Our article “Understanding
Sizing Factors for Centrifugal Separators” described what a
centrifugal separator is, the key design criteria required for proper sizing and
how to use our web-based sizing tools. In this article we explain the
sizing charts and formulas of which the web-based calculators are based upon.
The two articles combined enable you to both properly size a centrifugal vortex
separator and compare performance criteria of liquid removal and pressure drop
amongst several separator designs.

We use two charts for sizing separators, one for steam and another for all other gases. We will review sizing for steam applications first because it is more simplistic.

Over 50 years ago physical testing was performed in a laboratory using
steam and compressed air to optimize the efficiency of centrifugal separators.
The test results were plotted on graphs to be used for sizing separators and we
have since derived formulas from those performance curves to enable creation of
our web-based sizing tools. These charts and calculators indicate the
maximum capacity for removal of droplets and particles larger than 10 microns
with 99% efficiency.

The sizing charts have a X, Y and secondary Y axis. The X axis lists the mass flow rate of steam or the volumetric flow rate for air depending upon the chart. The Y axis lists absolute pressure (PSIa) and the secondary Y axis the corresponding differential pressure (∆P). The diagonal lines across the chart are the performance curves for standard separator sizes.

We use two charts for sizing separators, one for steam and another for all other gases. We will review sizing for steam applications first because it is more simplistic.

The sizing charts have a X, Y and secondary Y axis. The X axis lists the mass flow rate of steam or the volumetric flow rate for air depending upon the chart. The Y axis lists absolute pressure (PSIa) and the secondary Y axis the corresponding differential pressure (∆P). The diagonal lines across the chart are the performance curves for standard separator sizes.

The first step is to draw a horizontal line from the design (maximum) PSIa across to the differential pressure (secondary Y axis).

Next draw a vertical line from your design (maximum) flow rate of steam in lbs/hr units. The separator size required is indicated by the line to the right of where your two lines intersect.

To calculate the corresponding differential pressure for your application, use the following formula:

Since centrifugal vortex separators have an infinite turndown ratio they
maintain their separation efficiency at lower pressures associated with start-up
and shut-down.

If you require a lower differential pressure, rewrite the equation above
to solve for maximum capacity flow, find that value on the X axis and draw a
vertical line upwards to intersect with the PSIa line; the required separator
size to satisfy your specific ∆P requirements is the separator size to the right
of that intersection.

The full-size steam sizing chart is hyperlinked to the image on this page
and it has a sizing example written on it already.

Moisture Separator Sizing for Air and Other Gasses

The procedure for sizing air and other gas applications is the same with respect to drawing horizontal and vertical lines, the difference is that the volumetric air flow needed to use the chart is the “Equivalent Air Flow in SCFM (Q_{c})”.

Earlier I mentioned testing was performed many years ago using air and the
corresponding datapoints formed the basis of separator sizing. The
temperature of the air used for testing was 60°F. Therefore, if your
application happens to be for air at 60°F and you have the maximum flow rate in
SCFM, you can go right to the chart. If the air is at another temperature,
you have the mass flow or if it is a gas having a molecular weight different
than air (29 MW) then some additional calculations are required before you can
use the chart. These calculations result in Q_{c} “equivalent air
flow SCFM”.

The formula for Q_{c} is: Q_{c} = Q_{sg} x
F_{g} x F_{t}

Q_{sg} is the rate of flow in CFM

F_{g} is a correction factor for specific gravity (sg)

F_{t} is a correction factor for temperature

Moisture Separator Sizing for Air and Other Gasses

The procedure for sizing air and other gas applications is the same with respect to drawing horizontal and vertical lines, the difference is that the volumetric air flow needed to use the chart is the “Equivalent Air Flow in SCFM (Q

Q

We have tables and graphs for F

The MW for our calculations is 19 (75% of 16 + 25% of 28). And thus Qsq==

We have a table specifying F

Next is F

The intersecting lines indicate a 1½” separator is required and its maximum flow rate is 350 SCFM with a rated ∆P of 1.2 PSI. Thus the actual differential pressure is calculated as 0.715 ∆P.

Now that you know the size of the separator you will need to determine which model suits your application the best.

Additional criteria to consider include the volume of condensate to remove, your piping size and orientation, droplet size to remove and physical space available to install the separator. This aspect of separator selection is described in our article “Efficiency of Centrifugal Vortex Separators”.

Our web-based sizing tool automatically calculates Q

Although we have provided you with the fundamental information and tools to properly size a centrifugal separator for your application, we are here to assist you! Please contact us by phone, email or web-based inquiry form so we can put our experience to work for you!

Chris Pasquali has provided sales and engineering support for
Wright-Austin centrifugal separators since 2001