Thermodynamics IIChapter 6

Mixtures & Psychrometry

Mohsin Mohd SiesFakulti Kejuruteraan Mekanikal, Universiti Teknologi Malaysia

Objectives Differentiate between dry air and atmospheric air. Define and calculate the specific and relative humidity of

atmospheric air. Calculate the dew-point temperature of atmospheric air. Relate the adiabatic saturation temperature and wet-bulb

temperatures of atmospheric air. Use the psychrometric chart as a tool to determine the

properties of atmospheric air. Apply the principles of the conservation of mass and

energy to various air-conditioning processes.

Introduction• Psychrometry is the study of thermodynamic

properties of air and water vapor mixtures (moistair).

• Air is made up of five main gases i.e. Nitrogen78.03%, Oxygen 20.99%, Argon 0.94%, CarbonDioxide 0.03%, and Hydrogen 0.01% by volume.

• Water vapor adds some complexity because it mightcondense.

• Applications– Air conditioning– Evaporative cooling– Cooling towers

‒ Meteorology‒ Industrial processes (Drying)

Human Comfort &Air-Conditioning

We cannotchange theweather, but wecan change theclimate in aconfined spaceby air-conditioning.

A body feels comfortable when itcan freely dissipate its wasteheat, and no more.

Today, modern air-conditioning systems can heat,cool, humidify, dehumidify, clean, and evendeodorize the air–in other words, condition the airto peoples’ desires.The rate of heat generation by human bodydepends on the level of the activity. For anaverage adult male, it is about 87 W whensleeping, 115 W when resting or doing officework, and 440 W when doing heavy physical work.When doing light work or walking slowly, abouthalf of the rejected body heat is dissipatedthrough perspiration as latent heat while theother half is dissipated through convection andradiation as sensible heat.

A comfortable environment.

The comfort of the human bodydepends primarily on three factors: the(dry-bulb) temperature, relativehumidity, and air motion.The relative humidity affects the amountof heat a body can dissipate throughevaporation. Most people prefer arelative humidity of 40 to 60%.Air motion removes the warm, moist airthat builds up around the body andreplaces it with fresh air. Air motionshould be strong enough to remove heatand moisture from the vicinity of thebody, but gentle enough to beunnoticed.An important factor that affects humancomfort is heat transfer by radiationbetween the body and the surroundingsurfaces such as walls and windows.

In an environment at 10°C with 48km/h winds feels as cold as anenvironment at -7°C with 3 km/hwinds as a result of the body-chilling effect of the air motion(the wind-chill factor).

Air-conditioning Processes

Various air-conditioning processes.

Maintaining a living space or anindustrial facility at the desiredtemperature and humidity requiressome processes called air-conditioning processes.These processes include simpleheating (raising the temperature),simple cooling (lowering thetemperature), humidifying (addingmoisture), and dehumidifying(removing moisture).Sometimes two or more of theseprocesses are needed to bring theair to a desired temperature andhumidity level.Air is commonly heated &humidified in winter and cooled &dehumidified in the summer time.

Background (Gas Mixtures)

• Apply the rules for determining mixtureproperties to ideal-gas mixtures.

• Predict the P-v-T behavior of gas mixturesbased on Dalton’s law of additive pressures

• Understand the concept of partial pressures.

Gas Mixture - Dalton’s ModelPartial Pressures

Each gas existed alone at the mixture temperature andvolume.

Dalton’s Law

• A sample relationship exists between the totalpressure and individual partial pressuresMOLE FRACTION

Dalton’s LawDef:

Mole Fraction = mols of one substance = Xtotal moles

(XA + XB +XC =1)

Partial Pressure = pressure exerted byonly one gas in a mixture.

PA = nARuT/V or PA = XAPTOT

DRY AND ATMOSPHERIC AIRAtmospheric air: Air in the atmosphere containing somewater vapor (or moisture).Dry air: Air that contains no water vapor.Water vapor in the air plays a major role in humancomfort. Therefore, it is an important consideration inair-conditioning applications.

The cp of air can beassumed to be constant at1.005 kJ/kg·°C in thetemperature range 10 to50°C with an error under0.2%.

Water vapor in air behaves as if it existed alone andobeys the ideal-gas relation Pv = RT. Then theatmospheric air can be treated as an ideal-gas mixture:

Pa Partial pressure of dry airPv Partial pressure of vapor (vapor pressure)

Below 50°C, the h = const. lines coincidewith the T = const. lines in thesuperheated vapor region of water.

h = h(T ) since watervapor is an ideal gas

For waterhg = 2500.9 kJ/kg at 0°Ccp,avg = 1.82 kJ/kg · °C at 10 to 50°C range

In the temperature range10 to 50°C, the hg of watercan be determined from Eq.

14-4 with negligible error.

SPECIFIC AND RELATIVE HUMIDITY OF AIRAbsolute or specific humidity (humidityratio): The mass of water vapor present in aunit mass of dry air.

For saturated air, the vapor pressureis equal to the saturation pressure ofwater.

Saturated air: The air saturated with moisture.Relative humidity: The ratio of the amount ofmoisture the air holds (mv) to the maximumamount of moisture the air can hold at thesame temperature (mg).

The difference between specificand relative humidities.

What is the relative humidity ofdry air and saturated air?

In most practical applications, theamount of dry air in the air–water-vapor mixture remains constant, butthe amount of water vapor changes.Therefore, the enthalpy ofatmospheric air is expressed perunit mass of dry air.

The enthalpy of moist (atmospheric) air isexpressed per unit mass of dry air, not per unitmass of moist air.

Dry-bulb temperature:The ordinary temperatureof atmospheric air.

DEW-POINTTEMPERATURE

Constant-presssure cooling of moistair and the dew-point temperature onthe T-s diagram of water.

Dew-point temperature Tdp: Thetemperature at which condensationbegins when the air is cooled atconstant pressure (i.e., the saturationtemperature of water correspondingto the vapor pressure.)

When the temperature of acold drink is below the dew-point temperature of thesurrounding air, it “sweats.”

ADIABATIC SATURATION TEMPERATUREAND WET-BULB TEMPERATURE

The adiabatic saturation processand its representation on a T-sdiagram of water.

The specific humidity (and relative humidity) of aircan be determined from these equations bymeasuring the pressure and temperature of air at theinlet and the exit of an adiabatic saturator.

A simple arrangement tomeasure the wet-bulbtemperature.

The adiabatic saturationprocess is not practical. Todetermine the absoluteand relative humidity ofair, a more practicalapproach is to use athermometer whose bulbis covered with a cottonwick saturated with waterand to blow air over thewick.The temperaturemeasured is the wet-bulbtemperature Twb and it iscommonly used in A-Capplications.

For air–water vapor mixtures at atmosphericpressure, Twb is approximately equal to theadiabatic saturation temperature.

Sling psychrometer

THE PSYCHROMETRIC CHART

Schematic for a psychrometric chart. For saturated air, the dry-bulb, wet-bulb, anddew-point temperatures are identical.

Psychrometric charts: Present moist air properties in a convenient form. They are usedextensively in A-C applications. The psychrometric chart serves as a valuable aid invisualizing the A-C processes such as heating, cooling, and humidification.

DryBlub

RelativeHumidity

DewPoint

100%Saturation

EnthalpykJ / kg

WetBlub

Grains ofMoisture

SpecificVolume

Breakdown of the Lines

AIR-CONDITIONING PROCESSES

Various air-conditioning processes.

Maintaining a living space or anindustrial facility at the desiredtemperature and humidity requiressome processes called air-conditioningprocesses.These processes include simple heating(raising the temperature), simplecooling (lowering the temperature),humidifying (adding moisture), anddehumidifying (removing moisture).Sometimes two or more of theseprocesses are needed to bring the airto a desired temperature and humiditylevel.Air is commonly heated and humidifiedin winter and cooled and dehumidifiedin summer.

Most air-conditioning processes can be modeled as steady-flowprocesses with the following general mass and energy balances:

The work term usually consists of the fan work input, which is smallrelative to the other terms in the energy balance relation.

Energy balance

Mass balance

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Simple Heating and Cooling ( = constant)

During simple heating, specific humidity remainsconstant, but relative humidity decreases.

Many residential heating systems consist of a stove, a heat pump, or an electricresistance heater. The air in these systems is heated by circulating it through a duct thatcontains the tubing for the hot gases or the electric resistance wires.Cooling can be accomplished by passing the air over some coils through which arefrigerant or chilled water flows.Heating and cooling appear as a horizontal line since no moisture is added to or removedfrom the air.

Dry air mass balance

Water mass balanceEnergy balance

During simple cooling, specifichumidity remains constant, butrelative humidity increases.

Heating with HumidificationProblems with the low relative humidity resulting from simple heating can beeliminated by humidifying the heated air. This is accomplished by passing the air firstthrough a heating section and then through a humidifying section.

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Cooling with DehumidificationThe specific humidity of air remains constant during a simple cooling process, but itsrelative humidity increases. If the relative humidity reaches undesirably high levels, itmay be necessary to remove some moisture from the air, that is, to dehumidify it. Thisrequires cooling the air below its dew-point temperature.

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Evaporative Cooling

Water in a porous jug left in an open,breezy area cools as a result ofevaporative cooling.

In desert (hot and dry) climates, we canavoid the high cost of conventional coolingby using evaporative coolers, also known asswamp coolers.As water evaporates, the latent heat ofvaporization is absorbed from the waterbody and the surrounding air. As a result,both the water and the air are cooled duringthe process.

This process is essentially identical toadiabatic saturation process.

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Adiabatic Mixing of Airstreams

When two airstreams at states 1 and 2 aremixed adiabatically, the state of the mixture

lies on the straight line connecting the twostates.

Many A-C applications require the mixing of twoairstreams. This is particularly true for largebuildings, most production and process plants, andhospitals, which require that the conditioned air bemixed with a certain fraction of fresh outside airbefore it is routed into the living space.

Wet Cooling Towers

Power plants, large air-conditioningsystems, and some industries generatelarge quantities of waste heat that isoften rejected to cooling water fromnearby lakes or rivers.In some cases, however, the coolingwater supply is limited or thermalpollution is a serious concern.In such cases, the waste heat must berejected to the atmosphere, withcooling water recirculating and servingas a transport medium for heat transferbetween the source and the sink (theatmosphere).One way of achieving this is through theuse of wet cooling towers.A wet cooling tower is essentially asemi-enclosed evaporative cooler.

An induced-draft counterflow cooling tower.

Two natural draft cooling towers on aroadside. A spray pond.

Natural-draft cooling tower: It looks like a large chimney and works like an ordinarychimney. The air in the tower has a high water-vapor content, and thus it is lighter thanthe outside air. Consequently, the light air in the tower rises, and the heavier outsideair fills the vacant space, creating an airflow from the bottom of the tower to the top.Spray pond: The warm water is sprayed into the air and is cooled by the air as it fallsinto the pond,Cooling pond: Dumping the waste heat into a still pond, which is basically a largeartificial lake open to the atmosphere.

Summary• Dry and atmospheric air• Specific and relative humidity of air• Dew-point temperature• Adiabatic saturation and wet-bulb temperatures• The psychrometric chart• Human comfort and air-conditioning• Air-conditioning processes

– Simple heating and cooling– Heating with humidification– Cooling with dehumidification– Evaporative cooling– Adiabatic mixing of airstreams– Wet cooling towers