HX Design

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Effect of design temperature change on existing shell and tube heat exchanger design needs to be evaluated. Design per TEMA 1968. One HX is of type AEU while the other is AET. Design temperature change does not affect allowable stress values for any of the material involved but does slightly affect Modulus of Elasticity (E) and Mean Coefficient of Thermal Expansion (Alpha). If I am reading TEMA correctly, E and Alpha only affect Equivalent Differential Expansion Pressure (R-7.151) which needs to be evaluated only for HXs with tubesheets fixed to both ends. For AEU or AET type of HX since tubesheet is not fixed on both sides (Fixed on only one side), is it a correct conclusion to say that changes in E and Alpha do not impact HX design?

 

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No HX Experts in this forum?

 

[EdStainless]

E is critical when you are looking at vibration, such as from cross flow. It has no impact on pressure rating.
CTE will not matter if there is no stress that results from the expansion. This is why there are floating head and u-tube designs out there, to allow for expansion.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[Christine74]

Usually it's fairly easy to do a temperature-only rerate of a heat exchanger as long as you're staying within the linear range of the allowable stresses (for carbon steel, this is now the 0 - 400 F range), but there are at least four exceptions to this rule:

1) Fixed tubesheet designs, obviously. But this doesn't apply to either of your exchangers.

2) ANSI/AMSE flange pressure/temperature ratings. The allowable pressures for your nozzle flanges will start dropping at any temperature above 100 F, so this needs to be checked.

3) Removable channel covers. The required thickness for exchangers with two or more tube-side passes is typically governed not by ASME but by TEMA, which has a maximum allowable deflection which depends upon the modulus of elasticity of the material.

4) External pressure, such as with floating head covers. The required thickness for a floating head cover will likely increase with any increase in shell-side or tube-side design temperatures, per the ASME external pressure charts.

-Christine