Part 9 – Items Affecting Air Compressor Performance
There are 3 main factors that affect Air Compressor Performance:
- Altitude
- Moisture
- Temperature
Let’s take a look at each in detail and their impact on air compressor performance.
1. Effect Of Altitude
Recall that the density of air decreases with altitude. Therefore air needs to be compressed through an increased pressure range to maintain the same discharge pressure (absolute). Altitude causes a reduced volumetric efficiency and a reduced mass flow of air or free-air capacity.
Look at the chart to reference the reduction for each 1000m above sea level.
| Compressor Type | Free output Flow Rate % | Mass flow or Normal Flow % |
| Single Stage oil-free screw compressor | 0.3 | 11 |
| Two Stage oil-free screw compressor | 0.2 | 11 |
| Sigle Stage Oil-injected screw Compressor | 0.5 | 12 |
| Single-Stage piston compressor | 5 | 17 |
| Two-stage piston compressor | 2 | 13 |
| Multi-stage Centrifugal compressor | 0.4 | 12 |
Note: A rule of thumb for the altitude’s effect on the compressor at 7 (bar) working pressure and constant ambient temperature. Bear in mind that each compressor has a top pressure ratio that cannot be exceeded.
2. Effect Of Moisture
Water vapour is always present in atmospheric air and always occupies the same space as the air itself.
The Total pressure of the air is:
Sum of the water – vapour partial pressure and the partial pressure of the dry air
Cooling in an aftercooler, in distribution lines or air receivers usually causes precipitation of moisture. This may cause Rusting. Some carbon steel air receivers are protected with internal coatings of epoxy while others are constructed with stainless steel to avoid corrosion from rusting.
3. Effect of Temperature
Temperature of air determines the maximum quantity of water it can hold in suspension (saturated) – any moisture above this amount condenses out as liquid water. If saturated air is cooled, water immediately starts to condense out of it (lower temperature – less moisture in the air). The effect is that the cooler air improves compressor volumetric efficiency and less moisture removal in dryers.