Cooling hydraulic systems is necessary more often than heating them due to wasted energy from inefficiency and/or poor circuit design. A well-designed circuit eliminates most heat generation and may not need a heat exchanger. Use the same method to estimate how much heat a system generates as was used for the previous tank-cooling example.
When figuring wasted horsepower, see if there is any way to reduce or eliminate it so it does not have to be paid for twice. It costs money to produce the unused heat and it is expensive to get rid of it after it enters the system. Heat exchangers are expensive, the water that through them is not free, and maintenance of this cooling system can run high.
Items such as flow controls, sequence valves, reducing valves, and undersized directional control valves can add heat to any circuit. Are these items absolutely necessary? Can they be replaced with another valve or part that does the same thing with less pressure drop? Anytime these questions can be answered with a yes, the circuit is not ready to build.
Air-cooled heat exchangers
After calculating wasted horsepower, review heat exchanger manufacturers’ catalog to pick out a unit that will dissipate that amount of energy. Most catalogs include charts for given size heat exchangers that show the amount of horsepower and/or BTU they can remove at different flows, oil temperatures, and ambient air temperatures.
Figure 6-9 shows a typical air-cooled heat exchanger that may be used in place of a water-cooled unit in some applications. Air-cooled heat exchangers are not as efficient as water-cooled heat exchangers, but they require only an electrical outside hookup. They work well in cool atmospheres or when the amount of heat to be removed is low. Note that airborne contaminants such as heavy dust or water and coolant vapors can quickly reduce an air-cooled heat exchangers low efficiency to almost nothing. Some manufacturers offer a filter pack to take out airborne contamination before it clogs the heat exchanger’s radiator fins and tubes.
On circuits with pressure-compensated pumps, a small air-cooled heat exchanger often is used to cool case drain flow. On this type system, most of the heat generation is from internal leakage and control oil that flows to tank through the pump case drain. One type heat exchanger -- called a coupling cooler -- is a finned tube formed into a circle and wrapped around a blower that is driven by the motor turning the pump. A similar arrangement uses a small flat radiator attached to the intake end of the fan-cooled electric motor that drives the pump. Both units are low-flow, low-backpressure devices and dissipate only a small amount of heat.
Some systems use a water-cooled heat exchanger in the summer and an air-cooled one in the winter. This arrangement eliminates plant heating in summer weather and saves heating expense in the winter.
Water-cooled heat exchangers
Two popular water-cooled heat exchanger designs. As before, manufacturers’ catalogs will assist you in picking out a unit to dissipate the amount of wasted heat energy. For water-cooled heat exchangers, catalogs ask for information such as how much and what temperature water is available, how many horsepower or BTU of energy must be dissipated, what is the fluid flow in gpm, and how many passes will the water makes to get through the body. The more passes -- up to four maximum usually -- the greater the heat dissipation per gallon of water flow. Charts that use this information make it easy to pick the correct size heat exchanger.
Figure 6-10 shows two types of water-cooled heat exchangers commonly used for hydraulic systems. The shell-and-tube design is the most common one at present, but the plate-and-frame or brazed-plate type are coming along because they are much smaller and easier to maintain. It is important to use clean water in either type unit to keep from building up insulating deposits or corroding the tubes until they leak water into the oil. Treated water from a cooling tower works best.
Either type heat exchanger should have a thermostatic control to turn on the water or fan only when the fluid temperature rises above its normal operating range. Without a thermostatic control, fluid could be too cold and thick, while wasting the energy to operate the heat exchanger. Figure 6-11 shows one type of water-control valve that requires no electrical hookup. Heat-sensitive liquid in a thermometer-type probe in the tank expands and opens a water valve upon reaching a preset temperature. The temperature is fully adjustable to meet any requirement and it operates in all types of fluid. Another option is an electrically operated temperature sensor that controls a solenoid-operated water valve. This installation requires an electrical hookup but is able to maintain a fluid-temperature range at any desired setting.