Oil-to-Water (or simply water).
Oil-to-water coolers are commonly found in stationary applications where there is a ready source of cooling water available. These types of coolers are generally more efficient and more effective than air coolers. Oil-to-water coolers can be used to obtain lower fluid temperatures than sir coolers, are are normally more effective in providing a constant hydraulic fluid temperature over the long-term.
The term heat exchanger is usually considered to mean a “cooler.” A heat exchanger may be either a heater or a cooler. A cooler is required when the heat generation rate of a system (from either internal or external sources) exceeds the heat dissipation rate from the reservoir, piping, and hardware. On the other hand, a heater may be required to raise the fluid to some minimum temperature prior to a machine start-up to prevent freezing or separation of water-containing fluids, or to help in maintaining an acceptable operating temperature when the system is exposed to the extreme cold temperatures.