Greening Small Rooftop Packaged Units: Economizers

Introduction: Small rooftop packaged air conditioning units are sold in staggering numbers in the United States. As such, they represent a very large portion of the installed and future energy use in the built environment. This article on 'greening' rooftop packaged units is the first of a series that will address opportunities to increase the efficiencies of these units, and highlight JB products that can address these opportunities. Each article will discuss a different facet of efficient rooftop packaged unit design. This first installment will discuss the impact of effective economizers for rooftop packaged units

It is well established that air-side economizers save energy in the Pacific Northwest. And this stands to reason when you look at a graph of where the bulk of Seattle weather bin data lies:

(click for larger image)

The majority of the bin hours per year lie to the left of the 55º line, indicating that an economizer system would eliminate the need for mechanical cooling altogether during these hours. And nearly all of the hours are located to the left of the 75º line, where ambient temperatures would be lower than return air temperatures in a cooling system--allowing the system to offset some mechanical cooling load by using outside air.

When you consider that the use of outside air also brings IAQ benefits, it is clear why air-side economizers are such a compelling strategy for Northwest mechanical systems.

But there is a problem with air economizers in small packaged units: Too many of them don't work properly in the field. The reason for this is that for most small rooftop packaged cooling units do not have factory installed economizers. The standard of the industry is a bolt-on option that is shipped as a separate assembly to the jobsite for installation by the installing contractor. In some cases, they may not even be available at all for some duct configurations.

Typical small packaged unit economizer instalation

In practice, these economizers have a high rate of failure. The issue of non-functional economizers for small rooftop packaged units is significant enough that Puget Sound Energy includes re-commissioning of these devices in their Commercial HVAC Rooftop Unit Premium Service Rebate (program developed with the assistance of NEEC). And the Califorina Public Interest Energy Research program (PIER) goes further, recommending to owners and designers:

Specify reliable, factory-installed and -tested economizers with direct-drive actuators and low-leakage dampers.

That's exactly what Aaon provides on all of their units down to 1 ton.

(click for larger image)

Aaon's rooftop unit design provides inherent energy advantages over the competition. And factory-installed economizers are just one of many.

Extra: PIER software to estimate economizer savings.
Want free psychrometric software? See our offering here.

FREE Psychrometric Software

If you like the charts that I created to show psychrometric processes (like here), you're in luck.

Johnson-Barrow has made a deal to make this software available to our customers. We've also made some major aesthetic changes that we feel are a huge improvement: We've changed the chart colors and added a dynamic new logo!:




Pretty spiffy, huh?

The free version of the software allows you to do some simple analysis and process charting--allowing you to create high quality psychrometric charts for presentations or personal use. Additionally, a copy of the free version gives you a sizable discount off of the full version that is available here (chose HDpsychchart Pro Edition OEM upgrade SKU# HD1001, select Johnson-Barrow as OEM company).

The full version allows you to do the following:

• Model mixing, direct evaporative and humidification processes
• Create charts at any elevation
• Add climactic bin data
• Create flow charts
  
• Create detailed psychrometic process data points tables
• Vary the limits and extents of the chart axes
• Show ASHRAE winter and Summer comfort zones
• Project constant condition lines for ease of analysis

And many more tasks that make psychrometric chart analysis easy. The software also comes with additional tools like fan law calculators and even a loan payment calculator!

The free software is available for direct download on the toolbar to the right, on our www.jbarrow.com main page or right here.

UPDATE:

Some users have reported a problem with the software that prevents proper registration of the file. A new file that does not have this problem will shortly be uploaded.