Factory Engineering I - ISSCT

1 Factory Engineering I DEVELOPMENT OF CONDUCTIVITY AND CAPACITANCE SENSORS FOR PAN BOILING Reichard, R. Broadfoot and Wright Sugar Research Institute, MacKay, Queensland, Australia , / ' ., i ABSTRACT -. In the Australian sugar industry the massecuite condition' in vacuum pan boilings is mainly measured by conductivity transducers. This paper describes the hnsiderable developments which have taken place in the Fgn of these transducers since the early use of simple contacting, type, electrodes. Today radio-frequency types which allow the, measurement of the conduc- tivity and/or the capacitarice of the process material are being widely ,used.

2 Sensors have been developed to suit all pan boiling applications in the raw sugar Factory and also for all grades of sugar refinery boilings. A design of heated probe which prevents the formation, of sugar crust on the surface contacting the massecuite has been developed. This is particJlarly suitable for use in continuous vacuum pans boiling high purity massecuites. INTRODUCTION Conductivity has been widely used in Australia for the control boilings since its introduction from Java in the 1930s as the 'Cuitqmeter'. The )principles of the control method have been described elsewhere, Wright7.

3 Ban control in Australia is now almost fully automated;' with the feed control being largely based on conductivity sensing methods. However, there has been a gradual development of conductivity sensors from the early simple circuitry towards radio-frequency types which have special advantages. This paper briefly outlines these developments. CONTACTING ELECTRODE TYPES , 1 The first conductivity systems were based on measurements of the current flow in a ~ircuit containing contacting1 type massecuite electrodes poyered by 32V 50 Hz alternating current.

4 The pan electrodes were, of a type standardi- zed by' Smiths and known. in Queensland as the ,'Bureau type'. The ar- rangement for these electrodes is shown in, Figure l(a). This system consists of a pair of parallel metal cylinders, 16 mm diameter and 45 mm long, spaced about 75 mm apart and mounted at least 125 mm from the wall , Factory Engineering , . : ,. ! , , (a) Double probe 'Bureau' type. I TEFLON TEFLON 316 . I ,SPACER , SS,316 ' 'I> I:) , , I I. , , , , ,, ,\OD) Single Probe Type FIGURE 1. Examples ,of cqntacting r type elqctrodes. , I !f I /, s.

5 I/ 1 I, i of thec pan! Tlie conductivity 'cell constant: with sthi cml, and the opefating masseouite 'resistance?is around 250 ohms' for raw shgar high grade boilings and atound'350 ohms for 'low grade boilirigs. These electrode$ were widely used, St over brass, copper or mild stee18fot the '\I ,-!I I , !, l,3! q,, ;, t ' REICHARD, R. BROADFOOT AND WRIGHT A family of simpler single probe electrode units was introduced, into the Australian industry in the mid 1960s (Wright6). In these the active section of the probe is a 25 mm diameter metal tip which is separated from the earthed metal shank of the probe by a teflon spacer.

6 The original distance of separation was 50 mm to give approximately the same cell constant as the earlier double probe type. The single probe type has proven to be more versatile in finding a suitable location in the pan and easier to replace and to clean than the 'Bureau type'. Later it was found to be better (for strength reasons) to reduce the spacer distance to about 25 mm and to use the electrode with commercial instrumentation resistance transducers which use a more sensitive, higher frequency (1,000 Hz) circuit. A sketch of one arrangement of this electrode is shown in Figure l(b).

7 'CONTACTLESS' (RF) ELECTRODE TYPES One common defect with the contacting electrode types is the progressive change in the surface of the electrodes. Formation of scale or crust on the electrodes and erosion of metal by electrolytic action make frequent cleaning of the electrodes necessary to avoid drift in the operation settings. It was common practice in some factories that the electrodes be withdrawn and cleaned every eight hours, while other factories had a policy of 'not cleaning' which allowed the scaling to take place to a level where its effect was relatively constant.

8 To overcome these problems a new method for sensing conductivity at radio-frequencies (RF) was introduced by Reichard and Vidler". The method did not require direct contact of the metal to the massecuite in the pan and was consequently termed 'contactless' conductivity measurement. In this method the conductivity of the massecuite surrounding a probe is measured by sensing the degree of damping applied by the massecuite to a tuned circuit resonating at radio frequencies (about 10 MHz). The sensing probe which comprises an aerial encased in a thin walled teflon sheath, forms I I part of the tuned circuit, with the electrical energy being transferred to the massecuite by capacitive coupling.

9 The RF probe consists of a copper tube 1 aerial 20 mm in diameter and 100 mm long encased in a teflon sheath 2 mm thick. Current pulses of very short duration are generated with a repetition rate of a few hundred kilohertz. Each pulse generates a burst of damped oscillations~in the resonant circuit. An amplitude detector and signal condi- tioning circuitry are used to derive a 4-20 mA output signal that is propor- tional to the average amplitude of the ringing waveform. Little damping is experienced when the probe is immersed in a massecuite of high water content.

10 A more rapidly damped waveform is obtained when the probe is immersed'in a heavy massecuite (of low water content) and low conductivity. This type of transducer was first put into service by Sugar Research Institute (SRI) as a replacement for the conventional system in 1974. I Factory Engineering Other workers have also studied the electrical properties of massecuites at radio frequencies (Moller et ; radford and Cdx3) and have developed useful RF transducer systems. As well, at least one instrumentation company (Foxboro) market a RF conductivity transducer.