Improve and Maintain Cooling Tower Efficiency

Chemical Engineering Progress,  Apr 2005  by Willa, James L

Changing maintenance and/or operating procedures can significantly increase cooling tower performance without major capital expenditures. Here are some tips for ensuring proper design and operation of various tower components.

THE COOLING TECHNOLOGY INSTITUTE (CTI) helps industry improve the design of cooling towers and tests cooling towers to an accuracy of one-tenth of one degree Fahrenheit. Specifying engineers, likewise, go to great lengths to obtain the proper cooling tower. Yet, once towers are placed in service, they often do not get the attention they deserve and they begin to lose much more than tenths of a degree in performance. Such loss in thermal performance of cooling towers is unnecessary.

Debottlenecking projects often reveal the cooling tower as a (or the) production-limiting factor. Therefore, uprating of cooling towers has become the buzzword in the cooling tower industry for the past few years, and it appears that this trend will continue for some time.

Uprating means getting colder water back from the tower under current operating conditions, or maintaining current cold-water return temperatures under increased heat load, or various combinations of both. There are several valid approaches to this problem, many of which require substantial changes and expenditures.

Three factors affect a cooling tower's performance: the amount of water being pumped over the tower (L), the amount of air being pumped through the tower (G), and the "tower characteristic," which is a measure of the tower's heat transfer ability and is represented by the term K^sub a^V/L. A change in any of these factors will change the tower's thermal performance.

Total water flow can be altered by changing the pumping rate to produce a water flowrate above or below the design rate. In general, if this difference is excessive, the distribution system is adversely affected, either by flooding at high water flow or by improper spray patterns at low water flow.

Airflow can be affected by changing the static pressure against which the fan is pumping, fan speed, pitch or efficiency. It is usually not possible to change the speed of an operating fan. However, pitch and efficiency can be influenced. A change in static pressure is the most common cause of a change in airflow.

A change in the tower characteristic, or ability to cool water, is brought about primarily by changes in physical configuration in the wetted surface area.

The performance of the tower depends not only on maintaining the proper water/air ratio (L/C), but it also is based on the assumption that the water and air are thoroughly mixed or both properly distributed. However, in the real world, they very seldom are evenly and thoroughly mixed. The problem of proper distribution is the most significant factor affecting the thermal performance of a cooling tower, and will be discussed in detail in a companion article next month.

Louvers

Keep louvers free of algae, moss and scale (Figure 1 )to prevent increased static pressure and a reduction in airflow. One tower inspected had silica scale obstructing one-third of the total air inlet area, even though the vertical distance between louvers was 36 in.

New counterflow towers are built without louvers. On existing counterflow towers, two of every three louvers of the old style (1/2 in. by 6 in. by 3 ft) can be removed. On old straight-wall crossflow towers, two out of every three louvers can be removed (Figure 2).

Keep steam exhaust and/or other heat sources from entering the air inlets. Where steam-driven pumps are used, be certain the exhaust steam stack is tall enough to prevent its discharge from reaching the air inlets.

On crossflow towers, maintain the 1-ft-tall plywood air seal below the bottom louvers to prevent air from bypassing under the fill.

Casing

Any hole, crack or leak through the casing causes loss of air through the fill. The closer the leak is to the top of the tower, the more damaging the effect.

Missing corner rolls near the top of a counterflow tower can be the source of an appreciable bypass air leak into the plenum area.

Quite often, a new tower will have its casing installed leak-free with caulking. Then the fire protection contractor will cut a 1-ft-square hole through the casing for a 6-in. sprinkler supply line. Be sure that all holes and cracks are patched.

End-wall entrance doors on crossflow towers must be kept intact and closed.

Basin

Total cooling ability depends not only on proper cold water temperature, but also on getting this water to the right places in the right amounts. Therefore, the sump screens are an important, though almost universally neglected, factor in the tower's cooling ability.

All towers must have a double screen at every outlet to a sump or pump. The cooling tower, due to its size and negative pressure at the air inlets, acts as a huge vacuum cleaner and air washer - sucking up dust, plastic sandwich bags, foam coffee cups, paper, cardboard, plastic sheets, etc. The screens are designed to keep this material out of the heat exchanger tubes.

However, more than 90% of all screens do not have a debris-catcher on the first screen (the screens should be installed in sets of two). Thus, when the first screen is removed for cleaning, the trapped debris falls to the bottom, under the first screen to the second screen. When the second screen is removed for cleaning, the debris escapes under the bottom and travels through the pump and into the exchanger, where even one plastic bag can plug four or five tubes.

The lead screen must have a 6-in.-wide by 6-in.-deep screen-lined cup or catch on the bottom edge to catch debris when the screens are lifted for cleaning. Use stainless steel framing and a 1/2-in.-mesh stainless steel screen liner for this cup.

The screens must be cleaned regularly. A hoist, either hand-operated or electric, should be installed over the screens, and the tops of the screens should have adequate hooks or handles for attachment to the hoist.

Choose the location where the screens will be cleaned carefully. An inspection of a hyperbolic natural-draft tower revealed excellent stainless steel screens with cups, hoists, etc. However, they were moved by the hoist to one side, leaned against the louver face, and washed with a pressure hose. All the wood fiber and other debris went right back into the tower through the louvers. The hoist support rail must extend past the sump to a cleaning area away from the tower.

Extraneous streams are often dumped into the cooling tower basin. Streams such as steam condensate from steam-powered circulating water pumps, hot water from the steam condenser on the cold water pumps, and hot boiler blowdown used for makeup are detrimental to the cooling tower.

The most common extraneous stream, and usually the most detrimental, is side-stream filter water. This water is generally taken from the hot water line into the tower, then run through the side-stream filter and then back into the basin, since it has insufficient remaining head to be put over the top of the tower.

In one 10-cell, 6-cube double-flow crossflow tower, a stream of water was taken off the hot water header and run through an elaborate side-stream filter setup, which is good. However, the effluent from the side-stream filter was then piped back to the cold water basin. The thermal performance test on this unit revealed that a 0.7°F rise in the cold water temperature in this 107,000-gal/min cooling tower was due to this bypassing of hot water directly into the basin. The company had just paid approximately $250,000 to lower the cold water temperature by 0.9°F in a 4-cell crossflow tower to increase its production efficiency; yet it allowed the 10-cell tower to lose 0.7°F strictly for the lack of

a line running to the top of the tower and the addition of the side-stream filter water into the distribution basin rather than in the cold water basin. Even the addition of a small pump if necessary to accomplish this would be a minor expenditure, compared to the difference in cooling tower performance.

All extraneous streams above the cold water temperature should go over the top of the tower, where the effect will be minimal.

Do not allow hot water to bypass into the cold water basin. Often, a bypass line opened in the winter will be left open during the summer. Many counterflow risers have drain lines from above the valve into the basin. These lines are for winter use, to drain and prevent freezing in out-of-service risers. However, many are left open when the seasons change.

Add cold water to the acid dilution trough, not hot water. Remember, the "product" of the cooling tower is cold water - don't pollute it with hot streams, and don't waste it. Many plants blow down with cold water, whereas hot water should be used for blowdown.

Chlorine and other algicide/slimicide injection locations should be selected where these treatments are most needed. If slime in the exchanger is the most critical problem, then the chlorine should be added into the sump or somewhere near it. However, if algae growth on the distribution deck of a crossflow tower is causing poor distribution and loss of performance, or the film-pack fill is being plugged, then chlorine should be added to the hot-water supply line.

Water distribution patterns in the unit are most important. Many times, the spare cooling tower pump is run continuously. This results in more water flowing over the tower, which raises the cold water temperature. This rise in cold water temperature often outweighs the gain in heat exchange from the increased velocity. Avoid this "send more water syndrome."

Fill

This is where most of the heat transfer takes place. The fill must be kept clean from algae, slime, scale, product contamination, mud and debris, particularly film-pack-type fill.

Splash fill. Splash fill (Figure 3) must be kept level and in position. The first 6 ft of crossflow fill near the air inlet should be secured in place with plastic clips to the support grid, and its overhang past the last support grid near the louvers should never exceed 6 in. Ice-damaged fill slats near the perimeter must be repaired or repositioned. When icing threatens to be a problem, heavy-duty redwood fill should be used rather than plastic.

Crossflow and counterflow fills should not be used interchangeably. Crossflow splash fill should be positioned parallel to airflow in most cases. Missing fill should be replaced.

Film-pack fill. Film-pack fill (Figure 4) has a much larger surface area than splash fill, and is much more efficient per foot of height, which allows increased efficiency and reduced pumping head. However, film-pack fill does have limitations. It is much more susceptible to decreases in performance from poor distribution patterns, and is more susceptible to clogging or plugging. Indeed, all film-pack fill will clog or plug under adverse conditions.

During the 1950s and early 1960s, it was very difficult to convince industrial users to accept film-pack fill. The pendulum has now swung completely the other way. Now, film-pack fill is used in many, if not most, cooling towers. Many of these applications are not suited to film-pack fill due to water conditions and/or product contamination, which will result in its fouling or plugging. Therefore, a thorough investigation of the water and possible contaminants should be conducted before using film-pack fill.

The best defense against loss of performance due to fill problems is a good water-treatment program that will prevent accumulation of mud, algae, slime and scale, and a good screen design and operation to prevent debris accumulation.

Fan stacks

Holes in the fan stack beneath the fan cause air to bypass the fill. The most common holes in this area are slots in the fan stack at the drive shaft, oil line and mechanical equipment support steel. The hole for the drive shaft should not be a slot, but only a hole approximately 2 in. larger in diameter than the coupling on the drive shaft. The oil line, vent line and any other line or beam penetrating the stack should have holes only large enough for a tight fit around the line or beam. Vertical joints must be kept tight, and caulking or putty, where used, must be kept in place.

The backpressure against the fan can be reduced and the air delivery increased at the same horsepower by using a tall velocity-recovery fan stack that flares out after the throat area, thereby increasing the discharge area and reducing the backpressure.

The fan blade tip clearance should be kept to a minimum, such as 1 in. to 1-1/2 in. for a 28-ft-dia. fan. If the tip clearance is greater than this, a tip seal should be installed.

Fan deck

Any hole in the fan deck will leak air, which bypasses the water. Holes are common under the fan motor, and somewhat less common around the perimeter of a counterflow tower between the fan deck and the outside casing. Access hatches must be kept closed.

Often, when the fan deck is repaired, boards will be left extending into the fan stack area. These must always be cut off flush with the inside bottom flange of the fan stack to avoid obstructing the airflow. Pultruded glass-reinforced polyester panels should be used for any new or replaced fan deck.

On counterflow towers, a ladder should extend from the fan deck through the drift eliminators to the top of the fill. This will facilitate periodic checking of the distribution.

Fans

The fan blades must be pitched evenly. The degree of pitch must be high enough so that the motor draws a full current load or very near full load during summer operation. The blades may be left at this pitch all year.

Even though the colder (heavier) air of winter will increase the load on the fan, two compensating factors offset this. First, the air going through the fan is the hot air coming off the hot water, and its temperature does not vary nearly as much as the ambient air temperature. Second, the motor is being cooled by outside ambient (cold) air, which will prevent the motor from overheating during winter operation, even if it is drawing somewhat more than the rated amperage.

A very common mistake is to leave the center air disk or air seal off the fan. This allows easier access to the fan blade attachment area for mechanics. However, it also allows the air near the center to flow backwards or down through the fan. This can be a significant loss (up to 15%), which is reflected directly as a 15% loss in total performance.

The fans must be kept running at full speed for maximum performance. Mechanical equipment must be kept in good order, and vibration kept below the level that will trigger the vibration switch. The vibration switch should be set to allow starting torque and sensitive enough to prevent excessive vibration, but not so sensitive that it causes unnecessary shutdowns. The vibration switch should be mounted on the fan motor or on the motor end of the mechanical equipment support steel.

Fan blades must be kept clean and free of algae and scale, with smooth leading edges.

Mechanical equipment support

The fan stack must be sealed tight against the support steel. Torque tubes must be sealed on one end, preferably the outboard end, to prevent major air loss. Plastic caps on the outboard end are normal, but often are missing. A steel plate welded on the inboard end keeps saturated air out of the tubes.

Crossflow short wall

Any air leak into the tower reduces performance. The closer the leak is to the fan, the more air will pass through a given size leak, due to increasing negative pressure nearer the fan.

Missing boards or panels and holes in the crossflow short wall, i.e., the wall between the fan deck and the distribution deck, will significantly reduce the airflow through the fill. Seals are often left out between fan deck joists when new mechanical equipment supports

are installed, which causes major leaks. Access panels through this wall to the plenum area must be kept closed.

Drift eliminators

Any pluggage in the drift eliminators (Figure 5) can cause uneven or reduced airflow. This pluggage can be caused by algae, slime, moss and mud accumulation, which should be controlled by chlorine or other algicide and dispersants. It can be caused by scale, which can be controlled by proper pH, blowdown and dispersants. Objects on the drift eliminators such as scaffold boards, work platforms, collapsed partition walls, walkways, etc. can also restrict air flow.

Missing blades or sections of drift eliminators not only cause increased drift, but also induce uneven airflow patterns and loss of performance.

In counterflow towers, an access panel in the drift eliminators must be provided to allow inspection and work on the distribution system.

Be certain the seals at the top of the drift eliminators in crossflow towers are intact, to prevent air bypass.

Proper chemical treatment is the best way to keep drift eliminators clean.

Partitions

Wind checks in the bottom of counterflow towers are important, particularly in towers with open end walls. These partitions reduce the amount of water blown out of the bottom of the tower by wind (called windage) and help direct the airflow properly. Keep them intact by renailing when necessary.

Partitions between cells do not play an active role in performance as long as all cells are in operation. However, maintenance and load conditions often call for one or more fans to be out of service. Complete intact partitions with doors closed (on crossflow towers) are important to prevent active fans from pulling air from the plenum of adjacent inoperative cells, thereby bypassing the fill.

Distribution systems

All cooling tower calculations and rating systems must presume the air and water to be evenly and thoroughly mixed. However, in the real world, they very seldom are. Improper distribution is the most significant factor affecting the thermal performance of a cooling tower, and will be discussed in detail in a companion article next month.

JAMES L. WILLA

WILLA, INC.

JAMES L. WILLA has over 50 years of experience in the cooling tower industry. He has been a consultant for 18 years and is president of Willa, Inc. (18 Fortune Lane, St. Louis, MO 63122; Phone: (314) 965-5051; E-mail: jwilla@willainc.com), a firm engaged in cooling tower testing, inspecting, seminars, specification review, bid evaluations, and project engineering. He was vice president and president of a major cooling tower manufacturing company for 23 years. He worked extensively with the Cooling Technology Institute - he performed all CTI's testing and inspecting for 13 years, chaired the CTI Engineering Standards and Maintenance Committee and the Operating Seminar, and conducted research for CTI bulletins on wood maintenance, recirculation, performance testing, lumber standards, treating standards, certification program, and the performance curves "Blue Book." He served twice as president of CTI and several times as a board member, and has been a member of several technical organizations. He has written and published numerous papers, given many seminars, served on committees at CTI, ASME, American Wood-Preservers Association (AWPA), and the National Association of Corrosion Engineers (NACE), where he is listed as a corrosion specialist. He graduated from Rice Univ. with a BA and BS in chemical engineering.

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