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Saving Energy With Cooling Towers

Marley HP7000 Fan

Frank Morrison is supervisor, world strategy at Baltimore Aircoil Firm in Jessup, Md. He is chair of ASHRAE TC 3.6, Water Treatment, and a voting member of ASHRAE SSPC ninety.1, Vitality Commonplace for Buildings Except Low-Rise Residential Buildings.

Water-cooled techniques provide decrease power use than air-cooled alternate options. A few years in the past, the primary water-cooled systems used potable water directly within the condenser to supply heat rejection with the cooling water wasted to a drain. Cooling towers have been developed to recycle greater than 98% of this water, resulting in large reductions in water and power use as these methods grew in each size and popularity.

Since then, water-cooled systems have steadily improved their efficiency. As an example, the effectivity of a 500 ton (1757 kW) water-cooled centrifugal chiller has improved by over 50% since 1975 as indicated by the necessities of ASHRAE/IES Customary 90.1 (hereafter referred to as Normal ninety.1). Cooling towers have additionally advanced from centrifugal fan models to much more vitality-environment friendly axial fan designs with improved heat transfer surfaces, known as fill. As well as, impartial certification of thermal performance for open circuit cooling towers per the Cooling Know-how Institute’s Normal 201 has grow to be broadly accepted in the market and became required by Customary ninety.1 in the 2007 version.

Whereas the efficiency improvements of particular person system components have actually lowered total energy use, even greater improvements are potential by optimizing the best way cooling systems are designed and operated. For example, the complete load power use in a 500 ton (1757 kW) water-cooled chiller system, based on Commonplace 90.1-2013 minimum efficiencies, is roughly damaged down as follows: chiller 77%, cooling tower eight%, condenser pump 7%, and chilled water pump eight%. With the chiller accounting for nearly all of the energy use, many contend that it is sensible to function the cooling tower fan and condenser pump such that compressor energy use is reduced—since it’s by far the most important motor within the system.

For example, to lower chiller energy, the cooling tower is often operated at full fan velocity and flow until ambient situations allow the minimum condenser water temperature limit to be reached. Below this stage, the fan speed of the cooling tower is modulated, typically by a variable pace drive (VSD), to take care of the setpoint. This is actually the operating sequence for the water-cooled baseline buildings found in Appendix G of Customary ninety.1-2013, which makes use of 70°F (21.1°C) as the lower condenser water setpoint (though this value is above the low limit for almost all chillers). In practice, this lower restrict varies and relies on the type of chiller. The nearer to full load the system runs, the larger the vitality financial savings from such strategies. Nonetheless, most chiller systems function at lower than full load for the vast majority of time.

While it could appear counterintuitive, many designers and operators have discovered that using less cooling tower energy reduces total system energy at many off-design conditions. At such conditions, ancillary tools (condenser pumps and cooling tower fans) working at full design pace turns into a bigger portion of the system power use, especially when variable speed chillers are used. Decreasing cooling tower fan pace can reduce associated fan power considerably whereas increasing chiller energy only marginally. For example, slowing the tower fan speed to eighty% of design reduces tower fan power by about half, while only elevating the cooling tower leaving water temperature about three °F (1.7°C). Depending on the specific load point, the rise in chiller energy consumption from the upper condenser water temperature might or may not be lower than the reduction in cooling tower vitality. The secret is to stability the efficiency of the system elements so general efficiency is optimized. Articles corresponding to Taylor’s glorious series on chilled water system design present extra details on such strategies.

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