Moisture content at supply diffuser inside room

[Hasny]

Is it true that the moisture content of the air when it enters the conditioned room via supply air diffuser would be around 90%. I have designed the room at 50% RH but am bit confused after one of my college insiste that when cool air leaves the cooling coil of the AHU, it has 100% moisture content and by the time it reaches the room after travelling through the length of the duct, moisture content would remain around 90%. Something hard for me to agree. Please share your experience. Thanks

[beenthere]

Yes, the air coming out of the supply will be 80%+/_RH. Of course when that air temp rises to room temp, its RH will be much lower.

[thermojohn]

This is the perfect time to learn about the properties of air and how to read a psychrometric chart.

Get a wet bulb thermometer, and take a reading with it of the air while your air conditioner has been running. Use a dry bulb thermometer and take another reading simultaniously. Plotting those two numbers on that chart will give you the actual relative humidity. It will also show you that when the air is heated up to room temperature, the rh drops.

Once you understand how the properties of air are affected by the process of refrigeration, you will be hooked and want to learn more.

[teddy bear]

Also learning about the actual moisture content will help you understand %RH. The common moisture content of air is the dew point, the lbs. of moisture per lb. of air, and the grains of moisture per lb. of air.
An example of descriptions of moisture content of air are:
100%RH @ 32^F = 32^F Dew point = 25 grains of moisture per lb. of air = 24%RH@70^F= 50^F WB @ 70^F
There are 7,000 grains of moisture in a 1 lb. of water. There are many discriptions of this on the web.
Regards TB

[Hasny]

@beenthere,
Still 80% sounds too high. Don't you think if air with that % age of moist content will enter through diffuser inside the room, would make most of the diffusers look like sweating with moist. Also when you say RH afterwards comes down while mixing with comparatively warmer air of the room, meaning that both of the airs, one that enters the room and one that is already inside the room have big variation in their temperature. While I personally think that there is not much difference in both airs temperatures, and thus the air that enters through diffuser merely replaces the already existing air of the room. Do correct me if I sound wrong.
Thanks to thermojohm and teddybear for sharing useful theory relating to the subject.

[thermojohn]

Did not realize this was AOP forum as I was on my phone... Thought I was responding to a tech in the trade. I can't expect someone outside the trade to have a wet bulb thermometer and relate to a psychrometric chart.

About the diffuser sweating in the room due to the high RH.... Teddy Bear hit on a very important perspective. Dew point. If the temperature of the diffuser never reaches the dew point, it will not sweat. Relative humidity is humidity 'relative' to the temperature of the air. There is a lot to this... But it's simple once you get it.

A common condition of the discharge air at your diffuser outlet when the air conditioner is operating is 55° F, and 85% RH. Our psychrometric chart tells us that air is at 52° dew point. So if the conditions at your grille reach 52° F dew point or higher, your grille will sweat.

55° f dew point... A snapshot of room conditions that will make your diffuser sweat:

100° F @ 23% RH
90° F @ 30% RH
80° F @ 41% RH
75° F @ 50% RH This is why the bathroom diffuser and mirror sweats when someone takes a shower. The dew point always spikes at these typical bathroom conditions.
70° F @ 60% RH
60° F @ 81% RH

And that's it... Below 55° F, there won't be any sweating at all.

[beenthere]

@beenthere,
Still 80% sounds too high. Don't you think if air with that % age of moist content will enter through diffuser inside the room, would make most of the diffusers look like sweating with moist. Also when you say RH afterwards comes down while mixing with comparatively warmer air of the room, meaning that both of the airs, one that enters the room and one that is already inside the room have big variation in their temperature. While I personally think that there is not much difference in both airs temperatures, and thus the air that enters through diffuser merely replaces the already existing air of the room. Do correct me if I sound wrong.
Thanks to thermojohm and teddybear for sharing useful theory relating to the subject.

When the A/C is running the air when it leaves the evap coil may be at 50°F, and 80%RH. The diffusers ill also be at about the same temp. And the dew point for that air will be 43°F(rounded up), so the diffusers won't sweat because the RH is at 80%.

I believe you are over looking what RH is. Its the amount of moisture the air can hold at a certain temp. 50°F air at 80%RH only has 42.79 grains of moisture in it per pound of air. 70°F 50%RH air has 54.70 grains of moisture in it per pound of air. So even though the air coming from the supply has a higher RH in cooling mode, it has less moisture in it.

[Hasny]

Thanks a lot thermojohn and beenthere for explaining it so nicely. I am pretty clear now. Little extension to the same subject.....I have also been designing stream humidifiers / humidistats in the discharge side of the supply air ducts of AHU which served in an area with very low humidity levels. And it was done to ensure that the cool air reaches the room at the desired 50% RH, not that a higher humidity level was required. Don't you think based upon your explanation, I could have achieved the desired humidity levels inside the room even without the use of humidifiers.

[beenthere]

Probably. unless your in an area that has extremely low outdoor humidity. There are areas that need a humidifier to maintain RH when running the A/C. Those areas should also be running higher CFMs through the A/C coil so it doesn't remove as much moisture.

[Hasny]

Must say this thread has been a source of great info for me and gave me a chance to revise psychrometrics. Just little curious to know about adding humidifiers on the supply air way, if someone could clarify the moist air properties on its route from cooling coils upto the room with moisture being added on the way !!

[beenthere]

What would be the air properties entering the evap coil? Temp, RH?