In this article we will discuss about:- 1. Introduction to Air Compressor 2. Classification of Air Compressor 3. Single Stage/Single Acting Reciprocating Air Compressor 4. Work Input 5. Efficiency 6. Actual Indicator Diagram 7. Free Air Delivery (FAD) 8. Multistage Air Compressor 9. Capacity Control.
Contents:
- Introduction to Air Compressor
- Classification of Air Compressor
- Single Stage/Single Acting Reciprocating Air Compressor
- Work Input for Compressor
- Efficiency of Air Compressor
- Actual Indicator Diagram of Air Compressor
- Free Air Delivery (FAD) by an Air Compressor
- Multistage Air Compressor
- Capacity Control of Compressors
1. Introduction to Air Compressor:
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Air compressor is a power absorbing machine which provides high pressure air. It takes the air from the atmosphere, compresses it to a high pressure and high pressure air will be stored in a storage vessel (reservoir) from where it can be taken out for use.
There are many uses of high pressure air in industry. The main uses of compressed air are:
1. For inflating automobile tyres.
2. To clean workshop machines, generators etc.
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3. To operate air operated drills, hammers (Pneumatic tools).
4. To inject fuel in the diesel engine cylinder.
5. In spray painting.
6. To operate air brakes in automobiles.
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7. To operate compressed air engines/air motors in mines.
(In mines IC engines and electricity are not used because of fire risks. So high pressure air operated machines are used).
2. Classification of Air Compressor:
Air compressors may be broadly classified as given below:
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(i) Reciprocating and
(ii) Rotary.
The principle parts of a reciprocating air compressor are same as that of a reciprocating I.C. engine.
The air compressor may also be as follows:
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(i) Single Acting or Double Acting:
The air is admitted to one side of the piston or air is admitted to either side of the piston alternately.
(ii) Single Stage/Multistage:
In case of single stage air is compressors, air is compressed in a single cylinder completely, whereas in case of multistage air compressor, air where air is compressed in several stages. And each stage in pressure of air increases.
3. Single Stage/Single Acting Reciprocating Air Compressor:
It can be seen that the construction is just similar to reciprocating I.C. engine. In this case crank of the compressor is driven by means of electrical motor or engine output shaft.
During suction stroke, piston will be moving downwards and the pressure in the cylinder falls below the atmospheric pressure. As a result of which inlet valve opens and the air is drawn from the atmosphere. Then during compression stroke air will be compressed by means of upward moving piston and delivered through the delivery valve, when the pressure in the cylinder increases beyond the pre-specified value of delivery valve.
Compressor Valves and Types:
Compressor valves are automatic i.e., there is no valve operating mechanism provided, but they are operated because of the pressure difference.
Types of compressor valves:
(i) Reed types
(ii) Plate type
(i) Reed Type Valves:
Figure 15.3 shows the reed type valve during compression the pressure of air increases and the pressure developed at (1) increases beyond the delivery chamber pressure, (2) then the valve plate (which has 6-8 reeds) lifts and the air is delivered.
(ii) Plate Type Valves:
As shown in Fig. 15.4 valve plate is held between valve seat and valve guard. In between valve plate and valve guard spring is provided. When the air pressure increases at (1), then the valve plate is lifted against the compression of spring and high pressure air is delivered to side (2).
Indicator Diagram:
The events described for the single stage air compressor can be represented on P-V diagram as shown in Fig. 15.5. The diagram is drawn for the compressor without clearance volume.
Clearance Volume:
It is the volume of space provided between piston head and cylinder head when the piston is at TDC. This is provided in order to avoid striking of piston heat with cylinder head. Also in this clearance volume, during the delivery stroke some volume of air will be trapped, when the piston comes to TDC. This volume of air during the downward stroke expands and produces the necessary vacuum.
During suction stroke, the charge of air is drawn along 4-1 at constant pressure P1, which is slightly below Patm. At point 1 piston completes suction stroke and starts compression stroke. During starting both the valves being closed air will be compressed along 1-2. At point 2 pressure P2 is reached which is slightly higher than the pressure in the receiver.
The delivery valve at this point opens and the air is delivered along 2-3 into the receiver.
The network input required for compression and delivery of air per cycle is given by the area 1-2-3-4 on the P-V diagram.
The amount of work done on air depends upon the nature of compression curve. If the compression occurs very rapidly in a non-conducting cylinder so that heat transfer is zero, then the compression will be practically isentropic.
If it is carried out very slowly and heat of compression is extracted from air by jacket cooling water, then the compression will approach isothermal.
However in actual practice neither of these conditions can be fulfilled and the actual compression will be in between these two and is called as the polytrophic compression as shown in Fig. 15.6.
From Fig. 15.6 we can see that the isentropic work done = Area 4-1-2′-3 and Isothermal work done = Area 4-1-2″-3. So, if we achieve isothermal compression curve, the work input required will be minimum.
Note: For isothermal compression process PV= C, and n =1
For isentropic compression process PVγ = C and γ= 1.4 for air.
In actual practice we achieve the process in between these two and for water cooled cylinders the value of n is 1.3.
4. Work Input for Compressor:
Note:
All the expressions are arranged to give positive value, since we are interested in the magnitude of work input.
Expression for Work Input When Clearance Is Considered:
The work input required to compress the air with clearance is given by the area 1-2-3-4. (Ref. Fig. 15.9)
5. Efficiency of Air Compressor:
Isothermal Efficiency of Air Compressor:
It is defined as the ratio of-
Measures or Methods to Improve Isothermal Efficiency:
(i) Effective Cooling System:
This can be provided by providing fins on the cylinder and cylinder head and air blown on the fins. When the air is blown, it removes heat from the compressed air and dissipates it to the atmosphere.
Secondly cooling water jackets may also be provided around the cylinder and cylinder head for large sized compressors.
(ii) Multistaging with Intercoolers:
In this method, the compression of air is carried out in two or more stages i.e., two or more cylinders. The pressure of air is increased in each cylinder. It is a common practice to provide inter-coolers between the cylinders of multistage compressor for the purpose of cooling the compressed air to intake (or atmosphere) air temperature before it enters the next cylinder. It is this cooling between the cylinders that keeps the compression very near to isothermal.
(iii) Spray Injection:
In this method a small amount of water is injected or sprayed into the compressed air. It immediately vapourises by absorbing heat from air and thereby keeps the temperature of air low.
(iv) Suitable Cylinder Dimensions:
To have effective heat transfer, surface to volume ratio should be higher i.e., a cylinder with large dia. and short stroke should be selected. But this has the limitation since increased dia increases clearance volume and hence reduces volumetric efficiency.
Effect of Clearance on Volumetric Efficiency:
The volume of space provided between piston heat and cylinder head when the piston is at TDC is known as clearance volume. In actual compressors it is provided to safeguard the piston from striking the cylinder head.
The events taking place in a compressor with clearance are same as those taking place in a compressor without clearance.
Figure 15.9 shows the P-V diagram for a single stage/single acting reciprocating air compressor with clearances volume. In this case both compression and expansion curves are assumed to be polytrophic following the law PVn = C.
As shown in Fig. 15.9, 1- 2 is polytrophic compression, 2 – 3 is delivery stroke. At point 3, piston reaches TDC and delivery of high pressure air stops and some air is trapped in the clearance volume at the delivery pressure P2.
As the piston moves downwards this air which is trapped in the clearance volume expands according to PVn = C and produces the necessary vacuum. At point 4 the inlet valve opens and actual volume of air taken is along 4-1.
The variation of volumetric efficiency with clearance volume and polytrophic index is shown in Fig. 15.10.
We note from this figure that:
(i) As r increases ηvol decreases.
(ii) As n increases ηvol increases.
(iii) As c increases ηvol decreases.
Note:
(i) ηvol ranges from 60 to 85% and
(ii) Clearance ratio ranges from 4 to 10%
(iii) n ranges from 1.25 to 1.35 for air.
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. Actual Indicator Diagram of Air Compressor:
In actual air compressor, some air is present in the clearance volume. This air expands during suction stroke.
At point 4, the inlet valve will not open in actual practice because of valve inertia and pressure differential required to open the valve. Thus the pressure drops away until the valve is forced to open. Some valve bounce will taken place and slowly intake will become nearly steady. This negative pressure, is known as Intake depression.
A similar situation occurs at point 2 i.e., at point 2 pressure has to increase otherwise valve will not open. Actual work required to compress and deliver the air will be greater than theoretical work.
7. Free Air Delivery (FAD)
by an Air Compressor:
It is the actual volume of air delivered by an air compressor reduced to either NTP or STP conditions or intake conditions. We know that from continuity equation.
8. Multistage Air Compressor
:
A 2 stage air compressor with water jacketed cylinder and inter cooler is shown in Fig. 15.10 (i).
The suction in LP cylinder ends at 1 and the air is compressed polytropically to 2′. The LP cylinder then delivers the air along 2′ – P2 to the intercooler. Where air is cooled to initial intake temperature T1 at constant pressure P2 by circulating cold water in the inter cooler. When the air is cooled in the intercooler to initial temperature T1, the cooling is perfect.
The air when it is cooled at constant pressure, suffers reduction in volume from 2′ – P2 to P2 – 2. The cooled air is drawn into the HP cylinder along P2 -2 for II-stage compression, where it is compressed polytropically to final pressure P3 along 2-3 and then delivered at constant pressure P3 along 3 – P3. (Ref. Fig. 15.10 (iii)).
Figure 15.10 (iv) shows the combined indicator diagram for LP and HP cylinder of a single acting 2-stage air compressor with perfect intercooling.
LP diagram is shown as P1 – 1 – 2′ – P2 and HP diagram is shown as P0 – 2 – 3 – P3. The reduction of work required due to intercooling is shown by shaded are 2 – 3 – 3′ – 2′, when the intercooling is perfect, the point 2 lies on the isothermal line 1 – 3″.
Work required in LP cylinder/cycle.
Intercoolers and Aftercoolers:
In the intercoolers air is cooled in-between the cylinders of a multistage compressor. And in the afterwards air is cooled after the last cylinder and before it enters the reservoir.
By aftercooling there is no saving in work as such, but by aftercooling volume of air decreases. So more amount of air can be stored in the reservoir. Hence capacity (mass of air to be stored) of air increases.
Ideal Intercooler Pressure:
It may be noted that, saving in work increases as intercooling is increased. When the intercooling is perfect, saving of work is maximum and work input to compressor is given by Eq. (2).
It may further be noted that saving in work also varies with the chosen intercooler pressure P2. When P1 and P3 are fixed, the best value, of intercooler pressure P2 shall be fixed to give minimum work. This value of P2 can be found by differentiating the Eq. (2) w.r.t. P2 and equating it to zero.
9. Capacity Control of Compressors
:
Generally compressors are not operated at full capacity. Depending upon the demand for high pressure air, they are operated.
Various types of controls for compressors are:
(i) Throttle control
(ii) Blowing air to atmosphere
(i) Throttle Control:
When the demand for high pressure air is more and when less amount of air is present is the reservoir tank, then less pressure from the reservoir will act on the piston (1). Hence because of less pressure piston moves upwards and more amount of air is taken during suction stroke opposite operation will take place when the demand for high per air is less.
(ii) Blowing Air to Atmosphere:
When the pressure in the receiver increases beyond the required limit, then because of high pressure air piston (1) of relay cylinder (2) in lifted and high pressure air enters cylinder (3) and presses the piston (4) downwards and high pressure air from the compressor is directly blown off to atmosphere.
And when the pressure comes below normal in the receiver, then piston (1) of relay cylinder moves down because of dead weights (5). Because of compressor air pressure check valve (6) opens and air enters receiver (7).