CHAPTER 5 | Cooling Tower Piping
A well designed piping system is essential for the proper operation of a cooling tower. Two types of systems are examined -- open and closed. In the open cooling tower, system water is open to the atmosphere as it cascades over the wet deck and falls into the cooling tower basin. The closed tower --sometimes called a "closed circuit fluid cooler"-- has the system water contained within a coil and segregated from the water that is evaporating. This is covered near the end of this chapter.
In the open tower, all of the water in the wetdeck and water distribution system falls into the cooling tower basin when the system pump is turned off. It is the job of the system designer to minimize the drain-back of any additional water from the system by first locating the cooling tower as the highest element in the system. The tower should be elevated until all other system components are below the overflow level of the cooling tower except for any vertical risers to the tower inlet(s). Risers should be strictly vertical and turn into the cooling tower inlet(s) with a minimum horizontal run.
At pump shutdown, such a system will drain back only the water in the vertical riser above the overflow level and even this can be eliminated with the addition of a check valve at the discharge side of the pump.
Sometimes it is helpful to examine an absurd situation to better understand potential problems... for example, consider a cooling tower located at ground level with all the other system components installed above. The piping elbows vertically up from the tower to complete the piping connections. Such a system will quickly display two nasty problems:
-The sump will run out of water before it can fill all of the empty piping at start-up and, while the make-up valve may eventually add enough water for the system to operate, the pump may become air-bound -- an annoying problem.
-More than annoying, though, is the reverse situation. When the pump is shut off, excess water will cascade into the cooling tower basin, exceed its available volume, and run out all over the place.
When designing a system that requires any compromise to the ideal, the designer must calculate the amount of water the basin must accept at pump shutdown and determine that the sump has enough space available between its normal operating level and overflow level to accept this extra volume. The tower manufacturer can advise the available volume for each of their cooling towers.
Installed systems facing this problem can be helped by reducing the operating level in the cooling tower basin. The solution may be as simple as lowering the float or it may involve improving the anti-vortex device at the basin outlet to allow the float to be lowered even more.
Increasing the sump volume by equalizing to an extra vessel or by adding a "pocket" to the cooling tower side wall can do the trick but don't forget that the only effective volume is that which is added between the operating and overflow levels -- typically a space less than 5" tall.
Another approach is to tee off the suction line to an open top tank the top of which is at or slightly above the overflow level of the cooling tower. An up stream flow control valve is modulated to allow the tank to draw down during operation effectively adding its volume to the system at startup and accepting excess water at shutdown. This approach has problems, however, for systems with multiple pumps providing different flow rates because each flow requires a different flow control valve setting.
Digressing a moment, service technicians should take note of this "pulldown" phenomenon when setting the operating level of any cooling tower. Often the correct level is set only to find that the sump level dips too low at pump start-up. The technician sets the float higher and higher until the system starts without difficulty. The problem here is that when the float was initially set, the technician should have then held the float down filling the sump all the way to the overflow level before starting the pump. Upon reflection, filling the sump to a level that exceeds the operating level mimics the actual case of a system starting up after a shutdown. Likewise, the float should be held down and the basin filled completely after draining the tower for cleaning. For systems with electronic water level controls, overfill the basin with a garden hose before the initial start up.
In freezing climates, it is common to see the cooling tower arranged for "remote sump" operation. (See Ch.3). It is essential that the sump be sized with adequate volume between operating and overflow levels to accept all of the system water in suspension.
Don't utilize redundant remote sump drains, header them together, or make them exceptionally large. Multicell towers typically have air separation baffles that terminate several inches above the 'floor' and rely on the water level to form a seal between cells. A grossly oversize drain inhibits formation of a water level sufficient to form this seal. Oversized drains can also allow a considerable amount of air to escape passage over the wet deck. Blow thru towers allow air to escape out the drain while draw thru towers can pull air through the drain pipe. Drain lines can be 'trapped' before entering the remote sump tank to prevent unwanted air flow. Make sure any traps are freeze protected.
Another special system is the hot/cold well arrangement. It works good for industrial applications where the system water flow rate is variable or where increased water volume is beneficial to satisfy intense cyclical loads.(See Ch. 15 for more details.)
Piping between the cooling tower basin and the pump is most critical. Factory strainers are typically provided inside the cooling tower; However, debris can often bypass the factory strainers. Nuts, bolts, washers, chunks of scale, etc. can easily pass around many strainer designs. This is particularly true when strainers become old or damaged; It isn't uncommon for strainers themselves to go downstream. For these reasons, a large, low restriction, in-line strainer should be installed in the suction piping- Typically, just before the pump. Suction lines should be short in length. The velocity should be between 7-10 feet per second.
Cooling tower water is highly aerated. Pumps can usually accommodate this air as long as the bubbles remain small and evenly dispersed. Extraordinarily low suction line velocities should be avoided. This is especially prevalent when only one of several pumps operates or when a suction line is sized for anticipated future cooling tower cells. Low velocities allow small air bubbles to combine into larger bubbles that can migrate into a pump en masse and cause problems when ingested. Often, the problem occurs when a second pump is started and the rapid velocity increase causes a pump to suck in an air bubble that formed in the piping.
To reduce the formation of big air bubbles, eccentric fittings should be used in the suction line with the top of the pipe remaining "flat". The pipe can also be pitched slightly up toward the pump to encourage small bubbles to be steadily metered into the pump.
Avoid vertical risers into multiple pumps off a common header for pumps that cycle. The riser to an idle pump will slowly fill with air as its neighbor operates.
Discharge piping is less critical but like all piping should be installed as symmetrically as possible on multi-cell installations to provide a naturally balanced flow throughout the system.
Cooling towers with gravity flow hot water basins that accept water directly from elevated piping must have flow control valves to allow field adjustment. Even then, flow balance is typically diffcult. It is better to have the main discharge header below the tower with risers to the individual inlets. Systems with spray nozzles, on the other hand, display a natural tendency to balance and don't require flow control valves unless the towers are woefully asymmetrical or of different style or height.
When headering multiple cooling towers together, the system designer must connect the basins together with an equalizer line. This line is necessary no matter how symmetrical the piping. Even if the inlet and outlet flows to each tower are perfectly matched, it can be demonstrated that they will soon become imbalanced if one nozzle collects some debris or something like a plastic sandwich wrapper gets caught in a strainer. One basin would begin to overflow while the other became dry. A generously sized equalizer line will combat any imbalances and essentially force the sump levels to be nearly equal. It is common to assume for design purposes that an equalizer will flow 15% of the water to the largest cell , that a difference in level of 1/2" is acceptable, and that the equalizer line pressure drop is equivalent to 50' of straight pipe. The resultant equalizer sizes are tabulated here. This is one manufacturer's sizing criteria and it has been demonstrated to work over many years.
|Flow to each tower (GPM)
Equalizer size (in.)
|UP TO 122
Since the equalizer line will equalize the water levels, it is critical that the design operating level of each tower be at the same elevation. It may be necessary to locate the base of one cooling tower slightly higher than its neighbor when they are of different styles or made by different manufacturers.
The operating levels must be ascertained from the manufacturer(s). Maintenance manuals should give this information. Do not assume operating levels based on apparent overflow levels.
It is not appropriate to expect a suction line to perform as an equalizer- even if it is oversized.
When mixing blow-thru and draw-thru towers, locate the operating levels of blow-thru towers about 1in. below the operating levels of draw-thru towers to help account for the affect of the blow-thru tower’s fan on water level. This figure helps optimize the tower elevations. Fan cycling, two speed motors and VFD’s on blow-thru towers prevent selection of a ‘perfect’ elevation difference. When possible, run the blow-thru towers full speed and limit fan speed control to the draw-thru towers.
Equalizer lines that exit the cold water sump from the bottom tend to collect debris and become restricted over time. It is, therefore, a good idea to provide removable flanges at the ends of the horizontal pipe run for clean out.
The elevation and inlet pressure requirements for multiple water inlets should also be considered when mixing cooling towers. Open gravity hot water basins should always be very close in elevation. Otherwise, the flow will favor the lowest basins. And, while flow control valves can correct for small height differences, such valve settings will be good for a particular flow only and lower basins will again be favored under reduced flow conditions.
Multiple gravity flow tower inlets at different elevations can be made to balance properly by pumping the water to a point above the highest inlet and letting the water ‘fall’- not siphon- into the individual inlets. Siphoning can be prevented by venting the top of the inlet to the atmosphere.
Even when multiple gravity flow inlets are at the same level, equal flow distribution is hard to achieve. Keep the main supply header below the operating level of the tower’s cold water basin and branch off to the individual inlets. This way, the branch flow originates where there is considerable back pressure causing more even flow distribution.
Similarly, spray tree water distribution systems should be close to the same level and fitted with nozzles requiring the same spray pressure.
The worst case would occur when mixing crossflow (draw-thru, gravity water distribution) towers with counterflow (blow-thru, spray tree) towers. The designer must be extremely careful. In general, avoid mixing tower types whenever possible.
Turn-down is another consideration. This is the difference in maximum and minimum flow rate that any cooling tower can be expected to see. It is common for cooling towers to accommodate reduced flow rates during low load situations.
Cooling towers like to see the flow for which their water distribution systems were designed. They are typically forgiving for small increases or decreases in flow but the designer should always make sure that his maximum and minimum flows can be accommodated. It would not be correct to look in a manufacturer's brochure and assume that it is satisfactory to operate a cooling tower at any flow between the maximum and minimum figures indicated. What isn't apparent is that there may be as many as six different nozzle combinations required for the entire flow regime indicated. The astute designer will tell the manufacturer the maximum and minimum flow he expects for each tower and confirm that the application is appropriate. Manufacturer's can juggle nozzle sizes and types in order to optimize the water distribution system for the flow requirements of specific projects.
When the inlet flow rate to a cooling tower becomes too low, it is appropriate to shut off selected cells in order to force more water into others. This keeps the wetdeck drenched thereby preventing scale and ice formation. It also prevents a sharp rise in CFM due to the reduced restriction offered by reduced water flow reducing any tendency to "spit" -- fling water droplets out of the cooling tower. Finally, it will prevent possible fan motor overloading on centrifugal fan cooling towers.
Don't forget that when one water inlet is closed, the corresponding water outlet must also be closed. Otherwise, a major imbalance will occur between cells that even a generously sized equalizer cannot overcome. (Remember, equalizers are typically sized for 15% of the flow to one cell -- not 100%)
Bypass is an outmoded method of capacity control that isn't often seen. This is the practice of diverting a portion of the water from the tower inlet into the tower basin as a method of capacity control. Regulating airflow is the best way to control capacity, not water flow. Bypasses shouldn't be used because of their turn down limitations and inability to conserve fan horsepower. They are, however, acceptable for short lived situations for systems employing centrifugal chillers and the like to prevent unnecessarily cold water at start-up.
Don't skimp on isolation valves- particularly on projects with multiple cells where the system must run 24/7. The inability to fully isolate a cell can be a real problem when it is necessary to perform repairs- such as applying coatings- and water continues to seep into the tower from the inlet, outlet or equalizer.
Make sure there is enough city water pressure at the cooling tower to satisfy make-up and reasonable quick-fill requirements. 15 psi is the typical minimum.
Also, make sure the pressure isn't too high. If it exceeds approximately 50 psi, a pressure reducing valve should be installed. Too much pressure can pin a mechanical make-up valve open and can make solenoid make-up valves prone to water hammer.
Water hammer in the make-up line is difficult to predict. When it occurs, it can be addressed several ways:
-Add a globe valve or fixed orifice at the discharge end of the make-up valve to provide some back pressure.
-Add a vertical air column in the make-up line to act as an air cushion.
-Reroute the make-up line changing its length so as to cause a change in its natural frequency. Orienting this line in X, Y, and Z directions makes it more shock absorbent.
-Install a commercially available shock arrester.Or finally,
-Use a Cla or Griswold type pilot operated make-up valve; they are naturally slow closing and therefore less likely to cause water hammer.
Sizing the make-up line is relatively easy. Calculate the evaporation rate, double it as a generous allowance for bleed and select the line for about 7 feet/second velocity.
A quick-fill connection is a good idea to get the tower back on line quickly and assist in overfilling the sump prior to initial start-up.
Adding a bleed connection capable of handling the evaporation rate is a good idea to facilitate the water treater who will want a place to bleed water from the system. It should be high to keep from draining the system should the bleed valve stick open. To insure enough pressure, place this fitting about 6' below the inlet of a gravity water distribution tower.
Routing the bleed line to a convenient "open sight" drain will benefit the water treater allowing him to measure the bleed rate with a bucket and stopwatch.
When installing a tower, jack any vibration isolators down to the operating height before fitting the pipework. Then, when the tower is filled with water and the jack screws backed off, there will be no undue misalignment of the flex connections. Without flex connections, the rigid piping will "ground out" the isolation and defeat its effectiveness.
All piping connections to the cooling tower basin such as quick-fills, equalizers, filter connections, etc., should be factory ordered so as to maintain the integrity of the corrosion protection system on steel cooling towers. Welding to the basin can burn off protective zinc. When connections must be added in the field, it is best to weld the appropriate fitting to a plate, galvanize it (if the tower is galvanized), and bolt this subassembly to the tower to minimize damage.
Stainless steel basins have stainless steel connection fittings that can be welded to mild steel piping using welding rod 308-16.
CLOSED CIRCUIT FLUID COOLER piping considerations for closed systems are fairly straightforward.
First, select pipe that is large enough so as not to create excessive pressure drop. Typically velocities near 7ft/sec fill this need.
Also important is the need to provide fill and vent fittings in the piping. The vent must be at the highest point in the system.
The fill is typically at the bottom and the fluid pumped into the system to prevent 'burping' during filling.
The piping must be arranged so that it will fill without trapping air. Pitch the piping slightly to ensure that all air will migrate to the top during filling.
Secondary vents should be added at strategic locations where air traps cannot be avoided.
Include a pot feeder for the addition of chemicals. Connect it in parallel with the system pump and isolate it with valves. Opening the valves causes system fluid to circulate through the pot feeder thereby introducing chemicals into the system. Be sure to fill the pot feeder with water before opening the valves to avoid the introduction of air.
An expansion tank is needed to absorb the fluid volumetric changes that will occur due to temperature changes in a closed system. The ASHRAE Systems Handbook and various expansion tank manufacturers give descriptions of various tank types along with sizing methods.
Rather than make the calculation, call any expansion tank rep and be prepared to advise system volume, fluid (water, glycol, etc.), max temp, system height, initial fill temp, elevation, type of piping (copper or steel) and system pressure. They have computer programs that make tank selection easy.
Generally, it is best to use a diaphragm type tank to protect the integrity of the system fluid and because it doesn't have to be installed at the highest point in the system.
Unlike a cooling tower, a fluid cooler does not have to be installed as the highest system component.
For years it has been customary for system water to enter from the top of a closed circuit tower and be cooled as it progresses 'down' through the inside of the coil toward the outlet. Spray water covers the outside of the coil while fans direct air 'up' and around the coil's outer surfaces. The coldest air and spray water are at the bottom of the coil where the air quickly assumes near wet bulb temperature just after entry. This counterflow arrangement is the most efficient allowing the coldest air to extract maximum heat from the system water just prior to exiting.
Some newer closed circuit designs reduce the coil size, add conventional cooling tower wet deck and pump the cold water leaving the wet deck over the top of the coil. For this design, it is necessary to reverse the direction of water through the coil to maintain the most efficient counterflow arrangement.
Manufacturers place large 'inlet' and 'outlet' stickers near their coil connections but just remember piping is no longer a 'no brainer'.
To calculate the required volume of Ethylene Glycol as a percentage of weight or determine freeze/boil points, see the Glossary.