Basic Psych Rome Try (2)
Transcript of Basic Psych Rome Try (2)
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Basic Psychrometry
Compiled and Prepared by:
Engr. Alexander Hamilton S. Atienza
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Psychrometry
The study of the properties of mixtures of air and
water vapor
Definitions
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Definitions
Dry Bulb Temperature
- temperature of airas registered by an
ordinary
thermometer
- These lines are
drawn straight, not
always parallel toeach other, and
slightly inclined from
the vertical position.
This is the taxis,
the abscissa
(horizontal) axis.Each line represents
a constant
temperature.
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Wet Bulb Temperature
- temperature of air as
registered by a
thermometer whose
bulb is covered by a
wetted wick and
exposed to a current
rapidly moving air
- These lines are oblique
lines that differ slightly
from the enthalpy lines.
They are identically
straight but are not
exactly parallel to each
other. These intersect
the saturation curve at
DBT point.
Definitions
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Dew Point Temperature Line
-saturation temperature
corresponding to the actual
pressure of the steam in air
or the temperature at which
condensation of moisture
begins when air is cooled in
constant pressure
-it is also called saturation line
- From the state point follow the
horizontal line of constant
humidity ratio to the intercept
of 100% RH, also known asthe saturation curve. The
dew point temperature is
equal to the fully saturated
dry bulb or wet bulb
temperatures
Definitions
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Humidity Ratio
- The ratio of the mass of the
moisture present in the sample to
the mass of dry air present in the
sample. This quantity is also
known as the moisture content or
mixing ratio- These are the horizontal lines on
the chart. Humidity ratio is
usually expressed as pounds of
moisture per pound of dry air.
The range is from 0 for dry air up
to 0.03 (lbmw/lbma) on the righthand -axis, the ordinate or
vertical axis of the chart.
Definitions
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Relative Humidity- The ratio of the vapor pressure of
moisture in the sample to the
saturation pressure at the dry
bulb temperature of the sample.
- These hyperbolic lines are shown
in intervals of 10%. Thesaturation curve is at 100% RH,
while dry air is at 0% RH.
Definitions
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Specific volume-In psychrometrics, the term
quantifies the total volume of both
the dry air and water vapor per
pound of dry air.
-These are a family of equally
spaced straight lines that are nearlyparallel.
Definitions
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Specific enthalpy
-In psychrometrics, the term
quantifies the total energy of
both the dry air and water vapor
per pound of dry air
-These are oblique lines drawndiagonally downward from left to
right across the chart that are
parallel to each other. These are
not parallel to wet bulb
temperature lines.
Definitions
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Processes
Sensible Heating and Cooling-Addition or removal of sensible heat without change in absolute moisture
Definitions
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Processes
Dehumidification by cooling-cooling coil temperature reduces and saturation point(dew point) reached.
Definitions
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Processes
Dehumidification by cooling-cooling coil temperature reduces and saturation point(dew point) reached.
Definitions
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Processes
Temperature Reduction and Humidity Increase
Definitions
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Processes
Temperature Increase and Moisture Removal
Definitions
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Processes
Change in specific enthalpy
Definitions
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Comfort Zone
Statistical studies have beenperformed on large numbers ofsubjects of all ages, sexes and
nationalities to arrive at aquantaive description of human
comfort
Definitions
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Comfort Zone
Statistical studies have beenperformed on large numbers ofsubjects of all ages, sexes and
nationalities to arrive at aquantaive description of human
comfort
-majority of human beings arecomfortable when temperature isbetween 18-22C and RH is 40-65%
Definitions
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Humidity ratio
W=.622 Ps/(Pt-Ps)
Where W= humidity ratio in kg/kgPs= pressure of water vapor in kPa
Pt= atmospheric pressure in kPa
Problem: Compute for the humidity ratio of air at 60% RH when the
temperature is 30C(P is at 4.241 kPa). The barometric pressure is thestandard value of 101.3 kPa.
Solution: Since RH is 60%, the water-vapor pressure of air(Ps) is
0.60(4.241)=2.545 kPa.Solve forW : W=.622 (2.545)/(101.3-2.545)=.0160 kg/kg
Formula
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Enthalpy of mixture of dry air and water vapor
h=Cpt + Whg
Where h= Enthalpy of mixture of dry air and water vapor in kJ/kgCp=Specific heat of dry air at constant pressure=1.0 kJ/kg-K
t= temperature of air-vapor mixture in C
hg= enthalpy of saturated steam at temperature of air-vapor mixture
Problem: Compute for the humidity ratio given the enthalpy of mixture of dry
air and water is 95 kJ/kg while the saturated steam has 2592 kJ/kg at50C .
Solution: Since h=95kj/kg, hg=2592 kJ/kg and T= 30Solve forW : W=(h-Cpt)/hg=(95-1*50)/2592=.0174 kg/kg
Formula
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Specific volume of mixture of dry air and water vapor
v=RaT/Pa=RaT/(Pt-Ps)
Where v= specific volume of mixture of dry air and water vapor in m3/kgRa=gas constant of air
T= temperature of air-vapor mixture in K
Pa= Absolute pressure in kPaPt= Atmospheric pressure in kPaPs= Vapor Pressure in kPa
Problem:What is the specific volume of an air-water-vapor mixture havingtemperature of 24 C with pressure of 2.989kPa and RH of 20% at standardbarometer pressure
Solution: Since RH=20%,Ps=.2(2.982)=.5964 kPa= 596.4 Pa, R=287Solve for v : v=287(24+273.15)/(101300-596)=.85 m3/kg
Formula
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Definitions
Problem Solving
Calculate the specific volume of an air vapor mixture in m3/kg of dry airwhen the following conditions prevail at t= 30C and Pt=101.3 kPa with RHof 30% and water vapor pressure is 10kPa.