The idea of selecting fans of high efficiency is a concept
that every engineer doing air systems design is fully aware of and embraces as
a cornerstone of reducing energy use in these systems. The idea of mandating an efficiency that is
“acceptable” may slice against your grain but all would agree it’s a good and
noble idea. But it seems to me that fan
efficiency really is a pretty small part of the overall picture. Going back to the basics, the theoretical fan
power (and therefore energy as it operates) to circulate air in an air duct
system is given by the equation:
Fan
Power = [(cfm)(FTP)]/[(6356)(ηT)]
Where Fan Power = Fan
power needed not counting drive or motor losses (HP)
cfm = Volumetric
airflow rate (ft3/min)FTP = Fan total pressure (in wg)
ηT = Fan total efficiency (decimal)
6356 = Constant to convert to horsepower
So obviously, the higher the fan
efficiency, the lower the HP required.
So the FEG concept makes sense.
But there are still two more terms in the equation, the airflow required
in cfm and the pressure required to overcome the air handling unit and duct
system losses that establish the FTP.
The air quantity is set by the thermodynamics of the design and the load
that is to be offset. The designer has
some leeway with this term but not a great deal.
That leaves the last factor, the fan
total pressure. And while the
configuration of the building and the application may vary, this is a parameter
that is almost totally established by the design of the system – and therefore,
the designer. Time to wake up - that’s
us! Though ASHRAE 90.1 has tried to
weigh in on the topic of maximum fan pressure allowed, few designers understand
it and even fewer code officials enforce it.
The next edition of ASHRAE 90.1 in 2013 will include prescriptive
requirements for fan efficiency grade so get ready. But the problem is really more fundamental
than that. This time we’ll look at just
the system of ductwork and its design.
On another installment, we’ll look at how air handling unit design can
also have an impact on that fan total pressure term. But for now, let’s stick to ductwork.
With the types of projects we work on, we
get to see lots of air system designs that are done by other engineering
firms. And although the drawings today are
done in AutoCAD and are very professional looking, the thought that went into
them has often not kept pace with the technology that produced them. One of our partners has referred to duct
designs that appear to be “puked up” on the page. That may be a little too graphic (or too
gross) for some but it does convey the meaning pretty clearly – the duct
systems meander around the building so randomly that you wonder if there was
any planning in the overall duct layout at all.
Sure, I can hear you saying it now … duct
layouts, air distribution, layout organization and planning, it all sounds so
1970’s, it’s a waste of time - we’ve moved on to much more important stuff
now. We do have a lot more to think
about now. But I just want to remind you
that some of that boring stuff makes your systems work better and save
energy. Please don’t forget that the basics are still important!
I’m going to proceed now to give you what
I believe to be the five laws of air distribution and duct layout. There will be some naysayers in the group who
want to argue about these but I’m willing to take my chances.
Law No. 1
– KEEP IT SIMPLE
·
The shortest distance between two points
is still a straight line; the shortest possible route requires less ductwork
(less cost) and the lowest pressure drop (less fan power required)
·
There is nothing inherently wrong with a
45° elbow
·
An experienced sheet metal contractor I
remember from when I was a pup told me “Engineers almost always get themselves
in trouble when they run air past itself”
Law No. 2
– KEEP IT SYMMETRICAL
·
Attempt to use the duct geometry to
provide an inherently balanced design
·
Terminal boxes (VAV boxes, mixing boxes,
reheat boxes) should be located on the same side of the trunk duct as the area
they serve
·
Terminal boxes should be centered with
respect to the diffusers served to be more self-balancing
Law No. 3
– MAKE THE SYSTEM BALANCEABLE
·
Provide volume dampers to balance runouts
with differing pressure needs (which always exist)
·
Avoid diffusers mounted to the side of
ductwork to avoid noise and drafts
·
Design the ductwork so that the total
supply airflow can be measured with a pitot tube traverse at one or two
locations
·
Locate volume dampers as far from the
supply diffusers as possible; avoid the use of registers for supply or return
Law No. 4
– LET THE AIR DISTRIBUTION DESIGN
CONTROL THE DUCT DESIGN, NOT VICE VERSA
·
Don’t blow (or throw) from a ceiling
diffuser in any direction further than the ceiling height
·
For selecting diffusers for VAV systems,
look at diffuser performance at minimum flow as well as maximum flow
·
It may seem trivial but there is a lot to
know about air distribution; take some time to study Chapter 20 in the 2009
ASHRAE Handbook of Fundamentals
·
For each new air distribution situation
you encounter, test at least one space with the Air Diffusion Performance Index
(ADPI – calculation described in Chapter 20)
Law No. 5
– BE CAUTIOUS WHEN MIXING AIR STREAMS OF
DIFFERENT TEMPERATURES
·
Air streams of different densities caused
by their temperature difference have no intention of mixing unless they are encouraged
to do so; use direction and velocity to promote mixing
·
When mixing cold outdoor air and warm
return air streams, introduce the cold outdoor air from the top of the duct at
a 90° angle to the direction of the warm return air stream
·
Fans are not effective air mixing
devices; an entering stratified air stream will produce a stratified outlet air
stream (though it may be rotated or flipped in orientation)
If I can keep my brain pointed in the
right direction, next time we’ll talk about air handling units.
