Safety Factor in Hyraulic Calculations for Chilled Water Loop 2

[IBechir]

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.

 

[BronYrAur]

I have always found safety factory to be completely random. Most pumps in my experience were oversized to begin with due to conservative estimates of friction factors and pipe lengths. This is on top of load estimates that probably already had safety factor built into them. Then during actual pump selection, another safety factor was thrown in to cover someone's butt. Am I being too cynical? The remedy was to crank down the triple-duty valve during system balance.

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.

 

[IBechir]

Makes sense. Thanks.

 

[marcoh]

For a 20 storey tower you will have multiple chillers and multiple pumps. You pumps should match the chiller capacity which may or may not be the same as the connected AHU load.

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.