Downcomer loading

This discussion assumes that the cross-sectional area of the downcomer is adequate for reasonable vapor-liquid separation. If the downcomer loading (GPM/ft2 of downcomer top area) is less than 150, this assumption is okay, at least for most clean services. 

Adjust the downcomer flood approximately equal to the tray active area flood. With these two flood values equal or nearly equal, the tray is considered to have a balance of loading between the downcomer loading and the active tray area loading. This balance ensures that the tray will operate efficiently even if it has less than 50% flood loadings. Review these two flood values (downcomer and active area flood values) carefully and make adjustments, especially in new tray design, ensuring that these flood values are close to equal. 

There are two capacity limits related to liquid loading, which are the downcomer baekup limit and downcomer veloeity limit. The downcomer backup limit is set at 80% of liquid settling height based on the froth level. The downcomer velocity limit is 75% of maximum velocity allowed to avoid downeomer choke. The number of tray passes is the most important parameter affeeting the downcomer loading and thus these two downcomer limits. 

For 24-inch tray spacing, a maximum downcomer loading is 175 GPM per square foot of the downcomer top area. That s for clean services. For foaming services, I use 90 GPM. It does not hurt anything to make the downcomer too big. That is, there is no minimum downcomer loading. 

Certain boilers employ forced circulation, whereby a pump helps impart the circulation through the downcomer lines to the waterwaH header, particularly to improve or control circulation at low loads. Forced-circulation pumps are also required in high pressure and supercritical pressure boilers, because once the pressure within a boiler approaches the critical pressure, 22.1 MPa (3208 psia), the densities of the water and steam become similar, limiting or eliminating the potential for natural circulation. 

ADM = Minimum downcomer area, fT ATM = Minimum column cross-sectional area, fr CAF = Vapor capacity factor CAFo = Flood capacity factor at zero liquid load CFS = Vapor rate, actual ftVsec DT = Tower diameter, ft DTA = Approximate tower diameter, ft FF == Flood factor or design percent of flood, fractional FPL = Tray flow path length, in. 

VDdsg = Downcomer design velocity, GPM/fT Vioad = Column vapor load factor WFP = Width of tray flow path, in. pL = Liquid density, Ibs/ft pv = Vapor density, Ibs/ft ... 

GPM = Column liquid loading, gal/min Hj,. = Downcomer backup, inches of liquid hi = Condensing side film coefficient, Btu/hrft °F H,(j = Head loss under downcomer, inches of liquid H v = Weir height, ins. 

N,n = Minimum theoretical stages at total reflux Q = Heat transferred, Btu/hr U - Overall heat transfer coefficient, Btu/hrfP"F u = Vapor velocity, ft/sec U d = Velocity under downcomer, ft/sec VD(js = Downcomer design velocity, GPM/fL Vioad = Column vapor load factor W = Condensate rate, Ibs/hr Xhk = Mol fraction of heavy key component Xlk = Mol fraction of the light key component a, = Relative volatility of component i versus the heavy key component... 

A common type of distillation contacting device used in refinery applications is the sieve tray. In the early 50 s and for many years before, the bubble cap tray was the mainstay of the distillation field. A sieve tray consists of a flat plate with regularly spaced holes, normally 1/2 to 1 inch in diameter. Liquid flows horizontally across the tray and into a channel, called a downcomer, which leads to the tray below. The sieve tray exhibits good capacity, excellent efficiency, low pressure drop, and good flexibility i.e., it will operate quite efficiently at tower loadings which are 1/2 to 1/3 of design values. 

When how values exceed 1% to 2 in., consider special downcomers or down pipes to conserve cap area for high vapor loads. 

At high liquid loads (above 7-10 gpm/in.), downcomer flood is often the capacity limitation. This limitation is not predicted by the correlation. Caution is required. 

The ratio of oxygen in the economizer inlet to boiler waterwall downcomer must be controlled because this varies with load. The ratio typically is 4 1. 

Hydraulic load 600 N/m2 live load on the plate, plus 3000 N/m2 over the downcomer seal area. 

Effect of Downcomer Aeration. When only the central gas flows (No. 7 and No. 8 flows) were employed without downcomer aeration, the solids circulation rate depended primarily on the entrainment rate of the jets. The linear relationship for both bed materials (hollow epoxy and polyethylene) in Fig. 8 shows that the volumetric concentration of the solids inside the draft tube after acceleration (or the gas voidage) is approximately constant, independent of particle density. This can be readily realized by expressing the volumetric solid loading in the draft tube as follows ... 

A straight line relationship between LJpr and Gr as shown in Fig. 8 implies that the volumetric solid loading (j) is approximately constant because Ar is constant and er can be assumed to be approximately constant when the downcomer is not fluidized. More than 85% of the gas supplied through the central No. 7 and No. 8 flows in those experiments ends up in the draft tube as can be seen from the gas bypassing data presented in Fig. 7. 

In the spray regime, flooding (usually called jet flooding) is caused by excessive entrainment of liquid from an active area to the tray above. It increases the tray pressure-drop, and the entrained liquid recirculates to the tray below. The larger liquid load in the downcomer and the increased tray-pressure-drop together cause the downcomer to overfill so the tray floods. 

In the emulsion regime, there is little entrainment of liquid by the vapour. Instead, the high liquid load causes the downcomer to overfill and the tray to flood. 

An increase in reflux rate, assuming that the reboiler is on automatic temperature control, increases both the tray weir loading and the vapor velocity through the tray deck. This increases both the total tray pressure drop and the height of liquid in the tray s downcomer. Increasing reflux rates, with the reboiler on automatic temperature control, then will always push the tray closer to, or even beyond, the point of incipient flood. 

Weir load For trays (as distinct from downcomers), liquid load is normally defined as... 

Downcomer liquid load For downcomer design, the liquid load is usually defined as the liquid velocity at the downcomer entrance (m/s or ft/s) ... 

Antijump baffles (Fig. 14-24) are sometimes installed just above center and off-center downcomers of multipass trays to prevent liquid from one pass skipping across the downcomer onto the next pass. Such liquid jump adds to the liquid load on each pass, leading to premature flooding. These baffles are essential with proprietary trays that induce forward push (see below). 

Dual-Flow Trays These are sieve trays with no downcomers (Fig. 14-27b). Liquid continuously weeps through the holes, hence their low efficiency. At peak loads they are typically 5 to 10 percent less efficient than sieve or valve trays, but as the gas rate is reduced, the efficiency gap rapidly widens, giving poor turndown. The absence of downcomers gives dual-flow trays more area, and therefore greater capacity, less entrainment, and less pressure drop, than conventional trays. Their pressure drop is further reduced by their large fractional hole area (typically 18 to 30 percent of the tower area). However, this low pressure drop also renders dual-flow trays prone to gas and liquid maldistribution. 

The downcomer percent flood is a critical tray-loading factor. If it is over 90%, then tray flooding failure is likely. If it is below 20%, tray vapor blowthrough (downcomer side) is likely to happen. It is therefore important to keep the downcomer flood below 90% and above 20%. One more limiting downcomer flood value, the active area flood, is also important. The active area flood calculation is covered in the following section. 

Equations (3.89) and (3.90) equate the tray active area vapor loading VN to the maximum VM for determining the gas in liquid entrainment flooding of the tray. The early work of Souders and Brown [12], based on a force balance on an average suspended droplet of liquid, led to the definition of a capacity parameter VM- Both VN and Where refer to the active area of the tray. This active area is simply the net tower cross-section internal area less the downcomer areas. The downcomer areas include both the downcomer inlets and outlets. 

Tray liquid load definitions. For tray (as distinct from downcomer) design the liquid load is usually defined as... 

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