humidification and cooling towers - 2nd 2011 (1)

8/3/2019 Humidification and Cooling Towers - 2nd 2011 (1)

1/46

HUMIDIFICATION / DEHUMIDIFICATION

PROCESSES


2/46

PSYCHROMETRY

Psychrometry is concerned withdetermination of the properties of gas-vapor

mixtures. Most common:AIR-WATER VAPORsystem

For other systems, principles involved in

determining psychrometric properties are thesame as with air-water vapor system, exceptforpsychrometric ratio.


3/46

Review of TERMINOLOGY

28.97

18.02

pP

pH

_

The humidityorabsolute humidity(H) of anair-water vapor mixture is defined as the kgwater vapor contained in 1 kg of dry air.

P total pressure; p partial pressure of water vapor


4/46

The saturation humidity(HS)is

28.97

18.02

pP

p

H S_

S

S

P total pressure; pS vapor pressure of water vapor


5/46

Thepercentage humidity(HP)is

100pP

pP

p

p

H

H

H

S

SSP

Thepercentage relative humidity(RH)is

100p

pRH

S


6/46

The dew point(Td)of an air-water mixture isthe temperature at which a given mixture of

air and water vapor would be saturated.

The humid heat(cs)of an air-water vapor

mixture is the amount of heat in J (or kJ)required to raise the temperature of 1 kg ofdry air plus the water vapor present by 1 Kor 1oC.

airdrykJ/kg1.88H1.005cs

airdrybtu/lb0.45H0.24c ms


7/46

The humid volume(vH)of an air-water vapormixture is the total volume in m3 of 1kg of

dry air plus the vapor it contains at 101.325kPa absolute pressure and the given gastemperature.

d.a./kgmH18.02

1

28.97

1TK

273

22.41v 3H

d.a./lbftH18.021

28.971RT

492359v m

3oH


8/46

The enthalpy(h)of an air-water vapormixture is the total enthalpy of 1 kg of air

plus its water vapor in J/kg or kJ/kg dry air.If To is the datum temperature chosen forboth components then:

kJ/kgH

C)T1.88H)(T(1.005H

)T(Tch 0o

0

_

00

_

s

m0

o

0

_

00

_

s btu/lbHF)T0.45H)(T(0.24H)T(Tch

If T0

= 0oC = 32oF

kJ/kg2501.4H0)C1.88H)(T(1.005h _o0

m

_o

0 Btu/lb1075.4H32)F0.45H)(T(0.24h


9/46

ADIABATIC SATURATION TEMPERATURE, TS

Outlet gas

HS, TS

Inlet gas

H, T

Makeup H2O

TS

The adiabatic saturation temperatureis the steady-state equilibrium attained when a large amount ofwater is contacted by the entering gas.

If the entering gas at T has H < HS, then TS < T.

If equilibrium is attained, the leaving air is

saturated at TS with H = HS.

Adiabatic water vaporsaturatorTS


10/46

WET BULB TEMPERATURE, Tw

The wet-bulb temperatureis the steady-state non-equilibrium temperature reached when a small amountof water is contacted under adiabatic conditions by acontinuous stream of gas.

The wet-bulb temperature is often used to determinethe humidity of an air-water vapor mixture.

)AT(ThH)A(Hk'Q w_

C

_

ww

w

C

_

w

_

w

_

/k'h

TT

HH

12.1b Perrys

k = kGMBP

where:


11/46

gs

C

s

C

1.6kc

h

k'c

hPR

PSYCHROMETRIC RATIO (PR): ratio of heat-transfercoefficient to the product of mass-transfer coefficient andhumid heat.

0.56

S

0.56

C

PrScc

0.294Sc

k'

hfor air-water vapor

for other gases andliquids


12/46

If PR = 1 (e.g., for air-water vapor system):

For air-water vapor mixtures, PR is approximately0.96 1.005.

WS TT

numberLewisLePr

Sc

For air-water vapor system:

PrSc 1.005ck'

hS

C and


13/46

Psychrometric

Chart (1 atm)

Figure 12-3Perrys


14/46


15/46

Fig. 12-36 p. 12-28

(Psychrometric Chart, SI)


16/46


17/46

HUMIDIFICATION / DEHUMIDIFICATIONPROCESSES


18/46


19/46


20/46


21/46


22/46

Application of Simultaneous Heat

and Mass Transfer in the Design of

WATER-COOLING TOWERS


23/46

Cooling Towers


24/46

Cooling Towers


25/46

CLOSED-LOOP COOLING TOWER SYSTEM

Cooling towers represent a relatively inexpensive and dependablemeans of removing low-grade heat from cooling water.

Hot water from heat exchangers is sent to the cooling tower.

The make-up water source is used to replenish water lost toevaporation.

The water exits the cooling tower and is sent back to the exchangersor to other units for further cooling.


26/46

TYPES OF MECHANICAL DRAFT TOWER

Mechanical Draft Counterflow Tower Mechanical Draft Crossflow Tower


27/46

COOLING TOWER THEORY

Heat is transferred from water drops to the surroundingair by the transfer of sensible heatand latent heat.

Water drop with interfacial film


28/46

Cooling Tower Theory

Temperature and concentration profiles in upper part ofcooling tower:interface

Sensible heat in

liquid

TL

Ti

TG

Liquid water airHi

HG

Water vapor

Latent heat in gas

Sensible heat in gas


29/46

Cooling Tower Process Heat Balance


30/46

where:KaV/L = tower characteristicK = mass transfer coefficient (lb water/h ft2)a = contact area/tower volumeV = active cooling volume/plan areaL = water rate (lb/h ft2)T1 = hot water temperature (

0F or 0C)T2 = cold water temperature (

0F or 0C)

T = bulk water temperature (0F or 0C)ha = enthalpy of air-water vapor mixture at bulk water temperature

(J/kg dry air or Btu/lb dry air)hw = enthalpy of air-water vapor mixture at wet bulb temperature

(J/kg dry air or Btu/lb dry air)

This movement of heat can be modeled with a relation knownas the Merkel Equation:

T1

T2 awhh

dT

L

KaV12-8 Perrys


31/46

The tower characteristic value can be calculated by solving Equation12-8 with the Chebyshev numerical method:

A quicker but less accurate method is by the use of a nomograph (Figure

12-13 Perrys CHE HB)


32/46


33/46

Important three key points in cooling tower

design

A change in wet-bulb temperature (due to

atmospheric conditions) will notchange the tower

characteristic (Ka V/L)

A change in the cooling range will notchange Ka V/L. Only a change in L/G ratio will change Ka V/L.


34/46

Graphical Representation of Tower Characteristics

The following represents

a key to the figure:

C' = Entering air enthalpyat wet-bulb temperature,Twb

BC = Initial enthalpydriving force

CD = Air operating line

with slope L/G

DEF = Projecting the exiting air point onto the water operatingline and then onto the temperature axis shows the outlet air

wet-bulb temperature


35/46

Cooling Tower Design Considerations

The required tower size is a function of

Cooling range

Approach to wet-bulb temperature

Mass flow rate of water

Wet-bulb temperature

Air velocity through tower cell

Tower height


36/46

Figure 12-14 Perrys ChE HB

Utilizes the cold water temperature, wet bulb

temperature, and hot water temperature to find thewater concentration in gal/min ft2.

The tower area can then be calculated by dividing thewater circulated by the water concentration.

General rules are usually used to determine tower height

depending on the necessary time of contact:

Approach towet bulb (oF)

Cooling range(oF)

Tower height

(ft)

15

20 25 - 35 15 - 2010 15 25 - 35 25 - 30

5 - 10 25 - 35 35 - 40


37/46


38/46

Other design considerations toconsider

Fan horsepower

Pump horsepower

Make-up water source

Fogging abatement

Drift eliminators

WATER MAKEUP ( M k W t )


39/46

WATER MAKEUP (or Makeup Water)

Water losses include :

Evaporation0.00085*water flowrate(T1 T2)

Drift (water entrained in discharge vapor)

Drift losses are estimated to be between 0.1 and 0.2 %of water supply.

Blowdown (water released to discard solids)

Evaporation loss / (cycles 1)

cycles refers to the ratio of solids in the circulating water tothe solids in the make-up water

Total losses = Drift losses + Evaporation losses + Blowdown losses


40/46

CONTINUOUS COUNTERCURRENT

ADIABATIC WATER-COOLING TOWER

G1

G2

TG2

H2

Hy2

L2,

TL2

L1

Overall heat balance:G(Hy2 Hy1) = LcL(TL2 TL1) 10.5-2

Differential balance, neglecting sensible

heat terms (


41/46

EVALUATION OF TOWER HEIGHT:

y2

y1

H

H y

_

y i

y

GB

Z

0 HHdH

aPkMGZdz 10.5-13

y

_*

y

y

Hy 2

Hy1GB

Z

0 HHdH

aPKMGZdz 10.5-15


42/46

Minimum value of air flow

Hy2

Hy1

TL2TL1

Slope = LcL/G

Slope = LcL/Gmin


43/46

Problem 1

Air is to be cooled and dehumidified by counter-currentcontact with water in a packed tower. The tower is to be

designed for the following conditions:dry-bulb temp. of inlet air 82oF

wet-bulb temp. of inlet air 76.5oF

flow rate of inlet air 1,500 lb d.a./h

inlet water temp. 50oF

outlet water temp. 65oF

A. For entering air, find: H, RH, Td, h.

B. What is the maximum water rate which may be used tomeet design requirements, assuming a very tall tower?

C. Calculate the NTU required for a tower that meets designspecifications when 1,000 lb/h of water is used and if theliquid phase resistance to heat transfer is negligible.


44/46

Problem 2

1000 cfm of air (A) at 95oF dry-bulb / 74oF wet-bulb is mixedwith 2,000 cfm of air (B) at 65oF dry bulb / 54oF wet-bulb.Determine for the mixed stream.

A. T

B. TW

C. Cfm of mixed stream


45/46

PROBLEM 3

You have been requested to redesign a water-coolingtower that has a blower with a capacity of 8.30 x 106

ft3/h of moist air (80oF dry bulb and 65oF wet bulbtemperature). The exit air leaves at 95oF and 90oFwet bulb. How much water can be cooled in poundsper hour if the water to be cooled is not recycled,

enters the tower at 120o

F, and leaves at 90o

F?


46/46

PROBLEM 4

A cooling tower that uses a cold-water spray provides a method ofcooling and dehumidifying a school. During the day, the average

number of students in the school is 100 and the average heat-generation rate per person is 800 Btu/hr. Suppose that the ambient

conditions outside the school in the summer are expected to be100oF and 95% RH. You run this air through the cooler-dehumidifierand then mix the saturated exit air with re-circulated air from theexhaust of the school building. You need to supply the mixed air tothe building at 70oF and 60% RH and keep the re-circulated air leaving

the building at not more than 72oF. Leakage occurs from the buildingof the 72oF air also. Calculate:

A. The volumetric flowrate of air recirculation per hour in ft3/hr at 70oF

and 60% RH.B. The volume of fresh air required at entering conditions.

C. The heat transferred in the cooler-dehumidifier from the inlet airper hour.

humidification and cooling towers - 2nd 2011 (1)

Documents