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An old hacky way that can be used for estimation is to use two coils in series. The first coil is the waterside economizer coil and the second is a traditional chiller that comes on when the waterside economizer can't reach capacity.

Since you can specify the capacity of any cooling coil in IES, this will limit the off-coil temperature at some point, and the downstream coil will come on. If you want to get super-hacky, then you can also use a generic cooling source with a high COP to estimate the use of fans and pumps on the waterside economizer.

If you are concerned about shutting off this coil when outdoor conditions (wet bulb or dewpoint) are not favorable, then you can use a second controller monitoring outdoor air that can be hooked up to the first coil's controller to shut it off when outdoor conditions would make it impossible to make sufficiently cold water by evaporation alone. This is a quicker and dirtier way if you are stuck.