SECTION II STRUCTURAL COMPONENTS
B. Cold Water Basin
The cooling tower basin serves the two funda-mentally important functions of 1) collecting the cold water following its transit of the tower, and 2) acting as the tower’s primary foundation. Because it also functions as a collection point for foreign ma-terial washed out of the air by the circulating water, it must be accessible, cleanable, have adequate draining facilities, and be equipped with suitable screening to prevent entry of debris into the suc-tion-side piping.
1. Basin Types: Ground level installations, typi-cal of virtually all large industrial towers, utilize concrete basins (Fig. 44) almost exclusively, whereas elevated or rooftop installations are normally equipped with basins provided by the cooling tower manufacturer, compatible with the cooling tower framework. Typical materials include wood (Fig. 45), steel and, occasionally, plastic. In those cases, the cooling tower manu-facturer usually includes drain and overlow ittings, make-up valve(s), sumps and screens, as well as provisions for anchorage.
Concrete basins for wood or steel framed, ield-erected towers (Fig. 44) are usually designed and built by the purchaser, utilizing di-mensional and load information provided by the manufacturer. However, due to their integration into the overall tower structure, and because of the extensive site-related concrete work required, cold water basins for concrete towers (Fig. 46) are often both designed and built by the cooling
Figure 44 — Crossflow tower framework on concrete basin prepared for future tower extension.
tower manufacturer.
To insure proper functioning of the tower, the basin must provide a stable, level founda-tion. Generally, a well-drained soil with moderate bearing capacity will support mechanical draft towers of wood or steel construction. Concrete towers impose heavier loads on the soil and, in some cases, may require the use of piles or caissons. The soil should have a uniform bear-ing capacity under the basin to prevent uneven settlement. Footings must be below the prevail-ing frostline (Fig. 47), and construction practices should always conform to local codes.
Wood and iberglass towers may be equipped with wood, iberglass or steel basins.
Wood basins are normally lat, less than 2' deep, and equipped with depressed sumps to facili-tate pump suction. Joints are sealed to prevent leakage. Plywood basins typically require con-siderably less maintenance than do carbon steel
basins. Fiberglass basins are typically used with iberglass tower structures.
Steel basins may be of carbon steel (gal-vanized or painted), or stainless steel, and of either bolted or welded construction. If bolted, joints must be gasketed and sealed leak-tight.
If welded, the weld vicinity should be suitably Figure 45 — Plywood cold water basin floor. (Note
depressed sump)
Figure 46 — Basin and basic framework of an octagonal mechanical draft tower in concrete construction.
Figure 47 — Extreme depth of frost penetration (in.) based on state averages.
Figure 48 — Steel grillage supporting tower equipped with wood cold water collection basin.
Figure 49 — Water from the basin of this tower returns directly to the lake.
coated for corrosion protection. Steel basins also are normally lat, except for those under cer-tain factory-assembled towers (Fig. 12), which incorporate a depressed section to facilitate cleaning and improve outlow characteristics.
Being subject to oxidation, steel basins require more maintenance, and are more sensitive to water quality, than are wood basins.
2. Basin Support: A grillage of steel or concrete is normally utilized for support of a tower installed over a wood or steel basin. (Fig. 48) Grillages must be designed to withstand the total wet op-erating weight of the tower and attendant piping, as well as the dead loads contributed by stair-ways, catwalks, etc. It must also accept transient loads attributable to wind, earthquake, and main-tenance trafic. Grillage members must be level,
and of suficient strength to preclude excess de-lection under load.
In designing the grillage, the possibility of fu-ture extension of the tower should be considered as a means of minimizing future cost impact.
3. Basin Depth: As indicated previously, wood, i-berglass and steel basins are of relatively shallow construction, typically 14" to 20" deep. Although greater depths are possible, they are seldom re-quired or recommended. Suficient freeboard above the operating water level is included to accommodate the normal amount of transient water that collects in the basin at shutdown.
Greater design lexibility is afforded with the concrete basins typically utilized for larger tow-ers (Fig. 44), and adaptable for smaller towtow-ers.
Once the load points are accommodated at the proper elevation, the basin loor (slab) may be as far below the top of the basin wall (curb) as re-quired to satisfy design criteria. The basin must be deep enough to provide suficient hydraulic head for proper water low into the sump(s), and to accept the transient water and potential back-low at pump shutdown. Beyond this, the basin may be made deep enough to hold a reserve in case of interrupted make-up water supply; to stabilize water temperatures under highly vari-able loads; or to act as a reservoir to supply the plant ire protection system.
“Dry basins” are minimum depth basin which drain by gravity into adjacent lumes, vessels, col-lection ponds, or streams. The are so designated because the are intended to drain completely upon pump shutdown. Typical applications of this principle are the “indoor tank” (Fig. 135), and the “helper” tower. (Fig. 49)(Sect. V-L) Suf-iciently low water levels in dry-basin towers may necessitate air seals to prevent the reduc-tion in tower performance associated with air by-passing beneath the ill.
4. Basin Sumps: Sumps for towers with wood or steel basins are normally designed and furnished by the manufacturer. (Fig. 45) Concrete sumps (Fig. 50), provided by the purchaser, should be designed for water entrance velocities of less than 3'/second, and should be of suficient depth to satisfy pump suction head requirements.
Screens are usually vertical, of 12" square mesh, sized for 1'/second net velocity through the open area of the screen, and held in place by channels imbedded in the sump walls to allow for easy re-moval. Screens may be installed in duplicate to permit cleaning during continued operation.
5. Basin Cleaning Facilities: Because it is an area of relatively low low velocity, any water borne or borne particulates entering the circulating water system will tend to settle in the basin, where the resultant silt can be either periodically or continu-ously removed from the system. Periodic sludge removal usually takes place during normal shut-down intervals. Where towers are expected to
operate continuously, strategically located basin partitions can permit partial shutdown for sec-tional cleaning and maintenance.
Where possible, large capacity cleanout drains (Fig. 50) should be provided. Concrete basin loors should slope toward the sumps or drains at a rate of 1' per 100', to permit lushing of the sediment. Where drains cannot be provid-ed, basins should slope toward a cleanout sump from which sludge can be pumped, or removed manually.
Side-stream iltration (Sect. VI-E) has been found to be an effective means of maintaining suspended solids at acceptable levels in the cir-culating water system, and of reducing the costs associated with periodic silt removal. For most effective iltration, discharge low from the ilter should be returned to areas of low velocity in the basin in order to help maintain particulate sus-pension.
Figure 50 — Typical cross-section of concrete sump pit.
Figure 52 — Framework and joint detail in a well-designed cooling tower of wood construction.
Figure 51 — Factory-assembled towers of stainless steel construction are utilized in corrosive areas.