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"Symbols: A = cross-sectional area, ft2; d = tube diameter, ft; g = 32.17 ft/s2, L = tube length, ft; m = film thickness, ft; RL = liquid volume fraction (dimensionless); qL = liquid rate, ft3/s; t = time, s; z = plate gap, ft; T = liquid wetting rate, lb/s - ft; pL = liquid density, lb/ft2, pv = vapor density; lb/ft3; n = liquid viscosity, lb/ft s; y = local weight fraction of vapor (dimensionless). Source: Refs. 1 and 2.

"Symbols: A = cross-sectional area, ft2; d = tube diameter, ft; g = 32.17 ft/s2, L = tube length, ft; m = film thickness, ft; RL = liquid volume fraction (dimensionless); qL = liquid rate, ft3/s; t = time, s; z = plate gap, ft; T = liquid wetting rate, lb/s - ft; pL = liquid density, lb/ft2, pv = vapor density; lb/ft3; n = liquid viscosity, lb/ft s; y = local weight fraction of vapor (dimensionless). Source: Refs. 1 and 2.

falls below that of a falling film evaporator. In practice, the film thickness may be less than estimated by either method because gravity and vapor momentum will act on the fluid at the same time.

Once the volume fraction is known, the liquid residence time is calculated by equation 5. In order to account for changing liquid and vapor rates, the volume fraction is calculated at several intervals along the channel length. Evaporation is assumed to be constant along with channel length except for flash due to high feed temperature.

Table 3 shows a comparison of contact times for typical four-effect evaporators handling 80 GPM of feed. The tubular designs are based on 2-in.-diameter by 30 ft. Designs using different tube lengths, incidentally, do not change the values for a rising film tubular system.

The given values represent total contact time on the evaporator surface, which is the most crucial part of the processing time. Total residence time would include contact in the preheater and separator as well as additional residence within interconnecting piping.

Table 3. Residence Time Comparison

Contact time

Rising film tubular

Rising film plate"

Falling film tubular

Falling film plate

1st effect

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