In the last post we looked at how climate can vary across the country (and the world for that matter) and how these climate variations might impact the operation of an economizer equipped HVAC system. In this post we will look at the impact of an economizer that is stuck in the 100% outdoor air position on two identical systems, one of which is located in San Deigo, California, and the other of which is located in Key West, Florida.
The following plots paint a picture of the two climates similar to the picture in the previous post. The Mean Coincident Wet Bulb plot allows us to contrast the humidity of the two locations.
Plotting the data from these two illustrations on a psychometric chart allows us to paint a picture that takes both
humidity and temperature into account, as illustrated below.
The green constant enthalpy line on the psych chart is the demising line between outdoor conditions that make the outdoor air suitable vs. not suitable for cooling in an integrated economizer cycle, based on a space condition of 75°F/50% RH. Stated another way, if the outdoor air conditions lie to the left of the line, then, on a statistical basis, it will take less energy to cool a supply flow that is made up of 100% outdoor air when contrasted with cooling return air mixed with the minimum outdoor air required for ventilation purposes. If conditions lie to the right of the line, then the opposite is true and energy consumption will be minimized by terminating the economizer process. This analysis implies that, on a statistical basis, an economizer process should be terminated in San Diego when ambient dry bulb temperatures exceed 68°F (the point where the constant green constant enthalpy line crosses the red statistical San Diego climate data line). In Key West, economizer processes should be terminated when the ambient temperature exceeds 67°F based on this analysis.
Now, let’s return to the plots of dry bulb temperature vs. hours of occurrence for the two climates and superimpose the hours during which economizer operation is possible on them. The result is depicted below.
As can be seen from the upper plot, there are a significant number of hours when an integrated economizer cycle equipped air handling system in San Diego would be able to use outdoor air as a source of free cooling approximately 7,400 hours per year or 84% of the time). Many of those hours (approximately 5,700 or 65% of the annual hours) would be spent on 100% outdoor air for a system that was operating round the clock and had a discharge temperature requirement in the 55°F range.
In contrast, a similar system located in Key West would only be able to benefit from an integrated economizer cycle for approximately 1,500 hours per year or 16% of the time and virtually all of those hours would be spent operating at or near 100% outdoor air.
As a result, a commissioning provider or building operator who encountered a system with the economizer dampers stuck in a position that delivered 100% outdoor air in San Diego would have encountered an opportunity to reduce the load and perhaps improve performance on a peak day by returning the economizer to service. But, the energy savings would be modest since the system spends most of the hours of the year operating at or near 100% outdoor air.
On the other hand, a commissioning provider encountering the same problem in Key West would not only reduce the peak load and improve performance by returning the economizer to service. They would also generate significant energy savings because in Key West the economizer only provides benefit for a fraction of the time.
The bottom line is that the same problem in identical systems located in two very different climates yields a very different energy savings benefit when corrected due to the nature of the climate relative to the requirements of the HVAC process associated with the system.
In the next post, we'll take a look at the test results from the PEC AHU economizer in the context of the San Francisco climate.
