Achieving more accurate fryer control
For a producer of tortilla chip snacks the complex interaction between controlling the temperature, cooking time and oil replenishment of its fryers is critical to maintaining the high quality of its tortilla chip snacks.
VEGA radar sensors were already in use at the site, monitoring the cooking of raw corn in kettles, before it is softened, processed and shaped into the triangular tortilla shape.
The chips require precise frying to achieve a crispy, crunchy texture. They are cooked in vegetable oil inside a large fryer. The tortilla chips are fed in at one end, via a conveyor, floating through the hot oil on a recirculating current, and are then lifted out by another conveyor at the other end. Afterwards they are checked, seasoned and immediately bagged.
During this cooking process the quality of the chips is controlled via a complex mixture of cooking time, temperature, recirculation and oil volume. The oil depth and temperature also needs to be maintained and replenishment needs to be constantly and minutely controlled.
The fryer runs at an ideal temperature of 186ºC and there is a likelihood of some build up from the oil and other deposits. It changes in density and electrical properties through heating and product contamination. Ideally, any level measurement system needs to have no moving parts and should be very easy to be regularly cleaned.
The oil level from the fryer is fed into a complex PID algorithm, which seeks to maintain both oil temperature and quantity. The level measurement range is over 220mm and takes place inside a small chamber off the side of the large frying vessel, each mm represents 100’s of litres of cooking oil. It is important that the whole range is measured, both during charging of the oil and heating of the process as well as in full production. Heat input needs to be carefully controlled in any fryer system for optimal efficiency and safety.
Finding a solution
Engineers at the plant initially tried a high temperature guided wave radar. However, all guided wave radar sensors have a physical performance restriction when working with poorly reflecting liquids – such as oil-based products. Oil is a poor reflector and so is not easily detected at extremes of the range, resulting in only the middle part making an accurate measurement A capacitance probe was installed in its place on the fryer with some success; it has no moving parts and detected the oil over the full measuring range. However, the oil changes in dielectric (conductivity) as it heats and cooks the chips – due to changing carbon levels in the oil – which directly affects the level accuracy of this type of probe. Cleaning a rod-based device wasn't ideal either, but offered a better solution than a mechanical float system. The capacitance unit worked acceptably and, until recently, this was the method used - with a ‘best fit’ of oil dielectric in the calibration to maintain control. Ideally, the engineers wanted something more accurate.
At the same time, VEGA was looking at new challenges for its pilot 80GHz radar VEGAPULS 64, and the fryer application was considered to be a good test of its abilities. Engineers at the factory were interested in how accurate this device could be, whether it could deliver the incremental improvements they were seeking to help increase efficiency. When the radar was installed and set up, one thing was immediately apparent; it had the sensitivity to detect the oil almost as soon as the filling began. Sharp focusing meant it was also able to measure the level of liquid inside the chamber over the whole range with no interference from the side connections. The accuracy of control was very consistent and also ‘to the mm’, with no effect from the ‘dielectric’ change. Feedback from this and other pilot units, showed how VEGA could further enhance the performance of its antenna systems, which today are fully implemented on its production devices.
The consistency and better control provided by the VEGAPULS 64 now enables the factory to optimise both energy and oil input to an even greater degree and it is now looking to install the radar sensors on all of its fryers.>
Source: Control Engineering Europe - All Articles