Generally, the term "heat exchanger" must cover a wide variety of hydraulic and pneumatic conditions. Most systems whether hydraulic, lubrication, steam heating, etc…can be defined and categorized into a general application categories to simplify the selection process. Before attempting to apply calculations for selecting a heat exchanger, an understanding of the application will be foremost in determining the correct approach to the selection process.

Common systems

1. Open loop hydraulic system is best described as a system where all of the oil is returned to a common reservoir. Normally associated with fixed delivery pumps, the open loop system reservoir capacity is large normally 2 or 3 times the maximum oil flow rate of the system. Open loop systems normally can run between 110F to 140F depending upon conditions and manufactures requirements.
2. Close loop hydraulic system is best described as a system where a minimal flow (2-10 gpm) normally just pump and motor case drains are returned to the reservoir. The system reservoir is normally not larger than 2 or 3 times the maximum case-drain flow rate under maximum operating conditions. Closed loop systems are more frequently used in mobile equipment but also have industrial application. Closed loop systems frequently use pressure compensated pumps and normally can operate between 140F-180F depending upon application and manufactures requirements.
3. Off loop recalculation systems use their own transfer pump to circulate oil from the reservoir through the heat exchanger and back into the reservoir. The off loop or "kidney loop" system is used when return line oil is not available or when case drain flow is to low to effectively transfer the required system heat load. Though this has become more popular with system designers, the off-loop method requires a larger expense to implement due to its additional system requirements.
4. Gravity return is normally associated in lubrication systems where the head pressure or oil column is used as return line pressure for flow purposes.
5. Steam heating systems are generally straightforward with the liquid in the tube side and the steam in the shell side.