3 replies to this topic

[Propacket]

Dear All,

A question related to evaporative cooling. We have following air data:
Dry bulb temperature=131°F
Relative Humidity=60-90%
Atmospheric Pressure=14.43 psia
We want to cool a process stream to 105°F employing an air cooler. Keeping in view the ambient temperature, we can’t cool the process stream to 105°F unless we make use of evaporative cooing. Therefore, ewe contacted a renowned vendor dealing in evaporative coolers. He, the vendor, proposed that air under current set of conditions, air has a wet bulb temperature of 90°F. Hence, by using 15°F approach, we can easily achieve the required process temperature (105°F).
When I checked it with an online psychometric calculator (131°F dry bulb temperature is not available in most of psychometric charts available on internet), it gives following results.
With 60% RH, Wet Bulb temp=114.4°F
With 90% RH, Wet Bulb temp=127.05°F
Even with 60% RH, we can achieve a minimum of 114.4°F. Can anyone comment how the vendor is able to achieve 90°F wet bulb temperature?
Here is the online psychometric calculator i have used.
http://www.sugartech...ychro/index.php

Thanks All

Edited by P.Engr, 04 November 2010 - 05:55 AM.

[Zauberberg]

I would revert back to the vendor and confirm if we are on the same line, regarding humidity calculations. He might have misinterpreted your input data.

For 90 degF wet bulb temperature, and 131 degF dry bulb temperature, relative humidity equals to ~30% and not 60 or 90%.

[kkala]

Using the psychrometric card of Perry, 7th edition (Fig 12-3, Borometric pressure 760 mm Hg, differing a bit to the referenced), and starting from dry bulb of 131 oF:
-- For relative humidity (RH) 60%, wet bulb=115 oF, saturation at 113 oF.
-- For RH 90%, wet bulb=130 oC, saturation ~129 oF.
Both agree fairly well to the results of the Web calculator written.
Adiabatic humidification will follow the wet bulb temperature line, till saturation temperature is same as wet bulb at 100% humidity. Water evaporation alone cannot realize lower temperatures.
The vendor may mean that the wet bulb should be reduced to 90 oF, which results in ~60% RH at the desired dry bulb of 105 oF. But this could concern air conditioning, and would not be obtained through evaporative cooling. E.g. one could cool air to 90 oF (to remove excess water), then heat to 105 oC.
As Zauberberg says, you had better revert to the vendor for explanations.

Edited by kkala, 07 November 2010 - 02:09 PM.

[Propacket]

Response from the vendor is dumbfound. It's hard to believe that a renowned vendor specialist in evaporative coolers can do such a thing that he even didn't see the humidity data and proposed the wet bulb temperature merely based on 15°F approach as is normally recommended for air coolers. Our required process temperature was 105°F. Considering 15°F approach, he proposed 90°F wet bulb temperature without knowing what the actual wet bulb temperature is? He might have presumed that we have completed air side calculations on our side but as far as i know, air side calculations are always performed by the vendor.

Edited by P.Engr, 08 November 2010 - 06:22 AM.