Help with HVAC and Temp mixing

[wreckless192]

Hope somebody can help with this. i'm trying to found the dew point temperature.
the parameters are as follows:
Ambient Temp: 89F
Ambient Humidity: 210 gr/lb
Pressure: 14.7 psi
Hot air into tube: 575F (@ .1 lb/min flow)
Cold air into tube:-18.9F (@ 44 lb/min flow)

I know that first the mixed temperature (T3=Tout) needs to be calculated. then the dew point temperature can be calculated.

any thoughts on this...

thanks

 

[IRstuff]

Is this for school?

TTFN

FAQ731-376

 

[wreckless192]

No, this is not a school project. I am in the thermal and heat management industry, but we don't really deal with condensation applications.

 

[IRstuff]

Well, it's unclear from your problem description, what is mixing with what? Perhaps, a picture is in order...

TTFN

FAQ731-376

 

[IRstuff]

Also, please refrain from double posting.

TTFN

FAQ731-376

 

[wreckless192]

Hot air is mixing with cold air. A sketch is attached.

 

[MintJulep]

If you're looking for the dewpoint temperature of stream 3 then you're going to need to know the moisture content of streams 1 and 2.

 

[wreckless192]

The air is not steam. the hot air is coming off of an aircraft engine turbine. the cold air is is coming from outside.

 

[MintJulep]

You're the first to say anything about steam.

 

[IRstuff]

OK, but dew point is dictated by the moisture content of your air.

In any case, assuming perfect mixing, the mixture temperature is -17.6 °F.

TTFN

FAQ731-376

 

[IRstuff]

that's assuming identical densities, though...

TTFN

FAQ731-376

 

[wreckless192]

ok. In my calculations i got the same answer. now given that. is there a formula or psychrometric chart that i can determine the dew point temp?
my challenge has been with the units i have for ambient humidity (gr/lb). which is more of the mixing ratio units.
all the table and charts that i have found have the relative humidity in % and the mixing ratio as value less that 100.
what are your thoughts?

 

[MintJulep]

Our thoughts are that you need to know the moisture content and that you haven't told us the moisture content and that without the moisture content it is impossible to know what the dew point temperature is.

 

[IRstuff]

Particularly, since you've shown nothing that correlates the ambient humidity, for which you show great concern, and the two air flows that you supposedly are concerned about.

TTFN

FAQ731-376

 

[Zesti]

Maybe I am missing the point here, but if you are going to have air of -19°F inside "just a pipe" without any insulation in an 89°F environment then never mind the dewpoint, it is frosting that is going to happen on the outside of the pipe!

 

[Zesti]

Looking at your numbers:

Ambient Temp: 89F
Ambient Humidity: 210 gr/lb

Does "gr" mean "grams" or "grain" ???

If it is grams then these figures can not be right...
At 89F air can only hold about 30 gr/kg.

PS: I have said it before: you guys should really switch to metric - measuring airvelocities in furlongs/fortnight is just plain annoying

 

[wreckless192]

Thanks so far for all the feedback. Do forgive my lack of knowledge/experience in this area. here is a little more detail of my application.
This is an aerospace application where i'm looking to insulate a system. I typically deal with high temp. where we insulate to contain or protect from heat.
In this application i'm tasked with having to insulate a system with those conditions so that NO condensation will form on the outside of the ducts.
The parameters that i have listed is all that my customer has listed in their SCD (design reqm'ts). So i'm trying to find out what the dew point temperature is (if there is), in order to determine what insulation is needed to keep it from getting there to avoid any condensation.
As for the Grams/Grain, i will need to verify that and get back with you.

thanks

 

[mauricestoker]

If you have it available, the ASHRAE Handbook of Fundamentals,Chapter 6, Psychrometrics, page 6.13 should answer your question.

The Trane web site has a free download for mixed air streams. I would double check the numbers if going below 32* F.

 

[Zesti]

So now we know what we are after here:

We need the dewpoint of the ambient air, which is stated as having the properties:

Ambient Temp: 89F
Ambient Humidity: 210 gr/lb

I am not all that familiar with Imperial units of measure but it should be pretty straightforward for others around here.

From there it is a small step to figuring out what insulation will give you a surface temperature that is higher than this dewpoint temperature given the fact that you have an inside temperature of around -19F.

 

[Compositepro]

Please realize also that insulation alone will not prevent condensation unless there is a vapor barrier in the correct location. Some insulation materials are inherently vapor barriers but then must still be installed correctly to achieve the barrier. This is very different from hot insulation.