Gauge glass liquid circulation

Circulation 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. Slight 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. 

Circulation and heat transfer in this type of evaporator are strongly affected by the liquid level. Highest neat-transfer coefficients are achieved when the level, as indicated by an external gauge glass, is only about halfway up the tubes. Slight reductions in level below the opti-... 

Figure 9.2 Liquid circulation through a gauge glass.

In this technique much of the equipment shown in Fig. 2 is used, but the distilling flask and reflux condenser are replaced by an isoteniscope mounted in a glass thermostat as shown in Fig. 3. The water in the bath should be stirred vigorously (or circulated) to ensure thermal equilibrium. Place the liquid to be studied in the isoteniscope so that the bulb is about two-thirds full and there is no liquid in the U tube. Then place the isoteniscope in the thermostat (which should be at room temperature) and connect it to the ballast bulb and pressure gauge (which are assembled and connected as in Fig. 2). 

---