Compressor Unit Of Automatic Pneumatic Lift - Online Article

The compressor unit basically consists of a reciprocating compressor, electric motor, pressure gauge, pressure vessel, hose pipe and plastic tubes. The reciprocating compressor is used for compressing the air from the atmosphere and then transferring it into the pressure vessel. The electric motor is used to operate the compressor. This electric motor is of 1.5 HP single phase induction motor. The compressed air from the compressor is stored inside the pressure vessel. This pressure vessel is mounted with a pressure gauge for indicating the pressure maintained inside the pressure vessel. The box pipe and plastic tube are used to transfer the compressed air from the compressor to the pressure vessel and from pressure vessel to the working unit.

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Introduction

A compressor is a machine that compresses air or another type of gas from a low inlet pressure (usually atmospheric) to a higher desired pressure. This is accomplished by reducing the volume of the gas. Air compressors are generally positive displacement units and are either of the reciprocating piston type or the rotary screw type or rotary vane types.

Piston Compressor

In this project single-piston compressor is used because pressure provide by this, is sufficient to solve our purpose.

Piston compressor contains piston rings operating in precision-bored close-fitting cylinders. Note that the cylinders have air fins to help dissipate the heat. Cooling is necessary with compressors to dissipate the heat generated during compression. When air is compressed, it picks up heat as the molecules of air come closer together and bounces off each other at faster and faster rates. Excessive temperature can damage the metal components as well as increase input power requirements. Portable and small industrial compressors are normally air cooled, whereas larger units must be water-cooled. A typical small-sized, two-stage compressor unit, observe that it is a complete system containing not only a compressor but also the compressed air tank (receiver), electric motor and pulley drive, pressure controls, and instrumentation for quick hookup and use. This particular compressor unit also contains an air dryer, which provides a constant supply of high-quality dry air for application where moisture would be a problem. It is driven by a 10-hp motor, has a 120-gal receiver, and is designed to operate in the 145-to 175-psi range with a capacity of 46.3 cfm (cubic ft per min).

A cutaway view of a direct-drive, two-cylinder piston type compressor by providing forced air flow.

A single-piston compressor can provide pressure up to about 150 psi. Above150 psi, the compression chamber size and heat of compression chamber size and heat of compression prevent efficient pumping action. For compressors having more than one cylinder, staging can be used to improve pumping efficiency. Staging means dividing the total pressure among two or more cylinders by feeding the exhaust from one cylinder into the inlet of the next. Because effective cooling can be implemented between stages, multistage compressors can be dramatically increase the efficiency and reduce input power requirements. In multistage piston compressors, successive cylinder sizes decrease and the intercooling removes a significant portion of the heat of compression. This increases air density and the volumetric efficiency of the compressor.

Electric Motor

Introduction

The electric motor used in the air compressor is the single-phase induction motor. Single-phase induction motors are small-size motors of fractional-kilowatt ratings. Domestic appliances like fans, hair driers, washing machines, vacuum cleaners, mixers, refrigerators, food processors and other kitchen equipments employ these motors. These motors also find applications in air-conditioning fans, blowers, office machinery, small power tools, dairy machinery, small farming equipment etc.

Principle

A single-phase induction motor consists of a single-phase winding mounted on the stator and a cage winding on the rotor. When a single-phase supply is connected to the stator winding a pulsating magnetic field is produced. By pulsating field we mean that the field builds up in one direction, falls to zero, and then builds up in the opposite direction. Under these conditions, the rotor does not rotate due to inertia. Therefore, a single phase induction motor is inherently not self-starting and requires some special starting means. If, however, the single-phase stator winding is exited and the rotor of the motor is started by an auxiliary means, and the starting device is then removed, the motor continues to rotate in the direction in which it is started.

Starting Methods & Types of Single-Phase Induction Motors

We have seen that some means should be used to start the single-phase induction motor. Mechanical methods are impractical and, therefore, the motor is started temporarily converting it into a two-phase motor.

Single-phase motors are usually classified according to the auxiliary means used to start the motor. They are classified as follows:

  1. Split-phase motor.
  2. Capacitor-start motor.
  3. Capacitor-start capacitor-run motor.
  4. Permanent-split capacitor motor.

All these starting methods depend on two alternating fields displaced in space and phase.

The resultant of two fields is a rotating field. This rotating field reacts with a cage rotor to provide the starting torque. One field is produced by the main winding and the other by the auxiliary winding. The auxiliary winding is also called starting winding.

Capacitor Motors

Capacitor motors are single-phase induction motors that employ a capacitor in the auxiliary winding circuit to produce a greater phase difference between the current in the main and auxiliary winnings. There are three types of capacitor motors.

Capacitor-Start Motor

It has a cage rotor and its stator has two windings namely, the main winding and the auxiliary winding (starting winding).The two windings are displaced 90 degree in space. A capacitor is connected in series with the starting windings. A centrifugal switch is also connected.

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By choosing a capacitor of the proper rating the current in the main winding may be made to lag the current in the auxiliary winding by 90 degree. Thus, a single-phase supply current is split into two phases to be applied to the stator windings. Thus the windings are displaced 90 degree electrical and their mmf's are equal in magnitude but 90 degree apart in time phase.

Therefore the motor acts like a balanced two- phase motor. As the motor approaches its rated speed, the auxiliary winding and the starting capacitor are disconnected automatically by the centrifugal switch mounted on the shaft. The motor is so named because it uses the capacitor only for the purpose of starting.

Motor Characteristics

The capacitor-start motor develops a much higher starting torque (3.0 to 4.5 times the full-load torque) than does an equally rated resistance-start motor. The value of the starting capacitor must be large and the starting-winding resistance low to obtain a high starting torque. Because of the high VA rating of the capacitor required, electrolytic capacitors are used. The capacitor is short-time rated. The torque-speed characteristic of the motor is which shows that the starting torque is high.

Capacitor start motors are more costly than split-phase motors because of the additional cost of the capacitor.

Reversal of Direction of Rotation

The capacitor-start motor may be reversed by reversing the connections of one of the windings. The motor is first brought to rest for this purpose.

Applications

Capacitor-start motors are used for loads of higher inertia where frequent starts are required. These motors are most suitable for pumps and compressors, and therefore they are widely used in refrigerators and in air-conditioner compressors. They are also used for conveyors and some machine tools.

Pressure Vessel

Pressure Vessel are used in a large number of and in different industries including chemical and fertilizers, power plants, cryogenic application, gas storage and the like.

Pressure Vessel technology is considered an independent field and its importance are recognized by International Institute of Welding (IIW) by having a separate Commission for it.

Material Selection

All Pressure Vessels can be considered to consist of mainly the shell, the nozzles, and the tubes. There are four main factors which determine the selection of materials for pressure vessel shells:

  1. Fitness for the application
  2. Requirements of code
  3. Cost
  4. Availability

Fitness for the application

It means that the pressure vessel we use is whether fit for with standing the pressure or not. The type of design requirement required is provided by the pressure vessel or not.

Requirements of code

Certain code of according to the requirement is provided by the International Community to the pressure vessel.

Cost

Cost has always been the important aspect for starting any work. Achieving good quality with lesser cost is the very min objective while selecting the pressure vessel.

Availability

Availability of the pressure vessel should be easy. Even the requirements for maintenance afterwards should be easy to get.

Bourdon Tube Pressure Gauge

Pressure gauge using Bourdon tubes as the elastic pressure element are most commonly used for pressure measurement because of their simplicity and versatility. The Bourdon tubes find wide applications because of their simple design and low cost, and are most commonly used for local indications and for signal transmission to remote locations.

There are three types of Bourdon elements and they are,

  1. C type
  2. Spiral type
  3. Helical type

Bourdon tube elements has several distinct advantages and these include low cost, simple construction, high pressure range, good accuracy except at low pressure, and improved designs at this pressure for maximum safety. Their greatest advantage is that they are easily adapted for designs for obtaining electrical outputs.

The disadvantages of Bourdon tubes are their low spring gradient which limits their use for precision measurements up to a pressure of 3 MN/square meter, are susceptible to shocks and vibrations and are subject to hysteretic.

C Type Bourdon Tube

The C type of Bourdon element is most commonly used for local indication but it is also used for pressure transmission and control applications. The tube which is oval in section is formed into an arc of 250 degree and hence the name C for the configuration.

One end called the tip of the tube is sealed and is called free end. This is attached by a light link-work to a mechanism which operates the pointer. The other end of the tube is fixed to a socket where the pressure to be measured is applied. The internal pressure tends to change the section of the tube. The degree of linearity depends upon the quality of gauge from oval to circular, and this tends to straighten out the tube. The movement of the tip is ideally proportional to the pressure applied. The tip is connected to a spring loaded link-work and a geared sector and pinion arrangement which amplifies the displacement of the tip and converts into the deflection of the pointer. The linkage is constructed so that the mechanism may be constructed for optimum linearity and minimum hysteretic, as well as to compensate for wear which may develop over the time.

Plastic tubing & Flexible Hoses

Plastic Tubing

Plastic tubing has gained rapid acceptance in fluid power industry because it is relatively inexpensive. Also, it can be readily bent to fit around obstacles, it is easy to handle, and it can be stored on reels. Another advantage is that it can be color-coded to represent different part of circuit because it is available in many colors. Since plastic tubing is flexible, it is less susceptible to vibration damage than steel tubing.

Fittings for plastic tubing are almost identical to those designed for steel tubing. In fact many steel tube fittings can be used on plastic tubing.

Plastic tubing is used universally in pneumatic systems because air pressures are low, normally less than 100 psi. Of course, plastic tubing is compatible with most hydraulic fluids and hence is used in low pressures hydraulic applications.

Materials for plastic tubing include polyethylene, and nylon. Each material has special properties that are desirable for specific applications. Manufacturers' catalogs should be consulted to determine which material should be used for a particular application.

Flexible Hoses

Flexible hoses are important type of hydraulic conductor, which is used when hydraulic components such as actuators are subjected to movement. Examples of this are found in portable power units, mobile equipment, and hydraulically powered machine tools. Hose is fabricated in layers of elastomer (synthetic rubber) and braided fabric or braided wire, which permits operation at higher pressures.

The outer layer is normally synthetic rubber and serves to protect the braid layer. The hose can have as few as three layers (one being braid) or can have multiple layers to handle elevated pressures. When multiple wire layers are used, that may alternate with synthetic rubber layers, or the wire layer may be placed directly over one another.

Working pressures may vary from 100 to 5000 psi depending upon sizes.

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