Each grill will need to have it's own hood as it will be a solid fuel fired cooking appliance. And they will have to be taken out separately. I juest designed a similar one for an Asian restaurant that does demonstration Mongolian BBQ. It was a pain as this was an addition to an existing...
A couple of sources of significant heat loss:
1. Convection due to area winds. The formula that is commonly used in pool heater sizing is 10.5 x surface area x delta T.
2. Evaporation of the water due to your decorative spray head. Look for cooling tower makeup water charts for your climate...