×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Actual Power vs. Installed Capability

Actual Power vs. Installed Capability

Actual Power vs. Installed Capability

(OP)
I'm looking at acquiring a plant that includes a big air compressor.  The motor on the compressor is rated at 9,000 kW, but the compression requirement is closer to 6,000 kW.

When I'm estimating steady-state electrical cost will I be seeing the 9,000 kW motor-rating or will I be seeing the 6,000 kW compression load.  

David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

RE: Actual Power vs. Installed Capability

You'll see roughly the mechanical load plus the motor losses.
This will be much less than the 9000 kW motor rating.  

RE: Actual Power vs. Installed Capability

(OP)
Thanks, dpc.  That is the way I was starting, but I got nervous that my tiny knowledge of electricity would cause me to understate op costs by 1/3 (when the expected electric bill for the whole plant is over $30,000/day that mistake would be major).

David

RE: Actual Power vs. Installed Capability

Is this an existing facility? If so isn't there a way to get old bills?

Also I would think you have accounted for other loads in the plant. Also the electrical demand charges makes significnant part of industrial billing. Also demand charges are racheted, meaning once you reach ceritain maximum, you pay for it for next 3 or 6 months. The starting current of this motor will most likely determine the peak demand.

RE: Actual Power vs. Installed Capability

(OP)
rbulsara,
We're buying an existing plant that was in a facility with a huge co-gen (which we're not getting) so the old records are missing or bizare.

We're looking at several peak-shaving possibilities (we're leaning towards standby generators).  On the utility we're going to be on, the "demand charge" is set for the highest 15 minutes in a calendar year and it is set for the following calendar year.

Luckily the motor has a pretty effective soft start so it shouldn't be too hard to "peak shave" while starting it and several other very large loads.  

Right now we're at the "feasibility" stage (mostly doing economics), when we get to the design stage there will be competent electrical types involved.  Right now it is just a flange-head out of his depth.

David

David

RE: Actual Power vs. Installed Capability

I would expect starting current, being of short duration and low power factor, to be of little significance in the 15 minute integrated kW demand.

RE: Actual Power vs. Installed Capability

Would it be worth it to actually interlock the big loads so that no two start at the same time? Or no two start in the same 15 minutes?

RE: Actual Power vs. Installed Capability

I agree with jghrist that motor starting is rarely an issue for reducing peak demand.

Just a word of caution regarding "peak shaving" using generators - I would not base any business plan or cost-benefit analysis on money saved by peak shaving through on-site generators, especially when you have a 12 month ratchet and a 15 min demand window.

If your equipment has one bad day,or even one bad hour, you pay the price for the next twelve months.

I have seen a lot of generators installed with an eye toward peak shaving, and I've never seen a single installation that lived up to expectations with regards to demand reduction.  If the generator doesn't make sense from an energy-production standpoint, I would be very skeptical about its merits.   

On the other hand, if you can reduce your peak by shutting down non-essential loads, that is a strategy that is much more reliable and can provide some payback.  But even that is tough when you have that 12 month ratchet.

Just some observations based on scar tissue I have picked up over the years and, of course, YMMV.

RE: Actual Power vs. Installed Capability

(OP)
Thanks dpc, we'll be looking at options with a careful eye.

David

RE: Actual Power vs. Installed Capability

An exception to dpc's scepticism about using generation for peak shaving is when the demand charge is based on a coincident peak with the supplier's system demand.  This would be unusual for an industry, but not so unusual for municipal utilities.  

We have some municipal utility clients who have made very profitable use of peak shaving generation with a coincident peak rate structure.  They have real time access to their suppliers system load data and can cut a lot off of their demand charge with as little as 3-4 hours operation per month.  In one case, the utility has a ratched coincident peak rate structure and saved millions with a single summer hour of operation.

With a non-coincident peak rate structure, I agree with dpc.  You can eat up a lot of fuel trying to reduce your peak because you have to run the generators every time you approach your peak use.  Unless you have a well-defined, sharp load peak, you have to run a lot of hours.



Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources