Plant water vapor transportation, valve

Fig. 1.86. Scheme for the estimation of the water vapor transport in a freeze-drying plant. 1, Frozen product 2, vial or the end of a shelf 3, open surface (FI) for the water vapor flow between 2 and 4 4, chamber wall 5, valve with an open area F2 6, condenser chamber 7, cooling and condensing surface in the con-... 

In an expediently designed plant, one can expect to reach in the pressure range above 8 10-2 mbar a vapor velocity in the cross-section Fk of between 50 and 80 m/s (l/d = 2.5 to 5) However 90 m/s will be reached only, if the design uses special features, e. g. a funnellike connection between the chamber wall and the location of the valve, slow changes in the outline, and smooth surfaces without sharp edges or holes. It is also recommendable, to clarify the maximal amount of vapor transportable at several pressures in a plant specification, e. g. at pch = 1 mbar a minimum of 3 kg/h and at pch = 0.04 mbar a minimum of 25 g/ h flow of water vapor must be demonstrated during the acceptance test, while the condenser temperature is below -30 °C, respectively below -57 °C. Such measurements can be earned out practically with sufficient accuracy (see e. g. Fig. 2.19 and the related text). 

In pilot freeze-drying plants the connection between the chamber and the condenser has usually a diameter large enough for the transport of the water vapor. In large production plants this diameter has to be estimated. Theoretically the maximum flow density (g/h cm2) is achieved if the following conditions are met The diameter of the connection d has to be large, compared with its length /, / J, a smooth outlet from the chamber and a smooth inlet into the condenser with no obstacles in the connection (e.g., no valve plate) and s This flow density... 

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