In this screencast we are going to use humidity charts to find physical properties for an
air/water vapor mixture, and we will touch on the concept of relative humidity dew point,
wet bulb temperature. Alright our example we have air that is at 40 degree dry bulb, and 30% relative
humidity, and we are going to use the charts to find the dew point and wet bulb temperature.
Both of these quantities are related to the moister content, but mean different things
and may cause confusion. If we say that the dry bulb temperatures 40 degree that is the
temperature that we are used to thinking about. It can be measured with a thermometer. This
has nothing to do with the moister content of the air. On the other hand the relative
humidity is a function of how much water is in the air, and we can get a sense of what
the relative humidity represents. If we look at how it is calculated. So we calculate relative
humidity by multiplying 100. The partial pressure of water divided by vapor pressure, which
is a function of temperature. Partial pressure of water can be calculated by multiplying
the mole fraction of water times the total system pressure. If we are using the humidity
charts the total system pressure specified in atm. On the other hand the mole fraction
of water is related to the moister content, which is on the y-axis. There is several different
equations that we can use to calculate the vapor pressure as a function of temperature
such as antiones equation, but that is a whole lot outside the scope of this screencast.
For our purposes it is sufficient to say that if we increase the temperature then we are
also going to increase the vapor pressure of water. This is a non-linear relationship
, but it helps us understand how things are changing as we move on the humidity chart.
So first we will tackle the dew point. We will go straight to the humidity chart to
find were we are and how we find it. The dry bulb temperature is found on the x-axis on
the chart. Here we know that the dry bulb temperature is 40. The relative humidity can
be found on the swiping curves that move along the humidity chart, and here we know that
our relative humidity is 30%. So if we find an intersection of 40 deg of dry bulb temperature,and
30% relative humidity our error is at this point on the humidity chart. At this point
we want to know what the dew point is, which is the temperature at which liquid water will
start to condense if we cool the temperature. Liquid water will only form 1 place on this
graph, and that is if the relative humidity is 100%. This is also called the saturation
curve along the left most curve on this graph. We get to the dew point by moving horizontally
on the humidity chart. If we are moving horizontally the moisture content is not changing. So if
the moister content stays the same then the mole fraction as well as the partial pressure
of water is going to stay constant as well. So lets look at what is changing as we go
to the dew point. If we are moving left then we are obviously decreasing the dry bulb temperature.
We said that the vapor pressure is a function of temperature. So the vapor pressure is also
going to decrease as we decrease the temperature. If we are not changing vertically. Another
words if we are not changing the moister content of air then the partial pressure of water
is staying the same. So if we keep the partial pressure of water the same while decreasing
the vapor pressure that means our relative humidity must go up. That is what we see when
we move to the left. We start at 30% relative humidity here we are at 40% relative humidity,
and here we are at 50%. We will continue to increase the relative humidity until we hit
100% at the dew point. So continuing left all the way to the 100% saturation curve we
find the dew point at this curve. The temperature can be found along this saturation curve.
So here is 20 degree. We are a little bit less then that. So we are slightly over 19.
We can also find the temperature if we drop straight down to the x-axis. Here again we
can see that our temperature is slightly over 19. So in this case our dew point is about
19 degC. The dew point represents the point at which liquid will first start to form,
if we continue to lower the temperature. We would move along this saturation curve. So
if we are moving along the saturation curve while lowering the temperature we are continuing
to condense water. If we are moving down we are also lowering the moister content as that
water condenses. Now that we know the dew point we want to find the wet bulb temperature.
It is helpful to look at how this can be measured to understand this concept. Here a wet bulb
differ from a dry bulb, that it is surrounded by a width that is saturated with water, and
if we blow air at 40 degC at a relative humidity of 30% past that bulb surrounded by water.
It is going to cause some of the water to evaporate. As water evaporates heat is lost
from the wet bulb. So heat is being transferred from the wet bulb to the water as it evaporates
and that has an affect of cooling the wet bulb. So the temperature of the wet bulb is
driven down due to water evaporation. How far the wet bulb temperature drops is a function
of the moister content of the air. We can think of a limiting case to illustrate this
point. For example lets consider the case that the relative humidity is 100%. If we
are moving air past the wet bulb that is completely saturated with water. Then water can't evaporate
from the wet bulb. If there is no evaporation there is no heat lost from the wet bulb. Consequently
the wet bulb temperature wont change. So if the relative humidity is 100% then the wet
bulb temperature is equal to the dry bulb temperature. There will never be a situation
of the wet bulb temperature is greater than the dry bulb temperature. How much the wet
bulb temperature is lower than the dry bulb is a function of the moister content. So in
our case of 30% relative humidity we can look to humidity charts to find the wet bulb temperature.
The point on the chart were our error is 40 degC and our relative humidity is 30% is shown
again. We can find the wet bulb temperature by moving from this point along the lines
of constant wet bulb temperature. The diagonal lines that move across the chart, and we find
the wet bulb temperature along the saturation curve along this line. So here we are about
25 degC. The wet bulb temperature is substantially lower then 40 due to the relatively low moister
content at a relative humidity of 30%. To illustrate how this changes as a function
of moister content lets look at the example. We are at 40 degree and 50% relative humidity.
I have shown that part with a blue circle here. We find the wet bulb temperature at
this point the same way. By moving along different lines at constant wet bulb temperature. So
moving diagonal and up/left to the curve. We can see that the wet bulb temperature is
something greater than 30 degree. So between 30 and 31. As we would expect as we increase
the relative humidity that is going to increase the wet bulb temperature. If we continue to
increase the relative humidity from here, as we approach 100% then we know that the
wet bulb temperature is going to approach the dry bulb temperature of 40.