Psycrometric Calcation Help

[MechGuy22]

If you have the following conditions:

72 degree F drybulb
62% Relative Humidity

If the drybulb was changed to 75 degree F, what would be the new RH%?? I came up with 54-55%.

Thanks!!!!!!

 

[IRstuff]

There are lots of psychrometric calculators available on the web.

TTFN

FAQ731-376
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[MechGuy22]

I know how many calculators there are.

My real question is; if you have a dry bulb temperature and a relative humidity, can you just move horizontally right on the psyc chart to see what the relative humidity WILL be at a higher drybulb temperature?

Ie) If you have 72 degrees and 62% RH, can you plot that point and draw a horizontally and then draw a vertical line that the new drybulb temp of 75 degrees? Using my chart I come up with a new RH of 53% I just wanted to make sure I was doing everything exactly right.

I know there are a lot of calculators online but they dont really answer my question. In order to answer my question you have to assume that all the other properties of the air will stay the same. You would have to punch in my conditions of 62% and 72 degrees, then write down the wet bulb or one of the other factors, then RE-punch that in at the new drybulb of 75 to see what the new RH would be. Correct?

 

[IRstuff]

Yes, that's how it's supposed to work, isn't it?

There are also lots of Excel sheets where that can be easily done.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[MintJulep]

Yes, if you are doing a purely sensible heating or cooling just move along the line of contant absolute humidity.

 

[MechGuy22]

I am using TRANE'S free psycrometric calculator and I seem to be getting a odd output.

First I plot 72 DB and 62% RH which gives me a 63.2 WB temperature.
So then I plot the point 75 degree DB and plug in my 63.2 WB to see what my new RH conditions are @75DB and it gives me a output of a 52.3 RH which is as expected because as the temperature increases, the RH decreases. But why arent the points even? They are staggered?? They should have the same "Y" value correct? Please see attached screen shot.

 

[MechGuy22]

Actually I think I made a mistake. If I plot those points, when I go to find out the new RH at the new DB, my limit would be the WB temperature which is horizontal line with a negative slope so I would have to follow that line down until it crossed the X value of 75 degree DB....

 

[quark]

That is because you are following constant wet bulb process (or precisely, isenthalpic process) and this is a possibility with desiccant dehumidification.

Use constant absolute humidity as suggested by MJ. Here, you are reducing the RH by sensible heating.

 

[MechGuy22]

OK, so if I want to get the same answer as the TRANE program then I should follow the wet bulb line. Following the wet bulb line would simulate removing humidity by means of say a dehumidifier or "desiccant dehumidification"

OR

If I draw a horizontal line (left to right) to follow the absolute humidity and plotting the point where it crosses the new temperature of 75 DB then this would simulate increasing the temperature and decreasing the humidity by means of a sensible temperature change such as turning up the thermostat (sensible)


So to recap:

If I have 72 DB & 62% RH and I wanted to lower the humidity by increasing the thermostat in my house to 75, the new RH would be about 55% when the temperature reached 75DB. This would be illustrated on a psych chart by drawing a vertical line at 72 plotting the point when it crossed 62% to get my current condition. I would then draw a horizontal line from left to right to line myself up at a absolute humidity value. I would draw a vertical line at my new DB of 75, and where the absolute humidity line and the 75 DB line intersect I could read my humidity value. Which in this case would put be at exactly 56.0% RH.

Phew!

 

[IRstuff]

A constant dewpoint calculation would result in 54% RH, which, I think, would be the result from purely heating alone and no removal of moisture.

From a practical perspective starting with that high a humidity, you cannot reach a comfortable temperature without removing the moisture.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[MechEngNCPE]

55.5%RH is the answer

 

[IRstuff]

Every calculator seems to have a slightly different answer:
HSPsyChart5 --> 54.2
PsychromeTree --> 55.9
Psychro (excel) --> 56.1

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Drazen]

If I understand well, your biggest issue is how to present processes in psychrometric chart.

And that is really the issue, you should know which process you apply, than you will know which line or curve to apply.

For sensible heating, it is rather simple, and that is horizontal line you mentioned.

For cooling first you need to know whether you have dry cooling or not. As you mention your desire to lower humidity, apparently you deal with wet colling, than process line is not straight, it is curve which depends on cooling coil construction, and you can get prcecise result from manufacturer only.

Commercial fan-coils and similar devices will not have anything avaialable, as they are not intended for regulated dehumidification, so manufacters usual give some rough data on moisture removal.

You could construct process curve from their data if they would give several process points, but normally they declare moisture reomoval at referent conditions only.

Try this, it gives nice overwiev of curves related to processes:

http://www.hku.hk/bse/MEBS6006/DencoCH2.pdf

Pitty Trane and Carrier don't bother to provide such simple explanations while offering software for precise calculations.

 

[kashain]

The answer depends on what is the process from the point 1 to point 2.

 

[MechGuy22]

Thanks so much to everyone that gave me a hand and posted on this tread! I definitely learned a lot from this thread and hope others did as well!