unit iv psychrometry€¦ · psychometry process • in the design and analysis of air conditioning...

UNIT IV PSYCHROMETRY

•Properties of moist Air•Gibbs-Daltons Law

•Specific Humidity (Humidity Ratio)•Dew point Temperature•Dry Bulb Temperature•Wet Bulb Temperature•Degree of Saturation(•Relative Humidity•Enthalpy

•Psychometric Chart•Various psychometric process

Psychrometric

• Is the study of the properties of mixtures of air and watervapour.

• Atmospheric air is a mixture of many gases plus water vapourand a number of pollutants.

• The amount of water vapour and pollutants vary from place toplace.

• The concentration of water vapour and pollutants decreasewith 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 theair. Hence, what we process is essentially a mixture of variousgases that constitute air and water vapour. This mixture isknown as moist air.

Basic gas laws for moist air

( Gibbs-Dalton law)

• According to the Gibbs-Dalton law for a mixture of perfect gases,

The total pressure exerted by the mixture is equal to the sum of partialpressures of the constituent gases. Each gas obeys perfect gas equation.Hence, the partial pressures exerted by each gas, p1,p2,p3 … and the totalpressure pt are given by:

where n1,n2,n3,… are the number of moles of gases 1,2,3,…

Where p = pt = Total barometric pressure

pa = partial pressure of dry air

pv = partial pressure of water vapour

Important Psychometry Terms

• Dry air,Moist air,Saturated air,Sling Psychometer

• DBT(td)

• WBT(tw)

• DPT(tdp)

• WBD=(DBT-WBT)

• DPD=(DBT-DPT)

• Specific Humidity (Humidity Ratio)(W) kg of Mositure/Kg of

moist air)

• W=0.622x Pv/(Pb-Pv) & Ws=0.622x Ps/(Pb-Ps)

• Relative Humidity(Φ) ,Pv/Ps or Mv/Ms

• Absolute humidity(g/m3) of mosit air

• Degree of saturation(μ)=W/Ws

• Enthalpy(h)=1.022 td+W[hfgdp+2.3 tdp]

• Specific Volume(v)=RaTd/Pa

• Vapor density(ρ)=W(Pb-Pv)/RaTd

Important psychometric properties:

• Dry bulb temperature (DBT)

• is the temperature of the moist air as

measured by a standard thermometer or

other temperature measuring instruments.

Wet bulb temperature

• It is the temperature of air recorded by the

thermometer,when its bulb is surrounded

by wetted cloth exposed to the air.such a

thermometer is known as wet bulb

thermometer. Generally denoted by tw

Wet bulb depression

• DBT-WBT

• It indicate the relative humidity of the air.

Dew-point temperature:

• If unsaturated moist air is cooled atconstant pressure, then thetemperature at which the moisture inthe air begins to condense is known asdew-point temperature (DPT) of air. Anapproximate equation for dew-pointtemperature is given by:

Relative humidity (Φ)

• is defined as the ratio of the mole fraction of watervapour in moist air to mole fraction of watervapour in saturated air at the same temperatureand pressure. Using perfect gas equation we canshow that: Relative humidity is normallyexpressed as a percentage. When Φ is 100percent, the air is saturated.

Humidity ratio (W):

• The humidity ratio (or specific humidity) W isthe mass of water associated with each kilogramof dry air. Assuming both water vapour and dryair to be perfect gases, the humidity ratio isgiven by: Substituting the values of gas constantsof water vapour and air Rv and Ra in the aboveequation; the humidity ratio is given by:

Degree of saturation μ:

• The degree of saturation is the

ratio of the humidity ratio W to

the humidity ratio of a saturated

mixture Ws at the same

temperature and pressure, i.e.,

Enthalpy:

The enthalpy of 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.

Enthalpy(h)=1.022td+W[hfgdp+2.3tdp]

Psychometry Process

• In the design and analysis of air conditioningplants, the fundamental requirement is to identifythe various processes being performed on air.

• Once identified, the processes can be analyzedby applying the laws of conservation of mass andenergy.

• All these processes can be plotted easily on apsychometric chart.

• This is very useful for quick visualization andalso for identifying the changes taking place inimportant properties such as temperature,humidity ratio, enthalpy etc.

• The important processes that air undergoes in atypical air conditioning plant are discussedbelow.

•

SENSIBLE COOLING

Cooling of air ,DBT decreases at const

specific Humidity.ie (W1=W2)

• During this process, the moisture content of airremains constant but its temperature decreases as itflows over a cooling coil. For moisture content toremain constant, the surface of the cooling coilshould be dry and its surface temperature should begreater than the dew point temperature of air. If thecooling coil is 100% effective, then the exittemperature of air will be equal to the coiltemperature. However, in practice, the exit airtemperature will be higher than the cooling coiltemperature.

Figure 1 shows the sensible cooling process O-A on a

psychometric chart.

• The heat transfer rate during this

process is given by• Cpm= humid specific heat =1.002 kj/kgk

• Qc = ρVCpm(To-TA) Kj/min

=1.2xvx1.022x(To-TA) Kj/min

=0.02044 v (To-TA) Kj/sec or KW

Sensible heating (Wo=Wa)

• 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:

• where c pmis the humid specific heat

(≈1.0216 kJ/kg dry air) and ma is the mass

flow rate of dry air (kg/s).

Figure 2 shows the sensible heating

process on a psychrometric chart.

Cooling and dehumidification

• When moist air is cooled below its dew-point bybringing it in contact with a cold surface asshown in Fig. some of the water vapor in the aircondenses and leaves the air stream as liquid, asa result both the temperature and humidity ratioof air decreases as shown. This is the process airundergoes in a typical air conditioning system.Although the actual process path will varydepending upon the type of cold surface, thesurface temperature, and flow conditions, forsimplicity the process line is assumed to be astraight line. The heat and mass transfer ratescan be expressed in terms of the initial and finalconditions by applying the conservation of massand conservation of energy equations as givenbelow

By applying mass balance for the water:

Heating and Humidification

• During winter it is essential to heat and

humidify the room air for comfort. As shown

in Fig.5., 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.

Types of AC ystem

As a machine

• Window AC system

• Split or Ductable AC system

• Central AC system

Types of AC ystem

Classify according to operation

• Summer AC system

• Winter AC system

• Year around AC system

• Industrial AC system

• As shown in the figure, the air conditioning(A/C) plant acts either as a heat source (incase of winter systems) or as a heat sink (incase of summer systems). Air, water orrefrigerant are used as media for transferringenergy from the air conditioning plant to theconditioned space.

• A thermal distribution system is required tocirculate the media between the conditionedspace and the A/C plant. Another importantfunction of the thermal distribution system isto introduce the required amount of fresh airinto the conditioned space so that therequired Indoor Air Quality (IAQ) can bemaintained.

Selection criteria for air

conditioning systems:

1. Capacity, performance and spatial requirements

2. Initial and running costs

3. Required system reliability and flexibility

4. Maintainability

5. Architectural constraints