Monitoring Pressure in Milk Processing
Nestlé’s R&D center in Konolfingen, Switzerland, develops milk-based foodproducts and designs manufacturing processes. Monitoring pressure in heat exchangers provided some particular challenges.
Andrew Sim, Baumer -- Control Engineering, 10/1/2009
 New Nestle products come from 24 research facilities worldwide. |
In Konolfingen, Nestlé operates its Product Technology Center (PTC), one of 24 such research facilities worldwide. For more than 30 years, the company has been developing milk-based products and production processes there, with the main focus lying on infant food, special-diet nutrition, and health-care related products. Konolfingen developers research appropriate recipes and test processes for all operational Nestlé companies worldwide that require new products for these markets. Recently the company expanded the plant, which included adding a group of new heat exchangers.
Outfitting new heat exchangers
Following applicable European sanitary protocols, all measuring devices that contact product in the new testing facility had to be equipped with an aseptic screw connection according to DIN 11864-1. At various places around the plant, Baumer pressure transmitters of the ED701 series were already being used in other applications such as homogenizers, mills, and filling equipment. Since Nestlé had been satisfied with those devices, they were also chosen for pressure measurement for the new heat exchangers. Depending on the processes being tested, up to six new heat exchangers are used for heating or cooling the products, with temperatures reaching 150 °C (302 °F).
At high temperatures the products precipitate protein which then deposits on the heat exchanger’s metal plates. If there is too much sediment on the surfaces, exchange efficiency falls and the differential pressure between the inlet and outlet rises. Eventually the heat exchangers have to be cleaned. Given the nature of new product development, these exchangers see a wide variety of ingredients and process temperatures as developers test recipes and manufacturing techniques with different kinds of liquid products, ranging from milk to dessert crème. Among other results, the tests show the possible range of process temperatures that might be required during production, as well as the amount of time and product throughput the unit can run before the heat exchanger needs cleaning.
 Sensors in crowded places are a challenge, particularly where impluse line length must be kept at a minimum for cleaning. Source: Baumer |
ED701 for differential pressure
The ED701 measures differential pressure using a piezo-resistive pressure transducer with a flush stainless steel diaphragm suited to industrial and sanitary applications. It is based on a silicon sensor designed for high precision over a wide temperature range with downstream digital compensation to reduce signal drift. The piezo-resistive element is anodically bonded to a glass and stainless steel base for stability and thermal isolation. A stainless steel diaphragm and a specially processed fill liquid separate the sensor and process fluid.
The electronics are located within a hermetically-sealed transmitter housing, making the unit resistant to humidity, shock, and vibration. Depending on the choice of electrical connection, the protection class varies from IP 65 to IP 67. The signal processing electronics core piece is a micro-controller that compensates for drift effects on the sensor signal due to temperature changes while still maintaining a response time below 5 ms.
The ED 701 family includes a selection of pressure ranges and electrical connections. Most units use a 4-20 mA current loop, along with various voltage output signals. The hygienic design makes the ED 701 series most suitable for applications in food and beverage processing, as well as in pharmaceuticals and biotechnology. Any parts that contact process fluids are 316L stainless steel (1.4435) and all materials, including the white oil used as fill liquid, comply with U.S. FDA standards.
For the use with the heat exchangers, Nestlé specified pressure transmitters with an accuracy of ±0.4% full scale and a compensated temperature range of -10 to 125 °C on the cooling water side and a maximum temperature of 300 °C on the heating side. The measuring range of gage pressure covers 0 to 20 bar (0 to 290 psi).
Nestlé’s engineers wanted a new process connection for the milk tube, which they developed in cooperation with Baumer. As a result Baumer delivered the units with only a connection for the pressure-receiving side, and Nestlé made the connector for the product side.
 The custom fitting, designed in cooperation with Nestle, satisfied all the sanitary requirements. Source: Baumer |
“For the milk tube fitting, Baumer offered us the CombiConnect adapter, which we found most suitable thanks to its flush-mounted o-ring”, says Ernst Strahm, Nestlé measurement technologist at Konolfingen. The CombiConnect system provides a series of replaceable sanitary process connections made of stainless steel. Nestlé has used the same connection with FlexBar and ED701 pressure transmitters as well as CombiTemp temperature measuring devices. The connections were developed to fulfill dairy and food industry sanitary requirements, including FDA and EHEDG criteria.
Duplicating successes
Nestlé’s product developers had enough experience in other applications to know what worked well and what didn’t. Based on positive results with similar pressure transmitters elsewhere in the plant, the selection process was not difficult. When selecting specialized units for use with the new heat exchangers, Nestlé returned to the CombiConnect with its own purpose-built adapter. Strahm adds, “For a hygienic pressure measurement, the quality of the measuring devices is the decisive criterion. With the ED701, we have had consistently good experiences.”
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