Wet surface air coolers minimize water use by maximizing heat transfer efficiency

By Peter G. Demakos, PE, Niagara Blower Co.

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The cooling needs of thermoelectric power plants account for a significant amount of freshwater use in the U.S. The future promises increased competition for that water from agricultural, residential, and commercial users. (See POWER, April 2008, “New coal plant technologies will demand more water.”)

Given limitations on freshwater availability, the need for water-stingy cooling technologies is growing--an issue covered in the January 2008 issue of POWER (“Costlier, scarcer supplies dictate making thermal plants less thirsty”). One technology in particular--closed-loop, evaporative cooling--can be very cost-effective for the main heat transfer needs of simple- and combined-cycle power plants.

Closed-loop evaporative coolers (also known as wet surface air coolers, or WSACs) have an extensive and admirable track record at facilities in a wide variety of industries. Applications in the power industry include auxiliary fluid loop cooling, direct steam condensing, and inlet air chilling refrigerant condensing. In the U.S., a WSAC unit has been cooling a 240-MW plant near Springfield, Mass., since 1993, and many similar units are working throughout North America and overseas.

ABCs of WSACs

The basic operating principle of a WSAC is rejection of heat by evaporation. The fluid/vapor to be cooled or condensed flows through tube bundles in a closed-loop system. A large quantity of water (generally 7 to 10 gpm/ft² per coil face area) from the unit basin is sprayed downward over the tube surfaces (Figure 1). Simultaneously, fans induce air flow over the bundles in a co-current direction. Evaporative cooling takes place at the exterior tube surfaces. The saturated air stream leaving the tube bundle then makes two 90-degree turns into the unit’s fan plenum. The reduction in velocity returns almost all of the large water droplets to the basin. The air is then discharged out of the unit through the fan stacks.

1. Go with the flow. The co-current flow of air and water in a wet surface air cooler increases the system’s heat transfer efficiency, reduces scale deposition on active surfaces, and virtually eliminates drift of water droplets into the environment. Source: Niagara Blower Co.

Keeping the process stream inside the tubes is important for three reasons:

It maintains thermal performance.
It minimizes and simplifies maintenance.
Open-loop spray water never contaminates the process stream. This allows poor-quality water to be used as makeup and enables operation at higher cycles of concentration (see sidebar). It also eliminates exposure of the process fluid to the environment.

WSAC put to the test

Three summers ago, a wet surface air cooler (WSAC) from Niagara Blower Co. was independently tested for 147 days on the 550-MW Unit 3 of the 1,500-MW San Juan Generating Station (SJGS) in New Mexico to determine its ability to cool poor-quality circulating water. The project was funded by the Electric Power Research Institute and the U.S. National Energy Technology Laboratory.

A unique aspect of WSAC technology is the continuous flushing of tube surfaces by a dense flow of spray water, which prevents suspended matter and mineral scale from depositing on them. Scale and suspended solids eventually settle in the WSAC basin or on noncooling surfaces.

The purpose of the pilot test was to determine if WSAC technology could cool process water at cycles of concentration that would be considered excessive (highly scale-forming) for a mechanical-draft cooling tower. To accomplish this, blowdown from the Unit 3 cooling tower at SJGS was fed to the WSAC pilot as makeup.

Between July 5 and November 29, 2005, the WSAC was operated for 2,898 hours at an equivalent of 24 to 70 cycles of concentration (based on freshwater normally supplied to SJGS cooling towers). Ten cycles of concentration is considered a safe limit at this plant to control mineral scale formation.

During the testing, Unit 3’s cooling tower, with a capacity of 220,000 gpm, was operated within a range of parameters (see table). Continuous chlorination was used to maintain a residual of 0.1 to 0.2 mg/l of chlorine in the cooling tower’s circulating water. A phosphonate-type inhibitor was used for scale control, and an azole-type corrosion inhibitor was used to protect the copper-based admiralty brass metallurgy of the main condenser.

Operating parameters for test of a wet surface air cooler. Source: Niagara Blower Co.

The goal of the testing was to determine the degree to which blowdown from Unit 3’s cooling tower could be concentrated in the WSAC and still maintain heat transfer. At the outset of testing, it was decided to operate at 5,000 microSiemens/cm conductivity (blowdown from Unit 3 to the WSAC had a conductivity of about 2,500 µS/cm).

As testing progressed, the plan was to increase the conductivity control setting on a stepwise basis, with the intent of determining the cycles of concentration (and chemistry) at which heat transfer would start to deteriorate. This threshold was never reached, however, because conductivity control proved problematic. Nonetheless, the WSAC proved able to operate at very high cycles of concentration with significant levels of suspended matter without degrading heat transfer.

To summarize the major findings and conclusions of the pilot testing:

Throughout the testing, the WSAC tube exterior surfaces were visually clean and smooth to the touch, with no apparent mineral deposits or biological film. At the completion of testing, there was no visible scale on the heat transfer surfaces (tube externals). Cooling was sustained throughout the test period.
Cycles of concentration varied (somewhat erratically at times) from 3.5 to over 10 during the test period. The average for the test period was 4.4 cycles. This was equivalent to 24 to 70 cycles of concentration (30 cycles average), based on Unit 3’s cooling tower freshwater makeup. This was far in excess of the control level for the cooling system: 10 cycles of concentration (based on calcium sulfate saturation).
Heat transfer was maintained throughout the testing; the temperature difference across the bundles was steady. Heat transfer was only a problem when one or more of the unit’s three fans were taken out of service.
The bundles with Cu-Ni tubes exhibited slightly better heat transfer than titanium and 316 stainless steel bundles (even with signs of corrosion in the interior Cu-Ni surfaces).
Calcium and silica cycles of concentration paralleled flow-based and heat-based cycles, but they were always less. This was attributed to calcium and silica dropping out of solution to form mineral salts at elevated cycles.
There was no measurable benefit to the WSAC pilot from residual concentrations of the scale and corrosion inhibitors injected into Unit 3’s cooling tower blowdown.
The exterior surfaces of the tubes fared well with respect to visible scale and corrosion. Only the Cu-Ni tubes showed signs of corrosion on the internal surfaces (non-WSAC side).
Mineral scale and suspended matter (measured as turbidity) were very evident throughout the testing. Total suspended solids levels greatly exceeded cooling system standards, making it clear that solids control is very important to the use of WSAC technology in this manner. However, given the operating conditions (high cycles of concentration, formation of extensive mineral scale, and very high levels of suspended matter in the bulk cooling water), the WSAC performed well by maintaining heat transfer throughout the test period.

The co-current flow of air and water creates an unobstructed spray system that is fully accessible for observation and maintenance. Another plus of the design: Because the air passes over the water in the system before and during its contact with the tube bundle, the mixed water temperature remains above freezing. This protects the tubes from freezing even when the ambient air temperature is below zero. The co-current flow also ensures complete coverage of tube surfaces (there are no bare spots, as seen with counterflow designs), reducing further the potential for fouling and freezing.

A WSAC is able to cool process fluids to within 8 to 10 degrees Fahrenheit of the wet bulb temperature, which is always lower than the ambient dry bulb temperature. As a result, a WSAC can deliver cooling water at 80F even on a day as hot as 110F (ambient).

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