An air compressor delivers 10 m3min f.a.d. at a pressure of 7 bar. The average circuit demand being 7 m3min f.a.d. with an allowable fluctuation of delivery pressure of from 7 to 6 bar. Assuming that a receiver having a capacity of 4 m3 is used, determine the number of times the compressor comes on load per hour. Assume the air is fully cooled and its temperature is constant. All pressure are gauge, and atmospheric is 1 bar absolute.
Power Pneumatics Chapter 1: PNEUMATIC PRINCIPLES A body has a mass of kg What is its weight a) On earth in a gravitational acceleration of 9.81 m/s2? b) On a smaller planet ‘Pneumo’ whose gravitational acceleration is 6 m/s2? Solution: Weight on earth = Mass x Acceleration = kg x 9.81 m/s2 = 9.81 kg.m/s2 = 9.81 newtons (N) Weight on planet ‘Pneumo’ = Mass x Acceleration = kg x m/s2 = kg.m/s2 = newtons (N) A mass of kg is suspended on a spring balance in a lift cage The maximum acceleration of the lift both ascending and descending is m/s2 If the spring balance is calibrated in kg what will be its reading at maximum acceleration when a) Ascending? b) Descending? Solution: (a) With the cage ascending the spring balance will be accelerated upwards at a maximum rate of m/s2 To the spring balance it will appear that the mass is being pull downwards with a acceleration of m/s2 in addition to the gravitational acceleration Figure 1.1 Acceleration of the cage upward Total acceleration is 9.81 + m/s2 Weight will be x (9.81 + 3) N = 12.81 N As the spring balance is calibrated in kg in will read 12.81 N 1.306 kg 9.81 m/s (b) Effective acceleration on weight = (9.81 - 3) m/s2 This time the reading on the spring balance will be 9.81 9.81 0.694 A pneumatic cylinder with a bore of 100 mm is to clamp a component with a static force of 3000 N (see Fig 1.3) Determine the required air pressure Solution: = Area of cylinder = = D2 1002 mm 2 0.12 m = 7.85 103 m Figure 1.2 Arrangement of the clamp cylinder System pressure = = Force / Area 3000 N 7.85 103 m = 382 103 N / m = 3.82 105 N / m = 3.82 bar The system pressure acts on one side of the piston and is opposed by atmospheric pressure acting on the other side The system pressure has therefore to overcome the atmospheric pressure and exert a thrust of 3000 N The pressure registered on a gauge is the pressure above atmospheric Taking atmospheric pressure as bar absolute, then System pressure = 3.82 bar gauge = 3.82 bar+ bar absolute = 4.82 bar absolute A compressor delivers 500 m3 of free air per hour at a pressure of bar gauge and a temperature of 40 oC The atmospheric air at the compressor intake has a relative humidity of 80 percent and a temperature of 20 oC Determine the amount of water that has to be extracted from the compressor plant per hour Solution: At 20 oC and bar (gauge), 100 m3 of free saturated air contains 1.73 kg of water At 80 percent humidity the water content is 1.384 kg per 100 m3 f.a.d Air at the compressor plant outlet will be saturated Water content per 100 m3 f.a.d at bar gauge and 40 oC by interpolation is 0.728 0.567 0.647 kg Thus water content of air entering the compressor plant per hour is 1.84 kg Water content of air leaving the compressor plant per hour is 0.647 kg Thus, the amount of water that has to be extracted from the compressor plant per hour is (1.384 0.647) kg 3.68 kg A compressor delivers m3 of free air per minute at a pressure of bar gauge Assuming that the compression follows the law PV1.3 = const, determine the theoretical work done Solution: Work done = Where n = 1.3 P1 = atmospheric pressure = bar abs V1 = m3/min P2 = bar gauge = bar abs Work done = 0.61 3 0.61 / 10 8.15 13.58 10 10 / But N m/s = watt; therefore, work done =13.58 kW Calculate the work done if the air in Example 1.5 is compressed isothermally Solution: P1 = bar abs V1 = m3/min P2 = bar abs Work done = 10 ln / / / = 10.4 kW For a two-stage compressor delivering air at a pressure of bar gauge, determine the intercooler pressure for minimum input power Solution: 1 2.82 1.82 Estimate the pressure drop over 100 m of pipework of 50 mm bore with a flow rate of 100 l/s The mean pressure in the pipe maybe taken as bar gauge Take f as 500, then Solution: ∆ Where f = 500 L =100 m Q = 100 l/s D = 50 mm Pm = (5 + 1) bar abs Therefore, 500 ∆ 100 100 50 0.27 Any pressure drop in the pipework is a loss of energy and, as a consequence, an increase in operating costs Increasing the bore of the pipe will reduce the pressure drop but increase the cost of the pipe These must be balanced against each other for optimum conditions If the pipe bore is increased to 60 mm the pressure drop is given by 500 ∆ 100 100 60 0.107 A compressor delivers 200 l/s f.a.d at a pressure of bar Assuming the maximum flow velocity of m/s, estimate the pipe diameter needed Solution: The compressed volume of air flowing will be the free air delivery divided by the compression ratio, where Thus, in this case, taking atmospheric pressure as bar, 200 25 25 / 10 Where d is the bore of the pipe in metres Thus, 25 10 0.073 73 In this case an 80 mm bore pipe would be used Chapter 2: PRODUCTION AND DISTRIBUTION OF COMPRESSED AIR An air compressor delivers 10 m3/min f.a.d at a pressure of bar The average circuit demand being m3/min f.a.d with an allowable fluctuation of delivery pressure of from to bar Assuming that a receiver having a capacity of m3 is used, determine the number of times the compressor comes on load per hour Assume the air is fully cooled and its temperature is constant All pressure are gauge, and atmospheric is bar absolute Solution: To find the quantity of free air stored in the air receiver apply the characteristic gas equation: (a) At bar gauge P1 = bar abs V1 = ? P2 = (7+1) bar abs V2 = m3 32 (b) Similarly, at bar gauge P3 = bar abs V3 = m3 P4 = (6+1) bar abs V4 = ? 28 The difference in the volume of free air stored in the receiver between bar and bar pressure is m3 The compressor will come on load when the receiver pressure falls to bar and go to off load when it reaches bar The compressor delivers 10 m3/min f.a.d This leaves m3/min to charge the receiver Thus, 1.33 / The receiver discharges from bar to bar gauge supplying the system There is a m3 f.a.d during this time Thus, / 0.57 This is the total time between the compressor going on load is: 1.33 0.57 1.9 The represents 31.6 starts per hour, which is too many, so the receiver size must be increased to, say, m3 This will increase the quantity of air stored between the pressure of and bar gauge to m3 So / 0.86 / This gives a cycle time of 2.86 min, which is acceptable The cycle time can be increased by either increasing the capacity of the receiver or increasing the range of pressures over which the system can function A double-acting pneumatic cylinder with a bore of 100 mm, a rod diameter of 32 mm and a stroke of 300 mm operates at a pressure of bar gauge on both extend and retract strokes If the cylinder makes 25 complete cycles per minute calculate the air consumption Solution: Calculate the swept volume extending and retracting 100 300 Therefore, 100 2.355 300 32 100 300 2.114 Total volume of compressed air per cycle is: 2.355 2.114 4.469 4.469 111.7 / 25 To express this in term of free air, the volume the air would occupy at atmospheric pressure has to be calculated, taking atmospheric pressure as bar absolute Then, by the gas law, Assunming isothermal conditions and expressing the pressure as an absolute value, then P1 = bar abs V1 = Air consumption f.a.d P2 = bar gauge = bar abs V2 = 111.1 l/min And T1 = T2 Substituting in 111.7 782 / Calculate the percentage reduction in air consumption per minute for the cylinder in Example 2.2, if the supply pressure used during the retract stroke is bar gauge Solution: From previous calculation: Retract volume per stroke = 2.114 litres The retract stroke is at bar gauge, so volume of free air per retract stroke is given by Or 1 2.114 6.34 From previous calculation: Extend volume per stroke = 2.355 litres The extend stroke is at bar gauge, so volume of free air for the extend stroke is obtained using the formula Or 1 2.355 18.48 Total air consumption per cycle is 18.48 6.34 24.82 At 25 cycles/min the air consumption is: 24.8 25 620 / The rotation of gears in internal gear pump takes place in A same direction B different direction C none of the above How does the fluid flow in internal gear pump? A fluid enters the suction side between rotor, which is a large exterior gear and idler which is a small interior gear B fluid enters the suction side between rotor, which is a small exterior gear and idler which is a large interior gear C fluid enters the suction side between rotor and idler which rotate in different directions D none of the above What causes internal leakage in internal gear pump? A less tolerance level between the meshing surfaces B more tolerance level between the meshing surfaces C no tolerance between the meshing surfaces D none of the above What is the relation between pressure and overall efficiency for a gear pump? A as pressure increases, overall efficiency decreases B as pressure increases, overall efficiency increases C overall efficiency is not affected by change in pressure D none of the above What is the advantage of internal gear pump? A moderate speed B medium pressure C high viscosity fluids can be used D all the above In centrifugal pumps, the rotation of which inner element causes the liquid to pump out? A internal gear B rotation of the impeller C cylinder rotor D none of the above Which force causes vanes to come out of the rotor slots? A centripetal force B centrifugal force C friction force D none of the above Which of the following statements is true? A combination of stator with rotor is known as cartridge unit B combination of stator with vanes is known as cartridge unit C combination of rotor with vanes is known as cartridge unit D none of the above 381 XV Hydraulic System and Components – – MCQs with Answers What is the advantage of flexible vane pump? A they can handle solids which are of large size B they can create good vacuum C both a and b D none of the above Cartridge kits generate pumping chambers of various sizes, which A increase the flow rate B decrease the flow rate C increase and decrease the flow rate D none of the above Which of the following statements is false for vane pumps? A wear in contact surfaces occurs due to continuous contact between vane tips and the cam ring B different sizes of cartridge kits can be replaced in same vane pump C elliptical cam ring is replaced by round cam ring to reduce unbalanced forces D none of the above Balanced vane pumps are designed to have A fixed displacement B variable displacement C both fixed and variable displacement D none of the above Cam ring of unbalanced vane pump is A round B elliptical C both a and b D both of the above In pressure compensated vane pump, having variations in eccentricity between rotor and cam ring gives variable displacement as eccentricity between rotor and cam decreases, flow of fluid decreases as eccentricity between rotor and cam decreases, flow of fluid increases having variations in eccentricity between rotor and cam ring gives fixed displacement A and B and C and D and What is the relation between speed and flow rate for fixed displacement vane pump? A flow rate increases with increase in speed of rotor B flow rate decreases with increase in speed of rotor C flow rate is constant and does not change with change in speed D none of the above 382 In fixed displacement vane pump, A flow rate decreases with increase in working pressure B flow rate increases with increase in working pressure C flow rate is constant and does not change with working pressure D none of the above What effect does working pressure have on input power for radial piston pumps? A as working pressure increases input power decreases B as working pressure increases input power increases C pressure remains constant for different input powers D none of the above Radial piston pumps can have, A cylinder block rotating and cam stationary B cylinder block stationary and cam rotating C both a and b D none of the above XVI Hydraulic System and Components – – MCQs with Answers What is the difference between vane pump and radial piston pump? A in radial piston pump, radial slots in vane pumps are replaced by radial bores which accommodate pistons B in radial piston pump, radial slots in vane pumps are replaced by radial bores which accommodate swash plate C in radial piston pump, radial slots in vane pumps are replaced by radial bores which accommodate both swash plate and pistons D none of the above What is the advantage of multiple piston pump? A multiple piston pump not have pulsating flow B multiple piston pump have pulsating flow which is required How many strokes does a single piston pump need to discharge oil? A one stroke B two strokes C three strokes D none of the above How is the arrangement of pistons in piston pumps? A axially B radially C both a and b D none of the above In which of these pumps, swash plate is used to translate the motion of rotating shaft into reciprocating motion? A radial piston pumps B axial piston pump 383 C bent axis piston pump D all the above Which factors are considered while designing an axial piston pump? A use of swash plate B application in open loop or closed loop circuit C design of bent axis piston pump D all the above Angle of swash plate in axial piston pump is adjusted by A compensator B yoke C both a and b D none of the above In axial piston pump, the yoke is pushed away from cylinder block due to which, A yoke angle increases B swash plate angle decreases C both a and b D none of the above When the angle of swash plate decreases A flow rate increases B flow rate decreases C flow rate does not depend on swash plate angle D none of the above What will be the discharge of oil in axial piston pump, when the angle of swash plate is zero? A discharge of oil is maximum B discharge of oil is minimum C there is no discharge of oil D none of the above XVII.Hydraulic System and Components – – MCQs with Answers Which of the following statements are true? piston pumps are self-priming piston pumps require high maintenance piston pumps have low cost of production piston pumps have low volumetric efficiency A and B and C and D all the above A bent axis piston pump has A pump axis bent B cylinder block which is inclined at an angle to the drive shaft 384 C both a and b D none of the above In which of these pumps, swash plate is replaced by cylinder block? A bent axis piston pump B radial piston pump C axial piston pump D none of these What happens when the distance between flange and cylinder block is varied? A piston displacement cannot be varied B variable flow rate of fluid can be achieved C fixed flow rate can be achieved D all the above What is the maximum angle between cylinder block and shaft axis? A 30O B 50O C 45O D all the above When does holding piston keep the angle between yoke and cylinder block maximum? A when set pressure is greater than load pressure B when set pressure is less than load pressure C when set pressure and load pressure are same D all the above XVIII Losses in Pipe Valves and Fittings – MCQs and Answers What are the reasons causing pressure drop in hydraulic systems? long length of pipe Friction type of fluid losses in valves and bends A and B only C and D all the above Calculate area of a pipe if, flow rate is 20 l/min and flow velocity is cm/s A 66.66 cm2 B 60 cm2 C 62 cm2 D none of the above Which formula is used to calculate head loss in valves? A K2 (v / g) B K (v / g) 385 C K (v2 / g) D none of the above XIX Reynolds Number, Darley - Welsbach Formula – MCQs and Answers In Reynolds number (ρvd) / μ, the letter μ denotes A kinematic viscosity B absolute viscosity C coefficient of friction D none of the above The ratio of inertia force to viscosity is known as A Biot number B Reynold number C Cauchy number D Euler number The Reynolds number for laminar flow is A more than 2800 B more than 2000 C less than 2000 D between 2000 and 2800 A pipe has a diameter of 0.2 m in which a fluid flow with a velocity of 0.3 m3/s Determine whether the flow is laminar or turbulent calculating the Reynolds number Assume kinematic viscosity = 0.5 × 10-4 m2 /s A the flow is laminar having Reynolds number 1200 B the flow is turbulent having Reynolds number 2100 C the flow is laminar having Reynolds number 2200 D the flow is neither laminar nor turbulent Head loss hf due to friction is given by A ƒ l v2 / g d B ƒ l v2 / g d C ƒ l v2 / g d D ƒ l v2 / g d Head loss due to friction (hf) is a function of velocity head pressure head length to diameter ratio diameter to length ratio friction factor A 1, and B 1, and C 1, and D all the above 386 XX Flow of Hydraulic Fluids – MCQs and Answers How is laminar flow characterized in a straight pipe? A flow of high shear stress B flow of high velocity C flow of low-velocity D none of the above What is the characteristic of turbulent flow? A high velocity B the direction of flow and movement of particles is same C change in cross section does not affect the flow D all the above Which flow pattern gets affected when cross section of the pipe is changed? A laminar flow B turbulent flow C laminar and turbulent D none of the above Which of the following statements is/are true according to Bernoulli's principle? If a system has constant flow rate, change in cross-section area of the pipe affects the total energy of system The change in cross-section has no effect on the total energy of the system and is constant if it has varying flow rate A system has constant total energy, if flow rate is constant and change in cross-section has no effect on total energy A Statement B Statement C Statement D All of the above Speed of the actuator is affected by A cross-section area of the orifice B velocity of flow C pipe diameter D all the above Why according to Bernoulli's principle, change in cross-section area of the pipe has no effect on total energy of the system? A when area of pipe increases upstream, kinetic energy of fluid decreases as velocity of fluid increases and potential energy increases B when area of pipe increases upstream, kinetic energy of fluid decreases as velocity of fluid decreases and potential energy increases C when area of pipe increases upstream, kinetic energy of the fluid increases as velocity of fluid decreases and potential energy decreases D when area of pipe increases upstream, kinetic energy of fluid increases as velocity of fluid increases and potential energy decreases 387 In which of these applications Bernoulli's principle is widely used? A design of blowers B design of aircraft wings C design of propellers D all the above The total energy developed by the hydraulic oil in a system is given as A Total energy = (Potential energy + Pressure energy) B Total energy = (Potential energy + Kinetic energy) C Total energy = (Potential energy – Kinetic energy) D None of the above If a pump gives higher flow rate to the valve then, pressure drop in the valve A increases B decreases C remains the same D none of the above Match the following Group-1 with Group-2 and select the correct answer from the options below (A) pressure head (1) μ / ρ (B) velocity head (2) vd / ν (C) kinematic viscosity - (3) v2 /ρg (D) Reynolds number - (4) p / ρg -(5) v2 / g A A-(5), B-(3), C-(2), D-(1) B A-(4), B-(1), C-(3), D-(2) C A-(4), B-(5), C-(1), D-(2) D A-(3), B-(5), C-(2), D-(1) XXI Governing Principles of Hydraulic System – – MCQs and Answers Generally, liquids are non-compressible but when a large pressure of 70 bar is applied, petroleum oil can be compressed up to A 0.5% of its original volume B 1% of its original volume C 5% of its original volume D none of the above Can all hydraulic fluids be compressed when extremely large pressure is applied? A Yes B No The resistance offered to the flow of fluid inside a piston develops into A pressure B force C stress D all of the above 388 At low pressures, liquids are A compressible B non-compressible C unpredictable Which of the following statements are false? pressure is resistance to flow friction creates pressure load cannot create pressure on the system the pressure developed decreases if load offers more resistance A 1, and B and C and D none of these If no load is attached to piston rod, the movement of piston assembly is possible when A oil overcomes its self-weight B oil overcomes friction in the piston rod assembly C both a and b D none of these If capacity of pump is more, it pumps less oil per unit time A True B False XXII.Governing Principles of Hydraulic System – – MCQs and Answers Which factor helps in obtaining high speed of the piston rod in the hydraulic system? A decreased friction B pump capacity C increased flow rate D all the above During any operation in hydraulic system, oil prefers the path of A least resistance B maximum resistance C both a and b D none of the above In a hydraulic circuit a pump is provided with two outlet paths, one where load is attached and other to the reservoir Which path will the oil choose to flow first? A oil will flow to the path where load is attached B oil will flow back to the reservoir first C oil will flow through both the paths simultaneously D none of the above 389 Pressure applied on a fluid in a container is equally distributed in all directions and acts with A equal force on equal areas parallelly B equal force on different areas and at right angles C equal force on equal areas and at right angles D none of the above Which law explains the behavior of hydraulic fluids under pressure? A Charles's law B Newtons law C Pascal's law D None of the above Flow of oil in a pipe takes place due to A balanced forces B unbalanced forces C both balanced and unbalanced forces D none of the above Pressure drop in pipes, occurs due to A frictional resistance B load C flow pattern D none of the above XXIII Types of Fluid – MCQs with Answers Match the following group with group and select the correct option: Group Group High water fluids A water and synthetic chemical water in oil B fire-resistant fluid water glycol - C oil in water synthetics D tiny droplets of water in oil A C – D – A – B B D – C – B – A C C – B – D – A D D – C – A – B High Water Fluids contain A oil in water B water in oil C only water D none of the above Viscosity of High Water Fluids is A greater than water B less than water C nearby water D none of the above 390 Adding an additive to water glycol fluids improves A flammability B viscosity C oxidation D all the above XXIV Fluid Characteristics – – MCQs with Answers Which of the following statements are true? low viscosity fluids have higher leakage high viscosity fluids have high pressure drop low viscosity fluids consume more power low viscosity leads to wear of components A and B 1, and C 1, and D 2, and In industrial applications hydraulic fluids have viscosity grade ranging from A 20 to 50 B 70 to 95 C 46 to 68 D 15 to 44 High viscosity fluids have A low pressure drop B less power consumption C slow operation D all the above What is viscosity index? A effect of pressure on changes in viscosity B effect of temperature on changes in viscosity C effect of resistance between two surfaces D none of the above Highest temperature at which a fluid in hydraulic system flows is called its pour point A True B False Which property decides the behavior of fluid when mixed with water? A pour point B demulsibility C viscosity D oxidation For any operation in a hydraulic system the fluid should have pour point A 200F below the lowest temperature B 200F above the lowest temperature 391 C 200C below the lowest temperature D 200C above the lowest temperature XXV Fluid Characteristics – – MCQs with Answers What is the disadvantage of petroleum based fluids? A low flash point B low density C light weight D all the above How is the water content in High Water Fluids (HFA) compared to oil content? A more oil than water B oil and water are in same proportion C more water than oil D contains only water A fluid used in hydraulic systems should have A low oxidation resistance B high oxidation resistance C high oxidation enhancing ability D none of these At which pressure, petroleum oil used in hydraulic systems gets compressed by 1/2 %? A 70 bar B 40 bar C 30 bar D 95 bar What is the relation between temperature and specific weight for water glycol? A as temperature increases specific weight decreases B as temperature increases specific weight increases C temperature and specific weight vary linearly D none of the above What is the relation between temperature and viscosity for hydraulic oil? A temperature and viscosity vary linearly B as temperature decreases viscosity decreases at atmospheric pressure C as temperature increases viscosity decreases at atmospheric pressure D none of the above 392 XXVI Hydraulic Fluids – MCQs with Answers Which of the following reasons make water unsuitable to use as a fluid in hydraulic systems? poor lubrication high viscosity more leakage quickly evaporates A and B and C 1, and D all the above The heat generated in hydraulic systems can be absorbed by A lubrication B cooling C sealing D all the above In sliding spool valves hydraulic fluid acts as sealant to reduce leakage A True B False For which of the following purpose hydraulic film acts as a seal between the machined cavity and spool? A to reduce leakage B for cooling purposes C for lubrication purposes D all the above XXVII Fluid Power Systems – MCQs with Answers How is tilting action possible in trucks for unloading the gravel material using hydraulic power? Tilting action is possible if, oil flows at high pressure into the cylinder Tilting action is possible if, air flows at low pressure into the cylinder The truck body tilts when piston rod in the actuator is pushed out Tilting action is possible if air is compressed at high pressure A and B and C and D none of the above The hydraulic system is A less precise than pneumatic system B more precise than pneumatic system C both hydraulic and pneumatic systems are same on basis of precision D none of the above 393 Which energy is used to transmit power in hydrostatic system? A pressure energy B kinetic energy C potential energy D all the above Which system uses kinetic energy to transmit power? A hydrostatic system B hydrodynamic system C pneumatic system D none of the above XXVIII Fundamentals of Fluid Power System – MCQs with Answers Which fluid is used in hydraulic power systems? A water B oil C non-compressible fluid D all the above Pressure of bar is equal to A 14.5 psi B 145 psi C 12.5 psi D 145 x 10-6 psi Why is fluid power preferred in mobile vehicles? power can be transmitted without any delay when overloaded, fluid power systems stop without damaging the components speed variation cannot be achieved fluid is non-compressible A and B 2, and C 1, and D and What effect does overloading have on fluid power and electrical systems? A electrical components get damaged in electrical systems B fluid power system stops working without damaging the components C both a and b D none of the above How is power transmitted in fluid power systems? A power is transmitted instantaneously B power is transmitted gradually C both a and b D none of the above 394 Which of the following statements is/are false? A air is non-compressible B less power is developed in fluid power systems than conventional systems C mechanical linkages used for load handling purposes have high efficiency D all the above 395