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7/25/2019 Topic 1 Pyschrometry Notes
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Psychrometric properties of air
Topic 1
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1. Introduction
! In all engineering work, consistent units must be employed. In this subject,
as in the ASHRAE handbooks, two systems of units will be used.
! English Engineering System - most commonly used in the U.S. with
some modification, such as use of inches instead of feet. Also knownas inch-pound or IP system.
! International System or SI - use in engineering practice throughout
most of the world and widely adopted in the U.S.
! Reason: Equipment that was designed using IP units will be operational for
years (or even decades). It will be necessary for engineers to work in either
IP or SI systems of units and to be able to make conversion from one
system to another.
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1. Introduction
! Psychrometryis the study of the properties of mixtures of air and water
vapour.
! Atmospheric airis a mixture of many gases plus water vapour and a number
of pollutants.
! The amount of water vapour and pollutants vary from place to place. The
concentration of water vapour and pollutants decrease with altitude, and
above an altitude of about 10 km, atmospheric air consists of only dry air.
The pollutants have to be filtered out before processing the air.
! Moist air- the processed air which is a mixture of various gases that
constitutes of air and water vapour.
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1. Introduction
! The moist air can be thought of as a mixture of dry air and moisture. For all
practical purposes, the composition of dry air can be considered as constant.
In 1949, a standard composition of dry air was fixed by the International Joint
Committee on Psychrometric data.
! Based on the above composition the molecular weight of dry air is found to
be 28.966 and the gas constant R is 287.035 J/kg.K.
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1. Introduction
! At a given temperature and pressure the dry air can only hold a certain
maximum amount of moisture. When the moisture content is maximum, then
the air is known as saturated air.
! This condition is established by a neutral equilibrium between the moist airand the liquid or solid phases of water.
! For calculation purposes, the molecular weight of water vapour is taken as
18.015 and its gas constant is 461.52 J/kg.K.
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1. Introduction
! Sensible heat
- energy added to a substance that produces a change in its temperature.
! Latent heat
- energy added to a substance that produces a change in its phase.
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2. Properties of moist air
! Mixture of perfect gases according to Gibbs-Dalton law:
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2. Properties of moist air
! Dry bulb temperature (DBT)is the temperature of the moist air as measured by a
standard thermometer or other temperature measuring instruments.
! Saturated vapour pressure (psat)is the saturated partial pressure of water vapour at
the dry bulb temperature. It is readily available in thermodynamic tables and charts.
ASHRAE suggests the following regression equation for saturated vapour pressure ofwater, which is valid for 0 to 100oC.
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2. Properties of moist air
! Relative humidityis the ratio of the mole fraction of water vapour in moist air
to mole fraction of water vapour in saturated air at the same temperature and
pressure. Using perfect gas equation:
! It is normally expressed as a percentage. When the value is 100 percent, the
air is saturated.
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2. Properties of moist air
! Humidity ratio (W)(or specific humidity) is the mass of water associated with each
kilogram of dry air. Assuming both water vapour and dry air to be perfect gases,
the humidity ratio is given by:
! Substituting the values of gas constants of water vapour and air Rvand Rain the
above equation; the humidity ratio is given by:
! For a given barometric pressure pt, given the DBT, we can find the saturated
vapour pressure psatfrom the thermodynamic property tables on steam. Thenusing the above equation, we can find the humidity ratio at saturated conditions,
Wsat.
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2. Properties of moist air
! If unsaturated moist air is cooled at constant pressure, then the temperature
at which the moisture in the air begins to condenseis known as dew-point
temperature (DPT)of air. An approximate equation for dew-point
temperature is given by:
! Note: From its definition, the dew point temperature is the saturation
temperature corresponding to the vapour pressure of water vapour, it can
also be obtained from steam tables or using the equation of saturated vapour
pressure.
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2. Properties of moist air
! The degree of saturationis the ratio of the humidity ratio W to the humidity
ratio of a saturated mixture Wsat the same temperature and pressure:
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2. Properties of moist air
! The enthalpyof moist air is the sum of the enthalpy of the dry air and the
enthalpy of the water vapour. Enthalpy values are always based on some
reference value. For moist air, the enthalpy of dry air is given a zero value at
0oC, and for water vapour the enthalpy of saturated water is taken as zero at
0oC. The enthalpy of moist air is given by:
! The unit of h is kJ/kg of dry air. Substituting the approximate values of cpand
hg, we obtain:
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2. Properties of moist air
! From the equation for enthalpy of moist air, the humid specific heatof moist
air can be written as:
! Since (W.cpw) is very small compared to the first term, for all practical
purposes, the humid specific heat of moist air, cpmcan be taken as 1.0216 kJ/
kg dry air.K
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2. Properties of moist air
! Specific volumeis defined as the number of cubic meters of moist air per
kilogram of dry air. From perfect gas equation since the volumes occupied by
the individual substances are the same, the specific volume is also equal to
the number of cubic meters of dry air per kilogram of dry air:
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Example 1
On a particular day the weather forecast states that the dry bulb temperature
is 37oC, while the relative humidity is 50% and the barometric pressure is
101.325 kPa. Find the humidity ratio and enthalpy of moist air on this day.
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Example 1 (solution)
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3. Wet-bulb temperature
! The thermodynamic state of moist air is uniquely fixed if the barometric pressure and
two other independent properties are known. As such, at a given barometric pressure,
the state of moist air can be determined by measuring any two independent properties.
! One of them could be the dry-bulb temperature (DBT), as the measurement of this
temperature is fairly simple and accurate. The accurate measurement of otherindependent parameters such as humidity ratio is very di"cult in practice.
! Since the measurement of temperature is easier, it would be convenient if the other
independent parameter is also a temperature. The dew-point temperature (DPT) can be
considered, but it is observed that accurate measurement of DPT is di"cult.
! In this context, a new independent temperature parameter called the wet-bulb
temperature (WBT)is defined. Compared to DPT, it is easier to measure the WBT
of moist air. Thus knowing the DBT and WBT from measurements, it is possible to
find the other properties of moist air.
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3. Wet-bulb temperature
! If the percentage saturation of an air sample
is less than 100, i.e. it is less than saturated,
and it comes into contact with water at the
same temperature, there will be a di#erence
in vapour pressures.
! As a result, some of the water will evaporate.
The latent heat required for this change of
state will be drawn from the sensible heat of
the water, which will be slightly cooled.
! This drop in the water temperature provides
a temperature di#erence, and a thermal
balance will be reached where the flow of
sensible heat from the air to the water
provides the latent heat to evaporate a part
of it.
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3. Wet-bulb temperature
! The e#ect can be observed and
measured by using two similar
thermometers, one of which has its
bulb enclosed in a wet wick. The
drier the air passing over them, the
greater will be the rate of evaporationfrom the wick and the greater the
di#erence between the two readings
(DBT and WBT). The di#erence
between the two temp is termed wet
bulb depression.
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3. Wet-bulb temperature
! In brief, WBT is the temperature measured by ordinary
thermometer when its bulb is covered with wet cloth and
exposed to air
! The sling psychrometer is widely used for measurements
involving room air or other applications where the air
velocity inside the room is small.
! It consists of two thermometers mounted side by side
and fitted in a frame with a handle for whirling the
device through air. The required air circulation (~ 3 to 5
m/s) over the sensing bulbs is obtained by whirling the
psychrometer (~ 300 RPM). Readings are taken when
both the thermometers show steady-state readings.
! In the aspirated psychrometer, the thermometers remain
stationary, and a small fan, blower or syringe moves the air
across the thermometer bulbs.
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4. Adiabatic saturation
! Adiabatic saturation temperature- temperature at which water, by
evaporating into air, can bring the air to saturation at the same temperature
adiabatically.
! An adiabatic saturator is a device using which one can measure theoretically
the adiabatic saturation temperature of air.
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4. Adiabatic saturation
An adiabatic saturation device has
liquid water at the adiabatic
saturation temperature T2is
evaporated at a rate mwinto a moist
air stream flowing at a rate of
mda(1 + W) through the duct. Thusconservation of mass applied
separately to both dry air and water
components yields
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4. Adiabatic saturation
! Above equation maybe used to solve for the upstream (ambient) humidity
ratio.
! Note: da = dry air; wv = water vapour
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5. Psychrometric chart
! A Psychrometric chart graphically represents the thermodynamic properties
of moist air.
! Standard psychrometric charts are bounded by the DBT line (x-axis) and the
vapour pressure or humidity ratio (y-axis). The left hand side of the
psychrometric chart is bounded by the saturation line.
! Psychrometric charts are readily available for standard barometric pressure of
101.325 kPa at sea level and for normal temperatures (0-50oC).
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5. Psychrometric chart
R R
ASHRAE PSYCHROMETRIC CHART NO.1NORMAL TEMPERATURE
BAROMETRIC PRESSURE: 29.921 INCHES OF MERCURY
Copyright 1992
AMERICAN SOCIETY OF HEATING, REFRIGERATING AND AIR-CONDITIONING ENGINEERS, INC.
SEA LEVEL
10 15 20 25
30
35
40
45
50
55
55
60
60
ENTHALPY - BTU PER POUND OF DRY AIR
15
20
25
30
35
40
45
50
ENTH
ALPY
-BT
UPER
POUND
OFDR
YAIR
SATU
RATIONTE
MPE
RATUR
E-F
35
40
45
50
55
60
65
70
75
80
85
90
95
100
105
110
115
120
DRYBULBTEMPERATURE-F
.002
.004
.006
.008
.010
.012
.014
.016
.018
.020
.022
.024
.026
.028
10%RELATIVEHU
MIDITY
20%
30%
40%
50%
60%
70%
80%
90%
35
3540
4045
45 50
50 55
55 60
60
65
65
70
70
75
75
80
80
85WETBULBTEMPERATURE-F
85
90
12.5
13.0
13.5
14.0
VOLUME-CU.FT.PERLB.DRYAIR
14.5
15.0
HUMIDITYRATIO-POUNDSMOISTUREPERPOUND
DRYAIR
0
1.0 1.0
:-
2.0
4.0
8.0
-8.0-4
.0-2
.0-1
.0
-0.5-0
.4-0.3-0
.2-0.1
0.
10
.2
0.3
0.4
0.5
0.6
0.8
-2000
-1000
0
500
1000
1500
2000
3000
5000
- ::
SENSIBL E HEAT Qs
TOTAL HEAT Qt
ENTHALPY
HUMIDITY RATIO
Dh
DW
R R
ASHRAE PSYCHROMETRIC CHART NO.1NORMAL TEMPERATURE
BAROMETRIC PRESSURE: 29.921 INCHES OF MERCURY
Copyright 1992
AMERICAN SOCIETY OF HEATING, REFRIGERATING AND AIR-CONDITIONING ENGINEERS, INC.
SEA LEVEL
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Example 2
Temperature in a room is 25C. If the wet-bulb temperature is 20C, find
the RH, dew point, humidity ratio, specific volume and specific enthalpy at
that state.
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6. Psychrometric processes
! Sensible heating or cooling
! a psychrometric process that
involves the increase or decrease
of air temperature without
changing its humidity ratio.
! example: Moist air passing over a
room space heater
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Example 3
! Find RH and wet-bulb temp of the post-heated air.
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Example 3 (solution)
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6. Psychrometric processes
! Heating and humidifying
! a psychrometric process that
involves the simultaneous
increase of dry-bulb temp and
humidity ratio of the air.
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Example 4
2.5 m3of lumber is being dried at 60C dry-bulb temperature and 52C
wet-bulb temperature. The drying rate of the lumber is 12.5 kg of water per
hour. If outside air is at 27C dry-bulb temperature and 80% relative
humidity, how much outside air is needed per minute to carry away the
evaporated moisture?
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Example 4 (solution)
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6. Psychrometric processes
! Cooling and dehumidifying
! a psychrometric process that
involves the removal of water from
the air as the air temp drops
below the dew-point temp.
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Example 5
Moist air at 50C dry-bulb temperature and 32% relative humidity enters
the cooling coil of a dehumidification kiln heat pump system and is cooled
to a temperature of 18C. If the drying rate of 6 m3of lumber is 4 kg/hour,
determine the kW of refrigeration required.
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Example 5 (solution)
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6. Psychrometric processes
! Adiabatic / evaporative cooling
! a psychrometric process that
involves the cooling or air without
heat loss or gain.
! sensible heat lost by the air is
converted to latent heat in the
added water vapour.
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Example 6
Referring to the figure, air at state point 1 (65C DBT and 57C WBT)
experiences a temperature drop of 3C as it passes through the 1.2 m
wide stack of lumber. Determine the properties of the air at state point 2
and compare them with those at state point 1. If the air is flowing at a rate
of 2 ms-1, determine the drying rate, assuming that the volume of the stack
of 2.5 cm thick lumber is 2.5 m3
. The stack is 1.2 m wide x 3.6 m long, andthe boards are separated by stickers 3.8 cm wide x 1.9 cm thick that are
spaced 0.6 m apart.
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Example 6 (solution)
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6. Pyschrometric processes
! Adiabatic mixing of moist stream air
! a psychrometric process that
involves no heat loss or gain
during the mixing of two air
streams.
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Example 7
0.2 kg/s of moist air at 45C (DBT) and 10% RH is mixed with 0.3 kg/s of
moist air at 25C and a humidity ratio of 0.018 kgw/kgda in an adiabatic
mixing chamber. After mixing, the mixed air is heated to a final
temperature of 40C using a heater. Find the temperature and relative
humidity of air after mixing. Find the heat transfer rate in the heater and
relative humidity of air at the exit of heater. Assume the barometricpressure to be 1 atm.
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Example 7 (solution)
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Example 8
Hot, humid outdoor air is drawn into the system with a volumetric flow rate V1= 100
cfm (0.0472 m3/s). The temperature of the outdoor air is T1= 95oF (35oC) and it has
relative humidity $1= 0.7. In order to provide a comfortable environment, the air
must be both cooled and dehumidified. The air passes over a chilled water coil inorder to dehumidify it. The air exits the chilled water coil at T2= 45oF (7.2oC). The air
then passes over a steam coil in order to reheat it to a comfortable temperature. The
air exits the steam coil at T3= 65oF (18.3oC).
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Example 8
(a) What is the humidity ratio of the air entering the chilled water coil?
(b) What is the mass flow rate of dry air entering the chilled water coil?
(c) At what temperature does condensation begin as the air flows through chilled
water coil?
(d) What is the humidity ratio of the air leaving the chilled water coil?
(e) What is the volumetric flow rate of condensate that forms on the chilled water
coil?(f) What is the relative humidity of the air leaving the steam coil that is provided to
the conditioned space?
(g) What are the rates of heat transfer to the chilled water in the chilled water coil
and from the steam in the steam coil?
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Example 8 (solution)
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Example 8 (solution)
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Example 9
Moist air at 60oF (15.6oC) DBT and 20% RH enters a heater and humidifier at
the rate of 1600 cfm (0.755 m3/s), Heating of the air is followed by adiabatic
humidification so that it leaves at 115oF (46.1oC) DBT and a RH of 30%.
Saturated water vapour at 212oF (100oC) is injected. Determined the required
heat transfer rate and mass flow rate of water vapour.
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Example 9 (solution)
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