Specifications

91Environmental Automation
Smoke detectors are the most often
seen air quality monitors in homes and
hotel rooms. Now carbon monoxide
detectors are required by law in
California in dwelling units, especially
rentals, that have (1) a fossil fuel-
burning heater, appliance, or replace
(for example, a gas stove or water
heater), or (2) an attached garage.
HVAC systems used in BASs monitor
CO
2
levels to ensure that enough air
is injected from the outside to keep
CO
2
suciently low and oxygen levels
suciently high. Other poisonous gasses
may need to be detected in various
industries to assure worker safety from
fumes of toxic chemicals, particulates,
and other airborne pollutants (volatile
compounds, combustion products,
radioactivity, etc.). The instruments and
technologies used for detecting these
pollutants vary widely and will not be
covered here, but the outputs from these
detectors are often tied into HASs or
BASs. The responses to these detectors
may be as mundane as HVAC system
adjustments to increase outside airow,
or they may trigger alarms and response
tied into the re alarm system and/or
other emergency response systems.
Specialty ltering and ventilation for
some industrial processes are critical to
the success of the process. Clean rooms
for integrated circuit manufacturing
are a prime example. The outside
urban environment typically contains
35,000,000 particles per cubic meter that
are 0.5µm and larger. This corresponds
to an ISO 9 clean room. An ISO 1 clean
room allows no particles at all in that
size range, and only 12 particles per
cubic meter of 0.3µm or less. To meet
these standards, HVAC systems with
special lters are needed. Some clean
rooms also control humidity to low
levels, necessitating the use of ionizers
to prevent ESD problems. Air locks, air
showers, and positive pressure systems
are also used to maintain the air quality
standards needed for some clean rooms.
Detectors of minute quantities of
anything typically produce small, high-
impedance signals that require high
amplication before being digitized.
Since noise and oset are always
amplied along with the signal, a very
precise amplier with low input oset
voltage and low noise is needed at the
front end of the detector. (If the noise
and oset are high frequency, a DC-
blocking capacitor can be used, but
noise and oset are usually not high
frequency in these types of systems.)
Maxim has several new op amps to
address these needs. The MAX9632
single op amp, for example, or
the MAX44251 dual op amp oer
extremely low input noise and
oset voltage while operating over
wide supply voltage ranges.
Refrigeration
Mentioned earlier as a part in several
systems, a refrigeration unit can be
used to cool air temperatures, or it can
be a stand-alone refrigerator or freezer
for cold storage. No matter what the
size or intended use, the fundamental
operating principals are the same.
Compressor-based refrigerators,
which use the “vapor-compression
refrigeration” cycle, are very common
in the homes and commercial
establishments of developed
countries. In contrast, the “absorption
refrigeration” cycle is used when waste
heat is available or where electricity
is not reliable or available. The latter
is also silent, whereas the compressor
running the vapor-compression cycle
can produce signicant noise during
operation. In large industrial plants,
gas turbines to generate electricity and
hot water, and the “waste” heat can
be used by absorption refrigerators.
When all three of these are generated
by a single plant—electricity, useful
heat, and useful cooling—it is called
trigeneration. Cogeneration is the
simultaneous generation of electricity
and useful heat that can be the source
of heat in district heating systems,
for example. Large district heating
systems are found in Scandinavia,
Eastern Europe, and New York City.
Vapor-Compression Cycle
Vapor-compression refrigeration uses
a refrigerant that transitions between
a liquid and a gas at temperatures and
pressures for the particular refrigeration
application. In general, when a liquid
turns into a gas through evaporation,
signicant quantities of heat are
drawn from the surroundings, thereby
cooling the area. Conversely, when a
gas condenses into a liquid, the “heat of
vaporization” is released back into the
surroundings, which warms up. By using
a power source to run a compressor,
the heat can be forced to ow in the
opposite direction from its natural ow
from warmer regions to cooler regions.
This is essentially a “heat pumping”
process. Heat pumps use the same
refrigeration cycle as air conditioners
and refrigerators, but instead, they
retain heat to warm an interior space
and emit cool air into an exterior area.
The refrigerant is in a closed loop. In
the rst step, gas is compressed in the
compressor, resulting in a high pressure
vapor—the vapor-compression cycle.
Because it is compressed, this vapor has
an elevated temperature. It then enters
a condenser where the vapor is cooled
by either water or air, causing the gas
FAN
WARM AIR
EXPANSION
VALVE
LIQUID + VAPOR LIQUID
VAPORVAPOR
COMPRESSOR
COLD
AIR
*CONDENSER CAN BE WATER-COOLED OR AIR-COOLED.
EVAPORATOR CONDENSER*
Single-stage vapor compression refrigeration cycle.