If one of your streams tends to have or give off air or gas, you want to enter from the bottom and leave from the top so that the air/gas can be swept along and out of the Hx.
Then your other stream needs to go conter current or as counter current as possible to that stream.
Sometimes you are just stuck and have to make the best of the compromises you have to make because of equipment arrangement, etc.
If I have a water stream for example and I know that on heating up it wants to liberate air, I will try to have it flow from low to high.
I'd say that 99 times out of 100 you want the hot streams to enter the top and leave the bottom, and just the opposite with cold streams. Even if you're worried about vapor or gas being trapped in the exchanger, the vapor is likely to be generated during a heating process. Similarly any condensation is more likely to occur during cooling. In fact, I can't think of any single phase fluid applications I've seen that would violate this "rule".
