Under performance evaluation we understand that a heat exchanger and the process conditions are fully specified. The primary questions posed to the analyst are typically as follows.

1. Does the given heat exchanger perform the heat duty required, Q̇_req?
   
   
2. Does the actual heat duty Q̇_act differ from Q̇_req? Is the design on the safe or unsafe side, and how can this be interpreted?
   
   
3. Is the design efficient in terms of ideal versus actual heat transfer coefficients, utilizing the available pressure drop, temperature driving force, and their combined effects?
   
   
4. Are the basic layout and construction elements well chosen?
   
   
5. How will the exchanger perform on startup (no fouling) versus fouled conditions?
   
   
6. Are the pressure drops within acceptable limits?
   
   
7. Are there any suggestions to improve or investigate an alternative design (if the exchanger is in a design stage)? If calculated performance differs substantially from the required conditions, this item becomes of utmost importance.

To demonstrate the performance evaluation procedure we shall follow an example case.

A. Basic evaluation criteria

The actual heat duty produced by an air exchanger, Q̇_act (W), is referred to tube outside surface, designated by the subscript o: