Maximizing mixing

This letter is in response to the article “Breaking Tradition” by Roger Howard, which appeared in the November 2006 issue of Engineered Systems (page 52).

In the section titled, “Opposed Blades for Better Mixing and Efficiency,” the author is to be applauded for his explanation of the superior control characteristics for opposed blades dampers modulated independently. However, I must take issue with the idea that opposed blade control dampers should be considered to enhance the mixing of air in mixing boxes. Our experience has shown that neither orientation of opposed or parallel damper blades provide the adequate mixing needed to eliminate the problems caused by stratification. This is especially true on larger mixing plenums where a 6-in. damper blade does not provide any significant deflection of the airstream. Furthermore, the momentum of the main airstream does not allow the two streams to mix within the mixing plenum.

The control dampers primary function is to control the mass flow of the airstreams; they should not be called on to mix. Dual-function components may be a novel idea, but they typically end up not performing either function very well. It is far better to incorporate a carefully designed scaled mixing system to do the mixing and let the dampers control the airstreams. Perhaps a tradition that should be at least questioned, if not broken, is the concept that control dampers can be called mixing dampers and therefore be counted upon to mix outside air with return air.

Ross Lindgren
President
Blender Products, Inc.
Denver


Howard responds:

I agree with Mr. Lindgren that the air-mixing performance of traditional AHU “mixing boxes” is a cause for concern. As the article states, “A recent ASHRAE research project, RP-1045, pointed out that parallel-blade dampers … do not effectively mix the air.” I did not mean to imply that opposed-blade dampers would definitely perform more effectively. As the article states, “Additional research is needed on how much better opposed-blade dampers mix the air … .” In the meantime, whenever the stratification is a major concern due to large differences in airstream temperatures and/or the layout of the mixing box, it is certainly wise to design the AHU with a static mixing device downstream of the mixing box, as Mr. Lindgren suggests.

The art of commissioning

I read Rebecca Ellis’ “Commissioning” column in the November issue of Engineered Systems (“Rewards Revisited,” page 24) with interest. However, I believe incentives, no matter how well intentioned, do not seem practical, are able to be implemented realistically, or are necessary. How about the incentive, “do your job or don’t get paid?” I think the industry made a mistake when they started to reduce the contingency fund on contracts. I have seen contractors walk away from completing the job and giving up the small contingency left on a project rather than completing the work they are contracted to do. Why reward bad work with more money (incentives)?

Commissioning is a relatively new discipline. It is progressing, growing, and becoming better each time it is done by a qualified commissioning provider. As a commissioning provider, I see owners accepting commissioning and asking for more (a larger scope of commissioning services) even when commissioning is done correctly.

Let’s push for a national certification for the commissioning process and for the qualified commissioning agent. Let’s make sure commissioning providers all perform to these standards, and we can look forward to commissioning actually accomplishing what it was intended to accomplish. The Building Commissioning Association (BCA, of which I am a member) has developed a very good process, based upon the guidelines that ASHRAE has published. Their certification is based upon experience, understanding of the construction process, and understanding of the proper commissioning process.

Before we try to ‘fix’ commissioning with incentives, let’s be sure first that it is broken. When contractors begin to see commissioning’s value to themselves and their bottom line, they will work harder to accept this new discipline.

Commissioning can help the construction industry provide a better product. Good agents and a good commissioning product will resolve problems without litigation. Bad agents will provide a level of further problems. I believe that we should concentrate on improving the quality of commissioning agents and also on improving the commissioning process.

Frank A. Mauro P.E., LEED AP, CCP
Commissioning PM
Heapy Engineering
Dayton, OH


The latent side of HVAC

This letter refers to Kevin Dickens’ titled, “Southern Comfort - Part 1” in the December 2006 issue of Engineered Systems (page 48). Kudos for a very well written, much-needed article. For years, I have felt like John the Baptist (a voice crying in the wilderness) in trying to educate the engineering community about the need to pay attention to the latent side of HVAC design.

I would caution anyone who attempts to solve moisture infiltration by the use of positive pressure systems. Note the vapor pressure differential between ambient air and the 75°F, 50% rh setpoint for room conditions. I believe you will find a differential of approximately 3.5 in. w.g. We would need to inflate the building like a balloon to keep out the moisture!

Bill McClure
Owner
Allied Environmental Systems
Atlanta