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Air Compressor
[201 0] -: INTRODUCTION:-
Air compressors have relatively simple design and can be sized all the way from the extremely powerful to ones that can just drive small tools. They have few parts and hence last for a long time with little maintenance. The factors make them drivers of choice from heavy equipment on manufacturing shop floors, to cleaning areas in gas stations and now, inside homes and small garages as parts of handy gadgets. An air compressor works by transforming electrical energy in to kinetic energy in the form of air that is held in a highly compressed space before sudden release. The energy released when the compressed air is let out can be used for inflation, cleaning under pressure, turning, generating torque, driving or other similar movement by force. The principle of an air compressor’s functioning is like that of an internal combustion unit. A crankshaft moves a piston through a connecting rod, in a linear fashion along the length of a cylinder in which it fits snugly. Air is drawn in through an open valve as the piston moves upwards, providing ample space for a high volume of air. The valve closes and the piston moves
down,
compressing
the
air.
The
latter
has
kinetic energy as it gains tremendous pressure because of the restricted space in to which it has been forced. The valve opens again to release the energy.
ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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Air Compressor
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Cylinders may be placed in series, so that the compressed air is fed to an intake where it is compressed further. The pressure goes up further and hence the power is increased in proportion. All air compressors must have back up systems to release extra pressure in the event of a malfunction in the valve mechanism. This is required to prevent an accident such as in an explosion. The efficiency of air compression depends on air temperature, atmospheric pressure and relative humidity. The rating of an air compressor therefore depends on ambient conditions. This aspect should be taken in to if an air compressor is used in very cold weather, very wet weather or at an high altitude. Oil is used to lubricate the piston as it moves inside the cylinder. Rings act as seals to reduce the amount of oil that can mix
with
the
compressed
air.
Some
oil
contamination,
especially in atomized form, will inevitably escape ring seals and hence a more secure oil containment design is required in case the compressed air energy source has to be completely free of oil. Painting and other functions related to finishing a surface have this kind of requirement as even a faint oil trace can affect appearance. Air compressors have relatively few parts and hence require little maintenance. They last for years and are versatile and compact as well. This is why new uses for air compressors keep ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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Air Compressor
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surfacing so many years after the first unit was built. Some models have a second cylinder and sizes can be adjusted at the design stage to increase the range of applications and uses. Years ago, it was common for shops to have a central power source that drove all the tools through a system of belts, wheels and driveshafts. The power was routed around the work space by mechanical means. While the belts and shafts may be gone, many shops still use a mechanical system to move power around the shop. It's based on the energy stored in air that's under pressure, and the heart of the system is the air compressor. You'll find air compressors used in a wide range of situations— from corner gas stations to major manufacturing plants. And, more and more, air compressors are finding their way into home workshops, basements and garages. Models sized to handle every job, from inflating pool toys to powering tools such as nail guns, sanders, drills, impact wrenches, staplers and spray guns are now available through local home centers, tool
dealers
and
mail-order
catalogs.
The big advantage of air power is that each tool doesn't need its own bulky motor. Instead, a single motor on the compressor converts the electrical energy into kinetic energy. This makes for light, compact, easy-to-handle tools that run quietly and have fewer parts that wear out. ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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Air Compressor
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Air Compressor
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Air compressor
types
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Air Compressor
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While there are compressors that use rotating impellers to generate air pressure, positive-displacement compressors are more common and include the models used by homeowners, woodworkers, mechanics and contractors. Here, air pressure is increased by reducing the size of the space that contains the air. Most of the compressors you'll run across do this job with a reciprocating
piston.
Like a small internal combustion engine, a conventional piston compressor has a crankshaft, a connecting rod and piston, a cylinder and a valve head. The crankshaft is driven by either an electric motor or a gas engine. While there are small models that are comprised of just the pump and motor, most compressors have an air tank to hold a quantity of air within a preset pressure range. The compressed air in the tank drives the air tools, and the motor cycles on and off to automatically maintain
pressure
in
the
tank.
At the top of the cylinder, you'll find a valve head that holds the inlet and discharge valves. Both are simply thin metal flaps– one mounted underneath and one mounted on top of the valve plate. As the piston moves down, a vacuum is created above it. This allows outside air at atmospheric pressure to push open the inlet valve and fill the area above the piston. As the piston moves up, the air above it compresses, holds the inlet valve shut and pushes the discharge valve open. The air moves from ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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Air Compressor
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the discharge port to the tank. With each stroke, more air enters
the
tank
and
the
pressure
rises.
Typical compressors come in 1- or 2-cylinder versions to suit the
requirements
of
the
tools
they
power.
On
the
homeowner/contractor level, most of the 2-cylinder models operate just like single-cylinder versions, except that there are two strokes per revolution instead of one. Some commercial 2cylinder compressors are 2-stage compressors–one piston pumps air into a second cylinder that further increases pressure. Compressors use a pressure switch to stop the motor when tank pressure reaches a preset limit–about 125 psi for many single-stage models. Most of the time, though, you don't need that much pressure. Therefore, the air line will include a regulator that you set to match the pressure requirements of the tool you're using. A gauge before the regulator monitors tank pressure and a gauge after the regulator monitors air-line pressure. In addition, the tank has a safety valve that opens if the pressure switch malfunctions. The pressure switch may also incorporate an unloader valve that reduces tank pressure when
the
compressor
is
turned
off.
Many articulated-piston compressors are oil lubricated. That is, they have an oil bath that splash-lubricates the bearings and ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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cylinder walls as the crank rotates. The pistons have rings that help keep the compressed air on top of the piston and keep the lubricating oil away from the air. Rings, though, are not completely effective, so some oil will enter the compressed air in
aerosol
form.
Having oil in the air isn't necessarily a problem. Many air tools require oiling, and inline oilers are often added to increase a uniform supply to the tool. On the down side, these models require regular oil checks, periodic oil changes and they must be operated on a level surface. Most of all, there are some tools and situations that require oilfree air. Spray painting with oil in the airstream will cause finish problems. And many new woodworking air tools such as nailers and sanders are designed to be oilfree so there's no chance of fouling wood surfaces with oil. While solutions to the airborne oil problem include using an oil separator or filter in the air line, a better idea is to use an oilfree compressor that uses permanently lubricated bearings in
place
of
the
oil
bath.
A variation on the automotive-type piston compressor is a model that uses a one-piece piston/connecting rod. Because there is no wrist pin, the piston leans from side to side as the eccentric journal on the shaft moves it up and down. A seal around the piston maintains with the cylinder walls and prevents
air
leakage.
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Air Compressor
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Where air requirements are modest, a diaphragm compressor can be effective. In this design, a membrane between the piston and the compression chamber seals off the air and prevents leakage.
~:Compressor power:~ One of the factors used to designate compressor power is motor horsepower. However, this isn't the best indicator. You really need to know the amount of air the compressor can deliver
at
a
specific
pressure.
The rate at which a compressor can deliver a volume of air is noted in cubic feet per minute (cfm). Because atmospheric pressure plays a role in how fast air moves into the cylinder, cfm will vary with atmospheric pressure. It also varies with the temperature and humidity of the air. To set an even playing field, makers calculate standard cubic feet per minute (scfm) ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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as cfm at sea level with 68 degrees F air at 36% relative humidity. Scfm ratings are given at a specific pressure–3.0 scfm at 90 psi, for example. If you reduce pressure, scfm goes up,
and
vice
versa.
You also may run across a rating called displacement cfm. This figure is the product of cylinder displacement and motor rpm. In comparison with scfm, it provides an index of compressor pump
efficiency.
The cfm and psi ratings are important because they indicate the tools that a particular compressor can drive. When choosing a compressor, make sure it can supply the amount of air and the pressure that your tools need.
Air Compressors
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Air Compressor
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Graph of Pressure against volume in a reciprocating compressor Volumetric efficiency Vh = Actual suction volume Vx/ Theoretical suction volume Vs For greater efficiency air compression should be isothermal as this requires the minimum work input. In practice Isothermal compression is not possible, an ideal Isothermal cycle requires sufficient time to allow all the required heat to be transferred out of the cylinder, practicality dictates that the piston must have a relatively high speed to give a reasonable output, Cylinder cooling on a single stage compressor gives better efficiency but there is a limitation in the surface area to cylinder volume that can be used for cooling effect, but multistage compressors with an efficient extended surface interstage cooler gives cycle improved compression efficiency better approaching that of the isothermal. In theory the greater the number of stages the closer the curve will approach the ideal isothermal compression curve, however there is an ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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Air Compressor
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increase in cost, complexity, and the law of diminishing returns limit the number. Compression in stages has the following advantages; 1. The compression ratio at each stage is lower and so the final temperature is lower. This reduces problems with lubrication 2. . The machine is smaller and better balanced 3. water can be drained off at each stage 4. Compression better approaches the ideal isothermal It is important that the compressor clearance volume is kept small as possible in order to improve overall volumetric efficiency as the air trapped in this space must expand to below suction pressure before new air can enter, this is an effective loss of stroke. A clearance is required in order to prevent the piston striking the cylinder cover when starting or stopping off load. The clearance volume is sometimes referred to as the 'Bump Clearance'. Crankcase lubrication Lubrication of the crankcase in a compressor does not pose any specific problems and normally consist of splash lubrication with pressurised oil being fed to shell bearings. Where drip cylinder lubrication is used, this should be kept to a minimum conducive with liner wear. A standard mineral oil similar to that used in the main engine may be used, although due to carbon deposits, higher quality oils are generally used with the most effective being specifically designed synthetics which have allow a considerable reduction in maintenance but are costly.
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Air Compressor
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Mineral oils contain a blend of lighter elements such as paraffin's, and heavier elements such as asphaltenes. During compression the lighter elements are vaporised leaving the heavy ends, these coat the piston rings and discharge valves in combination with oxidised oil deposits. These deposits also coat age ways and coolers resulting in higher interstage air temperatures. Deposits on discharge valves cause them to become sticky and leak resulting in hot air being drawn back into the cylinder for recompression. This increases the temperature and hence causes greater oxidation and deposits, and so the condition deteriorates with increasing rapidity.
Temperature can become very high, this may result in oily deposits at the discharge valves carbonising. Eventually this carbon could glow red and cause detonation. It is more likely, however, that oily deposits will be carried over to the air receiver and air start manifold to be ignited by blowpast at the cylinder air start valve. Deposits at piston rings cause leakage allowing oil to enter the cylinder from the crankcase thus increasing the danger it is essential that crankcase lubrication be kept to a minimum compatible with an acceptable wear rate. Regular maintenance will minimize oily deposits build up and hence the risk of explosion Materials and design of a reciprocating compressor The compressor casing, cylinder covers and piston rings are generally of cast iron. Pistons may be of cast iron, steel of aluminium. Aluminium being the preferred material for use on the LP piston due to its larger diameter. Valves are usually made so that parts can be interchanged between the suction and discharge valves. Seats are of mild steel with small ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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diameter air ages to prevent the fragments of broken valve plate from entering the cylinder. Valve plates are of vanadium steel heat treated and ground to provided the required hardness and surface finish. Springs should be arranged such that they lift and seat squarely. Uneven spring force or deposits on the seat cause valves to bend resulting in fatigue cracking.
For compressors designed for starting air requirements a water jacket relief valve is fitted.
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Air Compressor
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Rotary Compressor
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The rotary compressor may be of the impeller type similar to that used in the turbocharger , scroll, twin rotating lobes or of the sliding vane type similar to the one shown above. In practice there would be several more vanes than shown. Rotary compressors are capable of handling large quantities of low pressure air much more efficiently than a reciprocating compressor. In order to produce increased pressures it is possible to stage rotary compressors but leakage problems increase at higher pressures as well as stress on the vanes. The sliding vane compressor consists of a slotted rotor with its axis offset from that of the cylindrical casing. Vanes fit in the slots and have with the casing On the suction side the space contained between the casing, the rotor and an adjacent pair of vanes is gradually increasing allowing air to be drawn in. On the compression side this same space is gradually reduced causing the pressure increase. When the leading vane uncovers the discharge port air will flow to outlet. Larger
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Air Compressor
compressors of this type are compressors tend to be air cooled.
[201 0] water-cooled,
smaller
The main problems related with sliding vane compressors concern wear at the vane tips and sealing of the ends.
Rotary/reciprocating Compressor Rotary compressors in general do not require internal lubrication but they are not suitable alone for providing air at a pressure for starting duties. They can, however, be linked to reciprocating stages to produce a hybrid compressor. The compressor is lighter, more compact and better balanced than an equivalent all reciprocating unit. In basic the rotary first stage supplies air to the reciprocating second and subsequent stages. All stages being driven by the same shaft
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Air Compressor
[201 0] Safety Valve
Materials Cast iron-Casing, Liners, Pistons( the LP piston is sometimes made from an aluminium alloy, Cylinder covers SteelCrankshaft, Conrods, Pistons, Valve seats Vanadium SteelValve plates Starting air compressor circuit Starting and stopping sequence is adjustable, the magnetic valves are open when the compressor is stopped so any residual pressure is blown off. On starting the magnetic valve are sometimes delayed to close so as to allow the compressor motor to reach full speed before the compressor is loaded up. The non-return valves prevent HP air leaking back from the receiver on which the filling is also of the non return type.
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Calculation of required cylinder compression for a multistage reciprocating compressor r = R = for r = R1/2 for r = R1/3
stage compression a
a
three
pressure pressure two
ratio ratio stage
stage
for example, a 3 stage compressor requiring a final pressure of 64bar would have the following interstage pressures 1st stage 1bar compressed to 4bar 2nd stage 4bar compressed to 16bar 3rd stage 16bar compressed to 64bar It would appear that most of the work is being carried out in the final stage, this is untrue as with the increase in pressure is a complimentary reduction in volume, if the temperature ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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conditions remain the same then work will be equally divided between the stages. By reducing the suction pressure, the cylinder is required to do more work on the air before the discharge valve opens. This means that the air will be delivered at a higher pressure. The higher temperature can lead to problems with the cylinder lubrication as well as a drop in efficiency as well as carbonising of the oil and increased deposits on the valves and piston rings and interstage ages. In the extreme it can lead to seizure and possible diesel detonation of the oil laden air. The reduction in pressure at the suction can be due to a partially blocked suction filter or partially choked suction valve. The lower pressure conditions in the cylinder at the start of compression can cause oil laden air to be drawn from the crankcase up the liner. This oil can lead to increased deposits in the compressor as well as further downstream in the distribution system (P1.V1)/ T1 = (P2. V2)/T2 and (P1/P2).(T2/T1) = (V2/V1) P1.V1g = P2.V2g and P1/P2 T2 g and P1 P2
= = if
V2g/V1g T1. we = =
From these (P2/P1) = take for and 0.4 1
we (g
-1)/g
1.4 example bar bar
ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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get;
Air Compressor
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Pf = 5 bar Tinitial = 300 K we end with final temperatures for the two compression's of T1= 617 K and T2 = 475 K from the graph it can be clearly seen that losses due to the bump clearance has increased and the period of constant pressure delivery has been reduced.
Coolers •
•
•
Plain Tubeo -easy to clean o -very effective due to large surface area of large number small diameter tubes o -plugging of failed tube allos cooler to continue in service with little loss in efficiency o -must allow for thermal expansion by having one tube plate floating 'U' tubeo -suitable for higher pressures than plain tube o -self compensating for thermal expansion o -efficent due to large nomber small diameter tubes o -failed tubes may be plugged o -more expensive than plain tube and diificult to clean Coil tube o -self compensating for expansion o -suitable for high pressures o -difficult to clean o -inefficicent due to large tube diameter o - not easy to plug o -expensive
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Reciprocating Compressor In a reciprocating compressor, gas is compressed by mechanical variation of the volume of space inside the cylinder, by reciprocating motion of the piston. For a cycle of operation, there are two strokes such as, 1.) Suction stroke, and 2.) Compression stroke
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As the piston moves down, air is sucked from atmosphere to the cylinder through suction valve (a non-return valve). As piston moves up, air is compressed and at the end of compression stroke, air is delivered through delivery valve (which is also a non-return valve). Topmost portion the piston can travel inside the cylinder is is called Top Dead Center (TDC), and bottom most portion the piston can
reach inside the cylinder is called as Bottom Dead Center (BDC). RECIPROCATING AIR COMPRESSOR Screw Compressor Screw compressors (also called as helical lobe compressors) are positive displacement machines in which gas is being compressed is forced through the casing by two screws. Unlike the reciprocating compressors which are also positive displacement machines, screw compressors does not typically require internal suction or discharge valves. In addition the flow from screw compressor is generally more uniform and has fewer pulsations than the flow from a reciprocating compressor.
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A twin screw compressor consist of two meshing helical rotors mounted on counter rotating parallel shafts that are enclosed within close-clearance casing. One screw is called driving screw which is coupled with a drive, say an electric motor, while the other screw is called as the driven screw, since it is driven by the driving screw. Gears used for driving the screws are called timing gears, since they are properly timed to maintain the close-clearance between the screws.
SCREWS OF A TWIN SCREW COMPRESSOR For screw compressors oil is injected into the screws while operation. There are mainly 3 functions for the oil, they are, 1) Sealing of the screws to prevent leakage of the gas 2) Lubrication of the parts, especially the screws, and 3) Cooling of the gas compressed, which results in increased efficiency of the system
Materials used for construction Reciprocating Compressor ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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Crank-case and body - Cast Iron Crankshaft - Spheroidal graphite cast iron or stainless steel Connecting rod - Forged Steel Piston - Aluminium Alloy or Cast / Ductile Iron Piston Rings - Cast Iron Screw Compressor Casing Screw
- Cast or Ductile Iron - Steel or Stainless Steel or Nickel Alloy
Reciprocating Compressor Reciprocating compressors are characterized with higher pressures and reduced mass flow rate. They are mainly used in high pressure applications since it can deliver air at about 30 40 bar. 1) For diesel engine starting, where electric motor starting becomes costly and impractical. 2) Refrigeration compressors are normally reciprocating type (Single Stage) with a discharge pressure around 10 bar. 3) Air conditioning systems also uses reciprocating compressors (Nowadays trend is changed to screw compressors). Screw Compressors Screw type compressors provide air at increased mass flow rate but with reduced discharge pressure around 8 bar. Hence applications are also in low pressure systems, such as, ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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Air Compressor
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1) Service air compressors used in industry (For cleaning air, etc.) 2) Air conditioning systems nowadays employ screw compressors. (which have low power consumption and increased mass flow rate as advantages) 3) For low pressure air required for running pneumatic tools, pneumatic-hydraulic equipments, etc. Cycle of operation Consider one cycle of operation in a reciprocating compressor.
ONE COMPLETE CYCLE OF OPERATION The process that occurs in a cycle 1-2-3-4-1 are explained below (3) - (4) - As piston travels from BDC to TDC air trapped inside the cylinder is compressed. (4) - (1) - As piston approaches TDC discharge valve opens ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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and compressed air is delivered. (1) - (2) – Undelivered air trapped in the clearance space is expanded as piston moves down. (2) - (3) - When trapped air in the clearance space is expanded to atmospheric pressure, further downward movement of the piston creates a vacuum inside the cylinder and thereby atmospheric air enters through suction valve.
Again cycle repeats. In fig. 'Va' indicates the volume corresponding to actual stroke of the piston from TDC to BDC (also called stroke volume). Similarly 'Ve' indicates the volume corresponding to the effective stroke of piston, when atmospheric air enters the cylinder. The ratio of effective stroke volume to the swept volume is known as the volumetric efficiency of the compressor, Voleff = [ ( Ve / Va ) * 100 ] % Then why clearance space? It was already seen from the above equation that volumetric efficiency is 100% when Effective stroke = Actual stroke In other words, no clearance volume exists. This is practically impossible, because some clearance space is required otherwise piston hits on the cylinder head as it travels. Also expansion of piston occurs as it travels and very little clearance may cause the same problem. Also increased clearance space reduces compressor efficiency and increase its running hours. So clearance volume must be kept around a reasonable value as instructed by the manufacturer.
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Air Compressor
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How to measure clearance volume The clearance between piston and cylinder head while piston at TDC is called bumping clearance. This can be measured in different ways. One common method is, remove the valves from the top of the piston. Put a lead ball of sufficient diameter into the cylinder. Slowly turn the flywheel one revolution by hand. Take out the lead piece and measure its thickness, which gives the bumping clearance.
Why cooling is required? Reciprocating compressors are generally cooled with air or water. The cylinders in air cooled compressors often include large external fins that increase the surface area available for heat transfer. In water cooled compressors, freshwater is circulated through jackets that are built into the walls of the cylinders and cylinder heads. The compression cycle is illustrated graphically down below.
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A SINGLE STAGE COMPRESSION Above cycle compresses gas from atmospheric pressure to 8 bar in a single stage. The area enclosed by the points 12341 represents the work of compression in a single stage compressor. Also see the cycle or Pressure-Volume diagram (PV) below which compresses gas from atmospheric pressure to 8 bar in two stages.
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TWO STAGE COMPRESSION Here first stage compresses gas from atmospheric pressure to 3 bar and then gas is cooled isobarically (at constant pressure, refer diagram above). Now gas is again compressed to 8 bar. Now we can see that the work of compression corresponding to the shaded area in the diagram is saved by incorporating an inter cooling between two stages. Hence when comparing with a single stage compressor work can be reduced by inter cooling in a multistage compressor. Work can be reduced further by increasing number of stages and inter cooling, but as the number of stages increases design becomes complex, constructional cost increases, maintenance cost also increase, which may nullify the effect of work saved during operation. This is the limiting factor for more number of stages. ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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So how it happens?
ISOTHERMAL, PLOYTROPIC AND ADIABATIC COMPRESSION The indicator card (PV diagram) above shows 3 types (or processes) of compression possible.
Isothermal Compression During the process of compression, whatever heat produced is taken away by a cooling medium. In other words, it is the compression , keeping temperature of the gas constant. For a process to be isothermal, the process must be very slow, which
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is impractical. From the indicator card, it is clear that, work of compression is minimum in isothermal compression. Also,
EQUATION FOR ISOTHERMAL COMPRESSION Adiabatic Compression Whatever heat produced during compression is kept inside the gas only, or heat transfer is zero in an adiabatic compression. For a perfect adiabatic process, process must be very fast. All the thermodynamic process resembles adiabatic process. It can be seen from the indicator card that, work of compression is maximum in adiabatic compression. Also,
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EQUATION FOR ADIABATIC COMPRESSION
Specific heat is defined as the heat energy required to raise the temperature of unit mass of substance by unit degree. Polytropic Compression Polytropic compression is neither isothermal nor adiabatic. It comes in between.Also,
EQUATION FOR POLYTROPIC COMPRESSION Work of compression can be minimized by isothermal compression. But compression is practically a fast process. So it better resembles an adiabatic process. Jacket cooling of compressor makes the compression polytropic. ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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Now the only way to make the compression more isothermal is, by dividing the process to a number of stages. Between every stage inter cooling of gas is done. Hence the work can be saved substantially. Refer the diagrams below.
SINGLE STAGE & MULTISTAGE COMPRESSION
Reference : http://www.popularmechanics.com http://www.essortment.com http:// wikipedia.com ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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[201 0] -: INTRODUCTION:-
Air compressors have relatively simple design and can be sized all the way from the extremely powerful to ones that can just drive small tools. They have few parts and hence last for a long time with little maintenance. The factors make them drivers of choice from heavy equipment on manufacturing shop floors, to cleaning areas in gas stations and now, inside homes and small garages as parts of handy gadgets. An air compressor works by transforming electrical energy in to kinetic energy in the form of air that is held in a highly compressed space before sudden release. The energy released when the compressed air is let out can be used for inflation, cleaning under pressure, turning, generating torque, driving or other similar movement by force. The principle of an air compressor’s functioning is like that of an internal combustion unit. A crankshaft moves a piston through a connecting rod, in a linear fashion along the length of a cylinder in which it fits snugly. Air is drawn in through an open valve as the piston moves upwards, providing ample space for a high volume of air. The valve closes and the piston moves
down,
compressing
the
air.
The
latter
has
kinetic energy as it gains tremendous pressure because of the restricted space in to which it has been forced. The valve opens again to release the energy.
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Cylinders may be placed in series, so that the compressed air is fed to an intake where it is compressed further. The pressure goes up further and hence the power is increased in proportion. All air compressors must have back up systems to release extra pressure in the event of a malfunction in the valve mechanism. This is required to prevent an accident such as in an explosion. The efficiency of air compression depends on air temperature, atmospheric pressure and relative humidity. The rating of an air compressor therefore depends on ambient conditions. This aspect should be taken in to if an air compressor is used in very cold weather, very wet weather or at an high altitude. Oil is used to lubricate the piston as it moves inside the cylinder. Rings act as seals to reduce the amount of oil that can mix
with
the
compressed
air.
Some
oil
contamination,
especially in atomized form, will inevitably escape ring seals and hence a more secure oil containment design is required in case the compressed air energy source has to be completely free of oil. Painting and other functions related to finishing a surface have this kind of requirement as even a faint oil trace can affect appearance. Air compressors have relatively few parts and hence require little maintenance. They last for years and are versatile and compact as well. This is why new uses for air compressors keep ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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surfacing so many years after the first unit was built. Some models have a second cylinder and sizes can be adjusted at the design stage to increase the range of applications and uses. Years ago, it was common for shops to have a central power source that drove all the tools through a system of belts, wheels and driveshafts. The power was routed around the work space by mechanical means. While the belts and shafts may be gone, many shops still use a mechanical system to move power around the shop. It's based on the energy stored in air that's under pressure, and the heart of the system is the air compressor. You'll find air compressors used in a wide range of situations— from corner gas stations to major manufacturing plants. And, more and more, air compressors are finding their way into home workshops, basements and garages. Models sized to handle every job, from inflating pool toys to powering tools such as nail guns, sanders, drills, impact wrenches, staplers and spray guns are now available through local home centers, tool
dealers
and
mail-order
catalogs.
The big advantage of air power is that each tool doesn't need its own bulky motor. Instead, a single motor on the compressor converts the electrical energy into kinetic energy. This makes for light, compact, easy-to-handle tools that run quietly and have fewer parts that wear out. ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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Air compressor
types
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While there are compressors that use rotating impellers to generate air pressure, positive-displacement compressors are more common and include the models used by homeowners, woodworkers, mechanics and contractors. Here, air pressure is increased by reducing the size of the space that contains the air. Most of the compressors you'll run across do this job with a reciprocating
piston.
Like a small internal combustion engine, a conventional piston compressor has a crankshaft, a connecting rod and piston, a cylinder and a valve head. The crankshaft is driven by either an electric motor or a gas engine. While there are small models that are comprised of just the pump and motor, most compressors have an air tank to hold a quantity of air within a preset pressure range. The compressed air in the tank drives the air tools, and the motor cycles on and off to automatically maintain
pressure
in
the
tank.
At the top of the cylinder, you'll find a valve head that holds the inlet and discharge valves. Both are simply thin metal flaps– one mounted underneath and one mounted on top of the valve plate. As the piston moves down, a vacuum is created above it. This allows outside air at atmospheric pressure to push open the inlet valve and fill the area above the piston. As the piston moves up, the air above it compresses, holds the inlet valve shut and pushes the discharge valve open. The air moves from ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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the discharge port to the tank. With each stroke, more air enters
the
tank
and
the
pressure
rises.
Typical compressors come in 1- or 2-cylinder versions to suit the
requirements
of
the
tools
they
power.
On
the
homeowner/contractor level, most of the 2-cylinder models operate just like single-cylinder versions, except that there are two strokes per revolution instead of one. Some commercial 2cylinder compressors are 2-stage compressors–one piston pumps air into a second cylinder that further increases pressure. Compressors use a pressure switch to stop the motor when tank pressure reaches a preset limit–about 125 psi for many single-stage models. Most of the time, though, you don't need that much pressure. Therefore, the air line will include a regulator that you set to match the pressure requirements of the tool you're using. A gauge before the regulator monitors tank pressure and a gauge after the regulator monitors air-line pressure. In addition, the tank has a safety valve that opens if the pressure switch malfunctions. The pressure switch may also incorporate an unloader valve that reduces tank pressure when
the
compressor
is
turned
off.
Many articulated-piston compressors are oil lubricated. That is, they have an oil bath that splash-lubricates the bearings and ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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cylinder walls as the crank rotates. The pistons have rings that help keep the compressed air on top of the piston and keep the lubricating oil away from the air. Rings, though, are not completely effective, so some oil will enter the compressed air in
aerosol
form.
Having oil in the air isn't necessarily a problem. Many air tools require oiling, and inline oilers are often added to increase a uniform supply to the tool. On the down side, these models require regular oil checks, periodic oil changes and they must be operated on a level surface. Most of all, there are some tools and situations that require oilfree air. Spray painting with oil in the airstream will cause finish problems. And many new woodworking air tools such as nailers and sanders are designed to be oilfree so there's no chance of fouling wood surfaces with oil. While solutions to the airborne oil problem include using an oil separator or filter in the air line, a better idea is to use an oilfree compressor that uses permanently lubricated bearings in
place
of
the
oil
bath.
A variation on the automotive-type piston compressor is a model that uses a one-piece piston/connecting rod. Because there is no wrist pin, the piston leans from side to side as the eccentric journal on the shaft moves it up and down. A seal around the piston maintains with the cylinder walls and prevents
air
leakage.
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Where air requirements are modest, a diaphragm compressor can be effective. In this design, a membrane between the piston and the compression chamber seals off the air and prevents leakage.
~:Compressor power:~ One of the factors used to designate compressor power is motor horsepower. However, this isn't the best indicator. You really need to know the amount of air the compressor can deliver
at
a
specific
pressure.
The rate at which a compressor can deliver a volume of air is noted in cubic feet per minute (cfm). Because atmospheric pressure plays a role in how fast air moves into the cylinder, cfm will vary with atmospheric pressure. It also varies with the temperature and humidity of the air. To set an even playing field, makers calculate standard cubic feet per minute (scfm) ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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as cfm at sea level with 68 degrees F air at 36% relative humidity. Scfm ratings are given at a specific pressure–3.0 scfm at 90 psi, for example. If you reduce pressure, scfm goes up,
and
vice
versa.
You also may run across a rating called displacement cfm. This figure is the product of cylinder displacement and motor rpm. In comparison with scfm, it provides an index of compressor pump
efficiency.
The cfm and psi ratings are important because they indicate the tools that a particular compressor can drive. When choosing a compressor, make sure it can supply the amount of air and the pressure that your tools need.
Air Compressors
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Graph of Pressure against volume in a reciprocating compressor Volumetric efficiency Vh = Actual suction volume Vx/ Theoretical suction volume Vs For greater efficiency air compression should be isothermal as this requires the minimum work input. In practice Isothermal compression is not possible, an ideal Isothermal cycle requires sufficient time to allow all the required heat to be transferred out of the cylinder, practicality dictates that the piston must have a relatively high speed to give a reasonable output, Cylinder cooling on a single stage compressor gives better efficiency but there is a limitation in the surface area to cylinder volume that can be used for cooling effect, but multistage compressors with an efficient extended surface interstage cooler gives cycle improved compression efficiency better approaching that of the isothermal. In theory the greater the number of stages the closer the curve will approach the ideal isothermal compression curve, however there is an ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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increase in cost, complexity, and the law of diminishing returns limit the number. Compression in stages has the following advantages; 1. The compression ratio at each stage is lower and so the final temperature is lower. This reduces problems with lubrication 2. . The machine is smaller and better balanced 3. water can be drained off at each stage 4. Compression better approaches the ideal isothermal It is important that the compressor clearance volume is kept small as possible in order to improve overall volumetric efficiency as the air trapped in this space must expand to below suction pressure before new air can enter, this is an effective loss of stroke. A clearance is required in order to prevent the piston striking the cylinder cover when starting or stopping off load. The clearance volume is sometimes referred to as the 'Bump Clearance'. Crankcase lubrication Lubrication of the crankcase in a compressor does not pose any specific problems and normally consist of splash lubrication with pressurised oil being fed to shell bearings. Where drip cylinder lubrication is used, this should be kept to a minimum conducive with liner wear. A standard mineral oil similar to that used in the main engine may be used, although due to carbon deposits, higher quality oils are generally used with the most effective being specifically designed synthetics which have allow a considerable reduction in maintenance but are costly.
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Mineral oils contain a blend of lighter elements such as paraffin's, and heavier elements such as asphaltenes. During compression the lighter elements are vaporised leaving the heavy ends, these coat the piston rings and discharge valves in combination with oxidised oil deposits. These deposits also coat age ways and coolers resulting in higher interstage air temperatures. Deposits on discharge valves cause them to become sticky and leak resulting in hot air being drawn back into the cylinder for recompression. This increases the temperature and hence causes greater oxidation and deposits, and so the condition deteriorates with increasing rapidity.
Temperature can become very high, this may result in oily deposits at the discharge valves carbonising. Eventually this carbon could glow red and cause detonation. It is more likely, however, that oily deposits will be carried over to the air receiver and air start manifold to be ignited by blowpast at the cylinder air start valve. Deposits at piston rings cause leakage allowing oil to enter the cylinder from the crankcase thus increasing the danger it is essential that crankcase lubrication be kept to a minimum compatible with an acceptable wear rate. Regular maintenance will minimize oily deposits build up and hence the risk of explosion Materials and design of a reciprocating compressor The compressor casing, cylinder covers and piston rings are generally of cast iron. Pistons may be of cast iron, steel of aluminium. Aluminium being the preferred material for use on the LP piston due to its larger diameter. Valves are usually made so that parts can be interchanged between the suction and discharge valves. Seats are of mild steel with small ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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diameter air ages to prevent the fragments of broken valve plate from entering the cylinder. Valve plates are of vanadium steel heat treated and ground to provided the required hardness and surface finish. Springs should be arranged such that they lift and seat squarely. Uneven spring force or deposits on the seat cause valves to bend resulting in fatigue cracking.
For compressors designed for starting air requirements a water jacket relief valve is fitted.
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Rotary Compressor
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The rotary compressor may be of the impeller type similar to that used in the turbocharger , scroll, twin rotating lobes or of the sliding vane type similar to the one shown above. In practice there would be several more vanes than shown. Rotary compressors are capable of handling large quantities of low pressure air much more efficiently than a reciprocating compressor. In order to produce increased pressures it is possible to stage rotary compressors but leakage problems increase at higher pressures as well as stress on the vanes. The sliding vane compressor consists of a slotted rotor with its axis offset from that of the cylindrical casing. Vanes fit in the slots and have with the casing On the suction side the space contained between the casing, the rotor and an adjacent pair of vanes is gradually increasing allowing air to be drawn in. On the compression side this same space is gradually reduced causing the pressure increase. When the leading vane uncovers the discharge port air will flow to outlet. Larger
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Air Compressor
compressors of this type are compressors tend to be air cooled.
[201 0] water-cooled,
smaller
The main problems related with sliding vane compressors concern wear at the vane tips and sealing of the ends.
Rotary/reciprocating Compressor Rotary compressors in general do not require internal lubrication but they are not suitable alone for providing air at a pressure for starting duties. They can, however, be linked to reciprocating stages to produce a hybrid compressor. The compressor is lighter, more compact and better balanced than an equivalent all reciprocating unit. In basic the rotary first stage supplies air to the reciprocating second and subsequent stages. All stages being driven by the same shaft
ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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Materials Cast iron-Casing, Liners, Pistons( the LP piston is sometimes made from an aluminium alloy, Cylinder covers SteelCrankshaft, Conrods, Pistons, Valve seats Vanadium SteelValve plates Starting air compressor circuit Starting and stopping sequence is adjustable, the magnetic valves are open when the compressor is stopped so any residual pressure is blown off. On starting the magnetic valve are sometimes delayed to close so as to allow the compressor motor to reach full speed before the compressor is loaded up. The non-return valves prevent HP air leaking back from the receiver on which the filling is also of the non return type.
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Calculation of required cylinder compression for a multistage reciprocating compressor r = R = for r = R1/2 for r = R1/3
stage compression a
a
three
pressure pressure two
ratio ratio stage
stage
for example, a 3 stage compressor requiring a final pressure of 64bar would have the following interstage pressures 1st stage 1bar compressed to 4bar 2nd stage 4bar compressed to 16bar 3rd stage 16bar compressed to 64bar It would appear that most of the work is being carried out in the final stage, this is untrue as with the increase in pressure is a complimentary reduction in volume, if the temperature ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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conditions remain the same then work will be equally divided between the stages. By reducing the suction pressure, the cylinder is required to do more work on the air before the discharge valve opens. This means that the air will be delivered at a higher pressure. The higher temperature can lead to problems with the cylinder lubrication as well as a drop in efficiency as well as carbonising of the oil and increased deposits on the valves and piston rings and interstage ages. In the extreme it can lead to seizure and possible diesel detonation of the oil laden air. The reduction in pressure at the suction can be due to a partially blocked suction filter or partially choked suction valve. The lower pressure conditions in the cylinder at the start of compression can cause oil laden air to be drawn from the crankcase up the liner. This oil can lead to increased deposits in the compressor as well as further downstream in the distribution system (P1.V1)/ T1 = (P2. V2)/T2 and (P1/P2).(T2/T1) = (V2/V1) P1.V1g = P2.V2g and P1/P2 T2 g and P1 P2
= = if
V2g/V1g T1. we = =
From these (P2/P1) = take for and 0.4 1
we (g
-1)/g
1.4 example bar bar
ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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get;
Air Compressor
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Pf = 5 bar Tinitial = 300 K we end with final temperatures for the two compression's of T1= 617 K and T2 = 475 K from the graph it can be clearly seen that losses due to the bump clearance has increased and the period of constant pressure delivery has been reduced.
Coolers •
•
•
Plain Tubeo -easy to clean o -very effective due to large surface area of large number small diameter tubes o -plugging of failed tube allos cooler to continue in service with little loss in efficiency o -must allow for thermal expansion by having one tube plate floating 'U' tubeo -suitable for higher pressures than plain tube o -self compensating for thermal expansion o -efficent due to large nomber small diameter tubes o -failed tubes may be plugged o -more expensive than plain tube and diificult to clean Coil tube o -self compensating for expansion o -suitable for high pressures o -difficult to clean o -inefficicent due to large tube diameter o - not easy to plug o -expensive
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Reciprocating Compressor In a reciprocating compressor, gas is compressed by mechanical variation of the volume of space inside the cylinder, by reciprocating motion of the piston. For a cycle of operation, there are two strokes such as, 1.) Suction stroke, and 2.) Compression stroke
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As the piston moves down, air is sucked from atmosphere to the cylinder through suction valve (a non-return valve). As piston moves up, air is compressed and at the end of compression stroke, air is delivered through delivery valve (which is also a non-return valve). Topmost portion the piston can travel inside the cylinder is is called Top Dead Center (TDC), and bottom most portion the piston can
reach inside the cylinder is called as Bottom Dead Center (BDC). RECIPROCATING AIR COMPRESSOR Screw Compressor Screw compressors (also called as helical lobe compressors) are positive displacement machines in which gas is being compressed is forced through the casing by two screws. Unlike the reciprocating compressors which are also positive displacement machines, screw compressors does not typically require internal suction or discharge valves. In addition the flow from screw compressor is generally more uniform and has fewer pulsations than the flow from a reciprocating compressor.
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A twin screw compressor consist of two meshing helical rotors mounted on counter rotating parallel shafts that are enclosed within close-clearance casing. One screw is called driving screw which is coupled with a drive, say an electric motor, while the other screw is called as the driven screw, since it is driven by the driving screw. Gears used for driving the screws are called timing gears, since they are properly timed to maintain the close-clearance between the screws.
SCREWS OF A TWIN SCREW COMPRESSOR For screw compressors oil is injected into the screws while operation. There are mainly 3 functions for the oil, they are, 1) Sealing of the screws to prevent leakage of the gas 2) Lubrication of the parts, especially the screws, and 3) Cooling of the gas compressed, which results in increased efficiency of the system
Materials used for construction Reciprocating Compressor ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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Crank-case and body - Cast Iron Crankshaft - Spheroidal graphite cast iron or stainless steel Connecting rod - Forged Steel Piston - Aluminium Alloy or Cast / Ductile Iron Piston Rings - Cast Iron Screw Compressor Casing Screw
- Cast or Ductile Iron - Steel or Stainless Steel or Nickel Alloy
Reciprocating Compressor Reciprocating compressors are characterized with higher pressures and reduced mass flow rate. They are mainly used in high pressure applications since it can deliver air at about 30 40 bar. 1) For diesel engine starting, where electric motor starting becomes costly and impractical. 2) Refrigeration compressors are normally reciprocating type (Single Stage) with a discharge pressure around 10 bar. 3) Air conditioning systems also uses reciprocating compressors (Nowadays trend is changed to screw compressors). Screw Compressors Screw type compressors provide air at increased mass flow rate but with reduced discharge pressure around 8 bar. Hence applications are also in low pressure systems, such as, ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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1) Service air compressors used in industry (For cleaning air, etc.) 2) Air conditioning systems nowadays employ screw compressors. (which have low power consumption and increased mass flow rate as advantages) 3) For low pressure air required for running pneumatic tools, pneumatic-hydraulic equipments, etc. Cycle of operation Consider one cycle of operation in a reciprocating compressor.
ONE COMPLETE CYCLE OF OPERATION The process that occurs in a cycle 1-2-3-4-1 are explained below (3) - (4) - As piston travels from BDC to TDC air trapped inside the cylinder is compressed. (4) - (1) - As piston approaches TDC discharge valve opens ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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and compressed air is delivered. (1) - (2) – Undelivered air trapped in the clearance space is expanded as piston moves down. (2) - (3) - When trapped air in the clearance space is expanded to atmospheric pressure, further downward movement of the piston creates a vacuum inside the cylinder and thereby atmospheric air enters through suction valve.
Again cycle repeats. In fig. 'Va' indicates the volume corresponding to actual stroke of the piston from TDC to BDC (also called stroke volume). Similarly 'Ve' indicates the volume corresponding to the effective stroke of piston, when atmospheric air enters the cylinder. The ratio of effective stroke volume to the swept volume is known as the volumetric efficiency of the compressor, Voleff = [ ( Ve / Va ) * 100 ] % Then why clearance space? It was already seen from the above equation that volumetric efficiency is 100% when Effective stroke = Actual stroke In other words, no clearance volume exists. This is practically impossible, because some clearance space is required otherwise piston hits on the cylinder head as it travels. Also expansion of piston occurs as it travels and very little clearance may cause the same problem. Also increased clearance space reduces compressor efficiency and increase its running hours. So clearance volume must be kept around a reasonable value as instructed by the manufacturer.
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How to measure clearance volume The clearance between piston and cylinder head while piston at TDC is called bumping clearance. This can be measured in different ways. One common method is, remove the valves from the top of the piston. Put a lead ball of sufficient diameter into the cylinder. Slowly turn the flywheel one revolution by hand. Take out the lead piece and measure its thickness, which gives the bumping clearance.
Why cooling is required? Reciprocating compressors are generally cooled with air or water. The cylinders in air cooled compressors often include large external fins that increase the surface area available for heat transfer. In water cooled compressors, freshwater is circulated through jackets that are built into the walls of the cylinders and cylinder heads. The compression cycle is illustrated graphically down below.
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A SINGLE STAGE COMPRESSION Above cycle compresses gas from atmospheric pressure to 8 bar in a single stage. The area enclosed by the points 12341 represents the work of compression in a single stage compressor. Also see the cycle or Pressure-Volume diagram (PV) below which compresses gas from atmospheric pressure to 8 bar in two stages.
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TWO STAGE COMPRESSION Here first stage compresses gas from atmospheric pressure to 3 bar and then gas is cooled isobarically (at constant pressure, refer diagram above). Now gas is again compressed to 8 bar. Now we can see that the work of compression corresponding to the shaded area in the diagram is saved by incorporating an inter cooling between two stages. Hence when comparing with a single stage compressor work can be reduced by inter cooling in a multistage compressor. Work can be reduced further by increasing number of stages and inter cooling, but as the number of stages increases design becomes complex, constructional cost increases, maintenance cost also increase, which may nullify the effect of work saved during operation. This is the limiting factor for more number of stages. ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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So how it happens?
ISOTHERMAL, PLOYTROPIC AND ADIABATIC COMPRESSION The indicator card (PV diagram) above shows 3 types (or processes) of compression possible.
Isothermal Compression During the process of compression, whatever heat produced is taken away by a cooling medium. In other words, it is the compression , keeping temperature of the gas constant. For a process to be isothermal, the process must be very slow, which
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is impractical. From the indicator card, it is clear that, work of compression is minimum in isothermal compression. Also,
EQUATION FOR ISOTHERMAL COMPRESSION Adiabatic Compression Whatever heat produced during compression is kept inside the gas only, or heat transfer is zero in an adiabatic compression. For a perfect adiabatic process, process must be very fast. All the thermodynamic process resembles adiabatic process. It can be seen from the indicator card that, work of compression is maximum in adiabatic compression. Also,
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EQUATION FOR ADIABATIC COMPRESSION
Specific heat is defined as the heat energy required to raise the temperature of unit mass of substance by unit degree. Polytropic Compression Polytropic compression is neither isothermal nor adiabatic. It comes in between.Also,
EQUATION FOR POLYTROPIC COMPRESSION Work of compression can be minimized by isothermal compression. But compression is practically a fast process. So it better resembles an adiabatic process. Jacket cooling of compressor makes the compression polytropic. ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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Now the only way to make the compression more isothermal is, by dividing the process to a number of stages. Between every stage inter cooling of gas is done. Hence the work can be saved substantially. Refer the diagrams below.
SINGLE STAGE & MULTISTAGE COMPRESSION
Reference : http://www.popularmechanics.com http://www.essortment.com http:// wikipedia.com ATMIYA INSTITUTE OF TECHNOLOGY & SCIENCE FOR DIPLOMA STUDIES
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