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There is an actuator for pumps called Pump Mass Flow Rate [kg/s] (cf EMS Application Guide) that you should use for this.

Chiller Condenser Heat Transfer Rate [W] as a sensor would probably work if you have only one chiller on the demand side of your condenser loop. Otherwise you can probably use the Plant Supply Side Cooling Demand Rate [W] as a sensor.

You'll probably also need to get the Entering Water Temperature (EWT) too, for eg looking at Plant Supply Side Inlet Temperature. If your SetpointManager:Scheduled isn't using a fixed value that you can hardcode in your EMS program, you also need to define a Sensor on the Schedule Value for the corresponding schedule, that will give you the Leaving Water Temperature (LWT).

I think Plant Supply Side Cooling Demand Rate is a positive number, in which case I guess you could try this:

$$\dot{Q}_{demand} = \dot{m} \cdot C_p \cdot (T_{EWT} - T_{LWT})$$

Dimensional analysis:

$$[W = J \cdot s^{-1}] = [kg.s^{-1}] \cdot [J.kg^{-1}.K^{-1}] \cdot [K]$$

So, solving for your mass flow rate:

$$\Leftrightarrow \dot{m} = \frac{\dot{Q}_{demand}}{C_p \cdot (T_{EWT} - T_{LWT})}$$

You can use @CpCW(Temperature) in ERL if you'd like, but it's pointless since it'll always return 4180.0 anyways (source code: Psychrometrics.hh)

Disclaimer: I have not tested any of the above, not sure whether it'll work, integrate well with what the cooling tower is doing, nor what calling point would be appropriate.