Digital communication in sample gas preparation

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When sample gas is extracted from the process, process-related impurities such as particles or moisture are inevitably also removed. These can influence the measurement results or damage the measuring cells. The sample gas must therefore be treated before it enters the analyser. To remove the moisture, the gas temperature is lowered below the dew point in the sample gas cooler. The condensate produced is discharged.

The TC-MINI sample gas cooler is typically intended for compact or portable gas conditioning systems designed for a sample gas with low dew points and flow and providing a 24 V DC supply.

The cooler is controlled by a microprocessor. If the temperature falls below or exceeds the set warning range (e.g. after switching on), this is signalled both by the flashing of the LEDs and by the status relay. The status output can be used, for example, to control the sample gas pump to enable the gas flow to be switched on only when the permissible cooling range is reached. By connecting a moisture sensor, the control can be extended to monitor condensate slips.

Up to now, the measured cooling block temperature could be transmitted via an analogue 4 - 20 mA signal. As digitalisation continues to progress rapidly, Bühler Technologies GmbH will in future provide the more advanced Modbus RTU communication protocol as an option for all sample gas coolers with semiconductor cooling elements. This digital interface provides a higher-level instance, such as the control system, with extended information about the sample gas cooler.

With the option of the Modbus RTU digital interface, the user now has direct access to the process and diagnostic data, for example the current cooling block temperature or information about a possible temperature error if it is outside the setpoint range. If a moisture sensor is connected, for example, a humidity slip can also be determined directly from the control station. All settings that could previously be made via the menu on the display of the sample gas cooler can now be made remotely from the control room and directly during operation. For example, the condensate pumps can be activated and deactivated again as required, the setpoint for the cooling block temperature can be adjusted and the alarm thresholds can also be set remotely. New functions also include the output of the chiller's utilisation (in %), which is helpful for evaluating the performance limit, and the output of the unit runtime, which can be used to realise maintenance intervals.

Modbus RTU is considered a very robust interface and enables remote access to the TC-MINI sample gas cooler for control and monitoring of the current status, robust and reliable communication between field devices and the control system, thus supporting safe and continuous process and plant operation.

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