L a t e s t Te c h n i q u e s f o r
Multiple-function High-quality
Centrifugal Chillers
KENJI UEDA*1KAZUKI WAJIMA*1

MASATAKA YANAGIDA*1

KAZUMA TAITO*1
TAKAAKI MIURA*2

The latest AART and AART-I series of centrifugal chillers are characterized by high performance with good load and coolingwater follow-up performance. Their energy-saving characteristics are due not only to the equipment’s basic performance, but also to
the dedicated microcomputer control panel with ample computing power. Their high level of ﬂexibility, which includes the optimized
control of six adjustable elements applicable to any set of conditions using numerical arithmetic control, satisﬁes user requirementsthat have previously not been met. We included an ultralow load area of almost 0% into the controllable range, thus making it
possible for the centrifugal chillers to deal with chilled water temperature regulation in a continued ultralow load area or for load
startup from an ultralow load area. In addition, the centrifugal chillers detect instantaneous voltage drops in a half cycle, making
automatic recovery possible while maintaining self-sustaining operational signals. Furthermore, by making maximum use of highvisibility liquid-crystal display panels and extended data communications functions, the centrifugal chillers have simpliﬁed user
support functions, such as daily monitoring and maintenance, and performance evaluation after installation.

1. Introduction

2. Basic control of centrifugal chillers

Mitsubishi Heav y Indust r ies, L td. ( MHI ) has been
p ro duc i n g c ent r i f u g a l ch i l ler s more t ha n 4 0 ye a r s ,a nd de velop i n g t he m w it h c ont i nuous p e r for m a nc eenhancement as a top priority after 2000. Improvement of
the coefﬁcient of performance (COP) at the rated point has
been the focus in the past. Over the past two to three years,
however, development has concentrated on the challengeof reducing energy consumption and improving the annual
energy efﬁciency under actual operating conditions.
Of the centrifugal chillers used in Japan, 60% to 70% arefor industrial applications, such as semiconductor factories,
while the remainder are for consumer applications, suchas building air conditioning. These applications are very
diverse, including heat source systems for clean rooms insemiconductor manufacturing and large scale thin panel
display factories, heat sources for district heating andcooling, medium- to large -scale shopping centers, carpainting facilities, chemical manufacturing, large-scalebuilding air conditioning, and process cooling in chemicalplants. It is extremely important to make products capable ofmeeting these diverse user needs.
In addition, other common requirements have recently
become prominent, including the need to reduce CO 2
emissions, save energy, reduce operating costs, and achieve
stable operating characteristics. Therefore, the essential
product characteristics have now become clear.
Here, we report careful control and greater functionality
in tune with actual operating conditions as technology tomeet user needs.

About 90 % of the energy used in a centrifugal chiller
is consumed by the compressor as it compresses the
ref r igera nt gas. T he rema i n i ng 10 % is lost t h roughelectromagnetic and mechanical losses in the electric motor
and windage loss due to gas stirring.
As a centrifugal chiller is designed for optimum operation
at the rated point, the loss in the refrigeration cycle isminimal at that point.
However, as the chilled water temperature, cooling watertemperature, and cooling capacity move away from the rated
point, the losses increase and the performance is degraded
unless properly controlled. T herefore, control is ver yimportant.
(1) Optimal control
In the latest A A R T ser ies and A A R T-I ser ies ofch i l lers , opt i m i z at ion is possible t h rough t he si x
cont rol l able element s out l i ned below, even when
operation is not close to the speciﬁcation point (Fig. 1).
• First-stage inlet vanes: These are aerodynamicallyva r iable blades loc ated di rect ly i n f ront of t he
impeller to perform a capacity control function.
• Second-stage inlet vanes: These complement theﬁrst-stage inlet vane, making it possible to improvethe controllability.
• Variable speed control: T his controls the rotar y
speed of the compressor making it possible to control
the pressure of the exhaust gas and air volume
during suction. This is achieved by controlling thefrequency of the power supplied to the electric motor(A ART-I series only).

*1 Air-Conditioning & Refrigeration Systems Headquarters*2 Nagoya Research & Development Center, Technical Headquarters

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 2 (Jun. 2008)26

Impeller

Inverter
3

2
M

N

1
M

Closed type
motor

Controlpanel

Inlet vanes

PC Hot gas

PEbypass valve

Tcout
F2
Tcin

Ts

Tout
F1

6 M

Condenser

Compressor

Economizer

PM

Evaporator
Tin

Sub-cooler
4 M

High pressure expansion valve

5 M

Low pressure expansion valve

Fig. 1 Schematic diagram of centrifugal chiller showing the refrigerant system,
water system, principal key elements, and each control element• High pressure expansion valve: This circulates theamount of refrigerant required for the refrigeration
cycle without waste. Only condensed refrigerantliquid ﬂows downstream from the condenser, and notuncondensed refrigerant gas.
• Low pressure expansion valve: This controls only therefrigerant liquid expanded in the economizer so thatit ﬂows downstream to the compressor and not to theevaporator.
• Hot g a s by p a s s : T h is c on ne c t s t he c ondens er
and evaporator directly with piping and provides
minimization control of the gas f low between thecondenser and the evaporator, as occasion demands.
Numerical arithmetic control of all of these control
elements is provided using an exclusive highly functional
microcomputer control panel. T he panel calculates therefrigeration capacity, the pressure in each section, theenerg y consumed, the weight f low rate of ref r igerant
circulated inside the chiller, the volumetric f low rate of
the circulating refrigerant, the thermophysical propertiesof the refrigerant in each area, and the sound velocity. In
addition, numerical arithmetic control includes the ﬂow rate
coefficient of each expansion valve, and the pressure andﬂow rate coefﬁcients of the compressor as characteristics ofthe elements, and checks them against operating conditions.
Optimization of the refrigeration cycle is a result obtained by
optimization of each control element. With this optimization,
conditions close to the design target can always be obtainedeven if the conditions or speciﬁcations are different.
(2) Transient control
At first glance, it may seem that numerical control
should concentrate on achieving optimal control given
the current conditions. In actual operation, however,numerical control is usually designed to control thetransition to a certain operating point. Therefore, given
the target point and the current point, the equipment
condition and operational values are brought to the target
operating point over an appropriate interval. This makes
it possible to include an appropriate transition in thecontrol process.
Centrifugal chillers are available in several sizes andcapacities. Inputting several pieces of information onequipment composition in the form of a constant makes it
possible to achieve optimization on the same level for all
centrifugal chillers.

3. Handling of superlow loads
The latest advanced numerical arithmetic control has
made it possible to control superlow load areas with a verysmall heating value of 10% or less.
The range of standard capacity control for centrifugal
chillers is 20%–100%. For loads of less than 20%, on-off control
serves the purpose for single-unit operation. Dependingon the specifications, there are cases in which 10% is theminimum. However, a detailed examination of actual use
showed that there are also cases where a capacity setting in
the range 0% –10% would be very effective for energy savingsin heat source systems. Two such cases are as follows:• During initial trial operation in new factory construction
or introduction of a new heat source system.
• Standby operation in cases in which the startup load is
known in advance according to the schedule.
3.1 Calibration of chilled water temperature
When the refrigeration capacity is near 0%, the inlet andoutlet chilled water temperatures are almost the same. Thus,
to measure loads close to 0% , it is necessary to pay close
attention to the accuracy of the chilled water temperature
measurements. This measurement accuracy is achieved
Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 2 (Jun. 2008)27

Installation of four temperature sensors at both the cooling inlet and the cooling outlet
Chilled water piping on the equipment side

Chilled water outlet
Chilled water inlet

Temperature sensors must be
installed in a straight pipe section
after chilled water passes throughat least one bent portion so thatmeasurements will be made at alocation where temperature
mixing has taken place.

Condenser
Evaporator

Fig. 2 Method of measuring chilled water temperatureby securing a portion of straight pipe and establishingtemperature measurement points where sufficient stirring
takes place, confirming the measurement variations inadvance (Fig. 2).
Table 1 shows ac t ua l measu rement va lues of t he
refrigerant charged state when the chiller is stopped. The
sensor that shows a value closest to the average value of four
points is selected, and calibration is done in such a way that
inlet and outlet temperature values are the same. Then, thisvalue is used.
3.2 No-load control during initial trial operation
In many cases, new inst allations or inst allations ofadditional equipment are completed close to the end of the

fiscal year. That is, installation of heat source equipment
often takes place during the winter months and before the
manufacturing equipment that involves thermal loads isready. In addition, during initial trial operation, there areoften no cold or hot loads at all. Trial operations of heatsource systems usually involve spending large sums to install
temporary boilers for continuous operation of centrifugal
chillers, along with the temporar y piping required fordrawing in existing steam.
Figure 3 shows the status of follow-up performance basedon superlow load operation during actual trial operations inwhich a load of almost 0% was required. Note that when thecooling water temperature is about 15°C, the load continues

Item

Range

Average

1

2

3

4

Chilled water inlet

0.11

14.783

14.85

14.77

14.74

14.77

Chilled water outlet

0.03

14.775

14.76

14.76

14.79

14.79

Cooling water inlet

0.08

14.295

14.26

14.34

14.32

14.32

Cooling water outlet

0.15

14.348

14.24

14.39

14.39

14.37

16

110

15

100
90
: Chilled water inlet temperature: Cooling water inlet temperature: Chilled water outlet temperature: Load factor

80

The temperature reached a specified value in several minutes,
and then the load moved toward 0%.

50

13
12
11
10
9

70
60
40

:10
10
:20
10
:30
10
:40
10
:50
11
:00
11
:10
11
:20
11
:30
11
:40
11
:50
12
:00
12
:10
12
:20

10

10

9:5

9:4

9:3

9:2

9:1

0
:00

0
0

10

5
0

6
0

20

0

30

7

0

8

Load factor (%)

14

9:0

Chilled water/cooling water temperature (°C)

Table 1 Variations in chilled water inlet temperature and outlet temperature (°C)

Time
Fig. 3 Status trend graph for 0% load operation
The load dropped to almost 0% and water temperature control continued.

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

28

9:40

: Chilled water inlet temperature: Cooling water inlet temperature: Chilled water outlet temperature: Load factor

Standby operation

Load that starts up rapidly

10:00

10:20

10:40

10:00

11:20

100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
15
10
5
0

Load factor (%)

Chilled water/cooling water temperature (°C)

25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5

11:40

Time
Fig. 4 Startup characteristics from standby operation
The status of follow-up from superlow load area to normal load area can be observed.

at almost 0% . By stopping the equipment once and thenstarting it at about 11:00, a load occurred temporarily,
but soon returned to 0 % . The chiller maintained watertemperature control without stopping. Due to this superlow
load handling function, a temporary steam installation was
unnecessary during trial operation.
3.3 No-load control during standby operation
As described in the preceding section, no-load operation
is often required. The main difference is that a load starts assoon as the superlow load handling function is deactivated.
This means that upon deactivation, a transition occurs fromsuperlow load to normal operation.
Figure 4 shows the state of follow-up under conditions
where the cooling water temperature is approximately 18°C.
T he transition of a start-up load exceeding 60 % from a3 – 4% superlow load condition was handled correctly. It was
confirmed that the chilled water fluctuation was also small
and thus temperature control was possible.

even if a momentary voltage drop or power failure occurs,chillers must return to normal operation as quickly as
possible.
Power failure phenomena in which the electric motor
driving a centrifugal chiller stops due to a power supply
abnormality include a power failure in which the voltage drops
to 0 or sags to some intermediate value. Such power failuresand voltage drops may be due to natural phenomena, such as alightning strike on an electric power transmission system. In
recent years, power transmission networks have been improved
to the point where it is possible to isolate a failed electrical powertransmission system using a high-speed breaker. This meansthat the time required for shut-off or switching has becomemuch shorter. The instantaneous voltage drop time is on theorder of 0.07–2 s depending on the transmission voltage. Thisis about 0.1–2 s for a 60-kV system and about 0.3–2 s for a 6-kV
system.
What is important here is to detect a power failure or
instantaneous voltage drop before the chiller closes down.
If the operating conditions can be transited to automatic
restart, the chiller returns to the normal state without
stopping after a failure, thereby minimizing the time during
which the refrigeration capacity is zero.
Ta ble 2 s how s a bnor m a l c ond it ions de t e c t e d bycentrifugal chillers due to a power failure or momentary
voltage drop, and the means of avoiding them for automatic

4. Automatic return from power failure/instantaneous voltage drop
I n cases where cent r i f uga l chil lers a re used i nsemiconductor or electronic device manufacturing factories,
or used as the heat source for clean rooms or for chemicalplant applications, loss of chiller operation has a very large
impact on the quality of manufactured products. Therefore,

Table 2 Means of recovering from power failures and instantaneous voltage drops
Details of effectMeans of avoidance

Failure due to power cut in control relay
Failure due to stopping of oil pump
Failure due to stopping of auxiliary machine
Effect on the breaker for the customer’s equipment due to inrush current caused
by instantaneous restart of the electric motor
Possibility of damage to the compressor due to excessive torque caused byinstantaneous restart of the electric motor

1. Change the power supply for the control
relay to an uninterruptible power system.
2. Detect a power failure before equipment
stops, and stop the chiller. Then, restart
the chiller in a controlled manner.
3. Detect a delay in the failure signals.

UPS : Uninteruptable Power System

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 2 (Jun. 2008)29

restart. Conditions for automatic restart result mainly by
combining the means of avoidance 1–3 shown in the table.
More det a ils a re descr ibed later. However, i n t heconventional microcomputer control panel, the upper limit
of detection is 45 ms and a voltage of 80% with the use ofa high-speed relay, whereas in the latest microcomputer
control panel, detection within 10 ms and 70% or less can beachieved because processing is done on the chip.
In the case of an instantaneous voltage drop where theelectrical power transmission system is the cause, if detectionof the transmission voltage in a 60-kV system can be achieved
within 0.1 s, it is possible to meet the requirements of manyusers to bring the chiller to a temporary safe stop, and thenperform an automatic restart.

double the clock frequency of the previous main CPUs. In
addition, a 2-CPU conﬁguration, a dedicated CPU for theliquid-crystal display processing not only enables highspeed graphic processing, but also reduces the load of themain CPU for the basic control.
(2) Handling of two or more models
The latest panel is compatible in its basic form with
the A A R T and A A R T-I series of centrifugal chillers,and in its minimum form, with the Micro Turbo series.
By providing commonality of control components, it ispossible to select the number and types of module to beconnected according to the chiller series, and this has
greatly improved the expandability. Even for chillers
with a special configuration, flexibility can be achieved
through the combination of highly reliable standard
control modules.
Furthermore, high-speed communication is used
bet ween modules to ma i nt a i n ex pa ndabil it y. Dueconsiderat ion is g iven to noise i m mun it y for highfrequencies and harmonic currents in cases where an
inverter is used (Fig. 5).
(3) Improvement of user interface
A 10.4-in liquid-crystal display is used for the display
interface, with automatic backlight shutoff in power-saving
mode, and automatic turn-on when human presence is
detected. Being able to conf irm a larger variet y of
information without having to touch the operationalsurface leads to a reduction of user stress and reduces thepossibility of erroneous operation.
Graphics, in combination with large letters, make iteasy to determine the state of equipment quickly to aid
rapid decision making. This improves safety by reducing
operator uncertainty (Fig. 6).
(4) Customer support function
Knowing the state of a chiller after it is installed isvery important for predicting defects and for preventive
maintenance. However, there are many cases where a
L A N or telephone line is not available in the machine

5. Highly functional customized microcomputer
control panel
MHI has used customized microcomputer control panels
for centrifugal chillers for more than 12 years. However,the latest added functionality requires significantly moreadvanced calculation and sensing f unctions than usedprev iously. T his sect ion descr ibes the f unct ions andperformance that the microcomputer control panels musthave. T he communications required for maintenanceand other functions to maintain the performance of theequipment are also discussed.
(1) Enhancement of CPU speed and decentralization ofprocessing
With the improvement of chiller performance, theamount of data to be handled has increased sharply
because of the increase in the number of sensing points
and the increased complexity of numerical operation for
revolution control. This makes it essential to enhance thearithmetic capacity of the main CPU. The latest panels
that MHI will supply in 2008 include a main CPU with

Display
substrate

AIO
substrate

AIO substrate
×2(A total of 3)

IF substrate

IF substrate
×2(A total of 3)

DI substrate

DO substrate

AIO substrateCPUsubstrate: Principal composition: Minimum composition: Expansion composition

Fig. 6 Display substrate
Display interface

Fig. 5 Composition of microcomputer control substrate

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 2 (Jun. 2008)30

room. Therefore, data recording or obtaining a log at thetime of a failure was an issue to be resolved. Therefore,a general purpose memory card slot was included as ameans of easily transferring large amounts of data to andfrom the chiller. In the past, it was necessary to performmeasurements using a portable computer on-site alongwith specialized tools if remote monitoring was notavailable. However, it is now possible to collect data on
trial operation easily with only a single card.
The historical log includes data on 16 events counting
back from the latest. For each failure event, informationon the time of occurrence, data on 256 analog items,
and data on 512 digital items are stored with 50 samples
before and 20 samples after the occurrence. This makes
it possible to analyze the detailed state of the equipment
at the time the failure occurred. In addition, the data
logging (sampling) cycle can be changed in incrementsof 1 sec. This makes it possible to collect data at a rate
suitable for the details of a failure.
Alarms (minor failures) that do not lead to failuresthat stop operation are also recorded. A total of 100
such alarms can be recorded counting back from thelatest one. Thus, it is possible to collect information on
past operations of a chiller to determine any signs ofabnormality and to plan maintenance.
(5) Reinforcement of voltage drop detection
As described above, short-time detection of power
failures and voltage drops is extremely important for
stable operation and restart of the heat source system.
The latest MHI control module has a very high degreeof accuracy, being capable of detecting instantaneous
power failures within half a cycle or less of the power
supply frequency, as it is equipped to monitor the ACvoltage directly.

(6) Environmental responsivenessThe latest control modules comply with regulations on
the restriction of hazardous substances. They use leadfree solder and component parts.

6. Conclusion
The improved centrifugal chillers should perform at least
as well as the original basic machines. However, advanced
and sophisticated control mechanisms are essential toachieve the highest performance in various situations.
For this reason, MHI has made advanced arithmetic
cont rol possible through the use of customi zed
microcomputer control panels to meet new user needs, suchas automatic restart from an instantaneous voltage dropor superlow load operation. We intend to pursue further
development to meet new user needs that will become
increasingly diverse and demanding in the future.

Kenji Ueda

Kazuki Wajima

Kazuma Taito

Takaaki Miura

Masataka Yanagida

Mitsubishi Heavy Industries, Ltd.
Technical Review Vol. 45 No. 2 (Jun. 2008)31