One frequently raised question is how designers determine the amount of recirculating air to use in a particular oven. The question usually will be be phrased "Why did they use that number of air changes?" or "How was the recirculating fan cfm figured?" (Air changes, for the uninitiated, are the recirculating fan flow divided by the internal volume of the oven.)

Every designer has guidelines and reasons for the fan capacity he or she chooses, and generalizing about them is risky business. However, the fundamentals are pretty much the same everywhere, and they're worth understanding. These descriptions are simplified because of space limitations, but they illustrate the point.

In recirculating ovens, the recirculating airstream is the medium that transports the heat from the heater box to the oven chamber. It releases some of the heat it carries to the oven and its load and then returns to the heater to be recharged, either by mingling with a burner's products of combustion or by passing over a heating element, which could be an electric heater, steam or hot water coils, fuel-fired radiant tubes or a heat exchanger.

The first step in the process is calculating the heat load (BTU/hr, kJ/hr or whatever other heat units suit your fancy). Most of this is the heat absorbed by the work, but heat load also includes heat absorbed by conveyors and conduction losses through the oven walls.

The second step is to determine the key performance criterion for the heating system. Is temperature uniformity the most important requirement? Heating speed? The need to evaporate and remove moisture or solvents?

Suppose the application has strict temperature uniformity requirements: No part of the load can be more than 10oF (5.6oC) different from the rest of the load. That means, in effect, that the recirculating airstream cannot lose more than 10oF of its incoming temperature as it passes through the oven and over the load. Otherwise, the product in contact with the air at the end of its pass through the oven might be more than 10oF colder than the product that first encountered the air. How do you achieve this? Among other things, by limiting the time the product is exposed to the hot air. Get the air in and out quickly; in other words, use a high recirculating volume (more air changes). An example is shown in figure 1.

An oven's heat load is 800,000 BTU/hr, and the incoming recirculating air temperature is 400oF (204oC). Maximum acceptable temperature differential is 10oF, so the air must return to the heater before its temperature drops below 390oF (199oC). At 400oF, one cubic foot of air contains 6.44 BTU, based on 0 BTU at 60oF. At 390oF, its heat content is 6.25 BTU, so every time one cubic foot of that air passes through the oven, it's permitted to leave behind (6.44 - 6.25), or 0.19 BTU. Divide that number into the heat load

800,000 BTU/hr / 0.19 BTU/ft3

and you get the recirculating volume required, 4,210,530 ft3/hr, or 70,180 cfm.

That requires a pretty big circulating fan, so this is where some horse-trading may come in. If, for example, a differential of 25oF (14oC) can be tolerated, the recirculating volume can be reduced to give longer residence time in the oven. The air has the opportunity to release more heat before it returns to the heater, so a smaller recirculating volume, in this case, 28,070 cfm, will do.

These calculations assume you can transfer heat to the load at the desired rate. Sometimes, however, that shouldn't be taken for granted. In recirculating ovens, the majority of heat transfer takes place by convection; that is, by the hot air scrubbing the load surfaces. The heat transfer rate is a function of several factors -- most importantly, the air velocity, its temperature and the way it strikes the load surface.

This can pose a problem in large, roomy ovens, where the hot air nozzles can't be located close to the load. The velocity at the nozzle exits may be fine, but by the time the air reaches the load surface, it has mingled with the air already in the oven chamber, and the average velocity of the mixture is too low to develop the heat transfer rate. One solution is using a higher pressure circulating fan, but that usually requires substantial increases in horsepower. Another is to increase the recirculating flow rate, raising the average velocity at the load surface. One consequence of this is a decease in residence time in the oven and lower temperature differentials, whether they're needed or not.

In drying or solvent ovens, the need to remove the evaporated liquid may come into play. Strictly speaking, it shouldn't -- the fresh air brought into the oven to maintain desired humidity levels or to keep flammable solvents safely diluted is determined by the capacity of the exhaust fan, not the recirculating fan (see "Winning the Fan Shell Game"). However, the fresh air gets to the exhaust via the recirculating fan, so in some cases, the recirculating fan may have to be sized to meet this need.