SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
1.0
INTRODUCTION The SOLTEQ ® Basic Cooling Tower Unit (Model (Model:: HE152) HE152) has been been design designed ed to demonstrate students the construction, design and operational characteristics of a modern cooling system The unit resembles a full si!e forced draught cooling to"er and it is actually an #open system# through "hich t"o streams of fluid (in this case air and "ater) pass and in "hich there is a mass transfer from one stream to the other The unit is self$ contained supplied "ith a heating load and a circulating pump %nce energy and mass balances are done, students "ill then be able to determine the effects on the performance of the cooling to"er by the follo"ing parameters:
a) b) c)
Temperature and flo" rate of "ater &elati'e Humidity and flo" rate of air ooling load dditionally, a *ac+ing haracteristics olumn (optional) is a'ailable for SOLTEQ® Basic
Cooling Tower Unit (Model: HE152) This column is designed to facilitate study of "ater and air conditions at three additional stations (, and ) "ithin the column This enables dri'ing dri'ing force diagrams diagrams to be constructed constructed and the determinatio determinationn of the haracteristi haracteristicc E-uation for the To"er
1
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
2.0
ENER!L DESCRI"TIONS
2.1 2.1 Tower Unit
Co#$ Co#$on onen ents ts o% o% t&e t&e 'E1( 'E1(22 Basi Basicc Cool Coolin ing g
The unit comes complete "ith the follo"ing main components:
i)
Loa* Tan+ The The load load tan+ tan+ is made made of stai stainl nles esss stee steell ha'ing a capacity of appro.imately / litres The tan+ is fitted "ith t"o cartridge heaters, 05 + and 10 + each, to pro'ide a total of 15 + cooling load ma+e$up tan+ is fi.ed on top of the load tan+ float type 'al'e at the bottom of the ma+e$up tan+ is to control the amount of "ater flo"ing into the load tan+ cent centri rifu fuga gall type type pump pump is supp supplilied ed for for circ circul ulat atin ingg the the "ate "aterr from from the the load load tan+ tan+ through a flo" meter to the top of the column, into the basin and bac+ to the load tan+ temp emperat eratur uree sen sensor sor and temp tempeeratu rature re cont contro rollller er is fitt fitted ed to load load tan+ tan+ to pre' pre'en entt o'erheating le'el s"itch is fitted to the load tan+ so that "hen a lo" le'el condition occurs, the heater and the pump "ill be s"itched off
ii) ii) !ir !ir Distr Distri, i,-t -tio ion n C&a#, C&a#,er er The stainless stainless steel steel air distribut distribution ion chamber chamber comes "ith a "ater collecting basin and a one$side inlet centrifugal fan The fan has a capacity of appro.imately 25 3M of air flo" flo" The air flo" rate is ad4ustable by means of an inta+e damper
iii) iii) Col-#n Col-#n an* "ac+in "ac+ing g %ne pac+ed column is a'ailable The column is a standard column that comes together "ith this unit The column is made of clear acrylic "ith a s-uare cross$sectional area of 225 cm 2 and a height of 0 cm t comes "ith eight dec+s of inclined pac+ing top column that fitted on top of the column comes standard "ith a sharp edged orifice, a droplet arrester and a "ater distribution system *ac+ed column: 110 m 26m
2
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
i) /eas-re#ents Temperature sensors are pro'ided to measure the inlet and outlet "ater temperatures as "ell as the ma+e$up tan+ "ater temperature n addition, temperature sensors ha'e been installed to measure the dry bulb and "et bulb temperatures of inlet and outlet of the air The follo"ings sho" the list of codes assigned to each temperature sensors T1 T2 T T9 T5 T T
et 7ulb Temperature of the %utlet ir 8ry 7ulb Temperature of the %utlet ir nlet ater Temperature %utlet ater Temperature et 7ulb Temperature of the nlet ir Ma+e$up Tan+ ater Temperature 8ry 7ulb Temperature of the nlet ir
n inclined manometer is pro'ided for the measurement of pressure drop across the pac+ed column %n the other hand, the inclined manometer and the orifice are also used to determine the air flo" rate flo" meter is pro'ided for the measurement of "ater flo" rate The flo" meter is ranged at 09 to 9 ;*M
3
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
1 2 9 5
%rifice ater 8istributor *ac+ed olumn 3lo" meter &ecei'er tan+
< / 10
4
ir 7lo"er 8ifferential *ressure Transmitter Ma+e$up Tan+ ontrol *anel ;oad tan+
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
2.2
T&e "rocess Inole* in t&e O$eration i)
ater Circ-it ater temperature in the load tan+ "ill be increased before the "ater is pumped through a control 'al'e and flo" meter to the column cap 7efore entering the column cap, the inlet temperature of the "ater is measured and then the "ater is uniformly distributed o'er the top pac+ing dec+ This creates a large thin film of "ater, "hich is e.posed to the air stream The "ater gets cooled do"n, "hile passing do"n"ard through the pac+ing, due to the e'aporation process The cooled "ater falls into the basin belo" the lo"est dec+ and return to the load tan+ "here it is re$heated before re$circulation The outlet temperature is measured at a point 4ust before the "ater flo"s bac+ into the load tan+ E'aporation causes the "ater le'el in the load tan+ to fall The amount of "ater lost by e'aporation "ill be automatically compensated by e-ual amount from the ma+e$up tan+ t steady state, this compensation rate e-uals the rate of e'aporation plus any small airborne droplets discharged "ith the air
ii) !ir Circ-it one$side inlet centrifugal fan dra"s the air from the atmosphere into the distribution chamber The air flo" rate is 'aried by means of an inta+e damper The air passes a dry bulb temperature sensor and "et bulb temperature sensors before it enters the bottom of the pac+ed column hile the air stream passes through the pac+ing, its moisture content increases and the "ater temperature drops The air passes another duct detector measuring its e.it temperature and relati'e humidity, then through a droplet arrester and an orifice, and finally lea'es the top of the column into the atmosphere
2.
Oerall Di#ensions Height idth
: :
125 m 0/1 m
5
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
8epth
2.
:
095 m
eneral Re3-ire#ents Electricity ater >upply
: 115=61$phase60H! : ;aboratory ater >upply
6
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
.0
SU//!R4 O5 T'EOR4 .1
Basic "rinci$le 3irst consider an air stream passing o'er the surface of a "arm "ater droplet or film f "e assume that the "ater is hotter than the air, then the "ater temperature "ill be cooled do"n by radiation, conduction and con'ection, and e'aporation The radiation effect is normally 'ery small and may be neglected onduction and con'ection depend on the temperature difference, the surface area, air 'elocity, etc The effect of e'aporation is the most significant "here cooling ta+es place as "ater molecules diffuse from the surface into the surrounding air 8uring the e'aporation process, the "ater molecules are replaced by others in the li-uid from "hich the re-uired energy is ta+en
.2
Ea$oration %ro# a et S-r%ace hen considering e'aporation from a "et surface into the surrounding air, the rate is determined by the difference bet"een the 'apour pressure at the li-uid surface and the 'apour pressure in the surrounding air The 'apour pressure at the li-uid surface is basically the saturation pressure corresponding "ith the surface temperature, "hereas the total pressure of the air and its absolute humidity determines the 'apour pressure in the surrounding air >uch e'aporation process in an enclosed space shall continue until the t"o 'apour pressures are e-ual n other "ords, until the air is saturated and its temperature e-uals the surface Ho"e'er, if unsaturated air is constantly supplied, the "et surface "ill reach an e-uilibrium temperature at "hich the cooling effect due to the e'aporation e-uals the heat transfer to the li-uid by conduction and con'ection from the air, "hich under these conditions? "ill be at a higher temperature @nder adiabatic conditions, this e-uilibrium temperature is the #"et bulb temperature# 3or a cooling to"er of infinite si!e and "ith an ade-uate air flo", the "ater lea'ing "ill be at the "et bulb temperature of the incoming air Therefore, the difference bet"een the temperature of the "ater lea'ing a cooling to"er and the local "et bulb temperature is an indication of the effecti'eness of the cooling to"er Thus, #pproach to et 7ulb#, an important parameter of cooling to"ers, is the difference bet"een the temperature of the "ater lea'ing the to"er and the "et bulb temperature of the entering air
.
Cooling Tower "er%or#ance study on the performance of a cooling to"er can be done "ith the help of a bench top unit >tudents shall be able to 'erify the effect of these factors on the cooling to"er performance:
7
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
(i) (ii) (iii) (i')
ater flo" rates ater temperatures irflo" rate nlet ir &elati'e Humidity The effect of these factors "ill be studied in depth by 'arying it n this "ay, students "ill gain an o'erall 'ie" of the operation of cooling to"er
.
T&er#o*6na#ic "ro$ert6 n order to understand the "or+ing principle and performance of a cooling to"er, a basic +no"ledge of thermodynamic is essential to all students brief re'ie" on some of the thermodynamic properties is presented belo" t the triple point (ie 00002 atm and 001A), the specific enthalpy of saturated "ater is assumed to be !ero, "hich is ta+en as datum The specific enthalpy of saturated "ater (hf ) at a range of temperatures abo'e the datum condition can be obtained from thermodynamic tables The specific enthalpy of compressed li-uid is gi'en by h = h f + v f ( p − p sat )
(1)
The correction for pressure is negligible for the operating condition of the cooling to"er? therefore "e can see that h ≈ hf at a gi'en temperature Specific heat capacity (C p) is defined as the rate of change of enthalpy "ith respect to temperature (often called the specific heat at constant pressure) 3or the purpose of e.periment using bench top cooling to"er, "e may use the follo"ing relationship:
∆h = C p ∆T
(2)
and h = C pT
()
here C p B 91< +C+g $1
..1
Dalton7s an* i,,s Laws t is commonly +no"n that air consists of a mi.ture of #dry air# (% 2, D2 and other gases) and "ater 'apour 8alton and ibbs la" describes the beha'iour of such a mi.ture as: a) The total pressure of the air is e-ual to the sum of the pressures at 8
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
"hich the #dry air# and the "ater 'apour each and alone "ould e.ert if they "ere to occupy the 'olume of the mi.ture at the temperature of the mi.ture b) The dry air and the "ater 'apour respecti'ely obey their normal property relationships at their partial pressures c) The enthalpy of the mi.ture may be found by adding together the enthalpies at "hich the dry air and "ater 'apour each "ould ha'e as the sole occupant of the space occupied by the mi.ture and at the same temperature The bsolute or >pecific Humidity is defined as follo"s: Specific Humidity , ω =
Mass of Water Vapour Mass of Dry Air
(9)
The &elati'e Humidity is defined as follo"s: &e lative Humidity , φ
=
Partial *r essure of Water Vapour in the Air Saturation *r essure of Water Vapour at the same temperatur e
(5) The *ercentage >aturation is defined as follo"s: Percentage Saturation
=
Mass of Water Vapour in a given Volume of Air Mass of same vol of Sat Water Vapour at the same Temp
() t high humidity conditions, it can be sho"n that there is not much difference bet"een the #&elati'e Humidity# and the #*ercentage >aturation# and thus "e shall regard as the same To measure the moisture content of the atmosphere, this bench top cooling to"er unit is supplied "ith electronic dry bulb and "et bulb temperature sensors The temperature readings shall be used in con4unction "ith a psychrometric chart
..2
"s6c&ro#etric C&art The psychrometric chart is 'ery useful in determining the properties of air6"ater 'apour mi.ture mong the properties that can be defined "ith psychrometric chart are 8ry 7ulb Temperature, et 7ulb Temperature, &elati'e Humidity, Humidity &atio, >pecific =olume, and >pecific Enthalpy Fno"ing t"o of these properties, any other property can be easily 9
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
identified from the chart pro'ided the air pressure is appro.imately atmospheric n the 7ench Top ooling To"er application, the air inlet and outlet sensor sho" the dry bulb temperature and "et bulb temperature Therefore, the specific enthalpy, specific 'olume, humidity ratio and relati'e humidity can be readily read from the psychrometric chart The psychrometric chart pro'ided "ith this manual is only applicable for atmospheric pressure operating condition (101 bar) Ho"e'er, the error resulting from 'ariation of local atmospheric pressure normally is negligible up to altitudes 500m abo'e sea le'el
.(
Ori%ice Cali,ration s mentioned abo'e, the psychrometric chart can be used to determine the 'alue of the specific 'olume Ho"e'er, the 'alues gi'en in the chart are for 1 +g of dry air at the stated total pressure Ho"e'er, for e'ery 1 +g of dry air, there is +g of "ater 'apour, yielding the total mass of 1 G +g Therefore, the actual specific 'olume of the air6'apour mi.ture is gi'en by: v a =
v a! 1 + ϖ
()
The mass flo" rate of air and steam mi.ture through the orifice is gi'en by m = 00137
" v a
(<) here, m B Mass flo" rate of air6'apour mi.ture v a B ctual specific 'olume and . B %rifice differential in mmH 20 Thus,
10
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
" (1 + ϖ )
m = 00137
v a!
(/) The mass flo" rate of dry air, m a
=
m a =
1 1
+ ϖ 1
1 + ϖ
× Mass flo rate of air 6 vapor mi"ture × 00137
m a = 00137
" (1 + ϖ ) v a!
" v a! (1 + ϖ )
(10) simplification can be made since in this application, the 'alue of ϖ is unli+ely to e.ceed 0025 s such, neglecting b "ould not yield significant error
.8
!$$lication o% Stea*6 5low Energ6 E3-ation onsider >ystem for the cooling to"er defined as in 3igure 1 t can be seen that for this system, indicated by the dotted line, a) b) c) d) e)
Heat transfer at the load tan+ and possibly a small -uantity to surroundings or+ transfer at the pump ;o" humidity air enters at point High humidity air lea'es at point 7 Ma+e$up enters at point E, the same amount as the moisture increase in the air stream
11
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
B
a m
E
E m
a m A
Work, P
Heat, Q
5ig-re 29 System A 3rom the steady flo" e-uation,
− H Q − P = H exit entry a hda + m s h s ) B − ( m a hda + m s h s ) A − m E hE Q − P = ( m (11) No!: The pump po"er, * is a "or+ input Therefore it is negati'e f the enthalpy of the air includes the enthalpy of the steam associated "ith it, and this -uantity is in terms of per unit mass of dry air, the e-uation may then be "ritten as: a ( h B − h A ) − m E hE Q − P = m (12) No!: m a) The mass flo" rate of dry air ( a ) through a cooling to"er is a constant, "hereas the mass flo" rate of moist air increases as the result of e'aporation process b) The term m E h E can usually be neglected since its 'alue is relati'ely small @nder steady state conditions, by conser'ation of mass, the mass flo" rate of dry air and of "ater (as li-uid or 'apour) must be the same at inlet and
12
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
outlet to any system Therefore,
( m ) a
A
=
( m ) a
B
(1) and
( m s ) A + m E = ( m s ) B
or
E = ( m s ) B − ( m s ) A m (19) The ratio of steam to air ( ϖ ) is +no"n for the initial and final state points on the psychrometric charts Therefore,
( m s ) A = m aϖ A
and (15)
( m s ) # = m aϖ # (1) Therefore,
E m
= m a (ϖ B − ϖ A ) (1)
13
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
>ay, "e re$define the cooling to"er system to be as in 3igure 2 "here the process heat and pump "or+ does not cross the boundary of the system n this case "arm "ater enters the system at point and cool "ater lea'es at point 8
B
a m
C w m E
E m a m A D
5ig-re 9 System # gain from the steady flo" energy e-uation, $ − P = H e"it − H entry
and
P =0
Q may ha'e a small 'alue due to heat transfer bet"een the unit and its surroundings
a h B + m w h D − ( m a h A + m w hC + m E h E ) Q = m
(1<)
&earranging,
a ( h B Q = m
− h A ) + m w ( h D − hC ) − m E h E = m a ( h B − h A ) + m wC p ( t D − t C ) − m E hE
E h E can be neglected gain, the term m
14
(1/)
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
.:
C&aracteristics Col-#n St-*6 n order to study the pac+ing characteristics, "e define a finite element of the to"er (d!) as sho"n in 3igure , the energy balances of the "ater and air streams in the to"er are related to the mass transfer by the follo"ing e-uation: C pW m W dT = % a dV ( ∆h )
(20)
"here C pW
B >pecific heat capacity of "ater
m W B Mass flo" rate of "ater per unit plan area of pac+ing T B ater Temperature % B Mass Transfer oefficient a B rea of contact bet"een air and "ater per unit 'olume of pac+ing V B =olume occupied by pac+ing per unit plan area
∆h
B 8ifference in specific enthalpy bet"een the saturated boundary layer and the bul+ air
T2
t2
WATER
H2
INLET
mw
h2 AIR ma OUTLET
dz z
WATER
T1
OUTLET
H1 mw
t1 h1 ma
AIR INLET
"#$%&! 4: >chematic &epresentation of the ir and ater >treams entering and lea'ing a 7loc+ of *ac+ing
n this e-uation, "e assume that the boundary layer temperature is e-ual to the "ater temperature T and the small change in the mass of "ater is neglected Thus, from E-uation 20, % a dV m W
=
C pW dT
∆h
(21)
15
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
ntegrating E-uation 21, %a V m W
T 2
= C pW
∫ h
T 1
dT
− ha
(22)
The numerical solution to the integral e.pression E-uation 22 using hebyshe' numerical method gi'es, %a V m W
T 2
= C p
W
∫ h
T 1
dT
− ha
=
T 2 − T 1 4
1 1 1 1 + + + ∆ ∆ ∆ ∆ h h h h 1
2
3
4
(2)
here %a V m W
∆h1
B To"er haracteristic B 'alue of h − h a at T 2
+ 0.1( T − T ) 1
2
∆h 2 B 'alue of h − h a at T 2 + 04(T 1 − T 2 ) ∆h 3 B 'alue of h − h a at T 1 − 0.4( T 1 − T 2 ) ∆h 4 B 'alue of h − h a at T 1
− 0.1( T − T ) 1
2
Thermodynamics state that the heat remo'ed from the "ater must be e-ual to the heat absorbed by the surrounding air Therefore, the follo"ing e-uation is deri'ed: '( T 2 − T 1 ) = &( h a 2 − ha1 )
(29)
or, ' &
=
( ha − ha ) (T − T ) 2 2
1
(25)
1
here, ' & B ;i-uid to gas mass flo" ratio T 1 B old "ater temperature T 2 B Hot "ater temperature h a 2 B Enthalpy of air$"ater 'apour mi.ture at e.haust "et$bulb temperature h a1 B Enthalpy of air$"ater 'apour mi.ture at inlet "et$bulb temperature
16
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
h "2 (Hot "ater Temp) Enthalpy
Enthalpy 3orce ater8ri'ing %perating ;ine (h"$ha) 8
ha2 (ir out) h "1 (old "ater Temp)
7
>aturation ur'e ha1 (ir in)
ir %perating ;ine ;6
pproach
&ange
T "b (n)
T1T "b 2 (%ut) Temperature
"#$%&! 5: raphical &epresentation of To"er haracteristics The follo"ing represents a +ey to 3igure 5: 7 B nitial enthalpy dri'ing force 8 B ir operating line "ith slope ;6 &eferring to E-uation 22, the to"er characteristics could be found by finding the area bet"een 78 in 3igure 9 ncreasing heat load "ould ha'e the follo"ing effects on the diagram in 3igure 9: 1 ncrease in the length of line 8, and a 8 line shift to the right 2 ncrease in hot and cold "ater temperatures ncrease in range and approach areas The increased heat load causes the hot "ater temperature to increase considerably faster than does the cold "ater temperature lthough the area 78 should remain constant, it actually decreases about 2 for e'ery 10 03 increase in hot "ater temperature abo'e 100 03 To account for this decrease, an #ad4usted hot "ater temperature# is used in cooling to"er design
17
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
.;
Use%-l In%or#ation 1
%rifice alibration 3ormula: Mass flo" rate of air and 'apour mi.ture,
= 0.0137 m
x(1 + ϖ ) v ab
The mass flo" rate of dry air,
a = 0.0137 m
x v ab (1 + ϖ )
here, " B orifice differential in mmH 20, v a B B specific 'olume of air at the outlet ϖ B humidity ratio of the mi.ture 2
*ump or+ nput B <0 (00<+)
olumn nner 8imension B 150 mm . 150 mm . 00 mm
18
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
.0
E<"ERI/ENT!L "ROCEDURES .1
eneral O$erating "roce*-res .1.1
eneral Start=-$ "roce*-res 1 hec+ to ensure that 'al'es =1 to =< are closed 2 3ill the load tan+ "ith distilled or deionised "ater t is done by first remo'ing the ma+e$up tan+ and then pouring the "ater through the opening at the top of the load tan+ &eplace the ma+e$up tan+ onto the load tan+ and lightly tighten the nuts 3ill the tan+ "ith distilled or deionised "ater dd distilled6deionised "ater to the "et bulb sensor reser'oir to the fullest 9 onnect all appropriate tubing to the differential pressure sensor 5 nstall the appropriate cooling to"er pac+ing for the e.periment Then, set the temperature set point of temperature controller to 50A >"itch on the 10 + "ater heater and heat up the "ater until appro.imately 90A >"itch on the pump and slo"ly open the control 'al'e near the flo" meter and set the "ater flo" rate to 20 ;*M %btain a steady operation "here the "ater is distributed and flo"ing uniformly through the pac+ing < 3ully open the fan damper, and then s"itch on the fan hec+ that the differential pressure sensor is gi'ing reading "hen the 'al'e manifold is s"itched to measure the orifice differential pressure / ;et the unit run for about 20 minutes, for the float 'al'e to correctly ad4ust the le'el in the load tan+ &efill the ma+eup tan+ "hen re-uired 10 Do", the unit is ready for use
i
ii
iii i' '
No!: t is strongly recommended that %D;I distilled or deionised "ater be used in this unit The impurities e.isting in tap "ater may cause the depositing in co'er to"er hec+ that the pressure tubing for differential pressure measurement is connected correctly (;ea'e = to atmosphere? connect the columnJs higher pressure tube to =9, orifice pressure tube to =5 and columnJs lo"er pressure tube to =) To measure the differential pressure across the orifice, open 'al'e = and =5? close 'al'e =9 and = To measure the differential pressure across the column, open 'al'e =9 and =? close 'al'e = and =5 l"ays ma+e sure that no "ater is in the pressure tubing for accurate differential pressure measurement
19
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
.1.2
eneral S&-t=Down "roce*-re 1>"itch off heaters and let the "ater to circulate through the cooling to"er system for $5 minutes until the "ater is cooled do"n 2>"itch off the fan and fully close the fan damper >"itch off the pump and po"er supply 9&etain the "ater in reser'oir tan+ for the follo"ing e.periment 5ompletely drain off the "ater from the unit if it is not in use
.2
E>$eri#ent 19 eneral O,seration o% t&e 5orce* Dra-g&t Cooling Tower O'!#*!: To obser'e the process "ithin a forced draught cooling to"er 1 *erform the general start$up procedures and obser'e the forced draught cooling to"er process 2 s the "arm "ater enters the top of the to"er, it is fed into channels from "hich it flo"s 'ia "ater distribution system onto the pac+ing The channels are designed to distribute the "ater uniformly o'er the pac+ing "ith minimum splashing The pac+ing surfaces are easily "etted and the "ater spreads o'er the surfaces to e.pose a large area to the air stream 9 The cooled "ater falls from the lo"est pac+ing into the basin and then is pumped to the simulated load in the load tan+ 5 8uring the process, some "ater is lost due to the e'aporation Thus, #ma+e$ up# "ater must be supplied to +eep the amount of "ater in the cooling system constant The ma+e$up is obser'ed flo"ing past the float$controlled 'al'e in the load tan+ Kdroplet arresterL, or Kmist eliminatorL is fitted at the to"er outlet to minimi!e loss of "ater due to escape of droplets of "ater (resulted from splashing, etc) "hich is entrained in the air stream This loss does not contribute to the cooling, but must be made good by #ma+e$up# The droplet arrester causes droplets to coalesce, forming drops that are too large to be entrained and thus the droplets fall bac+ into the pac+ing The fan dri'es the air up"ard through the "et pac+ing t air outlet, the air lea'ing the cooling to"er is almost saturated, ie &elati'e Humidity is 100 The &elati'e Humidity at the air outlet is much higher than the &elati'e Humidity at the air inlet The increase in the moisture content of air is due to the e'aporation of "ater into steam and the #latent heat# for this account for most of the cooling effect < hen the cooling load is s"itched off and the unit is allo"ed to stabili!e, it is found that the "ater lea'es the basin at temperature close to the "et bulb temperature of the air entering et bulb temperature is lo"er than the dry bulb temperature and this 'aries according to the local atmospheric conditions (ie pressure and relati'e humidity) / ith no load, the "ater "ould be cooled to the incoming "et bulb temperature Ho"e'er, the condition cannot be achie'ed since the "or+ done by the pump transfers about <0 to the "ater
20
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
.
E>$eri#ent 29 En* State "ro$erties o% !ir an* Stea*6 5low E3-ations O'!#*!: To determine the Kend stateL properties of air and "ater from tables or charts To determine Energy and mass balances using the steady flo" e-uation on the selected systems P&o!+%&!: 1 *repare and start the cooling to"er "ith according to >ection 911 2 >et the system under the follo"ing conditions and allo" stabili!ing for about 15 minutes ater flo" rate : 20 ;*M ir 3lo" : Ma.imum ooling load : 10 + 3ill up the ma+e$up tan+ "ith distilled "ater, record the initial "ater le'el and then start the stop "atch 9 8etermine the ma+e$up "ater supply in an inter'al of 10 minutes 5 n this 10 minutes inter'al, record a fe" sets of the measurements (eg temperatures (T1NT), orifice differential pressure (8*1), "ater flo" rate (3T1) and heater po"er (O1)), then obtain the mean 'alue for calculation and analysis 8etermine the -uantity of ma+e$up "ater that has been supplied during the time inter'al by noting the height reduction in the ma+e$up tan+ The obser'ation may be repeated at different conditions: i 8ifferent "ater flo" rates ii 8ifferent air flo" rates iii 8ifferent load ,--#$./!. 9 1 alculate the ma+e$up rate 2 alculate the energy and mass balances by using the steady flo" e-uation
21
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
.
E>$eri#ent 9 Inestigation o% t&e E%%ect o% Cooling Loa* on et B-l, !$$roac& O'!#*!: To in'estigate the effect of cooling load on Ket 7ulb pproachL P&o!+%&!: 1 *repare and start the cooling to"er "ith according to >ection 911 2 >et the system under the follo"ing conditions and allo" stabili!ing for about 15 minutes ater flo" rate : 20 ;*M ir 3lo" : Ma.imum ooling load : 05 + fter the system stabili!es, record a fe" sets of measurements (eg air inlet "et bulb and dry bulb temperature (T5 and T), "ater outlet temperature (T9), orifice differential pressure (8*1), "ater flo" rate (3T1) and heater po"er (O1)), then obtain the mean 'alue for calculation and analysis 9 ithout changes in the conditions, increase the cooling load to 10 + hen the system stabili!ed, record all data 5 >imilarly, repeat the e.periment at 15 + 3inally, measure the cross sectional area of the column The four tests may be repeated at another constant airflo" < The e.periment may also be repeated at different: i ater flo" rates ii ir flo" rates iii ;oad ,--#$./!. : 1 alculate the Kapproach to "et bulbL and total cooling load 2 *lot a graph to sho" that the relationship bet"een cooling loads and approach to "et bulb temperature
22
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
.(
E>$eri#ent 9 Inestigation o% t&e E%%ect o% !ir ?elocit6 on et ,-l, !$$roac& an* "ress-re Dro$ t&ro-g& t&e "ac+ing O'!#*!: To in'estigate the effect of air 'elocity on: (a) et 7ulb pproach (b) The pressure drop through the pac+ing P&o!+%&!: 1 *repare and start the cooling to"er "ith according to >ection 911 2 >et the system under the follo"ing conditions and allo" stabili!ing for about 15 minutes ater flo" rate : 20 ;*M ir flo" rate : Ma.imum ooling load : 10 + fter the system stabili!es, record a fe" sets of measurements (ie temperature (T1$T5 and T), orifice differential pressure (8*1), "ater flo" rate (3T1), heater po"er (O1) and pressure drop across pac+ing (8*2)), then obtain the mean 'alue for calculation and analysis 9 &epeat the test "ith different sets of orifice pressure drop 'alues (5, 50 and 25 of the ma.imum 'alue) "ithout changing the "ater flo" rate and cooling loads This can be done by ad4usting the opening of the fan damper 5 3inally, measure the cross sectional area of the column The test may be repeated at another constant: i ;oad ii ater flo" rate ,--#$./!.: 1 alculate the nominal 'elocity of air and find the Kapproach to "et bulbL 2 *lot a graph to sho" that the relationship bet"een Kapproach to "et bulbL and pac+ing pressure drop 'ersus nominal air 'elocity
23
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
.8
E>$eri#ent (9 Inestigation o% t&e Relations&i$ ,etween Cooling Loa* an* Cooling Range O'!#*!: To in'estigate the relationship bet"een cooling load and cooling range P&o!+%&!: 1 *repare and start the cooling to"er "ith according to >ection 911 2 >et the system under the follo"ing conditions and allo" stabili!ing for about 15 minutes: ater flo" rate : 20 ;*M ir flo" rate : Ma.imum ooling load : 05 + fter the system stabili!ed, record a fe" sets of measurements (eg temperature (T1$T5 and T), orifice differential pressure (8*1), "ater flo" rate (3T1) and heater po"er (O1)), then obtain the mean 'alue for calculation and analysis 9 ithout changes in the conditions, increase the cooling load to 10 + hen the system stabili!ed, record all data 5 >imilarly, repeat the e.periment at 15 + The tests may be repeated at other constant: i ater flo" rate ii ir flo" rate ,--#$./!.: 1 *lot a graph to sho" that the relationship bet"een cooling loads and cooling range
24
SOLTEQ® BENCH TOP COOLING TOWER UNIT (MODEL: HE152)
(.0
RE5ERENCES *erry, &H, reen, 8 and Maloney, C%, K*erryJs hemical Engineering Handboo+L, th Edition, Mcra" Hill, 1/<9
25
