Packaged Rooftop Unit (RTU)

Overview

A packaged rooftop unit puts an entire commercial air conditioning and heating plant into one weatherproof box that sits on the roof of a store, school, or office. Where a split system separates the coils, a rooftop unit packages compressors, both coils, blowers, heat, dampers, and controls together, ducting conditioned air straight down through the roof. One unit conditions one zone, and a big building simply uses many of them.

Everything mounts inside a double-wall Weatherproof Cabinet on a roof curb. The cooling circuit pairs the Scroll Compressor units with a Condenser Coil and an Evaporator Coil; the Supply Air Blower pushes conditioned air to the building while Condenser Fan units reject heat outdoors. For heating, the Gas Heat Section section warms the same supply air. An Economizer and a Filter Rack handle outdoor air and filtration, and the Unit Controller runs the whole sequence.

How it works

Cooling is vapor compression at building scale. A Scroll Compressor squeezes refrigerant vapor to high pressure; the hot gas condenses in the Condenser Coil as the condenser fans pull outdoor air across it; the liquid drops through the Metering Section expansion valve and boils in the Evaporator Coil, chilling the supply air. Using two staged scroll compressors lets the unit match part-load demand instead of running flat-out and short-cycling.

Air handling is what makes a rooftop unit a complete system. The Supply Air Blower is a belt-driven centrifugal fan sized to push air through long duct runs; its adjustable sheaves let an installer tune airflow to the building. Before air ever reaches the coil it passes the Filter Rack, and outdoor and return air are blended by the Economizer, whose motorized dampers open to cool, dry outside air for free cooling when conditions allow, cutting compressor run time.

Heating shares the airstream. In the Gas Heat Section section, burners fire a tubular heat exchanger and the supply blower carries that heat to the rooms, with electric strips and a rollout Thermal Fuse for safety and backup. Tying it together, the Unit Controller stages compressors, sequences heat, modulates the economizer, and reports to the building automation system over a network, so a facility manager sees and schedules every rooftop unit from one screen.