What is the working principle of a Rooftop Package Unit ?
A rooftop package unit (roof-mounted air conditioning unit) is essentially an "all-air" central air conditioning system installed on the roof , integrating functions such as cooling, heating, air supply, filtration, and electrical control. Its core working logic is: to cool or heat the air inside the unit, and then deliver the treated air into the room through ducts .
I. Core Refrigeration Cycle: Vapor Compression
Whether a rooftop package unit is used for cooling or heating, its core relies on a vapor compression refrigeration cycle. The main components include a compressor, condenser, expansion valve, and evaporator.
Cooling mode : The compressor compresses the refrigerant into a high-temperature, high-pressure gas and sends it to the outdoor heat exchanger (condenser) . The outdoor fan draws air out and removes heat, causing the refrigerant to condense into a liquid. The liquid refrigerant, after being throttled by the expansion valve, enters the indoor heat exchanger (evaporator) to absorb heat and evaporate, thereby cooling the air flowing through it. Finally, the refrigerant gas returns to the compressor, completing the cycle.
Heating mode : For heat pump rooftop package unit , the refrigerant flow is switched via a four-way valve. In this mode, the outdoor heat exchanger becomes an evaporator to absorb heat from the outside, while the indoor heat exchanger becomes a condenser to heat and deliver air.
II. Air Handling and Transportation
Rooftop package unit process the air itself, not the water.
Air supply : After the indoor return air is mixed with some fresh air, it passes through a filter to remove dust, and then flows through the evaporator (during cooling) or condenser (during heating) for heat exchange. Subsequently, the treated air is delivered into the room by a supply fan (usually a centrifugal fan) through ductwork.
Exhaust and Pressure Balance : To maintain indoor air quality and balance air pressure, rooftop package unit typically have independent exhaust ducts or pressure relief valves. When the indoor pressure is too high, some air will be exhausted outdoors through the exhaust valve to prevent fresh air from being unable to enter or doors from being difficult to open.
III. Key Operational Controls
Internal resistance management : Because rooftop package unit integrate multiple components such as filters, heat exchangers, and water deflectors, airflow resistance is relatively high. Some advanced models reduce internal pressure loss and lower fan energy consumption by controlling the opening of the bypass vent valve or adjusting the fan speed.
Condensate drainage : During refrigeration, condensate will be generated on the surface of the evaporator, which will be collected in the drip tray below and then drained. The design must ensure that the drain pipe is directed away from areas with strong winds to prevent poor drainage or water blowing due to fan suction.
