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Pneumatic Systems Flashcards

1
Q

Pneumatics history

A

Word pneumatic derived from Greek “pneuma” = breath of air

Originally coined to give name for the science of motions/properties of air

Pneumatics = applications of compressed air to power/control/regulate machines. Also classed as branch of fluid power technology.

Gas in pneumatics systems behave like spring as compressible

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2
Q

Pneumatics - applications

A

Used in all types of mechanical work & development of automation solutions.

Majority of time, used for one of following:
- to determine status of processors (sensors)
- information processing (processors)
- switching of actuators using final control elements (control)
- carrying out work (actuators)

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3
Q

Pneumatics - applications II

A

Some industrial applications:
- material handling (clamping, shifting, positioning, orienting)
- machining & manufacturing (drilling, turning, milling, sawing)
- general applications (packaging, filling, metering, locking, driving of axes)

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4
Q

Pneumatic circuits - basic components

A

Pneumatic circuits consist of a combination of the following

  • control valves
  • actuators
  • pressure regulators
  • receiver tanks
  • exhausts (silencers)
  • filters (main line filters)
  • electric motors
  • drive unit
  • compressors
  • 1st air treatments (air dryer/cooler, lubricator, filter, units)
  • pressure gauges/switches
  • flow switches
  • fitting and tubing
  • lubricator
  • restrict its
  • water tap
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5
Q

Pneumatics - standard values

A

Generally in all pneumatics al calculations w air follow standard states:

Tn = 273.15 K Pn = 101325pa or 1.01325 bar

In this module, unless otherwise stated

air temp = 293K (20c)
air density = 1.225 kg/m^3
R (gas constant) = 287 J/Kg K
Air is perfect gas so pressure can then be calculated

P(abs) - ρRT

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6
Q

Pneumatics - type of gas used

A
  • air is most common
  • choice of gas = depends on application
  • apart from aero applications where inert gas preferred e.g nitrogen
  • pure nitrogen also used if danger of combustion
  • air = 78% N 21% O2 & trace elements C02, Ar, H2, Xe, He, Ne, Kr
  • air = colorless, odorless, compressible, unless dried contains water
  • compressed air carries P.E
  • compressed air can be costly & from energy efficiency may not appear advantageous but due to other qualities it’s used.
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7
Q

Compressed air - advantages

A
  • available practically everywhere
  • can be transported in pipelines even over long distance
  • no risk of explosion/fire
  • can be stored in reservoir & used when needed
  • fast working medium = high working speeds
  • un lubricated exhaust air is clean + leakage doesn’t cause contamination
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8
Q

Compressed air - disadvantages

A
  • needs good prep, dirt & condensate can’t be present
  • can’t ensure uniform & constant piston speeds with compressed air all the time
  • economical only up to certain force requirement. Under normal working pressure (6/7bar) output force limit ≈ 40/50kN
  • air systems may run into condensation problems if temp variation large
  • noisy, exhaust air is loud, so need sound absorption/silencers
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9
Q

Pneumatics - standards

A

Standards are important as
- components should be interchangeable/ perform to known standards
- so symbols understood by everyone

Organizations for producing standards
- BS (British standards)
- ISO (international standard org)
- CETOP

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10
Q

Pneumatics - common components explained

A

Actuators - converts fluid power to mech power to do work
Compressor - compressed fresh air drawn from atmosphere
Valves - control direction of flow, flow rate, pressure of compressed air
External power supply (motor) - drives compressor
Piping system - carries pressurized air from one place to another
Storage reservoir - stores given vol of compressed air

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11
Q

Air compressors - definition & type

A

Device that converts mech energy to pneumatic energy

Increases air pressure by reducing vol (which also increases temp of air)

Selected based on operating pressure needed & delivery vol

Classified into two main types
- positive displacement
- dynamic displacement

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12
Q

Air compressors - positive vs dynamic

A

Positive displacement
- draw in + capture vol of air in chamber
- reduce vol to compress air
- types: piston, diaphragm, rotary screw/vane

Dynamic displacement
- instead of reducing vol, speed up air to high vol + then restrict air flow so reduction in velocity caused pressure increase
- types: axial, centrifugal

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13
Q

Positive displacement compressors - piston

A

Piston type compressed used for high pressers (>10 bar) and relatively low vol (<10,000m^3/h)

Types
- single or multi cylinders
- single or double acting cylinders
- single or multi stage cylinders

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14
Q

Piston compressors - single cylinder

A
  • simplest form, functions similar to internal combustion cylinders
  • gives one pulse of air pressure per piston stroke
  • gives significant pressure pulse at outlet port
  • using large receiver helps, but often isn’t enough.
  • pressure usually 3-10 bar
  • for higher pressure + smoother output, use double-acting or multi-stage piston cylinders.
  • multi -cylinder pistons classified as vertical, horizontal (in line arrangememts) or (more compact) V, Y, or W constructions.
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15
Q

Piston compressors - double acting

A
  • more even air supply obtained by double acting action of compressor.
  • consists of two sets of valves and crosshead to keep piston rod square.
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16
Q

Piston compressors - single stage

A
  • single-stage compressors directly increase atmospheric pressure to required level in one operation, therefore known as single-stage compression.
  • process approximates isentropic compression, causing air temp to increase.
  • for every 5 bar increase in outlet pressure, compressed air temperature can exceed 200°C
    —> rise in motor power needed + loss of energy efficiency.

*isentropic = no change in entropy/heat transfer so process is reversible & adiabatic

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17
Q

Piston compressors - multi stage

A
  • for pressures above few bar, multistage compressors with cooling between stages = cost-effective.
  • in two-stage compressor, air partially compressed through isentropic compression
  • then cooled in intercooler, making compression process closer to isothermal compression, = more efficient.

Multistage compressors can have multiple cylinders or a more compact design with a single cylinder and a double-diameter piston.

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18
Q

Air compressors - diaphragm type

A
  • in piston type, contact between piston + air —> may introduce small amount of lubricant oil from piston wall to ait
  • undesirable in food + chemical industries
  • common type used for giving totally clean air supply by incorporating flexible diaphragm between piston and air
  • these are small capacity compressors
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19
Q

Air compressors - Vane type

A
  • this type has spring loaded vanes seated in slots of rotor
  • vanes move because of rotating part. (rotor is eccentric to cam ring)
  • spaces between vanes decrease closer to outlet due to eccentricity —> causes air compression
  • this movement squeezes air, making more pressurized.
  • compressors free from pulsation
  • ≠ eccentricities ≠ air outlet flow
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20
Q

Air compressor - screw type

A
  • for applications with med flow (~ 10,00^3/h) + pressure <10bar
  • simple construction with few moving parts
  • air delivered is steady + no pressure pulsation
  • two inter-meshing screws, air from inlet trapped between screws + compressed
  • contact between two meshing surfaces = min 0.05mm gap so no cooling needed
  • these systems quite in operation compared to piston as less moving parts
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21
Q

Air compressors - lobe type

A
  • used when high delivery vol + low pressure needed
  • operating pressure limited by leakage between rotors/housing
  • as wear increases during use, efficiency drops significantly
  • small but definite clearances allow operation without lubrication
  • timing gears control relative position of rotors to each other
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22
Q

Air compressors - dynamic

A
  • when very large vol required (5000 m^3/min) & low pressure compressed air needed dynamic compressors used
  • i.e ventilators, combustion systems, pneumatic powder blower conveyors
  • made of impellor in circular housing, inlet valve on one side & outlet on other
  • impellor rotates at high speed = large vol/low pressure air due to centrifugal forces
  • this type of dynamic is more efficient that reciprocating
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23
Q

Air compressors - sizing

A
  • sized to supply all equipment like with 25-50% + capacity in case of future expansion
  • pressure selected (usually 80-140psi)
  • free air demand of all equip using the air totalled (1x for continuous + x% of usage for avg demand)
  • based on these two, correct size compressor selected
  • cannot be sized below avg value of air consumption bc even with large receiver supplying air in peak demand, compressor can’t recover
24
Q

Air treatment

A
  • Air may need removal of contaminants
  • for satisfactory operation of pneumatic system, compressed air —> cleaned & dried
  • contaminants may come from environment (dirt, chemicals, water vapour) or introduced by compressor (lubricant)
  • treatment in 3 stages:
    1.inlet filter
    2. cooler, main line filter, dryer (primary treatment)
    3. Air service unit (secondary treatment)
25
Q

Air treatment - inlet filter

A
  • air contaminants need filtering compressor isn’t damaged.
  • filter classified by particle size
  • two type of inlet filters:
    1. dry filters using disposable cartridge
    2. wet filters, incoming air passed through oil bath & then fine mesh wire filter. Dirt particles cling to oil during bubbling and then remove by mesh wire. Wet filter cleaned with detergent
  • both require regular servicing
26
Q

Air treatment - cooler

A
  • as air is compressed, temp increases —> needs cooling.
  • cooled with cooler (type of heat exchanger)
  • two types commonly used
    1. Air cooled type - ambient air used to cook high temp air
    2. Water cooled type - water used as cooling medium, counter flow coolers where cooling medium flows in opposite direction to air
  • during cooking water vapour will condense & can later be drained
27
Q

Air treatment - main line filter

A
  • main line filter used to prevent solid contaminants from entering system & condense/remove water vapour.
  • air entering filter swirls around due to deflector cone —> centrifugal action causes large contaminants & water to be flung out. Hits glass bowl & collects at bottom.
28
Q

Air treatments - dryer ( refrigeration)

A

1st type, refrigated
- uses refrigerant to cool incoming compressed air.
- as cools, moisture condenses & drains.
- cool exiting air then usually exchanges heat with incoming air to precool inlet air (reducing refrigeration needed)
- and reheats outgoing air (to move air temp away from dew point & prevent pipe sweating)

29
Q

Air treatment - dryer (membrane)

A

Second type, membrane
- these dryers filter water vapour molecules from other molecules containing air
- smaller molecules (O,N,H) pass through membrane and continue down stream
- larger molecules vent into atmosphere

30
Q

Air treatment - service unit

A
  • air compressed by compressor & delivered to distribution system.
  • to ensure good/usable quality, service equip used to prep air before applied to control system.
  • aspect needed to be considered in prep of service air:
    1. Air pressure, storage, cleanliness, humidity, lubrication
    2. Type of compressor
    3. Line and valve size
    4. Material selection
    5. Drainage points/exhaust outlets
31
Q

Air treatment - service unit II

A

Air service unit (conditioning unit) combination of:
- compressed air filter (with water seperator)
- compressed air regulator
- compress air lubricator

Correct combination, size, type of elements determined by application/ control sys needed

32
Q

Air service unit - compressed air filter

A

Removes all contaminants from compressed air flowing as well as condensed water

33
Q

Air service unit - compressed air regulator

A

Keeps operating pressure of system (secondary pressure) practically constant regardless of fluctuations in line pressure (primary pressure) and air consumption

34
Q

Air service unit - compressed air lubricator

A

Delivers metered quantity of oil list into part of distribution sys when necessary for operation.

35
Q

Bullseye symbol in pneumatics

A

Represents conditioned air supply.

36
Q

flow control valves

A

these valves limit fluid volume flow rate, can be used for:
- controlling velocity of cylinders & motors
- compensating pressure and/or temp changes
- allowing one fixed displacement compressor to supply two or more branch circuits fluids at diff flow rates

37
Q

flow control valves II

A

speed control of circuits done by:
- metering fluid supplied to actuator
- metering fluid returned from actuators
- bleeding excess fluid back to reservoir

38
Q

flow control valves - flow and pressure

A
  • since air compressible, flow rate measured by mass flow rate (m dot) and volume flow rate (Q)
  • rate of air flow through orifice determined by absolute upstream/downstream pressure.
  • air can only travel max up to speed of sound through origins regardless of how high pressure
39
Q

flow control valves - mass flow rate

A

As downstream pressure (pd) mass flow rate increases but only till m dot max ( max mass flow rate)
When flow rate is sonic (speed of sound) any further reduction in pd = no effect

40
Q

flow control valves - mass flow rate equation

A

c = sqrt(γRT)

For air
γ = 1.4
R = 287 J/K.kg
T = 293 K

So speed of sound in air = 342m/s

41
Q

flow control valves - mass flow rate equation II

A

mass flow rate product of volume flow rate & density

m(dot) max = Qmax x ρ

{Qmax = A x c
{c = sqrt(γRT)
{ρ = P/RT

—> m(dot) max = A(P/RT) sqrt(γRT) —> sqrt(γ/R) x A x P/sqrt(T)

So for air, m(dot) max = 0.0698 x A x P/sqrt(T)

42
Q

flow control valves - mass flow rate equation III

A

P/T not normally known, as pressure/temp inside orifices, equivalent equation can be written based on upstream P/T

m(dot)max = Cm x A x Pu/ sqrt(Tu)

Cm = (2/γ+1) ^(1/γ-1) x sqrt ( 2γ/R(γ+1)

For air cm = 0.0405 s.sqrtK/m

43
Q

flow control valves - mass flow rate equation (subsonic)

A

When flow is sub sonic (slower than speed of sound) term added to equation to give true mass flow rate.

m(dot) = Cm x A x Pu/ sqrt(Tu) x W

W = mass flow ratio (value between 1 & 0), depends on ratio of upstream/downstream pressures. Can be found easy in graph

44
Q

Pneumatic circuits

A
  • most pneumatic circuits need /1+ actuators/valves to operate in coordinated & controlled way
  • memory, speed, pressure, delay circuits are necessary in pneumatic applications
45
Q

Pneumatic circuits - signal flow

A

Signal flow diagrams indicate path of pneumatic signal input.

Power components (cylinders/motors)
^
Control elements (DCVs)

                       ^  Processing elements (DCVs, pressure valves, etc)

                       ^ Input elements (switches, sensors, push buttons) 

                        ^ Supply elements (compressors, air preparation unit)
46
Q

Pneumatic circuits - labeling

A

Label: n A m

n -
0: ground level line
1- n: pneumatic line no.

m -
1-m: component number in line

A -
A: actuator
S: switch
V: valve
Z: air preparation

E.g

0Z4 - no.4 ground level air prep unit

1S - only switch in line 1

2A1 - no.1 actuator in line 2

47
Q

One actuator circuit - direct control

A
  • simplest level of control for double/single acting cylinder
  • cylinder actuated directly either manually or with a mechanically actuated valve. No intermediate switching of additional DCV
48
Q

One actuator circuit - direct control uses

A
  • cylinders with piston diameter smaller than 40 mm
  • valves with connection size smaller than 6mm
    If port size/flow valve of valve too large, operating forces needed may be too much for direct manual operation
49
Q

One actuator circuit - direct control (single acting cylinder)

A
  • on operating push button, air passes through valve from port 1 to 2 via 1S
  • pressure builds, advanced piston rod against force of cylinder return spring
  • release of button = valve spring returns 1S valve to initial position + cylinder retracts
  • air returns from cylinder via exhaust port of 3 of 1S
50
Q

One actuator circuit - direct control (double acting cylinder)

A
  • in initial position, valve is actuated, piston is retracted
    -on operating push button, air passes through valve 1 to 4 + advances piston rod. Displaced air flows to atmosphere via ports 2 to 3
  • on release of push button, valve spring returns control valve to initial position
  • so it’s possible to change direction of movement without piston rod reaching initial/end position
51
Q

One actuator circuit - indirect control

A
  • Indirect actuation, where signal generated via second smaller valve.
  • second valve provides force necessary to switch control element
52
Q

One actuator circuit - indirect control uses

A
  • used for pistons with large diameter + high air vol required
  • for these cases control valve with high nominal flow rate must be used to actuate
  • valve actuation force would be too high for manual actuation, so indirect actuation used
53
Q

One actuator circuit - indirect control (single acting cylinder)

A
  • in initial position, 1A retracted. Port 1 of 1V closed and port 2 exhausted to atmosp via port 3
  • on operating push button pressure applied to control port of 1V, so 1V actuated against spring force
  • pressure builds up in cylinder, causing piston to extend
  • once piston rod reached end position returns, only when push button released.
54
Q

One actuator circuit - indirect control (double acting cylinder)

A
  • on operating push button, 1V pilot signal supplied + piston rod of cylinder 1A advances
  • if push button released, control port of valve 1V exhaust to atmosphere, valve then reversed and cylinder retracts
  • so it’s possible to change direction of movement without piston rod reaching initial/end position
55
Q

One actuator circuits - air throttling

A

Supply air throttling
- this speed controller is called meter in circuit
- exhaust air can escape freely through check valve of the exhaust side of cylinder
- no air cushion on exhaust side of cylinder piston w this throttling arrangement
- so considerable difference in stroking velocity even w small variation of piston rod load
- any load in direction of operating motion accelerates piston above set velocity
- used for single acting and small volume cylinders

56
Q

One actuator circuit - air throttling

A

Exhaust air throttling
- this speed controller is called meter out circuit
- exhaust air leaving cylinder throttled in both directions of motion of cylinder
- piston loaded between 2 air cushions while cylinder in motion
- hence smooth motion obtained
- 1st cushion effect due to supply air entering cylinder through check valve.
- 2nd is due to exhaust air leaving cylinder through restrictor at slower rate
- used for speed control of double acting cylinder

57
Q

Logic functions - valves

A

Two types

  1. Dual pressure (AND function)
  2. Shuttle valve (OR function)

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