With the aforementioned considerations to reduce thermal resistance, designers have successfully pushed the flow:volume ratio down to one. A cooler may be required when the ratio drops any lower, say in extreme environments that minimize the differential temperature, or mounting provisions that reduce the heat transfer coefficient. This also may occur with heavy duty cycle systems.

A cooler’s size often is based on approximate system efficiency at full power, where constant pump systems have an estimated efficiency of 70 to 75% and variable pump systems come in at 75% to 80%. Another means of sizing involves breaking the system down into its components by the anticipated volumetric efficiency of the rotary components and the anticipated pressure lost across controls at full system load. A more accurate means of estimating the heat load would break down the cycle by peak loads, and then use a time-weighted average to obtain the average heat load that determines the cooler’s size.

The worst-case scenario may be accounted for on the environment side, assuming the lowest temperature differential or film coefficient. Often, the worst case is considered several times during design by assuming steady-state max load, extreme environments (low ΔT), and poor performance of components (low h). This has a multiplying effect on the cooler size, which results in excess heat exchange capacity, size, and weight.

Proprietary heat exchanger designs, such as Parker's Cross Flow Reservoir System, can further reduce footprint and weight. These units integrate multiple components into one assembly, which minimizes the number of connections needed and provides a compact assembly. The challenge here is the limited flexibility to configure a system.

Systems like heavy-duty-cycle closed circuits demand such flexibility — full flow is required through a cooler, while only a portion is held in reserve. Another example involves any system with a large exchange capacity. However, when recognizing these limitations, these units prove convenient for maintaining thermal performance in demanding applications. Moreover, their capabilities continue to expand to fit the market.

As designers push hydraulic systems to their limits, it’s important to understand the thermal response, the effects on a system, and how to design for thermal performance. Old rules are becoming outdated due to more precise, new designs. Fully modeling the thermal response may not always be practical, but understanding the trend and being able to anticipate loading will help pinpoint optimal products. It will ultimately lead to easier assembly, better performance, and lower footprint systems that meet market demands.

As designers push hydraulic systems to their limits, it is important to understand the thermal response, the effects on a system, and how to design for thermal performance. Old rules are becoming outdated as new designs become more precise. Fully modeling the thermal response may not always be practical but understanding the trend and being able to anticipate loading will help to choose products that are optimal. This understanding and expanded product offering allows for easier assembly, better performance, and lower footprint systems that will serve the markets needs.

John Trott is Design Engineer with Parker Hannifin’s Hydraulic Filter Div., Metamora, Ohio. For more information, visit www.parker.com/hydraulicfilter.

To request a PDF file describing  Parker’s Cross Flow Reservoir System, e-mail  hfdtechsupport@parker.com and request brochure #SS-0008.

This article appeared in print as "Beat the Heat in Compact Systems" in the September 2013 issue of Hydraulics & Pneumatics.