Accumulators:
Definition : A hydraulic accumulator is a pressure vessel used to store hydraulic energy and on demand make the energy available again to the system.
Functions of accumulator :
(a) To meet peak demands of power :
When excessive power is available during lower capacity, there will be loss of power if pump runs continuously, hence during the slack period, some of the fluid energy is used to charge an accumulator. Then it is used to meet peak demands of power.
(b) To smooth out pressure surges/shocks :
When fluid flow is suddenly changed pressure surges are developed, moving fluid may cause fluid hammer and shock waves. Accumulator will absorb the pressure shocks and ripples and reduces vibrations and pressure surges.
(c) To provide emergency power source:
The fluid energy stored in an accumulator may be sufficient to give an emergency supply in case of power failure causing the pumps to stop.
(d) Holding high pressure :
Accumulator gives fluid energy back up for longer periods without keeping the pump running.
They are used in
(i) Large hydraulic presses.
(ii) Farm machinery, diesel engine starters.
(iii) Hospital beds, aeroplane landing mechanism.
(iv) Lifts, trucks, etc.
Types of Accumulators:
They are classified on the basis of element used to store oil under pressure :
(a) Dead weight type
(b) Spring loaded type
(c) Gas charge type
(a) Dead weight type:
It uses dead weights to store oil energy.
Construction :
● It consists of a piston loaded with dead weight and moving within a cylinder which exerts pressure on the oil.
● The stroke of the piston pressure will remain constant because the load remains unchanged.
● The dead weights may be of some heavy material such as iron, concrete block or even water.
● The piston is fitted precisely in to the cylinder to minimize leakage.
● They can be used to serve several hydraulic systems at a time are used in mill and central hydraulic system.
Working:
● When oil from pump enters through inlet port, it pushes the piston upwards against the load exerted by the dead weight placed on the platform.
● The dead weights are used to maintain oil pressure for charging of the accumulator.
● When piston moves from lower end to upper end, the accumulator get charged and when it is connected to the application point, the oil under pressure release energy through the outlet A.
● When pistons moves down completely, the accumulator is unloaded and discharged.
Advantages :
1. It supplies oil at constant pressure.
2. Simple in construction.
3. Large capacity, it can serve several hydraulic systems at a time.
Disadvantages :
1. It has larger size.
2. More space required hence not suitable where space is limited.
(b) Spring loaded accumulator :
It used spring force to store hydraulic energy.
Construction :
● A spring loaded accumulator consists of cylinder body, a movable piston and a spring.
● The spring applies a force to the piston. As dead weight accumulator requires more bulky weights, spring provides large force in a limited area for the same pressure.
● The spring used in this accumulator is compression spring which exerts force on the piston.
Working :
● When pressurized oil enter from the inlet at the bottom, it pushes the piston in the upward direction against the spring force.
● When piston moves from lower end to the upper end spring get compressed. When piston reaches the upper stop end, the accumulator is fully charged.
● When accumulator is connected to the application point, spring pressure forces the oil under pressure through the outlet port. When spring is fully expanded the accumulator is unloaded and discharged.
Advantages :
1. They are compact and smaller.
2. Mounting is easy.
Disadvantages :
1. They supply a small volume (less capacity).
2. Operate at low pressure.
3. The pressure is not uniform and does not remain constant.
(c) Gas charged accumulator:
They store energy with the help of force of compressed gas.
Construction :
● It consists of a synthetic polymer rubber bladder fitted inside a metal shell. The bladder is filled with compressed gas.
● A poppet valve is located at the discharge port get closed when accumulator is completely charged.
● The accumulator is pre charged when it is empty (oil is not present) with the help of gas charging inlet.
● Generally dry nitrogen gas is used to fill the bladder.
Working :
● Initially the accumulator is pre charged to the required pressure with the compressed gas in the bladder.
● When oil under pressure enters in the accumulator through inlet, it starts pushing the bladder with the oil pressure.
● As the gas inside the bladder is compressible, the oil push the bladder in the upward direction and compress the bladder when oil pressure is more than force of the bladder.
● The accumulator is completely charged when bladder is compressed sufficiently. Gas expands during unloading the accumulator to the required application device.
Advantages :
1. The accumulator is compact in design.
2. Quick response for loading/unloading of accumulator.
Disadvantages :
1. Constant pressure cannot be maintained due to compressible nature of gas.
2. Higher pressure ratio and higher temperature may cause failure of bladder.
3. Special care required to prevent air/oxygen to enter in the accumulator which may cause danger of explosion.
4. Limited capacity.
Necessity of accumulator for every huge hydraulic press :
● Oil requirement of the system is not continuous but intermittent. A hydraulic press needs the oil only during the lifting operation.
● While, during the lowering of load, holding the load or during idle period, it doesn't not require any supply of oil.
● During such period, the hydraulic pump has to stop or delivered oil must be drained back to the reservoir. But frequent starting and stopping of pump is not desirable as it reduces the pump life.
● Also, draining the pressurized oil is wastage of power which reduces the system efficiency. Hence use of accumulator is required in the system.
Heat Exchangers (Cooler) :
Definition : It is a fluid power device which is used to remove heat from the system for cooling purpose.
Sources of heat :
There are variety of reasons for heat generation in hydraulic components and system. Some of the sources of heat generation are:
Sources of Heat Generation
i) Pump
ii) Friction
iii) Blockage of filter, valves and other elements
iv) Incompatible oil
v) Use of relief valves and orifices
(i) Pump : There is some energy from electric motor to the pump which is not converted into work. The unutilised power may generated heat in the system.
(ii) Friction : Pipe friction, joints, line fittings, moving components with vibrations may generate heat in the system.
(iii) Blockage of filter, valves and other elements: Clogged filter causes interruptions in the fluid flow in the valves and other elements and heat is generated.
(iv) Incompatible oil : If oil viscosity high and its is not properly chosen, it may increase friction and generates heat.
(v) Use of relief valves and orifices : Oil passing through valve may generate he due to change in flow area and path.
Types of heat exchanger :
(a) Air cooled heat exchangers
(b) Water cooled heat exchangers
(c) Gas charge type
(a) Air cooled heat exchanger :
● Air cooler are used where water is not readily available and the air is 3 to 5° C cooler than the oil.
● Use of air cooled heat exchanger is comparatively cheaper and hence economical.
● Natural air or fan can be used to improve rates of heat transfer.
(b) Water cooled heat exchanger:
● Water cooled heat exchangers are more compact, reliable, efficient and high heat transfer rate.
● But water may cause corrosion of tubes, scale formation, etc. Disposal of warm water directly is also drawback.
Oil Heaters :
● Oil heater are used to heat the oil when hydraulic system is to be installed in cold weather conditions.
● Due to lowest temperature oil may get freeze and lose it viscosity.
● A thermostatically controlled heater is installed in the reservoir which heats the oil to its normal working temperature so that the oil can function effectively.
Muffler (Silencer):
● In pneumatic system, there is no need to provide return lines and the air is exhausted to the atmosphere directly through the exhaust ports.
● As air is low density in nature, it will produce high noise during its application as well exhaust.
● The high noise produced during exhaust can be reduced with the help of silencer i.e. noise reducer known as muffler.
● It is similar to the silencer fitted to the various automobile vehicles to reduce noise level.
● It consists of baffle plates arranged in a zig-zag manner to avoid direct exit of air and there by noise level is reduced by inserting obstructions in the path of air.
● They are mounted at the exhaust ports of D.C. valves from which air is exhausted.
