To master blowroom efficiency, you must treat air as a transport vehicle. If the vehicle is too slow, the "passenger" (cotton) falls out and chokes the line. If it’s too fast, the passenger gets "bruised" (fiber rupture and neps).
Here is the technical breakdown of how to calculate and optimize air velocity.
1. How to Calculate Air Velocity (v)
In a blowroom duct, we use the relationship between the volume of air being moved by the fan and the cross-sectional area of the pipe.
The Formula: v = Q / A
Where:
- v = Air Velocity in meters per second (m/s).
- Q = Air Flow Rate in cubic meters per second (m³/s). (Note: Most fan manuals give this in m^3/h; divide by 3600 to get per second).
- A = Cross-sectional area of the duct (m²).
Example Calculation:
If you have a transport duct with a diameter (d) of 0.3 meters 300 mm and your fan is moving 4500 m³/h:
Where:
- v = Air Velocity in meters per second (m/s).
- Q = Air Flow Rate in cubic meters per second (m³/s). (Note: Most fan manuals give this in m^3/h; divide by 3600 to get per second).
- A = Cross-sectional area of the duct (m²).
Calculate Area (A):
A = 3.1415 x r2 = 3.1415 x (0.15)2 = 0.0706 m'2
Convert Flow Rate (Q):
Q = 4500/ 3600 = 1.25m^3
Calculate Velocity (v):
v = 1.25 / 0.0706 = 17.7 m/s
This is too high for premium cotton! You should reduce the fan speed.
2. How Velocity Affects Your Fiber (The "Why")
Low Velocity (< 12 m/s: The "Rolling" Effect
- Settling: The air isn't strong enough to keep heavy tufts suspended. They drag along the bottom of the duct.
- Neps: As tufts drag and tumble, they roll into tight balls, creating mechanical rolling neps that the Carding machine cannot open.
- Choking: Material builds up at bends or "U" turns, eventually stopping the entire line (ruining your Stop-Go ratio).
High Velocity (> 15 m/s: The "Impact" Effect
- Fiber Rupture: Cotton hits the duct walls and "elbows" with extreme force. This shatters the delicate fibers, increasing Short Fiber Content (SFC).
- Static Electricity: High friction against duct walls generates static, causing fibers to stick to the metal, leading to irregular feeding.
- Power Waste: Energy consumption increases by the cube of the fan speed. A small reduction in velocity saves significant money.
3. The Audit: Practical Tips
The Pitot Tube Test
Don't rely solely on the fan's RPM. Use a Pitot Tube or a Digital Anemometer at a straight section of the duct (at least 5 diameters away from a bend) to get the "Actual" velocity.
The Material-to-Air Ratio
For a balanced blowroom (Uniflock to Unistore), follow this corporate thumb rule:
1 kg of Cotton requires 0.6 to 0.8 m³ of Air.
If you are pushing 1000 kg/hr, your fan system should be moving roughly 600 - 800 m^3/hr of air just for transport, plus extra for dust extraction.
Static Pressure Check
If you see your velocity dropping even though the fan is at full speed, check your filters. A clogged dust-room filter increases "Back Pressure," which kills your air velocity.
"Optimization doesn't stop at the duct. These are thumb rules & may vary , You put your own values & confirm .
Next time, I’m pulling back the curtain on Fan Speed vs. Power Consumption" chart to help you justify energy-saving upgrades to your management?
Don't let your profits fly away with the dust. See you in the next chapter."
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