Engineering Practice

Cooling Towers: Estimate Evaporation

Loss and Makeup Water Requirements
Applying mass and energy balance calculations yields critical operating insight
U. Vengateson FIGURE 1. Shown here is the
tin
National Petrochemical Co. typical variation of the water
(Saudi Arabia) temperature and the wet-
bulb temperature of the air

C
ooling towers are an impor- stream as the hot water inlet
tant unit operation in chem- stream flows down from the
top of the cooling tower and
ical process industries (CPI) the air stream flows upward Water Tw, out

Height of cooling tower, m

operations. Applying mass along the height of the cool-
and energy balance calculations en- ing tower
ables process engineers to evaluate tout
evaporation loss, blowdown and Air
makeup-water requirements, and
to evaluate the performance of the
cooling tower. In this article, an illus- Tw, in
trative study showcases an induced-
draft cooling tower and describes Range
Approach
several key parameters — range, (tout – Tw, in) (tin – tout)
approach and efficiency — and their
significance. Two methods are dis-
cussed to estimate evaporation loss.
Temperature, oC
Requirements for blowdown and
makeup water are also detailed. ization from the water, and thus the Case study
water is cooled. The CWR line from the process unit
Cooling tower operation As a rule of thumb, for every enters an industrial cooling tower at
The cooling of process streams 10°F (5.5°C) of water cooling, 1% 45°C and leaves at 33°C, as shown
and condensation of vapors are total mass of water is lost due to in Figure 2. The tower has three
important functions in CPI opera- evaporation. The humidity level cells, each operating at 2,500 m3/h
tions. The use of a cooling tower is of the up-flowing air stream in- of water flow. The total flow 7,500
the most common way of extract- creases, and once it leaves the m3/h is measured at the CWR line.
ing waste heat in CPI operations, tower the air stream is almost sat- The dry-bulb temperature and wet-
and water is the most commonly urated. The temperature profile of bulb temperature of the inlet air are
used coolant to remove waste heat the water and the wet-bulb tem- measured as 30.3°C and 29°C, re-
in the majority of such operations. perature of the air along the height spectively. The dry-bulb tempera-
A typical large petroleum refinery of a typical cooling tower is shown ture of the exit air is 41.5°C, and it
that processes 40,000 metric tons in Figure 1. is assumed to be 100% saturated.
(m.t.) of crude oil per day requires The cooled water is collected in This case study is aimed at calculat-
80,000 m3/h of cooling water. This the sump (or basin) of the cooling ing the unknown variables — that is,
is roughly equivalent to 25 barrels tower, and it is typically pumped to evaporation loss, air flow through the
of water for every barrel of crude oil the plant as the cooling-water-supply tower, blowdown flow, and the re-
processed [1]. (CWS) stream. After extracting heat quired makeup water flow. First, the
In a cooling tower, the hot water from the process units, this stream is important parameters — approach,
stream (typically called the cooling returned to the cooling tower, as the range and efficiency — are detailed.
water return) is introduced down- cooling-water-return (CWR) stream. Approach. The approach is defined
ward through spray nozzles into fills The heat load extracted from the as the difference between the water
inside the tower. There are different process unit is finally released to the temperature at the tower outlet (tout)
types of fills — splash, trickle and environment in the cooling tower. A and the wet-bulb temperature of the
film — that are aimed at creating cooling tower is designed to re- inlet air (Tw,in). The approach repre-
more surface area, to maximize con- move the total heat load that is ex- sents the cooling tower capability.
tact between the hot water stream tracted from the plant by reducing In general, the larger the tower, the
and air. As air rises inside the tower, the CWR temperature to the CWS smaller is the approach. In this case
it receives the latent heat of vapor- temperature. study, the approach is 4°C.
64 CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM APRIL 2017
 Theoretically, the extent of maxi- Components of the system:
1. Sump
Outlet air
eL = 132,000 kg/h
mum possible cooling that could be 2. Pump DBT (Tout) = 41.5oC
achieved through a cooling tower 3. Fan WBT (Tw,out) = 41.5oC
4. Spray nozzles Yout = 0.053 kg of water/kg of dry air
would be to produce a stream that 5. Fills Hout = 178 KJ/kg of dry air
Gout = 4,948,942 kg of air/h
is at the wet-bulb temperature of out = 1.09 kg/m3
the ambient air. However, to achieve
this theoretical maximum, the
3
tower would need to have infinite Inlet air Lin = 7,500,000 kg/h
G' = 4,699,850 kg dry air/h 4 tin = 45oC
height. So the practical limit of the DBT (Tin) = 30.3oC
CWS temperature is generally con- WBT (Tw,in) = 29oC Cooling-water return
Yin = 0.02492 kg water/kg dry air
sidered to be 4°C above the wet-bulb Hin = 93.95 KJ/kg of dry air 5
temperature of ambient air. For Gin = 4,816,970 kg air/h

Cooling-water supply
in = 1.148 kg/m3
design purposes, the worst sce-
nario — that is, the summer season Lout = 7,368,000 kg/h
tout = 33oC
wet-bulb temperature — needs to 1
be considered. Makeup water Cooling-water flow to
Range (∆T). The range is the differ- exchanger
Lm = 165,000 kg/h
tm = 33oC Heat load
ence between the water tempera- 2 Q = 105 MW
tures at the inlet and the outlet of the Pump flow Cold blowdown
cooling tower (tin – tout). In this case, Hot blowdown
Lp = 7,533,000 kg/h Lp = 33,000 kg/h
the range is 12°C. The range does tp = 33 oC tp = 33 oC Lb = 0 kg/h
tin = 45 oC
not represent the cooling tower ca-
pability; rather, the range is based on FIGURE 2. This schematic diagram depicts the parameters of the case study cooling tower system.
the cooling-water circulation flowrate Note: All three cells and three fans are lumped together and shown as a single unit
(Lin), and the sum of the heat loads
taken from the heat exchangers in Use Equation (3) to apply a mass
the process unit (Q), and it is not re- balance for the entire section of (5)
lated to the size or capability of the tower. As shown in Equation (3), the
cooling tower. On the other hand, an amount of water evaporated (eL) in
increase in range will cause an in- the down-pouring liquid is the dif- Solving both mass and heat bal-
crease in approach, if all other con- ference between the inlet liquid flow ance equations [Equations (3) and
ditions are not changed. The range (Lin) and the sum of the outlet liquid (5)] simultaneously, the evaporation
is shown in Equation (1): flow (Lout) and the drift loss (dL). It is
equal to the difference of moisture
content of air across the tower. NOMENCLATURE
(1)
cw = Specific heat of water, kJ/kgK
dL = Drift loss, kg/h
Cooling tower efficiency (). The (3)
eL = Evaporation loss, kg/h
cooling tower efficiency is the ratio of
G = Air flowrate (wet), kg of air/h
actual cooling (range) to the theoreti- Where:
G' = Air flowrate (dry), kg of dry air/h
cally possible maximum cooling (that G' = the quantity of dry air flow
h = Water enthalpy, kJ/kg
is, when the approach is zero), as (which remains the same at the inlet
H = Moist air enthalpy, kJ/kg
shown in Equation (2): and outlet air streams), kg of dry air
L = Liquid flowrate, kg/h
Y = absolute humidity, kg water/kg
OL = Other losses (seal leak, pipe leaking,
dry air/h
and so on) in the system, kg/h
(2) The subscripts in and out refer to the
Q = Heat load, kW
entry and exit locations.
t = Water temperature, °C
Theoretically, an approach of The overall energy balance is given
T = Air temperature, °C
zero means the tower is 100% ef- by Equation (4):
Y = Air humidity, kg water/kg of dry air
ficient. Industrial cooling towers  = Moist air density, kg/m3
typically have an approach tem- o = Latent heat of vaporization of water,
perature between 4° and 8.5°C, (4) kJ/kg
and an efficiency between 70 and  = Cooling tower efficiency, %
75% [2]; in this case, the efficiency Where: ∆T = Range, °C
is 75%. h = the liquid enthalpy, kJ/kg water Subscripts
H = the moist air enthalpy, kJ/kg d = Drift water
Evaporation loss and air needs dry air b = Blowdown
Method 1. The evaporation loss Substituting Lout from Equation (3) in = Inlet location
and air flow requirement through the into Equation (4), and assuming the m = Makeup water
tower can be evaluated by solving enthalpy of the drift water hd is hout, out = Outlet location
the mass and energy balance equa- and simplifying Equation (4), one p = Pump
tions simultaneously. gets Equation (5): w = Wet-bulb temperature

CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM APRIL 2017 65

 we
ratio of latent heat transferred (eLo)
t-b to the total heat released from the
ulb Exit
tem air
per
atu Latent heat water side is shown by on the left in
re, oC and mass the Equation (6), and this expression
transfer
is numerically equal to the ratio of la-

Specific humidity, Kg water/Kg dry air

HB tent heat added to the dry air to the
B YB total heat gained by the air, which is
we
t-b Ente shown on the right side in the Equa-
ulb rin
tem g a
per ir
tion (6):
atu
re, oC

HA
(6)

C YA
A
From Equation (6), eL is calculated
E D YD as 132,000 kg/h. It is to be noted
that in this method, the dry air flow
Sensible heat (G') is not required. Once eL is evalu-
transfer ated, G' is estimated from the mass
balance equation [(Equation (3)]. The
Dry-bulb temperature, oC
split of latent heat transfer and sen-
FIGURE 3. In this psychrometric chart, the relevant process condition from the case history is marked as sible air heating in this case is about
vector AB 85% and 15%, respectively.

loss (eL) and the dry-air require- the inlet air. But in the case of DB, Makeup water and blowdown
ment (G') are estimated as 132,000 the dry-bulb temperature of air is Makeup water (Lm) is added to the
kg/h and G' = 4,699,850 kg dry decreased and thus the air is cooled sump to compensate for the water
air/h, respectively. at the exit. In both cases, the wet- losses in the circuit. The water losses
Method 2. Depending on the tem- bulb temperature of the exit air will include evaporation loss (eL), drift
perature of the inlet air (whether it always be increased compared with loss (dL), blowdown (Lb), and other
is hot or cold), the air can be either that of the inlet air. So, the water leakage losses (OL) in the system,
heated or cooled as it travels along flowing through the cooling tower such as losses from the pump seal,
the height of a cooling tower. In the can be cooled by unsaturated air, ir- piping leak, washdown water and fil-
psychrometric chart shown inFig- respective of whether the air is hot ter backwash.
ure 3, the entering condition of the or cold.
air is denoted by point A, and the In this case study, from the field (7)
exit air (which is completely satu- measurements of DBT and WBT,
rated with water) is denoted by the psychrometric properties, such Drift loss. Small droplets that are
point B. The enthalpy difference of as absolute humidity, saturation hu- entrained by the upward-flowing air
dry air is (HA–HB). The vector AB midity and moist air enthalpy for the stream are collected in a mist elimi-
is the sum of the two components. inlet air and the outlet air, could be nator, where they accumulate to
The horizontal component AC rep- evaluated. The inlet air is marked as form larger drops that are eventually
resents the sensible heating of air, point A, and the outlet air is marked returned to the fill. In general, very
and the vertical component CB is as point B in the psychrometric little water in the form of droplets is
the latent heating of air. In a cool- chart. Another hypothetical point C carried along with the air, but those
ing tower, it is also possible to is marked in such a way that it has a droplets do results in water loss,
cool the air if the inlet air condition dry-bulb temperature similar to point called drift loss or windage loss.
is at D [3]. At point D, the air is hot B and absolute humidity similar to This drift water typically contains dis-
and dry, when compared to the air point A. It must be noted that the solved solids and may cause stain,
at point A. point C is a hypothetical and does corrosion or damage to nearby
The component DE is the sensible not correspond to any location in the buildings and structures. Drift loss is
air cooling, and the component EB is cooling tower; the point C is marked usually about 0.1–0.3% of the circu-
the latent heating of air. The net heat on the chart to see the horizontal lation water rate (Lin).
received by the air is the difference and vertical component of vector To compensate for the evapora-
between the latent air heating and AB. Moist air enthalpy for point C is tion loss and drift loss, additional
the sensible air cooling. calculated. makeup water is added. Since the
In the case of the AB process, the The total heat gained by the air makeup water typically contains dis-
dry-bulb temperature of the air is (HB–HA) has two components: the solved solids, these solids are typi-
increased at the exit — that is, the latent heat transfer (HB–HC), and the cally left behind in the sump water
exit air becomes hot compared to sensible heat transfer (HC–HA). The as the water evaporates in the cool-
66 CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM APRIL 2017
 350,000
ing tower. Meanwhile, since the
cooling water is a very effective air Makeup water requirements, kg/h
scrubber, dust and debris present in
300,000
the up-flowing air is washed out by
down-pouring water and collects in
the sump. As solids accumulate in
250,000
the sump, they increase the poten-
tial for scale corrosion and biological
fouling in the cooling-water circuit.

Makeup water, kg/h

200,000
By taking small amounts of water
continuously from the cooling tower
circuit (blowdown), the concentra-
150,000
tion of dissolved solids in the cool-
ing water can be reduced below the Evaporation loss, kg/h
upper limit of the acceptable range,
100,000
in order to meet the cooling-water
quality specification of the plant.
Blowdown. There are two ways
50,000
to remove the blowdown — as hot
blowdown and cold blowdown (Fig- Drift loss, kg/h
ure 2). Hot blowdown refers to the
0
continuous removal of water in the 0 2 4 6 8 10 12 14 16 18 20
Cycle of concentration (CC)
cooling-water-return line to the ef-
fluent. Since the water is hot at this FIGURE 4. Makeup water requirements versus the cycle of concentration are shown here
location, hot blowdown may not be
acceptable in some applications due Equation (11): are calculated using Equation (11)
to potential environmental impact; and Equation (12), as 33,000 kg/h,
in other cases, it is desired, since it (10) and 165,000 kg/h, respectively. Fur-
reduces throughput to the cooling ther, assuming 0.2% drift loss and no
tower and increases overall cooling system leak, makeup water needs to
performance. be considered as 180,000 kg/h. Q
Cold blowdown refers to the con- (11) Edited by Suzanne Shelley
tinuous removal of water from the
cooling-water pump outlet to the ef- Further, the amount of makeup References
fluent [4]. Drift loss and any leakage water needed is estimated, including 1. American Petroleum Institute, Programme in Learning
loss from the system are also con- drift, using Equation (12): Operating Techniques — Cooling Towers, 1995.
sidered as blowdown, since these 2. Huchler, L., Cooling Towers, Part 2: Operating, Moni-
toring and Maintaining, Chemical Engineering Progress,
streams contain dissolved solids (but Oct. 2009.
such losses are unintentional). 3. American Society of Heating, Refrigerating and Air Condi-
The amount of water blowdown (12) tioning Engineers, “ASHRAE Handbook,” Chapter 39 —
is established by calculating the HVAC System and Equipment, 2008.
cycle of concentration (CC), which The required makeup water mainly 4. Smith, R., ““Chemical Process Design and Integration,”
is defined as the ratio between the depends on evaporation loss and John Wiley & Sons Ltd., 2005.
amount of solids dissolved (mostly the CC calculated above. It is to be
chlorides) in the blowdown and in noted from Equation (11) that the Author
the makeup water, using Equation minimum value of CC to be consid- Uthirapathi Vengateson is a se-
(8): ered is 2, which requires blowdown nior process design engineer at
National Petrochemical Co. in
to be at the same amount of water Yanbu, Saudi Arabia (Phone:
as the amount lost in evaporation. +966 534878029; Email:
(8) Any attempt to reduce the CC below uvengateson@natpetpp.com,
2 results in a significant amount of drvengateson@gmail.com). For
17 years, Vengateson has been
makeup water, as shown in Figure 4. involved in process engineering
Assuming drift loss and leakage Higher CC means that Cm tends design, research and develop-
losses are negligible, and solving to zero (indicating good quality of the ment, and commissioning of chemical and petrochemi-
cal plants. Prior to this, he worked in Lurgi India Com-
the water-balance shown below in makeup water). But, this is achieved pany Ltd. in New Delhi, India. Vengateson earned a
Equation (9): at the cost of water treatment of the bachelor’s degree (B.Tech.) in chemical engineering
source water. A typical cycle of con- from Madras University, a master’s degree in petroleum
refining and petrochemicals from Anna University, and a
(9) centration (CC = 5) is considered in Ph.D. in chemical engineering from Indian Institute of
this case study for the optimum re- Technology, New Delhi, India.
The dissolved-solids balance quirement. Based on the evapora-
shown below in Equation (10), tion loss and cycle of concentration,
the blowdown is calculated using cold blowdown and makeup water

CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM APRIL 2017 67