Designing for healthcare patient and
critical environment spaces is strongly dictated by strict environmental and
safety standards. However, possibly one of the most important components that must
be taken into consideration is the one you can’t see. Effective airflow design
(EAD) not only helps meet airflow change and industry standards, but is
critical in limiting the contraction of airborne illnesses and can reap
considerable cost savings for facilities. When designing for healthcare
facilities, it is important to abide by airflow and air quality standards, in
particular for three priority rooms for EADs: Hybrid operating rooms, patient
rooms and isolation rooms.
Standards
and Approaches
To determine what airflow plan is right
for a space, engineers first meet with a designer and give them a general
layout for the room, diffuser size and placement, requirements for airflow and
other details. Although the designed layout has a big impact, the effectiveness
of an airflow design boils down to the velocity of the air through the space
and what direction it is flowing. In the majority of spaces within the
Healthcare environment the primary objective it to ensure the cleanest air is
supplied first to the patient then into the remainder of the room and that it’s
filtered before it circulates back into the area. As for requirements, most
states (42) have adopted some version of the Facility Guidelines Institute
(FGI) recommendations for healthcare facilities, but each administration has their
own rules and regulations so it’s critical for those involved to be aware of
what standard(s) they’re designing to.
Though it would not be a drastic shift,
this individualistic approach among states to regulations may change within the
next two decades as results of research projects are adopted into code. This
research, commissioned by ASHRAE, FGI, and others entails determining how much
airflow is needed to prevent contamination in certain spaces based on evidence rather than conjecture, which has been the standard practice. The International Code Council (ICC) has formed an
Ad Hoc Committee on Healthcare that is working to ensure standards and codes are
not increasing the cost of construction and operation purely based on assumptions
or outdated practices. This is a key area of focus, since 9 percent of the
annual energy usage in the United States is dedicated to healthcare spaces; of
that usage, HVAC is responsible for half.
Finally, thermal comfort for is
addressed in ASHRAE Standard 55. Since most regulations are concerned with
airflow and air quality, thermal comfort is not a priority. However, a room’s
temperature and humidity is important because it can impact recovery time of
patients as well as the performance of the facility’s staff – an
overly cold or warm environment makes it difficult for surgical staff to
perform at the highest level. ASHRAE Standard 170 also has stipulations as far
as minimum and maximum humidity temperatures. While the scope of Standard 170
includes occupancy comfort, it should not be assumed that meeting the prescriptive
design minimums will ensure compliance with ASHRAE Standard 55. Appropriate
step must be taken to realize thermal comfort in the space for patients, as
well as for visitors.
Hybrid Operating Rooms
Hybrid operating rooms (hybrid ORs) are
surgical areas equipped with advanced medical imaging devices such as CT and
MRI scanners. Incoming air should be HEPA-filtered to minimize the pathogens
entering the space. Hybrid ORs have 30 percent more air changes per hour (ACH) than
catheterization labs. The increased airflow and type of procedures in the space
dictate a different approach to EAD. Rather than conventional or radial flow
diffusers, hybrid ORs utilize unidirectional diffusers so air comes straight in
one direction. These diffusers introduce highly filtered air into a space right
above where critical work is happening. This air then expands out and pushes
the contaminants away. A body’s natural convection can also protect itself from
unclean air, so it is a best practice for diffusers to have very low velocities
that do not disrupt the wound’s convective plume. Recent studies have shown that in some surgery types there is not a thermal plume generated at the
wound site. In these instances delivering clean air at very low velocity is
critical to minimizing entrainment of contaminates since this natural defense
does not always occur.
Design specifications for hybrid ORs
call for diffusers to be located right over operating tables, and to satisfy ASHRAE
Standard 170 the diffusers must cover at least one foot beyond the table and
emit no more than 25-35 CFM/ft2. ASHRAE Research Project 1397: EXPERIMENTAL INVESTIGATION OF HOSPITAL OPERATING
ROOM (OR) AIR DISTRIBUTION results
showed that the unidirectional airflow collapses in towards the table and accelerates
into the operating room as a result of buoyant and gravitational forces. The
amount of collapse and acceleration is affected greatly by the temperature
difference between supply air temperature and room air temperature. Titus
recommends that the diffuser array extend two to three feet. Doing so will
allow for a smaller temperature difference, limiting the collapse and
acceleration so patients, nurses, surgeons and all surgical instrumentation are
covered by the sterile field. This practice helps reduce costly surgical side
infections (SSIs), which make up about 30 percent of all healthcare acquired
infections (HAIs).
Patient Rooms
Like hybrid ORs, patient rooms are
critical spaces that require a high standard of air quality. Designers do not
typically have many major issues designing these rooms; however, when using chilled
beams and displacement ventilation systems intuitive designs can lead to
airflow patterns that are less than ideal. This can be a concern as an
inefficient airflow design fails to minimize the amount of potential particles
and pathogens in the air being circulated or re-circulated through the room,
translating to higher levels of airborne contaminants potentially leading to
HAIs, and thereby raising costs. An EAD in these spaces means lower costs because
patients recover more quickly and there is a higher turnover rate. Facilities
also do not have to treat or retreat patients for something they acquired
during their stay.
Use of chilled beams can be a useful
means of developing an EAD within patient room spaces. The most intuitive design
is to place a 2-way active beam near the patient bed with the throw introduced
into the room perpendicular to the patient’s bed. This is typical for most
active beam deigns, placement over the occupant seeks to minimize air velocity and
create a uniform temperature around the patient for thermal comfort. Recently the
result a CFD study (Comparative Analysis
of Overhead Air Supply and Active Chilled Beam HVAC Systems for Patient Room)
showed that placement of a 1-way beam over the head of patient could
potentially create an airflow pattern that results a single pass system in
regards to airborne particulate in the room. A single pass airflow pattern or
reduced pass airflow patterns strive to minimize the airborne particulates in
the space to reduce HAIs.
Displacement ventilation design also
presents a challenge in some cases. The size and floor level installation of
these diffusers can lead to their installation in corners where they can be
easily blocked by furniture or belongings, significantly reducing their
efficacy. Placement of diffusers on the
wall adjacent to the foot of the bed results in the most effective airflow
pattern. Placing the exhaust above the patient’s bed at a 15 degree angle away
from the head of the bed and towards the foot will be most effective in
removing aerosolized saliva containing potentially viable viruses and bacteria
from the space. Additionally, it is
critical to have the transfer grille to the toilet space installed at least 6
feet above the finished floor to prevent short circuiting. Since the toilet room
is to be negatively pressurized and has a high air change rate a low level transfer
grille could lead to the low velocity air discharged from the displacement
ventilation unit being exhausted from the patient room without addressing the
load in the space.
So why are more facilities implementing displaced
ventilation and chilled beams for projects? Both systems are very effective at
getting air into spaces at the right temperature, exhausting and/or recirculating
it without bringing contaminants back into the occupied space – the primary
goals of EAD. In addition, displacement ventilation systems are extremely
effective in removing pathogens from patient’s bedside areas.
Isolation
Rooms
There are some specialized types of
patient rooms that rely heavily on EAD to achieve their individual goals. These
are Airborne Infection Isolation (AII) Rooms and Protective Environment (PE)
Rooms. PE specifically designed to prevent patients with suppressed immune
systems ( i.e. chemotherapy patients, bone marrow or other organ transplant recipients, AIDS patients). AII rooms are designed to
minimize transmission of airborne infectious diseases from an infected patient
to staff, visitors, and other patients.
To prevent infections in Isolation rooms,
ASHRAE Standard 170-2013 stipulates requirements to help achieve EAD. These
requirements include room pressurization, filtration, air change rate, and use
specific diffuser type and their location. To prevent migration of particles
into the isolation rooms a minimum requirement the room must maintain differential
pressure +/-0.01 in wc to the adjacent spaces. However, ASHRAE Research Project 1344: Cleanroom Pressurization Strategy Update --
Quantification and Validation of Minimum Pressure Differentials for Basic
Configurations and Applications has
shown that even when maintaining a pressurization of +/-0.01 in wc particles
can migrate into the room as people enter and exit the rooms. To minimize transmission
of particles into or out of isolation rooms differential pressurization of at
least +/- 0.04 in wc or use of a anteroom is recommended.
All air supplied to PE rooms must be
HEPA filtered. To further develop air distribution to reduce the chance of Healthcare
Acquired Infections (HAIs) use of non-aspirating, unidirectional diffusers are
to be installed directly over the patient with exhausts/returns grilles located
near the door the patient room. This is to create an airflow pattern within the
space where the cleanest air possible flows over the patient first before
moving into the rest of the room. However, to achieve effective airflow design
in PE room thermal comfort of the patient must also be considered. Patients are
going to have very low clo levels and met rates, so additional diffusers must
be used to keep the volume and velocity of the air flow out of the
non-aspirating diffusers to a comfortable level. Displacement ventilation would
complement the non-aspirating diffusers best in this space as it would not disrupt
the airflow pattern that is to be developed by the non-aspirating diffuser.
In AII rooms the goal is to prevent
transmission of infections from the patient to staff, visitors, or other patients.
As such, the location of the exhaust is to be directly over the patient bed or
in the wall at the head of the bed, and all air must be exhausted out of the
building. To establish effective air distribution in AII rooms, supply
diffusers should be installed near the entrance to the room with throw patterns
directed towards the patient.
Combination AII/PE Isolation rooms are
allowed by ASHRAE Standard 170-2013. Combined Isolation rooms must have an
anteroom and must be pressurized to both the corridor and the isolation room
itself. The differential pressure must be at least 0.01 in wc, and can be
either positive or negative. In combined isolation rooms, air distribution must
follow the same guidelines as PE rooms with diffusers located over the patient
and exhaust by the anteroom door. And, as with the AII rooms all of the air
must be directly exhausted out of the building.
Conclusion
Appropriate use of chilled beams,
displacement ventilation, and non-aspirating diffusers play a pivotal role in
establishing Effective Airflow Design across many different critical and
non-critical spaces. Designing a system that utilizes each piece in the best
way possible not only creates an environment that is safer and more
comfortable, but is also good for a facility’s bottom line. Lowering
readmission rates and reducing the number of Healthcare Acquired Infections are
goals all healthcare buildings should strive for; EAD helps make that happen.
Be sure to consult a designer before embarking on your next project to
determine which layout makes the most sense for your spaces.
Please direct questions toward Titus Communications (communications@titus-hvac.com) and/or Titus' CB/Critical
Environment Product Manager Matthew McLaurin (mmclaurin@titus-hvac.com).
