Advantages of RSTB Gas-to-Gas Heat Exchangers compared to Conventional Segmental Baffled Designs

• Higher heat transfer coefficient and lower pressure drop can be achieved due to the flexibility of adjusting  more design parameters during the optimization process. The tube spacing is an important parameter which can be adjusted because of tube-to-tubesheet joints being welded as opposed to expanded joints. Also, in each shell pass, the radially symmetrical flow pattern with no tubes at inner core and at outer annular space results in uniform crossflow and higher HTC.
• Lower shell side flow resistance is achieved mainly due to lower number of tube rows arranged in a rotationally symmetrical tube bundle performing the same duty.
• Uniform stress distribution in the tube sheet due to radially symmetrical distribution of temperature and load. Thermal expansion is uniform due to this symmetry. This prevents cracks in tube-end-welds. 
• In some cases, it is not surprising to see the area required is almost 20-30% less than the area available in conventional segmental baffled designs.
• More flexibility during installation and replacement with minimal or no interference with current ducting layout and foundation. Nozzles can be orientated to match existing duct tie points. These exchangers are generally lighter in weight and smaller in size.

The gas phase reactions during sulfuric acid production involves conversion of SO2 in SO2-rich gas stream into SO3 gas. The SO3 gas is then absorbed by weaker acid to sustain acid production. The process involves large amount of process gas flow at relatively high temperature and at very low pressure. The design pressure is very low around 200 inches WCG. The pressure drops are kept around 10 to 20 inches WCG to minimize pumping cost. Although the heat transfer coefficients on opposite sides of the heat transferring tubes are usually within a factor of 2 or 2.5 of each other, the absolute values are lower by a factor of 10 to 100 compared to liquid-to-liquid service; thus, a much larger volume of tube bundle is required to transmit a specified amount of heat. Because of low design pressure and large size, the weight of material often becomes the most significant load to be considered during mechanical design.

Since these gas exchangers operate below atmospheric pressure, with no limitation on size, they don’t need to be in conformity with ASME code. But even if the equipment does not fit within the limits of the ASME code it is common that much of the code requirements be applied. No ‘U’ stamp on nameplate or U-1 form as described in the code is required. A notable exception to code is to limit the pneumatic test pressure to maximum 2 psig to avoid explosion. Any rupture during the test can release large amount of stored energy of a compressed gas into kinetic energy which can result in sudden expansion leading to explosion and makes the test very unsafe. Low air test pressure for this type of ‘not-a-pressure-vessel’ application is proved to be more than adequate to check mainly any leakage at the tube-to-tubesheet welded connections. These welds are required to carry small load due to low operating pressure anyways.

Rotationally symmetrical-tube-bundle gas-to-gas heat exchangers in sulfuric acid plants are all welded, vertical shell-&-tube type with shell-passes separated by disk and donut baffles. The tube bundle is arranged with some rotational symmetry. Disk baffles direct the gas from inner empty core, through the tube bundle to the outer empty annulus. The opposite is true for the donut baffles. With no tubes at the inner empty core or outer empty annulus space, the flow across the tube bundle in each shell-pass follows uniform crossflow pattern resulting in efficient heat transfer.

In conventional segmental exchangers with NTIW designs, the tubes are bundled in a rectangular cross section with centerline (perpendicular to tube-flow direction) being a straight line of say length ‘L’. We can imagine the rotationally symmetrical-tube-bundle being generated by elongating this rectangular bundle along ‘L’ and at the same time rotating it into a circular band of mean circumference π.d > L. This leaves a bundle cross section of smaller ring width having lower number of tube-rows for equal number of tubes and tube-pitch resulting in lower pressure drop. We can capitalize on the extra pressure drop available in the segmental design by tightening the tube bundle, generating more turbulence with increased film coefficient resulting in smaller area.

For long, reliable service life of the gas exchangers, consideration for thermal discontinuities should be made during the design stage; steady process conditions should be maintained during plant operation; and selection of appropriate material, welding, inspection and proven manufacturing methods should be used during fabrication.