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5 HVAC Equipment, Systems,
and Selection
New options for HVAC equipment and systems are
being introduced continuously, and the design professional
must be knowledgeable about both existing and
emerging technologies. This chapter will discuss a number
of the most common types of equipment, beginning
with the simple and progressing to more complex equipment
and the systems in which it is applied. Designers
are concerned with selecting equipment that will meet
the performance needs of the building in the worst case
and during seasonal variations. Equipment performance
is often presented in manufacturers’ literature at conditions
that vary considerably from local design conditions.
For example, unitary cooling equipment
performance may be presented at near maximum recommended
indoor airflow rates to achieve optimum efficiency
values. In humid climates, operating air
distribution systems at lower airflow rates (300–
350 cfm/ton) is a viable option for improved equipment
latent capacity to enhance the system’s ability to maintain
lower indoor relative humidity. Data from manufacturers
must be available to correct for a variety of nonstandard
conditions, and engineers must know how to
use them effectively.
This text will provide performance data at a variety
of conditions to complement the diagrams and discussions
of various equipment and systems. Performance
data cannot be presented for all equipment and systems
in all ranges of available sizes and operating conditions.
However, a spreadsheet will be discussed that is able to
correct conventional equipment performance to within
reasonable accuracy. This information and spreadsheet
tool are provided to reduce the uncertainty in equipment
selection that can lead to equipment oversizing. Equipment
oversizing results in added cost not only in the
equipment but throughout the system (i.e., larger ducts,
larger pipes, larger electrical circuits, etc.). Furthermore,
inefficiency and lack of comfort will result due to frequent
on-off-on cycles (unitary) and/or operation at
near-minimum efficiency (low part-load) conditions.
Multiple organizations have responsibility for
developing and revising the test standards and ratings of
HVAC equipment (e.g., ASHRAE 2003, chapter 56).
ASHRAE develops both testing and rating standards for
a broad set of equipment, and the standards are
approved by ASHRAE as well as the American National
Standards Institute (ANSI). The Air-Conditioning and
Refrigeration Institute (ARI) has jurisdiction over
equipment that involves refrigeration devices (air conditioners,
refrigerators, freezers, coolers, etc.), including
equipment that heats air (heat pumps) and “heat pump”
water heaters. A parallel organization that represents
combustion equipment is the American Gas Association
(AGA), which develops standards in cooperation with
ASHRAE and ANSI. These organizations cover the
largest portion of the equipment discussed in this chapter.
Later chapters will discuss other organizations and
rating standards for other types of auxiliary HVAC
equipment.
UNITARY PACKAGED EQUIPMENT—
RESIDENTIAL AND SMALL COMMERCIAL
The term unitary suggests that all of the cooling and
heating equipment is housed in a single compartment
that requires no assembly at the site. Cooling-only systems
have all the basic components (compressor, evaporator,
condenser, expansion device, fans, motors,
controls, and cabinet) assembled and charged at the factory.
Heating and cooling systems may also have an
integrated furnace (electric or fossil fuel) or a reversing
valve so that heat pump operation is available. Many
packaged systems are nonducted and discharge air
directly to the conditioned space. Ducted models are
also common when multiple rooms are served by a single
packaged unit that is placed near the building or on
the roof (rooftop unit—RTU). Two advantages of packaged
units of all types are the reduced potential for
refrigerant leaks, system contamination, or improper
charge that may result with site assembly installations
and the reduced environmental impact of smaller refrigerant
charges typically required in unitary equipment.