Having reviewed the design of all existing structural elements, the digester structure was divided into areas of similar heat transfer characteristics, including the roof and above grade walls, below grade walls and floor slab. To develop the overall picture of the digester’s heat requirements under seasonal variations, the outdoor temperature was assumed to vary from -35oC – 40oC (for winter and summer, respectively).
Soil temperature and sludge recirculation pump flow were assumed to be constant over the entire review period. It was also assumed that heat dissipation from the roof and above grade walls due to radiation was a relatively small value. It was, therefore, accounted for in the applied safety factor.
Thermodynamic analyses conducted on the digester structure showed that heat dissipation does not vary significantly under seasonal ambient temperature variations. For proper functioning of the digestion process, recovery of the sludge temperature, due to digester structure heat loss, is the key factor. The temperature drop can be recuperated by transferring heat to the sludge in the heat exchanger.
Heat losses calculated for each season’s temperature (from -35oC – 40oC, with a 5oC increment) were presented as sludge temperature rises that should be provided in the heat exchanger in order to compensate for outward heat dissipation.
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The temperature rises needed when the recirculation pump is operating at the rated flow are shown in Figure 1. For comparison purposes, and in order to understand the heat loss trends, temperature rises were also calculated for several lower flows (85%, 80% and 70% of the rated flow).
A “24-hour shut-off” scenario, in which no heat is provided to the digester for 24 hours, was analyzed and is presented in Figure 2.
Digester structure heat loss distribution between the above grade and below grade structures was also estimated and presented in Figure 3. This shows the heat loss of the below grade structure throughout the year, as a percentage of the total digester heat loss.