AIR COMPRESSOR
AIR COMPRESSOR:
Compressors are utilized to increase pressure of a gas. Like pumps, compressors can be delegated either dynamic machines, which incorporates diffusive and pivotal compressors, or positive-uprooting machines, which incorporate responding and turning compressors. The packing medium or "gas" relies on upon the application, for example, if air is utilized it is named as an air compressor. Also if refrigerant is utilized, it is known as a refrigerant compressor. The kind of compressor, its release weight and release rate is directed by its utilization.
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 (P-V) below which compresses gas from atmospheric pressure to 8 bar in two stages
TWO STAGE COMPRESSOR:
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 limit number of stages.
Two Stage Compression
.
Refer figure below.
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 is impractical. From the indicator card, it is clear that, work of compression is minimum in isothermal compression.
Also,
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,
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,
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 poly tropic.
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.
Starting air lines on board ships may contain large amount of air along with fine droplets of oil mixed with it. These oil droplets are formed by carry over from air compressors.
Any source of heat when supplied to this rich mixture could result in catastrophic starting air explosion. So it is important to maintain starting air compressors in good condition. Compressor performance, oil separator operation, inter cooler and after cooler efficiency, etc. to be monitored periodically. Also drain all air bottles regularly to get rid of most of the water and oil.
Reasons for Oil Carry Over
Choking oil drain holes in the scraper rings
Worn out oil scraper ring
Wrong fitting of oil scraper rings
Worn out liner
Choking of crankcase breather (it results in crankcase pressurization and eventually oil carry over
Excessive cylinder lubrication in some types of compressors
How to Minimize it
Replace worn out scraper rings
Clear holes of scraper rings
Fit scraper rings properly
Clean and clear crankcase breather
Adjust lubrication to the top of the cylinder
Materials used for construction
Reciprocating Compressor
Crankcase 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 – Cast or Ductile Iron
Screw – Steel or Stainless Steel or Nickel Alloy
Where they are used? 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,
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 equipment, etc.
Cycle of operation
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.
TWIN SCREW COMPRESSOR:
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
COMPRESSOR ELEMENT
Shipboard air compressors may be centrifugal, rotary, axial flow screw, or reciprocating. The more common is reciprocating type of air compressor is generally selected for capacities from 100 to 800 cfm with outlet pressure ratings of 125, 600, and 3,000 psi, upto 5,000 psi.
The rotary lobe type of air compressor is selected for capacities up to 8,800 cfm and for pressures of no more than 20 psi.
The axial flow screw type of air compressor is selected for capacities up to 100 cfm and for pressures of no more than 125 psi.
The centrifugal type of air compressor is selected for capacities of 800 cfm or greater (up to 2,100 cfm in a single unit) and for pressures up to 125 psi.
SOURCES OF POWER
Compressors may be driven by electric motors, steam turbines, or gas turbines.
Aboard ship, most low-, medium-, and high-pressure air compressors that supply the ship’s service air are driven by electric motors.
The driving unit may be coupled to the compressor by one of several methods. When the compressor and the driving unit are mounted on the same shaft, they are close-coupled.
This method is usually restricted to small-capacity air compressor driven by electric motors..
When the speed of the compressor and the speed of the driving unit are the same, flexible couplings are used to mount the driving unit to the compressor. This is called a direct-coupled drive.
V-belt drives are commonly used with small, low-pressure, motor-driven compressors and with some medium-pressure and high-pressure compressor.
In a few installations, a rigid coupling is used between the compressor and the electric motor of a motor-driven compressor. In a steam-turbine drive, the compressors are usually (but not always) driven through reduction gears. Centrifugal high-speed compressors are usually driven through speed increasing gears.
Most general-service air compressors aboard ship are the reciprocating type. In the reciprocating air compressor, the air pressure is increased by the use of one or more cylinders. This is very much like the compression that takes place in an internal-combustion engine.
Most general-service air compressors aboard ship are the reciprocating type. In the reciprocating air compressor, the air pressure is increased by the use of one or more cylinders. This is very much like the compression that takes place in an internal-combustion engine.
Figure 14-1.-A simple two-stage reciprocating low-pressure air compressor (vertical V configuration).
RECIPROCATING AIR COMPRESSORS
All reciprocating air compressors are similar in design and operation. The following discussion describes the basic components and principles of operation of low-medium- and high-pressure reciprocating air compressors.
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
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 Centre (TDC), and bottom most portion the piston can reach inside the cylinder is called as Bottom Dead Centre (BDC).
RECIPROCATING AIR COMPRESSOR
Consider one cycle of operation in a reciprocating compressor.
ONE COMPLETE CYCLE OF OPERATION
(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 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:
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.
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.
CYLINDER ARRANGEMENT:
Most low-pressure reciprocating air compressors are of the two-stage type. They have either a vertical V (fig. 14-1), a vertical W (fig. 14-2), or a vertical in-line arrangement of cylinders.
The V-type and in-line compressor have one cylinder for the first (lower pressure) stage of compression and one cylinder for the second (higher pressure) stage of compression
Medium-pressure air compressors are generally two-stage, vertical, duplex, single-acting type. Many medium-pressure compressors have differential pistons. This type of piston is used in machines designed for more than one stage of compression during each stroke of the piston. (See view A of fig. 14-3.)
The W-type compressor has two cylinders for the first stage of compression and one cylinder for the second stage.
The vertical W cylinder arrangement is shown in view A of figure 14-3. Notice that the pistons in the lower-pressure stage (1) have larger diameters than the pistons in the higher-pressure stage (2).
Most high-pressure compressors are motor-driven, liquid-cooled, four-stage, single-acting units with vertical cylinders.
Figure 14-2.-Low-pressure reciprocating air compressor (vertical W configuration).
View B of figure 14-3 shows the cylinder arrangements for the high-pressure air compressors installed in Navy ships. Small-capacity, high-pressure air systems may have three-stage compressors. Large-capacity, high-pressure air systems may be equipped with four-, five-, or six-stage compressors.
As our example of the operating cycle, we will describe one stage of compression in a single-stage, single-acting compressor .
The cycle of operation, or compression cycle, within an air compressor cylinder includes two strokes of the piston: an intake stroke and a compression stroke. (Refer to fig. 14-4.)
The intake stroke (view A) begins when the piston moves away from top dead centre (TDC).
The air remaining in the clearance space above the piston expands rapidly (view B) until the pressure in the cylinder falls below the pressure on the opposite side of the inlet valve, which is at atmospheric pressure.
At this point (view C), the difference in pressure causes the inlet valve to open, and air is admitted to the cylinder.
Air continues to flow into the cylinder until the piston reaches bottom dead centre (BDC).
Figure 14-3.-Air compressor cylinder arrangements.
Figure 14-4.-The compression cycle.
The compression stroke (view D) starts as the piston moves away from BDC and continues until the piston reaches TDC again.
When the pressure in the cylinder equals the pressure on the opposite side of the air inlet valve, the inlet valve closes.
The piston moves toward TDC. When the pressure in the cylinder (view E) becomes great enough, it will force the discharge valve to open against the discharge line pressure and the force of the valve springs. (The discharge valve opens shortly before the piston reaches TDC.)
During the remainder of the compression stroke, the air that has been compressed in the cylinder is discharged at almost constant pressure through the open discharge valve.
The basic operating cycle just described is completed twice for each revolution of the crank-shaft in double-acting compressors, once on the down stroke and once on the up stroke.
COMPONENTS AND SYSTEMS:
Reciprocating air compressors consist of a system of connecting rods, a crankshaft, and a flywheel. These parts transmit power developed by the driving unit to the pistons as well as to the lubrication systems, cool-ing systems, control systems, and unloading systems.
COMPRESSING ELEMENT:
The compressing element of a reciprocating air compressor consists of the cylinders, pistons, and air valves.
CYLINDERS.-
The design of the cylinders depends mostly upon the number of stages of compression required to produce the maximum discharge pressure. Several common cylinder arrangements for low- and medium-pressure air compressors are shown in view A of figure 14-3. Several arrangements for the cylinders and pistons of high-pressure air compressors are shown in view B of figure 14-3. The stages are numbered 1 through 4. Notice that a three-stage arrangement and a four-stage arrangement are both shown. The basic stage arrangement is similar for the five- and six-stage compressor.
PISTONS.-
The pistons may be either of two types: trunk or differential. (See fig. 14-5.) TRUNK PISTONS are driven directly by the connecting rods (view A). The upper end of a connecting rod is fitted directly to the pistonby a wrist pin. This design produces a tendency for the piston to develop a side pressure against the cylinder walls. For the side pressure to be distributed over a wide area of the cylinder walls or liners, pistons with long skirts are used. The design of the trunk piston helps minimize cylinder wall wear. DIFFERENTIAL PISTONS, shown in view B of figure 14-5, are modified trunk pistons with two or more different diameters. These pistons are fitted into special cylinders, arranged so that more than one stage of compression is achieved by a singleupward stroke of the piston. The compression for one stage takes place over the piston crown; compression for the other stage(s) takes place in the annular space between the large and small diameters of the piston.
VALVES:
The valves are made of special steel and come in a number of different types. The opening and closing of the valves is caused by the difference between (1) the pressure of the air in the cylinder and (2) the pressure of the external air on the intake valve or the pressure of the discharged air on the discharge valve.
Two types of valves commonly used in high-pressure air compressors are shown in figure 14-6. The strip- or feather-type valve is shown in view A
It is used for the suction and discharge valves of the lower-pressure stages (1 and 2). The valve shown in view A is a suction valve; the discharge valve assembly (not shown) is identical except that the positions of the valve seat and the guard are reversed. At rest, the thin strips lie flat against the seat. They cover the slots and form a seal when pressure is applied to the guard side of the valve.
The following action works in either a suction or a discharge operation (depending on the valve service). As soon as pressure on the seat side of the valve exceeds the pressure on the guard side, the strips flex against the contoured recesses in the guard. As soon as the pressure equalizes or reverses, the strips unflex and return to their original position, flat against the seat.
The disk-type valve in view B of figure 14-6 is used for the suction and discharge valves of the higher-pressure stages (3 and 4). The fourth stage assembly is shown in view B. The valves shown are the spring-loaded, dished-disk type. At rest, the disk is held against the seat by the spring. It forms a seal when pressure is applied to the keeper side of the valve.
Figure 14-5.-Air compressor pistons.
Figure 14-6.-High-pressure air compressor valves.
The following action works in either a suction or a discharge operation (depending on the valve service). When the pressure on the seat side of the valve exceeds the pressure on the keeper side, the disk lifts against the stop in the keeper. This action compresses the spring and permits air to pass through the seat, around the disk, and through the openings in the sides of the keeper. As soon as the pressure equalizes or reverses, the spring forces the disk back onto the seat.
AIR COMPRESSOR:Compressors are utilized to increase pressure of a gas. Like pumps, compressors can be delegated either dynamic machines, which incorporates diffusive and pivotal compressors, or positive-uprooting machines, which incorporate responding and turning compressors. The packing medium or "gas" relies on upon the application, for example, if air is utilized it is named as an air compressor. Also if refrigerant is utilized, it is known as a refrigerant compressor. The kind of compressor, its release weight and release rate is directed by its utilization.
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 (P-V) below which compresses gas from atmospheric pressure to 8 bar in two stages
TWO STAGE COMPRESSOR:
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 limit number of stages.
Two Stage Compression
.
Refer figure below.
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 is impractical. From the indicator card, it is clear that, work of compression is minimum in isothermal compression.
Also,
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,
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,
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 poly tropic.
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.
Starting air lines on board ships may contain large amount of air along with fine droplets of oil mixed with it. These oil droplets are formed by carry over from air compressors.
Any source of heat when supplied to this rich mixture could result in catastrophic starting air explosion. So it is important to maintain starting air compressors in good condition. Compressor performance, oil separator operation, inter cooler and after cooler efficiency, etc. to be monitored periodically. Also drain all air bottles regularly to get rid of most of the water and oil.
Reasons for Oil Carry Over
Choking oil drain holes in the scraper rings
Worn out oil scraper ring
Wrong fitting of oil scraper rings
Worn out liner
Choking of crankcase breather (it results in crankcase pressurization and eventually oil carry over
Excessive cylinder lubrication in some types of compressors
How to Minimize it
Replace worn out scraper rings
Clear holes of scraper rings
Fit scraper rings properly
Clean and clear crankcase breather
Adjust lubrication to the top of the cylinder
Materials used for construction
Reciprocating Compressor
Crankcase 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 – Cast or Ductile Iron
Screw – Steel or Stainless Steel or Nickel Alloy
Where they are used? 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,
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 equipment, etc.
Cycle of operation
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.
TWIN SCREW COMPRESSOR:
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
COMPRESSOR ELEMENT
Shipboard air compressors may be centrifugal, rotary, axial flow screw, or reciprocating. The more common is reciprocating type of air compressor is generally selected for capacities from 100 to 800 cfm with outlet pressure ratings of 125, 600, and 3,000 psi, upto 5,000 psi.
The rotary lobe type of air compressor is selected for capacities up to 8,800 cfm and for pressures of no more than 20 psi.
The axial flow screw type of air compressor is selected for capacities up to 100 cfm and for pressures of no more than 125 psi.
The centrifugal type of air compressor is selected for capacities of 800 cfm or greater (up to 2,100 cfm in a single unit) and for pressures up to 125 psi.
SOURCES OF POWER
Compressors may be driven by electric motors, steam turbines, or gas turbines.
Aboard ship, most low-, medium-, and high-pressure air compressors that supply the ship’s service air are driven by electric motors.
The driving unit may be coupled to the compressor by one of several methods. When the compressor and the driving unit are mounted on the same shaft, they are close-coupled.
This method is usually restricted to small-capacity air compressor driven by electric motors..
When the speed of the compressor and the speed of the driving unit are the same, flexible couplings are used to mount the driving unit to the compressor. This is called a direct-coupled drive.
V-belt drives are commonly used with small, low-pressure, motor-driven compressors and with some medium-pressure and high-pressure compressor.
In a few installations, a rigid coupling is used between the compressor and the electric motor of a motor-driven compressor. In a steam-turbine drive, the compressors are usually (but not always) driven through reduction gears. Centrifugal high-speed compressors are usually driven through speed increasing gears.
Most general-service air compressors aboard ship are the reciprocating type. In the reciprocating air compressor, the air pressure is increased by the use of one or more cylinders. This is very much like the compression that takes place in an internal-combustion engine.
Most general-service air compressors aboard ship are the reciprocating type. In the reciprocating air compressor, the air pressure is increased by the use of one or more cylinders. This is very much like the compression that takes place in an internal-combustion engine.
Figure 14-1.-A simple two-stage reciprocating low-pressure air compressor (vertical V configuration).
RECIPROCATING AIR COMPRESSORS
All reciprocating air compressors are similar in design and operation. The following discussion describes the basic components and principles of operation of low-medium- and high-pressure reciprocating air compressors.
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
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 Centre (TDC), and bottom most portion the piston can reach inside the cylinder is called as Bottom Dead Centre (BDC).
RECIPROCATING AIR COMPRESSOR
Consider one cycle of operation in a reciprocating compressor.
ONE COMPLETE CYCLE OF OPERATION
(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 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:
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.
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.
CYLINDER ARRANGEMENT:
Most low-pressure reciprocating air compressors are of the two-stage type. They have either a vertical V (fig. 14-1), a vertical W (fig. 14-2), or a vertical in-line arrangement of cylinders.
The V-type and in-line compressor have one cylinder for the first (lower pressure) stage of compression and one cylinder for the second (higher pressure) stage of compression
Medium-pressure air compressors are generally two-stage, vertical, duplex, single-acting type. Many medium-pressure compressors have differential pistons. This type of piston is used in machines designed for more than one stage of compression during each stroke of the piston. (See view A of fig. 14-3.)
The W-type compressor has two cylinders for the first stage of compression and one cylinder for the second stage.
The vertical W cylinder arrangement is shown in view A of figure 14-3. Notice that the pistons in the lower-pressure stage (1) have larger diameters than the pistons in the higher-pressure stage (2).
Most high-pressure compressors are motor-driven, liquid-cooled, four-stage, single-acting units with vertical cylinders.
Figure 14-2.-Low-pressure reciprocating air compressor (vertical W configuration).
View B of figure 14-3 shows the cylinder arrangements for the high-pressure air compressors installed in Navy ships. Small-capacity, high-pressure air systems may have three-stage compressors. Large-capacity, high-pressure air systems may be equipped with four-, five-, or six-stage compressors.
As our example of the operating cycle, we will describe one stage of compression in a single-stage, single-acting compressor .
The cycle of operation, or compression cycle, within an air compressor cylinder includes two strokes of the piston: an intake stroke and a compression stroke. (Refer to fig. 14-4.)
The intake stroke (view A) begins when the piston moves away from top dead centre (TDC).
The air remaining in the clearance space above the piston expands rapidly (view B) until the pressure in the cylinder falls below the pressure on the opposite side of the inlet valve, which is at atmospheric pressure.
At this point (view C), the difference in pressure causes the inlet valve to open, and air is admitted to the cylinder.
Air continues to flow into the cylinder until the piston reaches bottom dead centre (BDC).
Figure 14-3.-Air compressor cylinder arrangements.
Figure 14-4.-The compression cycle.
The compression stroke (view D) starts as the piston moves away from BDC and continues until the piston reaches TDC again.
When the pressure in the cylinder equals the pressure on the opposite side of the air inlet valve, the inlet valve closes.
The piston moves toward TDC. When the pressure in the cylinder (view E) becomes great enough, it will force the discharge valve to open against the discharge line pressure and the force of the valve springs. (The discharge valve opens shortly before the piston reaches TDC.)
During the remainder of the compression stroke, the air that has been compressed in the cylinder is discharged at almost constant pressure through the open discharge valve.
The basic operating cycle just described is completed twice for each revolution of the crank-shaft in double-acting compressors, once on the down stroke and once on the up stroke.
COMPONENTS AND SYSTEMS:
Reciprocating air compressors consist of a system of connecting rods, a crankshaft, and a flywheel. These parts transmit power developed by the driving unit to the pistons as well as to the lubrication systems, cool-ing systems, control systems, and unloading systems.
COMPRESSING ELEMENT:
The compressing element of a reciprocating air compressor consists of the cylinders, pistons, and air valves.
CYLINDERS.-
The design of the cylinders depends mostly upon the number of stages of compression required to produce the maximum discharge pressure. Several common cylinder arrangements for low- and medium-pressure air compressors are shown in view A of figure 14-3. Several arrangements for the cylinders and pistons of high-pressure air compressors are shown in view B of figure 14-3. The stages are numbered 1 through 4. Notice that a three-stage arrangement and a four-stage arrangement are both shown. The basic stage arrangement is similar for the five- and six-stage compressor.
PISTONS.-
The pistons may be either of two types: trunk or differential. (See fig. 14-5.) TRUNK PISTONS are driven directly by the connecting rods (view A). The upper end of a connecting rod is fitted directly to the pistonby a wrist pin. This design produces a tendency for the piston to develop a side pressure against the cylinder walls. For the side pressure to be distributed over a wide area of the cylinder walls or liners, pistons with long skirts are used. The design of the trunk piston helps minimize cylinder wall wear. DIFFERENTIAL PISTONS, shown in view B of figure 14-5, are modified trunk pistons with two or more different diameters. These pistons are fitted into special cylinders, arranged so that more than one stage of compression is achieved by a singleupward stroke of the piston. The compression for one stage takes place over the piston crown; compression for the other stage(s) takes place in the annular space between the large and small diameters of the piston.
VALVES:
The valves are made of special steel and come in a number of different types. The opening and closing of the valves is caused by the difference between (1) the pressure of the air in the cylinder and (2) the pressure of the external air on the intake valve or the pressure of the discharged air on the discharge valve.
Two types of valves commonly used in high-pressure air compressors are shown in figure 14-6. The strip- or feather-type valve is shown in view A
It is used for the suction and discharge valves of the lower-pressure stages (1 and 2). The valve shown in view A is a suction valve; the discharge valve assembly (not shown) is identical except that the positions of the valve seat and the guard are reversed. At rest, the thin strips lie flat against the seat. They cover the slots and form a seal when pressure is applied to the guard side of the valve.
The following action works in either a suction or a discharge operation (depending on the valve service). As soon as pressure on the seat side of the valve exceeds the pressure on the guard side, the strips flex against the contoured recesses in the guard. As soon as the pressure equalizes or reverses, the strips unflex and return to their original position, flat against the seat.
The disk-type valve in view B of figure 14-6 is used for the suction and discharge valves of the higher-pressure stages (3 and 4). The fourth stage assembly is shown in view B. The valves shown are the spring-loaded, dished-disk type. At rest, the disk is held against the seat by the spring. It forms a seal when pressure is applied to the keeper side of the valve.
Figure 14-5.-Air compressor pistons.
Figure 14-6.-High-pressure air compressor valves.
The following action works in either a suction or a discharge operation (depending on the valve service). When the pressure on the seat side of the valve exceeds the pressure on the keeper side, the disk lifts against the stop in the keeper. This action compresses the spring and permits air to pass through the seat, around the disk, and through the openings in the sides of the keeper. As soon as the pressure equalizes or reverses, the spring forces the disk back onto the seat.
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