Apendix 4 - Compressor Theory Flashcards
What is an isothermal compressor?
The temperature of the air remains the same. This occurs by very slow compression. The heat produced is dissipated and not transferred to the air. This idea is very energy efficient but not very practical.
Start temp = final temp
It’s the ideal
What is an adiabetic compressor?
The reality of compressing. The heat of the compression is contained within the air.
As the temperature is greater after compression, cooling will occur
Pressure will fall as the air cools
In order to increase efficiency and approximate isothermal conditions the air is compressed in stages and cooled between each stage
What is a low pressure compressor?
Producing up to 50 bar - used for surface supply diving / recompression chambers
What is a high pressure compressor?
Producing over 50 bar used for refilling cylinders
What are the features of high pressure compressors?
They can be electric or petrol/diesel driven
Electric are oil free and conventional types - use the manufacturers recommendation
Working pressure of 200-400 bar
Charge rate - cfm
Purity quality
Portable sometimes
What is the design of a high pressure compressor?
Multistage compression
Air is passed from one cylinder to the next and compressed in stages
Cylinders become smaller in volume but higher in pressure
They lose heat in stages
What do duplex pistons do?
Pump air on both strokes
What happens to the air in each stage of the compression?
- Compressed
- Cooled
- Drained of water (condensate) gungey
- Gauged for pressure and pressure release valve
- Passed to the next stage
What are the ratios of multistage compression?
Ratio 1 2 3 4
1: 4 4 16 64 256
1: 6 6 36 216 -
1: 7 7 49 343 -
How many stages do most compressors have?
3 or 4
What are the compressor schematics?
- Inlet filter
- 1st stage - 7 bar
- Cooling coils
- Water separator
- 2nd stage - 49 bar
- Cooling coils
- Water separator
- 3rd stage - 343 bar
- Water separator
- Filter tower
- Charging panel
What are the maximum permitted air quality levels of Oxygen?
(21 +/- 1) %
What are the maximum permitted air quality levels of CO2?
< or = to 500 ml m-3 (ppm)
What are the maximum permitted air quality levels of CO?
< or = to 5 ml m-3 (ppm)
What are the maximum permitted air quality levels of oil?
< or = to 0.5 mg m-3
What are the maximum permitted air quality levels of moisture?
25 mg m-3
What are some of the sources of contamination?
Pollution drawn into the intake
- smoking
- vehicle exhausts
- compressor exhaust - must be downwind on a boat
Compressor overheating/failure
Filter tower overheating/failure
How can contaminates be removed from the air?
Liquid droplets - water separators
Small particles - felt pads
Moisture - silica gel / activated alumina
Oils and odours - activated carbon / charcoal
Oil and water - molecular sieve
What is the order of the filter tower?
Silica gel Felt pad Carbon/charcoal Felt lad Molecular sieve Felt pad
Moisture must be taken out first as the activated charcoal is less efficient if moist
When should you do air purity tests?
Check air purity regularly
Follow manufacturers recommendations and HSE legislations
Test frequency is usually based on running time and a set interval
Current maximum interval is 3 months
What are draeger tubes?
A hand pump is used to pass a set amount of air through the tube
Colour change indicates contamination
How do you fill a cylinder directly from a compressor?
Compression to maximum output pressure. Doesn’t require storage cylinders. Can be slow if cfm rate is low (portable compressors)
HP bank (decanting / cascading)
A large bank can fill many cylinders quickly. Compressor runs only when required to top up.
Larger static compressor installations
What are the procedures before charging a cylinder?
Look at log of compressor Check location of air intake Check oil levels Check prime mover (electric/petrol/diesel) Open drain valves Start compressor and allow to warm up Close drain valves Check cylinder
How do you decant?
Highest pressure diving cylinder and lowest bank pressure
What are the pre cylinder checks?
General visual inspection Check cylinder markings Check test date Check working pressure Check o rings
What do you check for during filling?
Watch for change in wind direction
Drain separators regularly
Be alert for overheating
Check prime mover fuel levels
How do you shut down the compressor?
Disconnect cylinders
Open drain valves
Run off load for a couple of minutes - cool down
Complete compressor log
Shut down compressor and clear away
Store as per manufacturers recommendations
How do you fill from the HP bank?
HP bank cylinders have high working pressures
Fill highest diving cylinder from lowest bank cylinder
What are the decanting calculations?
Final pressure = total free air / total water capacity
