Evaporative cooler perfomance

[TRASH101]

Hello everyone

I hope someone can help me with this.

I need to be able to calculate the air off dry & wet bulb temps from known air on temps and volume versus water temp. from a (none specifc) swamp cooler.

Anyone got a formula or software for this?

Thanks

[Abby Normal]

Plot outside air condition on a pyschrometric chart and determine wet bulb of the entering air

The entire process neglecting the heat from the fan and motor goes along the wet bulb line of the entering air.

This means neglecting the fan and motor heat the leaving wet bulb is the same as the entering wet bulb.

If you completely saturate the air, the leaving dry bulb is also equal to the entering wet bulb.

Typically the leaving dry bulb would get cooled by " 80% of the difference between the entering dry bulb and wet bulb"

Suppose outdoor air at 83F dry bulb and 60F wet bulb was drawn into a unit.

Typically you would try to cool the dry bulb off by 0.8 x (83-60) = 18.4F, so you would shoot for a supply temperature of 83-18.4 = 64.6F

So in this instance the supply air would be 64.6F dry bulb and 60F wet bulb. However the draw through fan could heat the air up by perhaps 0.5F, this increases the dry bulb by simple math but will also increase the wet bulb.

You need to move a lot of air to get the sensible cooling done as 64.6 F air is not overly cool, but once you work out your air flow you would divide this flow by the specific volume of the supply air.

This gives you a mass flow rate of air and you would then multiply by the change in humidity ratio of the entering and leaving air to get how much water you would be using per unit time.

[Abby Normal]

you need to give air a relief path out of the structure with evaporative cooling, such as open windows, a dedicatied damper/louver or perhaps "out through the kitchen hood" if this is a kitchen make up air application.

It is a process that uses a lot of outdoor air, so you need the relief to avoid humidity building up in the building.

If you do not use outside air and just recirculate the air, you are now functioning as a humidifier

[TRASH101]

Thanks for the reply Abby

It has become a little clearer but I am still troubled by one thing.

No matter how I look at a psychrometric chart I cant see how it is possible to get a decrease in the specific enthalpy when your adding further enthalpy gains with the evaporated water.



It seems to me that for any sensible cooling the real picture is energy gains.

Is this true??????

[Abby Normal]

you are converting sensible heat to latent heat. The process follows a constant wet bulb line.

Wet bulb line is damn near parallel to an enthalpy line

You supply air to the space cooler than what you want to maintain the space at so it gives you sensible cooling.

[TRASH101]

Thanks for bearing with me on this Abby

What I've concluded so far is that:

a) you'll never get a decrease in enthalpy

b) depnding on the "air on wet bulb" - "air off dry bulb" the enthalpy gain is insignificant - small

c) It is not only limited by "air on" qaulity, the high dew point is also a factor

d) It makes a damn good humidifier assuming you have the space

In summary, although large sensible duties can be achieved (depending on "air on" db & wb) the combined duty will be less than 0.

Is that about right?

All we need is a moisture elimination system and then we got a proper cooler

[Abby Normal]

It works by moving a higher volume of air than mechanical cooling.

It works better in a dryer climate and you have to use outdoor air for it.

When done right in a proper climate, you let air pressure relieve outside, it takes mositure with it and I doubt the space breaks 60% RH. A central supply and then windows cracked open in bedrooms etc, gives air a path out and encourages air distribution

In less than ideal climates it can still be useful on commercial kitchen make up air, industrial applications.

any enthalpy change is insignificant and has a lot to do with a changing mass flow rate.

When you apply it to recirculated return air, it becomes a pure humidifier