Reservoir Cooling
It is desirable to maintain the fluid temperature in a hydraulic system at about 430 C (1100 F) to 490 C (1200 F). This is not always easy; especially if the reservoir is placed where it is always hot. In addition, wasted pump work, cause d by pump losses, pipe losses, and control losses, generates large amounts of heat. Experience has shown that systems run cooler under the following conditions:
Sufficient fluid in the system
Sufficient reservoir surface area to radiate heat properly
A free standing reservoir that permits air circulation on all sides of the reservoir
A good baffle in the reservoir
Thin side walls on the reservoir, with sturdier bottom and top plates.
Location of the hydraulic system in cooler area of the shop
To permit efficient heat transfer, the reservoir walls should be as thin as possible. However, safety must come first. The wall thickness of a reservoir can vary from 1/16 inch for small tanks to 1/4 inch for tanks containing up to 500 gallons. Thicker tops and bottoms with adequate angle – iron reinforcement should be used to provide a firm mounting for the pump and its motor. The approximate amount of cooling provided by a reservoir of a given size is shown in Table 4 – 1. Because hydraulic fluids transfer heat at different rates, the BTU’s shown have to be multiplied by a correction factor to determine the actual change in degrees Fahrenheit of the fluid. The correction factor varies from approximately 0.10 to 0.40 depending on the fluid temperature, flow rate, and type of fluid used.
Many manufactures use reservoirs that are built right into the machine. These save space and shorten the hydraulic lines, but they have disadvantages – including improper cleaning and cooling. Hot reservoirs under precision machinery are undesirable, and directly affect the hydraulic system and the machine badly.
If hydraulic system operates with a high temperature level, even with all of the favorable conditions existing, some additional means of cooling the system and reservoir must be used. The following methods will usually aid in dispersing some of the heat.
Fans or blowers can be used to cool parts with direct streams of air.
The ventilation near the hydraulic system installation can be improved with exhaust or free – standing fans.
A fan – cooled fluid radiator can be used to cool the hydraulic fluid.
Finned pipes can be inserted in the lines to cool the fluid.
A water – cooled heat exchanger can also be installed to cool the hydraulic fluid.
The type of cooling method used will depend on the application and the amount of cooling required. The plant or maintenance engineer will probably select the cooling equipment required.
The operating temperature of the fluid in a hydraulic system can be lowered by cooling the reservoir with compressed air. For example, at room temperature without air cooling, the fluid temperature at the pump inlet may be 660C (1500F) and the temperature of the returning fluid 820C (1800F). Forced air cooling can reduce these to 410 C (1050 F) at the pump inlet and 570 C (135 0 F) in the return lines. This is a much more satisfactory range of operating temperatures.
Under normal circumstances, the fluid reservoir provides sufficient cooling of the hydraulic fluid. If additional cooling (heating) is needed, shell and tube heat exchangers like the one shown in Fig. 4 – 5 can be used. Shell and tube heat exchangers are efficient, economical, and compact. They consist of a bundle of tubes connected to a header plate at each end. A header cap is placed on each end and is connected to the cooling fluid, which circulates through the tubes. The shell, which surrounds the tube bundle, is connected to the hydraulic fluid lines. As the hydraulic fluid flows through the shell, interior baffles change the fluid direction several times. These baffles help prevent coating films from forming, increase heat transfer efficiency, and support the tube bundle. As the hydraulic fluid flows around the tubes in the shell, it give around the tubes in the shell, it gives up its heat to the cooling fluid in the tubes.
Most heat exchangers are of the Counter flow type. This mean that the fluid being cooled moves in one direction through the heat exchanger, while the liquid doing the cooling moves in the opposite directions. As shown in Fig. 4 – 5, the hydraulic fluid being cooled flows generally from right (inlet) to left (outlet) through the shell. The cooling liquid flows from left (inlet) to right (outlet) through the tubes, thus removing heat from the surface of the tubes. Usually, either the temperature or the flow of cooling water is controlled (to change the cooling rate of the fluid).
A heat exchanger used for cooling hydraulic fluid should be connected between the hottest return lines and the reservoir. If only a small amount of cooling is needed, a finned tube can be installed on the return lines. Using a finned tube helps cool the hydraulic fluid without requiring additional piping for a cooling fluid.
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