With parallel flow the fluids are travelling through the heat exchanger in the same direction where as a counter flow installation will have the fluids flowing against each other in opposite directions.
Counter flow heat exchanger design.
But of the three the counter flow heat exchanger design is the most efficient when comparing heat transfer rate per unit surface area.
The counter current design is the most efficient in that it can transfer the most heat from the heat transfer medium per unit mass due to the fact that the average temperature difference along any unit length is higher.
In counter flow heat exchangers the fluids enter the exchanger from opposite ends.
The heat exchanger design equation can be used to calculate the required heat transfer surface area for a variety of specified fluids inlet and outlet temperatures and types and configurations of heat exchangers including counterflow or parallel flow.
In the counter flow setup the fluids are travelling along.
Parallel and counter flow designs heat exchangers although ordinary heat exchangers may be extremely different in design and construction and may be of the single or two phase type their modes of operation and effectiveness are largely determined by the direction of the fluid flow within the exchanger.
Heat exchanger companies suppliers counter parallel flow heat exchanger calculator.
A primary surface heat exchanger is one which does not involve any secondary surfaces such as fins ribs dimples etc.