|
|
Scenario: |
Our process stream has a molecular weight of 18.4
g/mole and a pressure of 30 bara. We need to compress
50000 kg/hr to 120 bara.
What driver power is required
and what compressor is necessary?
|
| Solution: |
Run
Pro-M
|
Press the open
file button. Select the Demo.prm file and press
OK.
In the Compressor
Inputs screen, you will find the following
information:
|
Inlet Pressure (bara) |
30 |
|
Discharge Pressure
(bara) |
120 |
|
Inlet Temperature (C) |
30 |
|
Flow (kg/hr) |
50000 |
|
Gearbox |
Checked |
Open the
Gas Composition. The following values will be
present:
| |
Nitrogen (%) |
1 |
| |
Carbon dioxide (%) |
2 |
| |
Methane (%) |
88 |
| |
Ethane (%) |
7 |
| |
Propane (%) |
0.87 |
| |
i-Butane (%) |
0.5 |
| |
n-Butane (%) |
0.5 |
| |
i-Pentane (%) |
0.05 |
| |
n-Pentane (%) |
0.05 |
| |
n-Hexane (%) |
0.03 |
You are now
ready to conduct a run. Return to the Compressor Inputs screen
and press the Calculate button at the bottom left of the
window.
The Compressor
Design screen will be displayed. A summary of the key results
is as follows.
| |
Molecular Weight
(g/mole) |
18.44 |
|
Head (m) |
22412 |
| |
Flow (m3/hr) |
2128 |
|
Gas Power (kW) |
4202 |
| |
No. Impellers |
8 |
|
Compressor Power (kW) |
4647 |
| |
Impeller Diameter (mm) |
298.7 |
|
Gear Loss (kW) |
116 |
| |
Speed (rpm) |
14380 |
|
Driver Absorbed Power
(kW) |
4763 |
| |
Efficiency (%) |
73.5 |
|
Driver Rating (kW) |
5239 |
The compressor
design screen will look like this:
Question:
Why is the compressor power higher than the gas power?
Answer:
The gas power is the theoretical power needed to compress
the specified mass flow for the required pressure rise. It
does not take into account the effect of many practical issues
that impact on compressor power. One of which is leakage (including balance piston
leakage) which raises the flow
passing through the compressor and in turn increases the
compressor power.
You can now view
the operating point on the head map for the compressor that
has been designed by Pro-M. Open the Compressor Performance
Graphs Screen by pressing the Performance Graphs button. You
will see the window shown by the figure below:
The red dot shows
the operating points on the graphs. It shows that the designed
compressor has an operating point that is within the band of
peak efficiency (central band of the head map), and there is
further speed available should the compressor become fouled
(experiences head loss). | 
