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Reducing laboratory airflow
systems’ energy use through
automatic operator linked control
Welcome to our regular series of CPD modules, designed to help you broaden your professional
knowledge while you work. This module covers the reduction of laboratory airflow systems’ energy
use through automatic operator linked control and is sponsored by Critical Airflow Controls.
This free Building Services Journal reader
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and accessible manner.
Working in association with London
South Bank University, Building Services
Journal has devised these distance-learning
modules to help you meet the CIBSE CPD
requirement. All you have to do is read the
text supplied here (pages 65-67) and tackle
the multiple-choice questions on page 68 .
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paper for assessment.
The primary objective for laboratory
airflow systems is to maintain operator safety
while using hazardous materials in protected
areas such as fume cupboards. To ensure this,
room pressurisation must be maintained by
supplying and removing the correct amounts
of air. The traditional method of airflow
control was to use constant volume (CV) and,
subsequently, two-stage controls were used
to gain efficiency by reducing laboratory
airflow under specific conditions, such as
night setback. Latterly, variable air volume
(VAV) has been used where the position of
the fume cupboard sash door is used to
determine airflow rate and, most recently,
this has been refined with automatic operator
linked control. Automatic operator linked
control minimises the airflow while
maintaining a safe minimum level that is
increased only when needed by the presence
of a cupboard user.
Fume containment for safety
Fume containment is critical to the safety of
laboratory workers. Several factors are
building services journal 12/06
involved in the proper containment of fumes,
including face velocity, cross-drafts and
work practices.
Common industry guidelines [1] for face
velocity range from 0.3-0.7 m/s. In many
modern facilities, 0.5 m/s is accepted as the
standard for safe operation – the need for
this high velocity is due to the interference
caused by the operator to the airflow pattern
(Figure 1).
There is some interest in operating
cupboards below 0.3 m/s to save energy –
cupboards with sash-opening limits and
deeper cupboards have been tested for this
concept. Often the ASHRAE 110-95 test [2] is
used to test cupboards for containment. This
is a static test where tracer gas is released in
the fume cupboard and a non-moving
mannequin is used to test the breathing zone.
The amount of tracer gas sensed at the
mannequin determines the cupboard’s
containment level. If the level remains below
the maximum threshold (for example 0.1 ppm),
the cupboard may be deemed compliant. This
type of test does not assure containment
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under actual operating conditions. The static
nature of the test does not take into account
the dynamic conditions in a working
laboratory. The movement of people, high
supply-air cross-drafts, and operator work
habits all affect proper containment.
Containment under dynamic conditions is
currently difficult to quantify. However, visual
smoke tests under dynamic conditions do
show cause for concern at low face velocities.
For example, walk-by and hand movement
tests [4] show improved containment at
0.5 m/s compared with 0.3 m/s. The new EU
standard EN14175 seeks to address this issue
with the inclusion of some dynamic testing.
Requirements for proper airflow
To properly control airflow there are a number
of qualities that contribute to a successful
laboratory ventilation system.
■ System response time – The fume cupboard
exhaust airflow control device must respond
to the change in sash opening by achieving its
commanded value within 1 second of the sash
reaching its final value. Oscillations and
overshooting are not acceptable because
these may cause a loss of containment. A slow
response to a lowering of the sash can also
create hazards due to excess face velocities.
■ Pressure independent response time –
Where there is more than one VAV supplied
cupboard (or fume extraction device), rapid
changes in volume because of sash
movements will cause changes in duct static
pressure. The system must react quickly and
in a stable way to these perturbations to
guarantee a stable solution, since all the VAV
devices react almost instantaneously to
changes in duct static pressure.
■ Accurate turndown ratio – The fume
cupboard face velocity must be maintained
accurately over a wide range for safety and
energy-saving reasons. An accuracy of ±5% of
the desired airflow is important to maintain
the correct face velocity and proper room
pressurisation.
Concentration (ppm)
14
Person moving
12
Person still
10
8
6
4
2
0.3
0.4
0.5
0.6
0.7
0.8
Face velocity (m/s)
Figure 1 Example of the effect of operator
movement on fume cupboard containment [3]
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■ Stable control system – The control system
should exhibit less than a 5% overshoot or
undershoot when attempting to reach a
desired control value. Pulsations of face
velocity caused by these oscillations could
affect the cupboard’s containment. Control
approaches that measure volumetric airflow
are reportedly prone to this effect.
■ Insensitivity to inlet and exit conditions –
Laboratories typically have large volumes of
exhaust and supply airflow, which result in
congested, tight ceiling areas for the
corresponding ductwork. This ductwork can
be quite convoluted, leaving little room for
the long straight duct runs necessary for
typical airflow measuring devices to meet the
required accuracies.
■ Simplicity and reliability – Failsafe and
redundant features should be installed where
appropriate. Control system concepts and
equipment must be simple and easily
understood by the average maintenance
person. Troubleshooting should also be readily
undertaken by maintenance staff to prevent
quick “fixes” that could potentially create a
dangerous hazard.
Variable air volume performance
A VAV fume cupboard control system can be
used to vary the fume cupboard’s exhaust
volume as a function of the sash opening.
Thus, full flow will be commanded at full sash
opening (50% flow at 50% opening, etc),
down to a minimum flow (typically 20%) which
will be maintained for sash openings of 20%
or less. This type of control maintains a
constant average face velocity into the
opening of the fume cupboard to eliminate
the excess face velocities that occur in a
CV cupboard when the sash is lowered since
excess velocity can create turbulence and
eddy currents that can potentially release
fumes from the cupboard.
The VAV lab control system must maintain
negative room pressurisation – typically done
by controlling the supply or make-up air into
the room at a volume flowrate slightly less
than the total exhaust airflow. The total
airflow rate for a laboratory is dictated by the
highest of: total amount of exhaust from the
cupboards; minimum ventilation rates; cooling
required for heat loads.
At times, the amount of airflow commanded
by the cupboards is below the amount needed
to cool or ventilate the room. In these
instances, the room’s supply air volume must
be increased to provide the proper amount of
air. The laboratory control system must also
act to maintain the proper laboratory
pressurisation by exhausting this “excess”
supply air – potentially achieved by adding a
general exhaust to the room controlled by the
laboratory VAV system to maintain the proper
balance between the supply and exhaust from
the room.
Use-based solutions
Constant volume systems have high life-cycle
costs due to peak-load equipment sizing and
high energy use. This changed with the
introduction of VAV airflow control resulting
in a decrease in the fume cupboard and
laboratory airflow volumes from peak constant
volume levels to those based on both fume
cupboard sash position and laboratory room
thermal requirements. However, the energy
savings will not occur if sashes are left open.
There is the need for containment, but also
demands to reduce energy use and capital
costs. However, it is possible to get both the
safety from containment and the reduction in
HVAC system capacity by taking diversity, ie,
designing a system for less capacity than the
sum of the peak demands. Understanding
diversity in laboratories becomes critical for
safe designs that optimise savings.
Diversity
Diversity may allow existing facilities to add
fume cupboard capacity using current HVAC
systems and in new construction it allows
reduced capital costs. The diversity of use of
the installation will depend
on a number of factors.
Percentage of fume hoods with users present
(maximum) (10% presence probability)
■ Presence of an operator
100%
– It has been shown by a
90%
number of studies that the
80%
amount of time the fume
70%
cupboards are occupied
60%
during the day tends to be
50%
very short – often less than
40%
1 hour per day.
30%
■ Sash management –
20%
When users are in front of
10%
cupboards, they typically
1 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
have the sash open. When
Number of fume hoods
they leave the cupboard,
they may or may not close
Figure 2 The statistical quantity of operators at a given number
the sash. The level of sash
of cupboards based on a 10-hour day and 1 hour/day use
building services journal 12/06
Cpd collection
closure determines the sash management of a
facility. Sash management is hard to predict,
so the system is frequently designed for
full-time capacity.
■ Random use of fume cupboards – Research
conducted at over 35 sites [3] helped to
determine that fume cupboard use is random
throughout the day. This means that there is
no particular time throughout the day where
most or all cupboards will be occupied.
■ Quantity of fume cupboards – By combining
knowledge about the presence of an operator,
the random use of cupboards and the quantity
of fume cupboards on the system, a statistical
tool, such as the probability distribution
function, can be used (see Figure 2). This helps
to determine the quantity of operators that
will be in front of a given number of fume
cupboards at any time. Some designers are
hesitant to take diversity since the savings are
only realised when the fume cupboard sashes
are closed. Often, this has led to systems with
methods of “forced” diversity.
■ Mechanical sash stops – This “prevents” a
user from opening a sash beyond a preset
maximum setting. Unfortunately, users often
override these mechanical stops for everyday
activity and for setting up experiments. This
can create a dangerously low face velocity
profile if the controller is not sized for full
sash opening.
■ Sizing flow control based on low face
velocity settings – By lowering flows,
containment may be compromised.
With this knowledge of diversity, enhanced
control systems have been designed to sense
the actual usage of the fume cupboard by an
operator as opposed to just using sash
position. This allows a designer to predict
fume cupboard use and to assure a safe level
of diversity.
Control system options
Metres cubed per day
There are various options for the control of
fume cupboard airflow.
Constant volume systems are designed to
provide the airflow for all the fume cupboards
in a system (whether occupied or unoccupied,
with open or closed sashes) – the total flow
remains the same. There is no diversity
potential for this approach,
Variable volume
but limiting sash openings
control damper
with mechanical stops is
Sash
sometimes used to reduce
sensor
cupboard flows by perhaps
40%. Future system
changes, such as adding
cupboards, are limited.
Zone
Two-state systems differ
presence
significantly in flow design
sensor
Fume
and switching mechanisms.
hood monitor
Systems that interlock flow
with light switches or room
occupancy sensors will
Detection
reduce flow at night but not
zone
during the day. This results
in some energy savings but
Figure 3 Fume cupboard with integral VAV control
requires mechanical systems
and automatic operator linked control
to be full sized, since all
cupboards operate at full
flow during the day. Systems
with sash switches allow
m/s
(0.2
efficiency
High
400
face velocity) cupboard
each cupboard to control
350
- 24 hours at 12 m3/hour
flow based on sash position.
300
Unfortunately, sashes must
Poor management
250
be closed to realise the
VAV - 1 hour fully open,
16 hours part open,
200
benefits, making diversity
7 hours closed
unpredictable in most cases.
150
The best two-state
100
Good management
systems may be where the
VAV - 1 hour fully open,
50
7 hours part open,
cupboard flow is increased
0
16 hours closed
to a safe face velocity when
With operator
Standard control
linked control
an operator is present
but reduced to a safe
Figure 4 Example of “average use” fume cupboard daily
operation with and without automatic operator linked control
“unoccupied” velocity when
(OLC) based on 8 hour use (7 of which are unattended). For a
the cupboard is vacant.
typical VAV cupboard: Full flow = 30 m³/hr; part open (occupied
VAV systems are totally
and unoccupied without OLC) = 18 m³/hr; part open (unoccupied
dependent on an operator’s
with OLC) = 10.8 m³/hr; closed = 6 m³/hr.
building services journal 12/06
compliance in closing the fume cupboard sash
to realise the benefits. If operators leave the
fume cupboard sash open, the cupboard will
operate at high flows much like a CV system.
Determining the peak airflow demand is
difficult because any number of sashes may be
left open. With such unpredictability in
airflow, downsizing the building’s mechanical
equipment to take account of diversity is
difficult. With VAV systems, diversity beyond a
20 or 30% reduction in capacity is rarely
taken because the risk is considered too high.
Automatic operator linked control can
significantly lower the risk in downsizing the
building’s mechanical equipment. It senses the
actual presence at the fume cupboard of an
operator, not just the sash position (Figure 3).
When unoccupied, the fume cupboard
would operate in the standby mode of 0.3 m/s
(in practice most of the day) instead of
0.5 m/s, resulting in nearly a 40% reduction in
airflow. Furthermore, if sashes are closed, up
to 80% reduction in airflow is realised.
This means that higher flows are used only
at the cupboards that are occupied, for only
the time someone is present. When the
operator leaves, the flows are reduced,
assuring lower airflow rates. This type of
control can be applied to enhance VAV and
two-state systems.
Energy consumption comparison
Figure 4 indicates the total flows that may be
expected in a day when comparing a
high-efficiency cupboard with a VAV, and
a VAV with operator linked control.
Referring to Figure 4, greater energy
savings can be obtained from a well-managed
VAV cupboard than from a high-efficiency
cupboard with a 0.2 m/s face velocity. In the
example, the VAV cupboard with operator
linked control giving face velocities of 0.5 m/s
provides savings that would only be met by a
high-efficiency cupboard if it were to operate
below 0.2 m/s – a velocity that will not
provide the all-important operator safety. ■
© Tim Dwyer 2006
References
[1] CIBSE Guide B2 Ventilation and Air
Conditioning
[2] ASHRAE 110-95 (Method of Testing
Performance of Laboratory Fume Hoods)
[3] Ljungvist, Bengt, “Some Observations on
Aerodynamic Types of Fume Hoods”,
Ventilation '91, pp 569-572.
[4] Manufacturer’s notes – Phoenix Controls
Further reading
■ BS 7258, 1994 Laboratory Fume Cupboards
(now partly superseded)
■ BS EN 14175-2: 2003 Fume Cupboards
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