In food processing operations involving frying, many factors influence the type of heating system used. Among the variables to consider are the method of heat transfer (directly or indirectly heating the cooking oil), the heater configuration (two- or three-pass designs), flux rate, thermal efficiency and maintenance requirements.
Heating Method. There are two ways to heat cooking oil using a helical coil heater:
- Heat a food-grade thermal fluid with the heater, and then circulate the hot fluid through an external jacket on a vessel or kettle of cooking oil.
- Instead of using thermal fluid, heat the cooking oil directly by circulating it through the helical coil inside the heater.
Each approach has advantages and disadvantages.
The indirect heating method, using thermal fluid to heat a jacket on a vessel or kettle that holds the cooking oil to be heated, provides precise temperature control. Valves are used to vary the flow of thermal fluid through the external heating jacket, so temperature control of the fluid does not rely entirely on the firing rate of the burner. Moreover, this system facilitates using a heater sized for a wide range of heat loads. So, users are able to precisely control the heat for several kettles one day and only one or two the next day.
The use of food-grade thermal fluid is a safety precaution. It is required even though it is unlikely that the fluid would ever come into contact with the food or its cooking oil. Nevertheless, the thermal fluid is being used to heat the exterior surfaces of the kettle. As with almost any heat processing system, there is a remote possibility that something could go awry, and this could allow some heat transfer fluid to find its way inside the kettle before being discovered. The use of food-grade thermal fluid should prevent any adverse effect if the unlikely ever happens.
Heating the cooking oil directly eliminates the need for both a jacketed kettle and the thermal fluid. The cooking oil simply is heated while being pumped in and out of the kettle itself. Temperature control of the oil is achieved by modulating the firing rate of the burner. And while burners do not have infinite variations in their firing rate, they can do a great job if properly sized with suitable turn-down ratios.
Heater Configuration. Another consideration is the type of heater, whether vertical or horizontal, two-pass or three-pass. If the heater is used to heat thermal fluid, it can be either vertical or horizontal, depending on the floor space available. Vertical heaters have a smaller footprint, which may be a deciding factor. If the heater is used to heat the cooking oil directly, it must be a vertical in order to completely drain the cooking oil from it as frequently required.
Two-pass heaters have a single helical coil. Burner gases make two passes to heat the coil: They pass through the open central area of coil and travel back around its outer side before exiting the stack. Three-pass heaters have two helical coils: The gases make three passes around the coils before they exit the stack.
The choice between two-pass and three-pass designs largely depends on available space. Three-pass heaters are somewhat smaller for a given output, but their two coils usually have a shorter life expectancy than the single coil of a two-pass heater of comparable size.
Flux Rate. Another consideration is the design of the helical coil heater itself. Some helical coil heaters are better suited for food processing than others. This has to do mainly with the heater’s flux rate. The lower the flux rate, the longer the life of its heating coils and the fluid that passes through the coils.
Flux rate is the number of square feet of coil surface area in relation to the BTU per hour output of the heater. The larger the coil surface area for a given output, the lower the flux rate. A larger surface area transmits the same quantity of heat (or BTUs) as a smaller one, but does so at a lower temperature than coils with smaller surface areas. Flux rate is related to the film temperature of the fluid flowing through the coil, and film temperature is related to fluid life.
Film temperature is the temperature of the fluid in direct contact with the coil surface as it flows through the coil. As fluid flows through a heated coil, the portion of the stream in direct contact with the bare metal surfaces on the inside of the coil is heated to a higher temperature than the portion in the center of the stream. Consequently, the temperature of these surfaces is of concern. Lower flux rates mean lower temperatures of these surfaces. So what is the optimum flux rate?
The optimum flux rate is one that minimizes fluid breakdown, enabling it to have maximum life. The exact rate depends on the fluid being used. Fluids, including cooking oils, are available with a range of film temperature ratings, so it is not possible to cite one flux rate that works best for all applications.
However, here is a precaution about flux ratings cited by different heater manufacturers. Do not confuse radiant flux rate with average flux rate. Average flux rate is the average heat absorbed by the total surface area per square foot (both the radiant and convective sections). Radiant flux rate is the average heat absorbed by the surface area exposed to the burner flame per square foot (the radiant section only). The average flux rate will be lower and far more favorable than the radiant flux rate. So when you compare flux rates, make sure you are not comparing apples with oranges.
Incidentally, flux rates of two-pass heaters are nearly always more favorable than those of comparable three-pass heaters. Three-pass heaters typically have a much smaller surface area subject to the radiant energy. The coil in a two-pass heater is usually larger and longer and has a much greater surface area subject to the radiant energy.
Thermal Efficiency. Another consideration is the matter of thermal efficiency. Thermal efficiency is directly related to the amount of fuel used. Higher thermal efficiency produces greater fuel savings. Food processors should not buy a heater with a thermal efficiency lower than about 85 percent. Some heaters do even better, ranging up to 90 percent. Obtaining the higher efficiencies depends on heater design, the fuel used and whether the heater has a stack heat exchanger that captures exhaust gases that would otherwise be lost. Typically, the direct heating of cooking oil is more efficient than using thermal fluid.
Maintenance. Last, but not least, maintenance is a consideration. Two of the hallmarks of helical coil heaters are the ease of maintenance and their long life. They require less maintenance than systems that employ a boiler and use steam heating. Also, they do not require the presence of a licensed boiler operator while in operation.