Static vapor head

This unit is usually used as the reboiler for the distillation column and, in this service, operates by the thermosiphon action of the difference in static head in the column and in the vapor-liquid phase leaving the reboiler. When tied into the bottom chamber, the liquid is usually recirculated many times, vaporizing only 10-25% of the reboiler feed per pass however, when used as a draw-off from the bottom tray seal pan, the feed to the reboiler is not recirculated flow. The basic operation is the same, however. 

The absolute pressure at the inlet to the pump is usually the atmospheric pressure in the receiver, plus the static head from the water surface to the pump inlet and minus the friction loss through the pipes, valves and fittings joining the pump to the receiver. If his absolute pressure exceeds the vapor pressure of water at the temperature at which it enters the pump, then a net positive suction hand (NPSH) exists. If this NPSH is above the value specified by the pump manufacturer, the water does not begin to boil as it enters the pump suction and cavitation is avoided. If the water entering the pump is at a higher temperature, its vapor pressure is increased and a greater hydrostatic head over the pump suction is needed to ensure that the necessary NPSH is obtained. 

The net head or pressure measured in ft. or m that causes a liquid to flow through the suction side of a pump, enter the pump chamber, and reach the impeller. When the source of liquid is above the pump, NPSH equals the barometric pressure plus the static head, less the entrance head, frictional losses in the suction piping and vapor pressure of the liquid. When the source of liquid is below the pump, NPSH equals the barometric pressure less the static head, entrance head, frictional losses in the suction piping and vapor pressure of the liquid. NPSH is specific for each pump design and application and must be supplied by the manufacturer. 

This suggests that the pressure in a water drain can get so low, that air could be sucked out of the bathroom and down the drain. Of course, we all see this happen several times a day—typically when we flush a toilet. So much air is drawn into the water drainage piping, that we install vents on our roofs, to release this air. The only requirement, then, for vapors to be drawn into a flowing nozzle is for the nozzle exit loss to be larger than the static head of liquid above the nozzle. 

Circulation is promoted by the difference in static heads of supply liquid and the column of partially vaporized material. The exit... 

Thermosiphon Rehoiler — Flow of the vaporizing fluid depends upon the difference tit static head between the column of liquid flouing from the tower to Ihe rehoiler and the partially vaporized column of liquid returning from the exchanger to the unset. 

When liquids are being pumped, it is important to keep the pressure in the suction line above the vapor pressure of the fluid. The available head measured at the pump suction is called the net positive suction head available (NPSHA). At sea level, pumping 15°C (60°F) water with the impeller about 1 m below the surface, the NPSHA is about 9.1 m (30 ft). It increases with barometric pressure or with static head, and decreases as vapor pressure, friction, or entrance losses rise. Available NPSHA is the characteristic of the process and represents the difference between the existing absolute suction head and the vapor pressure at the process temperature. The required net positive suction head required (NPSHR), on the other hand, is a function of the pump design (Figure 2.121). It represents the minimum margin between suction head and vapor pressure at a particular capacity that is required for pump operation. Cavitation can occur at suction pres-... 

Circulation is promoted by the difference in static heads of supply liquid and the column of partially vaporized material. The exit weight fraction vaporized should be in the range of 0.1-0.35 for hydrocarbons and 0.02-0.10 for aqueous solutions. Circulation may be controlled with a valve in the supply line. The top tube sheet often is placed at the level of the liquid in the tower. The flow area of the outlet piping commonly is made the same as that of all the tubes. Tube diameters of 19-25 mm diameter are used, lengths up to 12 ft or so, but some 20 ft tubes are used. Greater tube lengths make for less ground space but necessitate taller tower skirts. 

CORROSIVE DUE TO PUMPING TEMPEB TURE DEG E VAPOR PRESS PSIA GRAVITY AI. JbC E SP GR API GR SP GP rSUC COND t,P GR AT OIS COND VlvCOSiTY AT DEG F SUCTION PRESSURE - - PSiQ DIFFERENTIAL PRESSURE PSI DESIGN HEAD NPSH AVAIL FT NPSH REQUIRED FT NPSH INCLUDES fT FT STATIC HEAD — LIFT ... 

The pressure drop over the plates is an important design consideration. There are two main sources of pressure loss that due to vapor flow through the holes (an orifice loss) and that due to the static head of liquid on the plate. 

The head vapor was liquefied in a condenser cooled by a chilled glycol solution. The condensate, forming two liquid phases as expected, was collected in a static decanter separating a light phase rich in hexane and a heavy phase rich in water and alcohols. A vacuum pump connected to the condenser maintained the system under a reduced pressure in the order of 200 torr to lower all temperatures to the end of reducing polymerization in the falling film evaporator. 

The simple form of vacuum crystallizer has serious limitations from the standpoint of crystallization. Under the low pressure existing in the unit, the effect of static head on the boiling point is important e.g., water at 45 F has a vapor pressure of 0,30 in. Hg, which is a pressure easily obtainable by steam-jet boosters. A static head of 1 ft increases the absolute pressure to 1.18 in. Hg., where the boiling point of water is 84°F. Feed at this temperature would not flash if admitted at any level more than 1 ft below the surface of the magma. Admission of the feed at a point where it does not flash, as in Fig. 27.10, is advantageous in controlling nucleation. 

Reboiler circuits may set tower elevations. A thermo-syphon reboiler requires enough liquid static head to provide a driving force so that the reboiler will work properly. This head determines the circulation ratio and the amount of vapor returned to the tower, thereby setting the entire tower gradient. Reboiler circuits must be considered with pump NPSH considerations as they set the tower elevation. 

Horizontal reboilers, with natural circulation, have a simple circulation system. Liquid flow from an elevated drum, tower bottom or tower trapout boot through a downcomer pipe to the bottom of the exchanger shell. The liquid is heated and leaves the reboiler in the return piping as a vapor or vapor-liquid mixture and flows back tc the tower or drum. There is no pressure difference between the inlet and outlet nozzles. The circulation is forced by the static head difference between the two liquid columns (see Figure 7-72 Use the exchanger centerline as a reference line. 

FoulingJ insufficient static head/excess friction in the pipes/on the tubeside the outlet nozzle area < total tube area/on tubeside the inlet nozzle area < 0.5 total tube area/rate of vaporization > 25 % of circulation rate/mass rate of vaporization > mass rate of drculation/natural drculation rate < 3 X vaporization rate/vapor-ization induced solids, and see Sedion 1.12 for generic fouling. 

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