Heat is generated in a hydraulic system because of the simple reason that no component can operate at 100% efficiency. Significant sources of heat include pumps, pressure relief valves and flow control valves. This can cause a rise in temperature of the hydraulic fluid above the normal operating range. Heat is continuously generated whenever the fluid flows from a high-pressure region to a low-pressure region, without producing mechanical work. Excessive temperatures hasten oxidation of the hydraulic fluid and also reduce its viscosity. This promotes deterioration of seals and packings and accelerates wear and tear of hydraulic components such as valves, pumps and actuators. This is the reason why temperature control is a must in hydraulic systems.
The steady-state temperature of the fluid depends on the rate of heat generation and the rate of heat dissipation. If the fluid-operating temperature is excessive, it means that the rate of heat dissipation is inadequate for the system. Assuming that the system is reasonably efficient, the solution is to increase the rate of heat dissipation. This is accomplished by the use of ‘coolers’, which are commonly known as heat exchangers. In certain applications, the fluid needs to be heated in order to achieve the required viscosity of the fluid in the system. For example, if a mobile hydraulic equipment is required to operate in sub-zero conditions, the fluid needs to be heated. In such cases, heat exchangers are termed as heaters.
The factors to be considered when sizing a heat exchanger are:
• The required drop in temperature of the hydraulic fluid
• The flow of the hydraulic fluid in the system
• The time required to cool the fluid.
There are two main types of heat dissipation heat exchangers:
1. Air-cooled heat exchangers and
2. Water-cooled heat exchangers.
Air-cooled heat exchanger
Figure 7.25 shows an air-cooled heat exchanger.
The hydraulic fluid to be cooled is pumped through the tubes that are finned. As the fluid flows through the tubes, air is blown over them. This takes away the heat from the tubes. A fan driven by an electric motor is incorporated in the heat exchanger to provide air for cooling. The heat exchanger shown above, uses tubes which contain special devices called turbulators whose function is to mix the warmer and cooler oils for better heat transfer.
Advantages associated with air-cooled heat exchangers are:
1. Substantial cost reduction because of the use of air for cooling purposes, as compared with water
2. Lower installed costs
3. Possibility of the dissipated heat being reclaimed.
Disadvantages of air-cooled heat exchangers are:
1. Relatively larger in size
2. High noise levels
3. Higher installation costs.
Water-cooled heat exchanger
Figure 7.26 is an illustration of a common type of water-cooled heat exchanger used in hydraulic systems.
This is typically a shell and tube-type heat exchanger. The cooling water is pumped into the heat exchanger and flows around the tube bank. The hydraulic fluid, which is to be cooled, flows through the tubes. While flowing through the tubes, the fluid gives away heat to the water, thereby reducing its temperature.
Advantages of water-cooled heat exchangers are:
1. They are very compact and cost-effective
2. They do not make noise
3. They are good in dirty environments.
Disadvantages associated with water-cooled heat exchangers are:
1. Water costs can be expensive
2. Possibility of mixing of oil and water in the event of rupture
3. Necessity for regular maintenance to clear mineral deposits.