Environmental Engineering Reference
In-Depth Information
Exhaust Air
Damper Actuators
Outdoor
Air Intake
Outdoor Air Damper
Mixing
Chamber
Fan
Filter
Freeze Stat
Mixed Air Stat
Heating Coil
Cooling Coil and Drip Pan
Return Air
Grille
Supply Air
Diffusers
Outdoor Air
Temperature Sensor
Humidifier
Air Handling Unit
Figure 11.7
HVAC system providing conditioned and ventilation air to a single
occupied space. (From USEPA,
Building Air Quality Manual
, EPA 402-F-91-102, Wash-
ington, D.C., 1991.)
to be manually adjusted. In most modern units, such adjustments are made
automatically by use of thermal or CO
sensors, or timers.
2
Buildings are climate controlled and ven-
tilated with a variety of HVAC systems. There are three basic system types
used in buildings, each with its own variations. These include all-air,
air-water, and all-water systems.
a.
Types of HVAC systems.
i. All-air systems. In all-air systems, air is conditioned as it passes
over heating and cooling coils. It is then delivered to occupied spaces
through a single duct or through individual hot and cold ducts to a mixing
box (and then to occupied spaces). Air flow through these systems may be
at a constant rate (constant-air-volume [CAV] systems), or air flow may be
varied to individual spaces (variable-air-volume [VAV] systems). Simplified
CAV and VAV system designs are illustrated in
Figures 11.8
and
11.9
.
In CAV systems, temperature is varied to meet heating and cooling
needs. Supply air flows at a fixed and constant rate. In VAV systems, air is
initially conditioned in an AHU. It is delivered to different zones or spaces
through VAV boxes which regulate air flow and temperature in response to
heating/cooling demands of individual zones. Both systems provide out-
door air for ventilation and are able to control the relative amount of outdoor
and recirculated air that spaces are provided. Variable-air-volume systems
were developed in response to energy management concerns and, when
properly installed and operated, can reduce building energy consumption
