CHAPTER 3 | Components/Materials

Redwood was an early, favorite construction material for cooling towers because of its natural tendency to inhibit decay.

As supplies of good quality redwood diminished, douglas fir was pressed into service. Its superior strength allowed for the use of fewer structural members but it deteriorated more easily. Treatments were developed to kill the decay causing micro organisms that depleted the wood. The lumber was essentially dipped in poisonous chemicals. The process was enhanced by incising and pressure treatment. Incising is a procedure where small longitudinal incisions are made into the wood to encourage the acceptance of chemical treatment.

Pressure treating has the wood placed in a vessel that is first evacuated to remove moisture from the wood followed by the introduction of chemicals under pressure. Simultaneous with the greater reliance on fir was the increased use of factory assembled steel cooling towers in ever increasing sizes. Galvanized steel performed well with the water treatment chemicals of the day- chromates and acid. Acid was used to lower the pH allowing higher cycles of concentration of the dissolved solids (see ch. 12) while chromates quelled the rampant corrosion that would otherwise occur.

Asbestos cement board casing side panels were popular on field erected cooling towers. Asbestos was also found in the wet deck and sealing compounds of many towers. Increasing environmental concerns in the 1970's saw the end of chromates except for a few very large facilities where they were removed from the discharge water at site treatment facilities. Without chromates, the low pH water was very corrosive and many cooling towers and piping systems were ruined in short time. Substitutes have never lived up to chromates for effectiveness and cost. Wood towers didn't escape intense environmental scrutiny. The potential hazards of wood treatment chemicals became more apparent causing revised formulations and tighter controls both leading to increased costs.

Asbestos also came into disfavor and was quickly 'designed out' of cooling towers as the manufacturers became more aware of the potential health and financial hazards. Type 304 stainless steel became more popular as the corrosion potential increased. Manufacturers simply substituted stainless steel for galvanized steel components. Due to cost constraints, just the cold water basin was typically up graded to SST. There were some unfortunate occasions where galvanized and stainless steels were fastened together below the water line causing rapid deterioration of the galvanized steel at the joint from galvanic action. Anyone considering mixing these materials must pay attention to the surrounding materials- particularly the fasteners. Such joints should never occur below the overflow level of the cooling tower.

Specifiers will sometimes call for type 316 SST. Manufacturers had difficulty complying because type 316 was hard on tooling. It is also difficult to form. For these reasons, type 316 has been slow coming on board.

The galvanized steel cooling tower has remained the factory assembled standard. The thickness of the steel has steadily declined with more economical designs but the thickness of the zinc layer has steadily increased to a current standard of G235. (Or, 2.35oz. of zinc per sq. ft.) from a 1970's standard of G90 (.90 oz/sq.ft.). This thickening of the sacrificial zinc layer has a very beneficial effect on cooling tower life.

Various enhancements to the galvanized steel in the form of barriers have been employed by some manufacturers. Their suitability largely depends on the local water quality. Concrete can be an excellent construction material for basins- even side walls, fan decks, discharge stacks, and mechanical support beams. Its use beyond basins, however is not typically justified for commercial applications. Extensive concrete construction is used for architectural reasons- where the tower is disguised to look like or blend in with a building- or, the cooling tower is designed as a structure with a life expectancy equal to the facility it serves such as a hospital or university.

Pultruded fiberglass is increasingly replacing steel in structural applications. These are composites with precisely located glass fibers that make the parts very strong. The process allows the addition of surface treatments that limit ultra violet degradation- an important requirement for cooling tower duty. Typical Components:

Fill- a.k.a. Wet Deck or Surface- is the heart of most cooling towers. Generally, it takes the form of PVC plastic film type surface. Water is made to spread out on this 'surface' maximizing it's contact area with air to encourage evaporation. It consists of individual vacuum formed sheets with proprietary patterns of ridges, bumps and wrinkles. When arranged vertically (side-by-side), the individual sheets space themselves apart leaving passageways for water and air. The sheets can simply press against each other or be glued together. Edges can be folded for increased strength. A 'block' of glued together film type fill can be placed on a table top and observed. If a marble were dropped through the fill it would follow one of the 'channels' that are formed between the adjacent fill sheets. These channels- or flutes- are typically at an angle to vertical to increase the 'hang time' of the water as it falls through the wet deck.

The goal of the film type wet deck designer is to maximize air/water contact while minimizing air flow pressure drop.

Reduced flute sizes increase thermal capacity limited by air flow pressure drop and clogging. Typical 'clean water' applications such as air conditioning can have 'marble' sized flutes. 'Dirty water' applications like tomato vacuum pans or steel mills, on the other hand, may require 'golf ball' or 'base ball' size flutes. Big flute wet deck may have less capacity under ideal conditions but is a whole lot better than clogged, small flute wet deck that doesn't work at all. This is an area where the industrial designer must pay attention. Cooling tower selection programs default to small flute wet deck because it has the highest capacity per cubic foot and less volume = less cost.

Clearly, crossflow and counterflow wet decks are designed differently. Anyone in the repair business with the attitude that 'fill is fill' will have some unpleasant surprises. Tile Fill is a bullet proof approach to wet deck. It is suitable for clean to moderately dirty water and has extraordinary longevity. Heat transfer efficiency is less than that for film type wet deck, however, requiring more volume or more fan horsepower for equivalent capacity.

Splash Bars are another method tailored to extremely dirty water applications. Instead of spreading the water into a thin film, the approach is to have the water splash into droplets as it cascades through the tower splashing off successive splash bars. Clearly, the total surface area of all the water droplets is far less and the thermal capacity is diminished vs film type fill. On the positive side though, considerable debris can be tolerated and cleaning is relatively easy.

The ultimate in dirty water towers is the 'Spray Fill' design. Here, there is no fill at all. Water simply sprays into the empty plenum area of a tower. This design is limited to counterflow type towers.

Eliminators are used to remove water droplets from cooling tower discharge air by imparting several rapid directional changes. The heavier water particles collide against the eliminator and drain back into the tower. Superior eliminator designs limit escaping water droplets- a.k.a. 'Drift'- to .002% -or less- of the recirculated flow rate while imparting minimal pressure drop to the airstream.

The Spray Tree is used to distribute water over the wet deck in counterflow cooling towers. It can consist of a single header fitted with Spray Nozzles or, it can utilize spray branches with nozzles for wider coverage. Spray nozzle designers seek minimal pressure requirements and uniform coverage over wide flow ranges.

Hot Water Basins are used to distribute water in crossflow towers. Here, water is pumped to an open pan over the wet deck fill. The bottom of the pan has holes through which water is distributed. Manufacturers will fit specially shaped plastic drip orifices into the holes to give the water an 'umbrella' shape for more uniform distribution. Different size orifices are used for different flow rates. Ideally, the basin will be almost full at maximum flow- perhaps 6" deep. This way, sufficient depth is retained for good water distribution as 'turn down' occurs. The turn down ratio can be extended by the addition of hot water basin weirs- a pattern of baffles perhaps 2" tall fastened to the basin floor- that insure good water distribution by selected nozzles at reduced flow. As full flow is restored, the water overflows the weirs to again engage all available orifices.

Cold Water Basins collect cooled water at the bottom of the tower. They are an integral part of factory assembled designs and are built in place- typically of concrete- for field erected towers. A Make-up Valve replaces water that exits via evaporation and bleed (See Ch.12) with fresh water. It operates somewhat like the valve found in a conventional toilet tank but is bigger and more heavy duty. Like toilet tank floats, they can function mechanically or hydraulically.

Cold Water Basin Heaters address freeze-up for cooling towers in cold climates. They are electric immersion heaters installed below the water level that add sufficient heat to prevent freeze damage. A 1,000 ton cooling tower may use something like 30KW. This is equivalent to 30 KW x 3,415 Btu/Hr/KW x 1/ (12,000 Btu/ Hr/ Ton) = 8.5 tons. Some designers expect immersion heaters to heat the system water as an aid to cold system start up but sump heaters are clearly inadequate for this task. Water cascading through a 1,000 ton tower- even with the fan off- will easily reject more heat than a small sump heater can possibly add. Sump heaters should be interlocked with the system pump and only operate when the pump is idle. Properly configured controls will include a thermostat typically set to energize the heater when the sump water temperature is below 40 DEGF. A low water cutout is definitely a good idea; Heaters will burn out if energized in the absence of water. Steam or Hot Water Coils can also be employed in lieu of electric heaters.

The Remote Sump is a freeze protection scheme where the tower drains by gravity to a separate sump located in a heated space. (See Ch.5 & 15.)

If there is a seasonal shutdown, the simplest approach is to drain the tower (Remember to heat trace the make-up line to prevent freeze-up).

It is never appropriate to add any type of 'anti-freeze' solution to an open cooling tower. Closed (Fluid Cooler) systems, however, can be protected from freeze-up by the addition of Ethylene Glycol or other fluids. (See Glossary for determining fluid mixtures by weight.)

Fluid cooler casing sections can also be insulated to reduce heat loss thereby protecting the coil from freeze-up. Counterflow, blowthrough towers tend to be more popular as the freeze potential increases.

Crossflow towers tend to freeze water on their air inlet louvers under extreme conditions; Fans (prop type) can be arranged to reverse direction on such towers to melt ice. This process should never be automated; The operator should weigh the situation and reverse the fan only as long as required. The designer must select components suitable for reverse rotation.

Fan Discharge Dampers are a capacity control accessory item for centrifugal fan cooling towers. They fit in the fan scroll. In the 'open' position, they are much like a thin piece of sheet metal in a moving airstream oriented parallel to airflow... The airstream doesn't know its there. As the dampers 'close'- the sheet metal becomes less parallel to airflow- turbulence disrupts the air stream. Airfoil dampers essentially ruin fan housing efficiency to achieve a reduction in airflow. Dampers can be set and locked when a 'manual locking quadrant' is specified but it is more common to use electric or pneumatic actuators that close the dampers as the exiting water temperature becomes too low. While reducing airflow is the correct method of reducing capacity, dampers are not the best approach. They offer the poorest energy savings and the actuating mechanisms tend to fail long before the average cooling tower life span.

System designers often think dampers block airflow and are suitable to prevent back drafts in idle towers. This is not the case; Airfoil dampers simply hamper fan housing efficiency- they do not block airflow.

Air Inlet Screens are always part of blow thru, counterflow towers to protect people from rotating equipment . Some designs can be a hazard (or ingest trash) when accessible from the underside and require the specifier to call out additional screening. They can be a worthwhile accessory when there are nearby trees even when not required for safety reasons. Air inlet screens should be eliminated on towers utilizing inlet ductwork. Inlet ductwork may also make it necessary to block extraneous air entry such as from the underside when towers are elevated. This is where a good sales engineer will tailor the tower to the duty.

A Vibration Cutout is a control device used to shut down the fan motor when excess vibration is sensed. They can be- and are- used on any tower with a fan motor either by choice or by code although they are only practical for towers employing large propeller fans. Typically, centrifugal fans do not fail in a catastrophic mode; Similarly, small prop fans don't cause enough damage to require such devices. Ladders and Handrails are necessary for large field erected cooling towers and make sense on some factory assembled designs. Often, just a ladder makes more sense. Or, nothing at all on small towers. Internal Ladders, Walkways, Platforms, etc. should be evaluated on a job by job basis.

Seismic Bracing options are available and should be specified in earthquake prone areas.

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