Condensation

The largest energy cost for freeze drying is that for the refrigeration used to condense vapor as ice. This cost can be reduced by removing water before drying and by operating the condenser at the highest temperature that still reliably prevents collapse. Higher condenser temperatures can be safely used when the pressure drop caused by vapor flow is low, as occurs when extract granules are dried.

In addition to removing heat of condensation (roughly the same as the heat of sublimation), the refrigerator also removes heat the vapor picks up from the product and the heating plate and heat transferred to the condenser by radiation. Heat shields or condenser enclosures that do not significantly interfere with vapor flow are used to reduce transfer of radiant heat to the condenser.

The temperature at the outer surface of deposited condensed ice is higher than at the condenser wall itself. When air inleakage is small, transparent ice forms on the condenser and the temperature difference across the ice is small. If inleakage is moderate, milky ice forms, and the temperature difference is greater. When inleakage is excessive, porous frost that conducts heat poorly forms and the temperature on the outside of the frost is markedly higher than in the condenser. When this occurs, the condenser operating temperature may have to be reduced to prevent Pa from rising enough to cause collapse. This raises refrigeration costs. Accumulation of air around condensers is minimized by having the vapor contacting the condenser efficiently sweep air toward the vacuum pump.

The drier shown in Figure 3 has two condensers, an online one that condenses vapor as ice, and an off-line one undergoing de-icing. The two condensers cyclically change roles with the aid of a pressure-tight slide gate. Thus, de-icing takes place almost continually while drying occurs. Ice buildup on the active condenser is 2 to 3 mm, at most. Up to 17 mm of ice may deposit on condensers subjected to de-icing only at the end of drying.

The Mediterranean Diet Meltdown

The Mediterranean Diet Meltdown

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