humidification and cooling towers - 2nd 2011 (1)
TRANSCRIPT
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HUMIDIFICATION / DEHUMIDIFICATION
PROCESSES
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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.
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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
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The saturation humidity(HS)is
28.97
18.02
pP
p
H S_
S
S
P total pressure; pS vapor pressure of water vapor
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Thepercentage humidity(HP)is
100pP
pP
p
p
H
H
H
S
SSP
Thepercentage relative humidity(RH)is
100p
pRH
S
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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
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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
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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
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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
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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:
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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
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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
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Psychrometric
Chart (1 atm)
Figure 12-3Perrys
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Fig. 12-36 p. 12-28
(Psychrometric Chart, SI)
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HUMIDIFICATION / DEHUMIDIFICATIONPROCESSES
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Application of Simultaneous Heat
and Mass Transfer in the Design of
WATER-COOLING TOWERS
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Cooling Towers
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Cooling Towers
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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.
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TYPES OF MECHANICAL DRAFT TOWER
Mechanical Draft Counterflow Tower Mechanical Draft Crossflow Tower
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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
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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
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Cooling Tower Process Heat Balance
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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
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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)
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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.
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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
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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
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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
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Other design considerations toconsider
Fan horsepower
Pump horsepower
Make-up water source
Fogging abatement
Drift eliminators
WATER MAKEUP ( M k W t )
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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
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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 (
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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
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Minimum value of air flow
Hy2
Hy1
TL2TL1
Slope = LcL/G
Slope = LcL/Gmin
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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.
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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
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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?
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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.