The Psychrometric
Chart
Calculating the air
temperature out of the cooler
| On the
psychrometric chart first locate the inlet conditions to
the cooler. As an example the chart below shows air at
30 0C 35%RH
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Next plot
a line parallel to the lines of constant enthalpy from
the ambient conditions to the saturation line. This now
shows the wet bulb temperature of the air which is
approximately 19 0C. The ‘wet bulb depression’ is 30
0C-19 0C=11 0C
The actual
cooling of the air is dependent upon the efficiency of
the pad. At 85% efficiency the cooling is .85x11=9.35 0C
which gives a cooled air temperature of 20.7 0C.
This can be plotted on
the chart as shown.
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Dry Bulb:
The ambient (surrounding air) temperature taken with a
thermometer .
Wet Bulb:
The lowest temperature level of the air that can be reached
by evaporatively cooling the air.
Wet Bulb
Depression:
The difference between the Dry Bulb and Wet Bulb
temperatures. I.E. if Dry Bulb is 30 0C and the Wet Bulb
is 22 0C, the Wet Bulb Depression is 8 0C. The Wet Bulb
Depression is used to determine the percent of efficiency of
the cooling media.
Cooling Efficiency:
The percent of the temperature drop across the media
compared to the Wet Bulb Depression. I.E. if the Wet Bulb
Depression is 8 0C (as in the above example) and the actual
temperature drop measured across the cooling media is 7 0C,
the cooling efficiency of the media is 87.5%. (7/8) =
.875). The cooling efficiency is also referred to as
"Saturation Efficiency" because it refers to the amount of
moisture that is packed into the air. 100% Saturation
Efficiency would indicate a temperature drop of 8 0C in the
above example of wet bulb depression.
Dewpoint:
The temperature at which moisture begins to form on a slick
surface indicating 100% saturation of the air with moisture.
That is how the dew gets on the grass in the morning. The
dry bulb air temperature drops to the wet bulb temperature
level.
Specific humidity:
Actual measurement of water vapour in the air.
Relative humidity:
The percent of moisture in the air compared to the amount
of moisture the air could contain. This is the most common
reading to be reported in the weather reports. The report
of "100 degrees and 12% relative humidity (RH)" is the same
as all the above examples. Using the analogy of a glass
half filled with water means the glass is 50% full. It
could hold an additional 50% at which point it would start
to overflow if more water was added.
Evaporation Rate:
The rate at
which water is absorbed into the air passing through the
cooling media. For practical purposes, this rate is
measured in gallons of water per hour (or minute).
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