Ferus Gas Industries originally planned to install an open cooling tower for summer process-heat rejection and an air-cooled heat exchanger for winter operation when makeup water for evaporation in the cooling tower is unavailable at the site. By using wet-dry closed-circuit cooling, a single piece of equipment handles all the plant's heat-rejection requirements: evaporative cooling in the summer, no water use in the winter, and less energy and water consumption than other options.
Ferus produces, markets, stores and transports liquid carbon dioxide and liquid nitrogen for industrial applications, primarily in Canada. Its liquid nitrogen plant in Strathmore, Alberta, can produce and store 300 tons per day of liquefied nitrogen. The liquefied nitrogen is used in the energy industry for well stimulation, well cleanup, pipeline purges, vessel blanketing and the production of natural gas from coal (NGC).
The air-separation plant produces liquefied nitrogen in a cryogenic process that separates the gas from the atmosphere through compression and cooling, so the raw materials are atmospheric air and electrical power.
Air-Separation ProcessThe composition of air in the atmosphere is roughly 78 percent nitrogen, 21 percent oxygen, 1 percent argon and small amounts of carbon dioxide, helium, krypton, hydrogen and xenon. Variable amounts of water vapor also are present, depending on local humidity plus other gases and particles produced by natural processes and human activities.
The air-separation process begins with filtering and compressing atmospheric air. Contaminants, including water and carbon dioxide, that would freeze and interfere with the process are removed.
Through repeated expansion and heat exchange, the gas is cooled to cryogenic temperatures of approximately -300oF (-185oC). Then, low-temperature distillation separates the gaseous air components to achieve the desired product purity. To produce a liquefied product, additional refrigeration is required (figure 1).
A cooling system, required for the product heat exchangers, compressor cooling and mechanical refrigeration system, usually takes the form of open- or closed-circuit cooling towers or air-cooled heat exchangers, depending on site conditions and the cost and availability of water.
The Strathmore plant, located in a rural farming community approximately 31 miles east of Calgary, Alberta, is ideally situated near areas of high coal-bed methane drilling activity, which requires nitrogen as a stimulation fluid to complete the methane wells. The nitrogen also is used by the energy industry in conventional oil wells and in the food industry to freeze and preserve foods. Electrical power is readily available at the site, but water for plant use only is available May through September from local irrigation canals. Because the canals freeze during the winter months, evaporative cooling cannot be used from October through April, when makeup water to replace the evaporation losses is not available.
As a result, the original plant design was based on using a conventional open cooling tower in the warmer months to take advantage of the cooler process water temperatures that are made possible by evaporative cooling. Then, in October, when the frozen irrigation canals shut off the water supply, the plant would switch over to finned-tube air-cooled heat exchangers that, thanks to the low ambient temperatures, could provide adequate cooling of the process water without consuming any water through evaporation. Although this system required duplication of cooling equipment, there did not seem to be any economical alternative.
Hybrid SystemSelecting and operating an air-cooled heat exchanger all year round was out of the question because the desired design process-water return temperature of 85oF (29oC) is essentially the same as the local design ambient dry-bulb temperature. At this point, the plant designer was introduced to the concept of a hybrid wet/dry, closed-circuit cooling tower (figure 2). It incorporates both dry finned-tube coil surfaces and wetted evaporative-coil surfaces to provide a combination of evaporative and dry cooling. If the tower could be sized so that the finned-coil surface could handle the entire duty in the winter months, it would have the performance advantages of the wet-surface evaporative cooling tower in the summer and those of the finned-tube air-cooled heat exchanger in the winter -- all in one piece of equipment.
During the warm months, the process fluid to be cooled flows first to the dry finned coil, where a portion of the heat rejection takes place, and then to the wetted evaporative coil, where the balance of the heat is rejected. Then, the process fluid returns to the system to pick up heat again. Spray water is drawn from the collecting basin and pumped to the water-distribution system above the wetted coil. Wetting this coil on the outside allows evaporative cooling of the fluid on the inside to occur. The spray water then falls from the wetted coil onto the wet-deck surface section, where it is cooled to enhance the evaporative cooling function.
Air is drawn across both the wetted-coil section and the wet-deck surface section, where it becomes saturated and picks up heat. This air mixture is still cool enough, however, to provide significant cooling as it passes through the finned coil installed in the air-discharge plenum above the fan. Even at peak summer conditions, approximately 30 percent of the heat rejection is handled in the dry section, reducing the evaporation proportionately by the same amount. In other words, at its maximum evaporation rate, the hybrid cooling tower has already reduced its water consumption by 30 percent compared to conventional open- or closed-circuit cooling towers.
As the ambient wet-bulb temperature drops below the summer design point, the water savings increase because the dry finned section handles a larger and larger percentage of the heat rejection, further reducing the evaporation rate. Eventually, the ambient temperature will reach a point where 100 percent of the heat rejection can be handled by the dry section, and the spray water can be turned off because no further evaporative cooling is required. This is called the dry-operation switch point and usually it is somewhat above the water's freezing point.
For the Strathmore site, in order to run completely dry from October through April, the dry finned-coil section had to be sized to provide adequate cooling at the maximum ambient dry-bulb temperature anticipated for those months. A design ambient switch-point temperature of 53oF (37oC) was selected. At that temperature, the spray water can be turned off and the basin drained. The process fluid still flows through both coils as before, but all of the heat rejection can be accomplished through sensible (nonevaporative) cooling from the coil surfaces.
Annual Water SavingsFor the operating ambient temperature range of May through September, figure 3 plots the typical wet- and dry-bulb temperatures for the plant site vs. the expected percentage of hours at or below those temperatures. It is a given that the hybrid unit will save water compared to a conventional open- or closed-circuit cooling tower operating year-round because it operates dry for approximately half the year. In this case, however, the hybrid unit achieved significant water savings compared to the alternative combination of a cooling tower/air-cooled heat exchanger (figure 4).
For the period from May through October, the predicted evaporation from the conventional cooling tower is 14.8 million gal. For the hybrid unit operating during the same period, the total evaporation is 9.1 million gal., a savings of 5.7 million gal., or 40 percent. Taking into account blowdown for a system operating at five cycles of concentration, the savings increases to more than 7 million gal. Because the plant is in an agricultural region that already is sensitive to demands on its groundwater supply, the designer appreciated that the plant could be a "good neighbor" with respect to water consumption.
By employing a hybrid wet-dry, closed-circuit cooling tower, Ferus Gas solved the dilemma caused by lack of available water during part of the year and saved on capital costs and annual water consumption at the same time. Ferus has since purchased an identical hybrid system for another installation.
For more information, contact Baltimore Aircoil. Call (410) 799-6200.
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