By Chad Edmondson
When designing a cooling tower, one of the first decisions an owner or engineer must make is whether choose a packaged (factory assembled) tower or a field erected tower.
Factory assembled cooling towers, as the name suggests, are pre-fabricated at the factory and arrive at the jobsite ready for placement and hook-up. Because these units ship by truck they can only be so large, which limits their applicability to projects where the heat rejection load is very high. However, factory assembled cooling towers are commonly used in most commercial or institutional applications, including office buildings, hospitals and universities.
Field erected systems are built onsite, using materials and custom-manufactured components that are shipped from the manufacturer. Field-erected towers are used to serve facilities with very high heat rejection loads like large chilled water plants, power plants, refineries and large manufacturing plants. As such, they are typically larger than factory assembled cooling towers. Since field erected cooling towers are built onsite, shipping does not impose any limitations on their size or their capacity.
Very generally speaking, if the facility’s heat rejection load is in excess of 5000 Tons, a field erected system might be the best choice. If the required capacity is 1400 Tons or less, a factory assembled module is the most practical choice. If the required capacity is somewhere in between, both options should be evaluated.
Factory assembled cooling towers are configured into “cells” which are bounded either by exterior walls or partition walls between cells. These cells are the smallest tower subdivision that can function independently in terms of air and water flow. Individual factory assembled cells are typically no larger than about 1400 tons. Beyond that the fan(s) and the space required for sufficient air flow become so large that the overall cell size exceeds standard shipping capability. Multiple cells can be piped together for higher loads, but this will increase the piping requirements and the number of components. At a certain point, a larger field erected system becomes the more practical and economical choice.