Application of the testo 6740

Application

The exact monitoring of the pressure dewpoint is becoming more and more important in industrial application such as compressed air systems or granulate drying systems. The main object is the avoidance of damage from humidity.

The testo 6740 serves to assure the quality of compressed air.

The quality of compressed air is defined in the international norm Norm ISO 8573 (see table below), Class 1 making the highest demands.

In addition to this, the testo 6740 enables the pressure dewpoint to be monitored, and can moreover be used as a supplier of the regulation parameter in humidity-regulated dryers.

Avoiding damage from humidity

Compressed air, air and gases are used in all areas of industry. Humidty is usually undesired, as it causes damage, such as corrosion or the conglutination of materials, which can raise the running costs or lower the quality of the end product, as is shown in the diagram below.

Assuring compressed air quality

The international Norm ISO 8573 determines seven classes of compressed air quality, and lays down which humidty, which oil content, which particle content etc. the compressed air may contain. Class 1 makes the highest demands. Class 4, for example, is fulfilled if the pressure dewpoint does not exceed 3 °Ctpd or 37 °Ftpd, and an absolute humidity of 6g of water vapour per m3 or 8150 ppmV (parts per million by volume) is not exceeded. The main measure taken to comply with a quality class consists in the installation of a suitable dryer. Its monitoring and, where applicable control, are handled out by the testo 6740.

For example, the trace humidity quality class 4 can already be implemented with a refrigeration dryer, whereas for the trace humidity class 1, a higher-performance adsorption dryer must be used (see illustration).

Monitoring / adjusting dryers

The testo 6740 serves to monitor refrigeration and membrane dryers and to monitor and adjust adsorption dryers.

Refrigeration dryers
Economic refrigeration-compressed air drying is divided into two phases. In the first phase, the warm compressed air flowing out is cooled by the already refrigrated air flowing in, in the air - air heat exchanger. In the second phase, the compresssed air flows through a refrigerant - air heat exchanger The cooling down to the required pressure dewpoint takes place here. The condensate trap is behind this heat exchanger system. The separation of the condensate from the compressed air takes place here. In the following air -air heat exchanger, the dry cold compressed air is re-warmed; a greater safety distance from the dewpoint is created (see illustration below).

The trace humidity transmitter testo 6740 is installed behind the refrigeration dryer.This ensures that, for instance, a blocked condensate drain cannot lead to an unnoticed humidity increase.

Adsorption dryers
Adsorption dryers extract the inherent humidity from the compressed air using a dessicant (e.g. silica gel). This is contained in two identical containers. While the adsorption (humidity is extracted from the air) takes place in the first container, in the second container the regeneration is carried out (humidity is extracted from the dessicant) with extremely dry air, by a pressure relief of the partial flow of the already dried air to atmospheric pressure (relative humidity sinks) (see illustration below)
The two chambers are switched over cyclically, whereas a humidity-dependent switch saves running costs as opposed to a time-dependent switch.

Humidity-dependent switching of adsorption dryers

In the example (see diagram below), the pressure dewpoint limit value of the process is –22 °Ctpd.
Container A is used for drying. As soon as the limit value is reached, container A is switched from the drying phase into the regeneration phase. Container B simultaneously switches over into the drying phase. As soon as the limit value is reached again, the tasks are switched once more.
The drying phase is represented by the blue bar, the regeneration in red. Because of the instruments' design, the regeneration phases are completed after a certain time. The drying phases, however, can as a rule last much longer than the regeneration phases. This means that there are time periods in which the regeneration is already completed, but drying can still be carried out (1). During this time, no compressed air is required for the partial flow (15%); the total volume flow can be switched down from 100% to 85%..