zama.... In my opinion, a corrosion allowance should NOT be applied to heat exchangers with properly selected tube materials. I have seen many heat exchangers in the process industry constructed with more expensive, thin walled, corrosion resistant tubes (usually SS) The other components of the same HX may be constructed of less costly ( but more corrosion prone) materials with a corrosion allowance included. The reason for these material choices is that a less expensive HX can be the result. My recollections only...... MJC