Handbook of air conditioning and refrigeration / Shan K

16.12 CHAPTER SIXTEEN
● To improve indoor air quality, medium- and high-efficiency filters should be used to provide an
acceptable IAQ and to protect coils and air distribution devices. Dirty coils and condensate pans
significantly degrade the IAQ. A prefilter must be installed to extend the service life of high- and
ultrahigh-efficiency filters, or gas absorbers.
● Use an air or water economizer to save energy. Use indirect and direct evaporative coolers to
replace part of the refrigeration if they are applicable and economical. Evaporative cooling is
discussed in Chap. 27.
● Select an AHU with adequate specific and safety controls and diagnostics.
16.4 PACKAGED UNITS
Types of Packaged Unit
Rooftop Packaged Units
A packaged unit (PU) is a unitary, self-contained air conditioner. It is also the primary equipment of
a unitary packaged system. A packaged unit not only conditions the air and provides the motive
force to supply the conditioned air to the space, but also provides gas heating, or electric heating,
and refrigeration cooling from its own gas-fired furnace and refrigerating equipment or from its
own heat pump. It is actually the primary equipment in an air conditioning system.
A packaged unit is always equipped with DX coil(s) for cooling. This characteristic is the
primary difference between a packaged unit and an air-handling unit. The portion that handles
conditioned air in a packaged unit is called an airhandler, the air system of the packaged system, to
distinguish it from an air-handling unit, as shown in Fig. 16.4. Another portion is a condensing unit,
the refrigerant plant. Refrigerants HCFC-22, HFC-134a, HFC-404A, HFC-410A, HFC-407A, and
HFC-407C are now used in packaged units. Most PUs are factory-built standard fabrication units.
A packaged unit can be either enclosed in a single package or split into two units: an indoor air
handler and an outdoor condensing unit. A packaged unit can also be a packaged heat pump; most
are air-source heat pumps. In a packaged heat pump, in addition to the fan, DX coil, filters, compressors,
condensers, expansion valves and controls, there are four-way reversing valves to reverse
the refrigerant flow when cooling mode operation is changed to heating mode operation.
The construction and size of a packaged unit depend mainly on its model and cooling capacity,
in refrigeration tons or Btu/h (W). Packaged units can be classified according to their place of
installation as rooftop packaged units, indoor packaged units, and split packaged units. Among
these units, the rooftop packaged units are most widely used in commercial buildings.
A rooftop packaged unit is mounted on the roof of the conditioned space, as shown in Fig. 16.4. It
is usually enclosed in a weatherproof outer casing. The mixture of outdoor air and recirculating air
is often conditioned in the rooftop packaged unit and supplied to the conditioned space on the floors
below. Based on the types of heating and cooling sources, rooftop packaged units can be subdivided
into the following:
● Gas/electric rooftop packaged unit. In this unit, heating is provided by a gas-fired furnace, and
cooling is provided by electric-driven reciprocating or scroll compressors.
● Electric/electric rooftop packaged unit. In this unit, heating is provided by electric heating coils
and cooling by reciprocating or scroll compressors.
● Rooftop packaged heat pump. In this unit, heating and cooling are provided by the heat pump,
with auxiliary electric heating if necessary.