Solutions for full-outdoor-air operation of air rooftop units with direct expansion circuit and integrated recirculation

Following the advent of the Sars – Cov 2 pandemic, HVAC systems have been affected by different interventions both in the emergency and in the medium term. These interventions were dictated by the need to maximise the risk reduction within air-conditioned environments and among these interventions one of the most applied has been the elimination of recirculation in favor of a full-out-door- air operation, specifically in multi-zone systems. Some plant, such as those served by roof-top units, however, provide a minimum share of recirculation that is necessary for the proper functioning of the integrated refrigeration unit. For these units, the elimination of recirculation has led to a significant increase in blocking situations. The research provides the basis for the application of integrated solutions designed to prevent blocking situations for new and existing systems through the insertion of a modulated bypass between the supply air and the external air intake so as to allow temporarily the recirculation function without, however, creating a situation of risk propagation. The proposed solutions have been applied to a real system, the regulation has been carried out in integration with the management logic of the unit and has been tested to be effective in cooling mode.