|Title of Invention||
AN AUXILIARY STARTER AND CONTROLLER ASSEMBLY FOR A COMPRESSOR SYSTEM
|Abstract||An auxiliary starter assembly for a compressor system, comprising a prime mover for driving the driver motor and compressor to a predetermined speed before switching on power supply to the driver motor; a controller operating the said system selectively on on-off control mode or constant speed control mode on sensing the rate of pressure rise by measuring the time taken for the receiver pressure to change over from cut-in level to cut-off level.|
This invention relates to an auxiliary starter and controller assembly for a compressor system, such as, a compressor system for producing compressed air or gases finding various applications including uses in the operation of pneumatic tools, refrigeration and so on. Reference in this specification to an air compressor will include a reference to a compressor for compressing gases other than air.
Basically, an air compressor system known to the art which is used to supply compressed air to the utilities consists of an air compressor and a receiver. The air compressor used may be a reciprocating type or a rotary type.
A reciprocating air compressor consists of a piston and cylinder. The piston located inside the cylinder is imparted reciprocating motion by rotation of the crank shaft through a slider crank mechanism. Usually, a flywheel is mounted on the crankshaft to avoid torsional
fluctuations. The crankshaft of the compressor is suitably coupled to a drive motor.
Suction and a delivery valves are provided in the reciprocating compressor. The suction valve is a unidirectional pressure operated valve, which would only allow the air to flow into the cylinder. The delivery valve is also a unidirectional pressure operated valve, which would only allow the air to flow from the cylinder to the receiver.
During the suction stroke, the piston moves away from the cylinder head causing vacuum pressure inside the cylinder. The suction valve opens due to this pressure difference and allows air from atmosphere into the cylinder.
In a reciprocating piston type compressor, during the compression stroke, the piston moves towards the cylinder head causing the pressure of the trapped air to rise. Once the pressure in the cylinder increases above the
receiver pressure, the delivery valve opens and allows air from the cylinder to the receiver.
In a rotary compressor air is compressed by two rotating intermeshing rotors. The action of the rotary screw compressor can be compared to a reciprocating compressor. Each of the slots in the female rotor acts as a compressor cylinder, while the point of contact between the two rotors acts as a piston. The air is forced along the helical volume around the female rotor towards the discharge end plate. The volume behind the point of contact is drawing air in through intake port.
The compressed air is stored in the air receiver before it is supplied to the utilities.
If the quantity of compressed air delivered from the compressor is higher than the demand of air from the utilities, the pressure of air stored in the receiver increases. Otherwise when the quantity of compressed air delivered from the air compressor is less than the demand of the utilities the pressure in the receiver decreases.
It is standard practice to maintain the receiver pressure between two pressure levels to avoid excess pressure variations in the compressed air supplied to the utilities. Also the maximum pressure level in the receiver is limited for safety. These two pressure levels are called as cut off level and cut in level.
Cut off level: It is the high pressure level in the receiver when reached the supply of air to the receiver should be cut off.
Cut in level: It is the low pressure level in the receiver when reached the supply of compressed air to the receiver is resumed.
When the plant demand is fluctuating, the pressure in the receiver also will fluctuate.
There are different types of controls presently used to maintain the pressure in the receiver within a bandwidth.
(i) ON-OFF control : If the pressure in the air receiver reaches cut off level the further supply of compressed air to the receiver is stopped by switching off the drive motor of the compressor. If the pressure in the air receiver reaches cut in level the supply to the receiver is resumed by switching ON the drive motor of the compressor.
(ii) Constant Speed Control: If the pressure in the receiver reaches cut off level, further supply of compressed air to the receiver is stopped by an unloading mechanism, In reciprocating compressors the unloader forcefully keeps the suction valve open. Since the suction valve is forcefully kept open, both during suction and compression strokes, air cannot be compressed inside the cylinder. Therefore compressed air
cannot flow through the discharge valves to the receiver. If the pressure inside the receiver falls to cut in level the unloading mechanism allows the suction valve to function normally and the flow of compressed air to the receiver resumes. Rotary compressors are unloaded by partially closing the inJet port and partially bleeding the discharge to atmosphere. Energy;
Out of the two types of controls the ON-OFF control is most energy efficient but not implementable in a high capacity compressor with power rating more than I5kW, due to the limitations in number of starts-stops of the motor per hour and large voltage drop caused in the power supply during starting of this motors.
Constant speed control is normally implemented in large capacity compressors. 30% to 40% of full load power is required when the compressor runs at
unloaded condition even though the delivery of compressed air in unloaded condition is zero.
An objective of this invention therefore is to implement ON -OFF control system in a large compressor to overcome the limitations in number of start-stops of large capacity motors, by using an auxiliary starter system.
Another objective is to operate the compressor selectively on ON-OFF control or at constant speed control to avoid frequent ON-OFF of the motor. The auxiliary starter and controller assembly for a compressor system, according to this invention, comprises a prime mover for driving the driver motor and compressor to a predetermined speed before switching on power supply to the driver motor; a controller operating the said system selectively on ON- OFF control mode or constant speed control mode on sensing the rate of pressure rise by
measuring the time taken for
the receiver pressure to change over from cut-in level
to cut-off level.
This invention will now be described with reference to
the accompanying drawings which illustrate, by way of
example, and not by way of limitation, one of possible
embodiments of the air compressor system proposed
Fig. 1 illustrating, among others, the air compressor C,
a reservoir R connected to the delivery end D of the
Fig. 2 illustrating the auxiliary starter and controller
The embodiment comprises an air compressor C, a
receiver R connected to the delivery end D of the
compressor. The suction end of the said compressor is
open to atmosphere through a filter.
Delivery and suction non-return valves are provided for the said delivery and suction ends. M is a driver motor for driving the said compressor.
The embodiment further comprises an auxiliary starter consisting of a prime mover P for driving the driver motor M to a predetermined speed before power supply to the driver motor is switched on. A controller E (such as, an electromechanical or microprocessor based system) operates the said system on ON-OFF control or constant speed mode by sensing the time taken for the receiver pressure through a pressure sensor &to change over from cut-in level to cut-off level. When the compressor is to be started the prme mover P will mechanically accelerate the compressor motor M and driven parts of the compressor C from zero speed to a rated speed. On reaching this predetermined rated speed which is sensed by speed sensor S the electrical power supply to the drive motor M is switched ON. By
bringing the drive motor M to the rated speed before supply of electrical power, the voltage drop in the mains supply during starting; and over heating of the motor are avoided, because the energy required for starting the compressor C and motor M is given externally by prime mover P. The motor can be started and stopped any number of times per hour. The unloader U keeps the suction valves forcibly open during the starting of the compressor to reduce the starting torque requirement. In constant speed mode unloader U is used to cut off and resume the air supply to the receiver.
Now the embodiment is explained with reference to Fig.2. The auxiliary starter and controller assembly consists of an auxiliary motor A. It can be hydraulic, electrical or pneumatic. Controller:
A controller J operates the compressd air system either in ON-OFF mode or constant speed mode depending
on the ratio of unloading operation time to loading operation time. El is the mains supply to the controller J. The motorM is powered through the electrical supply E2 from the controller J. The pressure sensor Q senses the pressure in the receiver R. Speed sensor S senses
whether the motor has reached the rated speed, U is the unloader.
If the air demand is significantly low compared to the air supply from the compressor to the receiver, the pressure in the receiver would build up quickly. In this condition the supply of air to the receiver would have to be cut off for sufficient period of time whereby energy savings can be achieved by stopping the compressor and drive motor. Therefore ON-OFF control is the preferred mode of operation.
If the air demand is considerable compared to the supply to the receiver the pressure build up will be at a slower rate. In this condition the supply of air to the receiver would have to be cut off only for a brief
period of time during which energy savings cannot be achieved by operating the compressor and drive motor
in ON-OFF mode. Therefore constant speed control is the preferred mode of operation.
The controller selects the mode of operation based on the rate of pressure rise by measuring the time taken for the receiver pressure to reach from cut in level to cut off level. If the pressure in the receiver increases from cut in level to cutoff level within a predetermined period the controller would choose ON-OFF control; otherwise it will choose constant speed mode.
It will be appreciated that this invention is not confined to the embodiment illustrated and described but that various other embodiments of the air compressor system proposed herein are possible without departing from the scope and ambit of this invention.
|Indian Patent Application Number||1608/CHE/2006|
|PG Journal Number||25/2010|
|Date of Filing||05-Sep-2006|
|Name of Patentee||KESAVAN PANNIR SELVAM|
|Applicant Address||LECTURER: MECHANICAL ENGINEERING DEPARTMENT NATIONAL INSTITUTE OF TECHNOLOGY, TIRUCHIRAPALLI 620015.|
|PCT International Classification Number||F04B 49/02|
|PCT International Application Number||N/A|
|PCT International Filing date|