Design Conditions are generated automatically by the program and also additional design cases can be added by the user in the Tubesheet>Design Conditions tab.
Design Conditions are possible operating scenarios which the exchanger may experience and hence the design must be performed for that condition eg. there can be an upset condition or shutdown condition when tubeside operating pressure can be 90% of design but shell side pressure would only be 25% of design, this is a special case specific for that exchanger which the designer may expect the exchanger to be operated at in the field.
Design Case are a set of cases specified by the code that must be considered for each and every Design Condition eg. for the example above for each of the 5 design conditions 3, 4, 7, 8, 9 there would be 7 cases to be computed as specified by the code, see the tubesheet calculation screenshot from the report below:
These design cases are automatically calculated by the software for each design condition, if a case is not needed it will not be computed eg. if user check differential pressure then a few relevant cases would not apply and hence not calculated. The above calculation is for fixed tubesheet heat exchanger hence 7 cases are calculated.
Observe that the design condition 4 is not there in the above report screenshot menu column in the left, this is because this specific zero tubeside shell side pressure case is automatically taken into account as a specific case for the loading condition 3 (look at case 4 in the report screenshot above).