optimization of cooling towers

8/14/2019 Optimization of Cooling Towers

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Optimization of Cooling

Towers

Prof. Dr. Javaid Rabbani Khan


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Introduction:

Cooling towers are used to reduce the temperatureof a water stream by extracting heat from water andemitting it to the atmosphere.

Main types of cooling tower are:Natural draft or Hyperbolic cooling tower

Cross flow tower

Counter flow tower

Mechanical draft Forced draft

Induced draft


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Cross flow natural draft cooling

tower:


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Counter flow natural draft cooling

tower:


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Forced draft cooling tower:


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Induced draft tower:


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Optimized cooling towers:

Cooling tower is a water-to-air heat

exchanger

Change in load causes the change inwater and air flow rates

In optimized cooling towers both flows are

controlled by variable speed devices e.g.pumps etc


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Savings due to variable speed

devices:


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Goal of cooling tower optimization:

Optimization of cooling tower is carried out

by:

Maximizing the amount of heat discharged into airper unit of operating cost invested

Minimizing the unit cost of cooling by minimizing

the operating speed of cooling tower fans and

pumps


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Minimizing unit cost of cooling by minimizing

operating speeds of CT fans and pumps:


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Minimizing Operating Cost:

Cost of fan operation can be reduced by:

Rising cooling tower water temperature

Increasing the approach (TctwsTwb) atwhich the tower operates

The approach can be increased to a point

at which the fans are off and theiroperating cost is zero.


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Optimum Approach:

As the approach rises, the temp difference

across all process coolers (TpTctws) is

reduced In order to reduce the process temp. Tp,

more and more water must be pumped

Consequently the pumping costs will rise.


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Total operating cost can be minimized by controlling the

range of the cooling tower at the value that corresponds to

the minimum cost of operation


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Supply Temperature Optimization:

An optimization control loop is required to

maintain the cooling tower water supply

continuously at an economical minimumtemperature

This minimum temperature is a function of

the wet-bulb temperature of theatmospheric air


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Optimum approach Ao:

Optimum approach is the one that will result in a

minimum total cost operation

It will increase if the: Load on the cooling tower increases Or

Ambient wet-bulb temperature decreases

Acan be obtained by continuous throttling if:

Cooling tower fans are centrifugal units or

Blade pitch is variable


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Benefits of Optimization:

Load-following optimization benefits because:

All cooling water valves in the plant are opened up as

the water P across the users is minimized Valve cycling is reduced and

Pumping costs are lowered

Valve cycling is eliminated when valve openings

are moved away from the unstable region nearthe closed position


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Starting Additional Pumps:

Additional pump increments are started whenthe pump speed controller set point is at itsmaximum

When the load is dropping, the excess pumpincrements can be stopped on the basis of flow

In order to eliminate pump cycling, the excess

pumping increment is only turned off when theactual total flow corresponds to less than 90% ofthe capacity of the remaining pumps


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Return Water Distribution and

Balancing: It is desirable to automatically distribute return

water flows to the various cells by operating theirassociated fans

Water flows to all cells whose fans are at highspeed should be equal and high

Cells with their fans off should receive water at

equal minimum flow rates. The normal water flow rate ranges from 2 gpm to

5 gpm per ton when the fan is at full speed


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Relationship between water flow and

water temperature (Tctws) or approach:


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Water distribution balancing is often done

manually, but it can also be done

automatically as shown:


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Cont

In above figure the total flow is used as the set

point of the ratio flow controllers

If the ratio settings are the same, the total flow isequally distributed

Ratio settings can be changed manually or

automatically to reflect changes in fan speeds

Naturally, the total of the ratio settings must

always be 1.


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Cont

The purpose of the control system inabove figure is to:

distribute the returning water between thecells correctly

make sure that this is done at minimum cost

Cost of pumping will be minimum when

the pressure drop through the distributioncontrol valves is minimum.


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CONCLUSIONS:

The operating cost of cooling tower motors, fans

and pumps can be cut in half by optimization.

Optimization is achieved by meeting the variablecooling load of the plant by the minimum water

and air flows that are needed.

Optimization also can include the cost-effective

balancing of the distribution of the returningwater among the tower cells.


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Cont

A desirable side effect of optimization is

the automatic indication of design defects:

in pipe

valve sizing and

the increased level of safety, by making sure

that no process cooling load is everneglected.


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Cooling tower specification sheet


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Cont


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Voidage Packing Correlation

whereL' = total water flow, Ib/hr

N' = no. of deck levels in tower

t1= water temperature at bottom of tower,0F

t2= water temperature at top of tower,0F

tL= water temperature of bulk of water,0F

v = tower volume, ft3/ft2plan area

iG= enthalpy of air saturated at wet bulb temperature, Btu/lb dry air

iL= enthalpy of air saturated at bulk water temperature, Btu/Ib dry air

K = overall enthalpy transfer coefficiem, lb/hr (ft2 transfer area)

(lb water/lb dry air)

(9 - 129)


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Values of A & n


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Ground Area VS Height

The economics of forced and induced draftcooling tower operation require a study of

fan and water pump horsepower and usually

dictate a fan static pressure requirement not

to exceed 0.75-1.0 in. of water.

Pritchardpresents an estimating curve indicating that as

packed height varies from 12-40 ft, the economics of

ground area suggest a G, of 2,000-1,400 respectively,being slightly less than a straight line function.


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Cont

The pressure drop for a given number and typeof packing deck is expressed

Pressure drop values, P//N/, per individual deck

range from 0.003-0.006 in. water for low L/and

G, rates to 0.03-0.06 in. water for high L/(3,500)

and G, (2,000) rates

Typical pressure drop curve is shown below


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Pressure Drop Curve


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Fan Horsepower for Mechanical

Draft TowerBHP= F psa/(6,356) (0.50)

Where

F = actual cfm at fan inlet, ft3/min

ps = total static pressure of fan, in. of water

This relation includes a 50% static efficiency of the fan

and gear losses, assuming a gear drive

Economical tower sizes usually require fan horsepowerbetween 0.05 and 0.58 hp/ft2ofground plan area andmotors larger than 75 hp are not often used due toinability to obtain the proper fansand gears in the spacerequired.


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Water Rates and Distribution

Water distribution must give uniform water flow over thetower packing.

Many towers use a gravity feed system discharging thewater through troughs and ceramic, metalor plasticnozzles.

Other systems use pressure nozzles dischargingupward, before falling back over the packing.

This latter method requires more pumping head due tothe pressure required at the nozzles.

Water rates usually run from 1 to 3.5 gpm/ft2of groundplan area.


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Blow Down and Contamination

Build-up the circulating water evaporates in passing through the

tower, the evaporated water vapor is pure.

This leaves behind and creates a concentration effect for

solids material dissolved in the remaining water. This concentration can aggravate the heat transfer

surfaces and develop corrosive conditions on manymechanical and structural parts of the tower.

To control and limit this build-up, a certain amount ofliquid is blown down to expel the concentrated materialand this quantity is replaced with fiesh make-up water


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Cont

The level to which the contamination can

concentrate in the circulating water is

And the rate of blow down is


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ContWhere

C = contaminant level in circulating water; number of concentration ratios

E = rate of evaporation, gpm (if not accurately known evaporation can beapproximated by multiplying total water rate in gpm times the coolingrange (OF) times 0.0008).

E (est)- (gpmT) (CR) (0.0008)

gpmT = total cooling tower water flow rate, gpm, (incoming to be cooledby tower)

DL = drift loss, water lost from tower system entrained in exhaust airstream,

measured as (a) % of circulating water rate, gpm, or (b) more precise

an L/G parameter and drift becomes pounds of water per million poundsof exhaust air; for estimating


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Cont

DL= (gpmT, as water flow rate) (0.0002)

CR = cooling range,OF, difference between hot water into

tower and cold water from the tower,0

FB = rate of blowdown, gpm. (Because an acceptable level

of concentration has usually been predetermined, the

operator is more concerned with the amount ofblowdown necessary to maintain the concentration,

L/G = ratio of total mass flow of water and dry air in cooling

tower, Ib/lb


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Guidelines for Cooling Tower

Recirculating Water

pH -Ideally 6.5-8.0; pH as low as 5.0 is acceptable if galvanizedsteel is not present.

Chlorides -Maximum 750 pprn (as NaCl) for galvanized steel;maximum 1,500 pprn for Type 300 stainless steel; maximum 4,000

pprn for Type 316 stainless steel; silicon bronze is the preferredmaterial if chlorides exceed 4,000 ppm.

Calcium -Ingeneral, calcium (as CaCO3) below 800 pprn shouldnot result in calcium sulfate scale. In arid climates, however, the

critical level may be much lower. For calcium carbonate scalingtendencies, calculate the Langelier Saturation Index or the Ryznar

Stability Index. Sulfates -If calcium exceeds 800 ppm, sulfates should be limited to

800 ppm, less in arid climates, to prevent scale. Otherwise, asulfatelevel up to 5,000 ppm is acceptable.


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Cont

Silica -Generally, limit silica to 150 ppm as Si02 to prevent silicascale.

Iron -Limit to 3 ppm. Note that excessive concentrations of ironmay stain cooling tower components, but these stains are not the

result of any rust or corrosion. Manganese -Limit to 0.1 ppm.

Total Dissolved Solids(TDS) -Over 5,000 pprn can adverselyaffect thermal performance and may be detrimental to wood in the

alternately wet/dry areas of the tower.

Suspended Solids -Limit to 150 pprn if the solids are abrasive.

Avoid film fill if solids are fibrous, greasy, fatty, or tarry. Oil and Grease -Over 10 pprn will cause noticeable thermal

performance loss.

Ammonia -Limit to 50 ppm if copper alloys are present.


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Cont

Nutrients -Nitrates, ammonia, oils, glycols, alcohols,sugars, and phosphates can promote growth of algaeand slime. This growth can cause tower problems,

particularly with film fill Organic Solvents -These can attack plastics and

should be avoided.

Biological Oxygen Demand(BOD)-Limit BOD to 25ppm, particularly if suspended solids exceed 25 ppm.

Sulfides -Should be limited to 1 ppm.

Langelier Saturation Index -Ideally, maintain between-0.5 and +0.5A negative LSI indicates corrosiontendenciesA positive LSI indicates CaC03 scaling

tendencies.


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Preliminary Design Estimate of

New Tower1. Determine the inlet water temperature to the tower. This

is approximately the outlet temperature from the coolingwater load.

2.Determine the heat load to be performed by the tower,based on required water inlet and outlet temperaturesand flow rates.

3. Establish the wet bulb temperature for the air at thegeographical site of the tower.

4. Prepare a plot of the saturation curve for air-water.

Establish the operating line by starting at the point

set by the outlet cold water temperature and the

enthalpy of air at the wet bulb temperature, and with

a slope L/Ga assumed between 0.9 and 2.7


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Cont

5. Graphically integrate, by plotting l/h-h vs. t, reading (h-h) from the operating-equilibrium line plot for variousvalues of temperature

6. The value of the integral is equal to the number oftransfer units, so set it equal to Equation 9-129 and solvefor the number of decks needed, N

7. If the number of decks required is unreasonable from aheight standpoint, the procedure must be repeated usinga new assumed L/Ga, or a new approach, or a new wet

bulb temperature, or some combination of these.8. For the assumed L/Ga and known L, calculate the

required air rate Ga.

G hi l i t ti t d t i b f


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Graphical integration to determine number of

transfer units


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Performance Evaluation of

Existing Tower1. Because the heat load, L, Ga and temperatures are

known for an operating tower, its performance asrepresented by the number of transfer units, or tower

characteristics can be determined. Solve Equation9-129 for Ka V/L, or use the modified Merkel diagram,

Figure 9-127. This is the number of transfer

units operating in the tower.


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Calculation of KaV/L factor


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Comparison of cooling efficiency of several

packing materials in terms of the coefficient of heat

transfer Ka.


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Cont

2. If it is desired to evaluate a change in performance on an existingtower, knowing the required conditions and numbers of decks andkind of packing, calculate KaV/L for we assumed values of L/Ga.

3.Plot this on the appropriate curve (good up to altitudes of 3,000 ft) forKaV/L vs. L/Ga for the proper wet bulb, range and at theintersection of the straight line plot with the approach value selectedor needed, read the L/Ga required to meet the performanceconditions.

4. Calculate the new Ga assuming that L is the important

value known. If on the other hand, it is desired to determine just howmuch cooling can be obtained, then for a fixed air rate, calculate theL that can be accommodated.

optimization of cooling towers

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