Basic Documentation

Siemens Industry, Inc. Page 5 of 14
Document No. 149-488
clean. If some activity contaminates the air to
unde
sired levels, the airflow automatically increases
until the air is clear. This demand controlled
ventilation concept is applied routinely in conference
rooms and class rooms, and is now finding support
in laboratories as well. Safety officials may endorse
lower airflow rates if they are confident that hazards
are detected and corrected. The key again is to think
from a safety perspective, about the relevant
contaminants, how we can detect them and how we
ought to respond. The response can include
automatic airflow adjustments, but that may be just
one aspect. We may need to coordinate that with
notification to the safety office or an indicator to the
lab users themselves. When we put the questions in
front of the whole design team, we find
comprehensive answers that cross organizational
boundaries, combining automation with operating
procedures.
Low Pressure Design
Cutting the airflow rate is the most productive step
toward reducing energy consumption, but not the
last one. After the initial focus on using less air the
design team turns to delivering it more efficiently.
A ventilation system employs fans to move air.
Supply fans draw air from outside, power it through
the conditioning equipment (filters, coils, etc.) push it
down the ducts, spread it out through the flow
control terminals and diffusers, into the individual
rooms. At every step along the way, it takes
pressure to make the air move; those individual
pressure losses add up to determine the sum that
the fan has to generate. Together, the airflow rate,
and the pressure (along with other efficiency factors)
determine the power consumed by the supply fan. A
similar set of pressure losses determines the power
the exhaust fans need to pull air from the room and
discharge it through the exhaust stacks.
The power consumed by a fan is proportional to the
flow rate and to the sum of the series of pressure
drops through the system. (Where the distribution
system splits into parallel paths, the path with the
greatest loss is the one that contributes to the fan
power.) Having already minimized the airflow rate,
the design team, concerned with energy
consumption turns its attention to pressure losses.
The Labs21 team published a Best Practice Guide
covering design tips and strategies to reduce
pressure losses at each component in the ventilation
system
8
It addresses sizing the ducts, coils and
other components to increase the cross section
available for airflow. It even suggests designing the
heating and cooling system to eliminate the need for
reheat coils in the space. All these steps reduce the
pressure losses in the system.
One step recommended is selecting airflow control
terminals with a low pressure loss.
Airflow control terminals are the valves needed at
each outlet of the supply system and each inlet to
the exhaust system. They accomplish the required
distribution of air through the facility, making sure
the right quantity is delivered in each location. There
are many mechanical designs for this purpose, but
they all work by restricting the path to regulate the
amount of air that flows through. An airflow control
terminal can be thought of as an adjustable pressure
loss. Its job is to dynamically adjust the opening to
use up enough pressure, so that only the desired
amount of air gets through. These mechanical
devices have a range of adjustability from the nearly
closed position (the maximum pressure loss) to
nearly open (the minimum pressure loss). This
characteristic, called the minimum operating
pressure varies according to the sizing parameters
and the basic mechanical design. When selecting a
fan, HVAC designer includes this minimum operating
pressure in the sum of losses that the fan has to
handle. Any increment of pressure required for the
terminal adds directly to the pressure needed at the
fan and to the power consumed.
The type of air terminal chosen makes a significant
difference. Most laboratory ventilation systems use
either single-blade dampers or Venturi air valves.
Figure 1 illustrates the air path in each type of
terminal. The
Venturi air valve simply does not open
as completely as the single-blade damper; it always
restricts the flow. Consequently, Venturi valves lose
more pressure and consume more energy than
single-blade dampers. Typically a single-blade
damper needs less than 0.1 in. WC of pressure
drop, while most Venturi air valves consume six
times that pressure. Some manufacturers offer low-
pressure Venturi air valves that operate down to 0.3
in. WC, but these products are rarely selected for
laboratory HVAC. (Product data sheets for single-
blade terminals often quote the pressure loss for the
terminal, including a reheat coil; data for Venturi
valves usually excludes the coil because it is
8. U.S. Department of Energy, Laboratories for the 21st
Century: Best Practice Guide, "Low Pressure Drop HVAC
Design for Laboratories" DOE/GO-102005-2042 (February
2005).