Heat transfer fundamentals (4 of 5)

Advantages of corrugation

• Can generate tubeside heat transfer coefficients up to 2½ times greater than the equivalent smooth tube with less than 2½ times increase in pressure loss.
• Does not obstruct the flow area of the tube to a significant extent so they can be used in safety for fluids with high solids or fibre content without fear of blockage.
• Increasing the tubeside heat transfer coefficient brings the temperature of the tube wall closer to the temperature of the bulk fluid on the tubeside this minimising any tendency to cause fouling due to "burn-on", freezing or chemical changes.
• With higher coefficients the heat exchanger size can be reduced and therefore minimises product hold up volumes and residence time within the heat exchanger as well as reducing the overall material content of the heat exchanger. When exotic materials are used this can have a significant effect on the overall cost of the unit and installation.
• The effectiveness of in-situ cleaning processes, Cleaning In Place, is increased because of the increased turbulence generated by the corrugated tube at traditional circulation velocities.
• The higher turbulence created in lower viscosity fluids will minimise any tendency for deposition fouling even at low flow velocities.
• If fouling does occur on the tubeside the deposits will normally be easier to remove as the corrugation leads to an uneven film thickness which experience has shown is less adherent than an equivalent deposit on a smooth tube.

The major advantages can be summarised as follows:

• Reduction in heat exchanger size
• Reduction in product hold up volume
• Reduction in processing time
• Reduction in fouling potential
• Tube wall temperature closer to tubeside fluid
• Increased cleaning potential
• More efficient processing of viscous fluids

Industries in which applications which would benefit in a positive way from any of the advantages listed could become users of the corrugated tube heat exchangers to provide size and weight savings and more effective processes.

Expansion bellows

Most of the standard HRS series heat exchangers are manufactured as fixed tube units and are normally fitted with a thin wall multiconvolution expansion compensator (or Bellows) to allow for the differential expansion between the shell pipe and the tubes. It is vitally important that the bellows unit is designed correctly and the method of design used by HRS Heat Exchangers is that recommended by the "Expansion Joint Manufacturers Association" of America which checks allowable stress values against those produced by the working conditions and gives a prediction of the number of working cycles that the bellows unit will withstand before failure through fatigue. It is important that the worst case conditions of pressure, temperature and differential expansion are identified (which may be a CIP or other non-working condition) for use in the design calculations.

The new Mechanical Design section of the HRS computer software includes these calculations. It must be stressed that the bellows units fitted to HRS heat exchangers are only intended to absorb the differential movement between the shell pipe and the tubes. The absolute expansion of the shell pipe (which can be as high as 20 millimetres on a 6.000 mm unit) must be allowed for by the installer of the equipment by allowing one end of the unit to expand freely with pipework movements being compensated for with an appropriate pipework bellows and sliding supports etc. Failure to do this will quickly damage the heat exchanger.

TEMA stesses that heat exchangers are not intended to act as pipework anchor points. If the pipework designer does not account for the expansions and contractions produced under all operational conditions and allows them to impose external loads onto the heat exchanger connections then both bellows and nozzle pipes can be damaged.

The dimensional standards used by HRS for the multiconvolutional bellows used on our standard fixed tube designs are as shown below. Sizes outside this range or for high pressure/temperature units or for units with very high differential movements are purchased externally from an appropriate manufacturer so dimensional details will sometimes differ from those shown.

Shell Diameter	Shell Thickness	Number of Ply	Ply Thickness
63,5 mm	1,5 mm	1	0,8 mm
76,1 mm	1,5 mm	1	0,8 mm
88,9 mm	2,0 mm	1	0,8 mm
104,0 mm	2,0 mm	1	0,8 mm
114,0 mm	2,0 mm	1	0,8 mm
129,0 mm	2,0 mm	1	0,8 mm
139,7 mm	2,0 mm	1	1,0 mm
154,0 mm	2,0 mm	1	1,0 mm
168,3 mm	2,0 mm	2	0,8 mm
219,1 mm	2,0 mm	2	1,0 mm