unit iii - psychrometry

7/29/2019 Unit III - Psychrometry

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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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CONDITIONING


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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.

unit iii - psychrometry

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