Freq drives on new chiller but not old? Problems?

[Dallas Duster]

If we were going to run the pumps at 60Hz why would they want VFD's on them? The whole point of VFD's is efficieny reason being they want to put the pumps on them. Sorry, your answer isn't making sense to me. Please elaborate.

Is water flow going to be varied thru the chiller? If yes VFD's are going to payoff for you if not then they are going to be a pretty expensive starter that will never pay for itself. I just assume since you are adding equipment to an existing system the answer just may be no. But who knows. Sounds like a good sized plant thou.

[Six]

If we were going to run the pumps at 60Hz why would they want VFD's on them? The whole point of VFD's is efficieny reason being they want to put the pumps on them. Sorry, your answer isn't making sense to me. Please elaborate.

Sounds like hes saying soft start alone isnt justifucation enough to replace a starter with a VFD.

I happen to agree. Ive got a Carrier water cooled 50 ton with VFDs on the CWP and it runs at 60 HZ.

Its a headache when that VFD throws a OV or OC alarm and shuts down the chiller because the drive is glitchy.

[Nuclrchiller]

If the new chiller is being added to the same system that the existing chillers are on, then more info is needed. It sounds like the existing chill water system may be a primary/secondary arrangement. Variable chill water flow thru the chiller is usually used on a variable primary flow system. Not sure how the two will be combined. With the cooling tower water flow, variable flow usually involves different tower temperature controls compared to constant flow. If cooling towers are piped thru a common header between the towers and the chillers, again, not sure how that will be accomplished. It sounds as though your job is about to get more... interesting.... Perhaps some of the other guys here have seen this done successfully. Good luck.

[ZeroTolerance]

Ya, Nucl its about to get very interesting haha. I think these engineers are even getting in over their heads.

All chillers currently are piped via a common header at the cooling tower then it runs down to the plant and branches to each chiller from there.

[ZeroTolerance]

Ok, mechanical engineers came back out again today. I stand corrected, they want to put a vfd on the compressor and not modulate the pumps. Pumps on new chiller will provide constant volume.

[Nuclrchiller]

Well, I think that will be better for you, as far as having a reliably running system goes. However, refer to Jayguy's post. The full energy saving potential of a vfd chiller probably won't be realized with your existing cooling tower arrangement. Perhaps the addition of head pressure controls on your existing chillers (i.e. individual condenser bypasses or throttling valves) will allow you to lower cooling tower temps to take advantage of all the energy savings potential of the new chiller. Personally, I'm more for rock solid reliability than I am for energy savings. Although a crashed chiller does consume very little power.....

[Six]

I was under the impression that variable compressor motors needed variable flow to maximize the efficiency of the chiller.

[jayguy]

...Although a crashed chiller does consume very little power.....

'OFF' is the green way to go!!

[CraziFuzzy]

VFD chillers have much better part load efficiencies, by being able to vary vanes and compressor speed, beyond the ability to run with colder tower water. That said, there are methods to use colder tower water for the variable chiller while still running the warmer water to the existing fixed chillers, but it would take modification to the existing chillers condenser piping (mixing valves can be used to temper up the condenser supply to the old chillers, while allowing shaving of a few degrees off the new chiller. Another option is a bypass pump added to the old chillers, that will pump water from the cond water return to the cond water supply, as required to warm up the water to the chiller. Whether either is feasible depends partly on the design of the condenser piping.

[Nuclrchiller]

Jay, I grinned when I read your post. But I laughed out loud when I saw your new avatar! That mlkwal is a funny guy! That is, when he's not leaping over tall buildings in a single bound and saving people.

[Nuclrchiller]

Crazi, you're right. I did not mean to imply that less than full energy savings would mean insignificant savings, or even less than desired savings.

[rtac]

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

[CraziFuzzy]

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

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.