For most industrial manufacturing, electric power generation and even air-conditioning needs, cooling towers are a critical element of the design. Cooling towers work on the principle of heat rejection: They extract heat to the atmosphere through evaporative cooling. The towers themselves are enclosed, steady-flow devices for cooling water by evaporation through direct contact with air. Cooling towers are used in water-cooled refrigeration, industrial process systems, and commercial HVAC air-conditioning.
A simple way to appreciate how a cooling tower works to consider the “beach effect.” On a 95°F (35°C) day with 95 percent humidity, a dip in the water is incredibly refreshing. When you get out of the water and are still damp, what happens when a breeze blows? You feel cool as the breeze evaporates the water on your skin. This is the basic premise behind evaporatively cooled equipment.
The essential element for a cooling tower to function is water, and water needs to be delivered through a piping system. It seems simple, but how effectively a cooling tower piping system is designed and installed can have a critical impact on system efficiency, functionality, and even safety.
Most larger cooling tower systems use a multiple-cell configuration. This configuration allows for the correct staging of equipment as the process or facility loads vary. It also affords facility managers the opportunity to operate their plants at peak efficiencies. Additionally, these multiplexed condenser water systems incorporate sufficient redundancies such that critical operations are never in jeopardy of being nonoperational.
Piping Scenarios for Multiple-Cell Cooling Towers
Cooling tower piping systems have evolved over the years from simplistic, dedicated hydraulic loops that lacked complexity to large-volume, multiplexed systems that offer peak operational efficiencies (figure 1). The evolution of a more sophisticated system is due in part to the advent of advanced controls and the operational flexibility of modern chillers, cooling towers and other mechanical equipment. Condenser piping systems are designed to be streamlined to keep capital costs low while yielding the most energy-efficient solution.
One note about condenser piping nomenclature: It is counterintuitive. The condenser return piping (CR) is the piping supplying the cooling tower but returning from the chiller. The condenser supply piping (CS) is the piping leaving the cooling tower and supplying the chiller. This seems backward from what might be expected, but it reflects the fact that the chiller is the central component in the system design.
Today, multi-cell cooling tower configurations have common headers for both the supply and return lines (figure 2). In these systems, the use of automatic control valves for proper operation is imperative at the inlet (CR) and outlet (CS) of each cooling tower.
Additionally, it is a better engineering practice (and the recommendation of this author) to provide automatic control valves at the outlet of each tower’s equalizing line. This additional layer of automated control typically spurs the most dialogue within the engineering community as compared to the more commonly used manual isolation valve. However, this design approach is easily justified. It provides better hydraulic control as you sequence on and off additional machines, and it is typically inconsequential to the overall project cost