Question-and-Answer Resource for the Building Energy Modeling Community
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The technologies are technically mutually exclusive as there is no reason to have both as they generally perform the same function.
You don't have to have a chiller connected to a water side economizer. Just make sure to shut off the chilled water pump when the water side economizer isn't able to meet the load.
That said, if you're putting in a chilled water loop, a condenser loop, pumps, and a cooling tower, then you might as well buy a chiller and avoid buying the DX units. Unless of course you're using a dry cooler as your water side economizer. Even then you'll have a cooling coil with water and a cooling coil with DX. That will increase the pressure drop and consequently the fan power.
In reality it would be simpler to have a DX unit with an air-side economizer or a chiller with a water side economizer as it's the least amount of equipment to install and maintain.
Hope that makes sense.
