Cooling Tower Fundamentals - 79 / 117 Pages
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Figure 116 — Weir by-pass arrangement to control hot water temperature to tower.
of disaster. In its most benign form, it may only increase the frequency of required silt removal from the cold water basin. More agressive composiͭtion, however, may clog water distribution systems, silt-up hot water basins, and obstruct pasͭsages in the fill and eliminators. Some forms of turbidity (such as iron oxides and sulfurous compounds) may build up and solidify on any availͭable tower component and place the tower in structural jeopardy.
In many cases, turbidity of the water arriving at the tower can be reduced by the application of a system similar to that shown in Figure 116, except water by-pass from the cold water basin would be required only if high temperatures were involved. Water returning from the process (or from the make-up water supply, if that is the source of turbidity) would flow into one end of a relatively large settling basin, wherein the flow velocity would be sufficiently low to allow particuͭlate matter to settle out, encouraged by a chemical precipitant if necessary. A separate pump, loͭcated at the other end of the settling basin, would deliver relatively clean water to the tower's water distribution system.
In the case of particulates whose tendency is to float, they could be skimmed off into an adjacent basin for waste or recovery, by means of either an overflow weir or a floating "skimmer".
Limited ground area availability, among other considerations, often precludes the use of effecͭtive settling basins, requiring that the cooling tower be designed to be as impervious as possible to the effects of excessive turbidity. A number of such designs have been developed to solve specific problems, one of which is pictured in Figͭure 117, and shown in cross-section in Figure 118. The service of this tower is blast furnace cooling at a large steel mill. As can be seen, the tower is spray-filled only (I-B-5), and all of the structural
An alternative method of accomplishing the same net result, occasionally utilized in conjunction with concrete basins, is the hot well—cold well, weir by-pass arrangement shown in Figure 116. Although the hot and cold wells are shown on opposite sides of the basin for clarity, they are usually adjacent to each other. Hot water from the process flows into a separate basin, or pit, where it is mixed with a measured amount of cold water from the cooling tower's basin, usually weir controlled. This combined flow of reduced-temͭperature water is then delivered to the tower's water distribution system by means of a separate pump. The amount of water to be extracted from the cold water basin for mixing with the process water flow is also calculable from formula (17).
Utilizing either of these methods for limiting incoming hot water temperature causes the tower's cooling "range" (Sect. I-E-4) to decrease which, in turn, causes the required cooling tower size to inͭcrease, as indicated by Figure 28. In the example case, instead of being sized for a 70°F range (160-90), the tower was sized for a range of 50° F (140-90). Therefore, the tower's design range was 71 % of what it would have been without by-pass, and Figure 28 shows that this increased the tower size an order of magnitude of 18%. Typically, however, the cost impact of a somewhat larger tower is less than that produced by the combination of premium materials, plus the long term deͭgrading effect of excessively high temperature water.
Because the piped by-pass method (Fig. 115) requires only one system pump, and because it afͭfords more positive control, it is usually the system of choice where control of incoming temperaͭture to the tower is the primary concern. 2. High turbidity, depending upon its character and content, can cause thermal and structural problems in a cooling tower, occasionally to the point