design a duct and choosing fan

sasha- · December 24, 2023, 02:50

Hi everyone, hope you are doing well!

I am embarking on my first industrial project and would like to ensure my proposed solution.
here is the problem:
I am tasked with designing a duct and selecting a fan for a device composed of a vertical long tube, a container at the end of the tube, and a horizontal short tube, placed in container, serving as the air outlet. The duct will be inlet and the fan is located in it, the airflow direction is from top to bottom, and the fan must draw a specified volumetric mass flow, transferring it to the long tube.
The inlet air flow temperature, pressure loss and volumetric mass flow in the long tube, relative humidity and temperature at outlet are known.

Do you have any idea how can I find the appropriate duct design based on this information, I can choose the fan by volumetric mass flow, but not sure if I just have to choose the length of duct by chance or can calculate it by pressure loss and darcy-weisbach equation, as the pressure loss in tube is known and pressure loss in fan can be obtained from its performance curve, but pressure at the inlet and outlet are not determined and the air is humid.
I appreciate any suggestion!

LuckyTran · December 26, 2023, 18:01

Fans operate on a design specific fan curve of dP versus voluemtric flow rate.

The rest of the system has certain loss characteristics and you can map out the system dP versus volumetric flow rate.

The intersection of these two fan curve versus system curve is what will actually happen in reality. In order to guarantee that the system operates at the targeted condition, you must make adjustments to the fan or the system such that the intersection occurs at the desired conditions.

sasha- · January 15, 2024, 07:11

thank you for the response
how can I map out system dp? I mean I understand that i can determine pressure loss in the system but how can i map it out with volumetric flow rate?

Also I have another question, I chose the fan which performs at the specified mass flow that i want thorough vertical tube(lets say 0.9 m^3/min), but when I measure the massflow at the outlet with a rotameter IT shows much less mass flow (0.5 m^3/min), is it normal due to pressure loss? or the fan is inappropriate?

teamworkhvac48 · January 24, 2024, 14:04

Hi! Congrats on your project! To design the duct and pick the right fan, try these steps:

Duct Size:

Use the Darcy-Weisbach equation to find the right duct size by considering pressure losses.
Fan Check:

Make sure the fan's performance curve matches your needed air flow. Check pressure loss in the fan against known values.
Handle Humidity:

Think about how humidity affects air. Use charts or software to factor in humidity.
Temperature Thoughts:

Consider how temperatures change along the duct. Ensure your fan and duct can handle these changes.
Experiment:

You might need to try different settings to get the best solution.
Connect with others and maybe talk to experts for more advice. Good luck with your project! Let me know if you need more help.

December 24, 2023, 02:50	design a duct and choosing fan	#1
sasha- New Member sasha Join Date: Jun 2022 Posts: 17 Rep Power: 4	Hi everyone, hope you are doing well! I am embarking on my first industrial project and would like to ensure my proposed solution. here is the problem: I am tasked with designing a duct and selecting a fan for a device composed of a vertical long tube, a container at the end of the tube, and a horizontal short tube, placed in container, serving as the air outlet. The duct will be inlet and the fan is located in it, the airflow direction is from top to bottom, and the fan must draw a specified volumetric mass flow, transferring it to the long tube. The inlet air flow temperature, pressure loss and volumetric mass flow in the long tube, relative humidity and temperature at outlet are known. Do you have any idea how can I find the appropriate duct design based on this information, I can choose the fan by volumetric mass flow, but not sure if I just have to choose the length of duct by chance or can calculate it by pressure loss and darcy-weisbach equation, as the pressure loss in tube is known and pressure loss in fan can be obtained from its performance curve, but pressure at the inlet and outlet are not determined and the air is humid. I appreciate any suggestion!

December 26, 2023, 18:01		#2
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,761 Rep Power: 66	Fans operate on a design specific fan curve of dP versus voluemtric flow rate. The rest of the system has certain loss characteristics and you can map out the system dP versus volumetric flow rate. The intersection of these two fan curve versus system curve is what will actually happen in reality. In order to guarantee that the system operates at the targeted condition, you must make adjustments to the fan or the system such that the intersection occurs at the desired conditions.

January 15, 2024, 07:11		#3
sasha- New Member sasha Join Date: Jun 2022 Posts: 17 Rep Power: 4	thank you for the response how can I map out system dp? I mean I understand that i can determine pressure loss in the system but how can i map it out with volumetric flow rate? Also I have another question, I chose the fan which performs at the specified mass flow that i want thorough vertical tube(lets say 0.9 m^3/min), but when I measure the massflow at the outlet with a rotameter IT shows much less mass flow (0.5 m^3/min), is it normal due to pressure loss? or the fan is inappropriate?

January 24, 2024, 14:04		#4
teamworkhvac48 New Member Jeff Henzler Join Date: Jan 2024 Location: Denver, CO Posts: 1 Rep Power: 0	Hi! Congrats on your project! To design the duct and pick the right fan, try these steps: Duct Size: Use the Darcy-Weisbach equation to find the right duct size by considering pressure losses. Fan Check: Make sure the fan's performance curve matches your needed air flow. Check pressure loss in the fan against known values. Handle Humidity: Think about how humidity affects air. Use charts or software to factor in humidity. Temperature Thoughts: Consider how temperatures change along the duct. Ensure your fan and duct can handle these changes. Experiment: You might need to try different settings to get the best solution. Connect with others and maybe talk to experts for more advice. Good luck with your project! Let me know if you need more help.