PRESSURE DROP IN EVAPORATORS

Pressure drop is one of the process variables which have the greatest effect on the size (cost) of an evaporator. Selection of optimum pressure drops involves consideration of the overall process. While it is true that higher pressure drops result in smaller equipment, investment savings are realized only at the expense of operating costs. Only by considering the relationship between operating costs and investment can the most economic pressure drop be determined. 

Pressure drop correlations are available which enable pressure drop to be confidently predicted. 

---

Quiben and Thome (2007a,b) presented an experimental and analytical investigation of two-phase pressure drops during evaporation in horizontal tubes. Experiments were performed under diabatic conditions in tubes of d = S and 13 mm in the range of vapor quality x = 0—1, mass velocity G = 70—700kg/m s, heat flux q = 6.0—57.5 kW/m. The test fluids were R-134a, R-22 and R-410A. The results... 

Exit air usually is maintained far from saturated with moisture and at a high temperature in order to prevent recondensation of moisture in parallel current operation, with a consequent lowering of thermal efficiency. With steam heating of air the overall efficiency is about 40%. Direct fired dryers may have efficiencies of 80-85% with inlet temperatures of 500-550°C and outlet of 65-70°C. Steam consumption of spray dryers may be 1.2-1.81bsteam/lb evaporated, but the small unit of Table 9.19(b) is naturally less efficient. A 10% heat loss through the walls of the dryer often is taken for design purposes. Pressure drop in a dryer is 15-50 in. of water, depending on duct sizes and the kind of separation equipment used. 

Loss of pressure drop due to restrictions and friction, the pressure drops in the pipelines. This causes the boiling temperature to decrease and thus a fraction of hydrogen to evaporate. This part can be calculated in dependency of the vapor fraction x ... 

For ordinary laboratory work, a modified Hickman still (Fig. 1-39) is convenient. Volatile impurities and gases present in the liquid to be distilled must first be removed so that frothing wiU not occur in the still. This is done by placing the liquid in an ordinary flask, which is then evacuated and gently warmed. After all of the volatile impurities have been removed, the liquid is transferred to the still. The latter is connected to the vacuum system by means of a large connector in order to minimize pressure drop in the system. The still is heated by an aluminum block which is bored to accommodate the bottom of the still to a depth of about 15 mm and which is wrapped with nichrome wire serving as a heater. The wire is best insulated from the block by asbestos paper, and a layer of asbestos paper should be placed between the still and the aluminum block. As the liquid is heated, it evaporates and collects on the condenser from which it drips... 

Calculate the frictional pressure drop in the evaporation zone of the evaporator tube in exercise 4.4. 

Oil applications are based on pressure atomization technology even if small power application solutions are available based on oil evaporation by heat. Electronic control of the fuel/air ratio previews an actuator to move a butterfly for controlling the air flow and an actuator to move the oil pressure regulator. For gas, two actuators control two butterflies, one for gas and one for air, even if fhe gas pneumatic valves (whose working principle is based on a pneumatic feedback) are widely used. More advanced burners control air flow changing the fan speed, reducing the pressure drop in the hydraulic pipes and valves. Despite this, a butterfly valve is often necessary to control startup operation, especially for high power applications. 

Pressure. The location of pressure sensors in evaporative crystallizers also affect their reliability. If the operating pressure measurement is made in the vacuum line or condenser, it will be affected by the evaporation rate of the solvent and pressure drop changes due to any incrustation in the lines or mist eliminator. Measurement of the pressure in the vessel headspace circumvents this interaction. The sensor should be equipped with a solvent flush line to ensure that incrustation does not distort the measurement. 

The method of supplying the electric flask heater with a constant current can be unreliable since it involves frequent checking of the load, especially if there are large differences in the heats of evaporation of the components being distdled. As the pressure drop in a column depends on the vapour velocity (section 4.11), it provides a convenient means for controlling the heater. The method also has the advantage of not being seriously affected by mains fluctuations or by variations in the gas pressure [13]. 

---