Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)38

New Suggestions for Centrifugal Chiller Technologies
to Realize High Efficiency and High Functionality

YASUSHI HASEGAWA*1

MASAHARU NITTA*1

KAZUKI WAJIMA*1

KENJI UEDA*2

The efficiency and functionality of centrifugal chillers has improved in recent years.
Centrifugal chillers have reached maturity as commercial products in terms of reduced energy
consumption of heat source equipment. However, there is still room for improvement in the extentof system operation-related interaction between the heat source equipment and the system in termsof energy saving, and Mitsubishi Heavy Industries Ltd. (MHI) considers that reinforcement oftechnology to support highly efficient operation of the heat source system and operational support
service is important. This paper introduces one way to enable further energy saving operation
through new operation support technology, including the expected COP, a load zone sign forhighly efficient operation, and a flow rate detector for chilled water and cooling water equipped as
standard, as well as operation support services including a heat source total control system andreal-time monitoring system on the web.

|1. Introduction
A centrifugal chiller is a large-capacity heat source system that can precisely control thetemperature of not only chilled water, but also low-temperature (below 0°C) brine or hot water, andsupply users with it. Because of its characteristics, the centrifugal chiller was historically used by
local heat supply service providers, industry heat source system users, and general large air
conditioning system users. It has recently been increasingly used as a replacement for absorption
water cooling and heating units, and is now also used in heating sources. In this way, thecentrifugal chiller has been widely used because its energy-saving ability is superior to that of otherheat source equipment. MHI has recently been working to improve the efficiency of centrifugal
chiller equipment and to develop technology for user support in the maintenance and operation of
the highly efficient equipment.
This paper presents a method to improve efficiency of a centrifugal chiller. It also introduces
technologies and functions to support the maintenance of highly efficient operation and explains
operation-monitoring technology.

|2. Improving the Efficiency of Centrifugal Chillers
MHI has been continually improving the efficiency of centrifugal chillers to meet recent
demand, stemming from the need to prevent global warming and ensure energy savings, especiallysince the introduction of the NART series of highly efficient centrifugal chillers in 2000. As theresult, the rated COP under JIS conditions was improved, to 6.4, in 2004 and the maximum COP ofa chiller equipped with inverter technology in part-load operation was improved to 24.2 in 2010
(Figure 1). Additionally, MHI has developed a small-capacity class (150-500 USRt) for
application in business air conditioning systems, which have been in great demand recently forenergy-saving modifications. That is the ETI series, which has both high mountability and highperformance, thereby improving performance and introducing high-efficiency machines with awide range of capacities 1. The following description focuses on technologies for improving theefficiency of centrifugal chillers; it begins by presenting technologies for basic performance (ratedperformance), and then presents technologies for part-load performance.
*1 Centrifugal & Absorption Chiller Department, Air-Conditioning & Refrigeration Systems Headquarters*2 Manager, Centrifugal & Absorption Chiller Department, Air-Conditioning & Refrigeration Systems Headquarters

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)39

Figure 1

2.1

History of centrifugal chiller performance

Technologies to improve the basic performance of a centrifugal chiller

The major technologies for improving the efficiency of a centrifugal chiller include:・ optimized refrigeration cycle (two-stage compression and two-stage expansion sub-coolcycle),・ high-precision machined blade,・ low-loss multiplying gear and low-loss bearings, and・ improved efficiency of the heat exchanger (evaporator and condenser) with optimized tube
shape and layout of the tube.
Through these technologies, chiller performance can be improved entirely from large- tosmall-capacity chillers; even the 200 USRt class (about the smallest centrifugal chiller) can attain avery high rated performance, with a COP as high as 6.0, based on electric power consumption,
including power for auxiliaries.

2.2

Technologies for further improving the efficiency of a centrifugal chiller

To further improve efficiency, MHI has developed technologies including a double
refrigeration cycle, which ensures optimized operation in each of the two compressors. Here,
one-path bidirectional-flow evaporator and condenser are positioned inside the centrifugal chiller
and the double refrigeration cycle is separated into a high-pressure side (the chilled water inlet andthe cooling water outlet) and a low-pressure side (the chilled water outlet and the cooling waterinlet) to reduce the compression ratio and the power required by the compressors. Use of thisdouble refrigeration cycle increased the rated COP to 7.0 Note and the maximum COP at part-load
operation increased to 29.1 2. This technology not only improves chiller efficiency, but also reduces
the power required to carry chilled water due to the increased difference in temperature of thechilled water, helping to improve the efficiency of the entire heat source system. Figure 2 displays
the ETI-40ES, which has the highest COP in the world.
Note : 400 USRt, chilled water 17/7°C, cooling water 32/37°C.

Figure 2

Appearance of ETI-40ES

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)
402.3Technologies to improve part-load performance of a centrifugal chiller

Recently, the way of thinking about performance improvement has been shifting to periodicefficiency, which involves performance under part-load conditions and low temperature coolingwater conditions, which is more typical of the majority of annual operation hours versus the COP
under the rated condition. In 2003, MHI also launched the variable speed controlled series with aninverter to enable effective reduction of annual energy consumption; this evolved into the current
AART-I series. Part-load performance is improved primarily though the following twotechnologies:(1) Improvement of part-load performance using an inverter; a variable speed control for thecompressor using an inverter can minimize the capability control by the inlet vane installedon the compressor, which generates loss.
(2) Improvement of part-load performance through a two-stage vane.
Regarding the capacity control mechanism of the compressor, an inlet vane is installed in
front of the two-stage impeller inlet in addition to the inlet vane installed on the inlet of thesingle-stage impeller to improve throttling ability (Figure 3). The capacity (air flow) control by
two vanes enables increased throttling ability at a certain vane opening, which can thereby
minimize loss generated when the vane opening is small.

Figure 3

One-stage/two-stage vane structure|3. Technologies to Support
Centrifugal Chillers

HighlyEfficientOperationofMHI has recently been focusing on developing technologies that allow users to maintainfavorable conditions in a centrifugal chiller the efficiency of which has been improved as discussed
above. Specifically, MHI provides indication of operating performance and the load zone in which
the optimal operation occurs and a function that continuously responds to variations in load andtemperature to maintain high performance.

3.1

Real-time indication of scheduled COP

It is important to be able to judge the appropriateness of current operation to ensure thecentrifugal chiller operates at high performance. In the case of the rated conditions, such a
judgment can be made through comparison with specifications. In actual operation, however,cooling water temperature and/or load differs from the rated specification conditions, soperformance (COP) to be expected is different from that under the rated conditions.
Although it is possible to estimate performance (COP) accurately using a method ofcalculation on a desk, based on actual measured operational data, this is not applicable to all cases,
which was a challenge. Thus, MHI has established a method of calculating partial load
performance, consisting of simple formulas based on theoretical principles 3.

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)41

Performance estimation involves separate methods for the inverter-driven centrifugal chiller
with variable speed control and the fixed-speed centrifugal chiller. The reason for this is that thetwo differ in their capacity control methods: one uses a compressor speed control, the other doesnot. For the fixed-speed centrifugal chiller, partial load performance is estimated based on the loss
characteristics along with the inlet vane control and the hot gas bypass, taking control of thecapacity. For the inverter-driven centrifugal chiller, a performance estimation formula was created
to reflect characteristics under all chilling capabilities and chilled water and cooling watertemperature conditions according to an index based on theoretical properties of the pneumatic
equipment in the centrifugal compressor; this performance can be estimated more generally
because loss at the inlet vane and the hot gas bypass valve can be identified. Regarding both control
types of centrifugal chillers, the difference between estimated and measured value was less than 3%
in rated power equivalent, so the results demonstrated that the electric power consumption of thechiller can be determined with a practical degree of accuracy (Figure 4).
The method of calculating performance can be installed on a microcomputer board of thechiller, which does not have as much computing power as a typical personal computer, and canindicate the chiller's feasible scheduled COP on the display screen of the chiller operation panel inreal time for comparison with the actual measured COP. Comparison of the results should result in
improved chiller operation and early detection of abnormalities.

Figure 4 Comparison between estimated and measured values of partial load COP

3.2
(1)

Chilled water and cooling water flow rate detector as standard equipment
Chilled water and cooling water flow rate detection
To reduce energy consumption of the entire heat source system, a variable flow control ofchilled water (or brine) and cooling water for reduction of the electric power consumption of
the chilled water and cooling water pumps has been widely spread. A variable flow control
usually involves electromagnetic flow meters mounted on the chilled water and cooling water
pipes to obtain flow rate signals to control the inverter frequencies of the motors driving thechilled water and cooling water pumps, based on the signals obtained. However, a centrifugal
chiller is a high-capacity heat source equipment with a large water pipe size and anelectromagnetic flow meter suitable for such a large pipe size is expensive. Thus, the chiller isequipped with a sensor, instead of a flow meter, to provide signals of chilled water and coolingwater used to control the flow of the pumps. Additionally, the expansion valve, the compressor
speed, and the hot gas bypass valve, excluding the inlet vane which performs PID control of thechilled water outlet temperature, are controlled based on cooling water inlet and outlet
temperature, cooling water flow rate, pressure at each point, and the refrigerant circulating
amount calculated with chilling ability to optimize control of the centrifugal chiller. For
variable flow control of chilled water, calculation of chilling ability requires chilled water flow

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)42

rate information, so the signal from the relevant sensor is used as a substitute for anelectromagnetic flow meter. Additionally, for systems not using variable flow control,robustness is ensured by allowing adjustable control with detection of departure or staggeringby continuously monitoring the flow rate of chilled water and cooling water, which affects
operational conditions.
(2) Detection of chilled water and cooling water stoppage
A chiller must be equipped with water stoppage detectors in both chilled water andcooling water systems to prevent freezing in the heat exchanger tube of the evaporator or rapid
increases in condenser pressure. In the past, differential pressure at the heat exchanger under aspecified flow rate of chilled water (or brine) and cooling water was used as the detection
criterion, and a situation in which differential pressure reached the value equivalent to a certain
flow rate was judged to be water stoppage. However, this detection method required anexpensive fine differential pressure detector when the flow rate was low. Thus, MHI used theflow rate measuring function described above to detect water stoppage under a wide range ofconditions, regardless of the specification conditions. This enables a wider range of chilled andcooling water flow control for the heat source system.
Although measurement of flow rate and detection of water stoppage involve the same
sensor, separate arithmetic processing for each signal can ensure both a quick response, whichis required for water stoppage detection, an equipment-protecting function, and accuracy, whichis required for flow rate measurement.

|4. Centrifugal Chiller Total Operation Support Service
In the past, most user needs for centrifugal chillers were related to improving efficiency andoperation stability. Today, users are also demanding sophisticated and diverse functions including(1) simple control for efficient operation, (2) the ability to check and evaluate operation data, and(3) the ability to determine optimized operation, based on feedback evaluation. In response to suchdemands, MHI continually works to develop and market products that can make heat source system
operation “sustainable, simple, and high performance.”

4.1

Technologies to support highly efficient operation

To support highly efficient operation of centrifugal chillers, MHI released the Ene-Conductor
heat source total control system in April 2010. This system can control heat source systems simply
and in an integrated manner by controlling a variety of heat source equipment and their optimum
load distribution, variable flow rate of chilled water and cooling water, and airflow of the cooling
tower, with the goal of improving COP in the entire heat source system 4. Figure 5 shows anexample estimation of Ene-Conductor’s energy-saving effect.

Figure 5An example estimation of the effect of operational support

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 2 (June 2011)43

For this estimation, simulations were conducted for equipment configuration of the heatsource system shown in Table 1; the assumed load was 24 hours a day and 365 days a year andresults were obtained for systems with and without optimized system control by theEne-Conductor. Application of chilled water and cooling water variable flow rate controls and acooling tower air flow control reduced chiller operation efficiency by approximately 9%, butsystem efficiency could still be increased by up to approximately 26% because electric power
consumption by the auxiliaries (the chilled water pump, cooling water pump, and cooling tower)
decreased significantly. This result demonstrates that Ene-Conductor is very effective in improving
system COP under conditions where performance improvement and energy-saving of heat source
equipment have already been maximized.
Table 1 Conditions for estimating the effect of operational support
Centrifugal chiller (inverter driven)
Cooling tower
Chilled water pump
Chilled water pump

4.2

Quantity
Chilling capacity
Rated COP
Quantity
Electric power consumption of fan
Quantity
Electric power consumption
Pump efficiency
Quantity
Electric power consumption
Pump efficiency

31,000 USRt
6.1
37.5 kW × 2
337 kW × 20.75
345 kW × 20.75

Indication and evaluation technology of real-time operation data

In response to the need to check and evaluate operation data and to optimize operation, MHI
has established an environment in which operation data about the centrifugal chiller can bedisplayed and checked by users in real-time on the web. Now, MHI is consolidating thisenvironment and the existing service that evaluates operation data to suggest ways to optimize
operation. MHI has always provided a menu in which remote monitoring is available, depending on
the user's maintenance contract, but only some users use this menu because the initial cost is high.
To allow all users to take advantage of this function, MHI will bear the expense of communication
devices, installation, and usage charge of the web system for the first year, which is the length ofthe chiller’s guarantee period. Thus, a user can check operation conditions of their chiller throughthe user portal site if they bear the expenses for the internet environment and provider fee.
Figure 6 shows a conceptual diagram for this service.

Figure 6

Conceptual diagram of the total operation support service

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44

MHI's scope of supply is scheduled to include aggregated management of documents such asspecifications and maintenance records and indications of operating conditions. Optional menus arealso scheduled to include the generation of periodic reports based on data obtained andaccumulated through the web, predictions of failure and performance degradation, and evaluationsof operating conditions and suggestions for optimized operation. Through these efforts, MHI canoffer highly value-added services, such as suggestions for optimized operation, to more users andestablishment of an interactive support system in which a heat source user and MHI can shareoperating information.

|5. Conclusions
Technologies that should be implemented on heat source equipment for improving theefficiency of heat source systems including centrifugal chillers are performance improvement andfunctionality improvement, and their latest technologies are as described above. In the past,development has focused on performance improvement. However, the major emphasis has beenshifting toward functionality improvement, such as optimization of heat source system component
operation and selection of additional functions to maintain high performance. In response to theincreasing need for functionality improvement, this paper introduced technology and services tosupport operations and demonstrated that they can contribute to highly efficient system operation.
The current trend is towards further functionality improvement, so additional functions of thechiller to improve functionality at both user or system components will likely be required in thefuture.

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