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Kilowatt calculations #169089 09/25/07 06:14 AM
Joined: Jul 2001
Posts: 311
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Frank Cinker Offline OP
Member
I would like to calculate the approx. kilowatt demand of a large walk-in freezer in a supermarket. It is of course thermostatically controlled, consequently it is not on full load constantly . My question is in general, should I apply 60%?, 70%? to my calculation?

Frank

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Re: Kilowatt calculations [Re: Frank Cinker] #169123 09/25/07 07:56 PM
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LarryC Offline
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What goes into it? Frozen sides of beef or pallets of warm beer? How big is the differential temperature between inside and outside? How often is the door opened and closed? How often does the defroster circuits operate? Does the compressor have a crankcase heater?

I would see if you can throw some sort of metering on it, to measure the total power draw over a week.

Re: Kilowatt calculations [Re: LarryC] #169221 09/29/07 02:45 AM
Joined: Oct 2000
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Scott35 Online Happy
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Expanding on what Larry posted:

* How well insulated is the Freezer? - walls, doors, ceiling, floor.

* Is the Refrigerant Ammonia or Freon?

* Is the Freezer large (+ 2000 square feet) or small?

* How high is the ceiling + what level of heat transfer can take place between the Freezer's ceiling and the Building's roof (are they very close to each other)?

* Are the Lighting Fixtures Freezer rated Fluorescents / HIDs, or simple Incandescents?

* Is the temperature -30°C or less?

* Is there a "Vestibule", or does the staff transfer stock directly from warm (ambient +30°C) areas, in bulk quantities?

* Are the door and floor heat tape strips self regulating + working properly?

and the most important questions...

* Is the Refrigeration System adequately sized for the area to be cooled?

* Is it working properly, or does it require some R&R,

* Are there any penetrations which need to be sealed?

* Is the integrity of the Refrigerant piping's insulation good?

* Does the staff leave doors open for extended periods of time?

If the environment is adequately sealed, the heat trace strips are not going "full throttle 24/7", the Refrigeration Assemblage has been / will be properly maintained, the Lighting Fixtures do not contribute excessive heat (other than what is required for Lamp Starting + Operation), and the staff does not prop open the door(s) for 10 minutes every hour, - and the Freezer is not really large, the Condenser(s) should cycle on for an hour at most.

The Evaporator(s) fans may run constantly - to de-ice the evap coils, or there may be other means to control evaporator coil icing (thermal defrost techniques).
These should be figured as an LCL.

I also agree with Larry's suggestion to monitor the load(s) for at least a 7 day period. Chart frequency of starts/stops, duration of running, time of day, day of week, ambient temp, etc.

Remember, the complete KW "Package" for this Freezer will include:
  • The Condensing Pumps,
  • The Evaporator fans ("Fan Coils),
  • Heat strips in the floor + at the door (-20°C and below),
  • Evap coil defrosting,
  • Humidity / Condensation control,
    and
  • Lighting.


Good luck.

Let me know if I missed the point you were going for, or if you have additional questions.

Scott


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Re: Kilowatt calculations [Re: Scott35] #169257 09/30/07 09:23 AM
Joined: Mar 2005
Posts: 1,213
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SteveFehr Offline
Member
Something like this is extremely hard to calculate. You could sit down with Manual J and calculate out all the BTU losses to come up with a fair estimate of how many BTUs you have to replace to maintain temperature, and thus how much enenergy you have to throw into your chillers, but you probably don't have any of the information availible to do those calculations. Also adding to the cost would be heating bills for the "normal" area of the supermarket, which will have increased energy costs to overcome all the heat losses into the freezer.

As an electrician, your best bet is to just put an logging ammeter on it for a while.


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