Safety Factor in Hyraulic Calculations for Chilled Water Loop
Safety Factor in Hyraulic Calculations for Chilled Water Loop
(OP)
Greetings
When it comes to hydraulic calculations for a chilled-water loop how is the safety selected? This is for the purpose of sizing the recirculation pumps for a twenty storey tower.
I wanted to learn how to use software for these calculations but found that it is more important to start with this question because a safety factor that is too large defeats the purpose of building a hydraulic model.
Is the safety factor a percentage multiplied with the calculated value or do we consider recommended aging factors and reported variations for each type of fitting? The Hydraulic Institute’s Engineering Data Book describes possible variations in fitting friction losses of +10% to +50% for different fittings.
When it comes to hydraulic calculations for a chilled-water loop how is the safety selected? This is for the purpose of sizing the recirculation pumps for a twenty storey tower.
I wanted to learn how to use software for these calculations but found that it is more important to start with this question because a safety factor that is too large defeats the purpose of building a hydraulic model.
Is the safety factor a percentage multiplied with the calculated value or do we consider recommended aging factors and reported variations for each type of fitting? The Hydraulic Institute’s Engineering Data Book describes possible variations in fitting friction losses of +10% to +50% for different fittings.





RE: Safety Factor in Hyraulic Calculations for Chilled Water Loop
You definitely don't want to short-change your pump selection. Better to have too much than too little. If you are using VFD's, then I am of the opinion that oversizing is not as bad as it used to be. You can set an upper limit on the drive to match the actual flow condition as opposed to cranking down a throttling valve. Your first-cost will still be higher, but you won't waste as much while operating.
If the heating/cooling load is accurate and the flow/head calculation is accurate, then safety factor is a good idea; but, you never know how accurate those calculations are until the job is installed and running on a design day. So it's a judgement call. Your flow number should be pretty solid. It's the head that is more difficult to determine, so if anything you would put a safety factor there.
I don't know of any "industry standard" or method of determining an appropriate safety factor.
RE: Safety Factor in Hyraulic Calculations for Chilled Water Loop
RE: Safety Factor in Hyraulic Calculations for Chilled Water Loop
Personally I would put the safety factor into the the AHU's and size the chillers/pumps flow rate to the calculated diversified cooling demand. I would be slightly conservative with the head calculation knowing that the variable speed drives will automatically ramp down anyway.
Does the build have primary/variable-speed-secondary pumping system, or a variable-speed-primary-only pumping system? For a new high rise tower I would prefer a variable-speed-primary-only pumping system which is easier, cheaper, and more energy efficient.