Questions on moving air?

[Hackjunkie]

Hello. I am working on a project to cool down an enclosed environment. The volume is approximately 300 cubic feet using a ventilation system mounted on the top. The environment is watertight but not airtight. It can reach temperatures up to 140 degrees Fahrenheit internally with approximately 100 degree Fahrenheit externally. I have two questions for this situation . Approximately how many cubic feet per minute would I need to move to achieve a temperature of approximately 80 degrees with no active cooling systems, and would this be feasible? And would it be easier to move air in then out, or just to evacuate air from the environment? I am an electrical engineer and do not know a whole lot about HVAC and related fields so I appreciate your help and replies. thank you

 

[berkshire]

With out active cooling, you are not going to achieve a temperature lower than the ambient air, if external air is 100*f then that's what you are going to get. The major question is , how much insulation does the building have? You may be able with insulation to delay the temperature rise of the building by temporarily sealing it ( Texas Cool ), however this will not work if you have heat generating equipment inside. Since you are electrical, I suspect that this may be an equipment room.
In which case you would need to know the wattage of any equipment working in the room, because you are going to have to remove the heat generated by that.
Sun radiation alone will bring an uninsulated room up to 40*f above the ambient air temperature, especially if it has windows the sun can shine into and it is painted with dark paint.
In some parts of the world, evaporative coolers can cool quite well at not too much cost, if the relative humidity is not too high.
Give us more information and you will get a more sensible answer.
B.E.

You are judged not by what you know, but by what you can do.

 

[LittleInch]

So this is a box approx 2m x2m x 2m?

I don't understand why you're trying to be so sophisticated here. Just stick a 4" fan on the inlet and a similar sized outlet and blow in relatively cold air.

Your temp won't fall below about 110F at best.

All depends on how much heat your equipment is generating.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[lukaiENG]

you could try evaporative cooling on the air intake also, this will help bring down the space temperature closer to the wet bulb temperature. This could get you down to around 80F in hot/dry climates. not so much in hot/humid climates.
This is of course an active cooling system which you were trying to avoid!

 

[HVACMech]

I agree with littleinch, there will be an approach temp that you reach, I usually find about 7 degrees difference to be where your CFM will get out of control in comparison the volume of the building. Without any active cooling (evap, DX, chilled water) you won't ever get to 80 degrees with an outside air temp of 100F.

 

[Hackjunkie]

Yeah I figured without an active cooling system attaining a temperature lower then the ambient air temp would be improbable. The shape is more irregular dome meant to look like a large pile of dirt without any real insulation. Its a small collection of sensors and a low power computer for data collection. Peak power consumption is about 25 Watts, with the average being about 15 Watts. Its being run from a 2.4 kW battery array. Its supposed to collect data for 5 day periods in a variety of locations. However the heat from baking in the sun is wreaking havoc on the batteries, and causing a few issues with the computer occasionally overheating. I am not opposed to active cooling, but I need to stay within certain power constraints. The sensor system is not mine. I was asked by a friend to help with creating a ventilation system with a small solar array. The largest single flat area is approximately 9 SQFT which could yield roughly 100 Wh under peak conditions. The ventilation system needs to be relatively low power, with low noise and vibration. It also needs to prevent water from being introduced into the interior during a rain storm. Along with the power provided by the solar array, the battery bank will be increased to accommodate the extra power draw. The ventilation system will be attached to a temperature sensor to prevent it from running constantly. So what would be the best way to approach this situation? Are there any good low power evap or other cooling systems, or would it be more efficient to just run a ventilation system to keep the interior closer to ambient air temperature? The original question from my friend was " My system keeps overheating and the batteries are only lasting 2 or 3 days. Could you make a cooling system ran from a solar cell?"

 

[HVACMech]

The battery life will still be shortened significantly due to the 100+ F temps. How would you do an evap system if you don't have access to water? One of my friends always said "A BTU is a BTU is a BTU", meaning it doesn't matter how you get from A to B, you still are going to need "x" amount of energy to do it. I would tell your friend to hire an HVAC consultant and have them help him work through his issue. In addition, depending on location, sounds to me like your friend is already violating code by not having ventilation for an enclosed space with batteries (things go boom with hydrogen evolution).

 

[berkshire]

If your structure is waterproof and you have water available, then a spray mister on the outside of the building, may be sufficient to get your temperature down. There are commercial roof spraying/misting systems available, however a residential system may be sufficient for your needs especially in southern California. You do need to get an HVAC specialist on site for the best advice.
B.E.

You are judged not by what you know, but by what you can do.

 

[LittleInch]

I think your best bet might be to look into mass metal coolers.

These work by running during the night when the air temp falls and blowing air through a large metal block. This gradually reduces the temperature of the block to the ambient air. As soon as the outside air temp increases above the block temperature you turn that fan off and start an internal fan which blows air through the block and around the enclosure. You might need quite a big block of metal, but is something I've looked at for remote solar powered instrument kiosks.

Your real issue though sounds like solar gain where some sort of insulation is a must otherwise you will always be chasing your tail. There are some very effective thin insulation materials around which will help a lot.

There are some solid state cooling units around with realtively low power needs and if the key itme to keep cool is the computer, try something like this

http://204.12.46.198/2010/applications/ac.htm

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Hackjunkie]

I was probably thinking of something else when referring to an evap system. I was thinking of the portable in home AC units that collect moisture from the room blow cold air out, and hot exhaust out a window. As far as breaking code in relation to battery storage that im not sure. I know the unit is not airtight, but that's about all I know in that. Im not sure if there are any vent tubes connected to the batteries. My only real involvement was building a voltage regulator and a few amps for some sensors. I'll have him contact an hvac specialist to get some more input. And spraying water in southern California could be a no go due to the drought.

 

[berkshire]

The portable units you are talking about are refrigerated air units and have a pretty high energy consumption rating.
B.E.

You are judged not by what you know, but by what you can do.

 

[Hackjunkie]

LittleInch, Ill have him look into these. I don't know what if any insulation is present. I don't even know what the outer shell or inner frame are made of. i just like to build circuits.

Berkshire, Oh i know they used to kill my electric bill. I dont really know much about active cooling systems, other then they usually require a significant amount of power.

 

[berkshire]

Hack junkie,
There are some small solid state air conditioners that are made for computer enclosures Most of them use Peltier diodes.
There are also some miniature 12 or 24 volt air conditioning compressors Originally made for military use that cool computer enclosures.
You may be able to repurpose one of these to cool your batteries. Talk to a professional.

http://www.bing.com/images/search?q...4d4c00620162c55c38a9629fb756eec1o0&ajaxhist=0

B.E.

You are judged not by what you know, but by what you can do.

 

[chicopee]

Did you ever asked the question as to the reason for reflective white paint on propane bulk storage tanks? About a fine mist of water spray over the areas where the boxes are located. You can also provide shades with inexpensive canopies over these boxes.

 

[Hackjunkie]

Berkshire, I was thinking about a peltier for the cpu,but they aren't that efficient and there would be nowhere for the transfered heat to go. I've played with them once or twice kinda fun. A small AC compressor would be a nice option if it's efficient enough. And a 12 volt compressor would be easy to wire in, but the sound and vibration would need to be isolated.

Chicopee, I understand why LPG tanks are white. They reflect heat. When I was asking my friend questions about his structure and why it was dirt brown his answer was "So it would not create to unnatural of an object". Using shade canopys isn't to bad of an idea since they can lower the temperature of a surrounding area, but I don't know what he's measuring and how it could effect it. I will suggest it though. Misting is a nice option, I like the misted lines at Disney land, but California is in a drought and that's not something im too knowledgeable on.