unit iii - psychrometry
TRANSCRIPT
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1. M.BALACHANDRAN 10TME002
2. K.BOOBALAN 10TME004
3. E.DEVAGURU 10TME005
4. S. JAKIR HUSSAIN 10TME011
5. W. JONSON 10TME0126. I. MANIKAM 10TME021
7. P. RAJAPRASANNA 10TME032
8. T.RAJA 10TME034
9. M.RAMESH BABU 10TME037
10. D.REXDEVARAJ 10TME040
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UNIT III - PSYCHROMETRY
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INTRODUCTION
In the design and analysis of air conditioning plants,
the fundamental requirement is to identify the
various processes being performed on air. Once
identified, the processes can be analyzed by
applying the laws of conservation of mass andenergy. All these processes can be plotted easily on
a psychrometric chart. This is very useful for quick
visualization and also for identifying the changes
taking place in important properties such astemperature, humidity ratio, enthalpy etc. The
important processes that air undergoes in a typical
air conditioning plant are discussed below.
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SENSIBLE COOLING
The moisture content of air remains constant but its
temperature decreases as it flows over a cooling
coil. For moisture content to remain constant, the
surface of the cooling coil should be dry and its
surface temperature should be greater than thedew point temperature of air. If the cooling coil is
100% effective, then the exit temperature of air will
be equal to the coil temperature. However, in
practice, the exit air temperature will be higher thanthe cooling coil temperature. Figure shows the
sensible cooling process O-A on a psychometric
chart
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SENSIBLE COOLING CONT.
The heat transfer rate during this process is given by
QC = ma(ho-ha)= ma cpm (TO-TA)
Sensible cooling process O-A on Psychometric chart
hoha W
DBT
A O
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SENSIBLE HEATING
During this process, the moisture content of air
remains constant and its temperature increases as
it flows over a heating coil.
The heat transfer rate during this process is given
by
Qh = ma (hB - hO) = ma cpm (TB-TO)
where Cpmis the humid specific heat (1.0216 kJ/kg
dry air) and ma
is the mass flow rate of dry air
(kg/s).
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SENSIBLE HEATING PROCESS ON A
PSYCHROMETRIC CHART
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COOLING AND DEHUMIDIFICATION
When moist air is cooled below its dew-point by bringing it in
contact with a cold surface as shown Fig, some of the water
vapour in the air condenses and leaves the air stream as liquid,
as a result both the temperature and humidity ratio of air
decreases as shown. This is the process air undergoes in atypical air conditioning system. Although the actual process
path will vary depending upon the type of cold surface, the
surface temperature, and flow conditions, for simplicity the
process line is assumed to be a straight line. The heat and
mass transfer rates can be expressed in terms of the initial andfinal conditions by applying the conservation of mass and
conservation of energy equations
By applying mass balance for the water:
ma. wo = ma. wc + mw
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COOLING AND DEHUMIDIFICATION
CONT.
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HEATING AND HUMIDIFICATION
During winter it is essential to heat and humidify the
room air for comfort. As shown in Fig., this is
normally done by first sensibly heating the air and
then adding water vapour to the air stream through
steam nozzles as shown in the figure.
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HEATING AND HUMIDIFICATION CONT.
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COOLING & HUMIDIFICATION
As the name implies, during this process, the airtemperature drops and its humidity increases. This
process is shown in Fig. As shown in the figure, this can
be achieved by spraying cool water in the air stream. The
temperature of water should be lower than the dry-bulbtemperature of air but higher than its dew-point
temperature to avoid condensation
(TDPT < TW < TO)
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COOLING
& HUMIDIFICATION CONT.
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HEATING AND DE-HUMIDIFICATION
This process can be achieved by using ahygroscopic material, which absorbs or adsorbsthe water vapour from the moisture. If thisprocess is thermally isolated, then the enthalpy of
air remains constant, as a result the temperatureof air increases as its moisture content decreasesas shown in Fig. This hygroscopic material canbe a solid or a liquid. In general, the absorption ofwater by the hygroscopic material is an
exothermic reaction, as a result heat is releasedduring this process, which is transferred to air andthe enthalpy of air increases.
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HEATING AND
DE-HUMIDIFICATION CONT.
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MIXING OF AIR STREAMS
Mixing of air streams at different states is commonly
encountered in many processes, including in air
conditioning. Depending upon the state of the
individual streams, the mixing process can take
place with or without condensation of moisture
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MIXING OF AIR STREAMS
Without condensation: Figure shows an adiabaticmixing of two moist air streams during which no
condensation of moisture takes place. As shown in
the figure, when two air streams at state points 1
and 2 mix, the resulting mixture condition 3 can beobtained from mass and energy balance
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MIXING OF AIR STREAMS CONT.
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MIXING OF AIR STREAMS CONT.
Mixing with condensation
As shown in Fig., when very cold and dry air
mixes with warm air at high relative
humidity, the resulting mixture condition maylie in the two-phase region, as a result there
will be condensation of water vapor and
some amount of water will leave the system
as liquid water.
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MIXING OF AIR STREAMS CONT
Due to this, the humidity ratio of the resulting
mixture (point 3) will be less than that at point 4.
Corresponding to this will be an increase in
temperature of air due to the release of latent
heat of condensation. This process rarely occurs
in an air conditioning system, but this is the
phenomenon which results in the formation of fog
or frost (if the mixture temperature is below 0oC).
This happens in winter when the cold air near theearth mixes with the humid and warm air, which
develops towards the evening or after rains.
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MIXING OF AIR STREAMS CONT
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Identify parts of the chart
Determine moist air properties
Use chart to analyze processes involving moist air
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This is a typical psychrometric chart
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This skeleton chart shows the
arrangement of the various
lines and/or coordinates:
1.saturation temperature
2. dewpoint temperature
3. enthalpy
4. relative humidity
5.humidity ratio (moisturecontent)
6. wet bulb temperature
7. volume of mixture
8. dry bulb temperature.
The chart is based on a standard barometric
(atmospheric) pressure of 101.3 kPa or 760 mm Hg.
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The main coordinates of theaverage psychrometric chart are:
saturation curve (100% RH) dry bulb temperature scale line(0% RH.)
moisture content or humidityratio scale.
The dry bulb temperature linesrun perpendicular to the basecoordinate. Each line representsone degree of temperaturechange, with the scale rangingfrom
-10 C to 55 C.
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The wet bulb temperaturelines extend diagonallydownward from thesaturation curve at anapproximate angle of 30 tothe base line.
Each line represents onedegree of temperaturechange, with a scaleranging from 10 C to33 C.
The temperature scale is
located on the saturationcurve.
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The dew pointtemperature scale isthe same scale as thewet bulb scale on thesaturation curve.However, the dew pointlines extendhorizontally to the
moisture content scaleon the right of thechart.
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The relative humiditylines follow
approximately thesame curves as thesaturation curve.
The saturation curveis actually the linerepresenting 100%relative humidity, withthe dry bulbtemperature scaleline representing 0%
relative humidity ordry air.
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The moisture contentor humidity ratio linesare the same as thedew point temperaturelines. However, thescale for the grams ofmoisture on the right ofthe chart is different
and reads from 0 to 33grams of moisture perkilogram of air.
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The specific volumelines run at a steep
angle from top left tobottom right.
The numerical values,along the bottom of thechart at the ends ofthese lines are given incubic metres perkilogram of dry air andrange from 0.75 to 0.95m/kg.
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Enthalpy is total heatcontent and is designatedby the letter h.
In psychrometric terms,enthalpy defines the heatquantity in the air and themoisture in the air.
It is measured in kilojoulesper kilogram of dry air.
The enthalpy lines on apsychrometric chart arethe same as the wet bulblines.
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The enthalpy scale islocated in convenient
sections adjacent to thesaturation temperaturecurve and ranges from10 to 110 kJ/kg of dryair.
The scale can be readby extending the wetbulb lines until theymeet the scale.
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If the value of any two of thepsychrometric properties isknown, the value of any otherproperty can be determined
from the psychrometric chart. Normal practice is for the dry
and wet bulb temperatures ofa sample of air to be takenand then these temperatures
are plotted on the chart. Thetwo lines representing thesetemperatures will alwayscross at some point and thispoint then represents thecondition of the air in thesample.
Once this point has beendetermined, values for otherproperties can be identified.
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GRAND SENSIBLE HEAT FACTOR (GSHF)
Grand Sensible Heat Factor is the ratio of the total
sensible heat to the grand total heat load that the
conditioning apparatus must handle, including the
outdoor air heat loads.
This ratio is expressed as:
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GRAND SENSIBLE HEAT FACTOR
The air which is passing through the AHU coil increases or
decreases the temperature and/or the moisture content. The amount
of rise or fall is determined by the total sensible and latent heat load
that the conditioning apparatus must handle. The condition of the air
entering the apparatus (mixture condition of outdoor and returning
room air) and the condition of the air leaving the apparatus is plottedon the psychrometric chart as shown below
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ROOM SENSIBLE HEAT FACTOR (RSHF)
The room sensible heat factor (RSHF) represents
the psychometric process of the supply air within
the conditioned space. Room Sensible Heat Factor
is the ratio of room sensible and room latent heat
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ROOM SENSIBLE HEAT FACTOR (RSHF) The supply air to a conditioned space must have
the capacity to offset simultaneously both the room
sensible and room latent heat loads. The process is
plotted on the standard psychometric chart as
below
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BYPASS FACTOR
In most cooling applications, the air leaving the
cooling coil is not entirely saturated since some air
does not come in contact with the cooling coil. The
fraction of air that misses the coil is called the
bypass factor, BF. The bypass factor can bedetermined from the temperature of water supplied
to the cooling coil and from the states of incoming
and exiting air.
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BYPASS FACTOR
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REQUIREMENTOF COMFORT AIR CONDITIONING
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REQUIREMENTOF COMFORT AIR
CONDITIONING
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REQUIREMENTOF COMFORT AIR
CONDITIONING
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REQUIREMENTOF COMFORT AIR
CONDITIONING
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COMFORT CHART
The comfort chart, shown in Figure, correlates the perception ofcomfort with the various environmental factors known to influence it.
The dry-bulb temperature is indicated along the bottom. The right
side of the chart contains a dew point scale, and the left side a wet
bulb temperature scale indicating guide marks for imaginary lines
sloping diagonally down from left to right. The lines curving upwardfrom left to right represent RHs.
ET* lines are also drawn. These are the sloping dashed lines that
cross the RH lines and are labelled in increments of 5F. At any point
along any one of these lines, an individual will experience the same
thermal sensation and will have the same amount of skin wetnessdue to regulatory sweating. CLO levels at which 94 percent of
occupants will find acceptable comfort are also indicated.
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COMFORT CHART
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EFFECTIVE TEMPERATURE
This factor combines the effects of dry bulb temperature
and air humidity into a single factor. It is defined as the
temperature of the environment at 50% RH which results
in the same total loss from the skin as in the actual
environment. Since this value depends on other factors
such as activity, clothing, air velocity and Tmrt, a StandardEffective Temperature (SET) is defined for the following
conditions:
Clothing = 0.6 clo
Activity = 1.0 metAir velocity = 0.1 m/s
Tmrt= DBT (in K)
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TEMPERATURE CONDITION
Inside design conditions for Winter:
Top between 20.0 to 23.5oC at a RH of
60%
Top between 20.5 to 24.5oC at a DPT of2oC
Inside design conditions for Summer:
Top
between 22.5 to 26.0o
C at a RH of60%
Top between 23.5 to 27.0oC at a DPT of
2oC
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TEMPERATURE CONDITION
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OUTDOOR DESIGN CONDITIONSFOR SUMMER
Selection of maximum dry and wet bulb temperatures at aparticular location leads to excessively large cooling
capacities as the maximum temperature generally persists
for only a few hours in a year. Hence it is recommended that
the outdoor design conditions for summer be chosen based
on the values of dry bulb and mean coincident wet bulb
temperature that is equaled or exceeded 0.4, 1.0 or 2.0 %
of total hours in an year. These values for major locations in
the world are available in data books, such as AHRAE
handbooks. Whether to choose the 0.4 % value or 1.0 %value or 2.0 % value depends on specific requirements. In
the absence of any special requirements, the 1.0% or 2%
value may be considered for summer outdoor design
conditions
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OUTDOOR DESIGN CONDITIONS FORWINTER
Similar to summer, it is not economical to design a winter airconditioning for the worst condition on record as this would
give rise to very high heating capacities. Hence it is
recommended that the outdoor design conditions for winter
be chosen based on the values of dry bulb temperaturethat is equalled or exceeded 99.6 or 99.0 % of totalhours in an year. Similar to summer design conditions,these values for major locations in the world areavailable in data books, such as AHRAE handbooks.
Generally the 99.0% value is adequate, but if thebuilding is made of light-weight materials, poorlyinsulated or has considerable glass or spacetemperature is critical, then the 99.6% value is
recommended.
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VENTILATION: HUMAN ISSUES
Comfort ventilation Perceived indoor air quality
Odors
Olfs, decipols
Health ventilation
Toxicity, illness
Exposure guidelines:
Threshold limit values (TLVs)
Permissible exposure limits (PELs)
Time weighted average (TWA) Short-term exposure limit (STEL)
Ceiling limit (CLG)
Biological Exposure Indices (BEIs)
?
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WHYVENTILATEABUILDING?
Comfort ventilation
Reduces odors
Improves thermal comfort
Health ventilation
Ilutes air contaminants
Provides fresh air
Structural cooling
Maintains integrity of building envelope and building
contents
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AIRFLOWPRINCIPLES
Air flows from areas of high pressure to areas of
low pressure.
Air flows from areas of positive pressure to areas of
negative pressure.
Blowing is easier than sucking!
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VENTILATION STANDARDS
European approach is to calculate ventilation requirements based
on CO2 levels(occupancy) and odors (olfs). The higher ventilation rate from both calculations is the one to be
applied.
Assumption is that olfs are predictive of IAQ, which in turn is
predictive of SBS.
minimum ventilation rates and other requirements for commercialand institutional buildings
Minimum ventilation rate - offices 5 cfmpp outdoor air
Breathing Zone Outdoor Airflow
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VENTILATION STANDARDS
Ventilation and Acceptable Indoor Air Quality in Low-
Rise Residential Buildings.
Nationally recognized indoor air quality standard
developed solely for residences.
Defines the roles of and minimum requirements for
mechanical and natural ventilation systems and the
building envelope intended to provide acceptable
indoor air quality in low rise residential buildings.
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VENTILATIONCOMMONASSUMPTIONS
Increasing the ventilation rate will reduce the
concentration of indoor air pollutants.
Increasing the ventilation rate will improve occupant
perceptions of IAQ.
Increasing the ventilation rate will decrease
complaints of odors.
Increasing the ventilation rate will decrease
complaints of the sick building syndrome
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COMFORTVENTILATION: ODORCONTROL
Ventilation is needed to control indoor odors.
People are a source of indoor odors (body odor).
Body odor (sweaty armpit smell) is caused by 3-
methyl-2-hexenoic acid, a metabolic byproduct of
bacteria that live in the armpit (lipophilic diptheroids)
and feed on apocrine secretions.
~90% of men and 67% of women have these
bacteria resident in the armpit, and women produce a
milder odor than men.
~5% of people cannot smell body odor.