AC in Electrical Rooms 2

[rickfer]

I'm looking for advise in order to select proper equipment for AC in a eletrical room.

I requiere about 48,000 BTU/H of cooling capacity, and I wonder,

Should I use a single AC unit? Or it is better to have 2 x 24,000 BTUH units?

I'm worry that if an AC unit fails, or needs maitenance, room will have not AC during this time, and electrical equipment may be on risk.

Is my concern valid? Or I'm just worring too much?

Thank you.

 

[Zesti]

> I requiere about 48,000 BTU/H of cooling capacity,
> Should I use a single AC unit? Or it is better to
> have 2 x 24,000 BTUH units?

Why have two units each at 50% ?
If one fails or needs maintenance you will not have enough cooling-capacity. You need three units of 24,000 to be safe in these circumstances (two running, one spare).
Depending on your budget you might also consider two units at, say 70% each. If one fails you still have 70% left instead of only 50%.

> Is my concern valid? Or I'm just worring too much?

If the electrical equipment will fail when not enough cooling is provided and if the electrical equipment is vital then you do need to worry about these things.

But, there is always a "weakest link" in the entire system and it might only be something small which might make everything fail after all.

(I am always amazed when I see double pumps (backup) in AC systems when then cooling pump on the emergency generator is only a single pump...)

For instance, consider this: where is the electric power coming from? If the electric power for the electrical equipment is coming from another source than the electric power for the AC equipment then this can stop working by a power failure even though the electrical equipment keeps running...

In short, when you start thinking about all the things that can go wrong and make the entire system fail, there's virtually no end...



PS: Do you really need 48.000 BTU/h ? Often electrical engineers exagerate the amount of heat given off by their equipment...

PPS: The usual approach in HVAC seems to be "let's just have two" instead of doing a serious analysis of the system and its components and identifying the components that are critical and most likely to fail...

 

[DRWeig]

Good stuff, Zesti.

My added comment would be to ask if the "electrical equipment" really needs cooling at all. If it's data processing equipment, you need to do all the analyzing that Zesti suggested. However, if it's just some motors and switchgear, they might be rated for very high temperatures and you can get by with just some ventilation air.

Perhaps you can give us a little more detail on the types of electrical equipment in the room?

Good on ya,

Goober Dave

 

[EEJaime]

Zesti: "exagerate"? Moi? Never...-ish.

rickfer,

The "need" for cooling varies with the type of equipment in the space. If as stated previously it is simply switchgear and transformers, they can withstand fairly uncomfortable temperatures for the most part. But if there is SCADA equipment, electronic breakers or other types of monitoring and control equipent, these are a little more temperature sensitive.

I give my HVAC guys a conservative number for heat load of about 3% of the full load kVA of all transformers in the space as heat loss. So at .8pf and 3.4 BTU/watt, it gives a total load. Again, you don't have to maintain 80 deg F, the space can easily operate at 95 deg F +.

It would help as DRWeig stated if the contents were known.

Regards,
EEJaime

 

[willard3]

I agree with DRWeig, what needs to be cooled?

If you are cooling computer equipment of any kind including controllers with computers, temperature is important.

Switchgear, panels, motor starters, conductors and etc are rated at 194F at the low end.

 

[cdxx139]

My understanding is that there is always some piece of equipment in an electrical room who's maximum working temperature is 104, anything more may damage the equipment and would be terribly uncomfortable for a worker. (and the more uncomfortable a worker is, the more unlikely the work will be done correctly)

We also use the 3% inefficiency for all transformers, plus a couple of hundred watts for panels and VFD's and any motor heat.

EEJaime - i never multiplied the power factor, but that makes sense. I will begin doing that, and consider my previous designs as having a safety factor.

and to require the full 48k cooling load, would require all transformers at full load, very unlikely, but upgrading from 2x50% to 2x70% is a smarter design, with minimal extra first cost.

However DRWeig, if the 48k load is correct, you would require almost 6000 cfm of OA w/ a 9 deg delta.

knowledge is power

 

[KiwiMace]

VFD's have always been the trigger for AC in electrical rooms in my plant work. We have typically provided 2 x 50% as you suggested. The reason for this is that the 2nd AC unit is only required part year in most climates. This gives plenty of time for maintenance and reduced risk of fail. Units should be set to duty cycle to share hours.

Also select a unit with a high sensible heat ratio - I usually strap a Bard external wall mount unit, often used in telco installations.