Effect of Gas Density on Fan Performance

Gas density has a major effect on fan performance, especially for ID fans, FD fans, and process gas fans.

Simple rule

A fan moves volume (m³/sec), but the pressure it develops and power it consumes depend on gas density.

Think of it like this:

  • Blowing hot light gas = easier
  • Blowing cold heavy gas = harder

Same fan, same RPM, same impeller → different results depending on gas density.

Fan laws related to density

For same fan size and speed:

1) Volume flow remains almost same

2) Pressure varies directly with density

3) Power varies directly with density

Where:

  • (Q) = flow
  • (SP) = pressure
  • (p) = gas density

Practical Example (ID Fan)

Suppose a fan is selected for:

  • Flow = 50 m³/sec
  • Static pressure = 500 mmWG
  • Gas temperature = 30°C
  • Density = 1.2 kg/m³

Motor power say = 400 kW

Now actual operation:

Flue gas temperature rises to 250°C

Density drops. – p2= p1 x T1/T2

Approx density:

Absolute temperatures:

30°C = 303 K  —- (273+30)
250°C = 523 K —- (273+250)

So: p2= 1.2 x (303/523) = 0.695 kg/m3

New pressure developed

Since pressure ∝ density:

SP2 = 500 x 0.695/1.2 = 290 mm wg

So same fan now develops only:

~290 mm wg instead of 500 mm wg

Big drop.

New power

Since power ∝ density:

= 400 x 0.695/1.2 = 232 kw – so power reduces too

What does this mean in plant?

If system actually needs 500 mmWG at 250°C, this fan will fail.

Because at hot gas density, same fan gives only 290 mmWG.

So correction required.

Required corrected pressure:

SP 2 = 500 x (1.2/0.695) = 863 mm wg

Meaning:

When selecting the fan at standard density, you must size it for 863 mmWG so that at operating hot density it delivers real 500 mmWG.

Real-world ID Fan lesson

This is why many people make a mistake:

They say:

“My duct loss is 500 mmWG.”

But forget that this is at hot gas condition.

Vendor selects fan directly at 500 mmWG (standard density basis).

Installed fan underperforms.

Thumb rule

Hotter gas → lower density → lower fan pressure

Colder gas → higher density → higher fan pressure + higher motor load

That is why ID fan sizing always asks:

  • Gas temperature
  • Gas composition
  • Altitude
  • Dust loading
  • Moisture

Because all affect density.