Page 119 - Air and Gas Drilling Manual

Chapter 4: Compressors 4-5
compressor intake. The pressure rating refers to the output pressure capability at
some standard condition, e.g., API, ASME, etc.
When a compressor is operated at surface location elevations above sea level, the
volumetric flow rate intake is referred to as actual cubic feet per minute (actual cfm
or acfm). Table 4-1 gives the average atmospheric pressure and average atmospheric
temperature for the middle northern latitudes (applicable to latitudes 30˚ N to 60˚ N)
[6]. Appendix D gives additional graphic data for surface elevation atmospheric
pressures and temperatures. This data will be used in follow-on examples.
Table 4-1: Atmosphere at elevation ( mid latitudes North America) above sea level [6].
Elevation (ft) Pressure (psia) Temperature (˚ F)
0 14.696 59.00
2,000 13.662 51.87
4,000 12.685 44.74
6,000 11.769 37.60
8,000 10.911 30.47
10,000 10.108 23.36
4.3 Continuous Flow (Dynamic) Compressors
The most widely used continuous flow compressors in industry are the
centrifugal and axial-flow compressors (or compressors that combine the two
designs).

4.3.1 Centrifugal Compressors
The centrifugal compressor was the earliest developed dynamic compressor.
This type of compressor allows for the continuous flow of the gas through the
machine. There is no distinct closed boundary enclosure in which compression takes
place. The compression of the gas results from the speed of the flow through a
specified geometry within the compressor. The basic concept of the centrifugal
compressor is the use of centrifugal forces on the gas created by high velocity flow
of the gas in the cylindrical housing. Figure 4-4 shows a diagram of a single-stage
centrifugal compressor [2]. The gas to be compressed flows into the center of the
rotating impeller. The impeller throws the gas out to the periphery by means of its
radial blades rotating at high speed. The gas is then guided through the diffuser
where the high velocity gas is slowed which results in a higher pressure in the gas.
In multistage centrifugal compressors, the gas is passed to the next impeller from the
diffuser of the previous impeller (or stage). In this manner, the compressor may be
staged to increase the pressure of the ultimate discharge (see Figure 4-5) [2]. Since
the compression pressure ratio at each stage is usually rather low, of the order of 2,
there is little need for intercooling between each stage (Figure 4-5 shows an
intercooler after first three stages).
The centrifugal compressor must operate at rather high rotation speeds to be
efficient. Most commercial centrifugal compressors operate at speeds on the order of
20,000 to 30,000 rpm. With such rotation speeds very large volumes of gas can be
compressed with equipment having rather modest external dimensions. Commercial