An Icy Situation
Cooling towers are especially susceptible to ice formation. Understanding temperature gradients is the first line of defense. A properly designed cooling tower promotes the maximum possible contact between air and water - and does so for the maximum possible time period. This effort on the part of the designer results in an efficiency that, although greatly appreciated in the summertime, has the capability to produce performance-degrading ice formations during winter operation. Therefore, means by which the cooling tower's efficiency either can be controlled or can be made to work toward the management of ice formations must be incorporated into its design, and they must be properly utilized by the operator.
While there are several methods of ice control, this article will closely examine temperature gradients and how they impact ice formation.
As on any outside structure, ice can form on a cooling tower in the wintertime purely by natural effect. In addition, being a device that moves air through water - cooling mainly by evaporation - a cooling tower can promote the formation of ice by its very operation.
Whether caused by nature or by the tower itself, the owner’s concern for an ice formation on a cooling tower should be a reflection of both its location and extent of icing. Ice on exposed working platforms can be a personnel hazard and should be corrected manually. Light random ice on the louvers, structure and the leading edges of fill is usually of minor concern. Ice allowed to form on fans and other mechanical equipment - not to mention the shrouds and control devices associated with that mechanical equipment - can lead to catastrophe.
Generally speaking, an acceptable level of ice formation is characterized by ice with a relatively thin cross-section and that forms on the louvers or air-intake structure of an induced draft tower. Figure 1 shows what might be considered an acceptable amount of ice formed inside the air-intake structure of a counterflow cooling tower, and it also shows a relatively light curtain of ice on the louvers of a crossflow tower. Because this amount of ice normally would be anticipated in a cooling tower’s design loading, it is customarily of little structural concern and. In some cases, its retardation of airflow through the tower achieves a result similar to the airside control procedures. (For more on airside control procedures, see the web exclusive “Airside Control,” posted on www.process-cooling.com.) However, although this ice may still be considered acceptable, it has proceeded to a point where measures that limit or remove it should be undertaken.
If allowed to grow unchecked, ice can achieve massive cross-section, encroaching upon the fill or totally blocking airflow (figure 2). Its weight alone can overload affected members and, when ice of such mass dislodges, it is obviously capable of doing significant damage.
Understanding how to anticipate and control ice requires some knowledge of the water temperature gradients that occur in an operating cooling tower. Without such knowledge, operators often assume that controls that will automatically cycle fans to maintain a leaving cold water temperature well above freezing are sufficient insurance against the formation of ice. Occasionally, they are bewildered to find ice beginning to form even before the cold water basin temperature has depressed to that presumably “safe” level.
The reason, of course, is the temperature gradients that occur transversely in all towers and longitudinally in multi-cell towers where fans are cycled progressively. Figure 3 indicates the typical transverse temperature gradients in a bank of crossflow fill. In this particular case, water is entering the tower at 64.5°F (18°C) and leaving at 44.5°F (7°C) - temperatures that would seem to indicate to an operator that a 12.5°F (7°C) “safe” zone exists between the operating point and freezing.
Obviously, such is not the case. As can be seen, the net outlet temperature of 44.5°F (7°C) results from a mixture of water temperatures varying from about 53°F (12°C) at the inboard edge of the fill to about 33°F (0.5°C) at the outboard edge. Consequently, the real margin of safety is only about 1°F (0.5°C) in this case.
It must not be assumed from this that 44.5°F (7°C) cold water temperature is the “magic” point of control for all operating situations. Water temperatures at the coldest point of the fill are very sensitive to the range (the difference between entering hot and leaving cold water temperatures through which the tower is cooling). At a given cold water temperature control point, reduced ranges (i.e., reductions in heat load at a constant water flow rate) will cause the water temperature at the coldest point of the fill to rise. Conversely, increased ranges (i.e., reductions in water flow at a fixed heat load) will cause lower water temperature at the coldest point of the fill.
To return to the example using the tower and fill shown in figure 3, if the tower was operating at a 10°F (5.5°C) range (cooling the water from 54.5 to 44.5°F, or 12.5 to 7°C), the entering wet-bulb temperature would be 29°F (-1°C), and the water temperature at the coldest point of the fill would be about 38.5°F (3.6°C). As wet-bulb temperature further reduces, measures would need to be taken to diminish airflow through the fill (by fan manipulation), and the coldest water in the fill would reduce only negligibly below that level.
There is also a longitudinal temperature gradient - actually, steps, rather than a gradient, as individual fans are manipulated - in a multi-cell tower. This is because cells with fans operating at full speed contribute much more to the tower’s overall cooling effect than do cells with fans operating at a reduced speed or off. For example, if water were entering the tower in figure 4 at 80°F (26°C) and leaving at a net 60°F (16°C) - achieved by having one fan running and one off - the actual water temperature produced by Cell #1 would be 50°F (10°C), and water at the coldest point of its fill would be at or near freezing.
Figure 5 indicates net performance - and thermal gradients - of a two-cell tower cooling through a 20°F (11°C) range, equipped with single-speed (top) and two-speed (bottom) fans. These curves are drawn on the premise that the operator will manipulate fans to prevent the net cold water temperature from going below 60°F (15°C), and the winter wet bulb temperature can routinely depress to 0°F (-17°C). The solid line indicates the net water temperature sensed by the operator’s thermometers or control devices; the dashed line indicates the net water temperature from the cell operating at the greatest fan speed; and the dotted line indicates the coldest water temperature in the fill.