Fouling evaporators

The maximum Inlet capacity of a single-stage rotary lobe compressor is about 30,000 cubic feet per minute which is equivalent to 60,000 Ib/hr of water vapor at atmospheric pressure. Positive-displacement compressors pump against whatever resistance Is developed by the evaporator discharge pressures will increase and evaporator rate will remain relatively constant as the evaporator becomes fouled. Evaporator capacity may be varied when using positive-displacement compressors by ... 

Work in connection with desahnation of seawater has shown that specially modified surfaces can have a profound effect on heat-transfer coefficients in evaporators. Figure 11-26 (Alexander and Hoffman, Oak Ridge National Laboratory TM-2203) compares overall coefficients for some of these surfaces when boiling fresh water in 0.051-m (2-in) tubes 2.44-m (8-ft) long at atmospheric pressure in both upflow and downflow. The area basis used was the nominal outside area. Tube 20 was a smooth 0.0016-m- (0.062-in-) wall aluminum brass tube that had accumulated about 6 years of fouhng in seawater service and exhibited a fouling resistance of about (2.6)(10 ) (m s K)/ J [0.00015 (fF -h-°F)/Btu]. Tube 23 was a clean aluminum tube with 20 spiral corrugations of 0.0032-m (lA-in) radius on a 0.254-m (10 -in)... 

Circffiation and heat transfer in this type of evaporator are strongly affected by the liquid level. Highest heat-transfer coefficients are achieved when the level, as indicated by an external gauge glass, is only about halfway up the tubes. Shght reductions in level below the optimum result in incomplete wetting of the tube walls with a consequent increased tendency to foul and a rapid reduction in capacity. When this type of evaporator is used with a liquid that can deposit salt or scale, it is customary to operate with the liquid level appreciably higher than the optimum and usually appreciably above the top tube sheet. 

Evaporators—These usually utilize a fin-tube design. Spirally finned tubes of 1.25 in to 2 in outer diameter (OD) with three to six fins per ineh are eommon. In the ease of unfired designs, earbon steel eonstruetion ean be used and boilers ean run dry. As heavier fuels are used, a smaller number of fins per ineh should be utilized to avoid fouling problems. 

Evaporation, Heat exchange, condensing vapours U.sually not May become necessary when processing thermally sensitive materials or if fouling of heat-transfer surfaces is possible. 

In forced circulation evaporators the liquid is pumped through the tubes. They are suitable for use with materials which tend to foul the heat transfer surfaces, and where crystallisation can occur in the evaporator. 

Evaporator type Very viscous > 1000 Medium viscosity < 1000 max Low viscosity < 100 Foaming Scaling or fouling Crystals produced Solids in suspension ... 

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