COOLING TECHNOLOGY INSTITUTE

Transcription

1 PAPER NO: TP09-02 CATEGORY: REPAIR AND CONSTRUCTION COOLING TECHNOLOGY INSTITUTE COMMON INDUSTRIAL COOLING TOWER ERRORS AND OMISSIONS JAMES L. WILLA WILLA, INC. The studies and conclusions reported in this paper are the results of the author s own work. CTI has not investigated, and CTI expressly disclaims any duty to investigate, any product, service process, procedure, design, or the like that may be described herein. The appearance of any technical data, editorial material, or advertisement in this publication does not constitute endorsement, warranty, or guarantee by CTI of any product, service process, procedure, design, or the like. CTI does not warranty that the information in this publication is free of errors, and CTI does not necessarily agree with any statement or opinion in this publication. The user assumes the entire risk of the use of any information in this publication. Copyright All rights reserved. This paper has been reviewed by members of the Cooling Technology Institute and approved as a valuable contribution to cooling tower literature; and presented by the author at the Annual Conference of CTI. Presented at the 2009 Cooling Technology Institute Annual Conference San Antonio, TX - February 8-12,

2 COMMON INDUSTRIAL COOLING TOWER ERRORS AND OMISSIONS I. Purchasing a Cooling Tower Design By James L. Willa Determine the total heat load first, then the required water flow. This will establish the range (hot water temperature cold water temperature). Then pick a reasonable cold water temperature required. Next consult the weather data and choose the 5% wet bulb for your area. That is the wet bulb that will not be exceeded more than 5% of the summer daylight hours (150 hours per year). It is unwise to specify a lower cold water temperature or a higher wet bulb than is actually needed. The bidders are well aware of the actual wet bulb. Closing the approach, which is either lowering the cold water temperature or raising the wet bulb temperature, increases the size and price of the cooling tower more than a one to one ratio. Raising the GPM requires increasing the size and price by a one to one ratio. Increasing the range (hot water temperature cold water temperature) increases the size and price by less than a one to one ratio. Therefore, if you want a straight forward safety factor in your design, take it in GPM, such as 110% GPM, which is a 10% safety factor. Specification Send out a complete specification with your inquiry for a cooling tower. Do not depend on the manufacturer to provide their own specification. When the bids are returned, ascertain that everything in your specifications is included. If not, contact the bidder who many times will confirm inclusion of the missing item(s), without a change in price. Bid Evaluation The lowest price may not always be the best. For example, does the low price include all items in the specifications? Check the drawings that you receive from the manufacturer carefully, as once you have approved these drawings they become the official document of what you will receive, not the specification nor the proposal. Be certain that freight included to plant means your plant, not manufacturers. Be suspicious of any fan efficiency above 75%. 2

3 Materials Always specify 304 stainless steel hardware. No galvanized allowed, except for mechanical equipment support steel. Straps can be stainless steel or fiberglass reinforced polyester plastic. Redwood has always been the construction material of choice. As the price of redwood continued to rise, the story was put out that proper grades were no longer available, and therefore, fir should be used. The redwood in proper grades has been and still is available, but at a higher cost than fir. This controversy has continued for three decades. However, the final answer has arrived, since the price of redwood has exceeded the price of pultruded fiberglass reinforced polyester plastic. This material is not subject to the many forms of wood deterioration, such as rot, surface attack, delignification, etc. While at the same time it is stronger than any lumber. Therefore, all new cooling towers and repairs should be done with all pultruded fiberglass reinforced polyester plastic, including framework, fan deck and distribution deck, so no lumber or plywood is utilized. Variable Frequency Drives For close temperature control on one and two cell towers, this equipment may be useful. However, a three cell tower with two speed fan motors has seven control steps, which is adequate for any situation. As the number of cells increases, the available steps increase. Such as a ten cell tower with two speed motors has 21 control steps. Further, a portion of the electricity savings is lost by the reduction in motor efficiency, as the speed is decreased. Also, sympathetic vibration frequency in the individual structure must be determined, and the controls blocked out at these speeds. The best way for proper electricity savings is to turn a motor to half speed or off, rather than slowing all of them down. II. Cooling Tower Construction Fan Deck Fan deck joints should be over joists, and should be staggered. If lumber is used, all field cuts and holes must be treated with a wood preservative solution. Fan deck must be cut clean at fan openings, no overhang into fan circle. Distribution Deck Be certain predrilled holes are in the right location, not over framework below. 3

4 Columns Do not over tighten bolts in FRP columns. In wood structure crossflow towers, install 4 x 6 columns under main hot water header until its diameter reduces to 30 or less. Install longitudinal diagonals in every transverse bay on both ends, and at least one W shape in center of tower. Without sufficient longitudinal diagonals, a single cell collapse turns into a complete tower loss. Fill All fill material should be PVC or other plastic. Use splash fill for crossflow towers, and for contaminated water conditions such as product leaks at refineries, slime, algae, scale and mud. Splash fill should be polygrid or equal. Use cross fluted film pack fill in counterflow towers with clean water conditions. PVC material should be 20 mil before forming or 15 mil after forming, not 10 mil. Adequate supports for film fill should be spaced a maximum of 3 apart. Supports should be strong enough to withstand the weight, if the bottom one foot of film pack fill freezes solid with ice. Crossflow fill hangers should be attached to framework every 6 vertical feet, to prevent overloading the hangers. Hangers of stainless steel or PVC coated steel should be used, not FRP or plastic hangers. The rough edges on polyester fiberglass hangers have a tendency to cut through the fill slats. Crossflow fill slats should be parallel to airflow. There should be a maximum of 4 fill slat overhang from outboard fill hanger in crossflow towers. Attach fill slats to hangers with plastic clips, stainless steel hog rings or other attachment devices, in crossflow towers. Drift Eliminators Drift eliminators must be PVC and properly supported. Counterflow supports should be on 3 maximum spans, not 6, in order to prevent sagging and falling out of position. Do not use distribution laterals for drift eliminator supports in counterflow cooling towers. In crossflow towers, the horizontal plywood seals at top of the drift eliminators should have a 2 diameter hole in each 4 section, to drain leakage from back wall of distribution basin. 4

5 Fan Stacks Fan stacks must be fiberglass reinforced polyester plastic (FRP). Stacks should be velocity recovery, to save approximately 5% of horsepower and reduce recirculation. Use LockTite nuts with nylon inserts, or double nuts. Many fan wrecks are caused from loose and/or missing fan stack bolts, particularly in access panels. Stacks should have 2 diameter sight holes at eye level, 90 from motor, to observe fan and drive shaft operation. To prevent air loss, drive shaft holes should be a maximum of 2 larger in diameter than the drive shaft coupling. Also, there should be tight holes not slots for mechanical equipment support steel and oil lines. The fan stack must have ½ holes drilled in the bottom of each vertical rib for water drainage, to prevent freeze damage. Top flange joints must be bolted with heads up and nuts down, so that if a nut is lost, the bolt will still remain in place. Do not allow cables inside top of fan stacks. Vibration breaks them and the loose ends can damage fan blades, causing a fan wreck. Remove cables now, if in fan stacks. Hold down bolts in bottom of flange must go through fan deck joists or framework between joists. Fans Fans should be fiberglass reinforced polyester, epoxy, or vinyl ester plastic (FRP), with a minimum of six blades (even number, not odd number), and a maximum of 22 horsepower/blade on 28 diameter fans. All fan hardware must be stainless steel. Gearboxes The service factor must be 2.0, in accordance with the CTI standard. Gearboxes should be coated with epoxy, and have a stainless steel oil line with flexible section at gearbox end, made of rubber or woven stainless steel. There must be an external gage with two permanent marks for proper oil levels for in and out of operation. There should be a vent line of ⅜ to ½ stainless steel or copper tubing from top of gearbox through fan stack, with outside open end pointed down. If not pointed down, it can take in rainwater which is sucked back into the gearbox, emulsifying the oil and blowing the gearbox. 5

6 Motors Motors should be two speed, TEFC, with a service factor of Drive Shafts Drive shafts should be carbon composite with stainless steel couplings and shim packs. Carbon composite shim packs are acceptable up to 100 horsepower. Above 100 horsepower, shim packs should be stainless steel. Access There must be stairs at one end of the tower, and a ladder with cage at the other end. Most stairs need X bracing on handrails and kneerails at landings, otherwise they become loose and unsafe. On crossflow towers, there must be a FRP ladder from fan deck to distribution deck on both sides of each cell. In longer crossflow towers, one ladder at each end on both sides with a walkway between, in the distribution area, is acceptable. In counterflow towers, there must be a FRP ladder in each cell from fan deck to drift eliminators, and a 3 x 6 platform with access through drift eliminators to distribution. The ladder should extend from distribution level to 3 above fan deck. Conduit Galvanized or cadmium plated conduit is not an acceptable material. Conduit should be aluminum or polyvinyl chloride (PVC). Support clamps should be stainless steel. Corroded and broken steel cuts into insulation and electrifies all of the tower conduit. Lightning Rods Lightning rods must have their own braided copper ground wire all the way down to a stake in soil at grade. This system should not be connected to the fan motor ground system, as this can subject the motor to lightning voltages. Lightning rods around top perimeter of fan stacks must be secured by double nuts or equivalent. Lightning ground wires from the top of the fan stacks must be firmly secured to the side of the fan stack. Sprinklers All sprinkler hangers and bolts must be stainless steel, and pipe joints coated with coal tar epoxy or equal. 6

7 Testing All new cooling towers should be tested by a qualified third party. An Owner consistently not performing an official acceptance test on new cooling towers will become known in the industry. This can result in getting smaller performing towers. Be alert to pulled curves (cold water versus wet bulb) that show unjustifiable higher cold water temperatures at lower wet bulb temperatures. These curves can give an optimistic performance result, particularly when testing appreciably below design wet bulb. Many cooling towers are not tested because they are successfully carrying the load. The Owner says the tower was supposed to cool the water 20 F and it is cooling the water 22 F, so everything is fine. Q Btu/hr = 500 x GPM x Range. The cooling tower is neither a heat source nor a heat sink. It is independent of heat load (Q). 500 is a numerical constant (pounds/gallon x minutes/hour), and therefore, independent of the cooling tower. Water flow (GPM) is determined by the operating pumps and the discharge pressure, which are independent of the cooling tower. Therefore, Range (hot water temperature cold water temperature) is independent of the cooling tower. The Owner usually does not have the instrumentation to measure wet bulb temperature, and therefore does not have an accurate approach (cold water temperature wet bulb temperature), which is the only measurement that is dependent on the cooling tower. III. Operating a Cooling Tower Distribution Poor water distribution accounts for more lost thermal performance than any other operating problem. Counterflow nozzles must be kept open and flowing. Usually the last one or two laterals on the far side from the risers are most susceptible to plugging. This not only reduces thermal performance, but also causes ice damage in this area due to reduced water flow. Sensitive pressure readings must be taken in each riser in order to properly adjust valves in the risers near grade, to maintain even water flow to each cell. One method is to place a short copper tube through a rubber stopper. Insert this stopper in first nozzle off first lateral in each cell. Place a Tygon tube from the copper tube through fan deck access hatch and attach to top handrail in each cell. Be certain all nozzles are clean. Now adjust valves on risers until water level is the same in all Tygon tubes. Counterflow mains should have a 2 PVC line from the bottom at the end of the header, on the far end from the riser. The line should go down through the tower with a PVC valve at louver height, within reach of an operator at grade. The valve should be opened for 5 to 10 minutes every month, to flush debris out of the main. This will prevent an accumulation of debris flowing into the laterals, and plugging the nozzles. 7

8 Crossflow nozzles must be kept clean, and water depth in distribution basins kept within + ½ of average depth. Basin depth should be between 4 to 6. There are nozzle rows behind the short vertical wall between fan deck and distribution deck in some towers. They are difficult to access, but are among the first to plug. They can be accessed by going through the access panel in the short wall. To help keep nozzles clean, install a fence around the flow control boxes. Fasten small FRP angle strips to the distribution deck around all four sides of the flow control boxes (being careful not to cover any nozzles), 6 out from the perimeter of the boxes. Attach ½ mesh S.S. wire 10 tall (vertically) to the inside edge of these angles, to form a fence around the boxes. This will greatly reduce nozzle pluggage by trapping all of the larger pieces of rust, scale and debris within the fence. This debris can then be cleaned out by hand by kneeling down on the flow control boxes, rather than having to walk in the water on the distribution deck. For safety, there must be handrails and kneerails on the outside edge of a crossflow distribution deck. If the bell and spigot pipe joints on crossflow towers start to leak, the joints must be mechanically held together. Stop any leaks in the distribution deck back wall or splash over the top of this wall in crossflow towers, to prevent this water from becoming drift loss. Performance The More Water Syndrome : Often when difficulty is encountered in maintaining cooling in say an overhead condenser on the Cat Cracker, the call goes out for more cooling water. This results in the cooling tower returning higher cold water temperature, causing increased trouble in the overhead condenser. Remember, condensing pressure is set by the cold water temperature. Reduce water flow to obtain a lower cold water temperature. Close drain lines from counterflow tower risers to basin, during summer operation. Also, close any other bypass lines from hot water to cold water basin, during summer operation. A missing FRP air seal over the fan hub can lose 15% of airflow. Air seal mount bolts must be kept tight. Fan tip clearance should not exceed 1 to 1½. Keep drain holes open in fan tips, by using a bent welding rod, or similar tool. Short Wall (Fan Deck to Distribution Deck on Crossflow Towers) Keep access panels closed, to avoid major air loss. There must be seals between fan deck joists at top of this wall, to prevent air loss. 8

9 Mechanical Equipment Support Many supports are 16 diameter steel pipe torque tubes sealed at the end. An open end is a major air leak. Work Grids Do not cover all of fan opening with fiberglass reinforced polyester plastic (FRP) grids, for mechanical work. Limit grid to area immediately around gearbox. Install 2 x 2 stainless steel angles, 2 apart, on fan deck framing, from fan stack access panel to gearbox area. When working in the gearbox area, slide 2 wide sections of FRP grid on these angles for a walkway, and remove these grids when work is complete. No other permanent platforms or walkways should be built closer than 5 to 6 vertical feet below fans. These can result in excessive vibration and fan blade failure. Drive Shafts A loose drive shaft coupling cover on the motor end is the number one source of cooling tower fires. Mount bolts must be kept tight, as loose or missing bolts will allow the cover to rub against the moving shaft. There must be two drive shaft guards per shaft, one near motor end, and one near gearbox end. Some towers have only one guard in center of shaft. These guards should be stainless steel. Fasteners must be kept tight. Fan Deck Fire extinguishers must be mounted on handrails at both ends of fan deck, plus one in middle on long towers. Any scaffold boards must be stacked on aluminum conduit, or other non-corroding, nonrotting material. Cold Water Basin The basin should be sloped towards the sump end. Winter heat exchangers in air conditioning tower basins, to prevent freezing in winter, must be turned off after freezing weather is finished. 9

10 Cold Water Basin Screens The lead screen must have a screen lined cup, 6 wide x 6 deep, on the bottom front side. Having no cup means debris goes under screens, through pumps and into condensers, when the screens are lifted for cleaning. There must be a chain hoist mounted on a steel I beam to lift screens. The I beam must extend past edge of sump so screens can be cleaned over open ground, or over concrete pad with drain. Screens should be ½ mesh stainless steel. Better still are screens that are continuously rotated and washed clean into a trough with high pressure water. Side Stream Filters Do not use cartridge filters. Filter cold water from pumps, and return to basin by gravity flow. For example, a large chemical plant tower had a side stream filter on the hot water. The filtered hot water returned directly to the cold water basin causing a 0.5 F increase in cold water temperature. This plant had just spent $500,000 on new fill, to achieve 0.5 F lower cold water temperature. Use open sand and gravel filter. Utilize blow down water for backwash. Acid Addition Do not turn acid loose into basin. Specific gravity differences sends acid stream directly to floor and eats out concrete on floor and/or curb. The best approach is to have a lead lined trough at least 6 long with baffles, supported under the fill above the water level. Use the falling cold water for the diluent, not hot water. Makeup Do not add heat in makeup water such as boiler blow down, or condenser water from cooling tower steam pumps. Blow Down The product of a cooling tower is cold water. Do not throw product away, blow down hot water, not cold water. 10

11 Winter Operation After all of the available advice, each winter results in some fill loss, and often complete tower loss. Crossflow Winter Operation At the beginning of the winter season, remove the outside two rows of distribution nozzles (next to air inlet louvers), and plug the inside two rows with stoppers or covers. If water level drops below 4, plug an additional back row of nozzles. The water must be left on all cells, as the valves at the flow control boxes will leak and form ice if they are closed. Maintain full water flow. An operator must walk around the cooling tower every few hours during freezing weather, to determine when ice starts to form in the air inlet louvers. When ice starts to form, start turning the fans back to half speed. If ice continues to form, then turn fans off. When turning fans to half speed or off, be sure to start with cell 1 fan etc. the first time. Start with cell 2 fan etc. the next time, so that all fans get the same number of slow downs or start-stops. Do not operate fans in reverse. Be certain to return the distribution nozzles back into original condition at the end of the winter season. Counterflow Winter Operation The water must be left on all cells. Maintain full water flow. If a riser valve is closed, it must have a drain line above the valve to put leakage back into the basin. An operator must walk around the cooling tower every few hours during freezing weather, to determine when ice starts to form in the air inlets. When ice starts to form, start turning the fans back to half speed. If ice continues to form, then turn fans off. When turning fans to half speed or off, be sure to start with cell 1 fan etc. the first time. Start with cell 2 fan etc. the next time, so that all fans get the same number of slow downs or start-stops. Do not operate fans in reverse. An example of an icing problem was a power plant unit off line, with no heat load on the tower. The pumps continued to put water over the tower with fans on for ten hours, in below freezing weather. All fill and a portion of the framework was lost in a ten cell tower from ice accumulation. 11

12 Winter Fan Operation (Do not run fans in reverse for deicing) Reasons not to run fans in reverse: 1. Saturated exit air is pulled through fan from adjacent operating cells, forming icicles on the trailing edges of the fan blades. These icicles can break off knocking holes in the fan stacks, and/or throw the fan out of balance, causing excessive vibration and a possible fan wreck. 2. When the gearbox is operated in reverse, the upper (output) bearing does not receive sufficient oil. Therefore, any extended (over 15 minutes) reverse operation can damage the output bearing. The only solution for this problem is an external oil pump, which usually has not been included in the specification. 3. When the gearbox is operated in reverse, the tooth contact between the pinion gear and the bull gear is on the back side of the teeth, not on the face hardened front side. 4. When the gearbox is operated in a forward operation, it has the tendency to push the input bearing and oil seal out. In reverse operation, it has the tendency to force the input bearing and oil seal in. The input shaft on the gearbox has two bearings. The inside bearing is larger than the outside bearing, as it takes the load between the pinion gear and the bull gear. In reverse operation, the smaller outer bearing takes most of the load, which shortens its life. 5. In reverse operation, cold air is forced through the drift eliminators and the top of the fill. In a crossflow cooling tower, this can force cold air through the lower drift eliminators and thus through the lower cold water area. Therefore, any icing in the fill will occur behind or under the drift eliminators, where it cannot be observed. 12

13 Vibration Vibration monitors are not enough, as they do not stop a fan in failing mode. Fans must have a vibration cut off switch mounted on the mechanical equipment support steel on the motor end near motor feet. A sensor mounted on the end of an oil line is not acceptable, because it senses vibrations in the oil line. A sensor mounted on the gearbox eventually shorts out, making it inoperative. Proper adjustment is to tighten down the sensitivity setting until it trips on starting torque, then back off setting counterclockwise one-half revolution. Vibration switches must have a local reset, not remote, so that the operator must be on top of the cooling tower to observe conditions, prior to reset. Sources of excessive vibration at fan speed frequency: 1. Uneven pitch between blades. Pitch on all the blades should be within + ¼ of average. 2. All blades not running in the same horizontal plane. For example, 28 diameter blades should tract the same horizontal plane within a tolerance of 1 from high to low blade. 3. Water in blades. Check that holes on ends of blades are open. They can be cleaned with a bent welding rod or similar tool. Excessive vibration at motor speed frequency: 1. Improper drive shaft alignment. 2. Drive shaft not straight. 3. Bad motor output bearing. 4. Bad gearbox input bearing. Excessive axial vibration: 1. Improper end clearance on drive shaft between gearbox and motor. References CTI How To Improve The Thermal Performance of Cooling Towers (J.L. Willa) CTI How To Make CTI Standards Work For You (J.L. Willa) CTI Cooling Tower Bid Evaluation (J.L. Willa) CTI How To Repair a Cooling Tower (J.L. Willa) CTI Proper Cooling Tower Operation Makes Money (J.L. Willa) CTI How To Inspect Industrial Water Cooling Towers (J.L. Willa) CTI Avoiding Cooling Tower Catastrophe (J.L. Willa) Willa Winter Operation of Crossflow Cooling Towers Willa Winter Operation of Counterflow Cooling Towers Willa Winter Fan Operation 13