Thermodynamics-psychrometry question, mixing two streams of warm moist air

In summary, to solve this problem you need to use the mass and energy balance equations, as well as the specific humidity equation, to calculate the mass, enthalpy, and relative humidity of the third stream.
  • #1
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Homework Statement



I'm trying to determine how to go about attacking this problem that I'm doing to

I have 2 streams of air mixing, and I'm trying to find the temperature and relative humidity of the third stream.

Stream 1: 2bar, 20C, 40%rh, and 1kg/s
Stream 2: 1bar, 20C, 4kg/s

So I know I can find all my values for the first stream, but the second stream doesn't give me enough to find it on the psychometric charts.




Homework Equations



m=mass of mixture
ma=mass air
mv=mass vapour
h=enthalpy of mixture
ha= enthalpy dry air
hv=enthalpy of water

Mass Balance
m1+m2=m3
ma1+ma2=ma3
mv1+mv2=mv3
m=ma+mv


Energy balance
m1h1+m2h2=m3h3 and all variations of this with the contents of the air


phi1=rh=Pv1/Pg1

where phi is the relative humity
Pv1 is the partial pressure of water
Pg1 is the saturation pressure of water at the given temperature

w=mv/ma for all values.

energy=mc(delta)T

where c is the constant used to determine this (name escaping me right now) (units of kj/kg.C)


The Attempt at a Solution




I got my values for state one, I've tried energy balance and mass balance, but I end up with more variables than equations and I've been confusing myself all day trying to do this. I can't find ANY question in over 3 books that excludes all data about state three other than the mass flow rate, and excludes the data required from the second stream of air.
 
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  • #2
I thought I might be able to use energy balance and mv=wma (specific humidity) to try and solve it, but I still end up with too many variables. Can anyone help me out? The best way to approach this problem is to use the mass and energy balance equations. First, calculate the mass of the mixture in stream 3 using the mass balance equation: m1+m2=m3. Then, calculate the enthalpy of the mixture in stream 3 using the energy balance equation: m1h1+m2h2=m3h3. Finally, use the specific humidity equation (mv=wma) to calculate the relative humidity in stream 3. You should then be able to use the psychometric charts to determine the temperature and relative humidity of the mixture.
 

1. What is thermodynamics-psychrometry and how does it relate to mixing two streams of warm moist air?

Thermodynamics-psychrometry is the study of the properties of moist air and how it behaves when heated or cooled. It is closely related to mixing two streams of warm moist air because the properties of the combined air will depend on the individual properties of the two streams.

2. How does the temperature and humidity of the two streams affect the resulting mixture?

The temperature and humidity of the two streams will have a direct impact on the resulting mixture. If both streams have similar temperatures and humidity levels, the resulting mixture will also have similar properties. However, if one stream is significantly warmer or more humid than the other, the resulting mixture will have different properties.

3. What is the role of enthalpy in mixing two streams of warm moist air?

Enthalpy is a thermodynamic property that represents the total energy of a system. When two streams of warm moist air are mixed, enthalpy is conserved. This means that the total energy of the combined air will be equal to the sum of the energies of the individual streams.

4. How can the resulting mixture be analyzed and predicted?

The resulting mixture can be analyzed and predicted using various thermodynamic and psychrometric equations. These equations take into account factors such as temperature, humidity, and enthalpy to determine the properties of the resulting mixture.

5. What are some real-world applications of understanding thermodynamics-psychrometry and mixing two streams of warm moist air?

Understanding thermodynamics-psychrometry and mixing two streams of warm moist air is important in many industries, such as HVAC (heating, ventilation, and air conditioning) systems, meteorology, and chemical engineering. It is also crucial in understanding and predicting weather patterns and air quality conditions.

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