Lag pump operation for chilled water

[Takket3]

Hi,

I don't know if any of you have any experience with pumps with integral VFDs such as the Armstrong design envelope series, but I just installed them for the first time on a chiller project. These pumps run lead lag and their own internal controllers decide when the lag pump needs to turn on and off.

Here is what's happening....

Chiller 1 and pump 1 are operating fine.

When chilled water temp rises, DDC calls for chiller 2 to activate.

Chiller 2's isolation valve opens to begin flow through the chiller.

The lag pump starts, but its VFD takes about 40 seconds to speed up

When the VFD is speeding up, chiller 1 loses flow (because pump 1 is basically pumping water through chiller 1 and chiller 2 while pump 2 is speeding up, thus the flow through chiller 1 is cut in half).

Chiller 1 shuts down because it is below the minimum flow.

Can anything be done to combat this? This is the first time I've used these pumps with integral VFds and I've never had a problem like this on them any chiller plants I've designed with a "traditional" pump and separate VFD.

FYI both chillers are air cooled rooftops, system is variable primary, AHU valves are all 2 ways, and there is a bypass to maintain minimum flow through the chiller during low flow conditions.

 

[RossABQ]

Offhand I think you need a more sophisticated control than what you have. The integrated pump controls are completely reactionary, with the ramp rate you have they'll do just what you're seeing. Is the ramp rate not adjustable? Can you force the lag pump on from a temperature signal, before the chiller valves start to open?