Lesson 9 - Gases SAQ
This lesson allows you to test yourself on the material covered in the Gases lesson and determine the level to which you have met the learning outcomes.
Suggested Response - Q1
* Differentiate between the following:
a) Oil-free and lubricated compressors
b) Single-stage and double-stage compressors
c) m3/s At NTP and m3/s at STP
d) Single-acting and double-acting pistons
Response:
a) Lubricated compressors have oil on the moving parts in the compressor which are in contact with the air. In oil-free compressors there is no oil used for this.
b) In a single-stage compressor, the compression takes place in a single step. In a double- or two-stage compressor, two compressors effectively work in series. The outlet pressure from one unit feeds the next unit.
c) One is normal temperature and pressure (NTP) conditions, the other is standard temperature and pressure (STP) conditions. Thus, if airflow in m3/s is specified at STP it uses 0°C and 100 kPa a (1 bar a) as the reference; if specified in m3/s at NTP, it uses 20°C and 101.325 kPa a as the reference.
d) A single-acting piston only has one compression chamber and thus only compresses in one direction. A double-acting piston has two compression chambers and thus compresses in both directions.