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.