High system fill pressure for chilled water system

hvacdoctor · Aug 31, 2004

Have a industrial chilled water/prop glycol 25% mix system with about 20' of vertical height and 70' of horizontal piping, fittings etc. Customer is running system around 40psi fill pressure. I say its overkill for this application. Using the 1psi=2.31ft of pressure 12 psi should be more than enough to run the system. Will the higher pressure restrict flow? performance? control valve operation?

btexpress · Aug 31, 2004

Most chilled water systems are set up to maintain approx. 12 psi on the suction side of the chilled water pump, while it is operating. The reason is that most expansion tanks usually come precharged at 12 psi. You would set your water press. reg valve (if auto. feed) to start feeding at about the same press. Hope this helps.

btexpress · Aug 31, 2004

To answer your original questions... at those pressures I would expect no detrimental effects on performance or components.

joegrind · Aug 31, 2004

less press = less stress on pipe and less wear of evap tubes, although 40 is not way out of line, we typically set them to run at 12 - 16 suction @ highest point of piping or pumps if there on the roof!

h2045 · Aug 31, 2004

Where are you looking at the pressure top or bottom of loop and what is the water temp? As the chiller starts and pulls down pressure will drop and you may over pressurize when the loop warms back up. Do you have waterside reliefs and backflow preventers? If filling to 40psig when chiller is operating w/o a BFP you could have 25% solution at your water fountains should the chiller fail.
Just a thought!

commissioning authority · Sep 1, 2004

Static pressure on a chilled water system serves the same purpose as it does on a hot water system.

The pressure that should be used is equal to the hieght between the fill point (in this case the Glycol Feeder) and the top of the system (the highest coil etc.) divided by 2.31 ft of head per lb. pressure plus 5 psi. This will provide sufficient pressure to keep the system in positive pressure when the pumps are on.

The pressure in the expansion tank should be set to the same formula except measuring from tank to the highest system point.

Also a Glycol system should never be connected to a fresh water make up because if there is ever a leak in the system it will dilute the glycol and defeat the purpose of the glycol IE Frozen chiller barrel or Air handler Coils

On multi story buildings the static pressure could easily be 40 psi or greater as 40 psi is only about what would be needed for 5 or 6 stories if the fll is in the basement.

Good luck on the building

High system fill pressure for chilled water system

hvacdoctor

btexpress

btexpress

joegrind

h2045

commissioning authority

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