Chapter 12

Ideal Gas Mixture and Psychrometric Applications

Learning Outcomes►Describe ideal gas mixture composition in terms

of mass fractions and mole fractions.►Use the Dalton model to relate pressure, volume,

and temperature and to calculate changes in U, H, and S for ideal gas mixtures.

►Apply mass, energy, and entropy balances to systems involving ideal gas mixtures, including mixing processes.

Learning Outcomes, cont.►Demonstrate understanding of psychrometric

terminology, including humidity ratio, relative humidity, mixture enthalpy, and dew point temperature.

►Use the psychrometric chart to represent common air-conditioning processes and to retrieve data.

►Apply mass, energy, and entropy balances to analyze air-conditioning processes and cooling towers.

Describing Mixture Composition

(Eq. 12.1)

►The mass mi, number of moles ni, and molecular weight Mi of component i are related by

i

ii M

mn

►Consider a system consisting of a number of gases within a container of volume V. The temperature and pressure of the gas mixture are T and p, respectively.►The composition of the mixture can be described by giving the mass mi or the number of moles ni for each component present.

►ni is in kmol when mi is in

kg and Mi is in kg/kmol.

►ni is in lbmol when mi is in

lb and Mi is in lb/lbmol.

Gas 1: n1, m1

Gas 2: n2, m2

Gas j: nj, mj

Sum: n m

…

Describing Mixture Composition

►The mass fraction is the relative amount of each component in the mixture. The mass fraction mfi of component i is

(Eq. 12.3)m

mmf i

i

►The sum of the mass fractions of all components in a mixture equals unity.

where m is the total mass of mixture.

Describing Mixture Composition►Alternatively, the mole fraction can be used to describe the relative amount of each component in the mixture. The mole fraction yi of component i is

n

ny ii

(Eq. 12.6)

i

j

iiMyM

1

(Eq. 12.9)

►The sum of the mole fractions of all components in a mixture equals unity.

►The apparent (or average) molecular weight M of a mixture is determined as a mole-fraction average of the component molecular weights:

where n is the total moles of mixture.

Describing Mixture Composition

Example: The molar analysis of a gas mixture is 50% N2, 35% CO2, and 15% O2. Determine (a) the apparent molecular weight of the mixture and (b) the analysis in terms of mass fractions.

(a) The apparent molecular weight of the mixture is found using Eq. 12.9 and molecular weights (rounded) from Table A-1

3215.04435.02850.0 M = 34.2 kg/kmol

Solution:

Describing Mixture Composition

►Then, the amount ni of each component, in kmol, is equal to its mole fraction, as shown in column (ii).►Column (iii) lists the respective molecular weights.►Column (iv) gives the mass mi of each component, in kg per kmole of mixture, obtained using mi = niMi (Eq. 12.1).►The mass fractions, listed as percentages in column (v), are obtained by dividing the values in column (iv) by the column total and multiplying by 100.

(i)Component

(ii)ni

× (iii)Mi

= (iv)mi

(v)mfi %

N2 0.50 × 28 = 14 40.94

CO2 0.35 × 44 = 15.4 45.03

O2 0.15 × 32 = 4.8 14.04

1.00 34.2 100

(b) Although the actual amount of mixture is not known, the calculations can be based on any convenient amount. We use 1 kmol of mixture.

Relating p, V, and T for Ideal Gas Mixtures►Many systems of practical interest involve mixtures where the overall mixture and each of its components can be modeled as ideal gases. For such mixtures the Dalton mixture model is commonly used.►The overall mixture is considered an ideal gas

V

TRnp (Eq. 12.10)

►The Dalton model also assumes each component behaves as an ideal gas as if it were alone at temperature T and volume V.

Gas 1: n1, m1

Gas 2: n2, m2

Gas j: nj, mj

Sum: n m

…

Gas 1: n1, m1

Gas 2: n2, m2

Gas j: nj, mj

Sum: n m

…

Gas 1: n1, m1

Gas 2: n2, m2

Gas j: nj, mj

Sum: n m

…

Relating p, V, and T for Ideal Gas Mixtures►Accordingly, with the Dalton model the individual components do not exert the mixture pressure p but rather a partial pressure denoted by pi:

V

TRnp ii (Eq. 12.11)

pyp ii (Eq. 12.12)

►By combining Eqs. 12.10 and 12.11 the partial pressure pi can be determined alternatively from

where the sum of the partial pressures equals the mixture pressure

(Eq. 12.13)

j

iipp

1

Gas 1: n1, m1

Gas 2: n2, m2

Gas j: nj, mj

Sum: n m

…

Gas 1: n1, m1

Gas 2: n2, m2

Gas j: nj, mj

Sum: n m

…

Gas 1: n1, m1

Gas 2: n2, m2

Gas j: nj, mj

Sum: n m

…

Evaluating U, H, and S for Ideal Gas Mixtures

►For an ideal gas mixture, the values of U, H, and S are evaluated by adding the contribution of each component at the condition at which the component exists in the mixture.

►Evaluation of the specific internal energy or specific enthalpy of a mixture component i requires only a single intensive property: the mixture temperature, T. ►Evaluation of the specific entropy of a mixture component i requires two intensive properties. We will use the mixture temperature, T, and the partial pressure, pi.

Evaluating U, H, and S for Ideal Gas Mixtures (Molar Basis)

►Accordingly, when working on a molar basis expressions for U, H, and S of a mixture consisting of several components are:

Evaluating U, H, and S for Ideal Gas Mixtures (Molar Basis)

►See Sec. 12.4 for applications using these expressions for U, H, S, and the specific heats.

►The mixture specific heats and are mole-fraction averages of the respective component specific heats.

vc pc

(Eq. 12.24)(Eq. 12.23)

Evaluating U, H, and S for Ideal Gas Mixtures(Mass Basis)

►When working on a mass basis the expressions for U, H, S, and specific heats of a mixture consisting of two components – a binary mixture – are:

Table 12.2

Engineering Applications of Ideal Gas Mixtures

►We encounter ideal gas mixtures in many important areas of application. Two of these are:

1. Systems involving chemical reactions and, in particular, combustion. For these applications we typically work on a molar basis. Combustion systems are considered in Chapter 13.

2. Systems for air-conditioning and other applications requiring close control of water vapor in gas mixtures. For these applications we typically work on a mass basis. Systems of this type are considered in the second part of Chapter 12.

Psychrometric Applications

►The remainder of this presentation centers on systems involving moist air. A condensed water phase may also be present in such systems.►The term moist air refers to a mixture of dry air and water vapor in which the dry air is treated as a pure component.

►The Dalton model applies to moist air.►By identifying gas 1 with dry air and gas 2 with water vapor, Table 12.2 gives moist air property relations on a mass basis.

►The study of systems involving moist air is known as psychrometrics.

Moist Air

►Consider a closed system consisting of moist air occupying a volume V at mixture pressure p and mixture temperature T.

►The Dalton model applies to the mixture of dry air and water vapor:

►In moist air the amount of water vapor present is much less than the amount of dry air:

mv << ma nv << na.

Moist Air

(Eq. 12.41b)

1. The overall mixture and each component, dry air and water vapor, obey the ideal gas equation of state.

2. Dry air and water vapor within the mixture are considered as if they each exist alone in volume V at the mixture temperature T while each exerts part of the mixture pressure.

3. The partial pressures pa and pv of dry air and water vapor are, respectively

pa = ya p pv = yv p

where ya and yv are the mole fractions of the dry air and water vapor, respectively. These moist air expressions conform to Eqs. (c) of Table 12.2.

Mixture pressure, p

,T

Moist Air

4. The mixture pressure is the sum of the partial pressures of the dry air and the water vapor:

p = pa + pv

5. A typical state of water vapor in moist air is fixed using partial pressure pv and the mixture temperature T.

The water vapor is superheated at this state.

Typical state of the water vapor

in moist air

Moist Air

6. When pv corresponds to pg at temperature T, the mixture is said to be saturated.

pTp

p

,g

v

(Eq. 12.44)

Relative humidity is usually expressed as a percent and ranges as 0 ≤ ≤ 100%dry air only

(pv = 0)saturated air

(pv = pg)

Mixture pressure, p

,T

Mixture pressure, p

,T

7. The ratio of pv and pg is called the relative humidity, :

Humidity Ratio►The humidity ratio of a moist air sample is the ratio of the mass of the water vapor to the mass of the dry air.

a

v

m

m (Eq. 12.42)

v

v622.0pp

p

(Eq. 12.43)

Since mv << ma, the value of is typically << 1.

►Using the ideal gas equation of state and the relationship pa = p – pv

a

v

m

m

TRVpM

TRVpM

/

/

aa

vvaa

vv

pM

pM

v

v

a

v

pp

p

M

M

18.02/28.97 = 0.622

Mixture Enthalpy

►Values for U, H, and S for moist air can be found by adding contributions of each component.

(Eq. 12.47)

►For example, the enthalpy H is

which conforms to Eq. (d) in Table 12.2.

►Dividing by ma and introducing , the mixture enthalpy per unit mass of dry air is

►For moist air, the enthalpy hv is very closely given by the saturated vapor value corresponding to the given temperature.

vvaava hmhmHHH (Eq. 12.45)

vava

va

a hhh

m

mh

m

H (Eq. 12.46)

Thh gv

Heating Moist Air in a Duct

Example: Moist air enters a duct at 10oC, 80% relative humidity, is heated as it flows through the duct, and exits at 30oC. No moisture is added or removed and the mixture pressure remains constant at 1 bar. For steady-state operation and ignoring kinetic and potential energy changes, determine (a) the humidity ratio, 2, and (b) the rate of heat transfer, in kJ per kg of dry air.

Heating Moist Air in a Duct

(a) At steady state, mass rate balances for the dry air and water vapor read:

Solution:

or)(water vap

air)(dry

v2v1

a2a1

mm

mm

a

v

m

m

Since the mass flow rates of the dry air and water vapor do not change from inlet to exit, they are denoted for simplicity as ma and mv. Moreover, since no moisture is added or removed, the humidity ratio does not change from inlet to exit: 1 = 2. The common humidity ratio is denoted by .

∙ ∙

Heating Moist Air in a Duct

The humidity ratio is evaluated using data at the inlet:

•The partial pressure of the water vapor at the inlet, pv1, can be evaluated from the given inlet relative humidity 1 and the saturated pressure pg1 at 10oC from Table A-2:

pv1 = 1pg1 = 0.8(0.01228 bar) = 0.0098 bar

•The humidity ratio can be found from:

v

v622.0pp

p

0098.01

0098.0622.0

air)(dry kg

(vapor) kg00616.0

Heating Moist Air in a Duct

(b) The steady-state form of the energy rate balance reduces to:

)()(0 v2va2av1va1acvcv hmhmhmhmWQ 0

•Solving for Qcv

∙

)()( v1v2va1a2acv hhmhhmQ

•Noting that mv = ma, we get

)()( v1v2a1a2a

cv hhhhm

Q

∙ ∙

Heating Moist Air in a Duct

)()( v1v2a1a2a

cv hhhhm

Q

(vapor) kg

kJ)8.25193.2556(

air)(dry kg

(vapor) kg00616.0

air)(dry kg

kJ)1.2832.303(

a

cv

m

Q

air)(dry kg

kJ)22.01.20(

a

cv m

Q

air)(dry kg

kJ32.20

For the dry air, ha1 and ha2 are obtained from ideal gas

table Table A-22 at 10oC and 30oC, respectively.

For the water vapor, hv1 and hv2 are obtained from steam table Table A-2 at 10oC and 30oC, respectively, using

hv ≈ hg

The contribution of the water vapor

to the heat transfer

magnitude is relatively

minor.

Dew Point Temperature►When moist air is cooled, partial condensation of the water vapor initially present can occur. This is observed in condensation of vapor on window panes, pipes carrying cold water, and formation of dew on grass.►An important special case is cooling of moist air at constant mixture pressure, p.►The figure shows a sample of moist air, initially at State 1,where the water vapor is superheated. The accompanying T-vdiagram locates

states of water.►Let’s study this system as it is cooled in stages from its initial temperature.

Dew Point Temperature►In the first part of the cooling process, the mixture pressure and water vapor mole fraction remain constant.

►Since pv = yv p, the partial pressure of the water vapor remains constant.

►Accordingly, the water vapor cools at constant pv from state 1 to state d, called the dew point.►The temperature at state d is called the dew point temperature.

►As the system cools below the dew point temperature, some of the water vapor initially present condenses. The rest remains a vapor.

Dew Point Temperature►At the final temperature, initially present plus saturated water vapor and saturated liquid.►Since some of the water vapor initially present has condensed, the partial pressure of the water vapor at the final state, pg2, is less than the partial pressure initially, pv1. ►The amount of water that condenses, mw, equals the difference in the initial and final amounts of water vapor:

the system consists of the dry air

mw = mv1 – mv2

Dew Point Temperature

►Using mv = ma and the fact that the amount of dry air remains constant, the amount of water condensed per unit mass of dry air is

21a

w m

m

where

and p denotes the mixture pressure, which remains constant while cooling occurs.

v1

v11 622.0

pp

p

g2

g22 622.0

pp

p

Dry-bulb Temperature and Wet-bulb Temperature

►In engineering applications involving moist air, two readily-measured temperatures are commonly used: the dry-bulb and wet-bulb temperatures.

►The dry-bulb temperature, Tdb, is simply the temperature measured by an ordinary thermometer placed in contact with the moist air.►The wet-bulb temperature, Twb, is the temperature measured by a thermometer whose bulb is enclosed by a wick moistened with water.

Dry-bulb Temperature and Wet-bulb Temperature

►The figure shows wet-bulb and dry-bulb thermometers mounted on an instrument called a psychrometer. Flow of moist air over the two thermometers is induced by a battery-operated fan.

►Owing to evaporation from the wet wick to the moist air, the wet-bulb temperature reading is less than the dry-bulb temperature: Twb < Tdb.►Each temperature is easily read from its respective thermometer.

MoistAir in

Psychrometric Chart►Graphical representations of moist-air data are provided by psychrometric charts.►Psychrometric charts in SI and English units are given in Figs. A-9 and A-9E, respectively. These charts are constructed for a moist air mixture pressure of 1 atm.►Several important features of the psychrometric chart are discussed in Sec. 12.7, including

Psychrometric Chart

►Dry-bulb temperature, Tdb.

Moist air state

Tdb

Psychrometric Chart

►Humidity ratio, .

Moist air state

Psychrometric Chart

►Dew point temperature, Tdp.►Since the dew point is the state where moist air becomes saturated when cooled at constant pressure, the dew point for a given state is determined from the chart by following a line of constant (constant pv) to the saturation line where = 100%.

Moist air state

Tdp

Psychrometric Chart

►Relative humidity, .

Moist air state

Psychrometric Chart

►Mixture enthalpy per unit mass of dry air, (ha + hv).

Moist air state

(ha + hv)

ha = cpaT

Fig. 12.9: T in oC, cpa = 1.005 kJ/kg-KFig. 12.9E: T in oF, cpa = 0.24 Btu/lb-R

The value of (ha + hv) is calculated using

Psychrometric Chart

►Wet-bulb temperature, Twb.►Lines of constant wet-bulb temperature are approximately lines of constant mixture enthalpy.

Moist air state

Twb

Psychrometric Chart

►Lines giving V/ma can be interpreted as the volume of dry air or of water vapor (each per unit mass of dry air) because in keeping with the Dalton model each component is considered to fill the entire volume.

►Volume per unit mass of dry air, V/ma.

Moist air state

V/ma

Psychrometric ChartExample: Using Fig. A-9, determine relative humidity, humidity ratio, and mixture enthalpy, in kJ/kg (dry air) corresponding to dry-bulb and wet-bulb temperatures of 30oC and 25oC, respectively.

Psychrometric Chart

Solution:

25oC = 67%

= 0.0181 kg water/kg dry air

(ha + hv) = 76 kJ/kg dry air

Analyzing Air-Conditioning Systems

►The next series of slides demonstrates the application of mass and energy rate balances together with property data to typical air-conditioning systems using the psychrometric principles introduced thus far.►Featured applications include

►Dehumidification►Humidification►Mixing of two moist air streams

►An application of psychrometric principles to a cooling tower is also considered.

Dehumidification

►The aim of a dehumidifier is to remove some of the water vapor in the moist air passing through the unit.►This is achieved by allowing the moist air to flow across a cooling coil carrying a refrigerant at a temperature low enough that some water vapor condenses.

Dehumidification

►The figure shows a control volume enclosing a dehumidifier operating at steady state.

►Moist air enters at state 1.►As the moist air flows over the cooling coil, some water vapor condenses.►Saturated moist air exits at state 2 (T2 < T1).►Condensate exits as saturated liquid at state 3. Here, we take T3 = T2.

ma, T1, 1

T3 = T2

21

3

2 = 100%, T2 < T1, 2 < 1

∙

mw∙

Dehumidification

►For the control volume, let us evaluate

►The amount of condensate exiting per unit mass of dry air: mw/ma and►The rate of heat transfer between the moist air and cooling coil, per unit mass of dry air: Qcv/ma.

∙ ∙

∙∙

T3 = T2

21

3

mw∙

ma, T1, 1∙ 2 = 100%,

T2 < T1, 2 < 1

Dehumidification►Mass rate balances. At steady state, mass rate balances for the dry air and water are, respectively

2vw1v

2a1a

mmm

mm

(dry air)

(water)

Solving for the mass flow rate of the condensate

2v1vw mmm

Then, with mv1 = 1ma and mv2 = 2ma, where ma denotes the common mass flow rate of the dry air, we get the following expression for the amount of water condensed per unit mass of dry air

21a

w m

m

(1)

∙∙ ∙ ∙∙

T3 = T2

21

3

mw∙

ma, T1, 1∙ 2 = 100%,

T2 < T1, 2 < 1

Dehumidification►Energy rate balance. With Wcv = 0 and no significant kinetic and potential energy changes, the energy rate balance for the control volume reduces at steady state to

With mv1 = 1ma, mv2 = 2ma, and Eq. (1), Eq. (2) becomes ∙ ∙ ∙∙

(2))()(0 v22va2awwv11va1acv hmhmhmhmhmQ

∙

Since heat transfer occurs from the moist air to the cooling coil, Qcv/ma will be negative in value.∙ ∙

w211va2vaa

cv )()()( hhhhhm

Q

(3)

Dehumidification

►Options for evaluating the underlined terms of Eq. (3) include

w211va2vaa

cv )()()( hhhhhm

Q

(3)

►1 and 2 are known. Since T1 and T2 are also known, ha1 and ha2 can be obtained from ideal gas table Table A-22, while hv1 and hv2 can be obtained from steam table Table A-2 using hv = hg.

►For the condensate, hw = hf (T2), where hf is obtained from Table A-2.

►Alternatively, using the respective temperature and humidity ratio values to fix the states, (ha + hv) at states 1 and 2 can be read from a psychrometric chart.

(ha + hv)2

(ha + hv)1

T1T2

Humidification

►The aim of a humidifier is to increase the amount of water vapor in the moist air passing through the unit.►This is achieved by injecting steam or liquid water.

Humidification

►The figure shows a control volume enclosing a humidifier operating at steady state.

►Moist air enters at state 1.►Steam or liquid water is injected.►Moist air exits at state 2 with greater humidity ratio, 2 > 1.

ma1

∙

3

m3∙h3,

Wcv = 0, Qcv = 0∙∙

Humidification

►For adiabatic operation, the accompanying psychrometric charts show states 1 and 2 for each case.

ma1

∙

3

m3∙h3,

Wcv = 0, Qcv = 0∙∙

►With relatively high-temperature steam injection, the temperature of the moist air increases.►With liquid injection the temperature of the moist air may decrease because the liquid is vaporized by the moist air into which it is injected.

Humidification

►For the control volume, let us evaluate

►The humidity ratio, 2, and

►The temperature, T2.

ma1∙

3

m3∙h3,

Wcv = 0, Qcv = 0∙∙

ma1∙

3

m3∙h3,

Wcv = 0, Qcv = 0∙∙Wcv = 0, Qcv = 0∙∙

Humidification►Mass rate balances. At steady state, mass rate balances for the dry air and water are, respectively

2v31v

2a1a

mmm

mm

(dry air)

(water)

Then, since mv1 = 1ma and mv2 = 2ma, where ma denotes the common mass flow rate of the dry air, we get

a

312 m

m

(1)

∙∙ ∙ ∙∙

ma1∙

3

m3∙h3,

Wcv = 0, Qcv = 0∙∙

ma1∙

3

m3∙h3,

Wcv = 0, Qcv = 0∙∙Wcv = 0, Qcv = 0∙∙

Since 1, ma, and m3 are specified, the humidity ratio 2 can be calculated from Eq. (1)

∙ ∙

Humidification►Energy rate balance. With no significant kinetic and potential energy changes, the energy rate balance for the control volume reduces to

With mv1 = 1ma and mv2 = 2ma, Eq. (2) becomes ∙ ∙ ∙∙

)()(0 v22va2a33v11va1acvcv hmhmhmhmhmWQ

Solving Eq. (3)

(3)

Since Wcv and Qcv are each zero in this case∙∙

)()(0 v2v2a2a33v1v1a1a hmhmhmhmhm (2)

)()()(0 v22a23a

3v11a1 hhh

m

mhh

3a

3v11a1v22a2 )()()( h

m

mhhhh

(4)

Humidification

►Options for determining T2 from Eq. (4) include►Use the psychrometric chart:

3a

3v11a1v22a2 )()()( h

m

mhhhh

(4)

11

(ha1 + 1hv1)

2

T1 T2

(ha2 + 2hv2)

2

Spec

ific e

ntha

lpy

of m

oist

air,

in k

J/kg

(dry

air)

•The first term on the right side of Eq. (4) can be read from the chart using T1 and 1 to fix the state.•Since the second term on the right is known, the value of (ha2 + 2hv2) can be calculated. •This value together with 2 fixes the exit state, which allows T2 to be determined by inspection.

Humidification

►Options for determining T2 from Eq. (4) include

►An iterative solution using data from Table A-22: ha(T) for the dry air and Table A-2: hv = hg(T) for the water vapor:

3a

3v11a1v22a2 )()()( h

m

mhhhh

(4)

• The value of the right side of Eq. (4) is known because the data are

either known or can be obtained from the indicated tables using T1.

• On the left side of Eq. (4), 2 is known from the mass rate balance.

• Accordingly, the only unknown is T2, which can be found iteratively:For each assumed value of T2, Table A-22 gives ha2 and Table A-2 gives hv2. This allows the left side to be calculated. Iteration with T2 continues until the calculated value on the left agrees with the known value on the right.

Adiabatic Mixing of Two Moist Air Streams

►In air-conditioning systems, a frequent component is one that mixes moist air streams as shown in the figure:

►For the case of adiabatic mixing, let us consider how the following quantities at the exit of the control volume, ma3, 3, and T3, can be evaluated knowing the respective quantities at the inlets.

∙

Adiabatic Mixing of Two Moist Air Streams►Mass rate balances. At steady state, mass rate balances for the dry air and water vapor are, respectively

3v2v1v

3a2a1a

mmm

mmm

(dry air)

(water vapor)

)( a2a13a22a11 mmmm

With mv = ma, these equations combine to give∙∙

Alternatively

31

23

a2

a1

m

m

(1)

These equations can be solved for 3 using known values of 1, 2, ma1, and ma2.

∙ ∙

Adiabatic Mixing of Two Moist Air Streams►Energy rate balance. Ignoring the effects of kinetic and potential energy, the energy rate balance for the control volume reduces at steady state to

)()()(0 v33va3a3v22va2a2v11va1a1cvcv hmhmhmhmhmhmWQ

Since Wcv and Qcv are each zero in this case∙∙

)()()( v33a3a3v22a2a2v11a1a1 hhmhhmhhm (Eq. 12.56c)

(2)

The enthalpies of the water vapor are evaluated using hv = hg. With ma3 = ma1 + ma2, Eq. 12.56c can be solved to give an expression with the same form as Eq. (1)

)()(

)()(

g33a3g11a1

g22a2g33a3

a2

a1

hhhh

hhhh

m

m

Using known data, this equation can be solved for (ha + hg)3, from which T3 can be evaluated.

∙ ∙ ∙

Adiabatic Mixing of Two Moist Air Streams

►From study of Eqs. (1) and (2) we conclude that on a psychrometric chart state 3 lies on a straight line connecting states 1 and 2, as shown in the figure

(2))()(

)()(

g33a3g11a1

g22a2g33a3

a2

a1

hhhh

hhhh

m

m

31

23

a2

a1

m

m

(1)

Adiabatic Mixing of Two Moist Air Streams

Example: For adiabatic mixing of two moist air streams with the data provided in the table below, use the psychrometric chart to determine(a) 3, in kg (vapor)/kg (dry air), and (b) T3 in oC.

State

12

T(oC)245

(kg (dry air)/kg (vapor))

0.00940.002

ma

(kg (dry air)/min)497180

(ha + hg)*

(kJ/kg (dry air))4810

*The values of (ha + hg) are read from Fig. A-9 using the respective temperature and humidity ratio values.

∙

Adiabatic Mixing of Two Moist Air Streams

(a) Inserting known values in Eq. (1),

Solution:

3

3

0094.0

002.0

180

497

we get 3 = 0.0074 kg (vapor)/kg (dry air).

T3 = 19oC

(b) Then from Fig. A-9

Adiabatic Mixing of Two Moist Air Streams

(a) Inserting known values in Eq. (1),

Solution:

3

3

0094.0

002.0

180

497

we get 3 = 0.0074 kg (vapor)/kg (dry air).

T3 = 19oC

(b) Then from Fig. A-9

Alternatively, Eq. (2) can be used to determine

(ha + hg)3 = 38 kJ/kg (dry air).

Then, from Fig. A-9

Cooling Towers

►Moist air principles also play a role in the analysis of cooling towers such as shown in the figure.

►Major events occurring within the control volume enclosing the tower include the following:

►The warm water to be cooled enters at 1 and is sprayed from the top of the tower.►Atmospheric air enters at 3 and flows counter to the falling water.

Cooling Towers

►As the liquid water and moist air interact within the tower, a fraction of the liquid evaporates, resulting in

►Since some of the incoming water has evaporated, an equivalent amount of makeup water is added at 5 so that the return mass flow rate equals the mass flow rate entering at 1.

• Liquid water that exits the tower at 2 with a lower temperature than the water entering at 1, which is the objective.• Moist air that exits the tower at 4 with a greater humidity ratio than the air entering at 3.

Cooling Towers►Mass rate balances. To evaluate the mass flow rate of the makeup water, apply mass rate balances to the control volume at steady state to get

4vw23v5w1

4a3a

mmmmm

mm

(dry air)

(water)

3v4v5 mmm

With mv3 = 3ma and mv4 = 4ma, where ma is the common mass flow rate of the dry air, this becomes

∙∙ ∙∙ ∙

)( 34a5 mm

►Energy rate balance. Application of the energy rate balance to such a cooling tower is demonstrated in Example 12.15.