Both the ambient pressure and temperature decrease with altitude above sea level. This lower inlet pressure impacts the pressure ratio, for the compressors as well as for the connected equipment, which, in practice, signifies an impact on both power consumption and air consumption. At the same time, the changes due to higher altitude will also affect the available rated power from electric motors and from combustion engines.
To choose the correct compressor where the ambient conditions differ from those stated on the data sheet, the following factors should be taken into consideration:
- height above sea level or ambient pressure - ambient temperature - humidity - coolant temperature - type of compressor - power source
These factors primarily affect the following: - max. working pressure - capacity - power consumption - cooling requirement The most important factor is the inlet pressure variations at altitude. A compressor with a pressure ratio of 8.0 at sea level will have a pressure ratio of 11.1 at an altitude of 3000 meters (pro-vided that the application's operating pressure is unchanged). This affects the efficiency and, consequently, the power requirement. The amount of change is dependent on the type of compressor and the design. The ambient temperature, humidity and coolant temperature all interact and affect the compressor performance to different degrees on single-stage or multi-stage compressors, dynamic compressors or displacement compressors.
A reduction in ambient pressure, temperature increase or reduction in humidity reduces the oxygen content in the air used for combustion and, consequently, the extractable power from the internal combustion (IC) engine. The degree of shaft power deration depends on the type of engine and its breathing method (naturally aspirated or turbocharged) as set out in the table below. The humidity plays a smaller role (de-ration <1% per 1000 m) when the temperature falls below 30°C. Note that the engine power falls more rapidly than the compressor requisite shaft power. This implies that for each compressor/engine combination, there is a maximum working height that will use the entire power margin of the engine over the compressor for use at sea level. Generally, suppliers should be entrusted with calculating and stating the specific data that applies to the compressor, engine and air consumption equipment in question. If anything is unclear or for any questions, the installer should always contact the equipment manufacturer.
Read more on the different aspects of dimensioning compressor installations below.
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