Biomedical Engineering Reference
In-Depth Information
absorption chillers—are the most common choices.
Other less common strategies are coupling a gener-
ator directly to the electric chiller or some form of
thermal storage. Any of these strategies should be
part of a larger energy strategy.
Trying to guess what energy prices will do over
the course of the next five years or even five
months is nearly impossible. The best recommen-
dation for managing the volatility of energy prices
over the life of an hvac system is the same advice
given for good fiscal planning—diversify. A care-
fully thought-out energy strategy that does not rely
on any single energy source for all heating or
cooling needs can have many benefits.
The ability to switch to the least expensive
fuel to produce chilled water during times of
peak demand will lead to lower utility costs than
the competing hospital who has decided to take
the lowest first cost approach to chilled water
and steam production. A secondary benefit to the
diversification of energy usage is the ability to
produce chilled water for the critical areas of the
hospital with minimal investment when compared
to the cost of electrically driven chillers or direct-
expansion equipment connected to the emergency
power system.
the emergency generator may be different than
systems connected to the normal power system.
AHUs, fans, and pumps controlled by VFDs will
contribute harmonic distortion into the emergency
power system that will lead to a degradation of the
generator. The emergency power system is much
less forgiving than the larger normal power system
and the harmonic distortion will reduce the service
life of the generator if the distortion is not properly
mitigated.
A separate harmonic analysis should be
conducted on the emergency power system to
ensure the system is in compliance with IEEE
519 “Recommended Practices and Requirements
for Harmonic Control in Electric Power Systems.”
18.7 Commissioning
A hospital is a complex building with many
systems that must interact for the facility to operate
under a number of different conditions. The conti-
nuity of hvac services during a power outage is
dependent on the interdisciplinary coordination of
design professionals, owners, and subcontractors.
This is a reality that sometimes yields unintended
results. Building systems commissioning can be
part of the solution to ensure that the hvac and
building systems perform according to the design
intent. These services can be a small price to pay
when you reflect on the potential consequences.
18.6 Emergency Generator
Coordination
Another issue that must be considered in the design
of a safe and reliable hvac system is the effect of the
major hvac systems on the generator. It is clear that
if the emergency generator is running and all of the
hvac motors were transferred onto the emergency
generator simultaneously, the generator would
probably shut down due to the inrush current.
To avoid this situation, it is advisable to inte-
grate a staggered restart into the sequence of
operation of the major hvac equipment after a
loss of normal power. A similar situation may
occur if the variable-frequency drives (VFDs) have
across-the-line bypass as an option. Larger motors
should have some form of reduced voltage starters
included as the bypass.
The design criteria for systems that have
motors controlled by VFDs and are connected to
18.8 Conclusion
Understanding the role of a hospital in the
community during an emergency situation where
normal power is not available and determining
what functions within the hospital must remain
operational in that emergency are the basis for
many of the hvac design decisions on a project.
The right decisions will ultimately provide the
hospital with a safe and reliable hvac system
during a loss of normal power. Commissioning
the building after the construction is complete
is the best insurance the owner, engineer, and
contractor have that the system will operate
as intended when the inevitable loss of power
occurs.
