Evaporative Cooling Ideas
Evaporative Cooling Ideas
(OP)
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.
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.





RE: Evaporative Cooling Ideas
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
RE: Evaporative Cooling Ideas
So the efficient application is in materials to construct and in fluid movement required, provided water (or other evaporative fluid) is cheep.
RE: Evaporative Cooling Ideas
Will keep posted,
T.M.
RE: Evaporative Cooling Ideas
RE: Evaporative Cooling Ideas
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.
RE: Evaporative Cooling Ideas
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.
RE: Evaporative Cooling Ideas
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.
RE: Evaporative Cooling Ideas
RE: Evaporative Cooling Ideas
Efficiency of these systems is defined as http://www.engineeringtoolbox.com/cooling-tower-ef... 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
RE: Evaporative Cooling Ideas
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.
RE: Evaporative Cooling Ideas
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.
RE: Evaporative Cooling Ideas
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.
RE: Evaporative Cooling Ideas
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.
RE: Evaporative Cooling Ideas
Thanks.
T.M.
RE: Evaporative Cooling Ideas
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