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reggie
28-11-2002, 08:00 AM
Can anyone tell me if there is a standard rule of thumb for calculating freezer/cool room equipment? I know without going into science many people swear by 100/120 watts/m when sizing AC plant comfort cooling. Is there a similar formula for cabinet sizing for both low and medium temperature applications?

herefishy
29-11-2002, 06:05 PM
yes, there are, and I am familiar with this information in the form of a table which lists ranges of "heay usage", and "average usage". There are so many variables in regard to what is being cooled (products, materials, entering temperature, loading, heat of respiration, pulldown time, etc.), location (indoor, outdoor, ambient temperature, etc.), type of construction (polystyrene, urethane, thickness, etc.), that any rule of thumb for one, would not apply for another application.

I recall evaluating a plant, for which a company did a load calculation and told the customer that there was no way that the existing refrigeration would attain the required conditions. The customer knew, "It has worked for the past 20 years", so called upon me for a second opinion. What I had determined was that the others had used the default entering product load and temperature ratings in their computer program (as opposed to interviewing the customer as to actual usage). Well, you might imagine what the typical load calculator might spit out for product load (rule of thumb) on a 30' x 40' x 20' high freezer store!!:rolleyes:

This customer has been my best customer for three years now!!

There are rules of thumb for each of the parameters of the calculation. There are many more variables in freezer/cool room applications than in the human comfort applications.

A knowledgeable person (who did author a chapter in published ASHRAE manuals) told me, "Despite all we know through all the research and mathematical formulas and tools that we use.... the best we can do is still only our best guess !"

subzero*psia
08-01-2003, 08:54 PM
I used to do technical support and during that time I would have argued that refrigeration was an EXACT science, and did argue with a few old time engineers that were and still are good friends....

Its like medicine... it isn't an EXACT science either although most would have you believe it is. If it were an exact science... cancer wouldn't be... and every procedure would be a success... every disease would be treatable. Like refrigeration... all it takes is an "UNKNOWN", or a flawed formula... and with all the conversions going on these days... there are plenty of flaws in them.... still. LOL!

What do you say we form a one world government?! Mark could pipe in on that one considering he is probably either the pope or the anti-christ? ..... hehehehheh! ;) :p

zolar1
31-03-2003, 07:14 AM
Here's the best formula there is -

Customer happy with results = customer pays the bill

LOLOL

I would rather slightly oversize an application slightly that put in the one that's called for (which in my humble opinion is usually way wrong)

For example, if comfort cooling called for a 2 1/2 ton a/c unit, I would try to put in a 3 ton unit if I could. That way, as global warming gets worse over time, the unit would still be adequate. The difference in cost isn't that great either.

But that's just my opinion.......

PS my parents have a 2 1/2 ton unit for a small house. It barely does the job. Any they just spent $5000 for new siding and insulation.

herefishy
31-03-2003, 03:39 PM
Bigger is not always better..... particularly in comfort cooling. Oversized A/C systems result in poor performance in addressing the latent load. This is particularly important in the Southern US which is Sub-tropical. You probably could get away with it in New Jersey.

But the mentality of Of oversizing comfort cooling applications in order to decrease run time - operating costs has resulted in dire economic straights relating to degradation of real property (mold) and devastation of the insurance market (homeowners insurance). The lawyers appreciate it though. :)