Chapter 17 - Pneumatic Systems Flashcards
1
Q
What are the general difference between pneumatic systems and hydraulic systems?
A
- fluid medium used is commonly air (which can be compressed)
- the supply is atmosphere
2
Q
What laws and theories can be applied to both pneumatics and hydraulics?
A
- Pascal’s Law
- Bernoulli’s Principles
- Law of Conservation of Energy
3
Q
Describe absolute zero temperature
A
- no molecular movement occurs
- 0 value in terms of Rankine and Kelvin Scale, equalling -460F and -273C
4
Q
Describe absolute zero pressure
A
- there is no pressure - 0 PSIA
5
Q
What is the normal atmospheric pressure at sea level?
A
- 14.7 PSIA
- Standard Air = 14.7 PSIA @ 68*F - 30% humidity
- Free Air = air at the location of intake
6
Q
Why are psia and psig?
A
- 14.7 PSIA = 0 PSIG
7
Q
How is vacuum measured?
A
- measured with a barometer
- measured in mm HG or “HG
8
Q
What are the properties of characteristics of compressed air?
A
- it can be compressed or reduced in volume
- air will expand to fill any container
- pressure in a confined, static (at rest) fluid acts the same and equally in every direction. It always acts at right angles to the containing surface (Pascals Law)
- there must be a pressure change to create air flow
- air flows from high- to low-pressure areas
9
Q
What are the ways of increasing pressure in a sealed container?
A
- reducing the volume to create more impact on a smaller wall area
- introducing more air into the confined space
- heating the air so the molecules travel faster and increase the intensity and amount of impact on the same wall area
10
Q
Which is a more efficient transfer of energy: laminar or turbulent flow?
A
- laminar flow
11
Q
What is …
- Boyle’s law?
A
- at constant temperature, absolute pressure (P) varies inversely to the volume (V)
- P1V1 = P2V2
12
Q
What is …
- Charles law?
A
- at a constant pressure, the volume (V) of a gas varies proportionately to its absolute temperature (T)
- V1/T1 = V2/T2
- for a constant volume of a gas, the pressure (P) varies proportionately to its absolute temperature (T)
- P1/T1 = P2/T2
13
Q
What is …
- the Ideal Gas law?
A
- Boyle’s and Charle’s laws combined
- P1V1/T1 = P2V2/T2
- P1V1T2 = P2V2T1
14
Q
How are the vacuum pumps used to create fluid power?
A
- removes the air rather than compressing it
- pressure differential induces a “HG rating
15
Q
How are compressors classified?
A
- Principle of operation:
- dynamic action - deliver large volumes of air at relatively low pressures
- positive-Displacement action - deliver moderate volumes of air at high pressures
- Motion classification:
- reciprocating - positive displacement action
- rotary
- – positive displacement - vane, lobe, screw
- – dynamic action - centrifugal
16
Q
Name and describe the methods used to control the maximum pressure output of a compressor
A
- unloading devices (fingers)
- consists of a set of fingers controlled y a pilot line from the system’s air pressure.
- -these fingers hold the intake valves open when the maximum system pressure is reached, allowing the inlet air to flow in and out of the chamber without compression.
- throttling the air intake
- allowing less free air to enter the compressor
- VSD
- at maximum pressure setting, the engine rpm is reduced and the centrifugal clutch disengages allowing engine to run at low rpm with no load
- pressure switch
- used to start and stop an electric drive motor within presets of a high-low range
17
Q
Describe the operating principles of a reciprocating compressor
A
1 - when the suction stroke begins, the movement of the piston creates a partial vacuum
2 - atmospheric pressure then unseats the inlet check valve, allowing air to fill the chamber
3 - the discharge check valve stays closed due to spring pressure and air pressure
4 - on the compression stroke, the inlet check valve closes and the piston advances, compressing the air in the cylinder
5 - it does this until the air pressure is greater than the combined line pressure and spring pressure of the check valve
6 - at that point, the discharge check valve opens
7 - this allows the compressed air to leave the cylinder
8 - the suction stroke begins and the cycle is then repeated
18
Q
Describe the types of reciprocating compressors
A
- single- or double-acting
- single- or multi-stage
- multistage more efficient (double-acting multi-stage most efficient)
19
Q
Describe the advantages and/or disadvantages associated with rotary compressors as compared to reciprocating compressors
A
- -Advantages
- generally smaller
- less vibration
- need less mass in a foundation for same volume of usable air
- Disadvantages
- very noisy and must be used with silencers and enclosed in a sound-deadening room
20
Q
Describe the operating principles of:
- a vane compressor
A
- composed of a rotor with sliding vanes mounted in an eccentric housing
- vanes move in and out under centrifugal force and form a seal against the housing
- intake air enters as compartments expand
- air is compressed as volume is reduced and discharges through the exhaust ports
21
Q
Describe the operating principles of:
- a lobe compressor
A
- work through the action of two closely meshed lobed rotors driven by timing gears
- air is trapped between the lobes and the casing and is carried around without a mechanical reduction in volume
- pressure is built up from restrictions at the discharge port and from the system’s resistance to flow
22
Q
Describe the operating principles of:
- a dry screw and a wet screw compressor
A
- screw compressors consist of two screws or lobes on a helix
- air is trapped between the meshing units and reduced in volume as it moves axially to the discharge port
- dry screw
- use two timed gears to prevent contact between the rotors
- gear backlash and bearing wear must be held to a minimum due to the small clearances between the rotors
- wet screw
- one rotor drives the other - no timing gears
- require oil to reduce wear between mating parts
- oil reduces the amount of air slip and removes some heat of compression, but must be removed downstream
23
Q
Describe the operating principles of:
- a centrifugal compressor
A
- uses dynamic action to build up pressure
- airflow enters the eye of the impeller and is discharged at its periphery cavities and into the eye of the next impeller and so on until the air reaches the discharge port
24
Q
Describe the operating principles of:
- an axial-flow compressor
A
- air is forced axially along the rotor by a series of fins and directed by the stationary vanes
- the impeller cavities decrease from one stage to another to compress the air as it moves towards the discharge port
25
What treatments are necessary to ensure the quality of air is suitable for a pneumatic system?
-ensures that the air is not harmful to the system; protecting against contamination, moisture and heat
- air treatment consists of:
- - filtering
- - cooling
- - removing moisture and oil
- - storage
26
What does the air-intake filter control?
- control the amount of airborne contaminants allowed to enter the compressor
- amount of filtration dependent on requirement of the compressor
- first line of protection
27
What do intercoolers control?
- heat generated due to compression
- used to cool the air between stages
- air-cooled intercoolers use a fan to force air over finned tubes through which compressed air passes. The fins and tubing must be kept clean for efficient heat dissipation
- liquid-cooled intercoolers use a nest of tubes immersed in recirculating liquid. Shell and tube heat exchanger.
28
Define dew point
- temperature at which water condenses as it is cooled
| - water vapour in compressed air settles out as free water if the intercooler temperature falls below the dew point.
29
Describe the location and purposes of water traps
- vital components in the removal of water from system
- can be placed in many locations to expel large amounts of water from the lines - wherever the compressed air is cooled and may fall below its dew point, a water trap is installed
- common areas are at/after the:
- - intercoolers
- - aftercoolers
- - air dryers
30
Describe the purposes and principles of the operation of aftercoolers
- heat exchangers which cool the air after it leaves the compressor and before it enters the receiver
- receivers can also act as an aftercooler
- aftercoolers permits the removal of most of the moisture and other entrained liquids which may be in the compressed air during the compression stages
- work on the principles of a shell and tube heat exchanger, with cooling happening due to counterflow of a air- or water-cooled aftercooling passing by compressed airlines
- moisture is collected and drained out of the system by manual or automatic water traps.
31
What are the functions of an air receiver?
- a storage tank for the compressed air before it enters the system
- acts as a reservoir to accommodate any fluctuations in the systems supply. This dampens pulsations from the compressor and provides a steady pressure to the system
- also reduces the velocity of the air. This allows any moisture carried over from the aftercooler to settle out. If an aftercooler is not used, the receiver acts as the aftercooler
32
How is an air receiver classified?
- an unfired pressure vessel (potentially explosive)
33
Where is the maximum pressure control located on a pneumatic system?
- located in, or on, the compressor, NOT the air receiver
| - maximum working air pressure is never higher than the maximum working pressure stamped on the receiver
34
What does the safety valve on the air receiver control?
- the safety valve on the air receiver is used in case of any failure in the system
- releases any excessive pressure, due to the failure of the unloading valve or ta a pressure surge transmitted back from the system
- usually set 5-10% above the systems maximum pressure
35
Where is the drain valve located on the receiver?
- must be installed at the lowest point on the receiver
- care must be taken to ensure the drain valve can drain the tank completely
- water and oil emulsions which settle out should be drained off by this valve regularly
36
What is the purpose of a fusible plug?
- will melt and release all the pressure in the receiver if the air temperature becomes dangerously high
- ensure the path in which the pressure would vent will not endanger workers nearby
37
How is the pressure in the receiver monitored?
- an air pressure gauge monitors the pressure inside the receiver
38
What are two types of compressed air used in industry?
- instrument air
- - must be clean, dry and free of oil
- plant air
- - should be clean and have a low moisture and oil content
39
Describe the operating principles of the following:
| - deliquescent dryers
- contain chemical desiccants, which absorb moisture
- the desiccants are consumed in the drying process, which means the chemicals must be replenished periodically and their disposal may be a problem
40
Describe the operating principles of the following:
| - regenerative desiccant dryers
- use a solid desiccant which absorbs water onto its surface
- usually silica gel, activated alumina, or a molecular sieve
- use two identical chambers in which the air is dried
- as one chamber becomes saturated, flow is directed to the other, and the saturated chamber is dried out
- heat may be applied to speed up the drying process, but the chamber must be cooled before it is ready to use again
- heat 75% - cool 25%
41
Describe the operating principles of the following:
| - refrigeration dryers
- condense moisture from compressed air by cooling the air in heat exchangers chilled by refrigerants
- moisture collects in a moisture trap and is drained
- the compressed air is then reheated to operating temperature before it enters the system, in order to prevent condensation from forming downstream from the dryer
42
Describe the operating principles of the following:
| - membrane dryers
- gas separation devices
- consist of permeable membrane surfaces that block nitrogen and oxygen molecules(air), but allow water vapour molecules to pass through
- because the membrane vents gas, not condensate, there is no need for regeneration or fear of freezing
- have no moving parts to wear out
- non-electric and suitable for most hazardous environments
- housing is constructed from plastic and aluminum to prevent corrosion
43
How is contamination produced within a compressed air system?
- construction, assembly, and maintenance debris
- oil carried over from the compressor
- operational wear particles, pipe scale, and rust generated within the line
44
Describe the operating principles of a typical filter that removes water, oil and particulate matter from compressed air
1 - a deflector plate swirls the air around the filter bowl
2 - a shroud ensures that the swirling action occurs around the filter bowl and not the filtering element
3 - this causes the larger particles and excess moisture to be thrown out the side of the filter bowl
4 - a baffle below the filtering element creates a quiet zone which allows the particles and moisture to collect and prevents them from becoming entrained in the air flow again
5 - the air is the forced through the filtering element which removes the smaller particles
6 - clean air then exits the filter and enters the system
45
What is the difference in filtering action between standard and coalescing filters?
- standard filters
- - filter solid particles and collect any excessive moisture that happens to condensate at that time
- coalescing filters
- - designed to remove all solid particles as small as 0.3 microns, together with nearly 100% of the oil and water vapours, from the air
- - they use a desiccant similar to the desiccant in air dryers
46
Describe the purpose and operating principles of the following:
- pressure regulator
- reduces the line pressure
- consist of a main piston or diaphragm which controls a poppet or valve by means of a connecting pin
1 - an adjusting screw preloads a spring on top of the piston
2 - the pilot passage is open to the outlet port
3 - as the down-side pressure increases, the piston rises, along with the poppet, and the flow decreases
4 - a pressure gauge is mounted on the down-side of the regulator so that any adjustment of the regulator can be read directly
47
Describe the purpose and operating principles of the following:
- lubricator
-
48
What is a FRL conditioning unit?
-
49
What system functions do pneumatics valves control?
-
50
Compare pneumatic valves to hydraulic valves
-
51
Describe the methods of sealing spool valves
-
52
How many ports are needed in DCV to operate a two-pressure and/or two-speed double-acting cylinder?
Figure 19
Figure 33
-
53
What precautions should be exercised when using a manual override?
-
54
Which device produces more infinite control: solenoid-controlled DCV or pilot-operated DCV?
-
55
Describe the purpose and/or principles of operation of the following:
- flow control valve
-
56
Describe the purpose and/or principles of operation of the following:
- quick exhaust
-
57
Describe the purpose and/or principles of operation of the following:
- shuttle valve
-
58
Describe the purpose and/or principles of operation of the following:
- muffler
-
59
Compare pneumatic actuators to hydraulic actuators
-
60
Compare pneumatic conductors and fittings to hydraulic conductors and fittings
-
61
What prevents the hose from collapsing with flareless fittings?
-
62
Describe quick-disconnect couplers
-
63
Describe the following piping systems:
| - grid system
-
64
Describe the following piping systems:
| - decentralized system
-
65
Describe the following piping systems:
| - loop system
-
66
How is moisture prevented from accumulating in the lines?
-
67
Identify the following symbols:
-
68
Describe the principle of operation of the two-hand or safety circuit in figure 32
-
69
Why are two flow control valves used to control the speed of the actuator in Figure 33?
-
70
Describe the operation of a quick-exhaust valve and state where it is placed
-
71
State the direction the actuator will turn, as pictured in Figure 35
-
72
What parts tend to freeze in cold weather in a compressed air circuit?
-
73
How are these parts thawed out and prevented from freezing again?
-
74
What are the causes of the following problems:
| - excessive noise in a compressor?
-
75
What are the causes of the following problems:
| - low oil pressure in a compressor?
-
76
What are the causes of the following problems:
| - excessive air pressure in the receiver?
-
77
What are the causes of the following problems:
| - compressor overheating?
-
78
What are the causes of the following problems:
| - sticky valves in a compressor?
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