Pneumatic Systems Flashcards
Pneumatics history
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
Pneumatics - applications
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)
Pneumatics - applications II
Some industrial applications:
- material handling (clamping, shifting, positioning, orienting)
- machining & manufacturing (drilling, turning, milling, sawing)
- general applications (packaging, filling, metering, locking, driving of axes)
Pneumatic circuits - basic components
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
Pneumatics - standard values
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
Pneumatics - type of gas used
- 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.
Compressed air - advantages
- 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
Compressed air - disadvantages
- 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
Pneumatics - standards
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
Pneumatics - common components explained
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
Air compressors - definition & type
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
Air compressors - positive vs dynamic
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
Positive displacement compressors - piston
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
Piston compressors - single cylinder
- 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.
Piston compressors - double acting
- more even air supply obtained by double acting action of compressor.
- consists of two sets of valves and crosshead to keep piston rod square.
Piston compressors - single stage
- 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
Piston compressors - multi stage
- 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.
Air compressors - diaphragm type
- 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
Air compressors - Vane type
- 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
Air compressor - screw type
- 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
Air compressors - lobe type
- 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
Air compressors - dynamic
- 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
Air compressors - sizing
- 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
Air treatment
- 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)
Air treatment - inlet filter
- 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
Air treatment - cooler
- 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
Air treatment - main line filter
- 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.
Air treatments - dryer ( refrigeration)
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)
Air treatment - dryer (membrane)
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
Air treatment - service unit
- 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
Air treatment - service unit II
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
Air service unit - compressed air filter
Removes all contaminants from compressed air flowing as well as condensed water
Air service unit - compressed air regulator
Keeps operating pressure of system (secondary pressure) practically constant regardless of fluctuations in line pressure (primary pressure) and air consumption
Air service unit - compressed air lubricator
Delivers metered quantity of oil list into part of distribution sys when necessary for operation.
Bullseye symbol in pneumatics
Represents conditioned air supply.
flow control valves
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
flow control valves II
speed control of circuits done by:
- metering fluid supplied to actuator
- metering fluid returned from actuators
- bleeding excess fluid back to reservoir
flow control valves - flow and pressure
- 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
flow control valves - mass flow rate
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
flow control valves - mass flow rate equation
c = sqrt(γRT)
For air
γ = 1.4
R = 287 J/K.kg
T = 293 K
So speed of sound in air = 342m/s
flow control valves - mass flow rate equation II
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)
flow control valves - mass flow rate equation III
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
flow control valves - mass flow rate equation (subsonic)
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
Pneumatic circuits
- most pneumatic circuits need /1+ actuators/valves to operate in coordinated & controlled way
- memory, speed, pressure, delay circuits are necessary in pneumatic applications
Pneumatic circuits - signal flow
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)
Pneumatic circuits - labeling
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
One actuator circuit - direct control
- 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
One actuator circuit - direct control uses
- 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
One actuator circuit - direct control (single acting cylinder)
- 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
One actuator circuit - direct control (double acting cylinder)
- 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
One actuator circuit - indirect control
- Indirect actuation, where signal generated via second smaller valve.
- second valve provides force necessary to switch control element
One actuator circuit - indirect control uses
- 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
One actuator circuit - indirect control (single acting cylinder)
- 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.
One actuator circuit - indirect control (double acting cylinder)
- 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
One actuator circuits - air throttling
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
One actuator circuit - air throttling
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
Logic functions - valves
Two types
- Dual pressure (AND function)
- Shuttle valve (OR function)
