Specifications

86 Control and Automation Solutions Guide
usage needs, such as known low
DHW usage in a building during the
night, it can allow the temperature
to drop, which could save energy.
Boilers create steam that is often used
by the HVAC system. Some “combi”
boilers not only create steam, but also
DHW. There are many types of boilers
and steam generators with a variety of
instrumentation and control needs.
In the past, boiler explosions were
quite common because there were
no sensors or indicators to warn of
dangerous conditions. Boilers that are
allowed to run dry become extremely
dangerous as they overheat quickly. If
any water is introduced while the boiler
is still hot, the water instantly ashes
to steam and its rapid expansion can
cause a violent explosion. One of the
purposes of implementing sensors
is to reduce the risk of these failures.
Today boiler explosions are rare due
to the use of sensors and automated
controls provided by a BAS. Boilers
can be monitored completely and
continuously, and problems responded
to automatically. Some boiler types
operate at very high pressures and
temperatures. There are “superheated
steam boilers” and “supercritical steam
generators” operating in the 1500°C
range and at pressures in the 3000psi
range. A boiler is specied by stating
the steam production rate. In the
U.S., the metrics for a boiler’s output
might be 1000lb/hr at 100lb/sq in and
500°F. Multiple-story tall boilers in
industrial plants can produce as much
as 300,000lb/hr at 350psi and 600°F. The
steam generated from these is most
commonly used to power turbines that
run alternators to generate electricity.
Boilers need water level sensors, pH
sensors, temperature sensors (like the
MAX6603 platinum RTD-to-digital
converter), pressure sensors (see the
Pressure Sensors and Weigh Scales (Force
Sensing) section in the Sensors chapter),
steam ow-rate sensors, water ow-
rate sensors, and valve position sensors
(like the MAX9621 Hall-eect sensor
interface and the MAX9924 VR sensor).
There are numerous types of valves used
(low-water cuto valve, overpressure
safety valve, blowdown valve, check
valves), circulating pumps, pressure
regulators, chemical injection systems,
fuel-feed systems, and rebox controls.
Signals from the sensors and controls
to these components are handled by
PLCs or similar controllers built into
the boiler (see the Programmable Logic
Controllers (PLCs) chapter). Operating
data and system level controls are
communicated to and from the BAS.
Heat Exchangers
In some locations, “district heating” is
used. In these cases, “waste” heat from
industrial installations produces enough
heat for the heating of water and space
in local neighborhoods.
Heat exchangers transfer the heat
from the supply system to potable
water systems in the residence,
thereby preventing the two
water supplies from mixing.
To maximize eciency of heat transfer
across the boundary, heat exchangers
often contain many small tubes carrying
one uid while being immersed in
another uid. These can foul over time.
Partial clogging can cause signicant
pressure drop across the inlet to
outlet ports on either side of the heat
exchanger. Pressure-drop sensing
is needed in commercial and larger
residential applications. Pumps can be
employed to overcome the inherent
ow restriction in heat exchangers,
and variable-speed drives for these
pumps can be used to provide more
control over the speed of the ow
to optimize the heat transfer and
the energy used by the pump motor
(see the Motor Control chapter).
Boiler room in a commercial facility.