Reciprocating Compressors MCQ Quiz in বাংলা - Objective Question with Answer for Reciprocating Compressors - বিনামূল্যে ডাউনলোড করুন [PDF]

Explanation:

Compressor

• The function of a compressor is to take a definite quantity of fluid usually gas and most often air and deliver it to the required pressure.
• The figure shows the theoretical indicator diagram i.e., the P-V diagram of a single-stage reciprocating compressor without clearance.

The pressure-volume (P-V) diagram of an ideal reciprocating air compressor more carefully:

Two Isobaric Processes: In an ideal reciprocating air compressor cycle, there are typically two isobaric processes:

• The process of intake (where air is drawn into the cylinder at constant atmospheric pressure).
• The process of exhaust (where compressed air is expelled at constant higher pressure).

One Adiabatic Process: This is the compression process where the air is compressed adiabatically (ideally, without heat transfer).

One Constant Volume Process: This typically represents the instant just after the intake valve closes and just before the exhaust valve opens, where there is a negligible movement in the piston, considered as a constant volume process for simplicity.

Description

4-1 - Suction stroke: The inlet valve opens and is air is sucked in the cylinder up to the end of the suction stroke.

1-2 - Return stroke: The valves are closed and the air is compressed till pressure reaches the final delivery pressure.

2-3 - Delivery stroke: The valves open and the air is expelled at constant pressure. 

3-4 -Exhaust stroke: The pressure in the cylinder falls to atmospheric pressure at constant volume as the remaining air is expelled, ready for the next cycle to start.

Explanation:

For maximum efficiency work consumption by the compressor should be minimum.

Intercooler:

• The intercooler is a constant pressure device which cools the working fluid at constant pressure.
• In a two or multistage air compressor, an intercooler is always placed between the low pressure (L.P.) and high pressure (H.P.) cylinder.
• The purpose of an intercooler is to reduce the work input given to the compressor to reach the same pressure as can be reached by a single compressor.
• Here in the below diagram, it can be clearly seen that using the intercooler green shaded region is the amount of work saved while compression by multi-staging. 

where P1 = Intake pressure of air, P2 = Intercooler pressure, and P3 = Delivery pressure of air.

Minimum work input with perfect intercooling

For two-stage compressor

\(W = \frac{{2n}}{{n - 1}}{P_1}{V_1}\left[ {{{\left( {\frac{{{P_2}}}{{{P_1}}}} \right)}^{\frac{{n - 1}}{n}}} - 1} \right]\)

Now, \(\frac{{dW}}{{d{P_2}}} = 0\)

we get, \(P_2^2 = {P_1} \times {P_3}\)

\({P_2} = \sqrt {{P_1} \times {P_3}} \)

and also, \(\frac{{{P_2}}}{{{P_1}}} = \frac{{{P_3}}}{{{P_2}}}\)

Hence, The pressure ratio per stage is equal and the intermediate pressure is the geometric mean of both the extreme pressures.

Explanation:

Volumetric efficiency:

The volumetric efficiency of an engine 

• It is defined as the ratio of the actual air capacity to the ideal air capacity.
• It is also the mass of air that enters in suction stroke to the mass of free air equivalent to the piston displacement at intake temperature and pressure conditions.

\({\eta _{vol}} = \frac{{{V_{actual}}}}{{{V_{swept}}}}\) which can be further written as

\({\eta _{vol}} = \frac{{\dot m\ \times\ {V_1}}}{{\frac{\pi }{4}{D^2}L \ \times \ \frac{N}{{60}} \ \times \ K}}\)

where \(\frac{\pi }{4}{D^2}\) represents the area of the piston

From the above formula, we can observe that it depends on

• The engine speed (N), as it increases, volumetric efficiency decreases.
• The valve area, as it increases, more air can enter into the cylinder, and the mass flow rate increase.
• The valve timing, as through the control of valve timing and matching of the intake manifold system volumetric efficiency can be changed.

[Note: The valve area should not be confused with the Piston area ]

Ignition lag:

• Ignition lag is the time interval in the process of a chemical reaction during which molecules get heated up to self-ignition temperature, get ignited, and produce a self-propagating nucleus of a flame i.e. the time lag between the first igniting of fuel and the commencement of the main phase of combustion.
• It is a chemical process not a period of inactivity.
• The ignition lag is generally expressed in terms of the crank angle which is 10° to 20°.

Ignition lag depends upon:

• Fuel: Depends upon the chemical nature of the fuel, the higher the self-ignition temperature, the longer the initial lag.
• Mixture ratio: Ignition lag is the smallest for the mixture ratio which gives the maximum temperature.
• Initial temperature and pressure: Ignition lag is reduced if the initial temperature and pressure are increased.
• Turbulence: Not much affected.

Additional Information

Few important points about volumetric efficiency are,

• Volumetric efficiency is a measure of the success with which the air supply and thus the charge is inducted into the engine.
• It indicates the breathing capacity of the engine.
• Supercharging is the process of supplying air or fuel-air mixture at a pressure higher than the normal atmospheric pressure. By supercharging the volumetric efficiency increases. Its value lies between 100 - 110 % and in some cases, it can go up to 130 %.

Explanation:

Compressor

• The compressor is a device which is used to increase the pressure of air from low pressure to high pressure by using some external energy
• There are basically two types of compressors
  • Positive displacement
  • Continuous flow

Air compressor terminologies

• Inlet pressure: This is the absolute pressure of the air at the inlet to the compressor
• Discharge pressure: This is the absolute pressure of the air at the outlet of a compressor.
• Single-acting compressor: Suction, compression, and delivery of the air take place on one side of the piston only in the single-acting compressor. There is only one delivery stroke for one revolution of the crankshaft.
• Double-acting compressor: Suction, compression, and delivery of the air take place on both sides of the piston. There are two delivery strokes per revolution of the crankshaft.
• Single-stage compressor: A compressor in which compression takes place from initial pressure to final pressure in one cylinder only is known as Single stage compressor.
• Multistage compressor: A compressor in which compression takes place from initial pressure to final pressure in more than one cylinder is known as a Multistage compressor.

Explanation:

Volumetric efficiency of reciprocating compressor

\({\eta _v} = 1 + C - C{\left[ {\frac{{{P_2}}}{{{P_1}}}} \right]^{\frac{1}{n}}}\)

C = clearance volume

From equation as C decreases η_vol will increase and therefore as C increases η_vol decreases.

Cooling intake air will increase its density, hence more mass flow rate and for same volume and thus volumetric efficiency increases.

Explanation:

Consider a two-stage reciprocating air compressor with perfect or complete intercooling:

Work done by single-stage air compressor (PVⁿ = C):

\(W = \frac{n}{{n - 1}}{P_1}{V_1}\left[ {{{\left( {\frac{{{P_2}}}{{{P_1}}}} \right)}^{\frac{{n - 1}}{n}}} - 1} \right]\)

So here:

Work was done per cycle in L.P. cylinder:

\({W_1} = \frac{n}{{n - 1}}{P_1}{V_1}\left[ {{{\left( {\frac{{{P_2}}}{{{P_1}}}} \right)}^{\frac{{n - 1}}{n}}} - 1} \right]\)

Work was done per cycle in H.P cylinder:

\({W_2} = \frac{n}{{n - 1}}{P_1}{V_1}\left[ {{{\left( {\frac{{{P_3}}}{{{P_2}}}} \right)}^{\frac{{n - 1}}{n}}} - 1} \right]\)

If the intercooler is perfect: P₁V₁ = P₂V₂

Total work is done per cycle:

W = W₁ + W₂

\(W = \frac{n}{{\left( {n - 1} \right)}}{P_1}{V_1}\left[ {{{\left( {\frac{{{P_2}}}{{{P_1}}}} \right)}^{\frac{{n - 1}}{n}}} + {{\left( {\frac{{{P_3}}}{{{P_2}}}} \right)}^{\frac{{n - 1}}{n}}} - 2} \right]\)

Condition for minimum power in two-stage compressors with perfect intercooling:

\({P_2} = \sqrt {{P_1}{P_3}}\)

\(W = \frac{{2n}}{{n - 1}}{P_1}{V_1}\left[ {{{\left( {\frac{{{P_3}}}{{{P_1}}}} \right)}^{\frac{{n - 1}}{{2n}}}} - 1} \right]\)

For an N stage compressor with perfect intercooling compressing air from P₁ to P_N+1:

\(W = \frac{{Nn}}{{\left( {n - 1} \right)}}{P_1}{V_1}\left[ {{{\left( {\frac{{{P_{N + 1}}}}{{{P_1}}}} \right)}^{\frac{{n - 1}}{{Nn}}}} - 1} \right]\)

For a three-stage compressor:

\(W = \frac{{3n}}{{\left( {n - 1} \right)}}{P_1}{V_1}\left[ {{{\left( {\frac{{{P_4}}}{{{P_1}}}} \right)}^{\frac{{n - 1}}{{3n}}}} - 1} \right]\)

Explanation:

Reciprocating compressor:

Volumetric efficiency:

• The volumetric efficiency of the reciprocating compressor is defined to account for the difference in the displacement volume or swept volume and the suction volume of the air to be compressed.
• \({\eta _{volumetric}} = \frac{{Suction\;Volume}}{{Swept\;Volume\;}}\)
• Also, Volumetric efficiency of reciprocating compressor

​\({\eta _v} = 1 + C - C{\left[ {\frac{{{P_2}}}{{{P_1}}}} \right]^{\frac{1}{n}}}\)

• C = clearance volume
• The volumetric efficiency for reciprocating air compressors is about 65 to 85%.
• So, in a reciprocating compressor, the value of clearance volume "C" has a direct impact on volumetric efficiency.

Concept:

Volumetric Efficiency:

The volumetric efficiency of a compressor is defined as the ratio of the actual volume sucked by the compressor at the inlet to the swept volume.

\(η_{vol} = 1-C[(\frac{P_2}{P_1})^{\frac{1}{n}}-1]~\)

Calculation:

Given:

Bore diameter d = 150 mm = 0.15 m

Stroke length L = 200 mm = 0.2 m

η_vol = 85 %

 Now, the volume of compressor chamber V₁ = \(\frac{\pi}{4}× d^2× L\) = 0.003534 m³

Therefore, swept volume V₂ = V₁ ×  η_vol = 0.003534 ×  0.85 = 0.003004 m³ = 3004 cm³

Explanation:

The pressure range of the Reciprocating compressor.

Reciprocating compressor:

• An air compressor is a machine that takes in atmospheric air, compresses it with the help of some mechanical energy, and delivers it at higher pressure.
• An air compressor increases the pressure of air by decreasing its specific volume using mechanical means.
• Thus compressed air carries immense potential energy.
• The controlled expansion of compressed air provides motive force in air motors, pneumatic hammers, air drills, sand-blasting machine, and paint sprayers, etc.
• It is also called an air pump.
• The construction and working principle of the reciprocating compressor (single-stage) is shown in the figure.

​

• The compressor pressure of this type of compressor can be very high and is limited by the strength of various parts of the compressor and the power of the driving motor.
• In a reciprocating air compressor, airflow is intermittent and not continuous like centrifugal and axial flow compressors.

Additional Information  

Multistage Compression:

If high pressure is to be delivered by a single-stage compressor then the following problems will be encountered

1. The pressure ratio will increase so the mass flow through the compressor decreases.
2. Delivery temperature increases due to the increase in delivery pressure. (T₈ > T₅ > T₂). So if high-temperature air is not desired then any increase in temperature represents an energy loss.
3. The high-pressure ratio of the single-stage compressor needs heavy working parts to sustain and this will increase the balancing problem and the high torque fluctuation will require a heavy flywheel installation.

Advantages of Multistage compression:

1. High volumetric efficiency: Intercooling between the stages improve volumetric efficiency.
2. Reduced driving power: Due to intercooling, work input for compression is reduced.
3. Better mechanical balance and uniform torque: is achieved with multi-stage compression.
4. Size and strength of cylinders: can be adjusted according to the volume and pressure of a gas.
5. Lubrication difficulties and explosion hazards are reduced because the maximum temperature reached during the compression process is greatly reduced.
6. Leakage losses are reduced considerably.

​ Important Points

The economical value of pressure ratio per stage should be in the range of 3 to 5 

Explanation:

For reciprocating air compressor, the law of compression desired is isothermal and that may be possible at very low speeds.

• Due to minimum work input required in isothermal compression, it is the ideal process for compression.
• Constant pressure line 4-1 represents the suction stroke.
• Area 1234 represents the adiabatic work.

Isothermal compression:

• If the compression is carried out isothermally, then it follows the curve 1-2 which has less slope than both isentropic and polytropic processes.
• This work done that is area 1234 in isothermal process is considerably less than that due to adiabatic compression.
• Thus, the compressor will have higher efficiency if compression follows the isothermal process.
• It is not possible in practice to achieve an isothermal process, as the compressor must run very slowly.
• In practice, compressors run at high speeds which results in a polytropic process.
• The cold-water spray and multi-stage compression are used for approximating to isothermal compression while still running the compressor at high speeds.