6 Replies Latest reply on Jul 27, 2009 4:57 PM by Joe Galliera

# Pressure drop

What is the best way to get the pressure drop of a piping system with straight pipes and 45 / 90 bends? I have an air fan in inlet end with certain static pressure and volume flow. What outlet boundaries should I use in order to get the dp of this system and the actual static pressure in outlet?
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I would like to see some hits on this topic. Very interested myself...
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In any conditions, the volume at inlet should be equal to volume at outlet. So apply the static pressure at inlet and volume flow at outlet and then you can find the pressure drop.
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Apply Intlet Fan as a boundary condition at Inlet using either pre-defined or user fan (from Eng.database) with the perfomance curve ( Pressure-CFM), and static or environmental pressure at Outlet.
The image file shows the Online Help Basic Fan information explaining how the pressure drop and flow rate is determine depending which option was selected. The user selected the "Static pressue increase (Pst)" which if you look at the image file I uploaded the pressure drop determined is Ptotal(face) - Pambient for an External Inlet fan. The user created a Surface Goal for the static pressure but not the Total Pressure at the inlet face.
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Hi Guys....

Which the correct method to simulate pressure drop ?
Recommendation from Igor or Deepak ?

Thanks
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First, don't get caught up in using CFD to calculate simple shapes that have been figured emperically every which-way. (http://www.pressure-drop.com/)  check out the online calculator (or ASHRAE books).

One trick is to measure the bulk average pressure (Static or Total) at station locatons which are sufficiently far from your boundary conditions.  Sufficient that the flow has developed (or insert known flow profile, or use fully-developed parabolic flow option).  See the attached BMP image.  I apply a velocity at one end, and environmental (static at an outlet) at the other.  Where I am taking measurements is a face of a extrusion (disabled via component control) that represents stations at which I will take emperical data.

There are no 'right' boundary conditions, pressure drop will vary depending on volumetric flow rate and velocity.  Typically you will apply a known flow rate or velocity at one end, and pressure on the other.  You create surface goals for bulk average pressure, and then create yourself an equation goal to complete the arithmetic.

On a side note, I've found a good reference on pressure loss is found in Chapter 34 of the ASHRAE Fundamentals Handbook (2001).

Brian

• ###### Re: Pressure drop

Pressure drop in a pipe is linear.  Wording in attached PDF says valve, but it could be anything.

Now place a black box in the tube, call it a valve, it changes the linear pressure profile. Linearly extrapolate the average static pressures back to the planes of the valve inlet and outlet, this is your pressure drop or difference due to the valve in the pipe.

Easiest way to do this is in Flow Simulation is to create an XY Plot.