Water coolers normally consist of a series of pipes inside a casing, with hydraulic fluid flowing through the inside of the pipes and water along the outside (Fig. 4.15). The water supply to the cooler will normally be controlled by a thermostatically controlled valve, which will only switch the water supply on when the oil temperature reaches a certain value. Best results (i.e. the greatest cooling effect) are obtained when the water and oil flow through the cooler in opposite directions (known as ‘counterflow’).
The size of cooler is determined by the amount of heat it is required to dissipate and the temperature of the cooling water available. However, coolers are often fitted into the return lines of systems (to cool the oil after it has been heated up in the system), where the flow rate may be greater than just the pump flow alone (as with return- line filters). The peak return-line flow rate may, therefore, also determine the size of cooler required.
POINT OF INTEREST Fluid temperature will increase by approximately 1°C for every 17.5 bar, which is approximately equivalent to 1°F per 140 psi.
As mentioned previously, an off-line filtration system, if one is used, is an ideal location for the cooler, because it has a steady and predictable through-flow under all conditions. In such arrangements the cooler is usually placed after the filter so that the oil is filtered while hot and then cooled down.
Any internal damage to a water cooler will create the possibility of water leaking into the hydraulic fluid, or vice versa. A protective low-pressure relief valve (often a spring-loaded bypass check valve) is normally fitted around the cooler to guard against sudden flow surges or pressure peaks.
When no cooling water supply is available, an air blast cooler is the usual alternative (Fig. 4.16). Here, hot oil is passed through a cooling element while air is blown through by means of an electric or hydraulic motor driven fan. On some vehicles the hydraulic cooler is mounted alongside other heat exchangers (e.g. main engine, turbocharger or air-conditioning unit) that all share a common cooling fan.
WATER
TEMPERATURE
OIL
THERMOSTATICALLY CONTROLLED VALVE COUNTERFLOW
WATER
OIL
Fig. 4.15 Water cooler
COOLER
Fig. 4.16 Air-blast cooler fitted to a power unit
As mentioned previously, the reason why a cooler may be required in a hydraulic system is to dissipate the heat created by system and component inefficiencies.
Careful design of the system, however, can often reduce the amount of inefficiency (and therefore heat), but this may sometimes involve a higher initial cost. For example, variable-displacement pumps or variable-speed drives can often provide a more efficient system than one using simple fixed-displacement pumps, but usually involve an initial cost penalty.
The design of the system therefore always involves finding the best compromise between capital cost and running cost. But, again, operating conditions will influence this decision; a system that is used for 10 minutes every day will have different priorities to one that is operational 24 hours a day.
Energy-recovery arrangements involving accumulators are now also a practical means of increasing system efficiency, but again may involve a higher initial cost.
POINT OF INTEREST Traditionally, power-transmission efficiency has not been a strong point of hydraulic technology.
However, times change, and now the emphasis in many applications is on making the hydraulic system as efficient as possible in order to reduce running costs, emissions and waste-heat generation.
INTRODUCTION
In Chapter 1 a hydraulic system was defined as a means of transmitting power from a prime mover to the various functions of a machine where the power is required.
In most industrial applications, and some mobile applications also, the prime mover is an electric motor of some description. The electric motor provides input power to drive the pump, which converts the mechanical shaft rotation of the motor into hydraulic power in the form of flow and pressure.
For many years, electrically operated directional valves have been used to control and distribute the hydraulic power to where it is needed. In many cases, these are simple on/off devices, but increasingly there is a demand for proportional controls, where valves are capable of modulating their output, as opposed to simply switching it on or off. An analogy can be made with a simple on/off light switch and a dimmer switch. The first is only capable of switching the light on and off, while the second can vary the amount of light produced, from fully off to fully on and anywhere in between.
Once proportional valves had been developed to respond to analogue electronic signals, it was then a relatively small step to introduce digital electronic controls, thereby adding some ‘intelligence’ to the control function. As a result, hydraulic components are now available with built-in electronics that not only sense what the component is doing but also can make control decisions about how to react and then communicate their actions back to a centralised control station.
ELECTRIC MOTORS
Most electric motors used in industrial systems are three-phase induction motors.
These are sometimes referred to as squirrel-cage motors due to the configuration of their rotating component. They are typically powered by a three-phase AC supply at a relatively high voltage of around 400 V (480 V in the USA), which is the normal industrial supply voltage. By contrast, the domestic electricity supply is around 220 V (110 V in the USA) and single phase.
A single-phase supply involves just two wires, where the voltage in one is constantly changing relative to the other in a sinusoidal manner. The rate at which the voltage changes is known as the supply frequency (i.e. the number of complete sinusoidal cycles per second). In Europe, the supply frequency is a nominal 50 cycles per second (50 hertz (Hz)), whereas in North America and some other countries it is 60 Hz. A three-phase supply involves three wires, where the voltage in each varies sinusoidally relative to the other two (Fig. 5.1).
In an induction motor, the electrical supply is connected to windings in the stationary part of the motor known as the stator. The cyclical nature of the voltage and the