Hello,
Where can I find/download pressure loss coefficient information for ductwork bends, tees etc.
I would like to know what the basis of the calculated pressure drops are, eg ASHRAE, CIBSE etc
Reason for query is that we are seeing very high pressure drops are tee's in particular.
Some of these are universal tees which we understand have high PD, however some are just ordinary shoe type branches.
Help appreciated.
Hi Ross,
The K factors used directly in the calculations are available here:
Pipe and Duct fittings - K values - Hevacomp Wiki - Hevacomp - Bentley Communities
Others for bends and tees this is not available, as ductwork sections and bends are not calculated separately, eg as with CIBSE based calculations, and the K value's are determined according to ratio's of velocity, flow rate, or cross sectional area. Standard data is based on that available in CIBSE Guide C and other data is provided by manufacturers but is still selected on the above criteria based on the data provided. Thus, a breakdown of separate pressure drops across ductwork/bends is not given by the software, as these are computed hand-in-hand.
However there are approaches you can take to reduce pressure that I would suggest trying and re-running the calc to see the effects, eg:
-Unfix any fixed duct sizes
-Change the Tee types - eg as here
Please let me know how you get on.
Shane Regan | Principle Building Performance Support EngineerHevacomp to OpenBuildings - A complete guide for every user | Bentley Communities
Answer Verified By: Shane Regan
Hi Shane,
Thanks for the quick response, its appreciated. The challenge we are facing is that we have used Hevacomp to check the design static provided by the designer, hence the ductwork sizes/configuration etc is all fixed.
The Hevacomp result is significantly greater than the manual calculation from the designer which we are advised is based on CIBSE Guide C. Unfortunately we cannot demonstrate the basis of our loss for the fitting without more detailed info.
Is their evidence from Bentley/Hevacomp that their software calculation is more accurate than a manual calculation?
For example are their complexities that the software can account for that are unlikely to be possible manually?
If the sizes are fixed this would suggest that there are constraints enforced on the system that are causing the high pressures. I understand that the sizes are needed as you have specified, however it would be worth testing this identify if it is the main root cause -eg you could quickly unfix a section that you are worried about and re-run the calc to see the effect, and then revert to your fixed sizes.
Other possible related constraints are likely to be coming from your system marker settings - chiefly those set in Sizing Criteria as below, so please check these too:
Also, should you need a full breakdown of the calculation methods you can view these in the manuals, eg as below: