Towers skirt

Very stable and easy to control. Has no two-phase flow. Allows low tower skirt height. This type is expensive, however. 

Rags and debris in tower skirts provide a fuel source. 

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. 

A vertical reboiler has very little piping and its length determines the height of the tower skirt. Supports at grade are saved but supports on the tower must be added. 

Skirts are used in vessels and towers. They transmit high axial and bending loads and offer favorable geometry for thermal gradients. In piping, when loads are beyond the capacity of lugs and tmnnions, skirts are often favored. 

Horizontal natl. circ. Ease of maintenance Lower skirt height than vertical Less pressure drop than vertical Longer tubes possible Less cost than kettle No theoretical tray Extra space and piping as compared to vertical Fouls easier as compared to vertical Accumulation of higher boiling point components in feed line, i.e., temperature may be slightly higher than tower bottom... 

Historical data on similar vessels and fractionation towers can best be used by correlating the costs of this equipment vs. weight. Many methods can be found in the literature for estimating the weight and costs of vessels and fractionators (References 8. 9, 10, and 11). Make sure the estimated weight is complete including skirt, ladders and platfonns, special internals, nozzles, and manholes. 

For the supports of self-supporting skirted columns, towers, and similar vessels by ... 

Figure 18.14. Methods of supporting vessels, (a) Saddle supports for horizontal vessels, usually of concrete, (b) Bracket or lug supports resting on legs, for either vertical or horizontal vessels, (c) Bracket or lug supports resting on steel structures, for either vertical or horizontal vessels, (d) Straight skirt support for towers and other tall vessels the bearing plate is bolted to the foundation, (e) Flared skirt for towers and other tall vessels, used when the required number of bolts is such that the bolt spacing becomes less than the desirable 2 ft.

FIG. 14-82 Fabricated costs and installation time of towers. Costs are for shell with two heads and skirt, but without trays, packing, or connections. (Peters and Timmerhaus, Plant Design and Economics for Chemical Engineers, 4th ed., McGraw-Hill, New York, 1991.)... 

The purchased cost for plate and packed towers can be divided into the following components (1) cost for shell, including heads, skirts, manholes, and nozzles (2) cost for internals, including trays and accessories, packing, supports, and distributor plates and (3) cost for auxiliaries, such as platforms, ladders, handrails, and insulation. 

Fabricated costs and installation time of towers. Costs are for shell with two heads and skirt, but without trays, packing, or connections. 

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