 3 Control range map
Optimum control method at each range determined by refrigeration
performance and cooling water inlet temperature.

2.2 Optimum control of variable speed
In a centrifugal compressor, inlet guide vane control
and hot gas bypass valve control are adopted for load
control. In inlet vane control, since resistance loss at
the inlet of the compressor increases in a small opening
area, the control must be performed at a large opening
area to reduce loss. In the hot gas bypass valve control
used in a low load region, a cyclic loss is produced in the
opening direction and, therefore, the control must be
performed in such a way that the opening is minimum.
Based on operational data obtained from plant verification tests, it was possible to prepare a control map
shown in Fig. 3
3. In addition, the limit conditions for
surging occurring on the small air flow rate side of thecompressor were calculated, and operation control was
performed at the optimum rotational speed to minimize
the lowering of performance by the hot gas bypass, while
inlet vane control was performed throughout the entire
operating range to allow highly efficient operation in awide load range.
2.3 Microcomputer control panel and high functional
communication technology
The characteristic equations used for realizing optimum control are very complex and the number ofdetection points is also large. Thus, a high-speed calculation capacity is required in order to perform
calculations without delay in the temperature control of
the chiller. For this purpose, MHI adopted a high performance CPU in the heart of the next-generation control
panel. Also, a seven-inch TFT LCD device was furnished
as a standard to increase visibility and ease of operation.
In addition, a 24-hour monitoring function has been
installable in the control panel for optimum operational
control and enhanced servicing. The following functions
can also be incorporated, as needed.
Routine monitoring of operation data
Storage of data when an abnormality occurs and automatic notice to the service person
Sampling of data in real time

Fig. 4 Appearance of high-voltage inverter
panel (L 2.4 m x W 2.6 m x H 2.7 m)

The data shown in Table 1 can be outputted for eachequipment device so that, by making use of the standard communication function, the heat source machinesof the user can be administrated easily. This function
can be realized by connecting up to a maximum of six
chillers to the MHI smart communication tool, or SCT,
(additionally installed) through the RS485 cable.
2.4 High-voltage inverter panel
For a large capacity machine with a class capacity
exceeding 800 USRt, the input voltage of the turbo
chiller's motor reaches a high-voltage class of 3 kV to 6
kV. Therefore, the introduction of inverters in theseclasses was requested. In the NART-I series, MHI was
the first in the industry to realize the driving of the inverter for all capacities through the adoption of a 3 kV
Fig. 4
to 6 kV class high-voltage inverters (Fig.
4).

3. Results of performance verification
In order to verify the cooling water temperature follow-up control and the partial load follow-up control of
the inverter machine, the test equipment at the MHI
plant was improved so that cooling water of 11 oC could
be produced, even in summer. The verification tests were
performed once these improvements were satisfactorily
completed. A flow chart of the equipment loop is shownin Fig. 5
5.
The partial load performances of the NART standard
machine and the inverter machine (NART-I series) against.
.
.

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.41 No.2 (Apr. 2004)3

Verificationequipment

Condenser
Pc

T4
T3

Cooling
water
pump

Fc
Fs

Monitor PC
Hot gasbypassvalve

Operating
panel
N
Power
supply
400V
50Hz

INV

V1 M1

M4

IntermediatecoolerHydraulic
Po To pump

Digital powermeter(motor input)

T5
Expansion
valve(high stage)
M2

Pm

M
V2

Digital power
meter(inverter input)

Compressor

SubcoolerPe

Expansion
M3
valve(low stage) T2

T1

Evaporator

Fe

Chilledwater
pump

Fig. 5 Flow chart of verification equipment at MHI plant
Detailed control of cooling water inlet temperature is realized even in summer by connection to another heatsource machine.
Table 2 Main specification of verification unit
Item

Specification value

Table 3 The lineup of NART-I series
40 I
NART(Refrigerating capacity) (400 RT)

Type of chiller

NART-401

Refrigerant

HFC134a400 V

1 407 kW

3 kV class

Refrigerating capacity
o

70I
100I
145I
200I
(800 RT) (1130 RT) (1610 RT) (2 000 RT)oChilled water temperature

7 C out/14 C in

Cooling water temperature

32 C in/37 C outo50I(570 RT)o

6 kV classcooling water temperature are shown at the upper portionof Fig. 6
6. The performances shown in Fig. 6 indicate theactual performance values measured using the NART-40I
2 Control of thespecified in the specifications of Table 2.
performance measurements was performed automaticallyas shown in the previous section.
As can be seen in Fig. 6, performance could be increased
remarkably in the cooling water temperature region of 20oC
or below where the control could not be performed sufficiently by the inlet vane control of the conventional fixed
speed standard machine. Moreover, stable control was
possible. The cooling water inlet temperature was allowed
to drop to approximately 13oC. The maximum COP reached17.8 for the machine having a chilled water outlet temperature of 7oC, which is the operating condition of generalchillers in Japan, and surpassed 20 for the machine usinga chilled water temperature of 9oC.

air temperature conditions in the Tokyo area were used as
the requirements for calculation. When the effect is converted into annual power consumption and the volume ofCO2 discharge, they are reduced by 40 % compared withthe high-efficiency standard machine (fixed speed). Thisis equivalent to 22% in terms of electricity charges.
In the plant loading pattern, the effect reduced is equivalent to 45 % in terms of the power consumption and thevolume of CO2 discharge and 35 % in terms of electricity
charges, respectively.
The increased amount of investment in terms of initialcost resulting from the addition of the inverter can be recovered within approximately one year for a machine thatis operated throughout the year in a plant process. The
increased amount can be recovered within about three years
even for a general air conditioning machine that is only
operated during the summer.
In terms of maintenance, the overhaul period is extended
to 50 000 hours or longer, and the load on the rotation system during operation can be reduced by rotation control. In
addition, a longer overhaul period could be possible throughoperation control in which the communication function of
the advanced microcomputer control panel is used. The
lineup of NART-I series by voltage level is shown in Table 33.

4. Energy savings effect on operation throughout the year
The calculation results of the energy saving effect during variable speed operation by the inverter drive
throughout the year are shown in the lower portion ofFig. 6.
The general air conditioning load pattern and outdoor

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.41 No.2 (Apr. 2004)

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Performance of standard machine

20
18
16
14
12
10
8
6

20
Temperature atcooling water inleto: 13oC: 15oC: 20oC: 22oC: 25oC: 29oC: 32 C

4
2
0

20

40 60
Load (%)

80

100

10.0
8.0

10.16

13.04

Load (USRt)

Annual rate (one million yen/year)

12.0: Meter rate
: Basic charge

5.96

3.55

6.61

Temperature atcooling water inleto: 20oC: 22oC: 25oC: 29oC
Remarkable: 32 C
improvement in
performance20

40 60 80 100
Load (%)

Cool heat load pattern by month for general air conditioners
900
Annual peak
: November
800
August: December
700 September: January: February
600
July: MarchJune
500
October
400
May
300
April
200
100
0

6.0
4.0

18
16
14
12
10
8
6
4
2
0

General air conditioner Example: 400USRt x 3units
14.0

COP (-)COP (-)

Performance of inverter machine

1

3

5

7

9

11 13 15 17 19 21 23
Time (h)7.08Power charges / utility rates

22% down

2.0Power consumption & volume of CO2 emissions
0.0

Inverter machine

40% down

Standard machine

Fig. 6 Performance comparison of inverter machine and standard machine; annual energy saving effect at general air
conditioning load
Inverter machine realizes remarkable improvement in performance at partial load and large annual energy saving effect.

Since it was first marketed in January 2003, the NARTI series has been well received mainly by utilities, plantheat source users. Dozens of machines are now in operation in Japan. In addition, MHI has also shipped machines
in the series to overseas users, as well.

low-up performance. In addition, a power reduction of 45% (under plant load conditions) could be achieved throughout the year compared with conventional standard
machines. The functions of the new NART-I series havebeen expanded remarkably by improving the control panel
in order to provide excellent operability and communication function. As for the power source, total lineup inverter
driving could be realized through the adoption of a highvoltage inverter panel that is the first to be applied as achiller in the industry.
The NART-I series won an Energy Conservation Award
by Ministry of Economy, Trade and Industry) in 2003, andis being received well for its high level of technology andperformance.

5. Conclusion
MHI has commercialized a turbo chiller capable of providing a COP of 17.8 (chilled water temperatures of 12oC/
7oC) under partial loads using the refrigerant HFC134a,
which has the world's highest level of performance.
Remarkably highly efficient operation has been madepossible throughout the year by optimizing rotational speed
control, load follow-up performance, and cooling water fol-

Wataru Seki

Kenji Ueda

Kazuma Taito

Yoshinori Shirakata

Jun Koga

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol.41 No.2 (Apr. 2004)

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