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Rel humidity / psychometrics question

Hello, all.

I'm not an engr (though I had originally planned to become one!), and my Dad who was a ME has passed away now, so I don't know whom to ask this question.

Can anyone point me to some SIMPLE charts, or a simple formula, for figuring out how relative humidity changes with a change in temperature?

I tried looking at Dad's psychometric charts (he was a plant engr for a bone gelatin factory way back when, and they had some sophisticated progressive drying systems), but those charts require the use of a sling psychrometer, and get into all kinds of variables, when all I want to do is change one variable - temperature ? and see how it changes RH.

Here's the application:

If I have air at 70°F which is 100% RH (i.e., saturated), and I heat that air to 130°F without adding any water to the air, and without changing the air's barometric pressure, how can I figure out the relative humidity of the resulting air?

Would the change in relative humidity be inversely proportional to the increase in volume, by any chance? (That seems intuitive to my liberal arts sensibility...)

Thanks in advance for any clues.

Jones

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Jones
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To clarify and simplify:

If I had one cubic foot of air which was 100% RH and I heated that air sufficiently that I doubled its volume to 2 cubic feet, and I didn't add any water to the system, and the barometric pressure of the air remained the same, would that mean that the RH of the resulting air would be (1/2)x(100%)=50% ?

Thanks again.

Jones

Reply to
Jones

Dear Jones:

Assuming the heating is not due to combustion in the gas stream (which adds more humidity)... Using the psychrometric chart, locate 70 °F on the saturation line (100% humidity, looks like the function e^x).

The axis that is labelled "mass of water vapor per mass of dry air" has lines that come near this point. Travel parallel to these lines until you reach 130 °F (my chart stops at 120). This means you are heating the entire air stream and not adding or removing either water or air.

Find the closest distance to the relative humidity lines on either side of it. That is your relative humidity.

David A. Smith

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