**4. Acoustic sensor for in-line monitoring of a manufacturing process**

In certain industrial processes it is often difficult to access useful information in real-time due to the conditions imposed on the mechanical and thermal parameters, pressure, hygiene..., conditions which require a specific installation of the sensor with regard to its environment. The difficulty thus arises of an integration taking into account both the process constraints and the acoustic constraints. This is the case of a plate heat exchanger which can be considered as a typical example in this category (Figure 18).

Fig. 18. Standard plate heat exchanger

### **4.1 Sensor selection criteria**

For the exchanger, the sensor selected is not cumbersome and is sensitive over a temperature range reaching over 100°C (Figure 19). The excitation and synchronisation

Low Frequency Acoustic Devices for Viscoelastic Complex Media Characterization 233

The second column shows the modal frequencies obtained from the analysis of the

The mean standard deviation between the frequencies obtained by modal analysis and those obtained experimentally is 5 %. The good correlation between these results indicates that the numerical modelling provides a good estimation of the resonance frequencies of the sensor.

As an indication, figures 20a and 20b show the temporal and frequency responses of the

**4.1.2 Excitation by mechanical shock: estimation of the frequency range**  The mechanical excitation in question is ensured via the core of an electromagnet.

impedance of the sensor mounted on a heat exchanger.

Fig. 20a. Temporal response of a mechanical shock

Fig. 20b. Spectral response associated with the shock

sensor.

modes remain the same as the previous case (disc sensor). The principle of the measurement is to excite a vibration mode in one or several plate exchangers and to analyse the evolution under the effect of fouling by measuring the response of the plates using a receiver.

A bivariate system-sensor study enabled the geometry of the latter to be defined over the same vibration frequency range as the system (exchanger).

Fig. 19. Positioning of the sensors on an exchanger plate
