Limits of Evaporative cooling on air 1

[woody81]

I am undertaking a basic cost benefit study of an air cooled condenser. Part of this includes looking at the maximum theoretical benefit of a spray/misting cooling system. Based on evaporative cooling, I am trying to provide data showing theoretical cooling of air from a start temperature for a range of Relative humidity's.

e.g. plot possible temperature change of air at 30 degC from 0-100% Rel Humidity start points.
(Obviously start at RH 100% = 0 degC temp change, but start at RH 50% = ? temp change)

Can I do this from steam tables and saturated air pressures?
Trawling the I-net has confused me. Any thoughts?

 

[woody81]

Just for a little further clarification. The table on this webpage is almost ideal (

http://www.engineeringtoolbox.com/humidity-measurement-d_561.html)

, although I want to extend its range. What I am really after is how to mathmatically generate this table?

 

[AbbyNormal]

Maybe review how evaporative cooling process follows the constant wet bulb line of the entering air.

In your case, you want to lower the dry bulb temperature of the condenser air.

Sometimes when a condenser is on a roof the air can be a lot hotter than the ambinet at the ground. Makes a opportunity climate for evaporative cooling even in humid climates.

Cool down that air that has been heated to 120F on the roof before it hits your condenser coil.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[AbbyNormal]

maybe work from the approach that the wet bulb stays the same, the amount of dry air in a cubic foot is reduced, displaced by water vapour and look at some saturated steam tables for the heat of vapourization of water at the wet bulb temperature. The water draws sensible heat from the air to evaporate.

So would have to look at the basics of pyschrometrics -- dry bulb temperature and the pounds of moisture per pound of dry air.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[woody81]

Yes, the recomendation of Pyschrometrics seems to be the way to go. Does anyone know where I can find pyschrometric charts for various dry bulb air temperatures without downloading software?

 

[AbbyNormal]

There are tables in the ASHRAE HOF

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[woody81]

Sorry for my ignorance - new starter. ASHRAE HOF?

 

[AbbyNormal]

American Society of Heating Refrigerating and Air Conditioning Engineers' Handbook of Fundamentals.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[quark]

Ideally, the air follows isenthalpic path. The latent heat required to vaporize the water is taken from the sensible heat of the air.

However, practically, 70 to 75% of wet bulb depression is generally considered for evaporative coolers. You can download psychrometric charts from the first two links below. The third link is a spreadsheet.

http://www.uigi.com/psychrometry.html

Psychrometric Chart
Psychrometric Spreadsheet

 

[AbbyNormal]

Wet bulb lines almost parallel to the enthalpy lines. Munters taught me to shoot for 80% depression.

I always like the 11x17 trane chart, that is laminated. Basic examples on the back of it, easy to read the dewpoint from.

Take the "V" out of HVAC and you are left with a HAC(k) job.

 

[Golestan]

Hi,
Another suggestion is that if you agree with the data on the tables, you could use excell, to give you an equation (best curve fit) for each set of data in terms of simple X and Y.

Then you can use the equation in your modeling calculations.
regards,