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

[Topher]

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