Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)
1Development of Japan’s First Air-source Circulation TypeHeat Pump Using R454C

YUJI OKADA*1

TAKACHIKA MORI*2

TORU KUROIWA*2

Mitsubishi Heavy Industries Thermal Systems, Ltd. worked on product development for anair-source circulation type heat pump using environmentally-friendly low-GWP refrigerant that
contributes to energy saving in the industrial field, and successfully launched a product thatrealizes the hot water temperature at outlet of 75°C at an outside air temperature as low as -20°C,while featuring a high efficiency of the rated COP of 3.3. We also actually introduced this productas a heating source of hot water for washing production equipment and confirmed its economicefficiency and environmental effects. Through the introduction of a hybrid system of this productand an existing gas-fired steam heater, in which this product covers the base heat load, it isexpected that the running cost and CO2 emissions will be reduced by about 50%, leading to a
substantial reduction in environmental load.

|1. Introduction
In machine parts manufacturing factories, steam boilers and electric heaters using fossil fuels
are widely used to produce hot water in the production processes of the degreasing and washing ofparts after cutting. Recently, the use of heat pumps, which contribute to energy saving through theirsuperior efficiency, has been increasing. Many conventional high-temperature heat pumps,
however, are water-source heat pumps that recover heat from hot wastewater from plants. They
require the installation of water pipes and pumps for the circulation of cold and hot water, resulting
in high installation cost and difficulty in securing installation space.
For conventional heat pumps, refrigerants with GWP values in the range of 1430 to 2090 areused. Along with the enforcement of the Law Concerning the Discharge and Control of
Fluorocarbons, the use of refrigerants with lower GWP values has been demanded from theviewpoint of reducing the effects of global warming.
To solve the problems of high installation cost and installation space, which have been
obstacles to the introduction of conventional heat pumps, we developed the “Q-ton Circulation”
circulation type heat pump, which uses air as a heat source and low-GWP refrigerant to reduce
environmental load. The equipment achieves high energy efficiency and the hot water temperatureof 75°C at outlet at an outside air temperature as low as -20°C.

|2. Overview of the developed heat pump
Table 1 lists the specifications of this product, and Figure 1 depicts its appearance. "Q-ton
Circulation" is an air-source circulation heat pump that uses R454C low-GWP refrigerant and it can
supply hot water of 75°C at an outside air temperature in the range of -20°C to 43°C. The heating
capacity and COP of this product are given in Figure 2. The COP value is 3.3 at an outside airtemperature of 25°C, which is the rated condition.

*1

Chief Staff Manager, Air-Conditioner Designing & Engineering Department, Mitsubishi Heavy Industries Thermal Systems,*2

Air-Conditioner Designing & Engineering Department, Mitsubishi Heavy Industries Thermal Systems, Ltd.

Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)2

Generally, the heating capacity of an air-source heat pump is reduced as the outside airtemperature is lowered. In this product, a two-stage compression refrigeration cycle was adopted,
which allowed the heating capacity of 30 kW to be secured at an outside air temperature of -20°C.
Furthermore, the hot water temperature of 75°C at outlet can be realized at an outside airtemperature of -20°C.
Table 1

Specifications

ModelOutput
kW
Rated COP−
Adjustable range of hotwater temperatures at°Coutlet
Hot watertemperature at °C
Operatingoutlet
range
Outside air°C
temperature
Dimensionmm
Refrigerant−
Flow rate
m3/hFigure 1

EQA40140 (Max. 50)
3.340 to 75
40 to 75
-20 to 43
2048 × 1350 × 720
R454C (10.8kg)
1.72 to 9.00

Appearance of the productFigure 2 Relationship between COP and outside air temperature|3. Selection of low-GWP refrigerantFirst, it was decided that a low-GWP refrigerant that could achieve the hot water temperature
at outlet of 75°C at an outside air temperature in the range of -20°C to 43°C, which is arequirement of the product specifications, should be adopted as the refrigerant for this product.
Figure 3 presents critical temperatures, boiling points and GWPs of various refrigerants. To satisfy
the aforementioned requirement of the product specifications, it was desirable to adopt a refrigerantwith a performance equivalent to that of the current R407C refrigerant. Therefore, low-GWP
refrigerants R454C and R455A were selected as candidates that meet the requirement, and Table 2
lists their properties.
Next, the candidates for refrigerant were compared in terms of the temperature glide, which
is important to an air-source heat pump. Figure 4 shows the temperature glides of the R32/HFO
mixed refrigerants. The temperature glide of R455A is 11.9 deg. Since it has a large temperature
glide, an evaporation heat exchanger may frost at low outside air temperatures. On the other hand,
the temperature glide of R454C is 7.7 deg, which is less different from the temperature glide ofR407C of 6.2 deg. Therefore R454C can be handled in the same manner as R407C.
According to the above results, R454C was selected as the low-GWP refrigerant for thisproduct.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)
3Figure 3

Properties of the candidates for refrigerant

Table 2

Properties of the candidates for refrigerant (calculated using the NIST REFPROP Ver.10)

Refrigerant
GWP
Mixing ratio (wt%)
ASHRAE classification
Boiling point @1 Atm (°C)
Critical temperature (°C)

R407C
1620
R32/R125/R134a
23/25/52
A1-36.6
86.1

R454C
146
R32/R1234yf
21.5/78.5
A2L-37.8
85.7

R455A
145
R1234yf/R-32/R-744
75.5/21.5/3
A2L-39.2
85.2

Figure 4 Temperature glides of the mixed refrigerants(Calculated using the NIST REFPROP Ver.10. Use conditions: Vapor, Saturated gas temperature of 5°C)

|4. Two-stage compression cycle
As previously described, high-efficiency heat pumps are widely used as alternativetechnologies to electric heaters and steam boilers in fields requiring the production of hot water. On
the other hand, when an air-source heat pump like this product is used to realize a hot watertemperature at a low outside air temperature, the refrigerant compression ratio becomes very high,resulting in reduced efficiency or limited hot water temperatures at outlet. To solve this problem, atwo-stage compression cycle with two compressors being connected in series was adopted in thisproduct.
Figure 5 shows the circuit diagram for the refrigerant in this product and the compressor
efficiency characteristics in a single-stage compression cycle and a two-stage compression cycle. Ina single-stage compression cycle, the pressure ratio in one compressor became high and theefficiency of the compressor was reduced. The adoption of a two-stage compression cycle allowed
the compression ratio in one compressor to be reduced and the compressor to be operated at themost efficient point. As a result, the efficiency was increased by 13% compared with operation in asingle-stage compression cycle.
This product is also equipped with two inverter compressors, thereby controlling theintermediate pressure of the refrigerant with the inverters so that the best operational efficiency isobtained. Furthermore, gas injection is performed to prevent the reduction of the heating capacity at
low outside air temperatures.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)
4Figure 5 Refrigerant circuit diagram and comparison of compressor efficiency in single-stagecompression and two-stage compression|5. Optimization of the compressor for two-stage compression cycle
Generally, lower-stage compressor in a two-stage compression cycle requires a larger
displacement compared with the higher-stage compressor due to the characteristic of economizer
cycle. On the other hand, the R454C refrigerant has a low density, and when a conventional
compressor is used, the displacement is insufficient, causing the reduction of the heating capacity.
If a larger-size and larger-volume compressor is used, the reduced capacity can be compensated,
but it leads to increase in cost for such a compressor.
To solve this problem, the three-dimensional scroll compressor (1) was adopted in thisproduct. The increased volume prevented the reduction of the heating capacity due to low density.
To achieve this without changing the outside dimensions, the structure was designed as follows:(1) Adoption of three-dimensional scroll compressor (3D scroll)(2) Redesign of scroll involute(3) Optimization of rotation axis balance(4) Expansion of ejection port

Figure 6Outlines of the scrolls

Figure 6 illustrates the outlines of the scrolls. The displacement of (b) three-dimensional
scroll was increased by 1.4 times compared with that of (a) two-dimensional scroll, but further
increase of displacement was required for a large-volume system. To increase a displacement,
generally, the lap height and the diameter are increased, but the durability may be reduced or theoutside dimensions may be increased. Therefore, we reviewed the scroll involute and the number ofwindings, so that the displacement was further increased. Figure 6 (c) gives the new scroll involute.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)
5The number of windings was reduced to optimize the internal volume ratio. If any change is madejust for the purpose of increasing the displacement, over-compression is caused in the compression
process on the first stage, and this results in the reduction of the efficiency. As a result of changing
the scroll involute and the number of windings, the compression ratio could be reduced while thedisplacement was increased.

|6. Reliability, safety, serviceability
The R454C refrigerant used in this product is a slightly flammable (A2L) refrigerant. R32
which is generally used in air conditioners for household and business falls under the same class.
Unlike R32 and R1234yf, however, R454C is not designated as a specified inert gas. Therefore,
concerning handling of the refrigerant, we established the risk assessment to secure safety. We
have notified customers about the content of the notification required for execution of recovery or
filling of the refrigerant and the necessity for the installation of a refrigerant leakage detector and aventilator when this product is placed indoor.
The legal refrigerant ton is less than 3 tons (2.99 tons), and this product is not regulated bythe High Pressure Gas Safety Act. Therefore, no prior notification is required in installation orservice.
In the service aspect, a 24-hour remote monitoring system was adopted, and even if an
unexpected trouble occurs, the recovery work at the field can be conducted immediately.

|7. Energy saving
We have been conducting a field test of this product for the purpose of heating a washingtank of a food factory since September 2018. The prediction result for the annual running cost andthe reduction of CO2 emissions based on the operational data for the period of September 2018 toMarch 2019 is shown in Table 3. The electricity rate was calculated from the actually measuredamount of electric power consumed by this product. In addition, the running cost before theintroduction of this product was calculated by back calculating the fuel consumption of the existingboiler from the heating capacity of this product and the comparison of the energy saving effects
was conducted. The system diagram in the field test is presented in Figure 7.
Table 3

Prediction result for running cost and CO2 emissions

Gas
Energyconsumption
Electricity
Running cost
Energy consumption in crude oilequivalent
CO2 emissions

Figure 7

Nm3
kWh×1000JPY

Gas
boiler369
24

Heatpump828
12Reduction

49.8%

GJ

158

46.1%

ton-CO2

0.8

0.4

53.0%

Comparison of the washing tank heating systems for food factoryMitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)
6The result showed that the running cost was reduced to 50% of that of the conventional
system and the CO2 emissions were reduced to 53% of that of the conventional system. It isexpected that the COP can be improved by increase of the continuous operation time and both theannual running cost and CO2 emissions can be reduced to about 54% in the future.
In addition, the energy consumption in crude oil equivalent can be reduced by over 40%.
Introduction of this product brings to customers the merit of contributing to energy reduction
required by the revised energy saving law.

|8. Conclusion(1)

The refrigerant R454C with the GWP value of 146 was adopted for the first time in Japan, sothat environmental load was substantially reduced.
A two-stage compression refrigeration cycle was adopted, and the hot water temperature atoutlet of 75°C was realized at a wide range of outside air temperatures from -20°C to 43°C.
In addition, the substantial energy saving was realized at the high efficiency of the rated COPof 3.3. It is expected that opportunities to introduce heat pumps in the industrial and businessfields will increase.
The adoption of an air-source heating system allowed the easy installation without securing
hot wastewater and conducting installation and piping works of pumps, and thus solved theproblems of high installation cost and installation space, which were obstacles tointroduction.

(2)

(3)

Reference(1)

M. TANIGUCHI, H. SATO, Y. TAKASU1 and Y. KIMATA, Development of Large Capacity 3D Scroll
Compressor for First Stage of Two-stage Compression System, 24th International Compressor
Engineering Conference at Purdue, July 9-12, 2018, Paper 1232.