Cooling tower size also plays a major role in approach. Unlike
flow, cooling tower size varies inversely with approach as shown
in Figure 2. A shorter approach requires a larger cooling tower.
Typically, below a 5°F approach, the effect on cooling tower size
begins to become asymptotic. For this reason, it is not customary
in the cooling tower industry to guarantee any approach of less
than 5°F. This is not because cooling towers are unable to produce
a 5°F approach but because any errors in measurement become
very significant when performance is calculated or tested.