Dextermech,
Single-pass exchangers can possibly be counterflow. However, it really depends on how they are constructed.
For example, if you use a long shell and tube exchanger with a single pass on the tube side, and a single pass on the shell side in the opposite direction, then it it pretty much counterflow.
Now think of a shell and tube exchanger with U-tubes (2-pass in this case) and a single-pass shell. In half of the exchanger you have counter flow, but in the other half you have co-current (also called Parallel) flow. This is where things begin to fall apart with a temperature cross.
If you have a rectangular exchanger (a more typical air-to-air configuration), with one side single pass and the other side multi-pass, then it is actually counter-cross flow, and also approximates counterflow.
A simple cross-flow exchanger will not work any time there is a temperature crossover required.
Since I first mentioned an overall heat transfer coefficient in the range of 2 to 3, I need to clarify that it's based on the assumption of fairly low velocities on both sides of the exchanger. In an air-to-air exchanger velocity is everything, since it determines the component heat transfer coefficients on both sides (film coefficients). The higher the velocity, the higher the heat transfer coefficient. However, this is at the cost of pressure drop. That is, higher velocities mean higher pressure drops, but a more compact exchanger.
Also, you should take a look at your process requirements. Remember that the overall size is inversely proportional to the LMTD (log mean temperature difference). If you start with a 10 deg F LMTD, the exchanger will be twice as large as one with a 20 deg. F LMTD (with all other factors being equal, but they never really are).
Regards,
speco