|
— by James Piper
New equipment and systems that incorporate technology not available just a few years ago are
helping maintenance and engineering managers improve the performance of their building HVAC
systems. These advances offer managers a way to achieve better energy efficiency, improved
system reliability, decreased maintenance costs and better environments for building
occupants.
These new applications have impacted nearly every aspect of maintenance, from training to the
equipment mechanics use in diagnosing system problems. There are a few areas, however, where
the impact of technology has been particularly great — the design and operation of boiler
systems, the performance of building temperature control systems and the operation of HVAC
system motors.
For a discussion of advances in chiller technology, see the accompanying article.
Building temperature control systems
Temperature control systems have gone digital. Microprocessors have replaced pneumatic relays
and controllers in most new building construction and renovation projects. Mangers are
converting many existing buildings to gain the benefits offered by these direct digital
control (DDC) systems.
DDC systems offer managers many advantages over their pneumatic counterparts. Their higher
level of accuracy all but eliminates such errors as offset, setpoint overshoot and setpoint
drift. The results are increased comfort for building occupants and energy savings for the
maintenance manager.
The systems can accept input from a wide range of sensors, so they can perform more complex
control strategies than were possible with pneumatic-based systems. They use software rather
than hardware to initiate control actions, making them flexible and easy to change.
Since the systems are microprocessor based, their information can be tapped into practically
anywhere in the system. Centrally located operators can monitor and control any activity
anywhere in the system. Mechanics can tap into the system with portable computers or
diagnostic equipment when servicing equipment. Remote operators can tie into the system by
telephone modem and perform any function normally performed by the central operators.
The widespread success of DDC systems has led to another major innovation in temperature
controls — interoperability. Building operation is typically supported by a number of
independent systems — fire safety, building security, telecommunications, maintenance
management and temperature control. Each system requires its own set of sensors,
microprocessors and data transmission system. Information in one system generally is
unavailable for use by another.
Advances in building automation technology are changing this situation. Standards have been
developed to govern how components in systems function and communicate with other components
and their controlling microprocessor. Systems that adhere to the standard can integrate these
independent building functions into one comprehensive building automation system that can
share information across functions. This ability will result in the installation of
interoperable systems.
Interoperable systems offer several advantages over current independent systems:
By sharing information over one system, installation costs will be reduced.
Since components from different manufacturers can be mixed and matched under interoperability
standards, managers will have a wider selection of options available at more competitive
prices.
A common interface reduces training time.
System capabilities are enhanced as the result of the availability of a wider range of data.
Motors and drives
A significant advance that will impact HVAC system energy use is the development of high-
efficiency motors. One focus of energy saving programs has been conserving fan motor energy
by shutting down fan systems when they are not required. Now, through the use of high-
efficiency motors, maintenance managers can save additional energy while the fan is operating.
The Energy Policy Act of 1992 (EPACT) addressed a number of energy issues, including motor
efficiency. EPACT established minimum efficiency standards for general-purpose polyphase
induction motors rated between 1 and 200 horsepower — the type and size of motors commonly
found in building HVAC fan systems.
Any motor made after October 1997 must conform to the standards. The amount of the
improvement varies with motor horsepower rating, typically 2-6 percent. HVAC systems are
ideal candidates for these higher-efficiency motors, as they have a large number of medium to
large-horsepower motors that have a high number of operating hours per year. Even a 2 percent
increase in motor efficiency translates to significant energy savings both on an annual basis
and over the life of the motor.
The use of high-efficiency motors in HVAC applications will produce significant energy
savings. Facilities can achieve even greater savings by using variable frequency drives
(VFD). In most HVAC applications, single-speed motors are used to drive fans, pumps and
chillers.
As the system load changes, valves, dampers and inlet guide vanes control the flow of the air
or fluid. Restricting the system’s flow rate reduces the amount of energy drawn by the motor,
but only slightly. VFDs change the voltage and frequency of the power to a motor to slow the
motor down and produce larger savings.
With most HVAC systems operating 90 percent of the time under part-load conditions, savings
achieved through VFDs can be very significant — typically, 15-25 percent on an annual basis
for pumps, fans and chillers.
VFDs offer an important side benefit to maintenance managers. Every time a motor is started,
its coils are subjected to stresses as the result of heating. Over time, these stresses
contribute to the breakdown of the insulation in the motor, resulting in failure of the
windings.
VFDs can be programmed to perform a soft start, where voltage and frequency are reduced
during motor startup to control torque and speed. The result is lower temperatures in motor
windings and reduced stress in the load coupling.
Boiler technology
The most important change that technology has brought to boilers in the past few years is the
development of digital controls. Boilers, one of the last strongholds for manual and
pneumatic control systems, are finally going digital. Today’s boiler control systems make
extensive use of microprocessors to monitor and control equipment performance. The result is
boiler systems that offer:
safer operation
enhanced energy efficiency
lower maintenance costs
greatly improved diagnostic capabilities.
The systems also allow operators to monitor and control multiple boilers at a central
location, removed from the boiler installation.
One key feature of digital controls for boilers is their ability to accurately manage the
operation of a boiler over its range of operation, both when the load is steady and when it
is changing. Conventional pneumatic and manual control systems are inclined to lag actual
operating conditions, resulting in errors that increase energy use. The typical digital
boiler control system provides between 10 and 100 times more accurate levels of control than
can be provided by the best pneumatic system.
The constant monitoring of conditions in the boiler provides the control system with feedback
necessary to closely regulate fuel and air flow to the burner, resulting in more efficient
fuel combustion.
Conventional pneumatic control systems typically are required to operate with a minimum of 10
percent excess air to prevent boiler sooting during sudden load changes. Digital control
systems, with their constant monitoring of combustion conditions and ability to rapidly
respond to changing conditions, can operate with excess air rates as low as 5 percent. Each
percentage point decrease results in a 0.25 percent increase in boiler efficiency.
Digital control systems for boilers also monitor between 10 and 20 times more operating
parameters than conventional pneumatic systems. This higher level of feedback to boiler
operators and maintenance personnel enhances their ability to detect, diagnose and correct
problems.
Tracking the data over time helps establish baseline performance characteristics for the
boiler. This baseline data can be used to evaluate boiler performance over time, identifying
slowly developing problems before they become costly to correct.
One key benefit of digital boiler control systems is the accessibility of boiler operating
data. Gone are dedicated panels that monitor and control the operation of a single boiler.
All boilers in a facility can be monitored and controlled from one location.
Systems can be accessed by remote PC and telephone line. System data can be made available to
other systems, such as energy-use monitoring and billing systems. Boiler controls can be
integrated with the operation of building automation systems, allowing even more efficient
operation.
Boiler control systems are not likely to stop in their evolution. As system capabilities
grow, additional sensing points will be added to help improve the operation of boiler
auxiliary equipment, such as feedwater pumps and fans. Even closer levels of control will be
implemented, resulting in improvements in operating efficiency and emission controls.
Diagnostic software will be enhanced to more closely track performance, helping identify
problems while they are still relatively small and inexpensive.
Future developments
New technology applications have made impressive progress in the effort to reduce building
HVAC system energy requirements. Today’s systems are performing more effectively and
efficiently than those installed as recently as five years ago.
Tomorrow’s systems will do even better. Boilers will be fitted with more intelligent control
systems to improve operating efficiency and diagnostic abilities.
Building control systems will become more fully interoperable, leading to even more energy
efficient operations. Manufacturers will improve motor designs beyond what is required by
EPACT, resulting in even higher efficiency motors. VFDs will continue to evolve, providing
even higher levels of part-load efficiencies for motors.
James Piper is a freelance writer and facilities management consultant based in Bowie, Md.
Savvy Refrigerant Strategies
Many maintenance managers have moved quickly to convert or replace chillers using CFC-based
refrigerants, but others have not. It is estimated that more than two-thirds of the large-
building chillers in operation today still require CFC-based refrigerants. With supplies of
CFC refrigerants dwindling and prices increasing, it is important that managers take the
initiative and develop a plan for the conversion or replacement of their remaining units.
The major elements of a refrigerant management plan include the following:
provide mechanic training and certification
develop an list of CFC-using equipment
keep accurate records to track refrigerant use
assess equipment condition and efficiency
prioritize equipment conversion or replacement
implement a leak testing program for all chillers
implement operating and maintenance practices to boost chiller efficiency and save
refrigerants.
An aggressive program for refrigerant management is required to meet environmental
requirements, to ensure that any capital investments in chiller systems will provide the most
positive return and to make current supplies of CFC-based refrigerants last as long as
possible.
— James Piper
|
