Handbook of air conditioning and refrigeration / Shan K

AIR SYSTEMS: EQUIPMENT—AIR-HANDLING UNITS AND PACKAGED UNITS 16.13
FIGURE 16.4 Cutaway view of a typical rooftop packaged unit. (Source: the Trane Company. Reprinted with
permission.)
A rooftop packaged unit is a single packaged unit composed of two main components: an air handler
and a condensing unit. Its cooling capacity may vary from 3 to 220 tons (10 to 774 kW), and
its supply volume flow rate may vary from 1200 to 80,000 cfm (565 to 37,750 L/s). An air handler
of a typical rooftop packaged unit consists of mainly a casing, an indoor fan, DX coils, filters, a
mixing box, and controls; a gas-fired heater, a relief or return fan, and a humidifier are optional. The
construction and characteristics of the casing, fans, filters, and outdoor air intake and mixing box
are similar to those discussed in AHUs.
Curb. A rooftop packaged unit is mounted on a curb which is a perimeter frame supporting the
unit. A curb is often made of galvanized or aluminized steel sheets or angle iron and sits on a deck.
It usually has additional structural support at or beneath the deck. On the top of the curb, there is
always a mating flange that matches the size of the rooftop unit with a sealing gasket to provide a
weatherproof joint. Curbs are either factory-prefabricated or field-assembled.
Curbs should be tall enough above the structural support for a sloped roof. The manufacturer
and the structural engineer should be consulted to ensure that the unit can resist the local anticipated
wind pressure. The requirements of National Roofing Contractors Association (NRCA)
should be followed.
DX Coils. For a specific model and size of rooftop packaged unit, the coil surface area is a fixed
value. DX coils are usually of two, three, and four rows (except makeup units) with a fin spacing of 12
to 17 fins/in. (1.5- to 2.1-mm fin spacing). For large units, two separate refrigerant circuits with their
own coils, associated expansion valves, and distributors are often used for a better capacity control.