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Lc_shi
04-07-2007, 02:43 AM
Hi Sirs
Energy saving and environment protection make the indirect/direct evaporative cooling tech revive in China and there're lot of swamp coolers manufacturers. How about the future of this tech application and can it be alternatives for some conventional compressed AC system? Hope your comment and share your experience.

thanks and regards
LC

Samarjit Sen
04-07-2007, 03:58 PM
Hi LC,

The Evaporaive Cooling has been there for many a years. It was put to great use during IInd World War. With the electricity charges going up every where, this technology does have a future. But this system is very efficient during the hot and dry months. I have seen a couple of large industrial and commercial application. If the system is designed properly they work wonders.

theboss
12-07-2007, 01:26 AM
using an aluminum sheet metal honeycomb structure you can suck air through it in two directions using two separate blower. Now introduce water jet to this and forced evaporation will cool the honeycomb metal which will in turn absorb heat from air passing through it. Evaporative cooling without raising indoor humidity.

If cooling is proportional to rate of evaporation then once can easily maintain constant rate regardless of relative humidity in forced evaporation scenario by varying the amount of air moving over the medium.

rough schematic of idea is attached.

Samarjit Sen
12-07-2007, 02:19 AM
Either there is a wet medium or a water spray banks through which the fresh air is sucked in and discharged to the area to be cooled. It is a very effective system to provide comfort conditions inside a room.

Abby Normal
12-07-2007, 02:54 AM
its like combining an evaporative cooler and an HRV

mohamed khamis
12-07-2007, 12:20 PM
using an aluminum sheet metal honeycomb structure you can suck air through it in two directions using two separate blower. Now introduce water jet to this and forced evaporation will cool the honeycomb metal which will in turn absorb heat from air passing through it. Evaporative cooling without raising indoor humidity.

If cooling is proportional to rate of evaporation then once can easily maintain constant rate regardless of relative humidity in forced evaporation scenario by varying the amount of air moving over the medium.

rough schematic of idea is attached.

Hi guys

The evaporative cooling as it is known is very effective in hot and dry climates because the wet bulb temperature is rater small. However, in hot humid country like Malaysia it is ineffective but have idea and i would like to discuss here in this forum and any thought is highly appreciated. My idea is during the sunny time the dry bulb is still high 32 to 34C and also the humidity is of 72 to 78% therefore at this time i will pass the fresh air over a liquid desiccant this will absorb humidity and the same time heat the air up after that this air is cooled by evaporative cooling either direct or indirect or use the honeycomb as theboss said. The efficiency of desiccant liquid is regained by energizing heater immersed inside the liquid each when the set point temperature inside the room is settled. In the night the ambient drops to 24C and 26C but the humidity pops up to 80 to 90% Another heater or the same heater will work for a certain time to ensure the liquid is warmed each certain interval to enhance the efficiency of dehumidifications and assist to heat the air to some extent which simulate the daytime case and then the air is cooled evaporatively. this heater will be energized by ambient air sensor. Any thought will greatly appreciated.

Cheers:)

Ravi
13-07-2007, 05:17 AM
Dehumidification by desiccants and then using direct evaporative cooling is sort of constructing a perpetual motion machine. As both the processes are isenthalpic(or constant wet bulb), you will be just oscillating on a straight line from your start condition to end condition.

Indirect evaporative cooling is a bit different and you can cool air sensibly(constant humidity ratio). But even this doesn't seem to make sense as I took up one sample calculation.

The conditions are 34C(93.2F) and 78%RH. Generally, direct evaporative coolers can drop air temperature upto 75 to 80% wet bulb depression. For calculation sake I took the same for indirect evaporative cooling also, but this is not the case in practice. If you want to reduce your air temperature to 24C (75.2F) then the WBT of dehumidified air should be 93.2-[(93.2-75.2)/0.75] = 66.5F. RH corresponding to 93.2F DB and 66.5F WB is 24.1%. However, you can't reduce the WBT of the air by desiccant dehumidification. So, though you reduce the humidity ration by going upto 24.1%, the WBT remains constant. At this point, your air DB will go up by about 122F (50C). When you sensibly cool it by indirect evaporation, the RH corresponding to 93F(34C) is 57%, 85F (29.4C) is 72%, 80F (26.6F) is 85%.

Drykor is one company that uses liquid desiccant systems (ofcourse Kathabar is the pioneer) but they use DX system for sensible cooling. LiCl is very corrosive and I have seen almost all installations where ducts were corroded like hell despite the guarantee given by the manufacturers. I myself replaced two such systems with solid desiccant system.

My final suggestion will be to drop this idea for the value of time and money.

Good luck,

mohamed khamis
13-07-2007, 09:55 AM
My final suggestion will be to drop this idea for the value of time and money.


Well that's ok..i do appreciate ur view but i have some questions i want to discuss with u if u don not mind. firstly i swear to u i have read ur post more than 3 times
but it sounds it has very sophisticated technical points i could not understand it well plus u sometimes use SI and sometimes British and sometimes the both. Secondly it sounds u did not understand what the order of the proposed idea i mentioned "adsorption then evaporative cooling". Ok now my questions and pls bear with me:




The conditions are 34C(93.2F) and 78%RH. Generally, direct evaporative coolers can drop air temperature upto 75 to 80% wet bulb depression.


One-by-one, u mentioned that "direct evaporative coolers can drop air temperature upto 75 to 80% wet bulb depression" and this is true. firstly as aforementioned i will make the process of desiccant " adsorption" first then the evaporative cooling. however, i will walk with ur calculations.




For calculation sake I took the same for indirect evaporative cooling also, but this is not the case in practice. If you want to reduce your air temperature to 24C (75.2F) then the WBT of dehumidified air should be 93.2-[(93.2-75.2)/0.75] = 66.5F.


Firstly i think u meant that u want to say the dry bulb temperature of the humidified air is 66.5, right or wrong?. Secondly it is not possible obtain 24C from direct evaporative cooling at this conditions which is 34C and 78% (by the way this conditions are also fictitious in Malaysia because take it as a general rule of thumb the maximum dbt corresponds to minimum relative humidity for the surrounding, so it supposed to say 34C and 72% as i cited in the previous post) because the wet bulb of this condition is 30.5C and u demonstrated by this great phrase
"Dehumidification by desiccants and then using direct evaporative cooling is sort of constructing a perpetual motion machine. As both the processes are isenthalpic(or constant wet bulb), you will be just oscillating on a straight line from your start condition to end condition." . Therefore, tell me how can i get 24 and the min. temperature is of 30.5C and by the way, ur calculation product is 79.7 F (26.5C) not 66.5 F (19.2 C).





RH corresponding to 93.2F DB and 66.5F WB is 24.1%.



I mince my self to find out an explained answer to how air at 34c and 78% relative humidity when it humidified adiabatically "evaporative humidity" the exit air relative humidity is decreased and becomes 24.1% " it does make sense at all, am i right or wrong?".

As u mentioned the wet bulb is constant, therefore by ur calculation the outlet air conditions from the evaporative cooler by 75% depression is

= (34 - (34-30.5)*0.75) = 31.37C and this corresponding to Rh = 91.8% and this will exacerbates the cooling in Malaysia, so i repeat my idea is dehumidification with heating "by adsorption" then evaporative cooling.



However, you can't reduce the WBT of the air by desiccant dehumidification. So, though you reduce the humidity ration by going upto 24.1%, the WBT remains constant. At this point, your air DB will go up by about 122F (50C).

Really i can not understand this.....:confused::confused::confused::confused:




Drykor is one company that uses liquid desiccant systems (ofcourse Kathabar is the pioneer) but they use DX system for sensible cooling. LiCl is very corrosive and I have seen almost all installations where ducts were corroded like hell despite the guarantee given by the manufacturers. I myself replaced two such systems with solid desiccant system. Good luck,

ok this can be taken into account and once again i appreciate greatly ur participation and thought.

Ravi
14-07-2007, 07:45 AM
Hi Mohamed,

I did undersand that you meant first dehumidification and then evaporative cooling (aka humidification).

I will try to explain the process without using numbers so that the issue at hand will not be too complicated.

The processes of humidification and dehumidification (by desiccants) are isenthalpic. That means the enthalpy of air essentially remains constant during the process. During humidification, when you spray water into the air, the latent heat required to convert water droplets into water vapor is absorbed from air itself and thus sensible heat of air gets reduced. (Only exception to this is steam humidification where latent heat is already supplied). When you remove moisture out of air, air absorbs the heat of condensation and the sensible heat goes up. Part of this heat gets added into the desiccant as heat of dilution, incase of liquid desiccants.

You can plot the dehumidification process on a psychro chart by identifying the initial condition and then going down and to the right (to that of initial state point) along the constant enthalpy line. Let us say you have identified the two state points as A and B. Now the moisture is removed from the air and you are going for evaporative cooling. Starting from point B, you add moisture to the air and then you will reach point A again as the process of evaporative cooling is also isenthalpic. So, that is why I said you will be oscillating on a same line.

On the other hand, indirect evaporative cooling is constant humidity ratio process (i.e only sensible cooling). This can be plotted on a chart by a horizontal straight line from the initial state point. In the example of my first post, I was trying to show you various dry bulb temperatures and RH conditions that you can achieve.

Considering the fictituous 34C as initial dry bulb temperature and if you want to reduce it to 24C, if I consider the maximum wet bulb depression as 70%, then this process is possible when the WBT of 34C DBT air should be 19.71C. Now, the total wet bulb depression is 34-19.71 = 14.29C. If you consider 70% of this then 14.29*70/100 = 10C. So, you can reduce air DB by 10C when the initial conditions are 34C DBT and 19.71 WBT. The RH corresponding to 34C DBT and 19.71 WBT is about 26%. This is the RH condition that you should acheive by dehumidification so that you can cool your air by indirect evaporative cooling.

After making these assumptions, I tried to calculate to what extent I should dry the air by desiccants. Considering 34C and 78% RH, I marked it on psychro chart. My second condition is 26% RH. Now, drawing a line from 34C and 78% RH to meet 26% RH line gives you the end process of dehumidification. This condition is 47.5C DBT and 31C WBT. Draw a horizontal line from this point and you will get various DBT and RH conditions and you can stop at any point favourable to you. But, following this line, I found out the following state conditions. DBT of 30C has RH of about 75% and DBT of 25C has RH of 100%.

I hope I answered your questions, at least partly, and I didn't mean anything offensive in my earlier post.

The link below gives you some good graphics about both direct and indirect evaporative cooling.
http://www.wescorhvac.com/EvapWhitePaper.htm

Good luck,

Abby Normal
16-07-2007, 02:24 AM
The 'coolerdaos' tend try and saturated the outside air and then running it through a crossflow HX. Room air passes through, gets sensibly cooled without the mositure.

I went to the 2006 ASHRAE show, was a guy there promoting evaporative cooling. I was commenting about how I am now in a hot humid environment, and it was useless. He pointed out that roof top equipment is subjected to high ambients still, and evapoartively cooling condenser air has some merit.

Hot humid climate you get 27 to 28C dewpoints, but in the persian gulf they are 33 to 35C, sort of mind boogling that deserts are the most humid.

mohamed khamis
16-07-2007, 08:16 AM
Hi Ravi

I repeat my thanks to you for the calculations and this website. I have realized what u wanted to say and by making various throughout and calculations i found that the idea could not be effective for some what extent. I found from rough calculation i need to use a heat exchanger with effectiveness of 90% and the water which will cool the air indirectly should be cooled by direct evaporative in a separate well-insulated water spray (to avoid sunrise temperature increment" then it is sprained over the dehumidified air after "dehumidification process". Anyhow this is just an idea has been come up when "theboss" offered the idea of honeycomb heat exchanger (which may satisfy this condition of 90% heat transfer effectiveness", May when i finish my study i think more deeply about it. Once again i do appreciate ur patience and kindness with me.

Best regards:)

Darshi
17-07-2007, 08:11 AM
Dear sirs, it is becoming a very enlightened discussion on the alternative cooling technologies.For a good comparison it will be very helpfull if some one can compare indirect /direct evaporative cooler with
Sensible effectiveness 60%
Adiabatic effectiveness 90%
and conventional air cooled split air conditioner with highest EER :
Design Data
Ambient: 47 deg C and 20% Rh
Space conditions: 26 Deg C and 60% Rh
Heat load 60000 Btu/hr.
Application - Disco and Bar
Occupancy 20 persons(smoking)
Fresh air 500 cfm
SHF 0.8
comparison could include Energy input/ water input/EER/carbon emissions/IAQ
Above summer design data is for many locations in India , Pakistan ,Iran and Iraq

satoof
12-12-2007, 11:48 AM
Hey all;
please, whom have any idea about the wick materials used for the liquid desiccant distribution, instead of cotton. any information will be valuable; books, web address..........
thanks

Peter_1
12-12-2007, 11:23 PM
Ravi, you did it very well, posting both SI an IP units (the other units between brackets)
So can you post be read by everyone.
;):cool: