What's the Best Duct Structure for Air Cooling?

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aikhh Power-User Join Date: Mar 2009 Posts: 130	What's the Best Duct Structure for Air Cooling? 04/09/2009 8:37 PM
	Dear All Gurus, I have one unit of SP 12000Pa ,volume : 600m³/min centrifugal blower which driven by 185Kw 415V 3Ph 50Hz motor,starting current is 1000A by Auto transformer and running is 230A. Now i want to supply this blower cooling air to a machine cooling which is 80m distance from blower outlet comprise of 5 set of 90deg elbow and distibute to 10 units of dia 200mm outlet.If i use circular shape duct of dia 1200mm (thickness 3mm GI plate) as main duct Or i use rectagular shape 1400mm x 1400mm (thickness 3mm GI plate) . Please advise which shape is much better and less pressure drop or air turbulance ? Or any software to calculate the above mentioned ducting design. Thank you and Best Regards
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XMech Power-User Join Date: Jan 2009 Location: Western Canada Posts: 295 Good Answers: 51	#1 Re: Duct structure for cooling air 04/10/2009 12:42 AM
	From ASHRAE & Huebscher the relationship between rectangular ducts and round ducts that is used to determine size equivalency based on equal flow, resistance, and length is: which, if I did my math right (it's late), leads to an equivalent diameter of the 1400 mm square duct of 1530 mm. So simply looking, with everything else being equal, at pressure drops/flow, you would be better off with the square duct. The drawback to square/rectangular ducts is the pressure retaining strength for a given thickness compared to a circular duct and the need for stiffeners - which drives up the cost. Have you determined the stiffener requirements with 3mm to have enough strength for 12000 Pa? And also the cost between the two duct styles? __________________ A little knowledge is a dangerous thing.
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sceptic Guru Join Date: Oct 2008 Location: Cairns, Qld, Australia Posts: 968 Good Answers: 65	#2 Re: What's the Best Duct Structure for Air Cooling? 04/14/2009 3:12 AM
	Note that circular cross section has the smallest surface area for a given cross sectional area, hence least friction loss. If you are worried about pressure drop, it is easier to put turning vanes in to a rectangular or square duct and so reduce drop at the bends. This drop is frequently dominant. With 80m run, friction loss is probably not too bad if your duct is a reasonable size. Another nuisance with circular ducts is that they are often difficult to physically fit into a plant layout, because the diameter for a given cross section is fairly large compared to a square duct. Cross breaking the steel of your duct usually provides enough stiffness to allow square duct to be used at quite high pressures, as you have. Do the calcs and choose to suit your particular situation.
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