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What is the relation between the ducting length allowable and fan external static pressure?
I have a problem regarding one concealed unit, which we can select to be either high static (0.83) or low static (0.2). How can I calculate the maximum ducting length allowable to ensure that the flow is sufficient to reach the end of the duct I design?
THanks..
In the first instance you need to consider the ESP or External Fan Pressure which is defined as the sum of the dynamic losses exterior to the AHU/Fan.
If you refer to ASHRAE or CIBSE Guides they both produce relationship charts plotting volume flow rate against pressure loss per unit length.
So to progress your question you will need to confirm the maximum flow rate in meters cubed per second, confirm by inspection the size and construction of the duct (is it square/round/rectangle made of galv steel/plastic or flexible with or without wire support.
By returning to the charts you should be able to determine the losses per metre and given that this does not exceed the units maximum ESP it will provide the conditions you require.
Do not forget you will also need to consider the losses caused by dampers and terminal devices in the distribution chain and if the duct is flexible you will need to confirm with manufacturers for exact results.
Frequently in your situation our experience is that the maximum setting will be used and then the volume measured at the grille to confirm that the conditions will be met.
As a rule of thumb I always design at an ideal loss rate of 1 Pa/m & always avoid velocities over 5m/s to avoid noise issues.
tmm
Thanks tmm for your prompt reply..
Is there any "rule of thumb" calculation for the ducting length? I was told by one engineer that 0.1 in.wg of fan static pressure equals to 100ft of duct, 0.2 in.wg equals to 200ft of duct, and so on...
Is it correct?
weeliang82. The obvious answer to your question unfortunately is no. If you take a typical example your ducted unit will have either one large rectangular connection or possibly circular spigots. If you consider the rectangular condition first you typically convert this to a suitable size for the volume of air flowing down the duct- a transformation section. This element like every other until the air flows into the room has a resistance against which the unit must operate. If spigots are fitted these should be sized for the flow which makes the process easier but again every fitting from elbow to tee and grille carries a resistance against which the unit must operate.
To provide additional details I would need to know the unit specifications, the type of duct used, spiral/flex or rectangular & sizes, length of run and grille details.
tmm
A search on the forum will provide a lot of threads already discussing this, such as--
http://www.refrigeration-engineer.co...ead.php?t=7077
http://www.refrigeration-engineer.co...ad.php?t=12830
http://www.refrigeration-engineer.co...ad.php?t=18703