Monday, April 22, 2013

Fan Efficiency Grade: New Tool or Just Another Example of Relearning Something We Already Knew

I know last time I promised that my next contribution would be an explanation of why the supply air temperature wasn’t necessarily going to be 55°F.  I’ll get back to that later – I promise.  But right now, my thoughts seem to be heading in a new direction.  I just reviewed an excellent paper for HPAC Engineering Magazine on the concept of Fan Efficiency Grade (or FEG as it is being referred to).  Within a few days of that review, I heard an excellent talk at the St. Louis ASHRAE Chapter meeting by the President of AMCA, Mr. Vic Colwell, on the same topic.  For those of you looking to learn about one more constraint on your air handling systems design, ANSI/AMCA 205-12 Energy Efficiency Classification for Fans will provide some good bedtime reading (www.amca.org/feg/codes-and-standards.aspx). 

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. 1KEEP 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. 2KEEP 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. 3MAKE 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. 4LET 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. 5BE 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.

Article Author: Jerry Williams
jwilliams@8760engineering.com
Blog Post: Michael Mosbacher
mmosbacher@8760engineering.com

1 comment:

  1. This content is simply exciting and creative. I have been deciding on a institutional move and this has helped me with one aspect. Viking Range Cleaning Tips

    ReplyDelete