Start sequencing

[techreptdh]

Question to all,
Is there any reason I would want to delay start up of chillers by 15 or 20 minutes each thru the BAS program? For instance after a power outage the lead chiller will start up after a 20 minute delay. It will immediately want a second unit to start but has to wait 15 more minutes. And keep in mind this is after the first chiller has waited 20 minutes. Meaning it will be over 30 minutes before 2 chillers are on line. Thats a long time. Anybody have any thoughts on this sequencing? Carrier 19xrvs are the units in question.

[CCSPIERCE]

The 19xrv has an IR2 parameter that calculates heat content within the motor windings. This parameter protects from a machine having been @ full load, stoppage and restarts. The normal internal restart time delay for restart typically accounts for motor temp safety. If however the machine had a strong phase imbalance prior to stopping then the IR2 will delay it longer due to one or two windings heating more. Typically the vfd drive will prevent phase imbalance. So in your case, IF the problems for stoppage are line voltage service faults then you are pretty well covered by the machines safety calculations.
Programmed time delays from the BAS system usually are placed there to prevent the chillers from restarting while all other equipments within the facility are restarting. The second chiller start time delay will be for demands to prevent two machines from starting and ramping up simultaneously. If you can live with what you already have programmed leave it in place so all equipment can ease back up on line. If you decide to speed things up, evaluate the building power service switchgear, demand charges, etc. to be sure you don't push things too quick.

[spinning wheel]

The chiller is ready for a restart after a short time delay, This is the BAS trying to not start more chillers than the bldg needs. If a power failure happens, just start the chillers by the local command if it bothers you that much. You could get the control company to shorten the first chillers delay however.

[CraziFuzzy]

It is also commonly used to prevent excessive demand charges. (which are usually calculated on 15 minute intervals). By making sure the starting load isn't both in the same 15-minute period, it will spread out the amp draw, and prevent excessive demand charges.

[techreptdh]

Thanks for the input everyone. I am inclined to narrowing that initial chiller start time, but can see the benefits of the delay.