Laboratory
facilities are expensive to build and even more costly to operate.
Lab buildings with even moderate concentrations of chemical fume
hoods can increase both construction costs and operating costs.
This article presents some viable, higher-first-cost alternatives
that generally result in significantly lower operating costs.
The first alternative is to use fume hoods with face velocities
of 60 ft/min, which are variously referred to in the trade as “low-flow,”
“low-exhaust-volume,” or “high-efficiency” fume hoods. The second
alternative is to use variable volume controls on standard VAV fume
hoods. A third alternative is to use low-flow fume hoods in conjunction
with variable volume controls.
Before the analysis, a reality check: This article is aimed at fume-hood-heavy
buildings. If you have one fume hood per 1,000 ft2 of laboratory
or less, you do not have a fume-hood-heavy building. Many biology
labs fall into the “not fume-hood-heavy” category, especially if
using biological safety cabinets that are not hard-ducted to the
exhaust system.
A fume-hood-light lab might still identify reasons to use low-flow
fume hoods and/or variable volume controls, but such a choice won’t
represent a major first cost. However, variable air volume can still
save major operating costs even if the fume hood density is light.
A special case of variable air volume operation is two-position
constant volume. This type of operation is often the most cost-effective
control system of its type, especially when the hours of operation
of the laboratory can be predicted ahead of time. This system is
often used as an occupied/unoccupied control system.
A short history of fume hood energy conservation A
standard 6-ft hood takes 1,200 ft3/min (cfm) of exhaust air to operate
at full open sash. The cost of a single cfm that needs to be heated,
cooled, filtered, and moved every hour of the year, in an average
electrical rate area, is about $3/cfm annually. The yearly air-conditioning
bill for a single 6-ft fume hood can be $3,600/yr or more.
In the 1960s, the solution to this problem of high operating cost
was the “auxiliary air” or “add-air” fume hood design. This design
used raw outside air, cold in the winter and humid in the summer,
introduced on the head of the fume hood user. I strongly recommend
that lab users not select this type of hood. (Readers wanting more
explanation can order a technical paper from ashrae.org on the disadvantages
of auxiliary air fume hoods.)
Variable air volume (VAV) fume hoods became a major player in laboratory
construction in the early 1980s. The main goal of VAV was to reduce
operating costs, but properly executed designs also had major safety
benefits. Safety benefits were achieved by improving flow monitoring
accuracy and providing more accurate fume hood alarms. VAV fume
hoods have sophisticated air-conditioning controls that reduce airflows
whenever the fume hood sash is lowered. The average fume hood, including
those using variable air volume controls, operates with face velocities
of 100 ft/min (fpm).
Low-flow hoods in Gwinnett
University Center, Lawrenceville, Ga. Photo: Mike Sinclair,
courtesy of Labconco. Click
to enlarge.
The new kid on the block is the low-flow hood. These fume hoods
have made significant impact in the construction of lab buildings
just in the past three years. Low-flow hoods use advanced aerodynamic
design to contain chemical vapors using a smaller quantity of exhaust
airflow. All of the major manufacturers have come out with fume
hood models in this category. The hoods are most commonly operated
at lower face velocities, such as 60 fpm. (Some individual cities,
along with the entire state of California, mandate higher face velocities,
but most jurisdictions will allow 60 fpm to be used.) Only in the
past few years have fume hood manufacturers offered models that
they would heartily endorse for use at this low an airflow rate.
Another special case is limiting the fume hood’s opening area. Horizontal
sashes are one example of this. Another is the use of vertical sashes
with sash locks at 18-in.” openings. These restricted sash opening
designs can save first costs and energy but should be used only
when the hood users can live with their smaller work openings. In
older designs of fume hoods that require 100 fpm for safe operation,
some safety problems have resulted when hood users have widened
the openings in a restricted-area hood so that they operated at
less than 100 fpm. If you have a hood that is only safe above 100
fpm, operating it at 50 fpm by defeating the 18-in. sash locks and
opening it to 36 in. is not a good idea. In these cases, owners
should consider using low-flow hoods operating at 100 fpm so that
if safety devices are defeated by users the resulting 50 fpm face
velocity is relatively safe.
Below, I compare low-flow fume hoods operating at a constant volume
in Alternative 1 with VAV hoods (operating, of course, with VAV
controls) in Alternative 2. Some lab projects will be able to afford
both low-flow hoods and variable air volume. This is alternative
3. This choice often represents the best of both worlds and will
have an attractive simple payback.
Alternative 1: Choose a low-flow fume hood Despite
the higher first costs involved, I have been impressed by most of
the new low-flow hoods. Some of the new designs are significantly
better than others, but I believe, on average, that the new low-flow
hood designs are safer at 60 fpm than most of the older models at
100 fpm.
The
standard fume hood, and even one of the low-flow designs, creates
a whirling mass of air inside the hood. Airflow visualization looking
in the side of the hood would show the contaminated exhaust inside
the hood rolling in a circular vortex many times before exiting the
hood. This leads to a buildup of contaminants inside the hood, although
containment can be good with these designs.
Protector XStream
hood airflow diagram. Image courtesy of Labconco.Click
to enlarge.
Berkeley Low-Flow
hood design. Image courtesy of LBNL.
Click to enalrge.
Starting almost 10 years ago, a team led by Geoffrey C. Bell and Dale
A. Sartor at the Lawrence Berkeley National Laboratory, Berkeley,
Calif., revolutionized low-flow hood design. With several million
dollars from the state of California and the Dept. of Energy, they
came up with a design with an absolute minimum of turbulence and no
whirling vortices. The diagrams above show their original design as
well as a commercial model that reflects its influence.
One of the effects of this design, besides working very well at 60
fpm, is that levels of contaminants tend to build up less inside the
hood. If there is a release of hood air due to operator movement or
cross-drafts, a lower amount of hazardous contaminants would be entrained
with it.
I have reviewed detailed factory and field data from a number of the
top manufacturers on the performance of their low-flow fume hoods.
For the most part, their performance is very impressive, and I recommend
the use of these fume hoods with face velocities of 60 fpm, where
allowed by code. However, not every fume hood manufacturer’s product
will come up to the exacting standard of performance required by low-flow
fume hoods.
Like newer, better equipment in every category, advanced low-flow
fume hoods have a higher first cost. A standard 6-ft hood with accessories
such as service fixtures, electrical receptacles, epoxy worksurface,
and supporting acid and flammable base cabinets generally runs in
the range of $1,200/ft. Low-flow fume hoods, such as the Labconco
Xstream, are generally 20 to 25% higher, and will cost approximately
$1,500/ft, installed. “If the high- performance fume hood is operated
at a reduced face velocity, the payback period can be very quick,”
according to Tom Schwaller, VP/sales, Labconco Corp.
For the purposes of payback analysis in this article, I assumed that
the entire additional cost of $1,800 per 6-ft hood is reflected in
the building cost. This will not be true on many projects. If your
building has a lot of densely packed fume hoods, the choice of low-flow
hoods at 60 fpm may so reduce the air-conditioning cost that the low-flow
hood will be “free” to the project.
Also for the purposes of example, I assumed that the fume hood is
operating at full sash height: 28-in. Fume hoods with restricted openings
of 18-in. or less achieve cost savings of their own. If the 18-in.
height is standard operating procedure, it reduces the potential savings
from low-flow fume hoods or variable volume. Restricted-opening fume
hoods have a long and successful history in laboratory buildings.
Forcing the user to work in a smaller opening can save first cost
in constructing the building, reduce operating costs, and improve
safety. It is always safer to reduce the working opening of the fume
hood in terms of promoting chemical vapor containment.
The only possible drawback to the reduced-opening hood is that many
scientists will modify it to open 28 in. instead of 18 in., so as
to gain greater access to their work. With most air-conditioning systems
and most standard fume hoods, a hood operated at 28 in. when it is
meant to operate at 18 in. will not be safe.
Many building maintenance directors like the low-flow fume hoods because
they achieve first cost and operating cost savings with almost absolute
reliability. The low-flow fume hood requires little more maintenance
than standard fume hoods. Because these savings can be achieved without
variable volume air-conditioning controls, the level of complexity
of maintenance is reduced.
Alternative 2: choose variable volume controls With
more than 20 years of installed history, there is quite a bit of solid
experience with using variable volume fume hood controls in the laboratory.
In some lab retrofits, installation of VAV has resulted in one-year
paybacks and 50% reduction of air-conditioning costs.
The use of variable volume controls has made many labs easily adaptable
to changing scientific requirements. A lab can be easily rebalanced
by reprogramming the direct digital controls (DDC). In an old-fashioned
constant volume building, a similar change in ventilation levels would
mean a long and cumbersome process of changing fixed dampers and making
field measurements.
The DDC systems needed for laboratory VAV entail better safety alarms
and better recordkeeping and documentation capabilities. The monitoring
function can reduce liability to the owner by verifying safe conditions
are being met. It is also invaluable in responding to emergency conditions
in the lab and promoting safety overall.
The main continuing resistance to VAV in the laboratory is its high
first cost. Although VAV will recover its high first costs within
one to three years of operation, most owners maintain that utility
cost budgets are not related to construction cost budget. Accounting
practices often make first costs hard to hide and tough to justify,
whereas high operating costs might be easier to “bury.” Ironically,
owners often will not increase the first cost of a building even when
the operating cost savings over 20 years will pay for the increase
10 times over.
Another objection is to the increased complexity of maintaining these
control systems. This objection lacks a great deal of validity. At
its easiest level, a maintenance contract can be given to an outside
company to keep the controls up and running.
I have been a big fan of VAV controls and hoods from their very start
in early 1980s. They have many benefits to building owner and to the
fume hood user.
Alternative 3: both of the above It is both technically
feasible and cost-effective to combine low-flow fume hoods with
variable volume controls. There is a lengthened payback period,
and the decrease in costs is not great, but this is usually going
to be the safest possible system.
The box above provides a cost and payback analysis of a theoretical
50-fume-hood facility. In this facility, low-flow hoods offered
the fastest simple payback, but VAV and low-flow plus VAV schemes
had lower annual operating costs. Each client would obviously have
to weigh the first-cost, payback speed, and long-term operating
variables to make an appropriate decision. There’s no blanket “correct”
answer, but one thing is certain: the availability of new, advanced
hood designs provides an excellent opportunity to create safer buildings
that cost significantly less to operate.
Costs
and paybacks for a 50-hood lab building
The building uses a 330 ft2 laboratory planning module and has
one 6-ft. benchtop chemical hood per module. Assuming average
utility rates and Midwestern U.S. weather patterns, $3/cfm will
be used for 8,760 hr of fume hood usage. The base case is a
6-ft bench hood with 1200 cfm of exhaust, operating continuously
year-round (CV). The cost of this system will be set as zero,
and the other alternatives’ costs will be given as difference
in cost compared with the base case.
Base Case: Older-style CV hoods,
minimal controls
• 1200 cfm 3 50 hoods/yr at $3/cfm/yr = $180,000/yr operating
costs.
• Cost impact: Zero, since this is the base case and is
lowest first-cost alternative.
Alternative 1: Low-flow hoods
• 720 cfm 3 50 hoods/yr at $3/cfm/yr = $108,000/yr operating
costs.
• Cost impact: Assuming 6-ft hoods, additional first cost
of $90,000. This conservatively assumes no savings from downsizing
the air-conditioning system. It is also assumed that the air
conditioning does not need to be increased to meet minimum
ventilation rates in the lab.
•Simple payback: 1.25 years, with $90,000 repaid out of
annual energy savings of $72,000.
Alternative 2: VAV hoods
• 1200 cfm 3 50 hoods for 1,000 hr/yr; 600 cfm 3 50 hoods
for 1,000 hr/yr; 300 cfm 3 50 hoods for 6,760 hr/yr. $67,000/yr
operating costs.
• Cost impact: Assume $6,000 additional first costs per
fume hood for advanced safety controls and monitoring, or
$300,000. This is conservatively not taking into account potential
air-conditioning cost savings from downsizing the HVAC system
because of the diversity of fume hood use.
• Simple payback: 2.65 years, with $300,000 in increased
costs offset by $113,000 in annual energy savings.
Alternative 3: Low-flow
hoods
with VAV controls
• 720 cfm 3 50 hoods for 1,000 hr/yr; 300 cfm 3 50 hoods
for 7,760 hr/yr. $52,000/yr operating costs for an annual
savings of $128,000.
• Cost impact: $390,000 (combined cost of advanced hoods
and more expensive control system).
• Simple payback: 3 years.
Victor A. Neuman, PE, is senior mechanical engineer with LSW
Engineers in San Diego. (www.lswsd.com).
LSW is a 70-person mechanical-electrical consulting engineering
firm with offices in Phoenix and San Diego that specializes in laboratories,
hospitals, LEED-certified buildings, semiconductor manufacturing
facilities, and commercial buildings.
