Apendix 4 - Compressor Theory

Question 1

What is an isothermal compressor?

Answer 1

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

Question 2

What is an adiabetic compressor?

Answer 2

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

Question 3

What is a low pressure compressor?

Answer 3

Producing up to 50 bar - used for surface supply diving / recompression chambers

Question 4

What is a high pressure compressor?

Answer 4

Producing over 50 bar used for refilling cylinders

Question 5

What are the features of high pressure compressors?

Answer 5

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

Question 6

What is the design of a high pressure compressor?

Answer 6

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

Question 7

What do duplex pistons do?

Answer 7

Pump air on both strokes

Question 8

What happens to the air in each stage of the compression?

Answer 8

1. Compressed
2. Cooled
3. Drained of water (condensate) gungey
4. Gauged for pressure and pressure release valve
5. Passed to the next stage

Question 9

What are the ratios of multistage compression?

Answer 9

Ratio 1 2 3 4

1: 4 4 16 64 256
1: 6 6 36 216 -
1: 7 7 49 343 -

Question 10

How many stages do most compressors have?

Answer 10

3 or 4

Question 11

What are the compressor schematics?

Answer 11

1. Inlet filter
2. 1st stage - 7 bar
3. Cooling coils
4. Water separator
5. 2nd stage - 49 bar
6. Cooling coils
7. Water separator
8. 3rd stage - 343 bar
9. Water separator
10. Filter tower
11. Charging panel

Question 12

What are the maximum permitted air quality levels of Oxygen?

Answer 12

(21 +/- 1) %

Question 13

What are the maximum permitted air quality levels of CO2?

Answer 13

< or = to 500 ml m-3 (ppm)

Question 14

What are the maximum permitted air quality levels of CO?

Answer 14

< or = to 5 ml m-3 (ppm)

Question 15

What are the maximum permitted air quality levels of oil?

Answer 15

< or = to 0.5 mg m-3

Question 16

What are the maximum permitted air quality levels of moisture?

Answer 16

25 mg m-3

Question 17

What are some of the sources of contamination?

Answer 17

Pollution drawn into the intake

• smoking
• vehicle exhausts
• compressor exhaust - must be downwind on a boat

Compressor overheating/failure

Filter tower overheating/failure

Question 18

How can contaminates be removed from the air?

Answer 18

Liquid droplets - water separators
Small particles - felt pads
Moisture - silica gel / activated alumina
Oils and odours - activated carbon / charcoal
Oil and water - molecular sieve

Question 19

What is the order of the filter tower?

Answer 19

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

Question 20

When should you do air purity tests?

Answer 20

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

Question 21

What are draeger tubes?

Answer 21

A hand pump is used to pass a set amount of air through the tube

Colour change indicates contamination

Question 22

How do you fill a cylinder directly from a compressor?

Answer 22

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

Question 23

What are the procedures before charging a cylinder?

Answer 23

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

Question 24

How do you decant?

Answer 24

Highest pressure diving cylinder and lowest bank pressure

Question 25

What are the pre cylinder checks?

Answer 25

General visual inspection
Check cylinder markings 
Check test date
Check working pressure
Check o rings

Question 26

What do you check for during filling?

Answer 26

Watch for change in wind direction
Drain separators regularly
Be alert for overheating
Check prime mover fuel levels

Question 27

How do you shut down the compressor?

Answer 27

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

Question 28

How do you fill from the HP bank?

Answer 28

HP bank cylinders have high working pressures

Fill highest diving cylinder from lowest bank cylinder

Question 29

What are the decanting calculations?

Answer 29

Final pressure = total free air / total water capacity