Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)1

Optimization of Process Cooling for Food Using Variable
Speed Centrifugal Chiller Operating at Ultra-Low Loadwith Low-GWP Refrigerant

KOHEI YAMAZOE*1

MASANOBU SAKAI*2

SATORU TANI*3

YOSHINORI HIDA*3

Along with the stricter regulations on refrigerants and the enforcement of the Law
Concerning the Discharge and Control of Fluorocarbons for the purpose of preventing global
warming, the need for low-GWP refrigerant (non-CFC) chillers has been increasing. On the otherhand, in process cooling systems at food factories where chillers are used, excessive starts andstops of chillers sometimes occur because the chillers cannot follow the load variation in batch
operations in which the load changes abruptly in a short time. Mitsubishi Heavy Industries
Thermal Systems, Ltd.(MHI Thermal Systems) introduced a two-stage cooling system using
temperature difference for which the variable speed centrifugal chiller operating at ultra-low load
was adopted, an exhaust heat recovery (hot water supply) system and other technologies topromote the delivery of high-efficiency systems that allow stable continuous operation at varyingloads.

|1. Introduction
Conventionally, in process cooling systems at food factories, the problem has often arisenthat as chillers cannot follow the load, the outlet temperature of chilled water is unstable, causing
excessive starts and stops of the chiller. To avoid this problem, MHI Thermal Systems proposes
optimal plant systems according to the operating conditions at customer plants by analyzing suchconditions that differ by line. We introduced a two-stage cooling system using the large
temperature difference produced in the manufacturing process, a heat recovery system in which theexhaust heat of the chiller is recovered to produce hot water so that the amount of steam used in theconventional manufacturing process is reduced, and cooling tower-free cooling (FC), thereby
realizing high-efficiency operation and contributing to the reduction of running costs.
This report provides an overview of the systems introduced at two factories, as well as thefuture outlook.

|2. Common matters
The features common to application examples (A) and (B) are as follows:(1) Energy saving
Through the processes of preparation for manufacturing, manufacturing operation andfinishing operation, chilled water/brine is supplied at stable temperatures/flow rates, so that
product quality is stabilized. The heat source equipment and related ancillary equipment (pump,cooling tower, etc.) are cooperatively controlled so that the operation is facilitated.
(2) Manpower saving
Through the processes of operation start-up, operation and finishing operation, theoperational data, preventive maintenance data, various report data, etc., can be automaticallytransferred to an IoT system, so that manpower is reduced. Since all the centrifugal chillersadopted are not regulated by the Law Concerning the Discharge and Control of Fluorocarbons,the burden of maintenance is reduced.
(3) Cost saving*1

Chiller & Heat Pump Engineering Department, Mitsubishi Heavy Industries Thermal Systems, Ltd.

*2

Sales Department, Mitsubishi Heavy Industries Thermal Systems, Ltd.

*3

Chief Staff Manager, Chiller & Heat Pump Engineering Department, Mitsubishi Heavy Industries Thermal Systems, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)
2The original plant system provided according to the operating conditions at each plantrealizes the reduction of the initial costs and running costs.
(4) Environmental performance
All the centrifugal chillers use new refrigerants with a global warming potential (GWP)
of 1 or less, so that the global environmental load is reduced and the amounts of electricity andgas used in the entire plant are also reduced, resulting in the reduction of CO2 emissions.
(5) Safety
Non-flammable or slightly flammable refrigerants with low toxicity are used for thechillers. The facility equipment is installed outdoors to eliminate concerns about noise in theplant.
(6) Space saving
Space saving is realized while maintenance space is kept within the limited installation
space.

|3. Application example (A)3.1

BackgroundTo a customer with a strong commitment to environmental conservation, such as advocating
a completely CFC-free factory, we proposed the ETI-Z low-GWP variable speed centrifugal
chiller, which uses the CFC-free refrigerant R-1233zd(E) with an ozone depletion potential of 0
and a global warming potential (GWP) of 1.

3.2

Issues

The chilled water return temperature differs by facility to which water is supplied. On oneline, it is returned at 10°C, while on another line, it is returned at 50°C or higher. Since different
lines are operated according to production schedules, the load variation is significant. In addition, if
a piece of equipment is selected at the maximum high-load operation, the heat source capacity of
the chiller and the overall size of the equipment become large, causing difficulties in arrangement
and efficient operation. To solve these problems, we introduced cooling tower-free cooling so thatthe heat load applied to the chiller is reduced and the load is stabilized.

3.3

Overview of the plant system

To effectively achieve cost saving and stable operation in process cooling systems for food, it
is necessary to understand the process characteristics and transfer heat stably and efficiently.
Therefore, we pinpointed the characteristics of a centrifugal chiller and established a system thatstably produces chilled water. In this section, the major technologies for the system are described.
(1) Cooling tower-free cooling (FC)To improve the aforementioned issues, we divided the return chilled water line into twolines based on the production line and the production time schedule of the customer's facility
and introduced a high-efficiency cooling system (cooling tower-free cooling system) in which
the high-temperature return water is cooled through heat exchange with cooling water via a heatexchanger. As a result, the leveling of the load applied to the chiller and the reduction of theheat source capacity have been achieved.
(2) Bypass control of chilled water at the chiller inlet (Patent No. 5412073, as of the issue of theTechnical Review)
The water temperature at the chiller inlet is reduced to or below the rated temperature of
the chiller to stably control the chilled water outlet temperature.
(3) Control of variable flow rate of chilled water
The flow rate of chilled water is adjusted so that the water temperature at the chiller
outlet is kept stable even if the water temperature at the chiller inlet rises.
(4) Constant control of discharge pressure of chilled water feed pump (secondary conveyingpump)
Inverter control is adopted to maintain a constant chilled water discharge pressure at thesecondary side irrespective of the operational state on the secondary side. A bypass circuit isprovided to follow the operational state on the secondary side.
The system flow image with (1) to (4) being reflected is shown in Figure 1. Through theaforementioned control, the system follows the load variation on the secondary side andprovides stable operation, contributing to stable manufacturing/production for the customer. In

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)3addition, the increase of the system COP, the reduction of running costs through energy savingand the downsizing of the equipment resulted in the reduction of the customer's initial costs forequipment introduction. From the aspect of facility management, the chilled water temperatureand flow rate are measured and the heat quantity is instantaneously calculated and recorded forthe visualization of energy consumption, which is useful for running cost management andmaintenance planning.

Figure 1

3.4

Image of plant flow

Results of the study

The advantages of the introduction of FC were estimated. When an FC cooling tower is not
introduced, the maximum load capacity is about 326 USRt. When an FC cooling tower isintroduced, the maximum load capacity is about 221 USRt, and the chiller heat source capacity wasreduced by 32% in the simulation. The electric power consumption was also estimated on theassumption that operations would be conducted for 24 hours a day, 365 days a year. As a result,when an FC cooling tower is not introduced, the electric power consumption is about 900 MWh,and when an FC cooling tower is introduced, it is about 541 MWh, achieving a reduction of about
40%. The estimated monthly electric power consumption is presented in Figure 2.

Figure 2 Comparison of electric power consumption between
system with and without FC

3.5Conclusion

With an FC cooling tower introduced, running costs were reduced by about 40%. Although aload variation occurred in which the actual load of the production facility on the secondary sideincreased by about five times in five minutes, the load applied to the chiller was reduced/levelledby the aforementioned system, and it was confirmed that the chiller could follow the load.
The system can supply chilled water at the temperature required by the customer,
contributing to stable production.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)4|4. Application example (B)4.1

BackgroundTo reduce environmental load, the ETI-Z chiller using the CFC-free refrigerant R-1233zd(E)and the GART-ZE chiller using R-1234ze(E) were delivered.

4.2

IssuesThere was a task of replacing the utility brine chillers on the old line and installing water
chillers for utilities on the new line. The problems were that the required utility temperature
differed by line and the cooling users were scattered at various points in the factory. To solve theseproblems, we introduced a centralized heat source system in which the chillers for multiple lines
were grouped by temperature zone and integrated and managed at a single location to supply
utilities to users located at various points.

4.3

Overview of the plant system

To effectively achieve cost saving in a food process cooling system, it is necessary tounderstand the characteristics of the process and transfer heat stably and efficiently. We pinpointed
the characteristics of the centrifugal chiller and established a system for stably producing chilledwater, brine and hot water. In this section, the major technologies for the system are described.
(1) Production of hot water and reduction of warmer gas consumption through recovery of
exhaust heat from chiller (exhaust heat recovery)In the food process studied in this application, there is a product heating stage
downstream of the upstream product cooling stage in a series of processes and the cooling andheating loads are generated at the same time. This characteristic is used to recover the exhaust
heat from the chiller produced for the upstream cooling stage (which is normally released into
the air via a cooling tower), store it in a hot water tank and use it in the downstream heatingstage, so that the amount of steam usage and the amount of CO2 emissions generated in theproduction of steam are reduced. (Figure 3(a))

Figure 3(2)

Image of (a) Exhaust heat recovery system and (b) Pre-cooling system

High-efficiency cooling technology on the process side (Pre-cooling system)
In the upstream cooling stage shown in (1), it is necessary to stably cool products down
to almost 0°C. Conventionally, products were cooled by using brine of 0°C or lower. Inchillers, the smaller the temperature difference between the cooling side and the exhaust heatside is, the higher the efficiency becomes. This characteristic is used to increase the systemefficiency by dividing the cooling of products into two stages: (1) pre-cooling to 5°C using
chilled water; and (2) temperature compensation using brine for cooling to 0°C or lower.
(Figure 3(b))(3) High-efficiency chilled water production technology (two-stage cooling system of thevariable speed centrifugal chiller)

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)
5In the production of chilled water of 5°C shown in (2), the performance characteristics ofchillers – the smaller the temperature difference between the cooling side and the exhaust heatside, the higher the performance – is used to increase the efficiency by cooling the processreturn water at two stages where the outlet temperatures of chilled water are 11.5°C and 5 °C.
Assuming that low-load operation would be required when the kind of product to be
manufactured is changed, only inverter-driven chillers were adopted to improve the efficiency
at low-load operation. (Figure 4, Figure 5)

Figure 4

Image of two-stage cooling

Figure 5

Image of performance characteristics of variable speed centrifugal chiller(4)

Stable production technology for chilled water and brine (Centrifugal chiller operating at
ultra-low load + tank system)
To prevent excessive starts and stops of centrifugal chillers in batch operations in which
the load changes abruptly in a short time, we used a centrifugal chiller operating at ultra-low

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)6load and a tank system to allow stable operation at low load without stopping the chiller. Inaddition, multiple centrifugal chillers for chilled water of 5°C and 11.5°C were installed, so thathigh-efficiency operation and risk dispersion were achieved simultaneously.

4.4

Results of the study

The advantages of the introduction of the centralized heat source system with theaforementioned features were estimated. In the simulation, this system reduced the power
consumption by about 43%, compared to the case where the chillers on the old line were notreplaced and latest chillers were additionally installed on the new line. The estimation results forthe respective power consumption are given in Figure 6.

Figure 6

4.5

Comparison of power consumption between new and old systems

Conclusion

For the utility facilities of the factory which had faced the task of replacing old equipment
and additionally installing new equipment, we introduced a centralized heat source system forproducing and supplying brine, chilled water and hot water to multiple lines. This system reduced
the power consumption by about 43% compared to the conventional system.

|5. Conclusion
Our company has developed variable speed centrifugal chillers using low-GWP refrigerants
to contribute to environmental conservation, and two models have been made into a series fordifferent temperature zones and heat source capacities. This report described the application
examples in which two models using low-GWP refrigerants were delivered to two factories. There
are also other factories that have been introducing chillers with low-GWP refrigerants. The demand
for such chillers is expected to grow in the future, and we will continue delivering this series ofchillers. Since the operating conditions and the equipment installation state differ by factory and byline, "individualized drawing system" engineering is required for optimization. We will makeefforts to offer optimal plant systems for production facilities with varied load patterns, while
carrying out discussions with customers.