[Greenbuilding] state of the art stove/bathroom exhaust fans?

Reuben Deumling 9watts at gmail.com
Fri Jan 2 22:08:21 CST 2015


Wow. Thank you!
Michael O'Brien started me on this the other day in a private message, and
I very much appreciate both of these tutorials. I have to study this more
to fully absorb all of it. But this is an excellent introduction.

On Fri, Jan 2, 2015 at 10:29 AM, Topher <topher at greenfret.com> wrote:

> On 12/27/2014 3:26 PM, Reuben Deumling wrote:
> > I'm a little fuzzy on all of these parameters. I know the
> > terminology but am not clear on the exact relationships, for
> > instance, between temperature and (relative) humidity and the
> > dewpoint. Anyone have a good primer they can recommend?
>
> Challenge accepted.  On the principle that one doesn't know a thing
> until one can teach it, [and to make up for getting it totally backwards
> once on this list.]
>
> Basic fact #1:  The amount of water vapor that air can hold is limited,
> that limit is dependent on temperature, and it is not a linear
> relationship.
>
> Saturation Point: When the air (at its current temperature) is holding
> all the water vapor it can.
>
> Psychrometric Chart: A graph with dry bulb temperature on the X-axis,
> Humidity Ratio on the Y-axis, Saturation point curve on the left side,
> and various other metrics plotted on it (including wet bulb temperature,
> enthalpy, relative humidity, specific volume, dew point temperature,
> etc.).  Atmospheric pressure is also a factor, so they are calibrated
> for a given elevation.
>
> Metrics (unit type given in parenthesis)
>
> Dew Point (temperature):  The temperature at which air (at it current
> level of water vapor) will be at its saturation point.  Dew point is a
> measure of absolute humidity (though the scale is not linearly related
> to either mixing ratio or partial pressure).
>
> Relative Humidity (percentage): For a given temperature, the percentage
> of the amount of water vapor actually present, compared to what it would
> take to be at the saturation point.
>
> Absolute Humidity [Mixing Ratio] (mass / volume): The mass of the water
> vapor in a volume of air.
>
> Dry Bulb Temperature (temperature): Standard temperature, named to
> differentiate it from Wet Bulb Temperature.
>
> Wet Bulb Temperature (temperature): Temperature read from a thermometer
> with a wet sock on it, being swung through the air.  Sling psychrometers
> are generally how it comes.  This can be used to determine humidity
> levels (in conjunction with a psychrometric chart).  Wet Bulb
> temperature is closely correlated with Enthalpy but don't worry about that.
>
> Humidity Ratio: (mass of water vapor / mass of dry air)  Another measure
> of absolute humidity.
>
>
> So, looking at a psychrometric chart (there are plenty on the web) for
> your elevation, you can find your current conditions by locating the
> current temperature on the x-axis, and the humidity level, either by wet
> bulb or relative humidity (depending on your measuring device) to
> establish a point on the Y-axis.  Wet bulb lines slope negatively (down
> to the right) on the chart.  Relative humidity (since it is a percentage
> of saturation temperature) follows the saturation curve.
>
> Warming the room involves moving that point to the right.
> Sensible cooling (only) involves moving the point to the left.
> Humidifying (only) involves moving it up.
> Dehumidifying (only) involves moving it down.
> Evaporative cooling moves it up and left.
> Chemical dehumidifying moves it down and right.
>
> If the point ever hits saturation curve, there is danger of
> condensation, and further movement in that direction will need to follow
> the curve.  For example if one is cooling (only) and reaches the
> saturation point, further cooling is only possible by condensing some
> moisture out of the air.
>
> Looking at the chart tells us that your 60% at 70°F is a dew point of
> 56°F.  Extending that point to the left, it hits 90% RH at 59°F, so the
> temperature outside needs to be lower than that for air mixing to reduce
> your inside humidity.
>
> If the temperature outside is, say, 40°F at 90%RH, that is a dew point
> around 38°F, warming it to 70°F gives it a relative humidity of 30%.
> Mixing can be approximated by averaging the humidity ratio, so 50%
> inside air at 60%RH and 70°F, and 50% outside air at 90% RH and 40°F,
> mixed, and heated to 70°F, gives humidity ratios of 0.0096 and 0.0048
> respectively, and a mixed relative humidity around 46%.
>
>
> Thank You Kindly,
>
> Topher
>
> --
> Topher Belknap
> Green Fret Consulting
> Kermit didn't know the half of it...
> http://www.GreenFret.com/
> topher at greenfret.com
>
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