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How Much Cooling Is Required To Cool A kW Of Electrical Load In A Space

09/29/2014 2:34 AM

Hi,

New to the group but excited to meet and talk to all of you budding and experienced engineers and technologists out there in the sustainability sector of engineering. I'm looking for a rough rule of thumb for the amount of cooling required to cool 1 w or 1 kW or 1 btu/hr of electrical equipment load in a space. I knew this from a long time ago but have now forgotten and am second guessing myself. If I remember correctly, it's almost a 1:1 ratio so if I had a wigit in my space that required a power draw of 1 kW constantly (thereby consuming 8,760 kWh annually)assuming that 100% of the heat generated by that machine stays in the space, I would need 1 kW of cooling to maintain a reasonable temperature range 75-80 F (24-26 C). Basically what I'm saying is I would need to install an air conditioner with a minimum capacity of 0.3 tons of cooling to maintain my setpoint mentioned above. Obviously this doesn't mean the same for energy consumption of the cooling equipment as the EER or COP would need to be factored in. For this example my COP is 3 thus 1 kW of cooling requires 0.3 kW of electricity and roughly 2920 kWh/year to run the air conditioner. If anyone else has a better rule of thumb to do quick rough calculations to size cooling based on the power draw and heat extraction, I'd be very interested to hear what others use for quick and dirty on the fly estimates. I myself am an energy modeler and find myself needing these quick and dirty calcs more and more often as modellers are wearing the designer cap at the same time.

If there is flaw in my math please point it out, I'm working on no sleep for over 24 hours now and am not thinking completely straight :). Happy Monday to all who read this question sometime today.

Cheers,

BEPSimGrrl3.412

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#1

Re: How much cooling is required to cool a kW of electrical load in a space

09/29/2014 3:04 AM

Did you model it?

Your rambling seems incoherent alright.

You'll need to remove as much heat as the space accumulates, given the relative efficiencies of the devices in that space, the power they consume and whatever is outside it. A BTU is a good unit to work with.

Sleep on it.

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#2

Re: How much cooling is required to cool a kW of electrical load in a space

09/29/2014 3:45 AM

Cooling is achieved by ventilation, either natural or forced, Guvnor. Air conditioning is for personal comfort control.

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#10
In reply to #2

Re: How much cooling is required to cool a kW of electrical load in a space

09/30/2014 10:44 AM

I've built several panels with air conditioning where we had to pack way too much heat into a too small cabinet in order to stay inside a customers given "foot print". This usually involves several large VFD's and a transformer and a PLC all packed into what little space we were given to work with. But, yes, several panel manufacturers also sell air conditioners for panel cooling.

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#15
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Re: How much cooling is required to cool a kW of electrical load in a space

10/06/2014 3:41 PM

I agree mostly, but most control rooms where there is a lot of instumentation on a process requires several tons of AC to keep the electronics happy, not so much the personnel.

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#3

Re: How Much Cooling Is Required To Cool A kW Of Electrical Load In A Space

09/29/2014 8:45 AM

You need to add the electric used plus the loss from efficiency plus the building heat load...including number of occupants...

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#4

Re: How Much Cooling Is Required To Cool A kW Of Electrical Load In A Space

09/29/2014 1:06 PM

Just switch the bloody thing off. That would do it.

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#5

Re: How Much Cooling Is Required To Cool A kW Of Electrical Load In A Space

09/29/2014 10:30 PM

You are correct. I use about 240% efficiency.

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#6

Re: How Much Cooling Is Required To Cool A kW Of Electrical Load In A Space

09/30/2014 4:23 AM

Air conditioning? Is this a sealed unit? If not, there's not a lot of air in space.

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#7

Re: How Much Cooling Is Required To Cool A kW Of Electrical Load In A Space

09/30/2014 8:47 AM

It depends on what your "wigit" does when it is fed 1kW of power. If it is an electric motor, then it will successfully convert some 90% of that power to mechanical work, leaving you with only 100W of heat output to deal with.

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#8

Re: How Much Cooling Is Required To Cool A kW Of Electrical Load In A Space

09/30/2014 9:59 AM

There's a lot of mouth-sound gong on, but not clearly thought out. I'll cover the basics of heating and cooling as i remember them.

The watt is not only a unit of power it is also a unit of heat, a resister that dissipated 1 watt of power generates 1 watt of heat.

The BTU is another unit of heat, if memory serves equivalent to the heat generated by one Standard British Phosphate Match. Conversions get complicated, so we'll keep everything in watts here.

The human body on average generates 100 watts of heat while awake and at rest/slightly active (aka 'desk work,' or 'light construction,' work in space is pretty much 'light construction,' since everyone moves slowly and carefully, and you're not exerting to lift heavy loads against gravity.)

To maintain a temperature balance, every watt of heat generated needs one watt of cooling/ventilation/radiation/dissipation/convection. Not all 'cooling' needs to be active, in fact, in space active cooling is highly inefficient, since it needs to cool itself as well (on Earth, we cool our house with A/C units, which stick their 'hot end' out a window to dump their heat into the atmosphere).

Passive cooling is generally heat sinks (conduction of heat into the heat sink, radiation of the heat into the air/space, and convection to move the hot air away and draw in cool air.) or cooling packs (conduction of the heat into a circulating fluid, conduction of heat from the fluid into an ice pack)

If you're making a satellite, assume the only cooling you will have available is heat sinks into space, and keep the head sinks on the side that faces away from the sun, so they'll radiate more heat than they receive. Better yet, put them on the 'dark side' of the solar panels.

if it's going to be part of a capsule or a module for the ISS, just clearly indicate the amount of heat generated by the equipment when idle/normal load/full load so the system engineers can balance the vehicle's cooling system properly for it.

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#9
In reply to #8

Re: How Much Cooling Is Required To Cool A kW Of Electrical Load In A Space

09/30/2014 10:09 AM

I have a totally insulated room.
I place an ordinary kitchen refrigerator in the centre of the room and connect the plug to the power-outlet.
I now open the fridge door.
Will the room gradually get hotter or colder?

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#12
In reply to #9

Re: How Much Cooling Is Required To Cool A kW Of Electrical Load In A Space

09/30/2014 11:34 AM

Hotter.

The fridge it taking the heat from inside its box and moving it to the coils on the back.

Its equipment (which is outside its 'cooling box') is also generating heat.

So the net effect is Room - (cooling from fridge) + (cooling from fridge) + (heat from fridge equipment) = Room has a net heat gain of what the fridge's motors are generating.

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#11
In reply to #8

Re: How Much Cooling Is Required To Cool A kW Of Electrical Load In A Space

09/30/2014 11:13 AM

I'll agree with everything you say, but I think the OP is talking about an (enclosed) space on Earth rather than about outer space, which brings active cooling back into the discussion.

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#13

Re: How Much Cooling Is Required To Cool A kW Of Electrical Load In A Space

10/01/2014 4:13 AM

Your estimates are about right, assuming normal evaporating and condensing temperatures for air conditioning purposes.

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#14

Re: How Much Cooling Is Required To Cool A kW Of Electrical Load In A Space

10/06/2014 12:50 PM

You may review specs of equipment. Not all electrical load is converted to heat i could say, it depends onthe type of equipment, except heating equipment that is. Electrical load could either be coverted to work and heat was just a product of ineffeciency. Heat could be minimal to some devices.

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