Basic Psych Rome Try (2)

8/3/2019 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.

Basic Psych Rome Try (2)

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