Effective, accurate fryer control using new 80 GHz Radar

A world leading convenience food and beverage manufacturer in the UK has found a new, innovative solution for controlling an integral part of their process for manufacturing tortilla chip snacks.  There are always challenges to find ways to improve consistency and quality in the food industry, one of the key areas are the fryers, where a complex interaction between controlling the temperature, cooking time and oil replenishment is critical to making and maintaining the product to the highest quality. Small improvements can also quickly pay back in energy savings.

At this site VEGA radars are already used to monitor the cooking of the raw corn in kettles, before it is softened, processed and shaped into the familiar triangular tortilla shape. The chips require precise frying for that crispy, crunchy texture they are known so well for. They are cooked in vegetable oil  inside a very large fryer, where the tortilla chips are fed in at one end via a conveyor, floating through the hot oil on a recirculating current, and lifted out by another conveyor at the other end. Afterwards they are checked, seasoned and immediately bagged for freshness. The quality of the chips produced in this cooking stage is controlled via a complex mixture of cooking time, temperature, recirculation and oil volume. Of course, as each chip is cooked a tiny amount of oil is used, so the oil depth and temperature requires maintaining and replenishment needs to be constantly and minutely controlled.

The fryer is 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. Any level measurement system used needs to ideally have no moving parts and 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. It's a careful mix - too much or too little oil will see unwanted swings in the temperature - therefore, the more accurate the information the better the process control can work. 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.

 Figure (right) The radar installed on the chamber with only a measuring range of 220mm, however each mm represents a large oil quantity

The engineers initially tried a high temperature guided wave radar, but discovered on oil over a few hundred mm range, all guided wave radar sensors have a physical performance restriction when working with poorly reflecting liquids (oil based products). This is because oil is a poor reflector and not easily detected at extremes of the range, resulting in only the middle part making an accurate measurement over this short length. In its place a capacitance probe was installed 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 better 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.

When VEGA were looking at new challenges for their pilot 80GHz radar VEGAPULS 64, this was an ideal test.  The engineers at the factory were especially interested in how accurate this device could be, whether it could deliver the incremental improvement they were seeking to increase efficiency. 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. The test helped VEGA too, feedback from this test and others on the pilot units, showed how they could further enhance the performance of their antenna systems, now fully implemented on the production devices. The consistency and better control provided by the VEGAPULS 64 enables the factory optimise both energy and oil input to an even greater degree, producing not only an even better product, but also better for the environment. They are now looking to install these on all fryers at the site. 

Figure (left) The small dimensions of the new 80 GHz radar are ideal for small vessels in the food industry, it also has hygienic fittings and approvals