Process Parameters of AJM

The process parameters in AJM can be grouped into the following categories. The Ishikawa cause and effect diagram depicts the effect of various process parameters on the accuracy and quality of the machining operations by the abrasive jet machine.

1. The Abrasive: types, composition, strength, size, mass flow rate

2. The Gas: composition, pressure and velocity

3. The nozzle: geometry, material, stand-off distance (SOD), feed rate, inclination to work

4. The workpiece: Type of material

The selection of abrasive particles to be used in AJM depends upon the type of work material and type of machining operation which needs to be carried out. Different machining operations such as finishing, roughing require different types of abrasive for AJM operations. Commonly used abrasive for cutting include aluminum oxide and silicon carbide. In cleaning, etching and polishing operations glass beads and dolomites are recommended. The size of the abrasive particles also plays an important role in type of machining operations of AJM. Coarse grain particles are recommended for cuttingoperations while fine grains are recommended for finishing or polishing operations


The gas used in the AJM process must be non-toxic. It should be cheap and easily available. Common types of gas used in AJM applications are air, nitrogen and carbon. The recommended velocity of gas abrasive mixture ranges between 100 m/sec to 300 m/ sec depending upon the cutting or finishing operation.

The velocity of gas abrasive mixture is a function of nozzle design, nozzle pressure, and abrasive particle size. Stand-off distance (SOD) is a very important parameter. SOD is defined as the distance between the tip of nozzle and the work surface. The larger the SOD the poorer is the quality and accuracy of the cut. The effect of SOD on the accuracy of the cut is 10 – 30 micron Cutting, grooving

Abrasive Jet Machining (AJM)

In abrasive jet machining (AJM) material removal occurs on account of impact of high velocity air / gas stream of abrasive particles on the workpiece. The abrasives are propelled by a high velocity gas to erode material from the workpiece. As an outcome of impact of the abrasive particles on the workpiece, tiny brittle fractures occur at the surface of the workpiece and the carrier gas carries away the fractured fragments. AJM is also called as abrasive blasting process. It is also known by several other names such as abrasive micro-blasting, pencil blasting and micro-abrasive blasting. AJM is an effective machining method for hard and brittle materials such as glass, silicon, tungsten and ceramics. Typically the process is used for cutting intricate shapes or forms of specific edges. The process is inherently free from chatter, vibration and heat problems because the tool never touches the substrate. The schematic of AJM process set up is shown in Figure

Principle of AJM

The principle of machining / cutting by abrasive jet process is explained through the
following steps:

1. Abrasive particles of size between 10 m to 50 m (depending upon the requirement of either cutting or finishing of the work piece) are accelerated in a gas stream (commonly used gas stream is air at high atmospheric pressures).
2. The smaller abrasive particles are useful for finishing and bigger are used for cutting operations.
3. The abrasive particles are directed through the nozzle, towards the work piece surface where-ever cutting or finishing is to be done. The distance between the tip of the nozzle and the work surface is normally within 1 mm.
4. As the abrasive particles impact the surface of the work piece, it causes a small fracture at the surface of the work piece. The material erosion occurs by the chipping action.
5. The erosion of material by chipping action is convenient in those materials that are hard and brittle.
6. As the particles impact the surface of
7. The abrasive particles once used, cannot be re-used as its shape changes partially and the work piece material is also clogged with the abrasive particles during impingement and subsequent flushing by the carrier gas.

Advantages
 AJM process is a highly flexible process wherein the abrasive media is carried by
a flexible hose, which can reach out to some difficult areas and internal regions.
 AJM process creates localized forces and generates lesser heat than the conventional machining processes.
 There is no damage to the workpiece surface and also the process does not have tool-workpiece contact, hence lesser amount of heat is generated.
 The power consumption in AJM process is low. Disadvantages
 The material removal rate is low
 The process is limited to brittle and hard materials
 The wear rate of nozzle is very high
 The process results in poor machining accuracy
 The process can cause environmental pollution
Applications:
Metal working:
 De-burring of some critical zones in the machined parts.
 Drilling and cutting of the thin and hardened metal sections.
 Removing the machining marks, flaws, chrome and anodizing marks.
Glass:
 Cutting of the optical fibers without altering its wavelength.
 Cutting, drilling and frosting precision optical lenses.
 Cutting extremely thin sections of glass and intricate curved patterns.
 Cutting and etching normally inaccessible areas and internal surfaces.
 Cleaning and dressing the grinding wheels used for glass.
Grinding:
 Cleaning the residues from diamond wheels, dressing wheels of any shape and size.

Classification of Advanced Machining / Material Removal Processes:

These processes are referred to a typical group of advanced machining processes in which the excess material is removed by non-traditional source of energy arising from electrical, mechanical, thermal or chemical source. Most of these processes don’t use a sharp cutting tool, as in the conventional case. Advanced material removal processes are generally classified according to the type of energy used to remove material. The classification of these processes based on the energy is given as below The processes based on use of Electrochemical Energy are:
 Electro-Chemical Machining (ECM),
 Electro-Chemical Grinding (ECG),
The processes based on the use of Thermal Energy are:
 Electric- Discharge Machining (EDM),
 Wire-Cut Electric Discharge Machining (WEDM)
 Laser Beam Machining (LBM),
 Electron Beam Machining (EBM).
The processes based on the use of Mechanical Energy are:
 Abrasive Flow Machining (AFM)
 Abrasive Jet Machining (AJM),
 Water Jet Machining (WJM),
 Abrasive Water Jet Machining
 Ultrasonic Machining (USM),

Why are Advanced Machining / Material Removal Processes Needed?

With the advent of new materials and the requirements of complex features on them, there was a necessity to develop new processes. Some of these features are:
1. Related to material properties:
 High hardness  High strength  High brittleness 2. Related to workpiece structure:
 Complex shapes  Typical thin and delicate geometries  Parts which are difficult in fixturing 3. Related to requirements in high surface finish and tight tolerances.
4. Related to controlling of temperature rise and residual stresses.

Need For Advanced Material Removal Processes

Advanced Material Removal Processes represent one of the technologies, which emerged after the second world war to cope up with the demands of sophisticated, more durable and cost competitive products. With the advent of new materials such as metal-matrix composites, super-alloys, ceramics, aluminates and high performance polymers etc. and the stringent requirements to machine complex geometrical shapes with high precision and accuracy, a strong need existed for the development of advanced material removal processes. The processes in this category differ from conventional processes in either utilization of energy in an innovative way or, in using forms of energy that were unused for the purpose of manufacturing. The conventional machining processes normally involve the use of energy from electric motors, hydraulics, gravity, etc. and rely on the physical contact between tools and work components. On the contrary, advanced material removal processes utilize energy from sources such as electrochemical reactions, high temperature plasma, high velocity jets and loose abrasives mixed in various carriers etc. Although these processes were originally developed to handle unique problems in aerospace industry (machining of very hard and tough alloys), today wide range of industries have adopted this technology in numerous manufacturing operations.

Why dry compressed air?

The air we breathe contains contamination in the form of water vapour and airborne particles. During the compression process an air compressor concentrates these contaminants and depending on the design and age will even add to the contamination in the form of oil carry over.

Modern air compressors generally have built in after coolers that reduce the discharge temperature of the compressed air and with the help of water separators, remove the bulk of liquid water.
In some applications this may be sufficient, but the remaining dirt and moisture content suspended in aerosol form, can, if not removed, damage the compressed air system and cause product spoilage.
Air Contaminants lead to increase down time and reduced productivity. it lead to corrosion , damaged Tools , poor finish to painting Jobs etc .

Compressors & Compressed Air Systems - Post 3

Rotary compressor

Rotary compressors have rotors in place of pistons and give a continuous pulsation free discharge. They operate at high speed and generally provide higher throughput than reciprocating compressors. Their capital costs are low, they are compact in size, have low weight, and are easy to maintain. For this reason they have gained popularity with industry. They are most commonly used in sizes from about 30 to 200 hp or 22 to 150 kW.

Types of rotary compressors include:
Lobe compressor (roots blower)
Screw compressor (rotary screw of helical-lobe,where mail and female screw rotors moving in opposite directions and trap air, which iscompressed as it moves forward,)
Rotary vane / sliding- vane, liquid-ring, and scroll-type
Rotary screw compressors may be air or water-cooled. Since the cooling takes place right inside the compressor, the working parts never experience extreme operating temperatures. The rotary compressor, therefore, is a continuous duty, air cooled or water cooled compressor package.
Because of the simple design and few wearing parts, rotary screw air compressors are easy to maintain, operate and provide great installation flexibility. Rotary air compressors can be installed on any sur face that will support the static weight.

Dynamic Compressors
The centrifugal air compressor is a dynamic compressor, which depends on transfer of energy from a rotating impeller to the air. The rotor accomplishes this by changing the momentum and pressure of the air. This momentum is converted to useful pressure by slowing the air down in a stationary diffuser. The centrifugal air compressor is an oil free compressor by design. The oil lubricated running gear is separated from the air by shaft seals and atmospheric vents.

COMPRESSORS AND COMPRESSED AIR SYSTEMS - Post 2

TYPES OF COMPRESSORS
There are two basic compressor types: positive-displacement and dynamic.

In the positive-displacement type, a given quantity of air or gas is trapped in a compression chamber and the volume it occupies is mechanically reduced, causing a corresponding rise in pressure prior to discharge. At constant speed, the air flow remains essentially constant with variations in discharge pressure.

Dynamic compressors impart velocity energy to continuously flowing air or gas by means of impellers rotating at very high speeds. The velocity energy is changed into pressure energy both by the impellers and the discharge volutes or diffusers.

Positive Displacement Compressor
two types: reciprocating and rotary.

Reciprocating compressor

In industry, reciprocating compressors are the most widely used type for both air and refrigerant compression. They work on the principles of a bicycle pump and are characterized by a flow output that remains nearly constant over a range of discharge pressures. Also, the compressor capacity is directly proportional to the speed
Reciprocating compressors are available in many configurations, the four most widely used are horizontal, vertical, horizontal balance-opposed and tandem. Vertical type reciprocating compressors are used in the capacity range of 50 – 150 cfm. Horizontal balance opposed compressors are used in the capacity range of 200 – 5000 cfm in multi-stage design and up to 10,000 cfm in single stage designs
The reciprocating air compressor is considered single acting when the compressing is accomplished using only one side of the piston. A compressor using both sides of the piston is considered double acting.
A compressor is considered to be single stage when the entire compression is accomplished with a single cylinder or a group of cylinders in parallel. Many applications involve conditions beyond the practical capability of a single compression stage. Too great a compression ratio (absolute discharge pressure/absolute intake pressure) may cause excessive discharge temperature or other design problems. Two stage machines are used for high pressures and are characterized by lower discharge temperature (140 to 160oC) compared to single-stage machines (205 to 240oC).
For practical purposes most plant air reciprocating air compressors over 100 horsepower are built as multi-stage units in which two or more steps of compression are grouped in series. The air is normally cooled between the stages to reduce the temperature and volume entering the following
Reciprocating air compressors are available either as air-cooled or water-cooled in lubricated and non- lubricated configurations, may be packaged, and provide a wide range of pressure and capacity selections.

COMPRESSORS AND COMPRESSED AIR SYSTEMS - Post 1

INTRODUCTION

Industrial plants use compressed air throughout their production operations, which is produced by compressed air units ranging from 5 horsepower (hp) to over 50,000 hp. The US Department of Energy (2003) reports that 70 to 90 percent of compressed air is lost in the form of unusable heat, friction, misuse and noise . For this reason, compressors and compressed air systems are important areas to improve energy efficiency at industrial plants.
It is worth noting that the running cost of a compressed air system is far higher than the cost of a compressor itself . Energy savings from system improvements can range from 20 to 50 percent or more of electricity consumption, resulting in thousands to hundreds of thousands of dollars. A properly managed compressed air system can save energy, reduce maintenance, decrease downtime, increase production throughput, and improve product quality

Compressed air systems consist of a supply side, which includes compressors and air treatment, and a demand side, which includes distribution and storage systems and end -use equipment. A properly managed supply side will result in clean, dry, stable air being delivered at the appropriate pressure in a dependable, cost-effective manner. A properly managed demand side minimizes wasted air and uses compressed air for appropriate applications. Improving and maintaining peak compressed air system performance requires addressing both the supply and demand sides of the system and how the two interact.

Main Components of Compressed Air Systems

Consist of the Following

Intake Air Filters : Prevent dust from entering a compressor; Dust causes sticking valves, scoured cylinders, excessive wear etc.
Inter-stage Coolers : Reduce the temperature of the air before it enters the next stage to reduce the work of compression and increase efficiency. They are normally water-cooled. After-Coolers: The objective is to remove the moisture in the air by reducing the temperature in a water-cooled heat exchanger.
Air-dryers : The remaining traces of moisture after after-cooler are removed using air dryers, as air for instrument and pneumatic equipment has to be relatively free of any moisture. The moisture is removed by using adsorbents like silica gel /activated carbon, or refrigerant dryers, or heat of compression dryers
Moisture Drain Traps: Moisture drain traps are used for removal of moisture in the compressed air. These traps resemble steam traps. Various types of traps used are manual drain cocks, timer based / automatic drain valves etc.
Receivers : Air receivers are provided as stora ge and smoothening pulsating air output - reducing pressure variations from the compressor

Design Consideration for Pools

Design Consideration for Pools & Spas Swimming Pools

According to ASHRAE (1999a) the desirable temperature for swimming pools is 27c , however this will vary from the culture by so much as 5 degree Celsius. .If the geothermal water is higher in temperature then some sort of mixing or cooling by aeration or in a holding pond is required to lower the temperature . If the geothermal water is used directly in the pool , then a flow through process is neccessary to replace the used water on regular basis. In many cases the pool water must be treated with chlorine , therefore it is more economical to used a closed loop system for treatment water and have geothermal water provide heat through heat ex changer . The Water Heating System should be installed in the return line to the pool. Acceptable water circulation level vary from eight hours to six hours for a complete change of water. Heat exchanger must be designed to resist the corrosive effect of the chlorine in the pool water and scaling or corrosion from the geothermal water. This often requires in the case of plate heat exchanger using titanium plates .

Four Factors determine the sizing of the system for temperature and flow rate . These are
i) Conduction through the pool walls
2) convection through the pool surface
3 ) Radiation from the pool surface
4) Evaporation from the pool surface

Conduction is Least significant unless the pool is above ground or in contact with the cold underground water
Convection losses depends on the temperature difference between the pool water and the surrounding air and the wind speed.this substantially low for indoor pool also pool with wind speed breakers.
Radiation losses are greater at night for the outdoor pools , however their will be gain in temperature during daytime. A Floating pool Covers can reduces both radiation and evaporation losses. Evaporation loss constitute the greatest heat loss from pools -50 to 60 % in most cases. The rate of which evaporation occurs is a function of air velocity and pressure difference between the pool water and the water vapor in the air .
As the temperature of the pool water is increased or the relative humidity of the air is decreased evaporation rate increase.

The required Gethermal heating output q can be determined by the following two equations
q1 = Density of Water * Pool heat up *pool Volume * ( Desired Temp - intial Temp ) * Pool heat up time

q2 = Surface heat Transfer Coefficient * pool Surface area * ( Pool Temp - ambient temp)

then Q= q1- q2

if there is no heat up time which is typical for geothermal pools then equations (1) will be zero and only equation 2 will apply. Equation 2 will assume a wind velocity of 5 to 8 Km/h . For Sheltered Pool wind velocity factor less than 5km/h

The neccessary Heat to increase and maintain the temperature of an outdoor pool can be expressed as

H( Total) = h (Surface) + h (heat up)

h (heat up) = Volume *8.34 (lbs/gal) * ( intial Temp - Final Temp) * 1.0 / 72

72 = time required to Rise the temp of pool

h (Surface) = ks * dtw* A

where

ks = surface heat loss factor

dtw = Temp Difference between the air and surface water in the pool

A = Surface area of the pool