Specifications

90 Control and Automation Solutions Guide
a fuel-burning capability for heating
and a refrigeration unit for cooling.
MAUs dier from AHUs in that they
use only fresh outside air. They do not
use return air from the building. These
are often used for commercial kitchen
ventilation, laboratory ventilation, and
anywhere large amounts of fresh air are
needed to replace contaminated air.
Sensors for these systems include air
ow, inlet and outlet air temperatures,
air pressures at various points such as
across lters, coil temperatures and
pressures, humidity sensors (DS1923),
and fan speed sensors (see Position
Sensors in the Recommended Solutions
table at the end of this chapter). Controls
include pump and blower motor-speed
commands to the motor drive, heater
controls for RTUs, AC controls, humidier
controls, and damper settings. Filter
replacement needs are detected by
measuring the pressure drop from
the input side to the output side at a
nominal air ow rate (see the Pressure
Sensors and Weigh Scales (Force Sensing)
section in the Sensors chapter).
Variable Air Volume (VAV) Units
A single VAV unit typically supplies
heating or cooling to a number of zones.
Each zone has an air terminal unit that,
under thermostatic control, opens or
closes as needed to allow a variable
ow rate. The VAV unit usually gets its
air supply from an AHU. VAV units in
cooling mode typically cool the air to
a xed temperature (55°F, 13°C) while
having the ability to vary the ow rate
using variable-speed motor drives. The
combined eect of these terminal units
ow restriction results in a total ow
rate needed through the main plenum
from the VAV unit. The main plenum
pressure is simply sensed and controlled
to a constant pressure by varying
the speed of the VAV unit’s blower.
Legacy pneumatic controls have
largely been replaced with electronic
controls, digital communication,
and networking to the BAS.
Motor Considerations
Fan and blower motors have traditionally
been the AC induction type (when
3-phase AC power is available) and VFDs
to control their speed; their eciency
is quite good, but eciency loss is still
experienced when the motor is run
at less than full speed and less than
full power. The situation is even worse
when 1-phase AC power is all that is
available for small air-conditioning units.
To attain dierent speeds, use motors
with multiple taps or silicon-controlled
rectiers (SCRs). Either device will
delay the power pulse to the motor for
each AC cycle. However, both are less
ecient. Because of this, BDC motors
are increasingly being used. They oer
both highly controllable speeds and
high eciencies over a wide speed
and load range. They are also simple in
construction, with only the controller
portion being complex. Of course,
the AC line needs to be rectied, but
this is quite simple and inexpensive
to do with diodes and a capacitor.
Constant Air Volume (CAV)
VAVs are replacing CAV systems due to
the former’s higher eciencies in large
installations, but CAVs are still found
in smaller installations. Single-speed
motors in CAVs provide constant rates
of air ow, but the temperature of the
air varies. There are several methods
to control the air temperature, one
of which is mixing cooled air with
heated air in varying proportions.
Fan Coil Unit (FCU)
An FCU is a lower installed cost
alternative to central heating systems
and their associated duct work. FCUs are
stand-alone room units that contain a
fan, coil (for heating or cooling), electrical
power, and fuel-supply connections.
Simple thermostatic control is common,
with no connection to any central BAS.
However, connection is possible if
communications interfaces are provided.
Humidifiers
Humidiers are often needed in cold
climates where the outside air is very
dry due to moisture condensing (and
freezing). Increasing the humidity can
save energy. Because humid air feels
warmer than extremely dry air, room
temperature does not need to be as
high. Humidiers can use a variety of
methods such as evaporative water
trays, ultrasonic water atomization,
and steam generation “vaporizers.”
Hard water causes issues for all types
of humidiers. As water is converted to
steam in evaporative humidiers and
vaporizers, a residue accumulates in
the water reservoir as scale deposits.
Atomizer types send the hardness,
and any pathogens, into the air along
with the water, which, when the water
droplets evaporate, causes the calcium
dust to settle out and accumulate
on anything nearby. Distilled water
or water softening mitigates these
problems. Also, sensors are needed for
water level and humidity detection. In
building-sized HVAC systems, steam
is often available and this sterile
source of humidity can be added in
controlled amounts as needed. However,
as with other types of humidiers,
hard water creates challenges.
Dehumidifiers
Dehumidiers use mechanisms very
similar to refrigeration units. They
temporarily chill the air, which causes the
moisture to condense and be collected,
followed by the air being re-warmed
from the condenser heat that was
extracted. Then the dried, re-warmed
air is simply sent back into the room.
The water extracted is “distilled” water
of high purity, but low volume in all
but the largest systems in very humid
climates. It is often simply discarded.
Air Quality Monitor
Monitors of various types are
needed in homes and buildings to
protect the occupants’ health and
safety. Stagnant air can cause the
spread of sickness and allergies. In
cases of high humidity, insucient
ventilation can promote the growth
of bacteria and fungi such as molds.