On India's power outage

[Ed Pawlowski]

Typical guidelines for design.

It is important for building owners to understand the criteria by which air conditioning systems are typically designed. These criteria which have been established by ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.) and are referenced by most energy codes.

This memo presents the typical design criteria and explains their ramifications. It also explains why air conditioning systems are not typically designed to cool rooms below 75°F and why they may not appear to work properly on very hot days.

What are the criteria for AC system design? l Indoor design temperature: 75°F l Outdoor design temperature: 85°-90°F l Outdoor relative humidity: 50%

What does this mean? What this means in practical terms is that a properly designed air conditioning system will cool a building to 75°F on a day when it is 85°-90°F outdoors with 50% relative humidity (the temperature varies depending on the geographic location of the building).

What are the consequences of these design criteria? It is important to understand the following: l The system is not designed to cool a building to less than 75°F on the hottest days. l If the outdoor temperature and humidity exceed 85°-90°F and 50%, the building or certain areas of the building may not be able to be cooled as low as 75°F. l Temperatures and humidity may exceed these values for several days at a time during a "hot spell".

[G. Morgan]

Unless the shutdown unit also has a random delay built-in when restored.

[krw]

Ok, but you've still accomplished nothing unless you've raised the average delta-T. IOW, a ten-minute power off does absolutely nothing.

[The Daring Dufas]

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I don't know if it's still offered but Alabama Power installed a radio controlled cutoff unit on customer AC condensers that would interrupt the 24volt current to the contactor in the outdoor unit whenever the power company had a big demand and transmitted the cutoff signal. I don't know if it was for a certain period of time or what but it's been about ten years since I last noticed one on an air conditioner. ^_^

TDD

[Dean Hoffman]

The rural power companies here in Nebraska use something similar on irrigation wells. The electronics control some sort of relay. It's easy to bypass the relay for troubleshooting. It's only a problem if the power controller is causing intermittent shutdowns.

[The Daring Dufas]

Alabama Power offered the same thing to customers at one time but I haven't seen one in years. ^_^

TDD

[HeyBub]

It's been reported that 360 million Indians were without power. And that was in just one block.

[Jon Danniken]

If you have to run your AC 100% of the time to be comfortable, you have an incorrectly sized AC. Doesn't the power company take this into consideration when they decide if you are eligible for the discount?

Jon

[Jon Danniken]

Shifting the draw by ten minutes will make an effect if there are tens of thousands of units controlled centrally. In such a situation, staggering compressor draw will serve to even out the load across both a local, as well as a larger area. Since the grid is already pushed pretty far into its capacity, this helps to alleviate brownout conditions, which, from my understanding, is the rationale behind their scheme.

Jon

[Stormin Mormon]

Or, the AC you have is running far less than capacity.

Have it serviced.

Christopher A. Young Learn more about Jesus

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If you have to run your AC 100% of the time to be comfortable, you have an incorrectly sized AC. Doesn't the power company take this into consideration when they decide if you are eligible for the discount?

Jon

[trader4]

Whether it's running at 100% or not wasn't material to the discussion. I only gave it as one case of several possibilities in analyzing whether the utility turning it off on you is possible with it actually benefitting the utility without raising the temperature in the house.

And I'd say it's not necessarily incorrectly sized if it's running close to 100% on one of the historically hottest days

No, they don't. Do you really expect the power company to come out and do a full by the book load calculation? They can install their radio controlled widget in 30 mins, which achieves their objective to shed load when needed. And if they did the load calc, what would the point be to not installing the device anyway? Like somebody is going to say, "Gee, to get that device that really benefits the power company, I have to install a new AC system, so let's do it? "

[clare]

Actually, for maximum efficiency and maximum dehumidifying, an air conditioner that has to run constantly on the hottest anticipated day to maintain the desired temperature is sized "just about right". Mine is too big - just like my furnace - and runs about 8 hours a day when outside temps hover in the low 90s F - which is as hot as it usually gets around here. Running only roughly 30% of the time, humidity is not as well controlled as it should be - an additional dehumidifier extracts about a gallon or more a day with outside humidity in the 73% range - which is on the low side of normal for a sothwestern ontario hot summer day.

[Ed Pawlowski]

Some decades ago, my father was the maintenance supervisor for a printing plant. The plant was air conditioned to provide for better handling of paper and printing. To control the humidity on damp cooler days, they would run the heat at the same time as the AC.

I don't know the capacity of the AC, but I remember walking into the ducts where the filter system was. .

[krw]

Sure, you push the load back ten minutes. Now what?

[The Daring Dufas]

The air conditioning for data centers where something like a super computer is operated on a raised floor has air handlers which blow air down under the raised floor so cooling air can come up through open sections under equipment to cool it. The air handlers often have pans of water with electric heaters to add humidity to the air and will control humidity using a method called reheat which will reheat the cooled dehumidified air the bring it back up to room temperature. ^_^

TDD

[Ed Pawlowski]

I imagine they do that in blocks and each gets a ten minute segment. Peak load is usually between about noon and 5 PM so it would be a juggle during that period. Once factories go off line about 4 and offices close at 5, no need to juggle.

Of course, all any of us are doing is guessing. It would be interesting to see the real method and results.

[krw]

So what? Every one of those ACs that went off-line will use that much more power after coming back on line.

Ten minutes isn't going to do it. If they leave them off for hours, perhaps. Then the delta-T is reduced, saving something.

[krw]

Are any "supercomputers" air-cooled? The ones I've worked with were water- cooled, with heat exchangers under the covers for those parts that use air. Only peripherals, smaller computers, and test equipment used air from the floor as the cooling source.

[Han]

" [e-mail]" wrote in news: [e-mail]:

We've been getting solicitations for this PSE&G program:

It sounds attractive to the customer and features 15 min off periods when PSE&G experiences too high demand (as they define it). I called them once or twice and they never responded. Then the question was posed by the person in charge why should we trust them? That was the end.

Is there anyone here with experience with the PSE&G program?

[The Daring Dufas]

The one I was involved with back in 1988 was a Cray X-MP at the mission control center for SDI tests at The Quajalein Missile Range. The Cray I think, had Freon cooling system interfaced with the building's chilled water system. There were a number of DEC VAX systems too and the Liebert air conditioning units were blowing down into the raised flooring system. It was a very interesting experience to be involved in that project during those days. ^_^

TDD