Tadashi SATO*
Yukihiro FUKUSUMI*
Motoshi KOHAKU*
Tomoya MIYACHI*
Masahiro KAZUSA*
Takayuki SUZUKI**
*
EBARA REFRIGERATION EQUIPMENT&SYSTEMS CO., LTD.
**
Fluid Machinery & Systems Company
EBARA has developed and released a new centrifugal chiller model RTXF, featuring lower energy consumption, a compact size, and a shorter delivery period. Positioned as the successor to EBARA’s core model RTBF, the product structure was totally redesigned in order to achieve these features. It is characterized by a gearless-type compressor equipped with an inverter as a standard component. In an evaluation test, the new centrifugal chiller model RTXF proved itself to be equal or superior to the current model RTBF in performance.
Keywords: Centrifugal chiller, High-speed motor, Inverter drive, Gearless direct drive, Low-pressure refrigerant, Low-noise design, Small footprint
In January 2017, EBARA released the centrifugal chiller model RTXF, which features lower energy consumption, a compact size, and a shorter delivery period (Figure 1).
The new chiller is positioned as the successor to the model RTBF, the dominant product of “EBARA Refrigeration Equipment & Systems Co., Ltd.,” achieving lower energy consumption, a compact design, quiet operation and a shorter delivery period by incorporating the cutting-edge technology of the EBARA Group and totally redesigning the structure. In this literature, we explain our attempt to release the new product in the Japanese market of centrifugal chillers, a market in which centrifugal chillers with a cooling capacity of 220 to 300 USRT are the largest capacity in terms of the number of units sold.
Fig. 1 Centrifugal chiller model RTXF
Recently, greater attention has been focused on energy saving, and social demand for companies to take measures to “prevent global warming” and “reduce CO2 emissions” has been escalating.
Centrifugal chillers offer advantages in that they are superior in efficiency to other types of chillers, have a larger capacity, produce less noise and vibration than positive displacement type chillers, use no flame, and can be used for many years with proper maintenance.
Customers now demand more energy efficient centrifugal chillers, which form the most common type of large-size chillers. However in theory, less increase in efficiency can be expected with the continued use of conventional technology, and enhancing efficiency from the current level requires further sophistication and upsizing of equipment and an increase in the number of parts.
To achieve higher efficiency, we determined the specifications of the centrifugal chiller to be newly developed under the following product concept:
・Increasing the partial load characteristic by incorporating an inverter as the standard design
・Significantly reducing the footprint with a compact design
・Achieving a shorter delivery period by standardizing the product as a package
・Adopting a low-pressure refrigerant that does not fall under the High Pressure Gas Safety Act
The detailed structure of the new chiller based on this product concept is described below.
An outline drawing of the newly developed centrifugal chiller model RTXF is shown in Figure 2, and its main specifications are shown in the Table.
By minimizing the overall length of the chiller, its required area, including the maintenance space required to replace the heat transfer tube, is reduced by 24 % compared with the current model RTBF (Figure 3).
This chiller can be carried by dividing it into separate units. Even if the chiller needs to be installed in a location that is difficult to access, or there are weight restrictions when carrying the equipment (for example, elevators and cranes), it can be carried into place by dividing it into individual components and reassembling it on site. The chiller can be installed in a wider range of areas. An example of the new chiller divided into components for easy transportation is shown in Figure 4.
| Item | Value | |
| Cooling capacity | 300 USRT | |
| 1055 kW | ||
| Chilled water |
Temperature at inlet | 12 ℃ |
| Temperature at outlet | 7 ℃ | |
| Flow rate | 3020 L/min | |
| Cooling
water
|
Temperature at inlet | 32 ℃ |
| Temperature at outlet | 37 ℃ | |
| Flow rate | 3550 L/min | |
| Voltage | 400 V | |
| Mass | 6.0 t | |
| Dimensions | Width | 2300 mm |
| Length | 3300 mm | |
| Height | 2080 mm | |
Fig. 2 Centrifugal chiller model RTXF outline drawing
Fig. 3 Comparison of footprint with current model RTBF and new model RTXF
Fig. 4 Example of individual components for transportation
An overview of the flowsheet of the chiller is shown in Figure 5. This system is a refrigeration cycle capable of achieving a high COP *1 by means of an economizer. A two-stage centrifugal compressor is used to expand refrigerant liquid supplied from the condenser in the pressure reducing mechanism. By feeding only the lowtemperature refrigerant liquid to the evaporator, a cycle of increasing the refrigeration effect → reducing the circulated amount of refrigerant → saving compression power is accomplished, resulting in a high COP.
For the expansion valve (pressure reducing device) on both the high stage (condenser to economizer) and low stage (economizer) sides, a motor-operated valve is used and functions as a variable flow rate control mechanism. The amount of circulated gas, which makes only a minor contribution to the refrigeration cycle, is reduced by appropriately regulating the opening of the motoroperated valve, and a high IPLV *2 can be achieved by controlling the amount of circulated gas to secure a flow of refrigerant liquid.
*1 COP : Coefficient Of Performance (cooling capability per power consumption during rated capability operation)
*2 IPLV : Integrated Part Load Value (cooling capability per power consumption, including operation at the partial load)
Fig. 5 Flowsheet of two-stage compression economizer cycle
For the refrigerant, a low-pressure refrigerant, HFC245fa was adopted because it ensures relatively high cycle efficiency and does not require legal formalities in connection with the High Pressure Gas Safety Act nor the appointment of a refrigeration safety officer. The model RTXF is designed to enable it to be carried into place by dividing it into individual components and reassembling it on site, but does not require legal procedures concerning the refrigerant as is the case with the current model. The adoption of the low-pressure refrigerant and the separable components can reduce the burden on the customer, including to shorten the installation period.
As described earlier, the compressor is a two-stage compressor, and both the first-stage and second-stage impellers have a variable guide vane at the inlet (upstream) like the current model RTBF.
The peripheral Mach number at the outlet was set to approximately 1.0 to ensure impeller efficiency. In addition, an optimum combination of the flow coefficients of the first-stage and second-stage impellers was selected in the flow efficient range where the efficiency of the impellers, a cutting-edge technology of the EBARA Group, is high quality in order to improve the overall efficiency of the compressor. The measurement results in polytrope efficiency, the efficiency index of the fluid element and the polytrope head are shown in Figure 6. For reference, the polytrope efficiency of the current model RTBF measured under the same conditions is also shown. In Figure 6, the horizontal axis is the flow rate at the design point of the model RTBF, the left vertical axis is the polytrope head at the design point of the model RTBF, and the right vertical axis indicates the efficiency at the design point of the model RTBF. The efficiency of the compressor used for the new model RTXF is 3 % higher than that of the current model RTBF, and the flow rate range in which efficiency is high is also much wider. This compressor is the result of commercializing the design technology for the 3D inversion method 4).
The motor is directly coupled with the coaxial on which the impellers are mounted, and runs at high speed with the high-frequency power generated by the inverter. This structure made the gear required for direct drive at the commercial frequency no longer necessary.
The gearless structure and the reduced number of bearings contribute to a reduction of mechanical loss (frictional loss) and improvement of the COP. The reduced number of mechanical parts helps to reduce the size of the compressor, greatly contributing to the compact size of the centrifugal chiller body (Figure 7).
Fig. 6 Comparison of compressor performance with current model RTBF and new model RTXF
Fig. 7 Compressors for current model RTBF and new model RTXF
The new chiller does not use a sub-cooler, provided with the current model RTBF as standard. Although using a sub-cooler increases both the cooling capacity and cooling efficiency, the price of the product will be impacted as the number of parts increases. The fall in performance resulting from not using a sub-cooler in the new chiller was successfully supplemented by enhancing the heat transfer performance of the condenser that utilizes a new heat transfer tube with superior heat transfer efficiency. The overall length of the condenser shell was reduced from 3.9 m, the length of conventional models, to 2.5 m, to restrain the rise in the loss of cooling water pressure.
Outside air infiltrating the chiller from a fine leaking point in the negative pressure portion stays and accumulates in the condenser as a noncondensing gas and inhibits the exchange of heat input and heat output, causing the LTD *3 to increase. If the LTD increases, the cooling capacity of the chiller will fall, and a malfunction, such as a high voltage failure attributable to the rise of the head of the compressor, will occur. To prevent these problems, a chiller using a low-pressure refrigerant is required to promptly discharge (bleed) the noncondensing gas infiltrating it. In the development test of the model RTXF, we evaluated gas purge at several points of the condenser and determined an optimum position for gas purge. We confirmed that the gas purge method obtained through the development process thoroughly bled air under the actual operating conditions and reduced the LTD. The adoption of the new heat transfer tube and the optimum design of the position for gas purge increases the overall coefficient of heat transfer of the model RTXF by 30 % compared to that of the current model RTBF and considerably reduces the LTD [Figure 8 (a)].
*3 LTD : Leaving Temperature Difference (difference between the temperature of the water and the temperature of the refrigerant at the outlet of the heat exchanger; the smaller the difference, the higher the performance of the chiller)
Like the condenser, the overall length of the evaporator is reduced from that of the current model RTBF, and various ideas to increase heat transfer efficiency are also introduced. More specifically, the latest heat transfer tubes with high heat transfer efficiency are used and placed in an optimum pattern, so that the overall coefficiency of heat transfer is 8 % higher than that of the current model RTBF. The new chiller achieves a much lower LTD [Figure 8 (b)] and a smaller charged amount of refrigerant than that of the current model.
In general, the temperature difference in chilled water between the inlet and outlet of the evaporator is 5°C in general air conditioning applications. However, if it is necessary to accommodate a difference of 5 ℃ or over, this can be done by increasing the number of paths of the evaporator. The evaporator in the new centrifugal chiller adopts an appropriate placement pattern for the heat transfer tube for the number of paths, and the number of paths can, therefore, be switched simply by changing the structure of the water chambers at both ends of the evaporator. This mechanism enables the evaporator to extract chilled water with a large temperature difference without changing its structure. The adoption of the new evaporator shell capable of adapting the change of the number of paths as described thus far makes it possible to keep a stock of standard components and, as a result considerably reduce the delivery period.
Fig. 8 Comparison of heat exchanger performance between current model RTBF and new model RTXF
We augmented the capability of the current control panel, which has been used for 10 years since its release, and made modifications to cater for the need of various functions. The new control panel was required to achieve a structure that offers potential for development in keeping with the trend toward further improvement of control capability, multi-function and an increasing number of input and output points. We also developed a new control panel compatible with any chiller type or system.
The features of the control panel are as follows:
a) Early determination of specifications
We can present to customers in advance standard and optional specifications determined on the basis of control specification surveys taken over the past 5 years.
b) Reduced design lead time
The control panel can be assembled by classifying the parts to be used into a group of standard parts, a group of optional parts and following the patterned procedure for adding optional parts to standard parts.
c) Reduced manufacturing and delivery period
The control panel can be kept in stock for the intermediate state of incorporating standard parts.
In this section, we explain the details of the enhanced functions of the new control panel. CAN communication is adopted to connect printed circuit boards and the printed circuit boards can be extended in the form of a daisy chain. This design ensures easy adaptability to an increase in input/output points. It is also easy to establish CAN communication among printed circuit boards by placing a printed circuit board in each unit making up the chiller. Thus, the control panel can flexibly support different printed circuit boards of derivative chiller models.
The operation and display units are provided with a monochrome liquid crystal display as a minimum function and as standard. On the other hand, a large-size color touch panel is offered as an optional item for customers requiring high-grade equipment with superior visibility and operability (Figure 9).
Fig. 9 Standard operation panel (left) and large-size color touch panel (right)
The control panel offers Modbus communication, widely used in the industry, as standard for customers requiring remote monitoring through communication. Time series data on the operating status of the chiller is retained in the control panel, and storage capacity has been increased from 32 Kbytes in the current mode RTBF to 4 Mbytes. This data will be helpful, not only for investigating the causes of problems, but also for proposing maintenance based on protective maintenance. The internal data can be read from the USB port provided as standard and the operating status of the chiller on a tablet or other device by connecting a commercially available USB-Bluetooth converter.
An inverter and a circuit breaker on primary (upstream) side are offered for the chiller as standard and will be attached to the chiller at the time of delivery and transferred to the place of installation. The circuit breaker and the inverter unit for the current model RTBF is housed in an independent enclosure separated from the chiller, and this unit and the chiller need to be separately installed and then connected to each other. This configuration requires additional space for installing the abovementioned power supply equipment.
The new centrifugal chiller model RTXF only requires the power cable to be connected to the circuit breaker after installation of the chiller, which significantly reduces electrical work.
We confirmed that the COP and IPLV values obtained from the new centrifugal chiller model RTXF are equal to or slightly higher than those of the current model RTBF, as initially designated. The noise level during rated operation is 80 dB (A) at the highest, and the vibration amplitude is 5 μm or less, proving that the model RTXF makes less noise and vibration despite its large size.
In this report, we explained the centrifugal chiller model RTXF, which has realized lower energy consumption, a compact size, and shorter delivery period by upgrading the performance of the conventional model. In the future we plan to develop new models from a wider spectrum of perspectives, such as enhanced efficiency at rated and partial loads, increase capacity and optimum combined control of absorption and screw chillers.
Furthermore, EBARA is currently committed to research and development into the application of a refrigerant with extremely minimized global warming potential (GWP) to centrifugal chillers to reduce global warming.
1) Kensaku MAEDA, Satoru FUJIWARA, Teiichi MOCHIZUKI, “New Centrifugal Refrigeration Machine”, Ebara Engineering Review, No.137, P.30-37 (1987-5)．
2) Yuichi SATO, Toru TOKUMARU, Takahiro SENDA, “RTC Series Energy-saving Centrifugal Type Refrigerator”, Ebara Engineering Review, No.206, P.39-42 (2005-1)．
3) Tadashi YAMAGUCHI, Naoyuki INOUE, Tadashi SATO, Atsushi KANEKO, Shuichiro HONDA, Hiroyoshi WATANABE, “Development of High-efficiency Centrifugal Refrigerating Machine”, Ebara Engineering Review, No.224, P.3-9 (2009-7)．
4) Hiroyoshi WATANABE, Yumiko SEKINO, “Blade Optimum Design by using Three Dimensional Inverse Method and CFD”, Ebara Engineering Review, No.235, P.3-8 (2012-4)．
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