[Rotating Equipment] Chapter 3 - Compressors

109 flashcards · Other · by Ahmed Adil

What compressors do?
They move gas from one place to another while increasing the pressure and decreasing the volume
List 3 properties of gases
Gases can be compressed or reduced in volume Gases will expand to fill any container until pressures are equalized Gas always flows from high pressure to low pressure
What are 3 ways to increase the pressure of a gas?
Reduce the volume of the gas (compress the molecules) Heat the gas (molecules move faster creating friction and force) Add more gas into the space All three take place during the compression cycle
What happens to gas molecules at absolute zero pressure?
Gas molecules cease motion and perfect vacuum occurs
What is the formula for absolute pressure?
Atmospheric pressure (101.3 kPa) + Gauge pressure
What are some application of compressors?
AC / Fridges / Supercharge
manometer is used to?
Measure pressure
What does Pascal law say?
When pressure is applied to static fluid, the pressure is transmitted equally in all directions
Give one application of Pascal law
Hydraulic brakes
State Bernoulli's law
Whenever the velocity (kinetic energy) of a fluid increases, the pressure (potential energy) decreases
State Boyle’s law
At constant temperature
State Charle's law
At constant pressure
State Amonton’s law
At constant volume
State the ideal gas (combined gas) law
1) Dynamic 2) Radial or Centrifugal 3) Axial 4) Positive displacement 5) Rotary 6) Vane 7) Screw 8) Lobe 9) Reciprocating 10) Single acting 11) double acting 12) Trunk 13) Crosshead
Compressor classification Trap consecutive volumes of gas in an enclosure, reduce the volume thus increasing the Pr, & push the compressed gas out of the enclosure. Usually known as
Positive displacement compressor
Compressor classification
Dynamic compressor
Name some types of gases that are compressed
Air for compressed tool and instrument air systems Hydrogen, oxygen, etc. for chemical processing Light hydrocarbon fractions in refining Various gases for storage or transmission
Reciprocating compressor applications
Low flow and high pressure application such as: Reinjection pipeline Station gathering systems Small refrigeration loads
Centrifugal compressors applications
High flow applications such as: Large refrigeration units pipe-trans
List 3 prime mover used for compressors
Electric motors Gas/steam turbine Diesel engine
1) Suction pipe 2) Discharge pipe 3) Suction valve 4) Discharge valve 5) Piston 6) Cylinder 7) connecting rod 8) Crankshaft
Piston Compressor Single Stage operates to pressure up to 1000 kPa Double stage operates to pressure up to 7000 kPa Multi stage operates to pressure up to 50000 kPa
Reciprocating compressor parts What is the purpose of the frame? What material is it made of?
The main housing of the compressor; it houses bearing and crankshaft. Cast iron or cast steel
Reciprocating compressor parts What is the purpose of the crankshaft? What material is it made of?
It converts the rotary motion into reciprocating motion to drive pistons (up/down, or back/forth) Forged steel
Reciprocating compressor parts What is the purpose of connecting rod in case of Single acting Double acting What material is it made of?
Reciprocating compressor parts What is the purpose of piston? What material is it made of?
Responsible for the compression of gas. Forged steel
In single acting compressors the piston is connected directly to the con-rod with a wrist pin
Reciprocating compressor parts What is the purpose of Top piston ring bottom piston ring
Top ring is for compression Bottom ring scrapes the oil back into the crankcase Bottom ring is also called "scraper" or "wiper"
Which part of reciprocating compressor is this?
piston ring
Which part of reciprocating compressor is this?
Cylinder
Cylinders in reciprocating compressor are made from a variety of styles and materials to withstand the pressures and temperatures due to compression. Lower compression ratios can be air cooled. Higher compression ratios can be water cooled for more efficient cooling.
Inlet and outlet valves in reciprocating compressor Explain the principle of valves
the work similar to a check valve the sealing element is held closed by a light spring pressure. Both the inlet and outlet sides have to overcome the spring pressure to be open
Which part of reciprocating compressor is this?
Valves
Crossheads and guides in reciprocating compressor Crosshead has upper and lower bearing surfaces which slide in the crosshead guides Guides keep the piston rod centered through the packing to minimize leakage from the compression cylinder The crosshead is connected to the piston by the piston rod and the whole assembly (piston, piston rod and crosshead) travels back and forth as a unit
2 Advantages of reciprocating compressors
Most common compressor type It has the widest range of operating suction and discharge pressure
3 Disadvantages of reciprocating compressors
Many moving part that require higher maintenance Seals can leak Not good for low demand (cylinders will overheat and carbon will form)
What are the four parts of the reciprocating compressor cycle?
Expansion Suction Compression Discharge X-axis is Volume Y-axis is pressure
Explain isothermal compression
A compression in a non-insulated cylinder where the energy generated by the piston will be removed by cylinder walls therefore the internal energy of the gas will not increase and the gas temperature will remain constant
Explain adiabatic compression
A compression which is carried in high speed in a perfectly insulated cylinder where the energy generated by the piston will be supplied to the gas therefor the internal energy and the temperature of the gas will increase
How many stages is this piston air compressor? And what are the names for part number 2 and 7?
multi stage 2 cylinder head 7 finned intercoolers
Reciprocating compressors design What are the types of compressors based on cylinder number and arrangement
Either: a single cylinder 2 or more cylinders in a line 4 cylinders in a ‘v’ formation 3 or 6 in a ‘w’ formation 2 cylinders (one horizontal and one vertical…called an ‘angle’ arrangement)
Reciprocating compressors design Horizontal: Usually the largest type (double acting and water cooled) Either constant speed, automatic start/stop control, or combination Cylinders / heads provided with water jackets Shaft and rod seals and tight covers on all crankcase openings protect against leakage of oil and keep out dust or other contaminates
Reciprocating compressors design Vertical: Smallest type of the two Usually ‘air’ cooled Used when air demand is limited and pressure requirement is low
What is the purpose of intercooler?
A heat exchanger that cools the compressed gas/air between stages
Why high compression ratios would not be possible without the use of intercoolers?
because of heat generated during compression
List 2 advantages of intercooler
reduces the weakening of parts due to high temperatures reduces risks of explosions with certain gases or oil-misted
Intercooler is usually air cooled
air/water
What is the purpose of aftercooler?
A heat exchanger like the intercooler, but it has a different purpose (to remove moisture after compression is complete)
Where is the aftercooler situated?
After the last stage of compression
Explain the working principle of intercooler
Hot gas/air travels on the inside of the tubes, heat is transferred by conduction through the fins on the outside of the tubes and air currents (natural or fan) release the heat using convection
Aftercooler is usually water cooled
air/water
What is the working principle of aftercooler?
Because of the second law of thermo-dynamics (heat always travels from hot to cold), the heat from the compressed gas/air warms the water (the is water inside the tubes and hot gases/air on the outside of the tubes), therefore the gas/air cools down enough that it can’t hold moisture anymore
Effects of altitude An increase in altitude causes a decrease in atmospheric pressure In compressor, this decrease in intake pressure causes a decrease in power requirements
Effects of moisture Excess water in the system corrodes receivers and piping, and causes blockages It also prevents proper lubrication and shortens the life of most compressor components For every 20°C rise in temperature, the air can absorb twice the amount of moisture (the reverse is true) A properly designed system has moisture separators, air dryers and inter/after-coolers to keep moisture under control
Maintenance on a reciprocating compressor List 3 maintenance activities for cooling system (water cooled)
Cooling system must always be turned on before the compressor is started When in operation, frequent temperature readings of the water should be taken at both the inlets and the outlets Check for leaks at connections
Maintenance on a reciprocating compressor List 4 requirements for the air entering the compressor
Impurities in the intake air must be removed before it enters the cylinder of the compressor (to prevent damage to parts) The ‘outside drawn air’ should be as cool and clean as possible and should never be from inside the engine room Air filters should be replaced depending on: 1. Outside air quality 2. Type and capacity of the filter (preventative maintenance schedule required)
Maintenance on a reciprocating compressor List 4 maintenance activities for oil lubrication
Check oil levels, color, and condition Make sure the proper oil is used (proper viscosity) and that it is clean Check to make sure oil filters are of correct type and are clean Check for leaks
Maintenance on a reciprocating compressor List 1 maintenance activity for intercooler
Check water connections and gaskets/seals for leaks
Maintenance on a reciprocating compressor List 2 maintenance activities for electric motor/prime mover
Clean vent grills of any dust/dirt Check for shaft and coupling condition (warped, loose, scoring, etc…) and proper alignment
Maintenance on a reciprocating compressor List 1 maintenance activity for belts
Check belts for wear/fraying/cracking, tightness and proper alignment
Maintenance on a reciprocating compressor List 3 maintenance activities for air receiver
Blow out moisture Check moisture separators/dryers Check safety valves
Maintenance on a reciprocating compressor List 2 maintenance activities for electrical connections
Check for frayed/cracked wire coverings Check wire connections are loose
Maintenance on a reciprocating compressor List 2 maintenance activities for instrumentation
Check pneumatic plastic tubing for discoloration, cracks, moisture and connections Check all gauges and manometers (calibrations and connections)
What are the 2 functions of air receiver
Provide a storage for gas Eliminate any pressure pulsation (to ensure a steady air flow, gradual pressure changes, and smooth regulation of outlet air)
aftercooler receiver dryer receiver
Code requirements to remember: Receiver should be installed as close to the compressor as possible. A shut off valve should never be placed in the discharge line, unless a safety valve is installed between the compressor and the shut off valve. A safety valve must be installed on the receiver, as well as a pressure gage. In case of wet receivers a drain valve should be installed so that oil and condensed moisture from the aftercooler can be drained away. Regular testing of safety valves should…
Name some of the compressor auxiliary equipment
1. Air receiver (storage tank) 2. Safety valves 3. ‘Check’ or ‘non-return’ or ‘one-way’ valves 4. Oil and water separators 5. Air intake and filters 6. Pressure regulators (PRV’s) 7. Lubricator 8. Air dryers 9. ‘Un-loaders’ and ‘governors’
Air Compressor ‘auxiliary’ equipment What is the name of this equipment
Air receiver
Air Compressor ‘auxiliary’ equipment What is the name of this equipment
Aftercooler
Air Compressor ‘auxiliary’ equipment What is the name of this equipment
Safety valve
Air Compressor ‘auxiliary’ equipment What is the name of this equipment
Check / non-return valve
Air Compressor ‘auxiliary’ equipment ‘Check’ or ‘non-return’ valves: Normally fitted between the compressor and the receiver Gravity normally keeps valve closed
Air Compressor ‘auxiliary’ equipment What are the 3 functions of oil/water separator
FRL = Filter, regulator and lubricator
Air Compressor ‘auxiliary’ equipment What is the name of this equipment
Lubricator
Air Compressor ‘auxiliary’ equipment What is the name of this equipment
Air dryer
Air Compressor ‘auxiliary’ equipment Air dryers: 2 types… ‘Mechanical’ and ‘chemical’ The most common ‘mechanical’ one used is the ‘cyclone separator’ which causes a centrifugal force, forcing the moisture to the sides, to fall by gravity and exit through the bottom. The most common ‘Chemical’ one used is the adsorption/desiccant type dryer. 2 towers filled with desiccant material that attracts moisture Desiccant material can be regenerated in one tower while the other tower is in service.
Name 4 common methods of 'unloading'
a) Start/stop b) Inlet valve throttling (inlet valve stays open) c) Variable speed d) Bypass (air returns back to suction side…recirculates)
Methods of 'unloading' Variable speed control: Commonly used on rotary compressors, that uses clutch. When the clutch disengages the engine slow down, which slows the compressor. When demand increases, the engine picks up speed and the clutch engage to allow the compressor to deliver air again.
Methods of 'unloading' Bypass unloading: Used in larger systems (dynamic compressors) Allows compressor to run under full ‘load’ at all times. If demand is satisfied, a ‘bypass’ valve simply opens and sends air back to compressor inlet.
Why there should be as little bends and fittings as possible on the inlet piping of compressors
because they cause friction and turbulence
Even though air is filtered before entering the compressor, contaminates may be present from (3)
1. Construction, assembly and maintenance 2. Oil and ‘wear’ particles from compression 3. Scale and rust from within the system
What is the name of this equipment
Pressure regulator
Pressure Regulator An adjustable screw is used to vary the spring pressure on top of the diaphragm and a pilot hole on the bottom of the diaphragm controls the operation of the valve for smooth operation
Rotary compressors: 1. Sliding vane (pressures up to 400 kPa for ‘single’ stage and 1000 kPa for double stages, and speeds between 250 - 2500 rpm) 2. Screw (pressures up to 350 kPa for ‘single’ stage, 900 kPa for 2 stages, 1700 kPa for 3 stages and speeds between 2000 – 12000 rpm)
What is the name and type of this compressor
Positive displacement rotary sliding vane
What is the name and type of this compressor
Positive displacement rotary screw
Rotary compressor Sliding Vane Comprised of an eccentric ‘rotor’ which revolves within a ‘casing’ Rotor has slots in which the vanes are mounted with light springs As the rotor revolves, the vanes are forced outwards against the casing When a vane passes the inlet port, air is trapped between it and the preceding vane, compressed, and discharged through the outlet Incoming air, which has been filtered, flows into the inlet port and fills in the empty spaces between the vanes Sometime rings or l…
Typical sliding vane compressor system
A - Air drawn into inlet B - Air trapped between rotor vanes and wall C - Air compressed, oil injected (to cool, seal, and lubricate) D - First oil separator E - Final oil separator F – After cooler G - Oil recirculated by air pressure H - Air flow regulated by oil pressure
List 4 advantages of ‘sliding vane’ compressors
Deliver a larger volume of air for the physical size of unit Reduced vibration ‘Pulsation - free’ air Lower maintenance costs than reciprocating
List 1 disadvantages of ‘sliding vane’ compressors
Lower efficiency than reciprocating compressors
Rotary compressors Screw: 2 inter-meshing rotors (machined in the form of an axial screw) enclosed in a close-fitting casing The 2 rotors trap and smoothly compress the air until it passes the discharge outlet ‘Male’ rotor: Larger of the two (5 or 6 lobes) and is the driven ‘Female’ rotor: Smaller of the two (4 lobes) and is the driver Number of lobes depends on demand requirements There are no valves controlling inlet/outlet
List 4 advantages of ‘screw’ compressor
Good for mobile applications (small size) Vibration free No pulsations No ‘suction’ or ‘discharge’ valves to wear out
List 1 disadvantages of ‘screw’ compressor
Limited in pressure (even in multi-stage applications because of clearances between lobes and casing)
Dynamic compressors List one application for centrifugal compressor
used for large natural gas pipeline
Dynamic compressors Low end is 20 cubic meters/min High end (multi-stage units) is 14100 cubic meters/min
1) Volute casing / 2) Vane / 3) Impeller / 4) Diffuser / 5) Guide vane / 6) shaft / 7) suction / 8) discharge
Term used for centrifugal compressor suction
Suction eye
Dynamic compressors Centrifugal compressor: Has ‘impeller’ rotating at high speed inside a specially shaped ‘volute’ casing. As the impeller rotate, it throws the gas off the vanes, causing an area of low pressure at the suction eye. Gas is continually moved through the suction eye, being thrown off the rotating vanes at high speed, and finally being slowed down in the volute casing. This reduction of speed causes an increase in pressure (Bernoulli’s Law) This increase in pressure moves the gas…
Dynamic compressors List 5 advantages of centrifugal single stage compressor
Displace large volumes of air Simple construction Low maintenance requirements Produces steady pressure (no pulsations) No receiver (storage tank) required