Monitoring cleaning cycles of fouled ducts using ultrasonic coda wave interferometry (CWI).

Fouling in heat exchangers is the buildup of deposits on the solid surfaces. These deposits reduce the eco-efficiency of the processing equipment and increase the risk of subsequent surface contamination with the formation of biofilms. In the agro-food and water supplier sectors, which are our main concern, fouling on the hot walls of processing heat exchangers is a common occurrence and requires frequent cleaning cycles to ensure hygiene requirements are met. This results in a considerable ecological footprint. Sensors and diagnostic tools for monitoring fouling are thus of utmost importance to ensure the rational validation of the cleaning end-point and to decrease the environmental impact of the cleaning cycles. In this paper, a non-destructive ultrasonic monitoring technique using coda waves and the associated signal processing was tested to monitor the evolution over time of a deposit layer on a solid wall during cleaning. To ascertain the feasibility of the method, a piece of wax of controlled thickness was deposited to simulate the initial fouling state and a cleaning cycle was launched. The decorrelation coefficient was used as an indicator to monitor fouling. This article presents the principle of this unprecedented technique for measuring the degree of fouling. The results of the experiments show that this non-destructive monitoring technology is sensitive to changes in fouling and that the decorrelation coefficient curves are in agreement with the cleaning kinetics captured using a video camera, thus ascertaining the pertinence of the diagnostic tool proposed.

Contribution of the shear wave ultrasonic reflectometry to the stickiness measurements.

Today, non-invasive quantification of the adhesion of a deposit to a surface is always a challenge and, unfortunately, few tools are available in this area. This is an obstacle, in several industrial processes, to the identification of conditions limiting the fouling and to the establishment of eco-efficient cleaning strategies. In this paper, a non-invasive ultrasonic technique was developed in the aim of characterizing the adhesion of viscoelastic fluids or solid deposited on a substrate. We adopted the idea that the more a deposit is difficult to clean the more adherent it is. From this point of view the value of the reflection coefficient of an ultrasonic shear wave informs us about the adhesion of the deposit on a surface. A large bibliography on the adhesion measurement is given. Then the principle of ultrasonic test is presented and cares required for the measurement of the reflection coefficient are widely discussed. The ultrasonic reflection coefficients obtained with different controlled samples covering a wide range of interfaces (liquid/substrate, solid/substrate) are presented and compared with other indicators of adhesion. All the data on various samples showed that the ultrasonic test is a tool to discriminate non-destructively a large range of interface quality, allowing ranking according to the adhesive strength.

Inline high frequency ultrasonic particle sizer.

This paper reports the development of a new method of particle sizing in a liquid. This method uses high frequency focused ultrasounds to detect particles crossing the focal zone of an ultrasonic sensor and to determine their size distribution by processing the reflected echoes. The major advantage of this technique compared to optical sizing methods is its ability to measure the size of particles suspended in an opaque liquid without any dedicated sample preparation. Validations of ultrasonic measurements were achieved on suspensions of polymethyl methacrylate beads in a size range extending from a few micrometer to several hundred micrometer with a temporal resolution of 1 s. The inline detection of aggregate formation was also demonstrated.

Heat treatment of whole milk by the direct joule effect--experimental and numerical approaches to fouling mechanisms.

The development of alternative technologies such as the direct Joule effect to pasteurize and sterilize food products is of great scientific and industrial interest. Our objective was 1) to gain insight into the ability to ensure ultra-high-temperature treatment of milk and 2) to investigate the links among thermal, hydraulic, and electrical phenomena in relation to fouling in a direct Joule effect heater. The ohmic heater [OH; E perpendicular to v (where E is the electrical field and v is the velocity); P (power) = 15 kW] was composed of 5 flat rectangular cells [e (space between the plate and electrode) = 15 mm, w (wall) = 76 mm, and L (length of the plate in plate heat exchanger or electrode) = 246 mm]--3 active cells to ensure heating and 2 (at the extremities) for electrical insulation and the recovery of leakage currents. In the first step, the thermal performance of the OH was investigated vs. the flow regimen [50 < Re (Reynolds number) < 5,000], supplied power (0 < P < 15 kW), and electrical conductivity of fluids (0.1 < sigma(20 degrees C) < 2 S/m) under clean conditions with model fluids. This protocol enabled a global thermal approach (thermal and electrical balance, modeling of the temperature profile of a fluid) and local analysis of the wall temperature of the electrode. An empirical correlation was established to estimate the temperature gradient, T(w)-T(b) (where T(w) is the wall temperature and T(b) is the product temperature) under clean conditions (without fouling) and was used to define operating conditions for pure-volume and direct-resistance heating. In the second step, the ability of OH to ensure the ultra-high-temperature treatment of whole milk was investigated and compared with a plate heat exchanger. Special care was taken to investigate the heat transfer phenomena occurring over a range of temperatures from 105 to 138 degrees C. This temperature range corresponds to the part of the process made critical by protein and mineral fouling. The objectives were 1) to demonstrate the ability of an OH to ensure heat treatment of milk, 2) to study the thermal and hydraulic performance with an increasing power and temperature difference between the inlet and outlet of the OH, 3) to define and validate a criterion to follow heat dissipation efficiency, and 4) to compare the fouling propensity with the different configurations. A heat dissipation coefficient, Rh(CO), was defined and validated to monitor the fouling propensity through global electrical and thermal parameters. Finally, a numerical simulation was developed to analyze heat profiles (wall, deposit, bulk). Because of an increasing Joule effect in the static deposit, the simulation showed how wall overheating would definitively cause fouling to spiral out of control.