Towers are classified according to the type of “internals” in the tower, and according to the function performed by the tower.
Towers are similar to vertical process vessels in that they are erected vertically and they are cylindrical in shape with heads at each end of the cylinder. Towers are, however, normally much taller then vertical process vessels. Typically the length to diameter ratio of a tower ranges from 3:1 to 20:1. Towers typically range in diameter from 3 to 20 FEET and in height from 20 to 150 FEET.
Towers are commonly used for the following purposes:
A description of these items follows.
Trays — Trays may be divided into two major categories; crossflow trays and counter flow trays. Crossflow trays get their name because liquid flows across the tray to a downcomer while vapor rises through perforations in the tray deck. There are three types of crossflow trays in common use today. They are the bubble cap, sieve tray, and valve tray. The bubble cap trays were used almost exclusively until about 1950. Since then, the use of bubble cap trays has almost disappeared because their complicated construction makes them heavy (resulting in heavier and more expensive tray supports) and expensive to fabricate.
Bubble cap trays get their name because vapor rises through holes in the tray and is collected underneath bubble caps. Each cap has slots in it through which the vapor from the tray below bubbles into the liquid on the tray.
Sieve trays are the cheapest trays to fabricate because of their simple design. They consist of a perforated plate through which vapor rises from the tray below, a weir to hold a liquid level on the tray, and a downcomer which acts as a downspout to direct the liquid to the tray below. The operation of the sieve tray depends on the vapor velocity through the perforations being high enough to keep the liquid flowing across the tray and not down through the same perforations the vapor is rising through. The drawback to the sieve tray is that it has a narrow operating range compared to the bubble cap tray and the valve tray. Too low a vapor velocity and the liquid falls through the holes to the plate below - a condition called dumping. Too high a velocity and vapor doesn’t bubble through the liquid on the tray. Instead, the vapor pushes the liquid away from the hole so that there is no liquid-vapor contact. This condition is called coning.
Valve trays have liftable caps which operate like check valves. These caps make valve trays more expensive than sieve trays but they also increase the operating range of the tray. At low vapor velocities, the caps close and prevent dumping. The other major category of trays is the counterflow type. These trays have no downcomers. The liquid falls through the same openings in the tray that the vapor from the tray below rises through. This type of tray is not widely used. The most popular of the counterflow type tray is the Turbogrid tray.
Packings — The second major category of tower internals is packings. Packings serve the same purpose as trays; they bring a gas or vapor stream into intimate contact with a liquid stream. Trays accomplish this by providing a very large wetted surface area for the gas or vapor to flow by. Packed towers would normally be selected instead of tray towers in the following instances:
The major disadvantages of packed towers are:
The most common types of packings are: Rashig rings, Berl saddles, Intalox saddles and Pall rings.
Adsorption towers are packed towers; however, their function is to transfer a material from the liquid or gas phase onto the surface of the solid adsorbent. Adsorbents are not packing types.
Adsorbents are generally either a granular material or else spherical or cylindrical shaped pellets. Some common adsorbents are: Fuller’s earthes (natural clays), activated clay, alumina, activated carbon and silica gel.
The tower shell and heads are usually fabricated out of carbon or low alloy steel plate.
As the name implies, the primary alloying element in carbon steel is carbon. All the other alloying elements in carbon steel are limited to concentrations less than 0.5%. The most common materials of construction for towers are the carbon steels A515 and A516.
Low alloy steel contain one or more alloying elements besides carbon in concentrations from 0.5% to 10%. Alloying elements in concentrations greater than 10% make the steel a high alloy steel.
When extremely corrosive materials are to be handled, the tower may be fabricated out of a high alloy steel such as one of the stainless steels, a non-ferrous metal such as titanium or monel, or a non-metal such as FRP (fiberglass reinforced polyester). However, because these materials are either very expensive or else have design limitations such as low strength, claddings and linings are commonly used for corrosion resistance. Clad plate consists of a thin layer of corrosion-resistant metal permanently bonded to an inexpensive carbon or low alloy steel backing. Linings differ from claddings in that there is not a permanent continuous bond between the corrosion-resistant material and the backing material, and the corrosion-resistant material is usually not a metal. Common lining materials are brick, cement, rubber and glass.
Typically, many companies normally require that tower shells and heads be designed according to the latest edition of Section VIII Division 1 of the ASME Boiler and Pressure Vessel Code. Towers manufactured in the United States will carry the ASME code stamp certifying that the vessel has been designed and fabricated to code standards. Towers manufactured outside the United States are to be designed and fabricated according to code standards as well, but need not carry the code stamp.
Towers that are unusually large, or towers which are required to operate at a very high pressure may be designed according to Section VIII Division 2 of the ASME Code. Division 2 requires complete stress analysis of the process vessel. This complete analysis allows the vessel to be designed with much smaller safety factors. This results in a vessel which has a thinner shell and head and is therefore cheaper to fabricate than the same vessel designed according to the rules of Division 1. Since a Division 2 design results in a cheaper vessel, why aren’t all process vessels designed according to the rules of Division 2? Again it is a question of economics. A Division 2 design is so complex that the money spent in extra engineering time for the vessel can easily exceed the savings realized in the fabrication of the vessel. Only in very large or thick-walled vessels is the economic advantage of Division 2 clear-cut.
“Towers” and “columns” are interchangeable name for the same device. These devices have one of two functions. One is to separate a mixture into two or more desired parts. The other function is to transfer a material from one phase to another phase.
Towers are classified according to the function performed. Examples are distillation, stripping or extraction. Towers are also classified by the type of device installed inside (internals) so the tower can perform its desired function. Tower internals consist of either trays or packings.
Towers are always erected vertically. They are usually tall and cylindrical in shape. Sometimes they are designed with the top of the tower one diameter and the bottom a different (usually larger) diameter. This gives the tower a “Coke bottle” shape and is called a double diameter tower.
The cylindrically shaped body of the tower is called the shell. The shell is closed at both ends with dome-shaped covers called heads. There are three head designs in common use:
Which kind of head to use is an economic decision. The torispherical head is the cheapest to fabricate, but is the thickest for a given pressure. The ellipsoidal head is more expensive to fabricate than the torispherical, but is thinner at the same pressure. The hemispherical head is the most costly to fabricate, but is the thinnest for a given pressure. Thus, the material cost decreases from the torispherical to hemispherical because the head gets thinner, but the fabricating costs increase. At pressures below 150 PSIG the torispherical head is generally the cheapest. From 150 PSIG to 500 PSIG, the ellipsoidal is usually selected. Above 150 PSIG, the hemispherical head becomes an economically viable alternative.
Openings are provided in the shell and heads of a tower so that process fluids can enter and leave. Other openings in the tower are provided for drains, purge connections and sample connections. These openings into the tower are called nozzles. Nozzles range in diameter from 1 INCH for small drains, vents and sample connections to 24 INCHES [1,200 MM] or more for large process connections. The small (1 INCH) connections are usually made with pipe couplings, not with welding necks and flanges.
Workers must be able to enter the tower after it is erected to install and maintain the internals. Openings in the tower provided for this purpose are called manholes or manways. Manholes are just nozzles large enough for a man to pass through. Manholes range in diameter from 18 - 24 INCHES [1,200 MM].
A tower is normally supported by a steel cylinder the same diameter as the tower called a skirt. The skirt is welded to the tower at one end and bolted to the foundation at the other.
In addition to nozzles, manholes and skirts, other appurtenances may be attached to the tower. These other externals may include insulation clips for the support of insulation, lifting lugs which are eyelets to which rigging is attached so that the tower can be lifted and placed on its foundation, and various structural steel members for the support of platforms and ladders.