Psychometric chart question.

[SherlockOhms]

Homework Statement

The air emerging from a hot aqueous process at a temperature of 45°C, passes over a surface which is gradually cooled. It is found that the first traces of moisture appear on this surface when it is at 40°C. Estimate the relative humidity of the air leaving the dryer.

Homework Equations

None.

The Attempt at a Solution

I'm just wondering, is it possible using the psychometric chart I've attached to answer the above problem? To me, it just seems like the wet bulb line ends at 30 C and you can't read off the value of 40 C.

 

[SherlockOhms]

Here.

 

[Chestermiller]

Homework Statement

The air emerging from a hot aqueous process at a temperature of 45°C, passes over a surface which is gradually cooled. It is found that the first traces of moisture appear on this surface when it is at 40°C. Estimate the relative humidity of the air leaving the dryer.

Homework Equations

None.

The Attempt at a Solution

I'm just wondering, is it possible using the psychometric chart I've attached to answer the above problem? To me, it just seems like the wet bulb line ends at 30 C and you can't read off the value of 40 C.

It doesn't look like you can. So..., what's the saturation vapor pressure of water at 40 C and at 45 C. The partial pressure of the water vapor in the air is the saturation vapor pressure of water at 40C, since this is the temperature at which water starts to condense out. Do you know the definition of RH?

 

[SherlockOhms]

It doesn't look like you can. So..., what's the saturation vapor pressure of water at 40 C and at 45 C. The partial pressure of the water vapor in the air is the saturation vapor pressure of water at 40C, since this is the temperature at which water starts to condense out. Do you know the definition of RH?

Thanks for the reply. RH is the defined as the ratio of partial pressure to saturation partial pressure at a given temperature (multiplied by 100 for a percentage). I could always just use this, knowing the saturation pressure of water at the given temps, to find my final value of RH. I was really just looking to see if I wasn't using the psycho chart correctly though. I don't actually have to solve this problem. Thanks again for the help.

 

[rezeew]

As a scientist, it is important to use appropriate tools and techniques to solve problems. In this case, the psychometric chart can be a useful tool, but it may not be the most suitable for this specific problem. The chart is typically used to determine properties of moist air, such as relative humidity, at a given temperature and pressure. However, in this problem, the temperature of the air is changing as it passes over the cooling surface, making it difficult to accurately determine the relative humidity using the chart.

Instead, I would suggest using the Mollier diagram, which is a more precise tool for determining properties of moist air at varying temperatures and pressures. Using this diagram, we can plot the given temperature and the temperature at which moisture first appears (40°C) to determine the relative humidity of the air leaving the dryer.

Alternatively, we can also use the equation for relative humidity, which is the ratio of the actual vapor pressure to the saturation vapor pressure at a given temperature. We can calculate the actual vapor pressure using the given temperature and the dew point temperature (40°C) and then use the saturation vapor pressure at the given temperature (45°C) to determine the relative humidity.

In conclusion, while the psychometric chart can be a useful tool for solving problems involving moist air, it may not be the most suitable for this specific problem. It is important to consider other tools and techniques, such as the Mollier diagram or equations, to accurately solve the problem.