Pysch Rome Try

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Psychrometry• Atmospheric air makes up the environment in almost every type ofair conditioning system.

• Hence a thorough understanding of the properties of atmospheric air

and the ability to analyze various processes involving air isfundamental to air conditioning design.

• The moist air is a mixture of dry air (mixture of gases) and watervapour.

• While the composition of dry air is constant, the amount of watervapour present in the air may vary from zero to a maximumdepending upon the temperature and pressure of the mixture (dry air+ water vapour).

• At a given temperature and pressure the dry air can only hold acertain maximum amount of moisture. When the moisture content ismaximum, then the air is known as saturated air.



• ‘psychrometry’ is the study of air-water vapour mixtures.

• Basic gas laws for moist air:• Daltons law of partial pressures• P =P1+P2

• For a mixture, total pressure P is equal to the sum of partialpressures (P1 & P2)Applying this law to moist air

p = pt = pa + pvwhere p = pt = total barometric pressure

pa = partial pressure of dry airpv = partial pressure of water vapour

Molecular masses:

Ma = molecular mass of dry air = 28.966Mv = molecular mass of water vapour=18.016Gas constant:Ra = 0.2871 KJ/Kg.KRv= 0.461 Kj/Kg.K



Psychrometric properties• Dry bulb temperature (DBT)

• It is the temperature of the moist air as measured by a standardthermometer or other temperature measuring instruments.

• Saturated vapour pressure (psat)

• It is the saturated partial pressure of water vapour at the dry bulb

temperature (DBT). This is readily available in thermodynamictables and charts.

• Relative humidity (Φ)

• is defined as the ratio of the mass of water vapour in moist air to

mass of water vapour in saturated air at the same temperature andpressure.



• Humidity ratio (W):

• The humidity ratio (or specific humidity) W is the mass of watervapour associated with each kilogram of dry air

W = 0.624 (pv/pa)

• Dew-point temperature (DPT):

• If unsaturated moist air is cooled at constant pressure, then thetemperature at which the moisture in the air begins to condense isknown as dew-point temperature (DPT) of air.

• Degree of saturation µ:• The degree of saturation is the ratio of the humidity ratio W to thehumidity ratio of a saturated mixture Ws at the same temperatureand pressure.



• Enthalpy:

• The enthalpy of moist air is the sum of the enthalpy of the dry airand the enthalpy of the water vapour.

• The enthalpy of moist air is given by:



• Specific volume:

• It is the volume of mixture per kg of dry airv = mixture volume /kg of dry air.

• ‘wet bulb temperature’ T* (or the ‘saturation temperature ’):

• It is the temperature at which water evaporating into moist air at agiven dry-bulb temperature and humidity ratio can bring air to

saturation adiabatically at the same pressure p .



• consider a stream of unsaturated air-water mixture flowing through atunnel into which water is sprayed

• since the air-water mixture is not saturated, it is capable of taking inmore of water vapour.

• Some of the water from the spray will therefore evaporate and mix

• with the air. Of course, w keeps increasing and at some point themixture becomes saturated with water.



• When saturated, the air-water mixture will then be at the wet bulb

temperature T * .

• Thus, in general, for some non-saturated vapour mixture, the drybulb temperature T > T * and (T − T * ) is called the wet bulbdepression .

• Wet-Bulb Thermometer:• Used for determining wet bulb temperature



Calculation of psychrometric properties from p, DBT and

WBT:



• On a particular day the weather forecast states that the dry bulb

temperature is 37oC, while the relative humidity is 50% and thebarometric pressure is 101.325 kPa. Find the humidity ratio, dewpoint temperature and enthalpy of moist air on this day.



• At 37oC the saturation pressure (ps) of water vapour is obtainedfrom steam tables as 6.2795 kPa.

• Since the relative humidity is 50%, the vapour pressure of water inair (pv) is:

• pv = 0.5 x ps = 0.5 x 6.2795 = 3.13975 kPa

• the humidity ratio W is given by:• W = 0.622 x pv/(pt−pv) = 0.622 x 3.13975/(101.325−3.13975) =

0.01989 kgw/kgda

• The enthalpy of air (h) is given by the equation:• h = 1.005t+W(2501+1.88t) = 1.005 x 37+0.01989(2501+1.88 x 37)

= 88.31 kJ/kgda



• Moist air at 1 atm. (760 mm of hg) pressure has a dry bulbtemperature of 32oC and a wet bulb temperature of 26oC. Calculatea) the partial pressure of water vapour, b) humidity ratio, c) relativehumidity, d) dew point temperature, e) density of dry air in themixture, f) density of water vapour in the mixture and g) enthalpy of

moist air using perfect gas law model and psychrometric equations.



• Using modified Apjohn equation and the values of DBT, WBT and

barometric pressure, the vapour pressure is found to be:• pv = 2.956 kPa

• The humidity ratio W is given by:• W = 0.622 x 2.956/(101.325-2.956) = 0.0187 kgw/kgda

• Relative humidity RH is given by:• RH = (pv/ps) x 100 = (pv/saturation pressure at 32oC) x 100• From steam tables, the saturation pressure of water at 32oC is

4.7552 kPa, hence,• RH = (2.956/4.7552) x 100 = 62.16%

• Dew point temperature is the saturation temperature of steam at2.956 kPa.

• Hence using steam tables we find that:• DPT = Tsat(2.956 kPa) = 23.8oC



• Density of dry air and water vapour

• Applying perfect gas law to dry air:• Density of dry air ρa =(pa/RaT)=(pt−pv)/RaT =

(101.325−2.956)/(287.035 x 305)x103

• = 1.1236 kg/m3 of dry air

• Similarly the density of water vapour in air is obtainedusing perfect gas law as:

• Density of water vapour ρv = (pv/RvT) = 2.956 x103/(461.52 x 305) = 0.021 kg/m3

Enthalpy of moist air is found from the equation:

• h = 1.005 x t+W(2501+1.88 x t) = 1.005 x 32 +0.0187(2501+1.88 X 32)

• h= 80.05 kJ/kg of dry air



Psychrometric chart• the psychrometric chart is a thermodynamic chart like any other.

However, it is normally presented at atmospheric pressure only, as

most applications are at this pressure. Thus, we need only 2 moreproperties to determine the thermodynamic state – the pressure isalready given.

• Thus, if we have a psychrometric problem at non-atmospheric

pressure, the atmospheric psychrometric chart cannot be used.



• the ‘saturation line ’: this is the line of 100% relative humidity, andrepresents the locus of thermodynamic states that can hold themaximum amount of water in purely vapour form.



Psychrometric processes• In the design and analysis of air conditioning plants, the fundamental

requirement is to identify the various processes being performed onair.

• All these processes can be plotted on a psychrometric chart.



• Important Psychrometricprocesses

• Sensible cooling:

• During this process, the moisture

content of air remains constant butits temperature decreases as it

flows over a cooling coil.• If the cooling coil is 100%

effective, then the exittemperature of air will be equal tothe coil temperature.

• However, in practice, the exit airtemperature will be higher than

the cooling coil

• Figure shows the sensible cooling

process O-A on a psychrometricchart.

• The heat transfer rate during thisprocess is given by

• Specific humidity is constant

i.e WA

= WO

• Relative humidity increases from

ΦO to ΦA



• Sensible heating:

• During this process, the moisturecontent of air remains constantand its temperature increases as it

flows over a heating coil.

• Figure shows the sensible heating

process O-B on a psychrometricchart.

• The heat transfer rate during this

process is given by

• Specific humidity is constant

i.e WB

= WO

• Relative humidity reduces from

ΦO to ΦB



• By-pass factor

• Under ideal conditions, the dry bulb temperature of the air leavingthe apparatus should be equal to that of the coil (TCO).

• But it is not so, because of the coil efficiency. This phenomenon iscalled by pass factor (BPF)

• BPF for heating coil is given by

• BPF = (TCO) - ( TB) / (TCO) - (TO)

• BPF for cooling coil is given by

• BPF = (TA) - (TCO) / (TO) - (TCO)



• Cooling and dehumidification:

• When moist air is cooled belowits dew-point by bringing it incontact with a cold surface asshown some of the water vapourin the air condenses and leaves

the air stream as liquid, as aresult both the temperature andhumidity ratio of air decreases.

• This is the process air undergoes

in a typical air conditioningsystem.

• By applying mass balance andenergy balance equation

• the load on the cooling coil, Qt isgiven by



• the 2nd term on the RHS of the above equation is normally smallcompared to the other terms, so it can be neglected.

• It can be observed that the cooling and de-humidification processinvolves both latent and sensible heat transfer processes,

• hence, the total, latent and sensible heat transfer rates (Qt, Qland Qs) can be written as



• SHF (Sensible heat factor)

• By separating the total heat transfer rate from the cooling coil intosensible and latent heat transfer rates, a useful parameter calledSensible Heat Factor (SHF) is defined.

• SHF is defined as the ratio of sensible to total heat transfer rate.

• From the above equation, one can deduce that a SHF of 1.0corresponds to no latent heat transfer and a SHF of 0 corresponds

to no sensible heat transfer.• A SHF of 0.75 to 0.80 is quite common in air conditioning systems in

a normal dry-climate.

• A lower value of SHF, say 0.6, implies a high latent heat load such

as that occurs in a humid climate.



• Heating and Humidification

• During winter it is essential to heatand humidify the room air forcomfort.

• This is normally done by firstsensibly heating the air and then

adding water vapour to the airstream through steam nozzles

• Mass balance of water vapor for the

control volume yields the rate atwhich steam has to be added, i.e.,

mw:

• mw = ma (wD − wO).

• where ma is the mass flow rate ofdry air.

• From energy balance:• Qh = ma (hD − hO) −mwhw

• where Qh is the heat suppliedthrough the heating coil and hw isthe enthalpy of steam.



• Cooling & humidification

• During this process, the air temperaturedrops and its humidity increases.

• this can be achieved by spraying cool

water in the air stream. Thetemperature of water should be lower

than the dry-bulb temperature of air buthigher than its dew-point temperature to

avoid condensation (DPT < Tw < TO).

• If the temperature of the water sprayedis equal to the wet bulb temperature of

air, then the net transfer rate will bezero as the sensible heat transfer fromair to water will be equal to latent heattransfer from water to air

• If the water temperature is greater thanWBT, then there will be a net heat

transfer from water to air.

• If the water temperature is less than

WBT, then the net heat transfer will be

from air to water.



• Heating and de-humidification

• This process can be achieved by using

a hygroscopic material, which absorbsor adsorbs the water vapour from the

moisture.

• If this process is thermally isolated,

then the enthalpy of air remainsconstant, as a result the temperature ofair increases as its moisture contentdecreases

• This hygroscopic material can be a

solid or a liquid.• In general, the absorption of water by

the hygroscopic material is an

exothermic reaction, as a result heat isreleased during this process, which is

transferred to air and the enthalpy of airincreases.



• Mixing of air streams

• Depending upon the state of the

individual streams, the mixing processcan take place with or without

condensation of moisture

• Without condensation

• when two air streams at state points 1and 2 mix, the resulting mixturecondition 3 can be obtained from mass

and energy balance.

• From the mass balance of dry air and

water vapor:



• Mixing with condensation:

• when very cold and dry air mixes with

warm air at high relative humidity, theresulting mixture condition may lie in

the two-phase region, as a result therewill be condensation of water vapor andsome amount of water will leave thesystem as liquid water.

• Due to this, the humidity ratio of theresulting mixture (point 3) will be less

than that at point 4.

• Corresponding to this will be anincrease in temperature of air due tothe release of latent heat of

condensation.

• This process rarely occurs in an air

conditioning system, but this is thephenomenon which results in theformation of fog or frost (if the mixturetemperature is below 0oC).



• 0.2 kg/s of moist air at 45oC (DBT) and 10% RH is mixedwith 0.3 kg/s of moist air at 25oC and a humidity ratio of0.018 kgw/kgda in an adiabatic mixing chamber. Aftermixing, the mixed air is heated to a final temperature of40oC using a heater. Find the temperature and relative

humidity of air after mixing. Find the heat transfer rate inthe heater and relative humidity of air at the exit of heater.Assume the barometric pressure to be 1 atm.



I id d id d i



Inside and outside design

conditions• Selection of Inside design conditions

• The required inside conditions for cold storage andindustrial air conditioning applications vary widelydepending on the specific requirement.

• However, the required inside conditions for comfort airconditioning systems remain practically sameirrespective of the size, type, location, use of the airconditioning building etc., as this is related to the thermal

comfort of the human beings.



Thermal comfort• Thermal comfort is defined as “that condition of mind which

expresses satisfaction with the thermal environment”.

• Factors affecting human comfort

• Effective temperature: the degree of warmth or cold felt by a

human body mainly depends upon dry bulb temperature, relativehumidity and air velocity.

• In order to evaluate the combined effect of these factors effectivetemperature is used. It is the index which correlates the combinedeffect of the above factors. It is represented by the ‘comfort chart’

• Physiological factorssuch as age, activity, sex and health.

These factors influence the metabolic rate. It is observed that ofthese factors, the most important is activity.



• Moisture content of air.• Insulating factor due to clothing: The type of

clothing has strong influence on the rate of heat transfer

from the human body.

Selection of Outside design conditions



Selection of Outside design conditions

• The ambient temperature

• moisture content

• The design outside conditions also depend on the

following factors:• Type of the structure, i.e., whether it is of heavy construction, medium or

light

• Insulation characteristics of the building

• Area of glass or other transparent surfaces

• Type of usage

• Nature of occupancy

• Daily range (difference between maximum and minimum temperatures in a

given day)



Air conditioning systems• According to season of year

• Winter air conditioning system• Summer air conditioning system

• Year round air conditioning system



• In winter air conditioning, the air isheated, which is generally

accompanied by humidification• The outside air flows through a

damper and mixes up with therecirculated air (which is obtained fromthe conditioned space)

• The mixed air passes through a filter

to remove dirt etc.• The air now passes through a preheatcoil in order to prevent the possiblefreezing of water

• After that, the air is made to passthrough a reheat coil to bring the air tothe designed dry bulb temperature.

• Now the conditioned air is supplied tothe conditioned space by a fan.

• From the conditioned space, a part ofused air is exhausted to theatmosphere by the exhaust fan orventilators.

• The remaining part of the used air (known as circulated air) is againconditioned)

• The outside air is sucked and made tomix with circulated air, in order tomake up for the loss of conditioned air.

Winter air conditioning system



Summer air conditioning system• In this type air is cooled and

dehumidified.

• The outside air flows through the

damper, and mixes withrecirculated air (from theconditioned space)

• The mixed air passes through thefilter to remove dirt, dust.

• The air now passes through acooling coil.

• The cooled air passes through a

perforated membrane and losesits moisture.

• After that, the air is passed

through a heating coil which heats

upthe air slightly . This is done tobring the air tho the designed DBT

and RH.



Year round air conditioning system• The year round air conditioning

system will have equipment for

both the summer and winter airconditioning system

• In summer, the cooling coiloperates to cool the air to desiredvalue.

• The dehumidification is obtainedby operating the cooling coil lowerthan the dew point temperature.

• In winter, the heating coil operatesto heat the air.



Psychrometry of air conditioning process



Psychrometry of air conditioning process

• Room sensible heat factor

• It is the ratio of the room sensible heatto the room total heat.

• RLH = room latent heat, RTH = Roomtotal heat,

RSH = room sensible heat• In cooling and dehumidification

process, the temperature at which the

RSHF line intersects the saturationcurve is called the room apparatus

dew point (room ADP)• Si line is RSHF line. The slope of thisline gives the ratio of RSH to RLH.

• X is BPF



cmm means cubic meters or air per minute

• Grand sensible heat factor:



Grand sensible heat factor:

• It is the ratio of the total sensible

heat to the grand total heat whichthe conditioning apparatus isrequired to handle.

• TSH = total sensible heat = RSH +

OASH

• TLH = total latent heat=RLH+OALH

• GTH = Grand total heat = TSH +TLH = RSH+RLH+OATH =

RSH+RALH+(OASH+OALH)

Outside air sensible heat:



• Outside air sensible heat:

• OASH = 0.0204v1 (DBT (outside air)-DBT (room air)) kW• Outside air latent heat:

• OALH = 50v1 (W1 – W2) kW

• Outside air total heat:

• OATH = OASH + OALH = 0.02 v1 (h1 – h2) kW

• Where

• v1 = volume of outside air or ventilation in m3/min

• W1 = specific humidity of outside air in kg / kg of dry air

• W2 = specific humidity of room air in kg / kg of dry air

• h1 = enthalpy of outside air in kJ/kg of dry air

• h2 = enthalpy of room air in kJ/kg of dry air

Eff i R S ibl F



• Effective Room Sensible Factor

• It is the ratio of the effective room sensible heat to the effective roomtotal heat.

• ERSHF= ERSH/ERTH

= ERSH/ERSH+ERLH

ERSH = Effective room sensible heat = RSH + OASH x BPF

ERLH = Effective room latent heat = RLH + OALH x BPF

ERTH = effective room total heat = ERSH +ERLH

BPF = Bypass factor

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