Engineering Asked on July 6, 2021
I have an air duct 400×400 mm that will attach to an engine with a ventilation opening 981 x 167 mm. I assume a smooth funnel of about 500m length between the two shapes (that’s about as much space as there is).
The air duct will run horizontally along the ceiling (the direction is along the length of the opening), the ventilation opening is pointing upward.
I’m at loss how to calculate or estimate the pressure loss of this part – it’s not a reduction as the area stays approximately the same. Until a better approach comes along I will just assume a right angle joint in the 400×400 duct and a 981×167 duct for the last part.
This has not much bearing on the question, but to provide some context: The machine is a combined heat and power plant, drawing cooling and combustion air through the long opening (~2500m³/h). It will be placed alongside a gas powered boiler, so the machine may never draw air from the room, hence the ventilation duct. As of now it seems likely that the internal blower is sufficient, no extra blower is needed – but I want to confirm that.
Since there is relatively little area change and the shapes are both rectangular, I would not worry about the pressure loss that occurs transitioning between the two shapes. I would think the right angle bend assumption would account for more loss than the transition between two similar shapes. You could check the equivalent diameter between the two shapes to make sure there isn't a reduction or expansion in equivalent diameter. If you're looking for a more exact answer, I would try simulating the flow and geometry in CFD or in a 1D flow solver like Flowmaster. Handbook of Hydraulic Resistance by I.E. Idelchik may provide some help estimating loss coefficients for the geometry.
Answered by mechcad on July 6, 2021
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