Computer simulation has eliminated the need to rework potato fryers in the field to overcome problems with uneven oil distribution, saving hundreds of thousands of dollars per year
Uneven distribution of oil in the large fryers used by the food processing industry causes a number of problems, the largest being that it can move the product to one side of the oven, making it difficult to handle. Several years ago, Gem Equipment (Woodburn, Oregon), a producer and designer of custom food processing systems, had no way to predict the performance of headers that are supposed to evenly distribute the flow. The company had to spend a considerable amount of money performing field rework on the headers used in 40% of the machines it installed.
The company then had the idea of using computational fluid dynamics (CFD) to simulate flow through the fryer. The results of the CFD simulation made it possible to improve the performance and robustness of the header design. The oil flow through the fryers now moves in a continuous plug flow, which is the ideal condition.
Potato fryer operation
The fried potatoes sold in restaurants are typically precooked by the food processor in order to sterilize the product and help it stay fresh longer. When the potatoes first enter the fryer, they are heavier than water so they sink onto the conveyor. But as they are heated by the oil, the water boils off, the density of the potatoes drops below that of the oil, and the potatoes begin to float. At this point, the flow of the oil through the fryer becomes very important. The ideal flow pattern is for the oil to move in a continuous plug pattern in the same direction as the conveyor, maintaining a constant velocity across any given section of the fryer. If there are areas of high and low velocity or if the flow moves perpendicular to the conveyor, the potatoes could be pushed to one side of the fryer. This could cause the potatoes to be unevenly cooked, and also creates handling problems when the potatoes leave the fryer.
Problems in equalizing flows
In order to maintain even velocity, the oil inlet in previous fryers was a header consisting of pipe that stretched across the width of the fryer. Oil entered the header on either side of the fryer and left through holes spaced along the width of fryer. If all the holes were the same size, the natural tendency would be for the oil to come out of the header at a higher velocity near the ends where the oil enters the header. In order to equalize the outflows, perforation plates are placed inside the header that reduce the size of the holes in these areas. The problem in the past was that Gem Equipment engineers had no way to determine whether they had succeeded in equalizing flows through the header except to build the fryer and see how it worked. In many cases, flow was uneven when the header was first installed. In these cases, Gem Equipment immediately sent a service engineer to the customer's site. The service engineer would observe the fryer, try to visually determine where the problems were occurring, then based on experience, try different perforation plates until the problem was solved.
While the service engineers were able to solve the problem, the cost of the service calls and the inconvenience to customers caused Gem to look for another approach to designing headers. Attention focused on the technology of CFD, which involves the solution of the governing equations for fluid flow at thousands of discrete points on a computational grid in the flow domain. When properly validated, a CFD analysis allows engineers to determine the direction and speed of flow at any point in the flow domain. Unlike a physical model, the geometry of the CFD model can be changed quickly on the computer and re-analyzed to explore different options in project design or operation conditions.
Simulating header design
Gem Equipment engineers selected the FIDAP CFD code from Fluent Incorporated, Lebanon, New Hampshire, as their modeling and analysis tool. This software package uses the finite element approach and allows for the use of unstructured grids. Unstructured grids provide considerably greater flexibility in modeling the complex and irregular geometries involved in food processing equipment, and automate the otherwise impracticably tedious process of fitting elements to the complex geometries used in complex areas such as the headers.
Gem engineers began by modeling the oil flow through their existing header design. They imported a computer aided design (CAD) file containing the geometry of the fryer into FIDAP and created a three-dimensional CFD model with 500,000 cells. The most challenging aspect of the modeling process was accounting for the effects of the conveyor moving through the oil. Engineers tried using a special FIDAP element called a slip entity that makes it possible to specify the velocity at any desired points in the problem domain. By measuring the actual fluid velocity of the oil in the area affected by the conveyor and creating corresponding slip entities, they were able to achieve accurate results in less time.
New design provides optimal performance
The first model of the new header showed flow distribution problems, but it provided engineers with complete information on the flow through the device. This information gave them ideas on possible solutions. In particular, engineers made changes to the part of the model that represents the perforations used to release oil from the header. They tried four or five different designs and finally found one that works for a wide range of conditions.
The new design has demonstrated the ability to provide even flow distribution in every application in which it has been tried. Over the past two years in which Gem Equipment has used this new design, not a single header has required adjustment in the field. Gem engineers who were initially skeptical of the ability of computer simulation to predict flow through the complex fryer geometry have become believers. As a result, the company is now using CFD to optimize other elements of the fryer design, such as the oil return passageways.