The Determination of thermodynamic properties of moist air Greek
psuchron () meaning "cold metron () meaning "means of measurement
Primary Application Air Conditioning
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A Psychrometric Chart is an important tool for HVAC engineers to
carry out heat load or cooling load calculations and find solutions
to various air condition related problems.
Raoult Law: The partial pressure of A is equal to the mol
fraction of A in solution multiplied by the vapr pressure of pure
liquid A at the temperature of a solution.
pA=A0
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Daltons Laws of Partial Pressure
P= pA+pB A+B=1
Specific Humidity or Humidity Ratio Mass of Water vapor/ Mass of
dry air
=
=
=
28.96
18 =0.622
Mass of water vapor that is associated with one kilogram of dry
air-water vapor mixture
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Relative Humidity is the ration of actual water vapor pressure
() in the air to the vapor pressure () which would exist in the
saturated mixture at the temperature of air. Percentage Saturation
It is the ration of the specific humidities, actual versus
saturated at a given temperature T
Percentage Saturation =
Enthalpy of the moist vapor is equal to the sum of the
enthalpies of dry air and associated water vapor H = Ha + Hv
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Saturation Curve
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Relative Humidities
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Dew Point Temperature and Specific Humidity Lines
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Constant Wet Bulb Temperature Line
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Enthalpy Lines
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Specific Volume Lines
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An air duct having a surface temperature of 15 degree C runs
through a space at 32 degree dry bulb temperature and 25 degree wet
bulb temperature. Will the duct seat? Hint: Find the corresponding
dew point temperature. If the surface temperature is lower than dew
point temperature, it will form droplets of moisture of the surface
and thus sweat
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Mixing of Two Streams
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Psycrometry of the Mixed Air
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2. For 1 kg of air a. At 25 C and 0.012lg/kg of dry air is mixed
with 3 kg of air b. At 40 C and 0.02 kg/ kg of dry air to form
mixture M c. What is the condition of M
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Sensible Heating
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3. Air at 26 C and 50% RH is heated to 40C dry bulb temperature
by passing it over an electric heating element. Show the process on
the Psychometric chart and Determine and Corresponding wet bulb abd
dew point temperatures.
