Page 337 - Mechanical Engineers' Handbook (Volume 4)

326 Heat Exchangers, Vaporizers, Condensers

5.4 Rating, Design, or Simulation
Several types of solutions are possible by computer. The better standard programs allow the
user to choose. It is important to understand what the program is doing in order to properly
interpret the results. The above three types of calculations are described as follows.

Rating
This is the normal mode for checking a vendor’s bid. All geometry and all process conditions
are specified. The program calculates the required heat transfer area and pressure drop and
compares with the specified values. Normally this is done including the specified fouling
factor. This means that on startup the amount of excess surface will be greater, sometimes
excessively greater, causing severe operating adjustments. It is therefore advisable to review
clean conditions also.

Design
This mode is used by the process engineer to obtain a size based on process heat transfer
requirements. In this case, most of the geometry specifications still are not determined by
the program and must be determined by the designer based on experience. Required, but
unknown, specifications, in addition to the process requirements of temperatures, flow rates,
and pressure drops, include
• Exchanger type (shell and tube, plate-and-frame, plate-fin, air-cooled, etc.)

If shell and tube
• TEMA shell type (E, F, J, G, H, X, K)
• TEMA front and rear head types (flat, dished, fixed tube sheet, split ring, pull-through)
• Baffle type (segmental, double segmental, triple segmental, rod, etc.)
• Tube type (plain, low-finned, enhanced surface, etc.)
• Tube length (usually standard lengths of 8, 12, 16, 20 ft)
1
3
• Tube diameter (usually ⁄8, ⁄4,1,1 ⁄4 in. or 1.25 in.)
5
• Tube pitch (pitch ratios 1.25, 1.3, 1.5)
• Tube layout (30, 45, 60, 90 )
• Tube material (carbon steel, stainless steel, copper alloys, titanium, etc.)
• Exchanger orientation (horizontal, vertical)
As shown, even with a good computer program, an overwhelming number of combi-
nations of geometry parameters is possible and presently the engineer is required to select
the best combination based on mechanical considerations, process considerations, fouling
tendencies, and allowable pressure drop. Some general guidelines are given in Section 5.6.
Once the above parameters are specified to the computer program, it can proceed to calculate
the number of tubes required and the baffle spacing and number of tube passes consistent
with the required pressure drops for both streams.

Simulation
This mode of calculation is used most to predict the performance of a field heat exchanger
under different operating conditions. Usually the engineer ‘‘zeros’’ the program first by ad-
justing fouling factors and friction factor multipliers to match existing field performance.
Then the adjusted process conditions are imposed and the computer program predicts the