Wet bulb temperature of airWet bulb temperature of air
Dry bulb temperature of airDry bulb temperature of air
Cooling tower inlet water Cooling tower inlet water temperaturetemperature
Cooling tower outlet water temperatureCooling tower outlet water temperature
Exhaust air temperatureExhaust air temperature
Electrical readings of pump and fan motorsElectrical readings of pump and fan motors
Water flow rateWater flow rate
Air flow rateAir flow rate
9.6
9.6 Factors Affecting Performance Factors Affecting Performance
9.6.1
9.6.1 Design Design
9.6.1.1
9.6.1.1 Capacity Capacity
Heat dissipation (in kCal/hour) and circulated flow rate (m
Heat dissipation (in kCal/hour) and circulated flow rate (m33/hr) are not /hr) are not sufficient to understandsufficient to understand cooling tower performance. Other factors, which we will see, must be stated along with flow cooling tower performance. Other factors, which we will see, must be stated along with flow rate m
rate m33/hr. For example, a cooling tower sized to cool 4540 /hr. For example, a cooling tower sized to cool 4540 mm33/hr through a 13.9/hr through a 13.9°°C range mightC range might be larger than a cooling tower to cool 4540 m
be larger than a cooling tower to cool 4540 m33/hr through 19.5/hr through 19.5°°C range.C range.
9.6.1.2
9.6.1.2 RangeRange
Range is determined not by the cooling tower, but by
Range is determined not by the cooling tower, but by the process it is serving. The range the process it is serving. The range at theat the exchanger is determined entirely by the heat load and the
exchanger is determined entirely by the heat load and the water circulation rate through thewater circulation rate through the exchanger and on to the cooling water.
exchanger and on to the cooling water.
Equation 7 CT Range Def. 2 Equation 7 CT Range Def. 2
°° == ((//))
(())
Thus, Range is a function of the heat
Thus, Range is a function of the heat load and the flow circulated through the system.load and the flow circulated through the system.
Cooling towers are usually specified to cool a
Cooling towers are usually specified to cool a certain flow rate from one temperature tocertain flow rate from one temperature to
another temperature at a certain wet bulb temperature. For example, the cooling tower might another temperature at a certain wet bulb temperature. For example, the cooling tower might be specified to cool 48000 m
be specified to cool 48000 m33/hr from 44/hr from 44°°C to 34C to 34°°C at 26.7C at 26.7°°C wet bulb temperature.C wet bulb temperature.
((°°) ) == ((°°))−− ((°°))
As a generalization, the closer the approach to the wet bulb, the more
As a generalization, the closer the approach to the wet bulb, the more expensive the coolingexpensive the cooling tower due to increased size. Usually a 2.8
tower due to increased size. Usually a 2.8°°C approach to the design wet bulb is the coldestC approach to the design wet bulb is the coldest water temperature that cooling tower manufacturers will guarantee. If
water temperature that cooling tower manufacturers will guarantee. If flow rate, range,flow rate, range, approach and wet bulb had to be ranked in the order
approach and wet bulb had to be ranked in the order of their importance in sizing a tower,of their importance in sizing a tower, approach would be first with flow rate closely following the
approach would be first with flow rate closely following the range and wet bulb would be of range and wet bulb would be of lesser importance.
lesser importance.
The range increases when the quantity of circulated water and heat load increase. This means The range increases when the quantity of circulated water and heat load increase. This means that increasing the range as a result of added heat load requires a
that increasing the range as a result of added heat load requires a larger tower. There are larger tower. There are twotwo possible causes for the increased range:
possible causes for the increased range:
The inlet water temperature is increased (and the cold-water temperature at the exitThe inlet water temperature is increased (and the cold-water temperature at the exit remains the same). In this case it is
remains the same). In this case it is economical to invest in removing the additional heat.economical to invest in removing the additional heat.
The exit water temperature is decreased (and the hot water temperature at the inletThe exit water temperature is decreased (and the hot water temperature at the inlet remains the same). In this case the tower size
remains the same). In this case the tower size would have to be increased considerablywould have to be increased considerably because the approach is also reduced, and this is not always economical.
because the approach is also reduced, and this is not always economical.
9.6.1.3
9.6.1.3 Heat Load Heat Load
The heat load imposed on a
The heat load imposed on a cooling tower is determined by the process being served. Thecooling tower is determined by the process being served. The degree of cooling required is
degree of cooling required is controlled by the desired operating temperature level of thecontrolled by the desired operating temperature level of the process. In most cases, a low operating temperature is desirable to increase
process. In most cases, a low operating temperature is desirable to increase process efficiencyprocess efficiency or to improve the quality or
or to improve the quality or quantity of the product. In some applications (e.g. internalquantity of the product. In some applications (e.g. internal combustion engines), however, high operating temperatures are desirable. The
combustion engines), however, high operating temperatures are desirable. The size and cost of size and cost of the cooling tower is proportional to the heat load. If
the cooling tower is proportional to the heat load. If heat load calculations are low undersizedheat load calculations are low undersized
equipment will be purchased. If the calculated load is high, oversize
equipment will be purchased. If the calculated load is high, oversize and more costly,and more costly, equipment will result.
equipment will result.
Process heat loads may
Process heat loads may vary considerably depending upon the process involved. Determinationvary considerably depending upon the process involved. Determination of accurate process heat loads
of accurate process heat loads can become very complex but can become very complex but proper consideration can produceproper consideration can produce satisfactory results. On the other hand, air conditioning and refrigeration heat loads can be satisfactory results. On the other hand, air conditioning and refrigeration heat loads can be determined with greater accuracy.
determined with greater accuracy.
9.6.1.4
9.6.1.4 Wet Bulb TemperatureWet Bulb Temperature
Wet bulb temperature is an important factor in performance of evaporative water cooling Wet bulb temperature is an important factor in performance of evaporative water cooling equipment. It is a controlling factor from the aspect of minimum cold
equipment. It is a controlling factor from the aspect of minimum cold water temperature towater temperature to which water can be cooled by the evaporative method. Thus, the wet bulb temperature of the which water can be cooled by the evaporative method. Thus, the wet bulb temperature of the air entering the
air entering the cooling tower determines operating temperature levels cooling tower determines operating temperature levels throughout the plant,throughout the plant, process, or system. Theoretically, a cooling tower will cool water to the
process, or system. Theoretically, a cooling tower will cool water to the entering wet bulbentering wet bulb temperature, when operating without a heat load. However, a thermal potential is required to temperature, when operating without a heat load. However, a thermal potential is required to reject heat, so it is
reject heat, so it is not possible to cool water to the entering air wet not possible to cool water to the entering air wet bulb temperature, when abulb temperature, when a heat load is applied. The approach obtained is a function of thermal conditions and tower heat load is applied. The approach obtained is a function of thermal conditions and tower capability.
capability.
Initial selection of towers with respect to design wet
Initial selection of towers with respect to design wet bulb temperature must be made on thebulb temperature must be made on the basis of conditions existing at the tower site. The temperature selected is generally close
basis of conditions existing at the tower site. The temperature selected is generally close to theto the average maximum wet bulb for the summer months. An important aspect of wet bulb selection average maximum wet bulb for the summer months. An important aspect of wet bulb selection is whether it is spe
is whether it is specified as ambient or inlet. The ambient wet cified as ambient or inlet. The ambient wet bulb is the temperature, whichbulb is the temperature, which exists generally in the cooling tower
exists generally in the cooling tower area, whereas inlet wet bulb is the wet bulb temperaturearea, whereas inlet wet bulb is the wet bulb temperature of the air entering the tower. The
of the air entering the tower. The later can be, and often is, affected by dilater can be, and often is, affected by discharge vapours beingscharge vapours being re-circulated into the tower. Recirculation raises the effective wet bulb temperature of the air re-circulated into the tower. Recirculation raises the effective wet bulb temperature of the air entering the tower with corresponding increase in the cold water temperature. Since there is entering the tower with corresponding increase in the cold water temperature. Since there is no initial knowledge or control over the recirculation factor, the
no initial knowledge or control over the recirculation factor, the ambient wet bulb should beambient wet bulb should be specified. The cooling tower supplier is required to furnish a
specified. The cooling tower supplier is required to furnish a tower of sufficient capability totower of sufficient capability to absorb the effects of the increased wet bulb temperature peculiar to his own equipment.
absorb the effects of the increased wet bulb temperature peculiar to his own equipment.
It is very
It is very important to have the cold water temperature low enough to exchange heat or toimportant to have the cold water temperature low enough to exchange heat or to condense vapours at the optimum temperature level. By evaluating the cost and size of heat condense vapours at the optimum temperature level. By evaluating the cost and size of heat exchangers versus the cost and size of the cooling tower, the quantity and temperature of the exchangers versus the cost and size of the cooling tower, the quantity and temperature of the cooling tower water can be selected to get the
cooling tower water can be selected to get the maximum economy for the particular process.maximum economy for the particular process.
The Table 7.1 illustrates the effect
The Table 7.1 illustrates the effect of approach on the size and cost of a cooling tower. Theof approach on the size and cost of a cooling tower. The towers included were sized to cool 4540 m
towers included were sized to cool 4540 m33/hr through a 16.67/hr through a 16.67°°C range at a 26.7C range at a 26.7°°C design wetC design wet bulb. The overall width of all
bulb. The overall width of all towers is 21.65 meters; the overall height, 15.25 meters, and towers is 21.65 meters; the overall height, 15.25 meters, and thethe pump head, 10.6 m
pump head, 10.6 m approximately.approximately.
The design wet bulb temperature is determined by the
The design wet bulb temperature is determined by the geographical location. For a certaingeographical location. For a certain approach value (and at a constant range and flow range), the higher the wet bulb temperature, approach value (and at a constant range and flow range), the higher the wet bulb temperature, the smaller the tower required. For example, a 4540
the smaller the tower required. For example, a 4540 mm33/hr cooling tower selected for a16.67/hr cooling tower selected for a16.67°°CC range and a 4.45
range and a 4.45°°C approach to 21.11C approach to 21.11°°C wet bulb would be larger than the same C wet bulb would be larger than the same tower to atower to a 26.67
26.67°°C wet bulb. The reason is that air C wet bulb. The reason is that air at the higher wet bulb temperature is at the higher wet bulb temperature is capable of capable of picking up more heat. This is explained for the
picking up more heat. This is explained for the two different wet bulb temperatures:two different wet bulb temperatures:
Each kg of air entering the tower at a Each kg of air entering the tower at a wet bulb temperature of 21.1wet bulb temperature of 21.1°°C contains 18.86 kCal. If C contains 18.86 kCal. If the air leaves the tower at
the air leaves the tower at 32.232.2°°C wet bulb temperature, each kg of air contains 24.17 kCal.C wet bulb temperature, each kg of air contains 24.17 kCal.
At an increase of 11.1
At an increase of 11.1°°C, the air picks up 12.1 kCal per C, the air picks up 12.1 kCal per kg of air.kg of air.
Each kg of air entering the tower at a Each kg of air entering the tower at a wet bulb temperature of 26.67wet bulb temperature of 26.67°°C contains 24.17 kCals.C contains 24.17 kCals.
If the air leaves at 37.8
If the air leaves at 37.8°°C wet bulb temperature, each kg of air contains 39.67 kCal. At anC wet bulb temperature, each kg of air contains 39.67 kCal. At an increase of 11.1
increase of 11.1°°C, the air picks up 15.5 kCal per kg C, the air picks up 15.5 kCal per kg of air, which is much more than the of air, which is much more than the firstfirst scenario.
scenario.
9.6.1.5
9.6.1.5 Tower SizeTower Size If heat load,
If heat load, range, approach and wet-bulb temperature are held range, approach and wet-bulb temperature are held constant, changing the fourthconstant, changing the fourth will affect the tower size as
will affect the tower size as follows:follows:
a)
a) Tower size varies inversely with Tower size varies inversely with approach. A longer approach requirapproach. A longer approach requires a smaller tower.es a smaller tower.
Conversely, a smaller approach requires an increasingly larger tower and, at 5°F
Conversely, a smaller approach requires an increasingly larger tower and, at 5°F approach,approach,
the effect upon tower size begins to become asymptotic. For that reason, it i the effect upon tower size begins to become asymptotic. For that reason, it i s nots not customary in the cooling tower industry to guarantee any approach of less than 5°F.
customary in the cooling tower industry to guarantee any approach of less than 5°F.
Figure 4 Tower size v/s approach Figure 4 Tower size v/s approach
b)
b) Tower size varies inversely with Tower size varies inversely with wet bulb temperature. When heat load, range, andwet bulb temperature. When heat load, range, and
approach values are fixed, reducing the design wet-bulb temperature increases the size of approach values are fixed, reducing the design wet-bulb temperature increases the size of the tower. This is because most of the heat transfer in
the tower. This is because most of the heat transfer in a cooling tower occurs by virtue of a cooling tower occurs by virtue of ev
evaporation (which extracts approximately 1000 Btu’s for every aporation (which extracts approximately 1000 Btu’s for every pound of waterpound of water evaporated), and air’s ability to
evaporated), and air’s ability to absorb moisture reduces with temperature.absorb moisture reduces with temperature.
Figure 5 Tower size v/s wet-bulb Figure 5 Tower size v/s wet-bulb
c)
c) Tower size varies directly and liTower size varies directly and linearly with heat load.nearly with heat load.
Figure 6 Tower size v/s head load Figure 6 Tower size v/s head load
d)
d) Tower size varies inversely with Tower size varies inversely with range. Two primary factors account for this. First; increasingrange. Two primary factors account for this. First; increasing the range
the range——also increases the ITD (driving force) between the incoming hot wateralso increases the ITD (driving force) between the incoming hot water temperature and the entering wet-bulb temperature. Second, increasing the
temperature and the entering wet-bulb temperature. Second, increasing the range (at arange (at a constant heat load) requires that the water flow rate be decreased
constant heat load) requires that the water flow rate be decreased——which reduces thewhich reduces the static pressure opposing the flow of air.
static pressure opposing the flow of air.
Figure 7 Tower size v/s range variance Figure 7 Tower size v/s range variance
9.6.2
9.6.2 Fill media effects Fill media effects
In a cooling tower, hot water is di
In a cooling tower, hot water is distributed above fill media and is cooled down stributed above fill media and is cooled down throughthrough evaporation as it flows down the tower and gets in contact with air. The f
evaporation as it flows down the tower and gets in contact with air. The f ill media impactsill media impacts energy consumption in two ways:
energy consumption in two ways:
Electricity is used for pumping above the fill Electricity is used for pumping above the fill and for fans that create the air draft. Anand for fans that create the air draft. An efficiently designed fill media with appropriate water distribution, drift eliminator, fan, efficiently designed fill media with appropriate water distribution, drift eliminator, fan, gearbox and motor with therefore
gearbox and motor with therefore lead to lower lead to lower electricity consumption.electricity consumption.
Heat exchange between air and water is influenced by surface area of heat exchange,Heat exchange between air and water is influenced by surface area of heat exchange, duration of heat exchange (interaction) and turbulence in
duration of heat exchange (interaction) and turbulence in water effecting thoroughness of water effecting thoroughness of intermixing. The fill media determines all
intermixing. The fill media determines all of these and therefore influences the heatof these and therefore influences the heat exchange. The greater the heat exchange, the more effective the cooling tower
exchange. The greater the heat exchange, the more effective the cooling tower becomes.becomes.
There are three types of fills:
There are three types of fills:
a)
a) Splash fill media.Splash fill media.Splash fill media generates the required heat exchange area by splashingSplash fill media generates the required heat exchange area by splashing water over the fill
water over the fill media into smaller water droplets. The surface area of the media into smaller water droplets. The surface area of the water dropletswater droplets is the surface area for heat exchange with the air.
is the surface area for heat exchange with the air.
b)
b) Film fill media.Film fill media. In a film fill, water forms a thin film on either side of fill sheets. The surfaceIn a film fill, water forms a thin film on either side of fill sheets. The surface area of the fill
b) Film fill media.Film fill media. In a film fill, water forms a thin film on either side of fill sheets. The surfaceIn a film fill, water forms a thin film on either side of fill sheets. The surface area of the fill