The biggest problem that causes this is too many 3-way valves in the system. If every chilled water coil has a 3-way valve, then yes, there will be no reason for the pumps to slow down, as they have to flow the same water no matter how much load there is. Limit the 3-ways to only the furthest air handlers, and even at that, restrict the flow through the bypass to keep the water 'cold enough', and the vfd's will have some room to work with. Quite often these situations arise through retrofits/replacements of the plant equipment, without continuing the upgrades to the field systems. At least cutting back or eliminating 3-way's is relatively easy.
I would rather have three way valves on all the loads than a VFD. It just seems to be more reliable.
It takes three people to do anything around here. Two do the work, one explains to the crowd of people who showed up when they seen smoke and flames.
Seems like I have ran across a lot of new installs with VFD on the chilled water pump and it runs at 60 hz. No slowing down at all...seems like a waste of money These were on a RTAC in a public school
last summer, when i worked for the orange meatball, i was visiting many customers who were enabling all of the chillers at the same time...water running through all of them...but only 1 evaporator pump running through all of them. i asked and i was told that 'our' engineers were saying that it was cheaper to run water through all of them rather than just the 1 that was in the lead...i never did find out who was running around doing it...stupid engineers....i went to school to be a mechanical engineer. i am glad that i got out before being brained.
It's tube brushing season...44% done...only 9,249 tubes to go!
I was under the impression that variable compressor motors needed variable flow to maximize the efficiency of the chiller.
Untrue. York came out with a VSD for centrifugal chillers in 1980...long before variable primary flow had even been considered. A VSD on a centrifugal chiller can realize savings whenever it isn't running at design conditions (which is close to 98% of the time).
Got an update today from the engineers. We do not have a common header, each chiller relies on its own dedicated set of pumPs. They are still wanting to put vfds on both pumps of the new chiller and on the compressor. Thoughts? Problems/issues? If they can, they are also going to put vfds on one of our existing 600 ton tranes. If not, the only chiller that will have vfds is the new one.