Misting Coolers

you are atomizing the water. This process is simular to the refrigeration process. Do you have any more information such as what you need to achieve here. p911

Don't think of it as temperature, think of it as energy levels. Warmer water has a higher energy level than cold, it evaporates faster right? If you start with water at a higher energy level even if atomized at the same rate as colder water it will vaporize at a faster rate and less will hit your target and evaporate away causing the target to drop it's temperature.

There comes a point of saturation where you can have more water than is able to evaporate away as quickly as it is applied where the water may stabilize or even increase the temperature of your target.

The point I think you are after is warmer water will still have a cooling effect as long as it is not applied too heavily.

Drew

As the water mist evaporates, it cools both itself and the air. The more it has to cool itself, the less cooling capacity remains for the air. This results in two choices: 1) prechill the water, which may require a relatively expensive refrigeration process; 2) Use more water mist, which may require a somewhat larger pump and several more nozzles in your array. I think Option 2 is less expensive, but I haven't checked closely. (That would require some more info about desired capacity, water temperature available, wet-bulb design temperature at your location, etc.)

smaller the particle......larger the surface area in reference to volume.

I was at Prince Sultan Air Base in '98, we could have benefited from a mist cooling system there. As I recall, the four months I was there it got up to 120°F (July to Nov), the air was very dry. The only rainstorm I observed only a scattering of drops actually hit the dirt.

Drew

I was surprised how well they worked in Vegas. High 90's here right now, humidity running from 80-90%, nothing evaporates.

Thanks everybody for the excellent replies..i am now fully convinced that precooling water is not required in misting coolers.

cheers

The heat energy absorbed by the water mist will be the same as the energy needed to change the water to a vapor. That is why it is a very effective and economical method in a hot and dry climate for cooling. It takes a significant amount of heat energy absorbed to change water to a vapor. It is much cheaper to evaporate water than to produce the same amount of cooling mechanically. It is a change of state problem when determining how much of a cooling effect is achieved.

There are very efficient systems that fog roofs. Check the Internet for manufacturers. This uses water evaporation to cool the roofs down--- that stops the heat from penetrating into the building. This is a very efficient way to cool down single story warehouse and manufacturing buildings. It eliminates the need for many tons of mechanical cooling. A real green idea that has been around for many years.

One of the most important factors often overlooked is the "air change rate" required. Even if you are using the sprays outdoors on a patio or in stadium you may need a fan of sorts to keep the air moving or you will end up with a "bowl" of very hot very humid air that will condensate at night, soaking everything.

In a building the same applies. When using evaporative cooling ALWAYS maintain at least 5 air changes per hour. That can often be achieved using wind driven roof turbines that will extract the humid air without using electricity. That said, the air change rate should match the heat-load of the application, be it people of heat generating processes, plus the heat-load of the building itself.

The only challenge with sprays is that they cannot differentiate between high and low humidity periods. In high humidity periods there is a strong possibility that the area will be saturated because the water droplets cannot evaporate, which could damage structure or stock in the area. A safer system is using wetted filter pads like CELLdec from Munters, or Aspen wood fibre mats that water is circulated through and a fan drives the air through the filter. Provided your airspeed does not exceed the design flow of the filter you will never end up with droplets of water messing things up.

I have an evaporative cooler, in this house & have set up more than a few of this type,

these coolers generally have adjustable pulley's [sheaves], some have 2-speed motors. The fans are usually rated around 4000cfm, for a 2000sf house. I will adjust the speed down to around 20% & run on the lowest speed. The air needs to move across the pads slowly enough to have good effect

synthetic fiber pads are the easiest to deal with, paper the cheapest, shaved wood [excelcesior] the most effective.

There are also large single pad models the cooler media being a series of cardboard baffles, more permant, but less effective at removing the dust from the air.

the best method for using an evaporative cooler is to

run 1st thing in the morning, with a few windows open

as the heat climbs progressively close windows, until in the hottest part of the day

partially open a window in the room you are actually going to be

for these kind of coolers dumping a couple of bags of ice into the sump makes a noticeable difference.

i've seen indirect commercial models use standard refrigeration as a prechiller

I can see why this wouldn't work on a mister system as the quantity of water is much lower

I think the key to your experience here is the comment that you run the fans at their lowest speed. This seems to me to suggest that the air / water in your operation gets close to equilibrium ie the the theoretical coolest temp that can be achieved. It also means that the air is going to be close to 100% humidity.

I love the refrigeration technique of dumping in some bags of ice - this will clearly cool the water and as you have an equilibrium system reset the equilibrium at a lower temp which is why you see the effect.

With the mist systems the relatively small amounts of water mean that they are a long way from equilibrium and so resetting the theoreticallowest temp makes little difference to the actual temperature achieved.

With your house I wonder if you would achieve more cooling - although at the cost of a higher ambient temp by running the fans faster. However seeing as the system works for you dont fix it!

This discussion has brought back to mind the punkah wallah from the very old BBC series 'It Ain't Half Hot Mum' which followed a british army concert party in India / Burma. Wallah is an Indian word for 'someone who does something' ie the chai wallah brings the chai (Indian tea). A punkah is a large fan which was moved manually - the punkah wallah sits in the background pulling on a string to move the fan. Often there would be a water source near fan for a simple evaporative cooling. Like the design of houses that the Moors brought to Europe the idea originates in the Arab peninsula I believe.

part of the total effect is to cool off the house as much as possible in the morning

running the fan at a higher speed reduces the amount of time each given volume of air spends on the pads [retention time]. as you say equilibrium. once you pass that point it's just a fan blowing hot air in the house. Locating the cooler in the shade, also helps a bit. Leaving the water pump running on the pads, help reduce the growth of various nasties.

the bags of ice were a daily ritual for a portable unit when I was working in a steel building. 3 of us would split the cost of 15#s of ice at around 2pm on days when the temp was at it's highest

I've used the mist systems, they can be tricky to set up, it's definitely important to make sure the water is low in mineral content & the high pressure systems are more effective. the size of the nozzle & amount of air flow are critical. The worst case scenario can leave a wet slippery floor. There needs to be a off delay for the fans

In general I think precooling isnt that beneficial but might might be more appreciated in practical terms.

In energy terms what you are doing is using the sensible heat of the ait to evaporate the water thus cooling the air and turning the water from liquid to vapour. This takes ~2100 kJ per kg of water. If you precool the water there will be some sensible heating of the water before it evaporates at the rate of 4.2 kJ/kg°C so you can see that precooling only adds 0.2% per °C to the cooling effect for all the extra work you have to do.

Having said that people would probably prefer to feel cold water mists as the intuitive reaction is that cold water is much better and clearlyif you only look at sensible heat (ie without evaporation) it is much better. But as soon as you factor in the evaporation there is little difference.

Pre-cooled water might produce a slight amount of sensible cooling of the air and therefore a depression of the wet bulb which will give you a just a slightly lower air temperature from the evaporative cooling effect. The added expense and complication of mechanically cooling the water would be hard to justify.

He is talking about a dry fog system.

Something I am interested in for next summer!

The amount of heat energy absorbed to raise the cooled water to just below the evaporation point is insignificant compared to the amount of energy absorb when the water changes state and evaporates. Cooling the water may cause some decrease in the air temperature. The decrease will be little and not enough to warrant the additional cost of cooling it.