8.16
Cooling Tower Control B. G. LIPTÁK
(1985, 1995, 2005) Air at Twb
Cooling Tower
CWS
CWR
Flow sheet symbol
Types of Designs:
a) Mechanical draft towers a1. Induced (fan at air outlet) cross-flow a2. Induced counter-flow a3. Forced (fan at air inlet) cross-flow a4. Forced counter-flow b) Natural draft towers c) Evaporative condensers
Water Losses:
Blowdown: −0.5 to 3.0% of circulated flow rate Evaporation: −1% for each 12.5°F (7°C) of cooling range Drift: 0.02% to 0.1% of circulated flow rate
Fan Types:
Induced and forced draft fans, axial-flow propellers
Heat Load:
BTU = 500(gpm)(water ∆T)
Approach:
5 to 10°F (2.8 to 5.6°C) for mechanical draft 20°F (11°C) for evaporative condensers
Fan Power Savings at Reduced Speeds:
50% at 80% speed 85% at 50% speed
Costs:
Initial investment: $25 to $40 per gpm of capacity Energy cost of operation: 0.01 BHP/gpm, or about $6 to $10 per year per gpm
INTRODUCTION This section is devoted to the discussion of the control of cooling towers, while the next section deals with their optimization. Other related sections in this chapter describe the control and optimization of HVAC systems and of chillers. Cooling towers are water-to-air heat exchangers that are used to discharge waste heat into the atmosphere. Their cost of operation is a function of the water and air transportation costs. In conventional cooling towers, the air and water flows are often constant. In optimized cooling towers the water and air flows are variable and are adjusted as a function of the load. In this section the types of cooling tower design are described and the various techniques of load-following capacity 1794 © 2006 by Béla Lipták
controls are discussed. Cooling tower winterizing and blowdown controls are also covered. DEFINITIONS In order to make the discussion of the control aspects of cooling towers completely clear, first some related terms will 1 be defined: The difference between the wet-bulb temperature of the entering ambient air and of the leaving water temperature from the tower. The approach is a function of cooling tower capacity; a large cooling tower will produce a closer approach (colder leaving water) for a given heat load, flow rate, and entering air condition (Figure 8.16a). (Units: °F or °C.)
APPROACH
8.16 Cooling Tower Control
Drive shaft
Air outlet Fan (Tao)
Motor Drift eliminators
Louvers Fill
Air in (Tai)
Fill
Gear drive
Air
Tai
Percent distance through tower
Range °F (°C)
Range
Temperature
Approach
Two
r ate W Tao
26 (14.4) 22 (12.2) 18 (10) 14 (7.8) 10 (5.6)
Fan stack (cylinder) Water inlet (Twi) Air in (Tai)
BHP =
8 10 12 14 16 (4.4) (5.6) (6.7) (7.8) (8.9) Approach °F (°C)
The area between two bents of lines of framing members; usually longitudinal. BENT A line of structural framework composed of columns, girts, or ties; a bent may incorporate diagonal bracing members; usually transverse. BLOWDOWN Water discharged to control concentration of impurities in circulated water. (Units: percentage of circulation rate.) BRAKE HORSEPOWER (BHP) The actual power output of an engine or motor. COLD WATER BASIN A storage device underlying the tower to receive the cold water from the tower and direct its flow to the pump suction line or sump. COUNTERFLOW COOLING TOWER A cooling tower in which air flows in the opposite direction to the fall of water through the cooling tower. CROSS-FLOW COOLING TOWER A cooling tower in which the direction of the airflow through the fill is perpendicular to the plane of the falling water. DEW-POINT TEMPERATURE (TDP) The temperature at which condensation begins, if air is cooled under constant pressure (see Figure 8.16b). DOUBLE-FLOW COOLING TOWER A cross-flow tower with two fill sections and one plenum chamber that is common to both fill sections.
© 2006 by Béla Lipták
(motor efficiency)(amps)(volts)(power factor)1.73 746 8.16(1)
FIG. 8.16a The mechanical cooling tower is a water-to-air heat exchanger. A direct relationship exists between water and air temperatures in a counterflow cooling tower. An increase in range will cause an increase in approach, if all other conditions remain unaltered.
BAY
The water loss due to liquid droplets entrained and removed by the exhaust air. Usually under 0.2% of circulated water flow rate. DRIFT ELIMINATOR An assembly constructed of wood or honeycomb materials that serves to remove entrained moisture from the discharged air. DRY-BULB TEMPERATURE (TDB) The air temperature measured by a normal thermometer (see Figure 8.16b). EVAPORATION LOSS Water evaporated from the circulating water into the atmosphere in the cooling process. (Unit: percentage of total GMP.) FAN-DRIVE OUTPUT Actual power output (BHP) of drive to shaft. DRIFT
Water outlet (Two) Twi
1795
The angle that a fan blade makes with the plane of rotation. (Unit: degrees from horizontal.) FAN STACK (CYLINDER) Cylindrical or modified cylindrical structure in which the fan operates. Fan cylinders are used on both induced draft and forced draft axial-flow propeller-type fans. FILLING That part of an evaporative tower consisting of splash bars, vertical sheets of various configurations, or honeycomb assemblies that are placed within the tower to effect heat and mass transfer between the circulating water and the air flowing through the tower. FORCED DRAFT COOLING TOWER A type of mechanical draft cooling tower in which one or more fans are located at the air inlet to force air into the tower. HEAT LOAD The heat removed from the circulating water within the tower. Heat load may be calculated from the range and the circulating water flow. (Unit: BTU per hour = gpm 500 [Tctwr – Tctws ].) LIQUID GAS RATIO (L/G) Mass ratio of water to airflow rates through the tower. (Units: lb/lb or kg/kg.) LOUVERS Assemblies installed on the air inlet faces of a tower to eliminate water splash-out. MAKEUP Water added to replace loss by evaporation, drift, blowdown, and leakage. (Unit: percentage of circulation rate.) MECHANICAL DRAFT COOLING TOWER A tower through which air movement is effected by one or more fans. There are two general types of such towers: those that use forced draft with fans located at the air inlet and those that use induced draft with fans located at the air exhaust. NATURAL DRAFT WATER COOLING TOWER (AIR MOVEMENTS) A cooling tower in which air movement is essentially dependent upon the difference in density between the entering air and internal air. As the heat of the FAN PITCH
Control and Optimization of Unit Operations
0% 10 % 90 % 80 % 70 % 60 50 %
% 40
r y ai of d r r po und U. p e he a t-BT
Pressure of water vapor-PSI
Weight of water vapor in one pound of dry air-grains
35
es
40 .0
30
lb lin
.5
45
10
Wet bu
13
50
air
14 12
dry
16
lb 60
bu
de 65
.5
d
an
.5
12
0.00
32 30
et W 55
25
pe em 75 t t in po 70 w
34
28
20 18
80
13
0.10
22
s re tu a r
36
.0
0.20
24
40 38
14
0.30
26
85
46
42
14
0.40
50 48
44
90
f b. o er l .0 15
0.50
54 52
Barometric pressure 14.696 PSIA
20
25
30
35
40
45
50
60 65 70 80 55 75 Dry bulb-temperature-degrees F.
85
90
250 240 230 220 210 200 190 180 170 160 150 140 130 120 110 % 100 30 90 80 % 20 70 60 50 10% d i t y 40 mi 30 e hu v i t Rela 20 10 0 105 110 115 120 t. p
0.60
58 56
.f Cu
0.70
250 240 230 220 210 200 190 180 170 160 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0
Tot al
1796
95
100
Copyright, 1942, General Electric Company
FIG. 8.16b The psychrometric chart describes the following properties of air: Water vapor pressure, weight of water per pound of air, dry-bulb temperature, wet-bulb temperature, heat content per pound of air, volume per pound of air. The dew point temperature is the wet-bulb temperature when the relative humidity is 100%.
water is transferred to the air passing through the tower, the warmed air tends to rise and draw in fresh air at the base of the tower. NOMINAL TONNAGE One nominal ton corresponds to the transfer of 15,000 BTU/hr (4.4 kW) when water is cooled from 95 to 85°F (35 to 29.4°C) by ambient air having a wet-bulb temperature of 78°F (25.6°C) and when the water circulation rate is 3 gpm (11.3 lpm) per ton. PERFORMANCE The measure of a cooling tower’s ability to cool water. Usually expressed in gallons per minute cooled from a specified hot water temperature to a specified cold water temperature with specific wetbulb temperature. Typical performance curves for a cooling tower are shown in Figure 8.16c. This is a “7500 gpm at 105-85-78” tower, meaning that it will cool 7500 gpm of water from 105 to 85°F when the ambient wet-bulb temperature is 78°F. PLENUM The enclosed space between the eliminators and the fan stack in induced draft towers, or the enclosed space between the fan and the filling in forced draft towers. POWER FACTOR The ratio of true power (watts) to the apparent power (amps × volts).
© 2006 by Béla Lipták
An instrument used primarily to measure the wet-bulb temperature. RANGE (COOLING RANGE) The difference between the temperatures of water inlet and outlet, as shown in Figure 8.16a. For a system operating in a steady state, the range is the same as the average water temperature rise through the process loads. Accordingly, the range is determined by the heat load and water flow rate, not by the size or capability of the cooling tower. On the other hand, the range does affect the approach, and as the range increases, a corresponding increase in the approach will occur if all other factors remain unaltered (see Figure 8.16a). RISER Piping that connects the circulating water supply line from the level of the base of the tower or the supply header to the tower inlet connection. SPEED REDUCER A device for changing the speed of the driver in order to arrive at the desired fan speed. 3 STANDARD AIR Dry air having a density of 0.075 lb/ft at 70°F and 29.92 in. Hg. STORY The vertical dimension or area between two lines of horizontal framework ties, girts, joists, or beams. In wood frame structures, the story will vary from 5 to 7 ft (about 2 m) in height. PSYCHROMETER
8.16 Cooling Tower Control
1797
THE COOLING PROCESS 4°F Range = 2
110
Fans off
20
Cooling tower water temperature (°F)
16 12
100
90
80
Fans at half speed
= 24°F Range 20 16 12 24°F ge = Ran 20 16 12
Fans at full speed
70
60 60
65
70
75
80
Wet-bulb temperature (°F)
FIG. 8.16c Cooling tower performance curves give the temperature of the generated cooling water as a function of the wet-bulb temperature of the air, the fan speed, and the range (the difference between the inlet and outlet temperatures of the tower water).
SUMP (BASIN)
Lowest portion of the basin to which cold circulating water flows: usually the point of suction connection. TONNAGE See Nominal Tonnage. TOTAL PUMPING HEAD The total head of water, measured above the basin curb, required to deliver the circulating water through the distribution system. TOWER PUMPING HEAD Same as the total pumping head minus the friction loss in the riser. It can be expressed as the total pressure at the centerline of the inlet pipe plus the vertical distance between the inlet centerline and the basin curb. WATER LOADING Water flow divided by effective hori2 zontal wetted area of the tower. (Unit: gpm/ft or 3 2 m /hr · m .) WET-BULB TEMPERATURE (TWB) If a thermometer bulb is covered by a wet, water-absorbing substance and is exposed to air, evaporation will cool the bulb to the wet-bulb temperature of the surrounding air. This is the temperature read by a psychrometer. If the air is saturated with water, the wet-bulb, dry-bulb, and dew-point temperatures will all be the same. Otherwise, the wet-bulb temperature is higher than the dew-point temperature but lower than the dry-bulb temperature (see Figure 8.16b).
© 2006 by Béla Lipták
In a cooling tower, heat and mass transfer processes combine to cool the water. The mass transfer due to evaporation does consume water, but the amount of loss is only about 5% of the water requirements for equivalent once-through cooling by river water. Cooling towers are capable of cooling within 5–10°F (2.8–5.6°C) of the ambient wet-bulb temperature. The larger the cooling tower for a given set of water and airflow rates, the smaller this approach will be. The mass transfer contribution to the total cooling is illustrated in the simplified psychrometric chart in Figure 8.16d. Points A and D illustrate the conditions in which the ambient air might enter. After it transferred heat and absorbed mass from the evaporating water, the air leaves in a saturated condition described to point B. The total heat transferred in terms of the difference between entering and leaving enthalpies (hB − hA) is the same for both the AB and the DB processes. The AC component of the AB vector represents the heat transfer component (sensible air heating), and the CB vector represents the evaporative mass transfer component (latent air heating). If the air enters at condition D, the two component vectors do not complement each other, because only one of them heats the air (EB). Therefore, the total vector DB results from the latent heating (EB) and sensible cooling (DE) of the air. In terms of the water side of the DB process, the water is being sensibly heated by the air and cooled by evaporation. Therefore, as long as the entering air is not saturated, it is possible to cool the water with air that is warmer than the water.
Le a vin tem g we per t-bu atu lb re
Ent eri tem ng we per t-bu atu l re b
at he al sfer t To ran t
hA
hB
Latent heat and mass transfer
B
E
wB
wA
C
A
Specific humidity
D
wD
Sensible heat transfer Dry-bulb temperature
FIG. 8.16d Heat and mass transfer processes combine to cool water. Total cooling is the sum of the latent cooling caused by the evaporation of water and the sensible heat transfer, which can either add to or reduce the latent cooling as a function of the relative temperatures 2 of the entering air and water.
1798
Control and Optimization of Unit Operations
where K = overall unit of conductance, mass transfer between saturated air at mass water temperature, and main air 2 stream: lb per hour (ft )(lb/lb) 2 a = area of water interface per unit volume of tower: ft 3 per ft 3 2 V = active tower volume per unit area: ft per ft L = mass water flow rate: lb per hour G = airflow rate: lb dry air per hour n = experimental coefficient: varies from − 0.35 to − 1.1 and has an average value of − 0.55 to − 0.65
Air out
Eliminators
Fan Hot water in Sprays Air in
Fill
Air in
Cold water out
Louvers
Induced-draft counterflow Hot water in Air out Louvers
Air out Eliminators
Fill
Sprays
Hot water in
Cold water out
Fan
Air in
Fill Eliminators
Fan
Forced-draft counterflow
Force-draft crossflow
Air out Hot water in
Air in Louvers
Fan
Fan Eliminators
Fill
Air in
Fill
Characteristic Curves
Louvers Cold water out
Cold water out
Air out Eliminators
Fill
Air in
Cold water out
Induced-draft crossflow (double air entry)
Induced-draft crossflow (single air entry)
FIG. 8.16e Mechanical draft cooling towers are distinguished by the location of the air fan (forced or induced draft), by the direction of the air flow (cross- or counterflow), and by the nature of the air entrance 2 (single or double entry).
MECHANICAL DRAFT COOLING TOWERS The basic mechanical draft cooling tower designs are described in Figure 8.16e. For operational purposes, tower characteristic curves for various wet-bulb temperatures, cooling ranges, and approaches can be plotted against the water flow to airflow ratio in accordance with following equation:
n
KaV ( L ) ~ L (G )
© 2006 by Béla Lipták
From the characteristic curves of the cooling tower it is possible to determine tower capability, effect of wet-bulb temperature, cooling range, water circulation, and air delivery, as shown in Figure 8.16f. The ratio KaV/L is determined from test data on hot water and cold water, wet-bulb temperature, and the ratio of the water flow to the airflow. For air conditioning applications the thermal capability of cooling towers is usually stated in terms of nominal tonnage. This tonnage is defined as the heat dissipation of 15,000 BTU/h (4.4 kW) per condenser ton (kW) and a water circulation rate of 3 gpm/ton (0.054 l/s per kW) cooled from 95 to 85°F (35 to 29.4°C) at 78°F (25.6°C) wet-bulb temperature. It may be noted that the subject tower would be capable of handling a greater heat load (flow rate) when operating in
8.16(2)
10 8 7 6 5 4 3 2.5 2 1.5 KaV 1.0 L 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.25 0.2
Approach 2 3 4 5 6 7 8 9 10 11 12 14 16 18 20 22 24 26 28 30 35 40 45
KaV L
0.15 0.1 0.1
0.15 0.2
0.3 0.4
0.6 0.8 1.0 0.5 L G
1.5 2 2.5 3
4 5
FIG. 8.16f Characteristic curves of cooling towers (based on 64°F [17.8°C] air wet-bulb temperature and 18°F [10°C] cooling water temperature drop range).
8.16 Cooling Tower Control
1799
Total operating cost ($/MBTU)
250
Percent of tower cost
200
150
We t
bu
lb
Ap pr oa
ch
ge
Ran
100
Total cost (M1, M2 and M3)
50
5 (2.8)
10 (5.6)
5 (2.8)
10 (5.6)
65 (18.3)
70 (21.1)
15 20 (8.3) (11.1) Approach
25 (13.9)
30 (16.7)
20 (11.1)
25 (13.9)
30 (16.7)
75 80 (23.9) (26.7) Wet bulb
85 (29.4)
90 (32.2)
15 (8.3) Range
SC 1
M1 Air at Twb
Air at Twb
Approach (∆T)
FIG. 8.16g The smaller the cooling tower (the lower the initial investment), the higher will be the operating cost. If the tower cost is considered to be 100 when it is sized for an approach and a range of 15°F when the wet bulb is 75°F, the data in this figure give the comparative cooling tower costs for a range of other sizing basis. (Courtesy of Foster Wheeler Corp.)
a lower ambient wet-bulb region. For operation at other flow rates, tower manufacturers usually provide performance curves covering a range of at least 2 to 5 gpm per nominal ton (0.036 to 0.09 lps/kW). The initial investment required for a cooling tower is essentially a function of the required water flow rate, but this cost is also influenced by the design criteria for approach, range, and wet-bulb. As shown in Figure 8.16g, cost tends to increase with range and tends to decrease with a rise in approach or wet-bulb. The initial investment is about $25–$40 per gpm of tower capacity, whereas the energy cost of operation is approximately 0.01 BHP/gpm, or about $6–$10 per year per gpm. This means that the cost of operation reaches the cost of initial investment in 5–10 years, depending on energy costs.
CONTROLS On HVAC-type applications, the cooling tower controls usually consist of on/off or sequential load controls, safety interlocks, and blowdown and winter operation controls. In industrial applications, the controls tend to be continuous and are usually controlling sophisticated, variable-speed fans (Figure 8.16h) as discussed in the next section.
© 2006 by Béla Lipták
Fan cost (M1) Ao
Temperature °F (°C) 0
Compressor and pump costs (M2 and M3)
Optimized setpoint to TDIC-1 SP Approach (∆T ) = Tctws − Twb
TDIC 1
Tctws
Twb
FIG. 8.16h In order to minimize the total cost of operation, the speed of the cooling tower’s air fan can be so throttled so that the optimum approach (the approach corresponding to the minimum cost of operation) is maintained.
Load Controls The cooling tower is an air-to-water heat exchanger. The controlled variables are the supply and return water temperatures, while the manipulated variables are the air and water flow rates. Manipulation of these flow rates can be continuous, through the use of variable-speed fans and pumps, or can be incremental, through the cycling of single- or multiplespeed units. If the controlled variable is the temperature of the water supplied by the cooling tower and the manipulated variable is the fan speed, a change in cooling load will result in a change in the operating level of the fans. If the fans are singlespeed devices, the airflow rate is changed by cycling the fan units on or off as a function of load. This is illustrated in Figure 8.16i and can be done by simple sequencers or by more sophisticated digital systems. In most locations, the need to operate all fans at full speed is required for only a few thousand hours every year. Fans running at half speed consume approximately one-seventh of the design air horsepower but produce over 50% of the design air rate (cooling effect). Therefore in 98% of the cases, twospeed motors are a wise investment for minimizing operating costs.
Operating Interlocks Figure 8.16j describes a simple fan starter. A three-position “hand/off/automatic” switch controls the fan. When the switch is placed in “automatic” (contacts 3 and 4 connected),
1800
Control and Optimization of Unit Operations
Water temperature generated by cooling tower °F (°C)
On sequence with increase in control signal
100 (37.8)
8 7
Throttling band (adjustable at controller)
6 5 4 Stage no. 1 “on” 2 1
1 – all fans off 2 – 3 fans on 3 – 6 fans on 4 – all 9 fans on
3 Stage no. 1 “off ”
90 (32.2)
1
80 (23.9)
2
70 (21.1)
3
60 (15.6)
4
Temperature setpoint
50 (10)
Off sequence with decrease in control signal
−20 −10 0 10 20 30 40 (−28.9) (−23.3)(−17.8)(−12.2) (−6.7) (−1.1) (4.4)
50 60 70 (10) (15.6) (21.1)
Ambient wet-bulb temperature °F (°C)
FIG. 8.16i Illustrations of load-following control accomplished by fan cycling. The figure on the left illustrates how the output signal from the approach controller (TDIC-1 in Fig. 8.16h) sequences the fan stages on and off as a function of the load. The figure on the right shows the effect of fan cycling on a nine-cell tower that is designed for a full capacity of 160,000 gpm (608,000 l/m) water loading with an 84°F (29°C) inlet 3 and 70°F (21°C) outlet temperature, operating at 50% load.
the status of the interlock contact I determines whether or not the circuit is energized and the fan is on. The sequencing of this contact I is described in Figure 8.16i. The purpose of the auxiliary motor contact M is to energize the running light R whenever the fan motor is on. The parallel
hot lead to contact 6 allows the operator to check quickly to see whether the light has burned out. The amount of interlocking is usually greater than that shown in Figure 8.16j. Some of the additional interlock options are described in the paragraphs below. Safety Interlocks
Device mounted on front of starter box
H
H-O-A 1
2
3
4
N
OL’s M
I
M
5 6
Interlock contact for automatic control
R
Auxiliary motor contact Legend
Indicating light Coil of relay or starter Selector switch
Indicates coil of motor starter Overload relay OL’s contacts (3) Hand-off-auto H-O-A (selector switch) M
FIG. 8.16j The wiring diagram of a simple fan starter circuit, manually controlled by a three-position (hand-off-automatic) switch.
© 2006 by Béla Lipták
Most fan controls include some safety overrides in addition to the overload (OL) contacts shown in Figure 8.16j. These overrides might stop the fan if fire, smoke, or excessive vibration is detected. They usually also provide a contact for remote alarming. Most fan controls also include a reset button that must be pressed after a safety shut-down condition is cleared before the fan can be restarted. When a group of starters is supplied from a common feeder, it may be necessary to make sure that the starters are not overloaded by the inrush currents of simultaneously started units. If this feature is desired, a 25 sec time delay is usually provided to prevent other fans from starting until that time has passed. Larger fans should be protected from overheating caused by too frequent starting and stopping. This protection can be provided by a 0–30 min time delay guaranteeing that the fan will not be cycled at a frequency faster than the delay time setting. When two-speed fans are used, added interlocks are frequently provided. One interlock might guarantee that even if the operator starts the fan to operate at high speed, it will operate
8.16 Cooling Tower Control
for 0–30 sec in low before advancing automatically to high. This makes the transition from off to high speed more gradual. Another interlock might guarantee that when the fan is switched from high to low, it will be off the high speed for 0 to 30 sec before the low-speed drive is engaged. This will give time for the fan to slow before the low gears are engaged. When the fan can also be operated in reverse, further interlocks are needed. One interlock should guarantee that the fan is off for 0–2 min before a change in the direction of rotation can take place. This gives time for the fan to come to rest before making a change in direction. Another interlock will guarantee that the fan can operate only at slow speed in the reverse mode and that reverse operation cannot last more than 0–30 min. This limitation is desirable because when the fan is in reverse in the winter, ice tends to build up on the blades and the gears can wear excessively. If the same fans can be controlled from several locations, interlocks are needed to resolve the potential problems with conflicting requests. One method is to provide an interlock that determines the location that is in control. This can be a simple local/remote switch or a more complicated system. When many locations are involved, conflicts are resolved by interlocks that either establish priorities between control locations or select the lowest, highest, safest, or other specified choice from the control requests. In installations with multiple control centers, it is essential that feedback be provided so that the operator is always aware not only of the actual status of all fans but also of any conflicting requests that are possibly coming from other operators or computers.
EVAPORATIVE CONDENSERS The evaporative condenser is a special cooling tower type, shown in Figure 8.16k. In this unit the induced air and sprayed water flow concurrently downward, and the process vapors travel upwards inside the multipass condenser tubes. A pressure controller (PIC) modulates the amount of cooling, so that it will match the load. The continuous water spray on
1801
that makes the tube surfaces wet, improves heat transfer and maintains tube temperatures. The approach of these cooling towers is around 20°F (11°C), and the wet-bulb temperatures of the cooling air can be varied by the recirculation of humid (or saturated) air. The resulting condensing temperatures are lower than the condensers, which are cooled by dry air. The higher heat capacity and lower resistance to heat transfer make these units more efficient than the dry-air coolers. The turndown capability of these units is also very high, because the fresh-air flow can be reduced all the way to zero. Both the exhaust and the fresh air dampers can be closed while the recirculation damper simultaneously opens to maintain the internal air flow rate constant. As more humid air is recirculated, the internal wet-bulb temperature rises and the heat flow drops off. In Figure 8.16k the heat flow is 4 effectively linear with fresh-air flow.
SECONDARY CONTROLS In addition to the previously discussed load controls and safety interlocks, controls are also needed for winter operation, water quality, and blowdown controls and other miscellaneous purposes. Winter Operation In case of a power failure during subfreezing weather, the tower basin should be electrically heated and should be provided with an emergency draining system. Another method of freeze protection is to provide bypass circulation illustrated in Figure 8.16l. In this system, a thermostat (TSL) detects the outlet temperature of the cooling tower water, and
h Cooling tower
Exhaust FO FC
PIC
FO Induced fresh air Process condensate
Conditioned water
R/A
S3
S2 Heater
PT
Blowdown
Process vapor
FIG. 8.16k In evaporative condensers, the pressure of the process vapors can be controlled by modulating the proportion of the humid air, which is recirculated. (Adapted from Reference 4.)
© 2006 by Béla Lipták
S1 TSL
P2
Process cooling load
P1
FIG. 8.16l Protection against freezing can be provided in the winter by starting a bypass pump if the temperature drops to near freezing (TSL) and by sending this bypassed water through a heater and then back to the tower.
1802
Control and Optimization of Unit Operations
when it drops below 40°F (4.4°C), the bypass circulation (located in a protected indoor area) is started. This circulation is terminated when the water temperature rises to approximately 45°F (7.2°C). The thermostat is wired to open the solenoids S1, S2, and S3 first and then to start the circulating pump (P2), which usually is a small, 1/4 or 1/3 hp (186.5 or 248.7 W) unit. The bypass line containing solenoid S1 is sized to handle the flow capacity of P2, with a pressure drop that is less than the height of the riser pipe (h). This makes sure that when P1 is off, no water will reach the top of the column. In the northern regions, in addition to the S1 bypass shown in Figure 8.16l, there is a full-size bypass (not shown in Figure 8.16l). When the main (P1) pump is started in the winter, this full bypass is opened; it does not allow the water to be sent to the top of the tower until its temperature is approximately 70°F (21°C). Once the water reaches that temperature, the bypass is closed because the water is warm enough to be sent to the top of the tower without danger of freezing. As the water temperature rises further, fans are turned on to provide added cooling. When open cooling towers are operated at freezing temperatures, the induced draft fans must be reversed periodically in order to deice the air intakes. The need for deicing can be determined through visual observation, through remote closed-circuit TV inspection, or by comparing the load and ambient conditions to past operating experience, as have already been discussed under interlocks. All exposed pipes that will contain water when the tower is down should be protected by electric tape or cable tracing and by insulation. The sump either should be drainable to an indoor auxiliary sump or should be provided with auxiliary steam or electric heating. Closed cooling towers should either operate on antifreeze solutions or be provided with supplemental heat and tracing. If the second method is used, the system must be drained if power failure occurs. Blowdown Controls The average water loss by blowdown is 0.5–3.0% of the circulating water rate. The loss is a function of the initial quality of the water and the amount and concentration of the dissolved natural solids and chemicals added for protection against corrosion or build-up of scale on the heat transfer surfaces. Because the cooling tower is a highly effective air scrubber, it continuously accumulates the solid content of the ambient air on its wet surfaces, which are then washed off by the circulated water. The “normal” condition of the circulated water can be defined arbitrarily as: pH
6–8
Chloride as NaCl
under 750 ppm
Total dissolved solids
under 1500 ppm
© 2006 by Béla Lipták
The blowdown requirement can be determined as follows: B=
([ E + D] / N ) − D 1 − (1 / N )
8.16(3)
where B = blowdown in percentage (typically, for N = 2, 0.9%; for N = 3, 0.4%; for N = 4, 0.24%) E = evaporation in percentages (typically 1% for each 12.5°F [7°C] of cooling range) D = drift in percentage (typically 0.1%) N = number of concentration relative to initial water quality Problems and Solutions In cooling towers, the major causes for concern are delignification, which is the loss of the binding agent of the cellulose. It is caused by the use of oxidizing biocides, such as chlorine. It can also be caused by excessive bicarbonate alkalinity; biological growth, which can clog the nozzles and foul the heat exchange equipment; corrosion of the metal components (this corrosion should be less than 3 mils per year without pitting); general fouling by a combination of silt, clay, oil, metal oxides, calcium and magnesium salts, organic compounds, and other chemical products that can cause reduced heat transfer and enhanced corrosion; and scaling by crystallization and precipitation of salts or oxides (mainly calcium carbonate and magnesium silicate) on surfaces. The treatment techniques to prevent these conditions from occurring are listed in Table 8.16m. The blowdown must meet the water quality standards for the accepting stream and must not be unreasonably expensive. It is also possible to make the water system a closed-cycle system (no blowdown) by ion-exchange treatment. Miscellaneous Controls The minimum basin level is maintained by float-type, makeup level control valves. The maximum basin level is guaranteed by overflow nozzles, which are sized to handle the total system flow. In critical systems, additional safety is provided by the use of high/low level alarm switches. Torque and vibration detectors can be used to safeguard the operational safety of the fans and of the fan drives. Lowtemperature alarm switches can also be furnished as warning devices signaling the failure of the freeze protection controls. The need for flow balancing among multiple cells is another reason for using additional valves and controls. (Advanced water distribution controls will be discussed in the next section.) In conventional systems, the warm water is usually returned to the multicell system through a single riser, and a manifold is provided at the top of the tower for water distribution to the individual cells. Balancing valves should be installed to guarantee equal flow to each operating cell. If two-speed fans are used, further economy can be gained by lowering the water flow when the fan is switched to low speed. It is also advisable to install equalizer lines
8.16 Cooling Tower Control
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TABLE 8.16m Potential Cooling Tower Problems and their Solutions Problem
Factors
Causative Agents
Wood deterioration
Microbiological Chemical
Cellulolytic fungi Chlorine
Fungicides Acids
Biological growths
Temperature Nutrients pH Inocula
Bacteria Fungi Algae
Chlorine Chlorine donors Organic sulfurs Quatenary ammonia
General fouling
Suspended solids Water Velocity Temperature Contaminants Metal oxides
Silt Oil
Polyelectrolytes Polyacrylates Lignosulfonates Polyphosphates
Corrosion
Aeration pH Temperature Dissolved solids Galvanic couples
Oxygen Carbon dioxide Chloride
Chromate Zinc Polyphosphate Tannins Lignins Synthetic organic compounds
Scaling
Calcium Alkalinity Temperature pH
Calcium carbonate Calcium sulfate Magnesium silicate Ferric hydroxide
Phosphonates Polyphosphates Acid Polyelectrolytes
between tower sumps to eliminate imbalances caused by variation in flow or pipe layouts. Another reason for considering the use of two-speed fans is to have the ability to lower the associated noise level at night or during periods of low load. Switching to low speed will usually lower the noise level by approximately 15 dB.
References 1. 2. 3. 4.
CONCLUSIONS One of the least understood and most neglected unit operations in the processing industries is the cooling tower. Its pumps are frequently constant speed with three-way or bypass valves used to circulate the excess water. This excess should not have been pumped in the first place, because the process does not require it. Meeting a variable load with a constant supply by wasting the excess is also frequently practiced in fan operation. In some installations, fan speeds cannot be changed at all; in others, they can be changed only manually, which can require seasonal adjustments in some extreme cases. As the rating of the tower fans and pumps usually adds up to several hundred horsepower, their yearly operating cost is in the hundreds of thousands of dollars. As optimization can cut this in half, the added costs for controls are justified. This is the subject of the next section.
© 2006 by Béla Lipták
Corrective Treatments
Dickey, J. B., Evaporative Cooling Towers, Marley Cooling Towers Co., 1978. ASHRAE, “Cooling Towers,” in ASHRAE Handbook—HVAC Systems and Equipment, New York: ASHRAE, 2004. Wistrom, G., “Cold Weather Operation of Crossflow Cooling Towers,” Plant Engineering, September 8, 1975. Lipták, B. G. (ed.), “Cooling Towers: Their Design and Application,” in Environmental Engineer’s Handbook, Lewis Publishers, 1997.
Bibliography ASHRAE, “Chilled and Dual-Temperature Water Systems,” in ASHRAE Handbook, Equipment Volume, Atlanta, GA: ASHRAE, 1983. ASHRAE, “Cooling Towers,” in ASHRAE Handbook-HVAC Systems and Equipment, New York: ASHRAE, 2004. ASHRAE, “Heating, Ventilating, and Air Conditioning Systems,” in ASHRAE Handbook, June 2004. Baechler, R. H., Blew, J. O., and Duncan, C. G., “Cause and Prevention of Decay of Wood in Cooling Towers,” ASME Petroleum Division Conference, New York: ASME, 1961. Baker, D. R., “Durability of Wood in Cooling Towers,” Technical Bulletin R-62-W-1, Kansas City, MO: The Marley Co., 1962. Brownell, D. L., Handbook of Applied Thermal Design, Taylor and Francis, 1999. Donohue, J. M., “Cooling Towers — Chemical Treatment,” Industrial Water Engineering, May 1970. Frayne, C., Cooling Water Treatment, Chemical Publishing Co., January 2001.
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Control and Optimization of Unit Operations
Hartman, T., “Ultra-Efficient Cooling with Demand-Based Control,” HPAC Engineering, December 2001. Kern, D. Q., Process Heat Transfer, New York: McGraw-Hill, 1950, p. 563. Koch, J., Untersuchung und Berenchnung und Kuehlwerken, VDI Forschungsheft no. 404, Berlin, 1940. Lipták, B. G. (ed.), “Cooling Towers: Their Design and Application,” in Environmental Engineer’s Handbook, Lewis Publishers, 1997. Lipták, B. G., “Cutting in Half the Cost of Cooling,” Control, November 2003. Luyben, W. L., Process Modeling, Simulation, and Control, New York: McGraw-Hill, 1989. Mathews, R. T., “Some Air Cooling Design Considerations,” Proceedings of the American Power Conference, 1970.
© 2006 by Béla Lipták
McMillan, G., “Good Tuning: A Pocket Guide,” ISA, June 2000. Nottage, H. B., “Merkel’s Cooling Diagram as a Performance Correlation for Air-Water Evaporative Cooling Systems,” ASHRAE Transactions, Vol. 47, p. 429, 1941. Nussbaum, O. J., “Dry-Type Cooling Towers for Packaged Refrigeration and Air Conditioning Systems,” Refrigeration Science and Technology, 1972. Roper, M. A., Energy Efficient Chiller Control, BSRIA Ltd., September 2000. Walker, W. H., Lewis, W. K., McAdams, W. H., and Gilliand, E. R., Principles of Chemical Engineering, New York: McGraw-Hill, 1937, p. 480. Witt, M. D., “Successful Instrumentation and Control Design,” ISA, 2003.
