HEAT LOSSES
The heat loss that results from the evaporation of water is caused by the airflow entering the fermenter. The airflow is generally
not saturated with water, but becomes saturated as it passes through the fermentation broth. Heat is needed for this evaporation
to occur, and this is counted as a loss to the system. At this point, it is important that the moisture condition of the air
entering the fermenter is known. This can be determined by directly measuring the humidity of the air, or by tracing back
the air supply to the point where the specific conditions of the air stream is known. The conditions of the air at either
of those points can be used to identify the absolute moisture of the air using a psychrometric chart. The temperature of the
air exiting the fermenter is assumed to be at the same temperature as the broth, which allows for the absolute humidity of
the saturated exit air to be determined. The rate of water evaporation is determined by subtracting the water entering the
system in the air from the water leaving the fermenter in the exit air. The mass flow rate is used to determine the heat needed
to evaporate the water. This is calculated using Equation 4:
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in which:
RHev = heat of evaporation (W)
Cv = heat of evaporation (2.27 x 106 J/kg)
ΔH2O = water evaporation (kg/s)
The heat loss of the vessel greatly depends on the particulars of the vessel in question. The construction materials, tank
geometry, wall thickness, insulation use, and environmental conditions play roles in this determination. The heat loss is
generally only about 1–2% of the total heat balance, so this portion of the loss is ignored.
The heat loss caused by the increase of the broth temperature is another portion of the heat balance that rarely comes into
play. Most fermentations are held at a steady state, meaning that the temperature is maintained at a specified set point.
If the fermentation was not held at steady state, the effect of the varied temperature would have to be incorporated into
the heat balance.
The heat loss resulting from the cooling of a fermenter is the largest component of the negative portion of the heat balance
for most of the fermentation. Initially, when the fermenter has been inoculated, the losses to the environment, evaporation,
etc., outweigh the heat inputs, and the fermenter will have to be heated. Once the cells start to grow, the heating will have
to be switched to cooling. There are two primary methods that are used to cool a fermenter: coils, and jackets. At most fermentation
scales used in good manufacturing practices facilities, jackets are the sole cooling methodology used. At large scale (greater
than 20,000 L), both may be used in combination to maintain the steady-state temperature of the fermenter. The heat removal
by jackets and coils can be described by Equation 5:
in which:
RHC = heat flow due to cooling water (W)
LMTD = log mean temperature difference (°K)
A = cooling surface area (m2)
U = overall heat transfer coefficient (W/m2 – °K)
T
out
= outlet cooling water temperature (°K)
T
in
= inlet cooling water temperature (°K)
T
f
= fermenter broth temperature (°K)
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