System Resistance vs Fan Curve (Simple Explanation with Example in m³/sec and mmWG)
This is one of the most important concepts in fan engineering, but once understood visually, it becomes very simple.
Think of it like this:
Fan wants to push air.
System resists airflow.
Actual operating point happens where both agree.
That meeting point is called the operating point
1. What is System Resistance?
Every air/gas system resists airflow.
Resistance comes from:
- ducts
- bends
- dampers
- bag filters
- cyclones
- scrubbers
- stacks
- furnace passages
The more air you try to push, the harder the resistance becomes.
Important:
Resistance does NOT increase linearly.
It increases as square of airflow.
Formula:
[SP = KQ^2]
Where:
- SP = static pressure (mmWG)
- Q = flow (m³/sec)
- K = system constant
Meaning:
Double flow → resistance becomes 4 times.
Simple Example
Suppose:
At 10 m³/sec, pressure drop = 100 mmWG
Then:
At 20 m³/sec:
[100 \times (20/10)^2]
= 100 × 4
= 400 mmWG
At 30 m³/sec:
[100 \times (30/10)^2]
= 100 × 9
= 900 mmWG
So system curve rises sharply.
System Curve Table
Flow (m³/sec) | Resistance (mmWG) |
10 | 100 |
15 | 225 |
20 | 400 |
25 | 625 |
30 | 900 |
This forms an upward curved line.
2. What is Fan Curve?
A fan has limited capability.
At low flow:
fan gives high pressure.
At high flow:
pressure falls.
Because energy gets spent moving more air.
So fan curve slopes downward.
Example fan:
Flow (m³/sec) | Fan Pressure (mmWG) |
10 | 800 |
15 | 700 |
20 | 550 |
25 | 380 |
30 | 200 |
Meaning:
Fan can produce:
- 800 mmWG at low flow
- only 200 mmWG at high flow
Simple Understanding
Fan says:
“I can give less pressure if you ask for more flow.”
System says:
“I need more pressure if you want more flow.”
Conflict happens.
Actual point = where both match.
Combined Table
Flow | Fan Pressure | System Resistance |
10 | 800 | 100 |
15 | 700 | 225 |
20 | 550 | 400 |
22 | 480 | 484 |
25 | 380 | 625 |
At ~22 m³/sec:
Fan ≈ System
That is operating point.
Meaning of Operating Point
Actual fan delivery:
22 m³/sec at 480 mmWG
Not design guess.
Actual physical reality.
Visual Understanding
Imagine graph:
X-axis:
Flow (m³/sec)
Y-axis:
Pressure (mmWG)
Fan curve:
downward slope
System curve:
upward curve
Intersection:
OPERATING POINT
Like scissors crossing.
3. Why System Resistance Increases with Square?
Because air friction rises rapidly.
Higher velocity causes:
- more turbulence
- more wall friction
- more bend losses
- more filter losses
Example:
Driving car:
40 km/hr easy
120 km/hr much harder due to air drag.
Same principle.
4. Practical Example – Bag Filter ID Fan
The example of Bag filter ID Fan are as follow :
Suppose system:
Bag filter dust extraction.
Required design:
20 m³/sec
Losses:
Duct = 120 mmWG
Bag filter = 200 mmWG
Bends = 60 mmWG
Stack = 40 mmWG
Total:
420 mmWG
At 20 m³/sec
System constant:
[K=\frac{420}{20^2}]
[=\frac{420}{400}] = 1.05
So:
[SP=1.05Q^2]
Now fan curve:
Flow | Pressure |
15 | 600 |
18 | 520 |
20 | 450 |
22 | 370 |
25 | 250 |
System values:
At 18:
1.05 × 324 = 340
At 20:
420
At 22:
508
Comparison:
Flow | Fan | System |
18 | 520 | 340 |
20 | 450 | 420 |
21 | 410 | 463 |
Operating point ≈ 20.2 m³/sec
Meaning:
Fan works correctly.
5. What if Fan Too Small?
Small fan:
Flow | Pressure |
15 | 350 |
18 | 250 |
20 | 180 |
System needs:
420 at 20.
Fan can only give 180.
Impossible.
Actual operating point may become:
14–15 m³/sec
Production suffers.
Meaning:
Undersized fan.
6. What if Fan Too Big?
Oversized fan:
Flow | Pressure |
20 | 800 |
25 | 700 |
30 | 550 |
System curve same.
Operating point shifts high.
Maybe:
28 m³/sec
Problems:
- excess power
- duct noise
- erosion
- unstable process
- damper throttling losses
Meaning:
Oversized fan wastes money.
7. Dirty Filter Effect
New bag filter:
200 mmWG
Dirty bag filter:
350 mmWG
System curve shifts upward.
Earlier:
20 m³/sec
Now:
Maybe only 17 m³/sec
Fan unchanged.
Flow drops.
This is why dirty filters reduce production.
8. Damper Closing Effect
Closing damper increases resistance.
System curve shifts upward.
Operating point moves left.
Less airflow.
Higher pressure.
Example
Open damper:
20 m³/sec
Close partially:
15 m³/sec
Same fan.
Different system resistance.
9. VFD Effect
VFD changes fan speed.
Fan curve itself moves.
Lower speed:
less flow
less pressure
Higher speed:
more flow
more pressure
More efficient than damper control.
Final Simple Analogy
Think water pump + pipe.
Pump = fan
Pipe restrictions = system
More restrictions → less flow
Bigger pump → more flow
Actual result = balance point.
Final One-Line Formula
System:
[SP = KQ^2]
Fan:
Manufacturer curve
Operating point:
Where Fan Pressure = System Resistance
Final Industrial Understanding
Fan does not decide airflow alone.
System does not decide airflow alone.
Both negotiate.
Intersection is reality.