Fan and pump static calculation accuracy

[Waramanga]

Hi Guys,

I was wondering what peoples experiences with fan and pump static calculation accuracy are (with respect to the final commissioned values for the system installed as per the shop drawings). I have a few questions that the veterans amongst you might be able to help with.....
Where do they fail most regularly?
How pedantic does one need to be?
What safety is reasonable or advisable?
Any particular method/work flow to use? ie marking up water/air path on clean paper, numbering the fittings features.....etc....etc
I am particularly careful/conservative with fan installation/arrangement because I have some information (I wish I had more) on the effects of poor installation which basically say poor install = unpredicatble results.

Thank you in advance for any light that you can shed on the above, its the kind of stuff that I always have trouble finding in books.

 

[walkes]

For me it depends on the project and the required accuracy. There may be some installations that have critical flow requirements that would need detailed analysis.

A commercial building may just need to be +/- 5%. The airflow with a DX system if it is out will adjust the discharge temperature up or down to meet the capacity of the unit. If it is ventilation flow you may have an issue with the AHJ. For space conditioning you would satisfy the loads most of the time anyway.

On a hydronic heating system you may not notice a 10% deficit on flow.

I have been less exact on my calculations when pumping with a VFD as well, as I try to hit the design envelope for the pump.

At the end of the day it is still your best estimate, as there will be changes by the installing contractor that will inevitably have to be considered.

(I don't know if I actually said anything there)

 

[SAK9]

Pump head calculations are fairly accurate flow being incompressible Further pressure drop in major components such as chillers,coils,control valves etc are well documented with a high degree of accuracy.I usually use 10~15% safety margin on pump heads if I am fairly confident that no significant route changes are likely(for eg an office building.A higher safety margin may be necessary for campus style distribution with longer pipe runs where pipe and fiiting losses make up a large percentage of the overall pressure drop.

Fan static calculation is a different kettle of fish with issues such as system effects(caused by abrupt changes in direction or less than ideal fan connections)coming into play.Fan static calculation based on a good quality shop drawing has great relevance.If you can identify locations where system effect is likely to come into play,you can account for it in the calculation.Quite often it is a guestimate only.I have used safety factors as high as 50% where I have been unsure.Generally contractors get away because many elements of the pressure drop calculations are not effective at the time of commissiong ie clean filters,clean coils causing less than design pressure drops.VSDs have made life easy for commissiong agents these days.So I would tend to be more conservative with fan static calculations unless it is a well defined short run with plenty of headroom for the flexible connections to retain their shapes.

 

[Waramanga]

Thanks guys for your thoughts. I have been thinking about my initial questions some more and I think I can boil them down to one.

If I assume my fans are installed such that there are no system effects and the duct system doesn't have any funny duct arrangements for which there is no data, what is my likely error margin if I take each feature in a duct system into account?

Also, regarding pump calcs, is it normal practice to simplify the loss co-efficients of fittings to one number (a conservative one I assume) and then just use the number of fittings?

thank you

 

[willard3]

As SAK9 points out, water pressure drop calculations are pretty accurate because the flow is incompressible. Pipes usually aren't so large that they are changed by the contractor to make other stuff fit.

Ducts, on the other hand, should be calculated carefully and field checking must be thorough and complete. Every contractor thinks that, since duct systems only handle air, they can be changed any way they want with no result. AABC allows 10% variance in calculated volumes, but it is better to be right on the money with calculations.

War stories are full of failed air systems.....how many piping system war stories do you hear?