Vapor head

Calculated from measured steam chest and vapor-head pressures. 

The heating surface usually determines the evaporator cost and the vapor head the space requirements. The vapor—Hquid separator must have enough horizontal plan area to allow the bulk of the initial entrainment to settle back against the rising flow of vapor and enough height to smooth out variations ia vapor velocity and to prevent splashing directly iato the vapor outlet. Separators are usually sized on the basis of the Souders-Brown expression ... 

Vapor-Liquid Separation This design problem may be important for a number of reasons. The most important is usually prevention of entrainment because of value or product lost, pollution, contamination of the condensed vapor, or fouling or corrosion of the surfaces on which the vapor is condensed. Vapor-liquid separation in the vapor head may also oe important when spray forms deposits on the w ls, when vortices increase head requirements of circulating pumps, and when shoiT circuiting allows vapor or unflashed liquid to be carried back to the circulating pump ana heating element. 

Highest heat-transfer coefficients are obtained in FC evaporators when the liquid is aUowed to boil in the tubes, as in the type shown in Fig. 11-122 7. The heating element projects into the vapor head, and the hquid level is maintained near and usuaUy slightly below the top tube sheet. This type of FC evaporator is not well suited to salting solutions because boiling in the tubes increases the chances of salt deposit on the waUs and the sudden flashing at the tube exits promotes excessive nucleation and production of fine ciystals. Consequently, this type of evaporator is seldom used except when there are headroom hmitations or when the hquid forms neither salt nor scale. 

In a submerged-tube FC evaporator, all heat is imparted as sensible heat, resulting in a temperature rise of the circulating hquor that reduces the overall temperature difference available for heat transfer. Temperature rise, tube proportions, tube velocity, and head requirements on the circulating pump all influence the selec tion of circulation rate. Head requirements are frequently difficult to estimate since they consist not only of the usual friction, entrance and contraction, and elevation losses when the return to the flash chamber is above the liquid level but also of increased friction losses due to flashing in the return line and vortex losses in the flash chamber. Circulation is sometimes limited by vapor in the pump suction hne. This may be drawn in as a result of inadequate vapor-liquid separation or may come from vortices near the pump suction connection to the body or may be formed in the line itself by short circuiting from heater outlet to pump inlet of liquor that has not flashed completely to equilibrium at the pressure in the vapor head. 

Knitted wire mesh serves as an effective entrainment separator when it cannot easily be foiiled by sohds in the liquor. The mesh is available in woven metal wire of most alloys and is installed as a blanket across the top of the evaporator (Fig. ll-122d) or in a monitor of reduced diameter atop the vapor head. These separators have low-pressure drops, usually on the order of 13 mm [ M in) of water, and collection efficiency is above 99.8 percent in the range of vapor velocities from 2.5 to 6 iti/s (8 to 20 ft/s) [Carpenter and Othmer, Am. nsi. Chem. 

Single-Effect Evaporators The heat requirements of a singleeffect continuous evaporator can be calculated by the usual methods of stoichiometry. If enthalpy data or specific heat and heat-of-solution data are not available, the heat requirement can be estimated as the sum of the heat needed to raise the feed from feed to product temperature and the heat required to evaporate the water. The latent heat of water is taken at the vapor-head pressure instead of at the product temperature in order to compensate partiaUv for any heat of solution. If sufficient vapor-pressure data are available for the solution, methods are available to calculate the true latent heat from the slope of the Diihriugliue [Othmer, Ind. Eng. Chem., 32, 841 (1940)]. 

Hvp = the Vapor head of the fluid expressed in feet. It is a fiinetion of the temperature of the liquid. See Properties of Water 11 in this chapter. 

To determine the Ha, atmospheric head, you only need observe the vessel being drained by the pump. Is it an opened, or vented atmospheric vessel Or is it a dosed and scaled vessel If the vessel is open, then we begin with the atmospheric pressure expressed in feet, which is 33.9 feet at sea level. The altitude is important. The atmospheric pressure adds energ) to the fluid as it enters the pump. For closed un-pressurized vessels the Ha is equal to the Hvp and they cancel themselves. For a dosed pressurized vessel remember that every 10 psia of pressure on a vessel above the vapor head of the fluid will add 23.1 feet of Ha. To the Ha, we add the Hs. 

The Hvp, vapor head, is calculated by ob.serving the fluid temperature, and then consulting the water properties graph in this chapter. Let s say we re pumping water at 50° F (10° C). The Hvp is 0.411 feet. If the water is 212° F (100° C) then the Hvp is 35.35 feet. The vapor head is subtracted because it robs energy from the fluid in the suction pipe. Remember that as the temperature rises, more energy is being robbed from the fluid. Next, we mu.st subtract the Hf... 

Hvp Vapor Head. It is based on the feed water temperature. See Chapter 2, Properties of Water I and II. 

Figure 13. Steam outside tube evaporator (A) Shell (B) Tube sheets (C CJ Distributing plates (D) Vapor head (E) Baffles (F) Steam inlet (G) Condensate outlet (H) Non-condensed gas vent (J) Thick liquor outlet (K) Vapor outlet (L) Liquor feed box.

This section addresses the effects of BLEVE blasts and pressure vessel bursts. Actually, the blast effect of a BLEVE results not only from rapid evaporation (flashing) of liquid, but also from the expansion of vapor in the vessel s vapor (head) space. In many accidents, head-space vapor expansion probably produces most of the blast effects. Rapid expansion of vapor produces a blast identical to that of other pressure vessel ruptures, and so does flashing liquid. Therefore, it is necessary to calculate blast from pressure vessel mpture in order to calculate a BLEVE blast effect. 

Fig. 3. Triple-effect steepwater evaporator. The third effect (forced circulation) Is shown in background second effect (falling film, recirculating) is middle unit. The first effect vapor head is shown in foreground. (Swenson. Whiting Corp)...

These corrosion data indicate that deaeration of the sea water is essential to long life of steel sea water evaporators. Where reasonable corrosion allowances can be made, as in piping, vapor heads, etc., steel is the most practical material of construction. Its only uncertain application is for the evaporator tubes, which must be made thin for good heat conduction. Only long runs in a continuously operating plant can prove whether or not steel is the most satisfactory material for tubes. 

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