Heat Transfer Issues in Mechanically Agitated Fermenters? Aerobic fermentation generates a significant amount of heat, which must be removed to maintain a constant temperature. The main heat sources are metabolic heat generation, agitator power input, and compressor power input. Removing the heat is made more challenging by the fact that fermenters are operated at mild temperatures, typically between 30 and 40C, which is not much warmer than cooling tower water. To reach the target heat transfer rate needed, chilled water is often needed. Also, a significant heat transfer area is needed. Rarely is a tank jacket alone sufficient. It is common to use multiple banks of helical coils, or, preferably, vertical tube bundles. While vertical tube bundles are an excellent heat transfer surface, in that they have high process side heat transfer coefficients and also serve as anti-swirl baffles, they do affect the power draw. When anti-swirl baffles have an opposed area greater than that of standard baffling (4 baffles at 1/12 tank diameter in width), the agitator power draw decreases. The same is true of vertical tube bundles. To get enough heat transfer surface, the opposed area is almost always greater than standard baffling. 6-8 tube bundles are common, and they often project up to 20-25% of the tank diameter from the wall. Some installations have as many as 12 bundles. For such extreme geometry, power draw has been known to decrease as much as 45% compared to standard baffles. Even with 6-8 bundles, it can be 15-20% below normal. Some may ask, Why is this important? Wouldn’t the drop in power draw save energy? The answer is that the mass transfer rate depends in part on agitator power input. A decrease in agitator power will result in a decrease in mass transfer potential.
The “cure” is to model the system mechanically on a small scale, then make adjustments in speed or impeller diameter to compensate for the geometry effect. This modeling usually must be done in an agitator vendor’s lab. LEC has experts that can help arrange and supervise such testing.
In low viscosity fermentation, the agitated-side film coefficient is often substantially greater than the tube side. In such cases, it may be advantageous to use internally finned tubing or a pipe insert on the inside of the heat transfer tubes.
In extremely energetic fermentation, it may be necessary to use an external heat exchanger and pump-around loop. Though there would be no capacity limits for such a system, cycling organisms between warm and cold areas and between high DO and low DO areas may be deleterious to some organisms. On the other hand, tank internals may present CIP issues. We remain ready to help you navigate through such decisions.
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