Zephir3 has 2 main operating modes: with Constant Supply Control and at the Maximum Available Capacity. When to prefer one way to the other? 

Zephir3 is a unit for primary air handling and purification. It recovers the energy from the exhaust air thanks to the heat pump circuit provided for standard with inverter compressors and modulating hot gas re-heating coil. 
Thanks to these features, it can be easily adapted to different plant design and operation logics.

For example, considering a centralized primary air system (air distribution in different and independent areas) or a traditionally designed system, the Constant Supply Control operation is the best solution: in every operating mode (both summer and winter) the unit always works in order to guarantee the desired input conditions, both in temperature and humidity. In particular, in cooling operation, the unit modulates the capacity of the refrigeration circuit in order to meet the humidity level required, then manages the hot gas re-heating to supply air at the required supply temperature. 
In heating operation, the capacity of the unit is modulated in order to achieve the desired inlet air temperature, activating the humidifier when provided to increase the specific humidity ratio of the inlet air.

Whereas, considering a system with homogeneous areas (open space or several separate but homogeneous rooms from the loads profile point of view), the Maximum Available Capacity mode is very interesting.
In fact, with this configuration, the operation of the unit is strictly linked to the punctual indoor conditions, adapting its operation to contribute to satisfy the internal loads. 
In particular, in cooling mode, humidity control is always a priority, while the supply air temperature (thanks to the modulation of the re-heating coil) is as lower as the return temperature is far from the set point. 
Similarly in heating mode, the supply temperature is as hotter as the return temperature is far from the set point. 
As a direct consequence, part of the internal load can be satisfied directly by the Zephir unit thus allowing to faster reach achieve the indoor set point and to undersize the main system (eg fan coil, VRF) with a final economic saving on the management of the system. 

In addition to these functional advantages, Zephir solution, as a packaged and autonomous product, makes the primary air handling independent from external heating and cooling plants and from the relative auxiliary components usually designed in traditional systems: consequently, the system design and all installation, start-up and maintenance activities are greatly simplified.

Why do Clivet rooftops perform more when they work with a percentage of external air?

The cooling capacity of a rooftop increases with the increase of the quantity of external air due to the temperature difference between the mixture of air and the exchanger temperature: the higher is the difference the higher is the capacity produced by the heat exchanger. 

In this way, the system increases the efficiency because it produces greater cooling capacity with an almost unchanged electrical power input. 
When a percentage of external air is introduced in a confined environment, there is the simultaneous need to expel the exhaust air to ensure the internal pressure balance (doors that cannot be opened, drafts, whistles, …). 

By Clivet rooftops the air is not simply expelled, but is conveyed to the external exchanger for the thermodynamic recovery, obtaining a further increase in efficiency thanks to the better heat exchange in the coil. 

The rooftop (CCKP version) with ThOR technology is equipped with heat exchangers dedicated to energy recovery: it further amplify the thermodynamic effect.

The ThOR exchangers are integrated into the main cooling circuit.