Evaporative Cooling Ideas 2

[turtlemaster]

Hi!!

I got my hands on a demonstration kit of an evaporative cooler. It's just a sealed box with air intake holes on the sides, a fan on top, a downward water spray just beneath it, and a helical coil carrying hot water to be cooled. I am wondering how to show the kids that it can be improved to cool faster.The sales guy told me its about 50 to 60 % efficient (He was probably getting the deal done). I doubt if it can give me 15*C difference.

I plan on increasing the surface area of the coil and maybe fabricate some fins on it, but I would like to know about more options from the thermal experts.

Hope this one's in the right park,

T.M.

 

[zdas04]

"Efficiency" is an amazingly slippery concept. The evaporative cooler in my home uses about 80 W of power to lower the temperature in my home by 20°F. Most of the year that is enough to require a sweater. Running it all month at full power (which we never do, it is usually consuming about 20 W) would cost $4. The same amount of cooling with an air conditioner would cost about $300 according to my next door neighbor who has about the same size house, built about the same time, with about the same energy efficiency, and the same number of residents. So the question becomes, "What is efficiency?" The only definition that makes any sense at all is (Energy reduction in supplied air)/(energy input). Power input includes the pump and fan, both of which are reasonably easy to capture. Here is where the math gets difficult. The incoming air is 7% RH on average. The air in the house is 14-23% RH. If you can determine (with a psychometric chart probably) the specific heat of the 7% air and the specific heat of the 20% air at the new temperature then you can get the numerator, but those values are really slippery. I've tried to do this a couple of times but have been stopped by some vital data point that I couldn't come up with.

15C (27°F) temperature depression would be a challenge for any evaporative cooler. I've designed them for the waste heat side of fin-fan coolers on compressors and with all the money in the world the best I could do was 22F at an average RH of 6%. As ambient RH goes up, the available energy drops rapidly. At around 30% RH they stop making any kind of sense. Somewhere around 40% RH they stop working altogether. If you are anywhere close to sea level with a nearby water source you will be disappointed in the results.

As to cooling a water pipe, good luck. Thermodynamics is pretty unforgiving. Q=mCΔT regardless of our wishes. The mass flow rate of air in these systems is pretty low. Even a very small flow of hot water will be a very large mass flow rate (relative to the air). If you are able to lower the air temperature by 15C, you will be doing really well to lower the water temperature by 1C. Increasing surface area and adding fins might get that to 1.01 C. Your best bet would be to force the flow rate down until you get a ΔT=Q/m/c that will impress the observers. The specific heat of water is very different than the specific heat of air so you might need to turn the flow rate down to a trickle.

David Simpson, PE
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[cranky108]

What are you trying to cool? If it's air, an evaporative cooler works as above. If it's like a cooling tower to cool another fluid, it works by reducing the surface area needed for the heat transfer of the fluid to water instead of air. Thus the evaporative cooler may not be efficient, but it is made of much less costly materials, and takes much lower fluid movement to accomplish what a dry tower would take.

So the efficient application is in materials to construct and in fluid movement required, provided water (or other evaporative fluid) is cheep.

 

[turtlemaster]

thanks zdas04, well explained. cranky108 I am trying to cool hot water, but for demonstration purpose I might look at other fluids.

Will keep posted,

T.M.

 

[cranky108]

Understand that in some of what I said, the fluid might be air. The many small drops of water have a much larger surface area than a dry cooling tower.

 

[berkshire]

As I understand it, you have a coil in which a liquid to be cooled flows.
You are spraying water on that coil and using the evaporation of that water to take the heat away from the coil.
The remaining droplets evaporate in the air reducing the temperature in the box according to an enthalpy curve for the day you are doing this. = ΔE + PΔV
You might make the box deeper below the coil (I am presuming the fan is drawing air out), to give the air more time to cool off.
B.E.

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

 

[cranky108]

The water does evaprate, and that change of state requires lots of energy, but if you do not reduce the humidity levels the evaporations will slow down. So by moving dry air into, or humid air out, you allow the process of evaporation to continue. You could use a closed cycle, but that would require much more volume to cool the humid air. But if water 9or other fluid), is cheep, there is no reason to use a closed system.

Notice that modern AC systems work simularly, except the compressor makes the system much smaller. This is a closed system.

The change of state of the water or fluid is one of the keys to reducing the size of the exchange of heat.

Fact is leaving hot water in the air will cool it down, with no energy required to help it. The question is size and time required to do that.

 

[turtlemaster]

Ran the machine today, it barely cools down the water,might as well leave it standing for a few minutes. anyways, I will change the flow rate and , as berkshire suggested, will make box deeper.

On a different note, I wonder if I pump a hot engine coolant (Fuchs etc.) through the tube, will I see a drastic change in temperature.

T.M.

 

[cranky108]

Heat transfer rate depends on tempeture difference, and surface area. Add fins or reduce the air/water tempeture. Changing to a coolant won't help unless you are trying to change a viscosity or state change issue.

 

[EmmanuelTop]

You won't see a drastic change in temperature. Remember that the limiting factors are (1) air flow, and (2) maximum achievable approach to the wet bulb temperature. So if you have constant air flow and relatively humid air, you can run a 1,000 degF fluid through the coil and it will not cool down any lower as compared to the fluid in the first case (speaking of total heat duty). Thermodynamics of the process has been explained in details by zdas04.

Efficiency of these systems is defined as

http://www.engineeringtoolbox.com/cooling-tower-efficiency-d_699.html

i.e. as approach to the wet bulb temperature. When looking at this equation, you can figure out how much is the maximum approach you can achieve in your device, and see if the salesmen was lying to you. Efficiency of these devices is up to a great extent a function of relative humidity of the surrounding air. They will not perform the same with 10% or 50% or 80% relative humidity. The more humid is the air, the more surface area and more air flow is needed to achieve the same heat duty.







Dejan IVANOVIC
Process Engineer, MSChE

 

[chicopee]

Here is an application of an evaporative cooler and I have seen it when I was a kid in Morocco when my dad was stationed there. A leather pouch filled with water and mounted on the front bumper of vehicles when being driven. The water would ooze thru the leather and evaporate thereby cooling the content a few degrees below atmospheric temperature. You can do the same experiment with a fan blowing air over such bag. For effectiveness the relative humidity has to be low.
If you don't want to buy such material to conduct this experiment, remember that sweat from the human body during hard work outs will cool the human body thru evaporative cooling and all the kids in you class can relate to that.

 

[georgeverghese]

The fins should help, as would any other means of increasing helical coil surface area.

Also some kind of packed bed to increase contact time and contact surface between upgoing air and CW travelling downwards will help to chill the water some more. Obviously, if you ran this device on a humid day, you wouldnt get any chilling at all.

 

[turtlemaster]

Berkshire, I made the box deeper, the thermocouples don't display any difference at 47*C but dipping my fingers in both the tanks does give a sense of cooling. I will get new sensors next week.

Also, if I pump air through the tubing(instead of water), and use the same setup, would I need desiccant pads to dry it even further?

T.M.

 

[berkshire]

Turtlemaster
I do not understand your question about pumping air through the tubing. Are you talking about the tubing for the water spray, or the tubing for the heat exchanger. Something in this is changed from your original question. If you are pumping air through your heat exchanger coil, then you have a medium that is far less dense than your original liquid. Depending on the humidity of the air entering the coil, there is a chance that that air inside the coil would shed its moisture which would condense on the inside of the coil, the exiting air would then as it rewarms to room temperature have lower relative humidity.
B.E.

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

 

[turtlemaster]

berkshire, you correctly guessed it. Was carrying a thought experiment with that, but you explained it well.

Thanks.

T.M.

 

[gruntguru]

A diagram would be useful. Sounds a bit like a cooling tower with a hot coil half way up. If so this is not the best way to remove heat from the coil for two reasons
1. The outer surface of the coil does not have sufficient area to convect to the gas.
2. Much of the evaporative cooling is occurring below the coil where it can't help to cool it.

Re-arrange the system so that the water spray falls all the way through the rising air column and collects in a pan at the bottom. Submerge your hot coil in the cool water that collects in the pan. The water from the pan can be pumped back up to the spray.

je suis charlie