Leveraging Fermentation Heat Transfer Data to Better Understand Metabolic Activity - - BioPharm International
Leveraging Fermentation Heat Transfer Data to Better Understand Metabolic Activity
 Apr 1, 2008 BioPharm International Volume 21, Issue 4

The overall heat transfer coefficient is determined through the summation of resistances to heat transfer. The overall resistance is the inverse of the overall heat transfer coefficient.3

in which:

U = overall heat transfer coefficient (W/m2 – °K)

h1 = heat transfer coefficient of broth (W/m2 – °K)

df = thickness of fouling layer (m)

kf = heat conductivity of fouling layer (W/m – °K)

dw = wall thickness of cooling coil or wall (m)

kw = heat conductivity of cooling coil or wall (W/m – °K)

h2 = heat transfer coefficient of cooling water (W/m2 – °K)

 Figure 1. Raw heat removal data from the fermentation
Equation 6 can be better visualized if the concept of electrical resistance is brought to mind. The electrical resistance of a circuit can be determined by adding each of the ohm ratings for individual resistors in series to determine the total resistance. The individual resistances to heat transfer also follow that same concept. Equation 6 represents the resistance to heat transfer for the fermentation broth, the fouling layer on the wall of the coil or jacket, the coil, or jacket material, and the cooling water itself. When the overall heat transfer coefficient is determined experimentally, it is usually lumped together with the surface area available for heat transfer to create a term known as UA. The major challenge with using UA for these calculations is that as the fermenter continues to be used, the fouling layers will build up and change the UA values. The UA values also change as the fermenter fills during the run. It is instead much easier to measure the amount of heat removed by the jacket or coils using a heat balance on the coolant. If the flow rate and the temperature in and out for the coolant can be measured, Equation 1 can be again used to determine the amount of heat removed from the fermenter into the coolant stream in this case. The ability to measure the flow rate of the coolant and temperature as it enters and exits the jacket or coil is required for this calculation. The heat removal determined from this calculation would look similar to Figure 1.

Initially, there is a negative value for heat removal because the fermenter is being heated. The heating is necessary because the heat losses outweigh the heat inputs to the fermenter.