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Contents
- Introduction
- Refrigerant regulation trends
- Centrifugal chiller refrigerants and characteristics
- Refrigerant properties and centrifugal chillers’ features
- Technology for applying low-GWP refrigerants to centrifugal chillers
- Switching existing equipment in the market to low-GWP refrigerants
- Conclusion
1. Introduction
Centrifugal chillers are used in district heating and cooling and air conditioning for large buildings, the semiconductor industry, pharmaceutical and food factories, etc., and produce chilled water at cooling capacities ranging from about 150 to 5,400 USRt (U.S. Refrigeration ton).
When it comes to refrigerants, the use of CFC and HCFC refrigerants was restricted with the view to protecting the ozone layer, and in the 2000s there was a rapid conversion to HFC refrigerants such as R134a for use in centrifugal chillers. In contrast, HFC (hydrofluorocarbon) refrigerants have a high global warming potential (GWP), and in order to drastically address this issue, there was an urgent need to switch to low-GWP refrigerants. For this reason, centrifugal chillers have been using low-GWP refrigerants since around 2015, ahead of other air conditioning systems.
This report outlines efforts for switching to these low-GWP refrigerants, and the characteristics of each type of refrigerant.
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2. Refrigerant regulation trends
R134a, an HFC refrigerant conventionally used for centrifugal chillers, has a GWP of 1430(1), and at the 28th Meeting of the Parties (MOP28) to the Montreal Protocol held in October 2016, the phase-down of the production and consumption of HFCs was made mandatory.
In Japan, the Fluorocarbon Emissions Control Act was amended in January 2019, with centrifugal chillers designated as a regulated product. Restrictions were placed on manufacturers so that the average GWP weighted by the number of centrifugal chillers shipped will not exceed 100 from April 1, 2025. This means that most shipments of R134a whthin Japan, which has a GWP of 1430, will not be possible, and there will be a complete switch to low-GWP refrigerants.
3. Centrifugal chiller refrigerants and characteristics
Refrigerants currently in use for centrifugal chillers include the HFC refrigerant R134a, and low-GWP refrigerants R1234ze(E), R1234yf, R1233zd(E), R1224yd(Z), and R514A. The general classification and legal treatment of these substances vary depending on their physical properties, etc., as shown in Table 1. Notable features are as follows:
Table 1. Classification of centrifugal chiller refrigerants and legal treatment
| Conventional Refrigerant | Low-GWP Refrigerants | |||||
| High-pressure Refrigerants | Low-pressure Refrigerants | |||||
| R134a | R1234ze(E) | R1234yf | R1233zd(E) |
R1224yd(Z) |
R514A | |
| GWP | 1430 1) | < 1 2) | < 1 2) | 1 2) | < 1 2) | 2 2) |
| Refrigeration Safety Regulation under High Pressure Gas Safety Act | Inert gas | Specific inert gases | Inert gas [Not applicable for air conditioning applications as the pressure is less than 0.2 MPA (G)] |
|||
| Fluorocarbon Emissions Control Act | Applicable |
Not applicable |
||||
| ASHRAE Standard 34 | A1 | A2L | A2L | A1 | A1 | B1 |
A1: nonflammability/ lower toxicity, A2L: lower flammability/lower toxicity, B1: nonflammability/higher toxicity
3-1 Practical use of low-GWP refrigerants of around 1
Low-GWP refrigerants used for centrifugal chillers all have a GWP of around 1, which is low enough to meet the target values set under the designated product system, and thus help to limit global warming.
3-2 Practical use of various refrigerants
To handle high-capacity thermal output, centrifugal chillers use centrifugal type (single to multi-stage) compressors, which have a larger gas flow output than volumetric or screw type compressors, and a shell-and-tube type heat exchanger. This technical feature makes the practical use of refrigerants with significantly different saturation pressures, gas specific volumes, etc. possible. Specific examples are shown in Chapter 4.
4. Refrigerant properties and centrifugal chillers’ features
4-1 Refrigerant classification
Refrigerants that can be used for centrifugal chillers are low-pressure refrigerants and high-pressure refrigerants. Low-pressure refrigerants refer to refrigerants where the evaporator and other components reach atmospheric pressure or less within the operating range of air conditioning applications, while high-pressure refrigerants refer to refrigerants where the pressure does not fall below atmospheric pressure. Low-pressure refrigerants are liquefied gases, and the pressure in the operating range for air conditioning applications is under 0.2 MPa (G), so the High Pressure Gas Safety Act does not apply, however high-pressure refrigerants are subject to the High Pressure Gas Safety Act as the pressure is 0.2 MPa (G) or more. Even if low-pressure refrigerants are used, the High Pressure Gas Safety Act may apply if the pressure increases for heat pump applications, etc.
4-2 Refrigeration cycle
The refrigerant undergoes the repeated compression → condensation → expansion → evaporation cycle, outputting cold heat. The Coefficient of Performance (COP) of the refrigeration cycle is calculated as the ratio of the amount of heat exchanged (latent heat of evaporation) per unit of refrigerant circulation in the evaporator section, to the amount of adiabatic compression work in the compressor section, as shown in formula (1).
COP = Wr/Ws … (1)
Wr: Latent heat of evaporation [kJ/kg]
Ws: Adiabatic compression work [kJ/kg]
Thus the higher the latent heat of evaporation, the higher the theoretical COP. The theoretical COP calculated for a two-stage compression, two-stage expansion sub-cooling cycle, assuming an evaporation temperature of 6°C, condensation temperature of 38°C, and adiabatic efficiency of 90%, is shown in Table 2. Since low-pressure refrigerants have a higher latent heat of evaporation than high-pressure refrigerants, the theoretical COP of low-pressure refrigerants is about 3% higher than that of high-pressure refrigerants.
Table 2 Comparison of latent heat of evaporation and theoretical COP
|
|
Unit | Conventional Refrigerant | Low-GWP Refrigerants | ||||
| High-pressure Refrigerants | Low-pressure Refrigerants | ||||||
| R134a | R1234ze(E) | R1234yf | R1233zd(E) |
R1224yd(Z) |
R514A | ||
| Saturation pressure (6℃) 2) | kPa(G) | 260.7 | 167.3 | 283.9 |
-39.3 |
-29.4 |
-60.9 |
| Latent heat of evaporation (6℃) 2) | kJ/kg | 194.0 | 180.3 | 159.3 | 200.7 | 172.8 |
197.9 |
| Theoretical COP | 7.23 | 7.26 | 7.11 |
7.47 |
7.43 | 7.55 | |
4-3 Compressor
The compressor of a centrifugal chiller is a centrifugal type, so aerodynamic characteristics such as the gas flow and compression ratio of each stage can be expressed as non-dimensional numbers of the flow rate coefficient/pressure coefficient and flow rate variable/pressure variable. Therefore, it is possible to use refrigerants with significantly different refrigerant properties. For example, the ETI-Z series compressors that use the R1233zd(E) low-pressure refrigerant, which has a high gas specific volume, and the JHT-Y series com pressors that use the R1234yf high-pressure refrigerant, which has a low gas specific volume, use the same shape of impeller blade.
In contrast, low-pressure refrigerants require a larger compressor size than high-pressure refrigerants. Low-pressure refrigerants have a high gas specific volume, and as such the gas flow required to output the same cooling capacity also increases. Thus the impeller diameter increases in size when using a compressor with the same flow rate coefficient. The static flow path of the compressor is designed similarly based on the impeller diameter, so if a large impeller diameter means the compressor will also be large.
Table 3 shows the impeller diameter when assuming a cooling capacity of 500 USRt is output in a two-stage compression, two-stage expansion cycle using an impeller with a flow rate coefficient of 0.1 and a pressure coefficient of 0.5. The impeller diameter for low pressure refrigerants is approximately twice that for high pressure refrigerants. Impellers are available in a variety of types, including types made from cast iron materials and types that are fully machined—to ensure high performance, full machining is required, which provides high dimensional accuracy and a smooth surface roughness. The main lineup from manufacturers of general-purpose NC 5-axis machining centers (Numerically Controlled) are designed for around φ500 mm, and machining of impellers is also cost-effective if they are 500 mm or less, so low-pressure refrigerants are used in small and medium capacity systems.
Table 3 Comparison of gas flow and impeller diameter
|
|
Unit | Conventional Refrigerant | Low-GWP Refrigerants | ||||
| High-pressure Refrigerants | Low-pressure Refrigerants | ||||||
| R134a | R1234ze(E) | R1234yf | R1233zd(E) |
R1224yd(Z) |
R514A | ||
| Saturated gas specific volume (6℃) 2) | m3/kg | 0.0056 | 0.069 | 0.047 | 0.278 | 0.209 | 0.402 |
| Gas flow | m3/s | 0.57 | 0.76 | 0.59 | 2.67 | 2.36 | 3.91 |
| Impeller diameter | mm
|
220 | 259 | 236 | 474 | 462 | 576 |
4-4 Heat exchanger
The performance and size of heat exchangers are affected by the external heat transfer coefficient of the heat exchanger tube, the liquid column, and the flow rate through the gas-liquid separator structure.
(1) Evaporator, condenser: External heat transfer coefficient of heat exchanger tube
In a heat exchanger, cold water or cooling water flows inside the heat exchanger tubes, and the refrigerant evaporates and condenses outside the tubes. If there is any resistance, such as dirt within the tube or non-condensable gas outside the tube, there will be a decrease in heat exchange performance.
Low-pressure refrigerants are susceptible to the effects of lubricating oil. This is due to the fact that synthetic oil is suitable as a lubricant for high-pressure HFO refrigerants, and as its specific gravity is similar to that of the refrigerant liquid, it is less likely to accumulate and thus be less likely to affect the heat exchange performance. In contrast, mineral oil is suitable for low-pressure refrigerants, and as its specific gravity is lighter than that of the refrigerant liquid, it accumulates near the refrigerant liquid surface in the evaporator, and the heat exchange performance decreases. Thus the amount of oil lubricating the bearings, gears, etc. of the compressor leaking into the refrigerant system through gaps in the seals of the compressor needs to be controlled so that it is lower for low-pressure refrigerants than for high-pressure refrigerants.
Moreover, as the pressure inside high-pressure refrigerants is always higher than atmospheric pressure within the operating range of centrifugal chillers, there is no risk of air intrusion. In contrast, when low-pressure refrigerants are used, the pressure inside the chiller, such as in the evaporator, can sometimes drop below atmospheric pressure within the operating range of the centrifugal chiller—this can cause air to enter the chiller from the atmosphere and accumulate in the condenser, reducing the heat exchange performance of the heat exchanger tubes.
(2) Evaporator: Liquid column
In a flooded evaporator, a two-phase flow of refrigerant liquid and gas is supplied from the bottom of the evaporator to facilitate heat exchange with all the heat exchanger tubes (so that the heat exchanger tube group is immersed). Thus the evaporation temperature at the bottom of the evaporator decreases by the amount of the liquid height (liquid column).
As the pressure of low-pressure refrigerants is low, the evaporation temperature decreases due to the liquid column, which increases the power required for the compressor, and this can result in a decrease in the COP. This tendency becomes more pronounced with increased capacity.
(3) Evaporator: Flow rate speed through gas-liquid separator structure
In the evaporator, if there is a high flow rate of the refrigerant gas, droplets scatters from the refrigerant liquid surface and are sucked into the compressor (carry-over), increasing the power consumption of the compressor. Thus the flow rate through the gas-liquid separator structure affects the size of the heat exchanger, and low-pressure refrigerants tend to require larger evaporators and condensers. The layout of the heat exchanger tubes in the evaporator and condenser is determined by taking into account the amount of heat exchanged, the flow rate and pressure loss at each part. As shown in Table 4, low-pressure refrigerants with a large refrigerant gas specific volume often have a large shell to limit the flow rate speed.
Table 4 Comparison of specific gravity and specific volume
|
|
Unit | Conventional Refrigerant | Low-GWP Refrigerants | ||||
| High-pressure Refrigerants | Low-pressure Refrigerants | ||||||
| R134a | R1234ze(E) | R1234yf | R1233zd(E) |
R1224yd(Z) |
R514A | ||
| Saturation pressure (6℃) 2) | kPa(G) | 260.7 | 167.3 | 283.9 |
-39.3 |
-29.4 |
-60.9 |
| Saturated gas specific volume (6℃) 2)
|
kJ/kg | 0.0056 | 0.069 | 0.047 | 0.278 | 0.209 | 0.402 |
| Saturated liquid ratio (6℃)2) | Kg/m3 | 1275 | 1222 | 1157 | 1308
|
1412 | 1357 |
| Gas flow | m3/s | 0.57 | 0.76 | 0.59 | 2.67 | 2.36 | 3.91 |
4-5 Summary
This report presented the technical characteristics of the refrigeration cycle, compressor and heat exchanger, as well as the characteristics of each type of refrigerant. This section summarizes these and outlines the characteristics of centrifugal chillers that use high- and low-pressure refrigerants.
(1) Low-pressure refrigerant centrifugal chiller
Has a large latent heat of evaporation and good theoretical COP. In contrast, given that it has a high specific gas volume and large equipment size, a balance between performance, footprint, cost, etc. is often achieved only up to a cooling capacity of around 1,000 to 1,500 USRt.
Those characteristics can be achieved by using the latest technology. For example, as shown in Table 5, the ETI-Z series using the low-pressure refrigerant R1233zd(E) has a footprint that is within +15% of the ETI series that uses the high-pressure refrigerant R134a. The theoretical COP difference between R134a and R1233zd(E) shown in Table 2 is used to enhance the rated COP by 3%. Moreover, the airtight techniques developed for high-pressure refrigerants are also applied to low-pressure refrigerants to minimize air intrusion.
(2) High-pressure refrigerant centrifugal chiller
With a low gas specific volume, the size of equipment can be made small, which makes it suitable for large capacity applications, such as cooling capacities of 3,000 USRt or more. Moreover, as the inside of the chiller is always under positive pressure, there is no risk of air intrusion, and it can be operated without deterioration caused by rust on the heat exchanger, etc.
Table 5 Comparison of installation area
| Cooling capacity class | USRt | 150 - 250 | 250 - 350 | 400 - 500 | 500 - 700 |
|---|---|---|---|---|---|
| High-pressure refrigerant R134a ETI Series |
m2 | 5.6 | 6.3 | 8.4 | 8.8 |
| Low-pressure refrigerant R1233zd(E) ETI-Z Series |
m2 | 6.1 | 7.2 | 8.6 | 9.5 |
| Low-pressure refrigerant/High-pressure refrigerant | 1.10 | 1.15 | 1.03 | 1.07 |
5. Technology for applying low-GWP refrigerants to centrifugal chillers
As outlined in Chapter 4, high-pressure and low-pressure low-GWP refrigerants were used, so there is a broad range of physical properties. Even with the same temperature and cooling capacity output, the performance and size of equipment can vary significantly depending on the type of refrigerant and how it is applied. This chapter introduces the technologies used for achieving high-performance, compact centrifugal chillers while dealing with a wide range of physical properties.
5-1 Compressor
(1) CFD analysis factoring in actual gas
The aerodynamic characteristics of a compressor are often studied using CFD analysis, assuming an ideal gas. Until now, refrigerants were limited to R134a, so they could be properly assessed and used by correcting the aerodynamic analysis characteristics using ideal gases with actual measurement data. Yet low-GWP refrigerants have a wide range of physical properties, so a method was used that minimizes corrections to the measured data by using aerodynamic analysis characteristics that reflect the properties of the actual gas.
(2) High gas flow impeller
As outlined in Chapter 4, the impeller diameter for low-pressure refrigerants is approximately twice that of high-pressure refrigerants, so when factors such as equipment size are factored in, an impeller that can provide a higher gas flow with the same impeller diameter is required. Yet if the gas flow is increased with the same impeller diameter, the flow path becomes wider, making it difficult to achieve efficient gas flow, thereby reducing efficiency. CFD analysis was used to carefully optimize the impeller's leading and trailing edge shapes, blade angle distribution, flow path shape at the impeller inlet, and inlet guide vane shape. As a result, the gas flow increased by 60% compared to conventional models with the same impeller diameter, achieving both a reduction in compressor volume and improved insulation efficiency.
Furthermore, as shown in Chapter 4, the same aerodynamic characteristics of the compressor can be used regardless of refrigerant properties, so an impeller optimized for the low-pressure refrigerant R1233zd(E) can also be used for high-pressure refrigerants such as R1234yf, resulting in higher performance and a more compact design.
(3) Compressor directly driven by electric motor
In conventional R134a equipment, the compressor speed was increased relative to the motor speed by a speed-increasing gear. Yet as shown in Table 3, the low-pressure refrigerant R1233zd(E) has a high gas specific volume and a larger impeller diameter for the same cooling capacity, so the required rotation speed is about 40% lower than that of R134a.
To address this issue, the compressor's speed-increasing gear was removed and connected directly to the motor shaft, thereby making the compressor more compact and improving performance by reducing the number of bearings. Furthermore, electric motors can now be made smaller by increasing their rotation speed.
(4) Changing design points to match refrigerant properties
Low-GWP refrigerants have a wide range of physical properties, so even with high-pressure refrigerants and low-pressure refrigerants, the gas flow at which the compressor efficiency peaks (design point gas flow) differs. As the compressor has a characteristic where its efficiency decreases as the gas flow deviates from the design point, it is not realistic to make impellers available so that the compressor efficiency peaks for all gas flows for each refrigerant. Therefore, changing the blade height of an impeller with the same shape and diameter shifts design point gas flow to the lower gas flow side, making it possible to achieve a high efficiency for a variety of refrigerants and conditions with the minimum impeller master.
5-2 Heat exchanger
(1) Evaporator: Gas-liquid separator structure tailored to refrigerant properties
If the evaporator is filled with excess refrigerant, or if the evaporator volume is small and the refrigerant gas flow rate is high, liquid droplets from the refrigerant liquid surface will flow into the compressor, and liquid refrigerant that does not contribute to cooling will circulate, which lowers the COP. Therefore, a gas-liquid separator structure is positioned within the evaporator to suit the refrigerant properties to ensure stable, high-performance characteristics.
(2) Condenser: Heat exchanger tube group design with low-flow resistance
At the inlet of the condenser, the flow of the gas discharged from the compressor is rectified so that it spreads throughout the entire heat exchanger tube group. Therefore, the high gas specific volume and large amount of discharged gas, particularly with low-pressure refrigerants means the condenser performance is likely to decrease due to losses caused by flow resistance until the discharged gas reaches the entire heat exchanger tube group. Therefore, we are studying the optimal shapes of the air rectifying plates and heat exchanger tube group by gaining an understanding of the discharge gas airflow with CFD analysis and actual measurements.
(3) Condenser: Optimization of extraction location
When low-pressure refrigerants are used, air may enter the chiller when the inside of the refrigerator is at negative pressure, and may accumulate in the condenser, thereby reducing the condenser performance. In this case, the air is discharged outside the chiller using an purge unit, but the air purge performance varies significantly depending on where the air is purged from in the condenser. Therefore, we determined the air purge points by gaining an understanding of where air is likely to accumulate within the condenser's heat exchanger tube group with CFD analysis and actual measurements.
6. Switching existing equipment in the market to low-GWP refrigerants
6-1 Market stock of HFC refrigerants
HFC refrigerants such as R134a became popular as an alternative to HCFC refrigerants such as R123 in the early 1990s, when their ozone depletion potential began to draw attention. Since then, low-GWP refrigerants became more common since around 2015, but they are still used as the mainstream refrigerant for centrifugal chillers. The number of centrifugal chillers shipped throughout Japan is approximately 300 units per year, which means that approximately 9,000 HFC refrigerant centrifugal chillers had been shipped over around 30 years from 1990 to 2023. In addition, the standard life cycle of a centrifugal chiller is 15 years, but as many users perform appropriate maintenance such as regularly changing lubricating oil and performing compressor overhauls, it is generally believed that a 20-year life cycle is typical. Some equipment has even been in operation for 30 or 40 years or more.
The capacity of centrifugal chillers sold in Japan, excluding special models, is between 100 and 2,500 USRt, and the amount of refrigerant charge is an average of 600 USRt class (4), which comes to about 0.8 t per unit. Based on the assumption of about 9,000 units, there is approximately 7,200 t of HFC refrigerant in Japan.
6-2 Points to note when switching to low-GWP refrigerants
Switching to low-GWP refrigerants involves updating the chillers (removing existing equipment) or updating the refrigerants (changing the type of refrigerant without removing existing equipment, or disassembling the compressor to change the sealing material, etc., which is also referred to as retrofitting).
Chillers are updated when their operation period has exceeded the standard life cycle or when the type of refrigerant is changed. For example, low-pressure refrigerants cannot be used in chillers that uses high-pressure refrigerants because the cooling capacity is significantly reduced, and high-pressure refrigerants cannot be used in chillers that uses low-pressure refrigerants because the equipment design pressure increases.
Retrofitting requires disassembling the compressor to change the sealing material and replace the control program. Yet for refrigerants subject to Japanese High Pressure Gas Safety Act, the above modifications currently require the application of a license, so they can only be carried out in limited cases. In Japan, discussions about the technical safety of retrofitting have just started. As most centrifugal chillers on the market use the high-pressure refrigerant R134a, retrofitting with R1234yf, which has the closest physical properties, is likely to become increasingly necessary in the future.
7. Conclusion
In Japan, restrictions on shipments of HFC refrigerants will come into effect on April 1, 2025 under the designated product system of the Fluorocarbon Emissions Control Act.
Centrifugal chiller manufacturers have released lineups of low-GWP refrigerants, and the transition has been completed for newly installed machines. In addition, as it takes about two years from the designing stage of the a centrifugal chiller to actual delivery, the designing stage of the future projects to install new units also has been being switched to low-GWP refrigerants as of 2024.
HFC refrigerants are still available on the market, but as overall production volume decreases, they will become more difficult to obtain, making it necessary to switch to low-GWP refrigerants. Centrifugal chillers are often installed in production facilities that operate year-round and have a large capacity (amount of refrigerant charge), so switching refrigerants is a large burden, and it is likely to take time for switching refrigerants to take hold in the vast number of machines on the market. As most of the HFC refrigerant stock on the market is R134a, legislation and technological support for retrofitting to enable maximum utilization of existing equipment and switching to low-GWP refrigerants without removal is needed.
References
(1) Intergovernmental Panel on Climate Change (IPCC): “Fourth Assessment Report (AR4)” (2007)
https://www.ipcc.ch/assessment-report/ar4/
(2024)
(2) Intergovernmental Panel on Climate Change (IPCC): “Fifth Assessment Report (AR5)” (2014)
https://www.ipcc.ch/assessment-report/ar5/
(2024)
⑶ “NIST Reference Fluid Thermodynamic and Transport Properties Database (REFPROP) Version 10”
https://www.nist.gov/srd/refprop
(2024)
(4) The Japan Refrigeration and Air Conditioning Industry Association: "Centrifugal Chiller Shipment Record 1994-2021"
https://www.jraia.or.jp/statistic/detail.html?ca=1&ca2=10
(2024)
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