Airflow Control Fundamentals; Need For Airflow Control; What Is Airflow Control; Types Of Airflow Systems - Honeywell AUTOMATIC CONTROL Engineering Manual

AIRFLOW CONTROL FUNDAMENTALS

NEED FOR AIRFLOW CONTROL

Proper control of airflow is important to physiological
principles including thermal and air quality considerations. Air
distribution systems, containment pressurization, exhaust
systems, and outdoor air dilution are examples of airflow control
systems used to meet ventilation requirements. Life safety
requirements are also met with fire and smoke control systems
using airflow control functions. Therefore, an understanding
of airflow control is required to provide the various locations
in a building with the necessary conditioned air.
One means of maintaining indoor air quality is to dilute
undesirable materials (e.g., carbon dioxide, volatile organic
compounds) with outdoor air. It is important to understand the
control of outdoor air airflow rates in order to:
— Increase outdoor airflow rates when needed for dilution
ventilation
— Prevent excessive building and space pressurization
— Minimize outdoor airflow rates when possible to limit
energy costs

WHAT IS AIRFLOW CONTROL

In HVAC systems, a well designed combination of fans, ducts,
dampers, airflow sensors, static pressure sensors, air terminal
units, and diffusers is necessary to provide conditioned air to
the required spaces. The function of airflow control is to sense
and control the static pressures and airflows of the building.
The static pressures occur in ducts and building spaces; airflows
occur in ducted air supplies, returns, and exhausts.

TYPES OF AIRFLOW SYSTEMS

An air handling system can provide heating, cooling,
humidification, and dehumidification as well as variable
quantities of outdoor air. Air handling systems can be classified
as single-path or dual-path. The single-path system has all
heating and cooling coils in series in a duct. The single duct
supplies all terminal equipment. The dual-path system has a
cooling coil in one duct and a heating coil (or just return air) in
another duct. Both ducts supply dual-duct terminal equipment
or multizone dampers.
These systems are further classified (ASHRAE 1996 HVAC
Systems and Equipment Handbook) as follows:
Single-Path Systems:
Single duct, constant air volume
Single zone systems
Reheat systems, single duct-variable air volume
Simple variable air volume
Variable air volume, reheat
BUILDING AIRFLOW SYSTEM CONTROL APPLICATIONS
Single duct, variable air volume-induction
Single duct, variable air volume-fan powered
Constant fan, intermittent fan
Dual-Path Systems:
Dual duct, single fan-constant air volume
Single fan, constant air volume reheat

Variable air volume

Multizone
The more common types of air handling systems are:
Single duct, variable air volume
Single duct, constant air volume
VARIABLE AIR VOLUME
A Variable Air Volume (VAV) system controls primarily the
space temperature by varying the volume of supply air rather
than the supply air temperature (Fig. 1). The interior zones of
most large buildings normally require only cooling because of
occupancy and lighting loads. Air terminal units serve these
zones and operate under thermostatic control to vary the airflow
in the individual spaces to maintain the required temperature.
The perimeter zones can have a varying load depending on the
season and exposure. Heating may be supplied via reheat coils
that operate under thermostatic control while air terminal units
maintain minimum airflow.
Airflow in the supply duct varies as the sum of the airflows
through each VAV terminal unit varies. In light load conditions,
the air terminal units reduce the airflow. As more cooling is
required, the units increase airflow. Air terminal units typically
have controls to limit maximum and minimum airflow and
compensate for variations in supply duct static.
To ensure that all air terminal units have sufficient pressure
to operate, a supply airflow control system is required. To
monitor duct static, static pressure sensors are installed near
the end of the supply duct. When VAV terminal unit dampers
open, the static drops in the supply duct. The static pressure
sensor detects the static pressure drop and the airflow control
system increases the supply fan output. The opposite occurs
when the VAV terminal unit dampers close.
Another VAV system feature is that the difference in airflow
between the supply and return fans (and not the position of the
outdoor air damper) determines the amount of minimum outdoor
air ventilation delivered by the supply system. For example, Figure
1 shows a single duct VAV system for a building with the fans on
and outdoor air dampers at minimum position. The design
condition for the supply fan is 30,000 cfm and the return fan is
25,000 cfm. The 5,000 cfm difference is lost through kitchen and
restroom exhaust fans (2,000 cfm) and exfiltration (3,000 cfm).
Thus 5,000 cfm of outdoor air must be brought in through the
supply fan to make up the difference. Depending on mixed air
system pressure drops, even is all dampers in the mixed air system
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ENGINEERING MANUAL OF AUTOMATION CONTROL