Online Monitoring of the Oil Concentration in the Air Conditioning Circuit
Ensuring process effectiveness and quality.
In air conditioning systems or for the development of components, LiquiSonic® sensors are used to monitor the air conditioning circuit by measuring the oil concentration in the refrigerant. In the climate test facilities of the automotive supplier DENSO, the sensors are installed directly in the pipeline and continuously determine the oil content every second.
Importance of oil in the air conditioning circuit
Compression chillers are often used to air condition. In these machines different refrigerants such as R134a circulate, because of their thermodynamic properties as a heat transfer medium. In addition, refrigeration oils are in the air conditioning circuit in order to lubricate the compressor. A high percentage of oil is best for good lubrication, however, this reduces the efficiency of the refrigerator. Therefore, the aim is a perfect blend of oil and refrigerant. The LiquiSonic® analyzer detects the oil content in the refrigerant and so the air conditioning circuits are continuously monitored. This is necessary particularly in the development of air conditioning compressors or in the operation of large plants in order to ensure optimum efficiency and a complete documentation and quality assurance.
Industry expertise and technology know-how
The company DENSO Corporation is an international automotive supplier headquartered in Kariya, Japan. One business sector of the subsidiary, DENSO AUTOMOTIVE Germany GmbH, located in Eching near Munich, is the development of components and tuning of climate circuits for vehicle air conditioning. To optimize development processes, DENSO AUTOMOTIVE has integrated several LiquiSonic® analyzers into the climate test facilities. “We have decided in favor of LiquiSonic® because of the simple installation, the flexibility to switch between various oil-refrigerant combinations and no maintenance,” says Jan-Martin of Pozniak, senior technical manager. “An advantage has been also the air conditioning circulation station, developed by SensoTech and DENSO AUTOMOTIVE in SensoTech’s in-house laboratory, to measure different oil-refrigerant combinations.”
Because of the good experience and satisfaction of DENSO AUTOMOTIVE, even more subsidiaries of DENSO Corporation have installed the LiquiSonic® analyzers to improve the vehicle development.
In particular, the EU’s decision to ban the R134a refrigerant from new cars and to replace it with alternative refrigerants leads to different strategies in the development of vehicle components, which can be effectively evaluated with the LiquiSonic® analyzers.
In the field of air conditioning in buildings or industrial processes other refrigerants are used depending on the construction and application of the air conditioning system. Even in such processes LiquiSonic® analyzers can be integrated, because the sensors can measure in various refrigerants and are always tailored to the respective application.
Measuring inline, processing online
A LiquiSonic® analyzer consists of one or more sonic velocity sensors and one controller. A standard system includes furthermore a pressure sensor, which will be installed together with the ultrasonic sensor in the pipe between the condenser and the expansion valve.
The LiquiSonic® sensor measures highly accurate the sonic velocity and temperature in the refrigerant. In the running process, the oil concentration is calculated from the physical parameters sonic velocity, temperature and pressure. The results are updated every second with a measuring accuracy of ± 0.1 wt%. The liquid-contacting sensor component is made of stainless steel DIN 1.4571. The robust and completely enclosed design does not require seals or “windows” to the process, so that the sensors are entirely maintenance-free.
The LiquiSonic® controller displays and records the reproducible measuring results. The controller operation is intuitive and allows password-based user authentication. The possible trend view provides a quick overview of the process course. In the integrated log readings and events are stored, so that subsequent analyses are possible. The measured values can be passed to process control systems or PCs through analog or relay outputs as well as various fieldbus interfaces.
The product data sets, required to calculate the concentration, will be stored in the controller by SensoTech in advance. The systems are delivered as plug & play, so that a quick start-up is guaranteed. After the sensor installation, the controller displays immediately the current concentration value. If process conditions change or new applications are to include, the data sets in the controller can be adapted. The update can be done quickly and easily by the user. Alternatively, it is possible, that the SensoTech support carries out updates and configurations. For this, a remote connection via modem, network, Ethernet or web interface is made. “If we needed the support of SensoTech, we just connected our controller to a modem. That was very convenient,” says Senior Technician Andreas Lahr of DENSO AUTOMOTIVE.
Maximum safety through certification
Since the accuracy of the measured values is of great importance for vehicle development, DENSO AUTOMOTIVE opted for a certification of the LiquiSonic® analyzers. The typical verification by an offline reference measurement as part of an “Operation Qualification” here is not feasible, as the refrigerants are liquid only under pressure, so that sampling for laboratory analysis is not possible. However, it must be ensured that the measured values are reliable. Therefore, SensoTech offers a measurement certificate confirming that the LiquiSonic® analyzer provides the guaranteed measuring accuracy. For the proof, a functional test on the working range will be done, which documents that all values are within the specified tolerance range.
Technical Report--High Precision Oil Concentration Measurement in Refrigerants
In the development of air conditioning systems, process analytical technology is used to monitor critical parameters such as the compressor oil concentration in the refrigerant for an optimum oil circulation ratio.
Various industrial sectors such as automotive or building include cooling processes, which need to meet high requirements on performance and reliability to achieve maximum effectiveness and stability in the air conditioning. In compression refrigeration systems, refrigerants and refrigeration oils circulate. Whereas a high percentage of oil is ideal for good lubrication of the compressor, oil leaks in the refrigerant reduce the cooling capacity. Using inline analytical technology, the oil content in the refrigerant can be determined continuously and directly in the process, in order to keep the optimum ratio of oil and refrigerant. Any leaks which lead to reduced cooling capacity, instability or failures are detected immediately.
Since the refrigerant is liquid only under high pressure, manual sampling and offline analyses are a near impossibility. Therefore, inline analytical technology is used, that is integrated directly into the process and provides the measurement data online. Due to the high pressure in the air conditioning circuit, the online monitoring of oil content places highest demands on the analytical technology. Analyzers measuring the oil concentration in the refrigerant by sonic velocity meet these requirements.
Sonic velocity is a clearly traceable, physical value to determine concentrations in liquids. In practice, the LiquiSonic® analyzers by SensoTech have proved successfully in air conditioning systems of international manufacturers and developers. Figure 1 shows the analyzer consisting of one sensor and controller. The sonic velocity is determined by sending an ultrasonic signal from one side of the sensor to the other one. The time is measured until the signal reaches the receiver. The distance between ultrasonic transmitter and receiver is by design always constant, so that the sonic velocity can be calculated from the values of way and time. As sonic velocity is influenced by temperature and pressure, the LiquiSonic® sensor incorporates two Pt1000 temperature probes and a pressure transmitter providing a compensation signal. Consequently, from the temperature- and pressure-compensated sonic velocity values the concentration can be calculated. This relationship is stored as a mathematical calculation model in the LiquiSonic® controller, so that the oil concentration is displayed immediately.
Compared to other process liquids, refrigerants have very low sonic velocities that can be, for example, 300 m/s. The measurement accuracy is ± 0.1 wt% and the measurement values are updated every second. Figure 2 shows schematically an air conditioning circuit with integrated LiquiSonic® analyzer. The electronics housing may be apart from the sensor and mounted separately in small process areas. The sensors are made of stainless steel DIN 1.4571 (equivalent to 316 Ti).
The concentration measurement is possible in various refrigerating machine oils and refrigerants, including for example polyester oils, polyalkylene glycols or polyalphaolefins. Refrigerants can be, for example, hydrofluorocarbons (R134a, R1234yf), carbon dioxide (R744), propane or ammonia (R717). The latter is frequently used in large stationary plants. The automotive industry used to operate often with the refrigerant R134a in cars. However, since this refrigerant has a high global warming potential, in 2011 the EU have passed the motion replacing the refrigerant successively until 2017 with climate-friendly refrigerants. In the automotive sector, this results in developing different strategies that can be effectively evaluated with the LiquiSonic® analytical technology.
The LiquiSonic® controller displays and stores the measurement values. All critical process parameters and configurations are fully documented for complete traceability and analysis. With the LiquiSonic® software SonicWork, the memory can be read and process protocols are possible. Clear trend graphs provide a quick overview of the process course. The measurement values can be transferred to the process control systems or PCs via a number of free scalable analog or relay outputs as well as via various fieldbus interfaces.