Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)
1Development of HCCV2001M, the World's First
Large-capacity Condensing Unit equipped with two CO2Scroll and Rotary compressors

HISAO MIZUNO*1

KENICHI MURAKAMI*1

TAKAHIDE GOTO*2

AKIO KAWANISHI*2TETSUYA HAYAKAWA*2 ATSUSHI ENYA*3To prevent global warming, refrigeration and air conditioning equipment is required toreplace current refrigerants with those with low global warming potential (GWP) to achieve areduction in GWP. Mitsubishi Heavy Industries Thermal Systems Ltd. has developed theHCCV2001M, a large-capacity 20-horsepower condensing unit following the 10-horsepower
HCCV1001, both of which adopt natural refrigerant CO2 with a GWP of 1 as an alternative tofluorocarbon refrigerants R404A (GWP3920) and R410A (GWP2090), which are the mainstream
refrigerants used in commercial condensing units. This larger capacity was established by using
two Scroll and Rotary compressors, which expanded the capacity range while accommodating thecomponents in a cabinet the same size as that of the HCCV1001. As a result, the installation
flexibility was ensured. This paper describes the technological issues and applied technologies ofthis product.

|1. Introduction
In Japan, GWP reduction has been promoted by the Law Concerning the Discharge andControl of Fluorocarbons. Commercial condensing units equipped with a compressor with a rated
output exceeding 1.5 kW are obliged to adopt refrigerants with a GWP of 1500 or less by 2025. As
shown in Table 1, alternative refrigerant candidates aimed at reducing GWP include fluorocarbon
refrigerants R448A (GWP of 1387) and R463A (GWP of 1494), which are mixtures of Hydrofluoroolefin (HFO) and Hydrofluorocarbon (HFC). Since these are fluorocarbon refrigerants,
refrigerant leakage over a certain amount during use must be reported to the government. Inaddition, these refrigerants must be managed at each life stage of construction, use, maintenance,and disposal, e.g., recovery of the refrigerant during maintenance and disposal, issuance ofcertificates for filling and disposal, obliged regular inspection of the equipment, etc. Furthermore,these refrigerants have another disadvantage in that their GWP is as high as just under 1500.
Ammonia (NH3), a natural refrigerant, is difficult to handle in terms of maintenance andmanagement due to its flammability and toxicity, so it is not suitable for small refrigerationequipment. On the other hand, natural refrigerant CO2 is easy to handle because it has a high designpressure and a low GWP, and is a single-component refrigerant that is subject to fewer obligations
for refrigerant management unlike fluorocarbon refrigerants. Based on these advantages, we put theHCCV1001 10-horsepower commercial condensing unit, for which a natural refrigerant CO2 wasadopted, on the market in 2017. This condensing unit has been earning a good reputation.

*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.

*3

Chief Staff Manager, Heat Transfer Research Department, Research & Innovation Center, Mitsubishi Heavy Industries,Ltd.

Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)2Table 1

Natural refrigerant CO2 and HFC-based mixed refrigerant

Global warming potential(GWP)
Flammability
Legal refrigeration
capacity (tonnage) below
which qualification for
operation andmaintenance is
unnecessary
Toxicity

Carbon dioxide(CO2)

Ammonia(NH3)

Mixed refrigerant(R448A)

Mixedrefrigerant(R463A)1<113871494

Nonflammable

Slightly flammable

Nonflammable

Nonflammable

20 tons

5 tons

20 tons

20 tons

Not toxic

Toxic
Not toxic
Not toxic
Leak sensor,
detoxificationequipment,
Leak sensor
Leak sensor
Safety equipment
Leak sensor
seismoscope, andsecurity equipment
Source of mixed refrigerant GWP values: Global warming potential (GWP) 100-year value in “IPCC FourthAssessment Report”

Table 2ModelApplication
Power supply
Refrigerant used
Ambient operating
temperature
Operating temperature
Legal refrigeration capacity
Design pressure
TypeDrive system
Compressor
Number ofcompressors
External dimensions (mm)Product weight

Figure 1

HCCV2001M specifications
HCCV2001M(20 horsepower)

HCCV1001(10 horsepower)

Refrigeration
3-phase 200 V, 50/60 Hz
R744 (carbon dioxide)-15°C to +43°C-45°C to -5°C
5.96 tons
2.98 tons
High pressure 14 MPaG / Low pressure 8 MPaG
Two-stage compression Scroll and RotaryDC inverter
21

Width 1350 x Depth 720 x Height 1690
470 kg
340 kgScroll and Rotary compressor

As shown in Table 2, the newly developed HCCV2001M 20-horsepower large-capacitycommercial condensing unit achieved a large capacity through the installation of two Scroll andRotary compressors as illustrated in Figure 1 in a cabinet with the same dimensions as the10-horsepower HCCV1001. Table 3 compares the developed condensing unit with a condensing

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)3unit using R22 refrigerant, which will be completely abolished in 2020 under the Montreal
Protocol. The HCCV2001M was designed to reduce the external dimensions, weight, etc., incomparison to the condensing unit using R22, so that it can respond to demands for thereplacement of condensing units using R22.
The HCCV2001M uses both compressor number control and compressor rotation speed
control to deal with a wide range of loads. According to the load of the connected load equipment(showcases or coolers connected to the condensing unit), the compressor number control switches
between single-compressor operation and two-compressor parallel operation, and the compressor
rotation speed control adjusts the rotation speed of the compressors. Due to these controls, theHCCV2001M, in comparison to the HCCV1001, doubles the capacity while maintaining the same
lower limit of the load that can be dealt with.
Table 3

HCCV2001M specifications and comparison with condensing unit using R22
HCCV2001M(20 horsepower)

ModelApplication
Power supply
Refrigerant used
Ambient operating
temperature
Operating temperature
Legal refrigeration capacity
Design pressure
Compressor

TypeDrive system
Number ofcompressors

External dimensions (mm)Footprint
Product weight
Liquid pipeConnectingpipe
Gas pipe

HCA150M(20 horsepower)

Refrigeration
3-phase 200 V, 50/60 Hz
R744 (carbon dioxide)

R22-15°C to +43°C-45°C to -5°C
5.96 tons
6.78/8.08 tons
High pressure 14 MPaG / Low
High pressure 2.94 MPaG / Low
pressure 8 MPaG
pressure 1.27 MPaG
Two-stage compression Scrotary
Scroll
DC inverter
AC constant speed
22

Width 1350 x Depth 720 x
Height 1690
0.98 m2
470 kgΦ15.88
Φ25.4

Width 2700 x Depth 600 x
Height 1800
1.62 m2
500 kgΦ19.05
Φ41.28|2. Technological issues
This chapter introduces the technological issues for increasing the capacity through theinstallation of two Scroll and Rotary compressors.
(1) Control of excessive refrigerant for increase in capacity
To accommodate the components in the limited cabinet dimensions, the accumulator andthe receiver, which contain the liquid refrigerant and adjust the amount of refrigerant in thesystem, had to be made smaller. Therefore, it was necessary to prevent the excessive liquid
refrigerant from flowing into the compressor from the receiver.
(2) Control for securing refrigeration oil in compressor
To prevent a shortage of refrigeration oil in the compressor, in addition to in-pipe-oil
return operation, two compressors were connected by an oil equalization pipe to perform oilequalization control, a proven technique used in air conditioners. In the case of a Scroll andRotary compressor, the pressure in the oil storage area is the intermediate pressure after
low-stage compression, and a differential pressure is generated between the two units by theeffect of operation. Therefore, it was necessary to avoid excessive movement of therefrigeration oil through the oil equalization pipe caused by the differential pressure, while
securing the refrigeration oil in each compressor through oil equalization control.
The measures against these technological issues are explained in the following chapters.

|3. Control of excessive refrigerant for increase in capacity
An increase in the capacity of the system results in enlarged volume of the contained
refrigerant due to the increase the volume of loads and pipes to be connected. Therefore, it isnecessary to store excessive refrigerant, the amount of which changes depending on operating
conditions, in the receiver. In the case of the two-stage compression gas injection cycle that weMitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)4adopted, in normal operation as shown in Figure 2 (a), the refrigerant is separated into liquid andgas in the receiver and the refrigerant gas is led to the gas injection circuit. In operation withextremely low refrigeration capacity load, the amount of refrigerant circulation decreases andexcessive refrigerant may accumulate in the receiver beyond the allowable volume, causing thecompressor to directly suck the liquid refrigerant from the gas injection circuit. If liquid refrigerant
flows into the compressor, the refrigeration oil in the compressor becomes diluted and the viscosity
decreases, resulting in the deterioration of the lubricity, which has an adverse effect on thereliability of the compressor. A general method of preventing this is to sufficiently increase thereceiver capacity to increase the allowable volume. However, this method cannot meet the demand
for smaller size and lighter weight.
The HCCV2001M adopts the maximum-volume receiver that can be accommodated in amachine room the same size as that of the HCCV1001 to achieve the same dimensions as theHCCV1001. It also uses a liquid level sensor that monitors the liquid level in the receiver to control
excessive refrigerant as shown in Figure 2 (b). When the liquid level rises transiently and the liquid
level sensor detects that the liquid level has exceeded the allowable volume, the liquid refrigerant is
controlled to be temporarily released to the low-pressure side through the circuit of the internal heatexchanger (double pipe liquid-gas heat exchanger) upstream of the liquid pipe outlet of thecondensing unit. This control enabled continuous operation of the compressor and secured thenecessary cooling capacity. In addition, by adopting a control method that closes the injection
circuit and stops the compressor when a liquid level exceeding the allowable volume is detected in
low-load operation where the capacity is excessive, inflow of the liquid refrigerant into thecompressor is avoided. After the compressor restarts, refrigerating capacity is ensured by thecontrol that opens the injection circuit when the receiver liquid level decreases.

Figure 2 Control of excessive refrigerant in receiver|4. Control for securing refrigeration oil in compressor
Refrigeration oil in the compressor is secured by adopting oil return control – technology
proven in air conditioners – and oil equalization control. The oil return control returns refrigeration
oil in the system that has accumulated in the heat exchangers and gas pipes, which are loadequipment, to the compressor. The start condition of this control is determined in consideration of
the oil discharge amount from the compressor and the oil retention amount in the pipes.
Figure 3 illustrates the measured correlation between the compressor speed and therefrigeration oil circulation ratio, which indicates the ratio of oil discharged from the compressor to
the refrigerant circuit that cannot be separated by the oil separator (refrigeration oil circulation
amount / (refrigeration oil amount + refrigerant circulation amount)). Based on this figure, theamount of refrigeration oil discharge that varies depending on the evaporation temperature and thecompressor speed is carefully estimated, and the implementation of oil return operation isdetermined to prevent a shortage of refrigeration oil. When returning the refrigeration oil to theMitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)
5compressor, the refrigeration oil can be returned together with the refrigerant in cases where therefrigerant in the pipe is a liquid, but the refrigeration oil cannot be returned at the same flow
velocity as the refrigerant when the refrigerant is gaseous. This is because even compatible oil has
difficulty flowing when the density and flow velocity of the refrigerant, which serves as theworking fluid, are small. For this reason, in consideration of cases where the condensing unit isinstalled at a higher position than (or with a head difference from) the load equipment, the control
that returns the refrigeration oil to the compressor by securing a refrigerant flow that can move therefrigeration oil upward (flooding flow velocity or higher) was adopted. The specific operation ofthe oil return control is to temporarily increase the compressor rotation speed to flow therefrigeration oil accumulated in the system and return it to the compressor to secure this flooding
flow velocity. Verification of this control was performed by using a high head difference testing
facility in our factory, and its validity was confirmed.
To secure the refrigeration oil, oil equalization control that eliminates the oil deviation
between the two compressors was adopted. This is because when two compressors are operated in
the same refrigeration cycle, the refrigeration oil deviation to one compressor occurs due toindividual differences in the compressors and the system even if their operating frequencies are thesame. In addition, this product employs a compressor number and capacity control that activates
one compressor in some cases to enable continuous operation of the refrigerating equipment evenwhen the load is small, which may cause refrigeration oil deviation to one compressor. Such oil
deviation between the two compressors would result in a shortage of oil if continued, so the oil
equalization control used for air conditioners was applied. In the case of an air conditioner, thecompressors that store oil are low pressure regardless of whether each compressor is operating orstopped. Therefore, to avoid oil deviation between compressors, by connecting the twocompressors with an oil equalization pipe and operating them at different rotation speeds,
refrigeration oil is moved by utilizing the pressure difference between the two compressors.
However, in the case of the HCCV2001M condensing unit equipped with Scroll and Rotarycompressors, the oil storage area in the refrigerating equipment is intermediate pressure after
low-stage compression, and the inside of the stopped compressor is low pressure during
single-compressor operation. For this reason, it is necessary to avoid the refrigeration oil moving
through the oil equalization pipe from the intermediate-pressure operating compressor to the
low-pressure stopped compressor. With an oil equalization valve on the oil equalization pipe
installed, the movement of the refrigeration oil was avoided. When the compressors are operating,the oil equalization valve is closed to avoid oil movement between the compressors. When oilequalization control is required, synchronizing control (Figure 4 (a)) for equalizing the compressor
pressures is performed, the oil equalization valve is opened, and then the oil equalization control
shown in Figure 4 (b) starts.

Figure 3

Oil circulation ratio

Figure 4 (b) Oil equalization control
schematic diagramMitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)
6Figure 4 (a) Synchronization control schematic diagram

As a measure to further improve the reliability, an oil pod was installed to increase thestorage amount of refrigeration oil and a sensor was provided to detect the refrigeration oil level. In
addition, compressor failure is prevented by implementing oil return control and oil equalization
control when an oil level drop is detected.

|5. Securing same usage range as 10-horsepower condensing unit
The newly developed HCCV2001M large-capacity commercial 20-horsepower condensing
unit is assembled in a cabinet with the same dimensions as the 10-horsepower HCCV1001, and thefootprints of the two are the same. The minimum installation interval of the HCCV2001M ininstallation in a row is unchanged at 10 mm, so the installability with respect to the capacity has
been improved (Figure 5). Similarly to the HCCV1001, the HCCV2001M satisfies the usage andinstallation conditions of condensing units using conventional fluorocarbon refrigerant, and has
specifications that easily meet demands for replacing an existing unit.

Figure 5

Installation in a row

Usage conditions
1. The evaporation temperature can be set freely from -45°C to -5°C.
2. The condensing unit can be used in the outside temperature range of -15°C to +43°C.
Installation conditions
3. The connecting pipe length between load equipment and the condensing unit can bein the range of 10 m to 100 m, and the condensing unit can be operated even under acondition where the load equipment is positioned 22 m below (Figure 6).

Mitsubishi Heavy Industries Technical Review Vol. 56 No. 4 (December 2019)7

Figure 6

Installability (piping length and head difference)|6. Conclusion
The newly developed HCCV2001M large-capacity commercial condensing unit using naturalrefrigerant CO2 is not only environmentally-friendly, but can also be easily handled in the samemanner as units using conventional fluorocarbon refrigerant. We will contribute to global
environmental conservation through the reduction of CO2 emissions by expanding the application
range of these condensing units from refrigerating warehouses and supermarkets to factory
facilities.

Reference(1)Hisao Mizuno,et al. : The application of the CO2 Direct Expansion System for the Freezing Refrigerating
Warehouse JSRAE jornal, 94 (1097), 1.3 (2019)