We notice from the development in Part 1 that the equations relating relative and specific humidity, temperature (wet and dry bulb), pressure (air, vapor) and enthalpy are quite tedious and inconvenient. For this reason a psychrometric chart relating all the relevant variables was developed which is extremely useful for designing and evaluating air-conditioning and cooling tower systems.
At first appearence the psychrometric chart is quite confusing, however with some practice it becomes an extremely useful tool for rapidly evaluating air-conditioning processes. The most popular chart in common usage is that developed by ASHRAE (American Society of Heating, Refrigeration and Air-Conditioning Engineers), however we feel that the construction of a simplified version of the chart based on approximations of the various equations can be a very useful tool for developing an understanding of it's usage. This approach was suggested by Dr Maged El-Shaarawi in his article "On the Psychrometric Chart" published in the ASHRAE Transactions (Paper #3736, Vol 100, Part 1, 1994) and inspired us to produce the following simplified psychrometric chart:
The basic information used to construct the
chart is the water vapor saturation data (Tsat, Pg) which is obtained
from steam tables over the range from Tsat = 0.01°C through
50°C. The specific humidity 
is then evaluated using the relative humidity 
as a parameter to produce the various relative humidity curves
(blue
lines) as follows:
where P is the standard atmospheric pressure 101.325 [kPa].
The saturation curve (100% relative humidity) also known as the dew point curve is drawn as a red line. Notice that on the saturation curve the wet and dry bulb temperatures have the same values.
The major simplifying assumption in the construction of the chart is that the enthalpy of the mixture is assumed to be constant throughout the adiabatic saturation process (described in Part 1). This implies that the evaporating liquid added does not significantly affect the enthalpy of the air-vapor mixture, leading to the constant slope wet bulb temperature / enthalpy (red) lines defined by:
Note that on the
Finally, the specific volume of the air-vapor mixture (green lines) is given by
where the gas constant Rair = 0.287 [kJ/kg.K]
It is normal practice to separate out the overlapping enthalpy / wet bulb temperature lines allowing them to be separately evaluated. Thus we introduce an oblique enthalpy axis and enthalpy (black) lines as follows:
The four equations highlighted above were programmed
in MATLAB and used to plot the simplified psychrometric charts
shown above. Refer to the link:
MATLAB program for plotting
a Simplified Psychrometric Chart
An excellent NebGuide (University of Nebraska-Lincoln Extension Publication) on How to use a Simplified Psychrometric Chart has been provided by David Shelton, and is also available as a pdf file (968k). This guide reduces the confusion by separately explaining 4 of the 6 sets of curves which make up a psychrometric chart.
One of the major applications of the Psychrometric
Chart is in air conditioning, and we find that most humans feel
comfortable when the temperature is between 22°C and 27°C,
and the relative humidity between
40% and 60%. This defines the "comfort zone" which is
portrayed on the Psychrometric Chart as shown below. Thus with
the aid of the chart we either heat or cool, add moisture or dehumidify
as required in order to bring the air into the comfort zone.
We will review some of the examples in this section by Potter (Examples 12-9 through 12-14) and compare the process of using the psychrometric chart to the direct use of the relevant equations and steam tables. (We believe that Example 12-9 has a numerical error, correct answer is 21.8 kJ/s). Example 12.11 is an important example illustrating the dehumidification process, however we believe that it has an error, and since it is presented in English units, we will present a replacement example as follows:
Replacement Example 12.11: Outside air at 35°C and 60% relative humidity is to be conditioned by cooling and heating so as to bring the air to within the "comfort zone". Using the Psychrometric Chart neatly plot the required air conditioning process and estimate (a) the amount of moisture removed [11.5g-H20/kg-dry-air], (b) the heat removed [(1)-(2), qcool = 48kJ/kg-dry-air], and (c) the amount of heat added [(2)-(3), qheat = 10kJ/kg-dry-air].
Potter Example 12.12 (repeated here for convenience): Hot dry air at 40°C and 10% relative humidity passes through an evaporative cooler. Water is added as the air passes through a series of wicks and the mixture exits at 27°C. Find (a) the outlet relative humidity [45%], (b) the amount of water added [5.4g-H20/kg-dry-air], and (c) the lowest temperature that could be realised [18.5°C].
This type of cooler is extremely popular in hot, dry climates, and is popularly known as a Swamp Cooler.
An interesting and informative description on Psychrometric Chart Use for livestock and greenhouse applications has been presented in a University of Connecticut website by Michael Darre. Other websites that we found interesting is that of Wikipedia on Psychrometrics, and one by Sam Hui (Hong Kong University) on using the psychrometric chart in the 'Climatic Design of Buildings'.
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