Moisture content of cool air at supply air diffuser in room! 1

[Hasni2]

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

 

[Drazen]

your colleague is right, but you need to study psychrometrics to understand it, some post itself will not help.

it is matter of theoretical knowledge, not the experience. to understand it by experience you would need to duplicate mental path of one-century scientist who observed, analyzed and established field of thermodynamics called humid air.

it was not once that i had to redesign complete ahu installed by "experienced" contractor who copy-pasted it from some of his previous projects. and client calls when seeing supply duct flooded with water.

 

[KiwiMace]

When you cool air down, the RH goes up. When you mix that cool air with the warm air in the room, the air warms up and the RH drops back down. Follow a horizontal line from left to right on a psychrometric chart and look where it crosses the lines of RH.

 

[Hasni2]

@Drazen
I still think that what you have said may theoretically be correct, but practically I can't imagine it happening at the supply air diffuser.90% moist air at diffuser means that surely you will find that diffuser sweating with moist. Just how I think.

@KiwiMace
In a running system where temperature is kinda maintained in a room at 25 deg C, the air that enters the room through diffuser simply replaces the existing air which can not be called as warm air, hence there remains no question of 90% RH just coming back to 50% RH. Hard to believe. Again it can be theoretically correct but practically it rarely happens. Further do you think an air that has started its journey from AHU at 100% moisture content does lose only 10 % moisture content while travelling the whole length of the duct thus reaching at diffuser with 90% RH. Do correct me if I am wrong. Thanks

 

[jkauwale]

Your colleague is correct. It might be helpful to grab a psychrometric chart and plot the points of your air conditioning process. For example, plot (1) incoming AHU conditions, (2) leaving AHU conditions, (3) space conditions.

When your air leaves the AHU and travels the length of duct, how will moisture be lost? The amount of water in the air will not change. The relative humidity will change as the air is heated sensibly.

Sweating at a diffuser will occur when moist air comes into contact with a surface that is colder than the air's dew point.

Justin K, P.E.

http://www.engproguides.com

 

[KiwiMace]

Supply air is around 12 degC, not 25. It doesn't replace the air, it mixes with it. The air leaving the coil is close to saturated, but as cold air absorbs heat the air will only get warmer along the duct therefore reducing RH. Seriously, have you looked at the psych chart yet?

 

[317069]

What do you mean by moisture content?

 

[willard3]

Rather than trying to guess the psychrometric process and second guessing people who really do understand it, get a book that explains it and read.

 

[DRWeig]

I've dealt with diffuser condensation problems quite a few times. Measurements of temperature and relative humidity of the supply air just upstream of the diffuser are sometimes the first step, since we may not know the cooling coil performance. 90% rh is quite common in chilled water systems, and something above 90% in DX systems that have colder supply air at lower air flows.

Condensation will occur when the temperature of the diffuser is significantly above the dewpoint of the supply air. At 1°-2°F below, the diffuser may have a foggy-like condensation on its surface that does not accumulate enough to drip. At 9°F below, it can begin to rain in the space and cause walls to sweat.

In very humid environments (Singapore, Hawaii, Gulf coast of Florida are areas in my experience), it is important sometimes to reset the supply air temperature upwards to keep its dewpoint near the dry-bulb temperature of the room and its solid components like diffusers, ceiling tees, and walls. This is especially common on morning start-up after a period of night setback. An 80°F room with 60% rh (dewpoint 65°F) and a supply air temperature of 55°F with 90% rh (dewpoint 56°F) are not a good mix at all. I guess it might be a good thing if the room is a greenhouse full of plants. At 75°F and 50% rh, the dewpoint (56°F) is is the same as the supply air, and the problem won't occur.

Dewpoint sensors are not too expensive these days.

Caution! Do not remove the duct insulation in an attempt to warm up the supply air and help solve the problem. You'll need to change out the ceiling tiles regularly. I have seen people try this.

Get a copy of a psychrometric chart and a copy of an HVAC manufacturer's coil performance curves. The manufacturer's data will show you leaving air conditions versus entering air conditions. You'll understand when you go through them.

Best to you,

Goober Dave

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[Compositepro]

Hasni2, you are confused about the difference between relative humidity and absolute humidity. Study the difference between these two concepts. "Moisture content" is absolute humidity not RH. When temperature of air changes due absorbing heat the RH changes but the moisture content does not.

 

[Hasni2]

Thanks a lot Drazen, KiwiMace and specially DRWeig. I am much clear now. Want to add 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.
@DRWeig - while designing HVAC systems for the area with very high humidity levels, I used to select AHUs involving sub cooling of the fresh air phenomenon. Such AHUs used to have 3 coils, cool - heat - cool.

 

[317069]

DRWeig, I have some points:
- you said that condensation is because the difference between diffuser temp and supply air tem then in you example you show that the reason is the difference between diffuser temp and room conditions.
- How can diffuser temp be 9F less than supply air temp.?
- How can we reset air supply temp when for example we use a small constant RTU or residential unit less than 5 ton?
- if I change room setting from for example 75F to 78F or 73F does it mean condensation will happen because the supply air still 55F

 

[DRWeig]

Hi 317069,

Good points.

I wasn't thinking clearly -- please pardon me, it was late at night. I meant to say that the diffuser or other objects are 9°F++ cooler than the room air dewpoint, not that the diffuser was warmer or cooler than the supply air. Of course, a diffuser's temperature can't vary much from the air that is blowing through it. I was chief engineer for a humidity-sensor manufacturer and also a controls guy, which makes that write-up above especially embarrassing. I hope my old co-workers don't see it!

To think better about your first two points: Usually, this mode of condensation is only a short-term problem on start of occupied period after unoccupied setback. The room air and the room fixtures and walls have warmed near the setback temperature, often 80°++ with a high dewpoint in the air. The diffuser, being a thin piece of metal, quickly cools down to the supply air temperature. It has the warm, humid room air induced toward it from directly below as the diffuser spews the supply air to the sides. It is then that the room air's high dewpoint of 65° or higher leads to condensation on the diffuser that might be as low as 50°F with a DX system.

If you're getting a good long coanda-effect air flow, the cold supply air running across the ceiling and down the walls may cool their surfaces quickly to below the room air dew point and do some sweating too as the warm room air swirls into them. Your idea of a cool-heat-cool to dry the supply air will certainly help. The dripping will still be there, though, if the dry-bulb of the diffuser and other solid objects is brought down too quickly and the room air is still warm and moist.

When the room air finally gets cool enough, all becomes dry again.

If the condensation problem lasts all day, the problem is usually because of a high latent load in the room. In the mornings when I have a hot shower, it rains a bit in my bathroom and the only real way to stop it is to shut off the unit. We once found windows with broken-out panes in Hawaii. That was a bad one. Kitchens can be bad, especially in Italian restaurants with huge pots of boiling pasta all day.

To your third and fourth points, it is very difficult to reset supply air on small DX units. Putting face-and-bypass in a packaged unit would be quite a challenge, I think. I'm not sure if mixing some return directly into the supply will work without causing lots of condensation at the junction point. That's a tough one.

In a commercial space like a small store, sometimes the only good solution is to eliminate setback or maybe decrease the setback room temperature setting until you find a point that will fix it. I think that if the room moisture is coming from a source in the room or a leaky envelope, monkeying with the supply air temperature won't do the job without under-cooling the space. I'm not sure how to solve those. Maybe a room dehumidifier?

A problem I never could solve happens in surgery rooms where open-heart surgery is done, it's the reverse of cold supply air and a warm room. The surgeons tell me that they want the room temperature down to as low as possible. They would like 40°F, but the best I could ever do with controls was about 50°F. Maybe a refrigeration system in the room? Anyway, at the end of the case when they're closing the patient up, they want the room up as warm as possible as rapidly as possible. Usually there are big steam coils in the air handler. The really hot supply air running around the room with all that cold metal and cold walls makes a wet mess. Somehow the nurses and doctors just deal with it.

If anybody knows a solution to the operating room one, I'd sure appreciate hearing it!

Thanks much for your kindly-put correction.



Best to you,

Goober Dave

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[317069]

DRWeig
another point
in small application less than 5 ton constant voleum units or residential when the supply air temperature is almost constant, most applications have thier non- insulated supply duct in a return air plenum(for small stores) or expoused duct in the basements (for residential)
return plenum has the same room condition.
what about this situation?

 

[DRWeig]

I will have to leave that answer to the mechanical engineers like those who have answered above. It's a bit beyond my actual expertise. They will know the condensation prevention strategies better than I do.

My experience is in adapting controls to fix drippy situations, and in sensing humidity, dewpoint, and other psychrometric properties. My work is mostly in larger buildings, as well. I don't run into uninsulated duct, supply or return, unless it is the source of a problem.

Best to you,

Goober Dave

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[sprinkler1000]

317069
The closer the air gets to its saturated point is in the coil, which is the lowest temperature point. After that, as we are in a cooling mode, the air raises temperature and the relative humidity goes down.
The problem is if you have that supply air duct uninsulated in an unconditioned plenum with fresh air intake, because this plenum air will have a lot of humidity and the duct will surely sweat.
Otherwise, sweating is rare, in my experience, unless the room has open windows and outside air is coming in.
I have installed a lot of systems with uninsulated supply ducts exposed in the room with no condesation problems. Systems were dx, not chilled water.

 

[Drazen]

as drweig pointed, most of condensation issues occur at start-up, and one other scenario that i see is wrongly calculated cooling coil, especially if sensible capacity is too large and latent capacity too low in climates with very humid days. dry cooling of very humid internal space can create something like cold sauna, which is likely the most uncomfortable condition imaginable.

hasni, i still feel you are trying to reach some answers blind-folded. to know where your room air will come to, you need to draw process in psychro chart to find out supply air condition, and to know which supply air condition you need at all, you must calculate sensible and latent loads first, otherwise you cannot even guess.

so:
- calculate cooling loads to find out which supply air conditions you need. as there are more variables than solutions, you have to assume some of them, the most often that is dry bulb of supply air.
- draw psychro processes, than size the equipment based on calculated capacities.

later, in calculation process, you will have to check your assumptions (i.e. supply air temp. or similar), see how it influences other parameters and equipment selection, than to eventually modify it if needed.

 

[317069]

Sprinkler:
room air 75F 50%RH, its dew point is 54.4F with RH=1, now let say thsi air (75F,RH=1) is passing over a cooling coil, how much the surface temperature of the cooling coil which in touch with the air shloud be?

 

[sprinkler1000]

50º F aprox. Maybe I did not understand your idea.
Regards.

 

[317069]

My idea is to find out what would happen if I run an expoused duct through a condioned returen air plenum for the design setting (75F& RH=1)