L

280° F Compressor

Work 18 Btu/lb

Desuperheat spray

Enthalpy 1155 Btu/lb Pressure 18.7 psia Latent heat 963.6 Btu/lb

Condensate

Enthalpy 1155 Btu/lb Pressure 18.7 psia Latent heat 963.6 Btu/lb

Pressure 14.7 psia Enthalpy 1151 Btu/lb Latent heat 970.6 Btu/lb

Evaporator

Effective steam economy 970 + 18 = 54

Efficiency of drive & compressor 65 -» 72% Inefficiency due to by-pass 5 10%

.'.Actual effective steam economy 32 -» 35 Effects

Pressure 14.7 psia Enthalpy 1151 Btu/lb Latent heat 970.6 Btu/lb

Evaporator

Figure 20. Mechanical recompression evaporator.

steam side of the evaporator is 18.7 psia. This compressed water vapor condenses and gives up its latent heat, thus vaporizing more water from the liquid that is being concentrated. The latent heat, thus vaporizing more water from the liquid that is being concentrated. The latent heat of vaporization of water at atmospheric pressure is 970 Btu/h. Note that it requires a theoretical energy input of only 18 Btu/lb to raise the water vapor from 14.7 to 19.2 psia. The theoretical steam economy, therefore, is 970/18 = 54. When compressor efficiency is taken into account, this figure is brought down to between 32 and 35, which is another way of saying that the MVR system is equivalent to a 32-35-effect steam evaporator. Related to energy costs of 4 cents per kilowatt-hour for electricity for the compressor drive and $6.00 per 1000 lb of steam for a conventional steam evaporator, the MVR system then becomes the economic equivalent of just under a 19-effect evaporator.

The MVR has another definite advantage over steam. The condensate is available at high temperature and is ideal for evaporator feed preheating, particularly if the condensate rate is as high as 90% of the feed rate, ie, a 10:1 concentration ratio within the evaporator. There are many such evaporators in operation where the sole energy input to the system is through the compressor with steam requirements limited to approximately 15 min during startup.

Although Figure 20 illustrates an evaporator system that uses a compressor, it will be shown that the use of a turbo fan in place of the centrifugal compressor not only yields lower boost but also provides more surface area and better operating economics.

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