Commissioning and Operations meet HVAC Design Theory; Design vs. Extreme Conditions

In my previous post, I mentioned a histogram that compared recent conditions with seasonal norms and extremes.  The following table, extracted from the 2001 ASHRAE Handbook of Fundamentals, is a different way of saying the same thing that the histograms I mentioned are saying, but for a location in California.

In general terms, seasonal norms for the peak summer and winter seasons are similar to design conditions in that they reflect the environment that will typically be encountered at a given location for a given time of year. For most projects, of necessity, designers target a statistically based ambient design condition as the basis of their design. Something Bill Coad, one of my mentors once said about design conditions, very early in my career really stuck with me and is worth thinking about when considering these design targets.

Bill said something to the effect that the design load calculations we do are one the most useful, meaningless pieces of information we develop for a project.

  • They are  useful because they will become the basis for all of the equipment we select for the project and how we configure it.
  • But in terms of operating the systems we create, the conditions we target are not very meaningful because the systems will not spend much time operating at those conditions.

Think about it; what the 99% design condition in the table above really means is that statistically, there will be about 88 hours per year where the out door temperature is at or below 42°F at that particular location. Or, there will be about 8,672 hours in the year when the conditions are at or above 42°F.

Seasoned operators and facilities engineers will likely tell you that the real trick is getting the systems to operate in a stable efficient manner for the 8,672 hours as well as the portion of the 88 hours that are below the design condition; i.e. the extreme winter condition.

The extreme winter conditions are conditions where temperatures are below the design target. If you look at the table I inserted at the start of this post again closely, you will notice that for this particular location, the heating design conditions are above freezing, but the extreme winter condition is below freezing. Failing to take this into account for systems that are designed and intended to operate in this environment could lead to
painful consequences. For instance:

  • A preheat coil full of water and not piped like a preheat coil in a make up air system handling 100% outdoor air at temperatures below freezing would probably have problems on the extreme day.

If the design has not considered and addressed the extreme conditions it will be subjected to, when these events occur – and they will at some point – the operators and facilities engineers running the systems experience a nightmare that can ripple out beyond the immediate impact of the extreme condition. In the case of the cooling tower:

  • As temperatures drop below freezing, ice begins to accumulate on the fill and fan blades.
  • The accumulated ice changes the performance of the tower, perhaps making it incapable of rejecting the load, which may be about the same irrespective of the season, especially in the case of process loads.
  • The fill can collapse if the accumulated ice becomes extreme enough.
  • The fan can be thrown out of balance and maybe even destroy itself if the imbalance due to the ice becomes extreme enough.

Any or all of these failures can ripple out and cause the plant to loose the ability to handle the load, leading to comfort problems, environmental control problems in critical care areas in health care facilities, or loss of product in process facilities.

The frozen preheat coil alluded to previously will generate similar ripple effects, at least one of which will not become apparent until the outdoor air temperature rises back up
above freezing and the water frozen in the coil melts.

So, bottom line, take a few minutes as you contemplate the design and operation of a system to consider how it will perform not only on the design day, but also on the extreme days and all of the days in between. If you’re a designer, it will likely change your perspective on how the system needs to be designed and installed. And by addressing the challenges raised, you’ll  probably have more fun developing the design and also make some operators very grateful down the road.

If you’re on operator, taking a few minutes to assess how you think the system you are charged with will work under extreme conditions, and asking questions if you are worried, can avert a few surprises down the road. If  you identify a problem and can institute some improvements you will save yourself some heartache.  And, even if you
can’t get peoples attention, and maybe even doubt yourself when challenged by others, at least you will have contemplated the possibility.  And if what you fear comes to pass, you’ll be in a better position to deal with it as a result of having thought it through one time.

In any case, the result of such an effort will be a happier end user, which of course, is the ultimate goal of the building design, construction an operation process.


David Sellers
Senior Engineer – Facility Dynamics Engineering

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