Static Pressure vs Total Pressure (with Practical Examples in mmWG and m³/sec)
Understanding Static Pressure (SP) and Total Pressure (TP) is fundamental in fan engineering, system design, and troubleshooting. These terms are often misunderstood, yet they directly determine fan selection, power consumption, airflow performance, and system resistance.
1. Static Pressure (SP)
Static Pressure is the pressure required to overcome resistance in the system.
It is the pressure that pushes air or gas against:
- ducts
- bends
- dampers
- bag filters
- cyclones
- scrubbers
- furnace resistance
- stacks
It does not include velocity energy.
Think of static pressure as:
“The force needed to push air through obstacles.”
Unit:
mmWG (millimeter water gauge)
Practical Example 1 – Bag Filter System
Suppose a dust extraction system has:
- duct resistance = 80 mmWG
- bag filter pressure drop = 120 mmWG
- bends/dampers = 40 mmWG
- chimney loss = 30 mmWG
Total static pressure:
SP = 80 + 120 + 40 + 30
SP = 270 mmWG
Meaning:
The fan must generate at least 270 mmWG static pressure to move air through the system.
Water analogy
Imagine water flowing through a pipe with many restrictions.
Static pressure = pressure needed to overcome restrictions.
Same in air systems.
2. Velocity Pressure (VP)
Moving air has kinetic energy.
That kinetic energy creates velocity pressure.
Formula:
VP = V² / 16.34
(where V in m/s approximately for mmWG context)
This pressure exists only because air is moving.
If airflow stops:
Velocity pressure becomes zero.
Example:
Air velocity = 20 m/s
VP = 20² / 16.34
= 400 / 16.34
= 24.5 mmWG
3. Total Pressure (TP)
Total pressure is:
Static Pressure + Velocity Pressure
Formula:
TP = SP + VP
This is the complete energy the fan imparts to the air.
Example:
Static pressure = 270 mmWG
Velocity pressure = 24.5 mmWG
Total pressure:
TP = 270 + 24.5
= 294.5 mmWG
Meaning:
The fan actually develops 294.5 mmWG total energy, but only 270 mmWG is available to overcome system resistance.
Simple Understanding
Static pressure:
Pressure against resistance.
Velocity pressure:
Pressure due to motion.
Total pressure:
Complete fan energy.
4. Fan Practical Example (m³/sec)
Suppose a bagasse boiler ID fan handles:
Flow:
25 m³/sec
Duct diameter:
1.2 m
Area:
A = πD²/4
= 3.1416 × 1.2² / 4
= 1.13 m²
Velocity:
Q = A × V
V = Q / A
= 25 / 1.13
= 22.1 m/s
Velocity pressure:
VP = V² / 16.34
= 22.1² / 16.34
= 488 / 16.34
= 29.9 mmWG
If measured static pressure = 350 mmWG
Then:
TP = 350 + 29.9
= 379.9 mmWG
So fan selection:
25 m³/sec @ 380 mmWG TP
5. Practical Example – FD Fan
FD fan supplies combustion air.
Flow:
12 m³/sec
Duct size:
0.9 m dia
Area:
= 0.636 m²
Velocity:
12 / 0.636
= 18.9 m/s
Velocity pressure:
18.9² / 16.34
= 21.8 mmWG
If system static loss:
200 mmWG
Total pressure:
TP = 200 + 21.8
= 221.8 mmWG
Fan duty:
12 m³/sec @ 222 mmWG TP
6. Practical Example – Cement ID Fan
Cement kiln exhaust:
Flow:
80 m³/sec
Duct:
2.0 m dia
Area:
3.14 m²
Velocity:
80 / 3.14
= 25.5 m/s
Velocity pressure:
25.5² / 16.34
= 39.8 mmWG
Static resistance:
- cyclone = 250
- baghouse = 180
- ducts = 100
- stack = 70
Total static:
600 mmWG
Total pressure:
600 + 39.8
= 639.8 mmWG
Fan duty:
80 m³/sec @ 640 mmWG TP
7. Why Confusion Happens
Customers often say:
“I need fan for 300 mmWG.”
Question:
Static or Total?
Huge difference.
If 300 static:
actual total may be 340+
Wrong selection causes failure.
8. Which Pressure is Used for Fan Selection?
Usually:
Industrial fan OEM selects on Total Pressure
Because fan produces total energy.
System engineer calculates static losses.
Then velocity pressure added.
Example:
System SP = 400 mmWG
Velocity pressure = 35 mmWG
Selection basis:
435 mmWG TP
9. Field Measurement
Using pitot tube:
Measures:
- static pressure
- velocity pressure
- total pressure
Relationship:
TP = SP + VP
10. Real-Life Analogy
Truck moving uphill.
Static pressure:
Energy to climb hill.
Velocity pressure:
Energy due to truck speed.
Total pressure:
Total engine output.
11. Shortcut Formula
If Q known:
Velocity:
V = Q / Area
Then:
VP = V² / 16.34
Then:
TP = SP + VP
Example Quick Calculation
Flow:
40 m³/sec
Duct:
1.5 m dia
Area:
1.767 m²
Velocity:
40 / 1.767
= 22.6 m/s
VP:
31.3 mmWG
SP:
500 mmWG
TP:
531.3 mmWG
Final Comparison
Parameter | Static Pressure | Total Pressure |
Meaning | Resistance pressure | Total fan energy |
Includes velocity? | No | Yes |
Formula | SP | SP + VP |
Depends on duct losses? | Yes | Yes |
Depends on airflow velocity? | No | Yes |
Used in system calculation? | Yes | Yes |
Used in fan selection? | Mostly total pressure |
Final One-Line Understanding
Static Pressure = Force to overcome resistance
Velocity Pressure = Energy of moving air
Total Pressure = Static + Velocity = Actual fan duty