Calculate economizer savings with confidence in minutes with the economizer analyzer

Engineered Systems, Nov, 2004 by Michael Kjelgaard

The next software tool in line for us to evaluate is the economizer analyzer--a spreadsheet that quickly and accurately calculates air and waterside economizer savings. This time however, we'll take a more efficient approach. The step-by-step method used in this column to develop previous spreadsheets (heat recovery and ventilation), showing specific cell addresses and formulas was a little tedious and turned out to be more of a lesson in Excel programming than the subject. So now that we have the website up and running, we can just post the economizer analyzer online for download and use the column to concentrate on the fun stuff'. If you want to grab yourself a copy and follow along, it is now posted on the Reference/Tools page at www.ClimateQuest.Com.

ECONOMIZER COOLING

Most buildings require cooling year round, even during the winter. Interior office spaces that do not have exposure to the building envelope experience heat gain from lighting, people, and equipment regardless of the temperature outside. Computer rooms, data centers, and water-cooled equipment or process loads are additional examples of year round cooling applications.

Economizer cooling systems provide flee cooling to handle these loads whenever outdoor air conditions allow. The cost savings can be very significant, depending on the size of the economizer load and the hours of operation. There are basically two types of economizer systems: airside and waterside systems. Airside economizers mix out door air directly with return air to maintain the supply air temperature leaving an air-handling system (usually about 55[degrees]F). Waterside systems typically use a cooling tower and heat exchanger to provide chilled water in lieu of running a chiller.

There are many different economizer design options and approaches that need to be taken into consideration which have been discussed in previous columns. Examples include an evaluation of economizers with steam and adiabatic humidification (February 2003 issue), partial economizer cooling (October 2003 issue), choosing the best location for an economizer heat exchanger (November 2002 issue) and using chilled water coils for economizer cooling (November 2003). I would refer you to those issues to review these topics in more detail.

THE ECONOMIZER EVALUATOR

The engine behind this spreadsheet is the economizer load-hour data. Load-hours are the number of hours when the outside air temperature is below a user defined "economizer on" temperature. These setpoints are typically around 55[degrees] dry bulb for airside economizers and about 38[degrees] wet bulb for waterside systems. Economizer load-hours are a unique and powerful tool because they are generated from a user-defined operating schedule and load profile. The user selects which hours the economizer load would be active and the percentage of the load that would be expected for each hour. For every hour at 100% load, a load-hr value of 1.0 is assigned. For every hour less than 100% load, say 50%, a load-hr value of 0.5 is assigned. The output summarizes load-hrs by month, and the spreadsheet simply multiplies load-hrs by the maximum economizer load (in tons) to get ton-hrs saved.

Let's try an example calculation for a 500-ton data center located in Cut Bank. Go to the economizer load-hr generator in "Test Drive" and select normal data, all months. Then select Cut Bank, MT (Test Drive defaults to this city regardless of selection). For economizer system selection, select the waterside economizer option.

Let's assume the data center is only 50% loaded for hours 1 through 5 (1 a.m. to 5 a.m.) and 20 through 24 (8 p.m. to mid-night). The load at hours 6 and 19 is 75%. And the load is 100% for all remaining hours (7 through 18). In the load profile simulator, enter these values for all days of the week. In economizer control parameters, enter 38[degrees] wet bulb temperature. The program assumes economizer cooling is available below this value and only counts those hours. Input the values and assumptions along with the load hour results into the economizer analyzer as shown in Figure 3.

As would be expected for this type of application (and location), the savings are significant and would likely easily justify the installation cost of a heat exchanger and controls. Well, that's it. And it only required one column--a record for me (and much less work for both of us). Enjoy the new spreadsheet. I have a feeling you will find it very useful.