How To Size A Waterside Economizer Part 1: How ASHRAE 90.1 Updates Have (Or WILL) Change Chilled Water Design

By Chad Edmondson

Has your state adopted ASHRAE 90.1-2010 or beyond into its commercial building code? If so, then it is highly likely that you should be incorporating either waterside or airside economizing into your cooling design. The Standard has effectively made air or waterside economizing a requirement in virtually every state throughout the country.

At this writing, 21 states have adopted 90.1 – 2010 or better into their code and another 16 states are in the process of doing so. For the most current information you can follow this link to find the latest status of state energy code adoption.

Waterside economizers, the focus of this series, are best suited in climates where the wet bulb temperature is below 55°F for an extended period of time. According to Energy Star, that threshold is 3000 hours or more, which covers a pretty large swath of the country.

The latest ASHRAE guidelines are even more inclusive—and specific. Per ASHRAE 90.1-2010 & 2013, Sections 6.5.1,

• Water economizer systems shall be capable of cooling supply air by indirect evaporation and providing up to 100% of the expected system cooling load at outdoor air temperatures of 50°F dry bulb/45°F wet bulb and below.

• Economizer systems shall be integrated with the mechanical cooling system and be capable of providing partial cooling even when additional mechanical cooling is required to meet the remainder of the cooling load.

• Pre-cooling coils and water-to-water heat exchangers used as part of water economizer systems must either have a water-side pressure drop of less than 15 ft. or a secondary loop must be created so that the pressure drop will not impact the circulating pumps with the system is operating in non-economizer mode.

There are a couple of exceptions to these requirements, including tropical climates which only apply to the southern most tip of Florida, and data centers or computer rooms. (Make sure to check Sections 6.5.1 and any local codes to see what exceptions might apply.) Other than that – if you are in a state that has adopted ASHRAE 90.1-2010 or 2013 and you are choosing to use waterside economizing, then the above bullet points apply.

Changes to Waterside Economizing Design

So how will the above changes impact the way engineers design waterside economizers?

First, assuming your state has adopted these standards into code (which is going to happen if it hasn’t already), you can no longer pipe chillers and economizers in parallel. They must now be integrated! Throughout the country, chillers and economizers will have to be able to operate simultaneously if outdoor conditions are suitable for free cooling. It’s no longer a matter of shutting off one system to operate another. So if you’ve been accustomed to using just one set of pipes and simply redirecting flow to the economizer instead of the chiller for part of the year, you’re going to have to rethink your piping arrangement.

Second, control just got a tad bit more complicated. Keep in mind that economizers need cold water to operate efficiently. Chillers, on the other hand, have a minimum condensing temperature, so the water temperature can only drop so low or else they will not operate properly. This means that you may actually need two different cooling tower cold water supply temperatures.

Over the next several blogs we discuss control and piping strategies to help you meet these new requirements. We’ll also address other important issues, such as heat exchanger selection and sizing.

In the meantime, we suggest you review this earlier blog:

Designing Waterside Economizer Piping to Meet ASHRAE 90.1 - 2010

You should also bookmark our video on Waterside Economizers and ASHRAE 90.1 – 2010. You can use it to follow along with this series.

Finally, if you are new to mechanical design – perhaps just fresh out of engineering school—we highly recommend reading Cooling Tower and Condenser Water Design Part 1: The Refrigeration Cycle.