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Heat Exchanger Components
Thin stayed tubesheets are required to avoid excessive thermal stresses. They should be designed in accordance with ASME Boiler and Pressure Vessel Code, Section I. Tubesheet thickness is governed by the largest unstayed area, which is usually the annular space between the bundle and the shell.
Tube ends should be rolled and strength-welded to the tubesheet. Roll 95% of the tubesheet thickness to minimize the crevice between the tubes and the tubesheet. Rolling at or beyond the tubesheet thickness may damage the tube.
Tube support spacing and thickness should conform to TEMA rules. Bolt the tube supports to clips on the shell to avoid cracking due to pressure dilation of the shell. Provide peripheral cut-outs on the tube supports so axial flow in the annulus is unrestricted.
The purpose of tubesheet refractory is to protect the tubesheet from high tube side temperatures. Only about 1 inch of refractory is useful for insulation. More thickness is needed to keep the refractory in place when it cracks. See Insulation and Refractory Manual for recommendations.
The purpose of ferrules is to protect the tubesheet and seal welds from high temperatures, and to provide a hole through the hot tubesheet refractory. Ferrules are usually high alumina ceramic or an alloy, such as Inconel. Metal ferrules are preferred, if temperatures allow, because they can be made thinner than ceramic, and thus reduce the geometry discontinuity between the downstream end of the errule and the tube I.D. Wall thickness of ceramic ferrules should be at least 1/8 inch to facilitate their manufacture.
Both ceramic and metal ferrules should be wrapped with 1/32 inch thick ceramic paper insulation, glued in place. About 1 inch of the outlet end of metal ferrules should be expanded to the tube I.D. dimension before insertion into the tube. This snug fit keeps the ferrule in place during installation of the refractory and keeps the paper insulation in place during operation.
Downcomers and Risers
A single downcomer is usually sufficient. Multiple risers are usually required, and should be sized for annular or churn flow. Consider the axial steam production profile along the unit in selecting riser locations for horizontal units; i.e., put one riser near the hot tubesheet and shift the others toward the hot end as appropriate. Equations for calculating the axial steam production profile are given in Appendix F.
Internal vs. External Bypass
An internal bypass is preferred to an external bypass because internal bypasses are cheaper and more reliable. If the internal bypass is uninsulated, the surface area should be included in the dryout heat flux calculations. If dryout is a problem, provide an insulated internal sleeve, welded on one end only, for the first 2 feet. Check the thermal stress between the bypass pipe and nearby tubes. If thermal stress is a problem, the insulated sleeve can be extended for the full length.
If it is necessary to operate a steam generating bundle close to the dryout limit, it may be advantageous to install dryout sensors. Local dryout usually results in mechanical failure before reduction in thermal performance is noticed. When dryout occurs, the tube temperature jumps from very near the water/steam temperature to near the tube side fluid temperature. This large temperature change can be easily detected by a thermocouple.
A dryout sensor consists of a thermocouple tack welded to the tube O.D. in the high heat flux region downstream of the ferrule, with the lead being taken out through a high pressure fitting in the shell. This can only be done during manufacture or retubing. Several dryout sensors are usually installed in areas most prone to dryout.
Piping and Instrumentation
Section I of the ASME Code defines instrumentation and piping requirements.
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