KW demand vs. chiller operation 5

[Run1on]

We have been investigating our power bills and have discovered we are on a 15 min. demand timer arrangement. When our 300 ton chiler starts it develops an inrush current of approx. 800a ( sometimes 1000) As such the demand meter "sees" this point and registers it. Is it possible to install a soft start or VFD to the system to lower this inrush demand and consequentially reduce the demand charge. This piece of equipment is far and away the largest user in the system. The chiller is a Trane RHTB rotary water cooled unit.

Kevin

 

[busbar]

As I understand the situation, chillier starting should only involve the 800-1000 amp peak for a few seconds at most, which for a 15-minute demand interval would not likely make any “soft” starter or adjustable-frequency drive retrofit cost effective.

Kilowatt-demand metering 'averages' power use over the full demand interval, so a single (or several) momentary motor start(s) have only small effect on the kilowatt-demand reading. In the simplest sense, say the starting inrush lasts a total of 15 seconds in any 15-minute interval, then the demand register will only increase by 1/60th of the registered total. Let’s say that, very conservatively, the starting load is 480V·1000A·1.732=832kW. If the total number of starts in 15 minutes lasts for 10 seconds, the demand is only going to increase by 1.1% of that 832kW, or less than 10kW.

If the facility demand charge is $10.00/kW/month, then the monthly bill increases by $100. Offsetting the cost of a starter retrofit will take years.

 

[jbartos]

Suggestion: It may be worthwhile to explore an application of the automatic wye-delta starter which may add additional two contactors, if you are using one. The motor should have 6 leads available in its terminal box (6 coil ends) to implement this scheme.

 

[BJC]

What kind of starter do you have on this motor now? What Horse Power? 800 amps sounds like you already have some kind of reduced voltage starter in place. jbartos suggested a Y-D start which is good but you may have to run extra conduit and wire to your motor. If the starters are located on the unit no problem ( most of the time they are). If your looking at starters look at an autotransformer starter.
Soft starts and VFDs are available and would work. I have always though that varying the speed of a centrifical compressor was not good, but the following artical says different.
I have not priced or looked at softstarts in the 200-300 HP range lately so I don't know what they cost. Here is an artical on VFDs for a chiller.

http://www.automatedbuildings.com/news/sep01/art/abb/abb.htm

USing a VFD may be cheaper than a soft start. If so use it to ramp up to normal speed and use bypass contactors.
If the chiller manufacturer says there is a range of rpm that it will operate in, you may save energy cost as well as demand charges. Get the chiller mfg on board to help with the control scheme - they can be complex.

 

[don01]

hi all
bjc has hit on something that was highlighted with air compressors a little while back that stopping and starting some of these things is not the go but 1 start and varying the operation of the chiller can save a lot of money. You may consider direct control of the motor or control of the load system. PID loop control with control valve (not solenoid) as suggested above - annoy the suppliers and see what they have to offer.
now busbar has done the calcs on the start current and max demand cost. If that is the case (and I think he's right) can it be that-
1) your existing starter is malfunctioning (we took a hiding in our power bill before I discovered our air comp was starting dol. the star contactor was u/s)
2) your control system is starting the cooler too frequently and so bumping up your max d (and adding to the bill)

as busbar put it - whats the payback??

any way there's my thoughts for today hope they help a little
regards
Don

 

[electricpete]

One comment about the economic discussion. We calculated 832KW. IF that is assumed to last for 15 minutes (not actual duration of starting current) at 10 cents per kilowatt hour, then the charge for one start is

0.1($/kwhr)*1/4 (hr)*832(kw)= $23.00

If the chiller starts only once per day, that is
30 (starts/month)* 23($/start) = 660 $/month.

Maybe I'm looking at the rate structure wrong?

I did not understand in Busbar's discussion:
#1 - Does the demand timer "care" how long the peak lasts or does it simply record one peak value as representative of the entire 15-minute period? Or is it smarter than that?
#2 - If the demand charge per month is related to the peak load recorded, then how does a $10.00/kW/month demand charge combine with a 832kW peak increase to yield $100 per month?
I'm not arguing... just trying to understand how this would typically be billed.

 

[busbar]

electricpete, figure that the 1000-amp demand only lasts (likely less than) three seconds, and that there may be up to 3 starts over any 15-minute demand interval. That short-term kW demand would only increase the metered quantity by ~1.1% [10 seconds/15 minutes] and would not have a costly effect on the demand charge in the energy bill; making a starter retrofit economically unjustified.

I considered the additional energy (kW-hours) insignificant due to the relatively short interval of increased use during the starting interval.

So, #1, I considered the demand at ~1.1% of 832kW (not the whole 832kW) and, #2, making the 15-minute demand increase less than 10kW because it lasts only a small fraction of the metered-demand interval.

It this needs any more clarification, please ask.

 

[Run1on]

I appreciate all the useful info on the subject. For the record, we are also on a ratchet structure where the highest demand (15 min. interval) recorded during the previous year establishes our baseline for the following year. The chiller in question runs nowhere near its 100 % limit, typically in the 300a range (480v, YD starter configuration).

While I agree that the start when averaged into the 15 min frame is a small amount, I am looking for ways to reduce this start peak as demand kw of the bill is approx. $10.00 per demand kw, representing sometimes 40% of the total electric bill.

Run1on

 

[don01]

just a thought for this one
drag the electric company in and ask them for a load profile and how they calculate the max demand. Then armed with this data hit the manufacturer to see what they recommend. You should then be able to do a quick cost analysis and act on that. If there is a problem with the unit that will show up too.
the elec co shouldn't charge for the data or just a nominal fee for the drip.

regards Don

 

[busbar]

Off Topic here, but how did the earlier chiller current-imbalance problem work out?

 

[Run1on]

Busbar:

The current imbalance issue is still out to lunch. We installed metering at the utility transformer and watched for several weeks. There was no evidence of voltage imbalance incoming. We then proceeded to thermal scanning (IR camera) and found some problems that have been corrected. Our last step will be to hook up a PDMA (emax) motor testor and evaluate the motor. We believe most of the system imbalance is a result of 1 phase 277v loads requiring adjustment.

Run1on

 

[busbar]

Thanks for the update on the imbalance problem.

An associate with an ME background said that chillers spend a lot of time running at around one-half capacity, so that something like a VF drive is worth investigating. He also mentioned that it was time well spent measuring and trimming the airflow components, to make effective use of the chiller output. Apparently, more operations are happy with doing that type work in-house.

If the serving utility includes it in their rate schedule, chances are you have investigated power-factor improvement. Besides kVAR-hour metering (‘quadergy’), PF penalties can be ‘disguised’ as kVAR demand, kVA-hour metering, and kVA demand.