Is Your Oven Out of Control?
For example, in one style of direct gas-fired oven, the principal means of heat transfer are conduction and radiation, with convection playing a lesser role. This design utilizes a number -- sometimes in the hundreds -- of open flame ribbon burners mounted below a nonperforated conveyor belt or band. The heated band conducts heat to the product as it is conveyed through the oven. Conduction heat transfer is directly proportional to product thermoconductivity and product thickness. Similar rows of burners are mounted above the product to radiate energy to the product and to convectively heat the low velocity air around the product. Radiant heat transfer is proportional to the emissivity of the product being radiated.
In forced convection ovens, the hundreds of ribbon burners are replaced by a small number -- maybe five or six for an equivalently sized oven -- of burners that are used to directly heat the air. The air then is forced by a series of fans to contact the product, usually from both above and below, at velocities 100 to 1,000 times greater than with direct-fired gas ovens. The air then convectively heats the product as it is conveyed.
Forced convection ovens typically are supplied with open-mesh or perforated-plate conveyors to allow air to simultaneously contact both the top and bottom of products. There also are forced convection oven designs that use air to heat solid conveyor bands instead of ribbon burners. Convective heat transfer is a function of the velocity of the air in contact with the product.
In addition to direct-fired gas burners, air also can be heated by steam or hot oil as it passes through coils. Steam has the advantage of no live flames and is considered safer to use, but air temperature is limited to approximately 150oF (194oC). Hot oils have the advantage of high temperature without flames, which is important when handling volatile products. Electric heat is clean, available and easy to use, but it may be too expensive for some applications.
Controlling the Process ChamberOven control begins with an understanding of heat and mass balances. As a product moves through an oven, its surface temperature rises by one of the means previously discussed. Heat then is transferred, principally by conduction, from the product surface inwardly to raise the overall product temperature. Given this fact, it is easy to see why thin products heat more quickly than thick products and high thermoconductivity products such as metals heat more quickly than low conductivity products such as wood.
Many ovens utilize closed-loop air temperature control. Thermocouples or resistance thermometers sense air temperature and signal a process controller or PLC, which signals a control device such as a gas supply or steam control valve to open or close proportionally to maintain setpoint temperature. In modern applications, electrically operated control valves usually are selected. Gone is the need for air-operated valves with I-to-P converters. Temperature control is fast, accurate and easy to maintain. There are few issues with temperature control. Many people sense product temperature with infrared devices and record results, but few actually try to control by this means.
Moisture enters an oven airstream in three ways: evaporation of moisture from products, moisture contained in makeup air and moisture produced by gas combustion. As products are heated, moisture is removed. The amount and speed of removal is dependent on the moisture-holding capacity, or relative humidity, of the surrounding air. The lower the relative humidity, the more rapidly moisture is evaporated from the product. Humidity decreases as air is heated and increases as moisture is evaporated.
A second source moisture within the oven enclosure is makeup air. This lower absolute humidity ambient air includes all sources of air entering an oven, including that entering from the ends of the oven and all points of air leakage. In other words, it is atmosphere and you cannot control it, but you have to account for it.
The third source of moisture for all direct gas-fired ovens is the moisture produced by combustion of gas. More than two pounds of water vapor are produced for every pound of natural gas burned.
Oven humidity is controlled by balancing the amount of higher humidity air that is exhausted with the amount lower humidity air brought in for makeup. This is done by controlling the position of an exhaust damper or by controlling the speed of the exhaust fan. The oven design with the lowest humidity air -- and hence has the greatest moisture removal ability -- is a one-pass air design with no air recycle. This design is seldom used, however, because it is prohibitively costly, as too much heat is exhausted. In addition, in many cases, low humidity air is not be best condition for drying or to enhance the chemical reactions taking place. For those reasons, some amount of air always is recycled, and in the case of forced convection ovens, a large amount of air -- 70 to 80 percent -- is recycled. The recycled air is blended with makeup air and then heated.
Measuring humidity in low temperature ovens has been done successfully for many years but finding sensing elements that can operate at high oven temperatures (500 to 600oF [260 to 316oC]), for any length of time has been a problem. Closed-loop control of humidity also is difficult because one cannot completely control all the sources of makeup air input such as leakage or end effects. The result, unfortunately, for many processors is to do nothing about humidity control.
Control PanelsToday, the end user has a range of control systems from which to choose. Traditional hard-wired panels with separate controllers for each control loop, which have served the industry well for many years, are rapidly being replaced by PLC-based systems with sophisticated human interface screens. These systems give the operator a vast array of information previously not available. Graphic color depictions of the complete process easily are displayed along with touchscreen functions, data recording and information transmission. Alarm history, recipes and complete process setups and changes easily can be accomplished remotely. The operator has more information and can make better decisions.
In the search to cut costs and increase productivity, never lose site of the value of operator training, safety and equipment maintenance. Time and again, we find that better trained operators make the best operators. Operator training should be a blend of both theory and practice as an understanding of how the oven process works is invaluable when inevitable trouble occurs. A preventative maintenance program for all safety and control devices such as flame safeguards, purge timers and limit switches should be standard operating procedure. These simple actions will go a long way to keep your oven under control.