CHAPTER 5a | Winter Operation of Cooling Towers

(Considerable interest has been expressed in Cooling Tower Freeze Protection. While the subject is touched upon in several places in other chapters, Ch. 5a is added here to address the subject more fully.)

Cooling towers often operate in freezing climates. System designers must take this into account as this condition poses the potential for erratic operation and severe equipment damage.

Both the severity and length of temperature excursions below freezing are important to consider. Some climates like Denver, Colorado have fairly long periods in the spring and fall with wild temperature swings. For comfort cooling, full tower capacity can be needed one day followed by a period where there is virtually no load coincident with sub zero ambient temperatures the next. It can be difficult to configure a tower to work under all possible conditions without freeze-up.

This paper does not recommend solutions for specific circumstances but does point out equipment design considerations, system modifications, methods of applying auxiliary heat and the like to mitigate freeze potential.

First, it's a good idea to ask "What is everyone else doing?" Most areas of the country have designers who are familiar with local conditions and have installations- both good and bad- that have taught them what is needed. Sales representatives and cooling tower manufacturers are also good sources of information.

Next, consider the cooling tower. Is it an 'open' tower where the cooling water collects in the bottom and is pumped elsewhere or is it a 'closed circuit' design where there are two distinct cooling fluids. One that flows over a coil, collects in a basin and is pumped back over the coil while separate fluid resides in the coil and is pumped to a distant point.

For the closed circuit tower, an antifreeze solution such as ethylene glycol can be used in the closed loop to prevent freeze-up.

It is never appropriate to use an antifreeze solution in an 'open' tower or in the fluid loop exposed to the atmosphere in a closed circuit tower. Thermal capacity depends on evaporation and ethylene glycol solutions simply don't evaporate well.
-The cooling tower would cease to work properly requiring all of the solution to be removed
-Electronic water treatment controls would be rendered useless
-The wet deck surface would become slimy

In general, blow through towers are better suited for freezing ambients because the fans don't have to process air that is saturated with moisture. Add a bit of carry over (AKA 'spit') and a drawthrough fan can easily build ice on its surface.

Counter flow towers also have an advantage. This is because the entering ambient air first encounters uniform temperature water as opposed to the crossflow arrangement where the coldest air can encounter the coldest water causing ice to form (near the bottom of the fill at the air inlet face).

Many draw through, crossflow towers work just fine during freezing ambients but the advantages of blow through counterflow designs become more apparent when the load is diminished and/or the ambient temperatures are well below 0 DegF.

Reversing the direction of fan rotation on prop fan crossflow towers is a common method of melting ice on air inlet louvers. Be sure to check with the tower manufacturer for recommendations.

Naturally, if the tower can be shut down and drained during the cold season the differences become moot.

If the fan and pump can be turned off, it becomes a matter of keeping the stagnant water from freezing.

Exposed piping is typically insulated and heat traced.

At shutdown, the water falls into the cold water basin which can be protected by sump heaters. These are typically 3-phase electric units that add enough heat to protect the sump. Heaters must be sized based on the minimum design ambient temperature.

Steam or hot water coils can be used in lieu of electric heaters. Injecting steam directly into a galvanized steel basin is not recommended as the steam condensate is extremely corrosive. Even small leaks in a continuous steam coil can cause severe localized corrosion during prolonged cold spells where the tower is idle and condensate accumulates in the basin.

Localized ice may form in the basin even with added sump heat but the sump is protected because basin is not allowed to freeze solid.

Some system designers feel they can depend on sump heaters to warm the water sufficiently to satisfy the requirements of centrifugal chillers that don't want to see water that is too cold at start-up. Such use is not recommended. When the pump starts, a correctly wired sump heater will turn off. The water in the cooling tower basin will pass through the chiller in a matter of seconds and be quickly replaced with colder water.

Regulating air flow with a speed drive is the best way to manage cooling tower capacity and minimize freeze potential.

The 'remote sump' design is often employed in freezing climates. In this scenario, the traditional sump drains by gravity into a second sump that resides in a heated space below the tower. The make-up valve and strainer reside in the new sump. So, when the pump is shut off, the system water falls into the warm space where it is protected from freeze up. It is a good idea to keep the vertical separation between the 'old' and 'new' sumps at a minimum because the energy of the falling water is not recovered and the system pump must be sized to handle the increased system head. The biggest advantage of the remote sump is that they operate naturally and don't depend on auxiliary heat that can fail. The elimination of electric heaters saves energy and also does away with the corrosion typically found in cooling tower basins adjacent to where heaters would be located. Don't allow electric heaters to sag and come close to the basin floor; Prop them up with bricks if necessary.

Water Source Heat Pump Projects:

Special consideration should be given to systems employing water source heat pumps.

Unlike most industrial applications where the heat load is fairly constant, heat pump projects typically need the least amount of cooling when the ambient temperature is the coldest exacerbating the freeze potential.

One approach is to immobilize the spray pump and drain the basin so the coil operates 'dry'. With the proper concentration of glycol in the coil, the system is protected; However, dry operation is very inefficient (perhaps 10%) leading to poor performance.

Adding fins to the coil makes it more efficient during dry operation and very well may be the answer to installations with vastly changing climactic conditions. Manufacturers have selection programs that select the appropriate number of fins for the climactic conditions the system designer provides. Excessive fins are not recommended as they make water treatment more difficult. Similarly, wide fin spacing is preferred over closer spacing. [Note: Fins are added by the factory when the coils are made; They can't be added in the field.]

Operating the system 'wet' and utilizing sump heaters and heat tracing for freeze protection is the typical method of spring and fall operation.

It is customary to place positive closure dampers at the air discharge with 'spring return' actuators that close when the system is 'off' thereby reducing heat loss from natural convection at the coil. In addition, the walls of the casing and positive closure damper plenum are covered with 1" thick closed cell foam insulation. These steps reduce the load on the system boiler.

Another approach is to couple a conventional 'open' cooling tower with a heat exchanger- typically of 'plate and frame' design- to gain the same 'closed circuit' configuration. This is particularly helpful if plain water is used in the cooling loop. The heat exchanger is small in size making it more easily protected from freeze up. The small size also reduces heat loss when the boiler is activated. The open tower has no positive closure dampers or insulation on the casing. All this typically yields a simpler, less expensive installation not to mention the substantial weight savings that result from omitting the coil.

For additional reading, check this excellent article from SPX.

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