By Chris Edmondson
Gone are the days of optional air or waterside economizing. Gone also are the days of waterside economizers piped in parallel. In other words, the “either/or” operation of the chillers and economizers that we’re all accustomed to will soon be a thing of the past.
Per ASHRAE 90.1 -2010, set to be adopted into nationwide building codes for non-residential construction in October 2013, economizing (either water or air) will be mandatory, with some minor exceptions. Get ready for these big changes to the way you approach cooling system design:
- Geographically, only the southernmost tip of Florida will be excluded from the economizer requirement.
- Waterside economizer configurations must be capable of providing up to 100% of the expected ASHRAE Standards system cooling load whenever outdoor air temperatures are 50oF dry bulb/45oF wet bulb or below.
- Control of the economizer system and chilled water system must be integrated so that the economizer can provide at least partial cooling even if mechanical cooling is needed to meet the cooling load. In other words, both systems must be capable of operating at the same time.Figure 1 – Economizer piped in series with chillers
No More Parallel Piping of Economizers With Chillers
This last point spells significant change to those of you who are already accustomed to designing economizer systems because, in effect, it eliminates parallel piping of economizers with the chillers. In other words, it’s no longer possible (or practical) to link the economizer, cooling tower, and chillers together with one set of pipes, because you will no longer only be running the economizer as a stand alone substitute for the chillers when the outdoor temperatures are suitable. Initiating economizer operation can no longer be accomplished all of the time simply by isolating flow from the chillers.
Rather, the chillers and economizers must be able to work in concert with one another when conditions permit, with the economizer pre-cooling the return chilled water before it enters the chillers, thus alleviating some of the load. The benefit to this, of course, is that you are able to operate your economizer for more hours throughout the year.
Benefits of “In Series” Piping
At JMP, we advocate piping the economizer in series to meet these new codes. There are various ways to do this, many of which we explore in our webinar “Waterside Economizers and ASHRAE 90.1 – 2010”. For now, Figure 1 shows one possible solution.
You’ll notice that in this case we have given the economizer its own dedicated cooling tower cell. You may find this necessary, depending on the limitations of your chillers since economizers are most effective when supplied with the coldest possible water, but many chillers will experience operational problems when the condenser water supply drops below 55 oF degrees. Varying flow through the chillers may be one option for controlling the temperature, but this too can be problematic. You must check with the chiller manufacturer to make sure what the chiller can withstand, both at start-up and for continuous operation.
In terms of efficiency, we also like the dedicated cooling tower because throttling down flow from the cooling towers to the condensers to control temperature wastes quite a bit of pump and fan energy, negating the benefits of waterside economizing. The same can be said for throttling back on the valve to maintain sufficient head pressure through the condensers. A dedicated cooling tower, sized perfectly to operate in concert with the economizer eliminates these tricky control issues and leaves you with the operational flexibility that the new code demands.
We will be revisiting the topic of the impact of ASHRAE 90.1-2010 on waterside design in future blogs. In the meantime, check out these two webinars: