In the January 2003 report, we reviewed different options for locating a free cooling heat exchanger within a central plant. You may recall that the tower side of the cooling tower bypass control valve seemed to make the most sense because it essentially allows for dual condenser water temperature control.

When in the free cooling mode, the towers operate to maintain cold water to the heat exchanger with the bypass valve closed. As a result, there is no circulation on the chiller side of the valve and the water temperature stays at room temperature. A chiller can then be started without delay when changing back to mechanical cooling.

Alternatively, if the condenser water connection is on the chiller side of the bypass valve, a warm up period is required to raise the condenser water temperature to a reasonable level prior to starting a chiller.

Not only does the ability to maintain two separate condenser water loop temperatures provide for better overall plant operation, it also allows for significant additional economizer savings. Why not take advantage of wet bulb temperatures that are above the economizer mode setpoint but are still low enough to generate chilled water that is less than the main chilled water return? The only required change is to connect (decouple) the heat exchanger from the main chilled water return in lieu of simply piping it in parallel with the chillers (Figure 3). This modification allows for reducing the load on a chiller by precooling the chilled water return through the heat exchanger. A reduction in load can be achieved as long as the condenser water temperature leaving the tower is less than the main chilled water return temperature.

To illustrate, let's assume the water temperatures in Figure 3 can be obtained at 46 degrees wet bulb. The chilled water return is reduced from 60 degrees to 55 degrees by the heat exchanger. And the load on the chiller is reduced from 625 tons to 417 tons. The following sequences briefly describe how both economizer modes can work within this piping arrangement:

100% free cooling mode (below 40 degrees F wet bulb) - The HX-1 pumps are on and the chillers are off. The tower fans operate to maintain 40 degrees condenser water to the heat exchanger. The tower bypass valve is 100% closed, maintaining the condenser water on the chiller side of the valve at room temperature. The main secondary chilled water return enters HX-1 at 60 degrees and flows back at 45 degrees to the secondary pumps via the secondary bypass.

Partial free cooling mode (41 degrees to 49 degrees wet bulb) - The HX-1 pumps remain on and one chiller is on. The tower fans operate at 100% to maintain minimum condenser water temperature to the heat exchanger. The tower bypass valve modulates to maintain proper condenser water supply temperature to the chiller. The main secondary chilled water return enters HX-1 at 60 degrees and flows back into the primary loop at a lower temperature, reducing the load on the operating chiller.

How much additional savings can be realized? Assume the plant in Figure 3 is located in Boston and the 625-ton load is constant. The equipment cost is $.70 kW/ton at a $0.09/kWh rate. In Boston, there are 3,495 hours less than 40 degrees wet bulb. So the estimated 100% economizer savings are then 3,495 • 625 • .70 • $0.09 = $137,615 (less the tower fan and pumping costs).

Let's assume partial economizer cooling can be feasibly accomplished up to 49 degrees wet bulb, which translates into an additional 1,387 economizer hours at part load. A model of this system resulted in 543,604 ton-hrs or $34,247 of additional savings. That's a savings increase of about 25%. Note that there are costs associated with operating the heat exchanger pumps, which need be taken into consideration when determining the upper wet bulb temperature limit (49 degrees in this case).

Are there any downsides? Perhaps there is an issue with the configuration of a plant that presents a problem, like a bypass valve located too close to the tower. Notwithstanding, if there are no such prohibitive issues, a 25% increase in operational cost savings with no added construction costs make this approach look attractive. ES
NOTE: This report is generated from raw data furnished by the National Weather Service (NWS). Normal max/min and degree day values are from the historical record provided by the National Climatic Data Center (NCDC). Normal values for VLI and economizer hours were derived from the TMY2 data set compiled by the National Renewable Energy Laboratory (NREL). ASHRAE design hours are number of hours that meet or exceed the 1997 ASHRAE Fundamentals design conditions. Airside and waterside economizer cooling hours are the quantity of hours that the outdoor air was below 55 degrees dry bulb and below 40 degrees wet bulb. The cooling ventilation load index (VLI) is the total (sensible + latent) energy in Ton-hrs/cfm required to maintain 55 degrees discharge air temperature. Likewise, the heating VLI is the sensible heat energy in Therms/cfm required to maintain 55 degrees. The humidification VLI is the amount of water in gal/cfm required to maintain 30% rh at 70 degree space temperature.

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