Does Water Temp = Cooled Air Temp?

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Old 05-23-13, 04:08 PM
J
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Does Water Temp = Cooled Air Temp?

Greetings Gurus!

At the risk of sounding like an idiot, I submit this question due to the fact that I simply don't know.

Would the temp of the water circulating through the pads be the same as the cooled air being delivered from the cooler. I realize there are factors which affect the temp of the water in the pan, i.e., fresh water entering the holding pan via the inlet, outside temp, etc. I'm specifically talking about the unevaporated water dripping from the pads back into the pan.

I honestly don't know if this question is very simple or if it's complex.

And also, does the mineral content/build-up affect the water's ability to be cooled adequately? Or, to put it differently, does fresh water cool and/or evaporate quicker/easier than does water which has been in the pan for a while recirculating over and over again, thereby becoming increasingly more laden with minerals.

Thanks!

Steve
Chandler. AZ
 
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Old 05-23-13, 04:53 PM
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Well...no expert...but I'm one of the few regulars that has ANY experience with coolers...though mine is just a small unit for the garage.

That would also be good info to put in your next post...what brand and model of unit, whole house or room, etc.

As I understand it....the evaporation of water on the pads is what cools the air as the water absorbs heat from the air blowing across the pads. I'm guessing, but if the water was the same temp as the blowing air...not sure it would work correctly? There's also a wet bulb/dry bulb issue that I remember from my Navy days.

Mineral buildup will affect cooling as the fibers of whatever type of pad you use will become clogged and allow less contact between the water and air. Does high mineral content in the water affect the actual cooling effect...that I don't know. The mineral buildup is normally partially controlled by a bleed off valve or pump that dumps the used water out of the system allowing a fresher supply to dilute the minerals. I don't have a bleed valve or pump in my small unit...I just have a siphon set-up where I drain it out into a bucket every day during heavy use times.

A little bit more technical info is available here...Evaporative cooler - Wikipedia, the free encyclopedia

Our expert should be checking in w/i a few days...check back often!
 
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Old 05-23-13, 05:58 PM
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Thanks for the reply, GG.

My particular unit is a Mastercool 6500 cfm down-draft with 12" rigid media. It's mounted over the attached, insulated, finished garage because it's the only part of the house with a flat roof. The door from the house to the garage is kept open to allow the air to enter and exit the house.

From a non-engineering/scientific mindset armed only with a fair amount of logic and average reasoning abilities, it seems like the temp of the conditioned air would be a direct result of the cooled water over which it has just passed.

I came across the following formula, but I'm not sure that it directly relates to my original question:

With direct evaporative cooling, the dry bulb temperature is reduced while the web bulb temperature remains the same.

1. Temp drop achievable = (dry bulb - wet bulb ) x (efficiency* of the media)
Example: (86 degrees - 66 degrees) x .9 = 18 degrees

2. Achievable temp = dry bulb - temp drop achievable
Example: 86 degrees - 18 degrees = 68 degrees DB/66 degrees WB**

3. Starting DB: 86 degrees
Ending DB: 68 degrees


And also this explanation which, although waaaaaaaaay over my head, seems to me that, if I'm understanding it correctly (and the operative word here being "if"...) it's possible for the water to actually be cooler than the air being produced...

In a 73 degree F house with 50% relative humidity (assumed) and little direct radiation, your evaporation rate is around 2.7 mm/day (Penman-Monteith equation with an assumed net radiation value of 100 Watts/square meter). Penman-Monteith isn't the best approximation to use, since it was developed for agircultural water estimates, but it's a decent first order equation, and I already have a spreadsheet set up for it. A 30 gallon aquarium has a surface area around 30" x 12" (~2300 square cm), so he's losing about 630 cc of water/day - 0.63 kg.

Water has a latent heat of evaporation of 2,260 kilojoules/kg - this is the amount of heat consumed during the phase change from liquid water to gaseous water vapor. Evaporating that much water requires around 1,400 kilojoules (kJ) of energy - that has to come from somewhere. In this case, it comes from the energy stored (in the form of heat) in the water within the tank.

Water has a heat capacity of 4.18 Joules per cc per degree C - this means it requires 4.18 joules of energy to change the temperature of cubic centimeter of water by one degree celsius. A 30 gallon tank holds ~115,000 cc (30 gallons x 3.84 liters/gallon x 1,000 cc/liter). If you assume the tank is well mixed (reasonable, since there's almost certainly a bubbler or other circulating pump in the tank), removing 1,400 kJ of heat from 115,000 ccs of water reduces the temperature by 2.9 degrees C, or a little more than 5 degrees F.

73 ambient and 68 water temperature sounds pretty reasonable. If you want to double check (and you're feeling ambitious), find out how much water he adds to the tank and how frequently he adds it - you can recalculate everything with the actual evaporation numbers.
 
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Old 05-23-13, 07:43 PM
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Water cools when it evaporates, so adding Ice or colder water doesent really do anything for it.

What people dont understand, its not the water that makes the cooler blow cool air, its the ability of the media to evaporate water.
Celdek media is very absorbent, hence the very cool air we receive from it. Anyone ever use the blue foam pads in their coolers? They are plastic and plastic doesnt absorb water, they have to add something to it that absorbs water, to make it evaporate.

Just because a certain pad may hold a lot of water, it will not make your cooler blow cold, it has to have the ability to absorb the water and not just retain it.

I have known people for years that think more water being pumped through their cooler, makes it blow colder, wrong! I tested one once and the cooler actually blew 2 degrees colder after I shut the pump off. The pads only need enough water to stay damp, anything more than that and the water cannot evaporate properly.

So back to the original question before I got carried away. Adding colder water to the cooler will make no difference. Minerals will block the media from doing its job of absorbing water for evaporation.
 
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Old 05-23-13, 10:07 PM
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Thanks for your reply Sammy!

I have had those same conversations with people who will not be convinced otherwise regarding more water, type of pads, etc.

As for my original questions, I think you got side-tracked in your treatise.

What I'm actually wanting to know is if the temperature of the conditioned air leaving the unit and that of the water running through the pads is the same. And secondly, if there is any difference in the water molecule's ability to be cooled is diminished by the content of TDS within it. I'm more than aware of the pads losing their abilities to work efficiently the more mineral buildup there is. I'm never more aware of this fact than every 3 years or so when I have to plop down the $$ to replace them. Yeeouch!

Thanks again for your always informative and knowledgeable replies! I'm sure you and all the other mods don't hear often enough how much you and your time is appreciated.

Steve
 
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