Thermodynamics Gas Mixture - Dew Point Temperature

In summary: You know the volume of water vapour in the mixture, so you can calculate the mass flow rate. This is 0.1 kg/s.
  • #1
Reefy
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1

Homework Statement


View attachment 116989

Only Number 1, not number 2

Homework Equations


Dew Point Temperature T = Saturated Temperated at Vapor Pressure

Partial Pressure = (mole fraction) x (Mixture Pressure)

The Attempt at a Solution



The dew point temperature is only dependent on the pressure of the water vapor in the mixture. This pressure is equal to

P (vapor) = 0.15 (100 kPa) = 15 kPa. Using the saturation tables, the temperature corresponding to this pressure is 53.97 degrees Celsius.

The 2nd part is where I am having difficulty. The temperature cools down to 43.97 degrees Celsius. Now I have to find the mass flow rate of the water to determine how much time it will take to collect 10kg of water. This means I need to determine the mass flow rate of the vapor too? I know the mole fractions add up to 100%
 
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  • #2
What is the vapour pressure of water at 43.97°C? So what fraction of the water condenses in the heat exchanger? You know the flow rate of water vapour into the HE, so that gives you the rate of collection of liquid water.
 
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  • #3
mjc123 said:
What is the vapour pressure of water at 43.97°C? So what fraction of the water condenses in the heat exchanger? You know the flow rate of water vapour into the HE, so that gives you the rate of collection of liquid water.

The saturation pressure at that temperature is 9.14 kPa using linear interpolation with the saturation tables. Ok, so the rate given was for the mixture which is the same for all components. For some reason, I was thinking each component might have a different mass flow rate.

Then the part I'm confused is how to determine the fraction of water condensing. I know the fraction of water vapor and so the fraction of vapor and fraction of liquid water should add up to 0.15, correct?

Edit: The new vapor pressure divided by the mixture pressure will give me a different mole fraction, wouldn't it? So is the mole fraction no longer 0.15?
 
  • #4
Am I supposed to use the humidity ratio to solve this? I thought the 0.1 kg/s was the mass flow rate of dry air

ω = 0.622 x (Saturated Pressure)/ (Pressure mixture - Saturated Pressure)
 
  • #5
As I read it, 0.1 kg/s at 100 kPa is the mass flow rate of the CO2-H2O-N2 mixture. If it isn't, there's not enough information given to answer part 1. This rate is not "the same for all components"; it is the sum of the rates for the individual components, which are proportional to their mass fractions (note what you are given is the volume fractions).
So the entry pressure is 15 kPa, and the equilibrium pressure is 9.14 kPa. What fraction of the water vapour condenses? How much mass is that per second?
 

FAQ: Thermodynamics Gas Mixture - Dew Point Temperature

1. What is the dew point temperature?

The dew point temperature is the temperature at which a gas mixture reaches saturation and begins to condense into a liquid. It is also known as the saturation temperature.

2. How is the dew point temperature calculated?

The dew point temperature can be calculated using the partial pressure of water vapor in the gas mixture and the total pressure of the gas mixture, as well as the mole fraction of water vapor in the gas mixture. There are also various equations and charts that can be used to determine the dew point temperature.

3. What is the significance of the dew point temperature?

The dew point temperature is important in determining the moisture content of a gas mixture. It can also be used in industrial processes to prevent condensation and corrosion, as well as in weather forecasting to predict the formation of fog, dew, and other atmospheric conditions.

4. How does the dew point temperature affect the performance of gas systems?

The dew point temperature can impact the performance of gas systems by affecting the moisture content of the gas mixture. If the dew point temperature is too high, it can lead to condensation and potential damage to equipment. On the other hand, if the dew point temperature is too low, it can result in dry conditions that can also cause issues with equipment performance.

5. Can the dew point temperature change?

Yes, the dew point temperature can change depending on the composition of the gas mixture and the surrounding environmental conditions. It can also change as the gas mixture is heated or cooled, or as the partial pressure of water vapor in the mixture changes.

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