Condensation Resulting From Mixed Air Flows?

[packdad]

Help! I haven't touched a psychrometric chart since college.

Here's the question: If you have two air streams coming together and mixing such that condensation is produced, what does this look like on a psych chart?

I remember that, when you have two air streams mixing (say, Stream 1 and Stream 2), you plot both on the chart. Then, you calculate a combined property (enthalpy, dry bulb temp, or whatever) to find the final mixed point on the chart (Stream 3), which will be located somewhere on a straight line drawn between points 1 and 2.

In order for condensation to occur as a result of adiabatic mixing, would you need the point for Stream 3 to end up on or to the left of the 100% humidity (saturation) line on the chart?

I'm pretty sure this differs from dehumidification as a result of a cooling coil, which, I believe, is represented as any line that moves down and to the left.

I'll also give 10 bonus points if you can tell me how to determine the rate of condensation based on the flow rate of each air stream, assuming that condensation does occur.

Thanks.

 

[imok2]

Mr.Packdad, Please try this:

http://www.elitesoft.com/web/hvacr/psywindows.htm

 

[imok2]

OH, and if you like it do I get bonus points?

 

[packdad]

Hey, now that looks pretty slick! 10 bonus points just for the link. (Here, the points are like lyrics at a Britney Spears concert...they don't mean anything...)

I'm at work right now, and TECHNICALLY we're not allowed to download executable files to our desktop, so I may not be able to try out the demo version of that software. In the event that I cannot run the demo program, what would your opinion be of my question above regarding mixing of two air flows? That is, as long as the resultant air is less than 100% relative humidity, is it true that there would be no condensation/precipitation? (This may sound like a "stupid" question, but really, it's been a while since I've done HVAC calculations and I just want to make sure I'm not missing the boat here.)

 

[Yeldud]

In theory, as long as you remain to the right of the saturation curve you should not have condensation. In reality though, mixing is not an instantaneous and homogeneous process, so there may be some localized condensation, such as you see with the water vapor plume off a power plant stack. You get condensation causing the plume to form, followed by reevaporation as the plume mixes further with surrounding air.

 

[imok2]

I ran some calculations and I determined that if you have the same amount of air for each air stream and the wet bulb is within 3% of dry bulb you could/would be in a saturated condition. However, the chances of mixing the exact amount of air for each air stream is slim and none in most cases. Does that sound that I'm all wet?..slipery?

 

[packdad]

What you're saying makes sense to me - but I needed a sanity check.

I never mentioned specific numbers before now, but in this particular case, I have a 3600 CFM of air at around 56 DB and 52 WB and 400 CFM of air at 95 DB and 80 WB. Common sense would suggest that you don't have much in the way of condensation, and plotting this on a psych chart would suggest the same thing, but it never hurts to get a second or third opinion. In this case, I have no where near equal mass flows of air, and the resultant mixture is below saturation.

 

[Yeldud]

Now if the duct or conduit in which this mixture flows has an interior surface temperature below the air mixture dewpoint temp, then you will condense on the surface. I think you already know that though.

 

[ChasBean1]

Standard mix plot on a psych chart is a straight line between conditions 1 & 2. 1, 2, and final state 3 (residing between the two at 60 db/56 wb), all dodge the saturation line, but Yeldud nailed it 4 posts above... Macroscopically, no rain falling, but microscopic condensation that immediately gets re-mixed into the resultant airstream, like when you see your breath on cold days. Probably a straight line on a psych chart doesn't do real justice in representing the actual process.