• Minimum versus average wall thickness. Minimum wall tubes cost a bit more than average wall tubing. When it’s unnecessary to use minimum wall tubing, such as for high pressure or corrosive service where metal loss is anticipated, it may be more economical to permit the use of average wall welded tubing and specify additional NDE or corrosion evaluation of the tube seam.
• Tube pattern. Shell and tube heat exchangers are the workhorses of the chemical industry. These units typically are fabricated with one of four types of tube patterns — 30°, 60°, 45° and 90° (Figure 4). Duty, pressure drop, cleanability, cost and vibration all depend on which pattern is chosen. Consider process needs, not cost, when making the selection.
A 30° or 60° pattern is laid out in a triangle configuration. The main benefit is that approximately 10% more tubes can fit in the same area as a 45° or 90° pattern. There’s very little difference between the 30° and 60° patterns. Often a thermal designer will run analyses of both patterns and select the one that provides the best pressure drop and vibration results. The disadvantage of a 30° or 60° pattern is that it’s difficult to mechanically clean on the shell side. Therefore, such a pattern is chosen for cleaner services; frequently the bundle isn’t removable.
Figure 4 – More tubes can fit with 30°
The 45° or 90° pattern is selected if shell-side mechanical cleaning is required. Such a pattern also requires a removable bundle. The 45° is more common than the 90° because it provides more shell-side flow disturbance, which improves heat transfer. A 90° pattern is used to reduce pressure drop at the expense of duty and often is employed in boiling service to enable better vapor disengagement.
Make the right choices
In today’s chemical industry, too many engineers given the task of specifying welded equipment such as vessels, heat exchangers and tanks aren’t well versed in what’s necessary to develop an economic design that provides suitable safety and performance. Myriad choices must be made — and each will incrementally add to the final cost and schedule. When looking for savings, cutting the wrong corners may turn out to be very costly over the equipment’s service life.
Chip Eskridge is a principal engineer with Jacobs, Louisville, Ky. Mike James is a senior consultant, materials engineering, for DuPont, Houston. Steve Zoller is director of fabricated equipment for Enerfab, Cincinnati. Reach them via e-mail at firstname.lastname@example.org, Michael.M.James-1@USA.dupont.com and Steve.Zoller@enerfab.com.
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