Centrifugal compressors are considered as a subclass of dynamic turbo machinery. They achieve the pressure increase by adding kinetic energy to a continuous flow of fluid. This kinetic energy is converted to an increase in pressure in the diffuser where the kinetic energy is converted to pressure energy by decreasing the gas velocity.

1. Preliminary selection of centrifugal compressor
Centrifugal compressors are considered as a sub-
class of dynamic turbo machinery. They achieve
the pressure increase by adding kinetic energy to a
continuous flow of fluid. This kinetic energy is
converted to an increase in pressure in the diffuser
where the kinetic energy is converted to pressure
energy by decreasing the gas velocity.
Components of the centrifugal compressors are:
 Inlet/ suction
 Impeller (open, closed, semi-open)
 Diffuser
 Discharge

The minimum conditions which are required for
gas compressor selection are:
 Inlet pressure at suction of the compressor.
 Inlet temperature at the suction of the
compressor.
 Compressor discharge pressure
requirement.
 Gas flow rate
 Gas composition.
The first step in specifying the centrifugal
compressor in industry is by the following
preliminary steps for selecting of centrifugal
compressor:
1. Calculate the compression ratio based on
the suction and discharge pressure.
𝑟 =
𝑃2
𝑃1
2. Calculate the gas flow at standard suction
conditions using the ideal gas law.
𝑞1 =
𝑃𝑠
𝑃1
𝑇1
𝑇𝑠
𝑞
3. From the manufactures’ manuals, get the
polytropic efficiency
Generally, it can be calculated by the
following formula
𝜁 = 0.61 + 0.031 × log(𝑞1)
4. Calculate the polytropic ratio:
𝑅 𝑝 =
𝐾 − 1
𝐾
1
𝜁
5. Calculate the discharge temperature
𝑇2 = 𝑇1 × 𝑟 𝑅 𝑃
6. Estimate the gas compressibility factor
values at the suction and discharge
conditions using the reduced pressure,
reduced temperature and compressibility
factor figures.
7. Calculate the gas flow rate at the inlet
conditions using the real gas law and
compressibility factor.
 Trial and error till the convergence
of q1 with an accepted deviation
8. Calculate the required gas horse power
𝐻𝑔 = 𝑅𝑇1 (
𝑧1 + 𝑧2
2
) (
𝑟 𝑅 𝑃 − 1
𝑅 𝑝
)
𝐻 𝑝 =
𝑀 𝐹 𝐻𝑔
33,000 × 𝜁
9. Calculate the gas horse power by:
𝐻 = 𝐻 𝑝 + 𝐻 𝑚
The mechanical power losses is usually considered
as 20 HP for bearing and 30 HP for seal.
Nomenclature:
r: Pressure ratio
P1: suction pressure
P2: discharge pressure
Ps: standard pressure
Ts: standard temperature
q: flow rate
q1: compensated flow rate based on the inlet
conditions
K: ratio of specific heats
T1: suction temperature
T2: discharge temperature
Hg: Polytropic head
Hp: gas horse power
Hm: Mechanical power losses.
Reference: Petroleum production engineering,
second edition, 2017