optimization of cooling towers
TRANSCRIPT
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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.