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tent) of 188.2 Btu/lb while steam at the same conditions has an enthalpy of 1153.4 Btu/lb. The difference between the vapor and liquid is the latent heat or 965.2 Btu/lb. In other words, in order to produce one pound of steam at 220°F, 17.2 psia, from water at 220°F, 17.2 psia, the addition of 965.2 Btu/lb of energy is required.

In the case of the MVR evaporator, however, the vapor at 17.2 psia, 220°F (enthalpy 1153.4 Btu/lb) is produced from vapor at 212°F, 14.7 psia (enthalpy 1150.5 Btu/lb). Theoretically, only 2.9 Btu/lb of energy must be added.

The compressor, a fan in this case, operates such that the entropy of the discharge vapor be at least as high as the entropy of the inlet vapor. Since inefficiencies in the compression cycle will result in an exit entropy above that of the inlet entropy, the temperature of the existing vapor and energy input to the compressor must be increased.

Figure 33 is an enthalpy—entropy diagram for water vapor. The vapor at 14.7 psia, 212°F, is shown at point A. During compressions the entropy remains constant (ideally) or increases (actually). With typical efficiencies for this duty, the discharge temperature may be expected to rise to 243°F where the enthalpy is 1164.6 Btu/lb. The energy input from the compressor is 14.1 Btu/lb.

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