ASHRAE Journal - June 2011 - 16

letters
Discussing Affinity Laws
March’s “Affinity Laws: Why They Work & When They Don’t” by Richard W. Ford, Member ASHRAE, was a breath of fresh air. A mistake commonly made in pump and fan applications is to assume all motor speed control applications are based on controlling volumetric flow. In VAV applications it is standard practice to estimate savings as if the speed of supply and return fans were modulated to control flow, when the goal of supply fan control is actually to maintain duct static pressure. Supply flow is set at the VAV box level, and those box dampers represent a continuously changing system resistance and, therefore, variable system curve. If one uses the cube law to justify the project, the payback is likely to be far longer than anticipated. The return fan typically draws from ducts and ceiling plenums that have a fixed resistance and, therefore, a fixed system curve. The control system should be controlling return volume (never fixed pressure) to track the supply volume, in which case, it will put the pressures wherever they need to be to move the associated volumes. Because fan efficiency is not impacted by position on the curve to the extent that pump efficiency is, the savings on return fans fit the affinity laws much better. Ken Kolkebeck, Member ASHRAE, Waltham, Mass.

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BIM for Engineers
I sit on the ASHRAE BIM Steering Committee that reports to ASHRAE Tech Council, and the article in April, “BIM Test at ASHRAE HQ” by Steven F. Bruning. P.E., Fellow ASHRAE, does little to encourage members to use BIM. Therefore, I would like to specifically address some points within that article. Between 2004 – 09, I created test plans, wrote specifications, and described workflows in HVAC and piping applications for the smart building parametric software that was discussed in the article and worked with the software engineers who created functions and features required by engineers, including heating and cooling load calculations. Sadly, the software manufacturer has not progressed far enough (in my opinion) toward the goals for load calculations (including in the 2012 version) since I was last involved with that project. So, I sympathize and understand the frustration of the author. Yet, I have had far better experiences (although also far from a desired experience) in actually using a BIM for load calculations in projects during the last two years at my present firm than the experiences described in the article. Here are a few of my observations. It continues to be a timesaver for me to use the model geometry with the gbXML (Green Building XML) export tool to calculate loads and create a report using the more familiar software applications that provide detailed and easy to read outputs, which also allows a simple “forensic” analysis of the loads. The gbXML open schema was created to help building designers get information about the energy consumption characteristics of their building projects. No question that the heating and cooling loads in the BIM software’s integral detailed report is “voluminous and difficult to follow.” I hate that! Sadly, the cumbersome report has not yet been revised to an easily read, usable format. There are workarounds: copy/paste into a spreadsheet is one example. No argument, as well, that yes, it is necessary to pick a construction material U-value from a predetermined list. I hate that too! A far better option would be to input a real value. Yet, how excellent would it be for the software to be smart and use, as an example, the BIM glass manufacturer defined U-value within the model and to use the U-value of the glazing and to automatically populate that value in the heating and cooling loads? Similarly, why can’t the actual lighting loads be populated from the lights defined in the model? It is these goals and many others that I hope software developers are continuing to strive toward!
16 ASHRAE Journal

The Author Responds
Mr. Kolkebeck is correct. Fans also can have a friction load, which follows the cube law, and a static load, which does not. The same rules of thumb can help avoid over-predicting energy savings. Don’t count savings from slowing a fan to less than 50% or don’t predict cutting a motor’s energy use to less than 15% of its full load power. Richard W. Ford, Member ASHRAE, Buffalo, N.Y. I encourage ASHRAE members to keep up their use of BIM, and demand from their software vendors solutions to the issues pertinent to the MEP disciplines. Steven Rosen, Member ASHRAE, Boston

The Author Responds
Our intent was not to discourage use of BIM; we are strong supporters of BIM. However, we did find that the internal heating and cooling loads features of this particular BIM software left much to be desired, as confirmed in Mr. Rosen’s letter. Unfortunately, BIM vendor demos tend to gloss over the reality we found. Software developers should spend more effort on improving the engineering product versus overselling it. TC 4.1 is working on a follow-up seminar, which will be presented on June 26 at the ASHRAE Annual Conference in Montreal. We’ll be using a new, created-from-scratch BIM architectural model of the ASHRAE HQ to avoid all the IFC translation
ashrae.org June 2011
ASHRAE Journal - June 2011

Table of Contents for the Digital Edition of ASHRAE Journal - June 2011
