A rooftop handling unit (RTU) is a large scale version of an Air Handling Unit (AHU) that is designed for larger commercial use and placed on a roof to eliminate the need for a large indoor space and clearance. An Air Handling Unit is a device used to regulate and circulate air as part of a heating, ventilating, and air-conditioning system commonly called “HVAC system”. Smaller AHUs are called terminal units, which are the common, everyday air conditioners. A rooftop handling unit is considered a primary HVAC system because it is capable of fully heating and cooling in its entirety. It can also be considered a primary/secondary system when it does not utilize ductwork and simply blows/returns the air directly into and out of the building from a hole in the ceiling connected directly to the RTU blower/fan.  

 

Primay Components:

• Filters: air filtration is needed in order to provide dust-free/pollutant-free air to the building. The filters can be one or a combination of the following techniques: low-MERV pleated medium, HEPA, electrostatic, gas-phase or in some cases ultraviolet. Filtration is normally placed first in any AHU in order to keep all downstream components clean. In some units, there are multiple successive filtration banks that each filter a certain amount of particulates out of the air.

 

• Heating and/or cooling elements: RTUs may need to provide heating, cooling or both to change the supply air temperature and humidity depending on the location and desired application. This conditioning is provided by heat exchanger coils, either directly or indirectly relating to the medium providing the heating and cooling effect.

  • Direct heat exchangers: These include gas-fired fuel-burning heaters, or a refrigeration evaporator that are placed directly in the air stream.

  • Indirect heat exchangers: These use indirect coils that carry hot water or steam for heating, and chilled water for cooling. The hot water or chilled water is supplied from a broiler or chiller elsewhere in the building. The coils are manufactured from copper or aluminum which aid heat in heat transfer. Cooling coils use eliminator plates to remove and drain condensation. Sensors are employed downstream to monitor and control “on” and “off” coil temperatures along with a motorized control valve before the coils.

 

• Humidifier: a humidifier is necessary in colder climates where continual heating will cause the air to become “dry” which is an uncomfortable air quality and can cause static electricity. Different types of humidification can be used, including evaporative, vaporizer, spray mist, ultrasonic and wetted medium. All techniques utilize steam or water blown either directly or indirectly into the air stream to increase water content, or humidity, within the air being carried through the unit into the building

 

• Mixing chamber: This is where outside, cooler air is mixed with warm, return air to create the desired temperature for the air supplied back into the building system. Dampers within the mixing chamber control the ratio between return, outside and exhaust air.

 

• Blower/fan: RTU have a large squirrel cage blower driven by an AC induction electric motor to move the air. Multiple blowers can be present in large commercial handlers where the air needs to be moved to or from multiple locations within the unit. There typically is at least one blower at the end of the RTU and at the beginning of the supply ductwork to propel the air either into the space directly or into the ductwork. There are also often return air duct fans pushing the air from the building back into the air handling unit to be mixed with the outside air within the mixing chamber.

 

• Balancing: If the fan is unbalanced it can wobble and vibrate which is a major problem as it greatly reduces the air circulation at the vents which compromises efficiency and increases noise. Weights can be strategically placed to correct for a smooth spin.

 

• Heat recovery device: A heat recovery device may be fitted into any air handler between the supply and extract air streams for energy savings and heat recovery. There are a variety of heat recovery heat exchangers which have heat recovery efficiencies from 50 – 85%.  The highest heat recovery efficiency of up to 85% comes from a thermal wheel which is a slowly rotation matrix of corrugated metal that operates in both opposing airstreams. As heat is absorbed, air passes through the matrix in the exhaust airstream and then released during the second half rotation into the supply airstream.

 

• Controls: Controls are used to regulate every aspect of an air handler such as flow rate, supply air temperature, mixed air temperature, humidity and air quality. This may be as simple as an off/on thermostat or a complex building automation system.

 

• Vibration isolators: The blowers can create substantial vibration and the large area of the duct system transmits the noise and vibration to the occupants of the building. This is avoided by vibration isolators that are inserted into the duct immediately before and after the air handler and also between the fan compartment and the rest of the unit. The rubberized canvas-like material allow the air handler components to vibrate without transmitting the motion to the attached ducts. This is not as necessary in a RTU due to the location of the system on the roof and therefore the noise is not as easily heard by the occupants of the building.

 

Secondary Component:

The secondary component of a RTU is the ductwork that distributes the conditioned air through the building and returns it to the unit. Sometimes RTUs discharge (supply) and admit (return) air directly to and from the space served without ductwork. This is normally seen in large commercial or industrial spaces where there are multiple units on the roof each supplying air directly into the space at that location.

 

Interaction:

Outdoor air enters into the unit, then the air passes through the filters and through the heating/cooling coils. Then the air hits the blower and is blown either into ductwork or directly into the building. Return air is also suctioned from the building into the mixing chamber where dampers mix part of the return air with the outside air (and exhaust the rest of the return air) to reach a desired temperature and then the cycle begins again.