The HXV offers a combination of sensible, adiabatic, and evaporative heat transfer to significantly reduce any plume that may occur with conventional evaporative cooling equipment. During the coldest times of the year, when the potential for visible discharge is greatest, the HXV operates 100% dry, completely eliminating plume.
Water savings are achieved throughout the year with each of three different operating modes.
The finned dry coil tempers the incoming fluid, allowing higher inlet fluid temperatures than traditional closed circuit cooling towers.
The HXV provides heat rejection at the lowest possible energy input and maintenance requirements via:
Materials of Construction — Various materials are available to meet the corrosion resistance, unit operating life, and budgetary requirements of any project.
|Operation Mode||Dry Finned Coil Fluid Flow||Wet Prime Surface Coil Fluid FLow||Spray Pump||Fan(s)|
In this mode, the fluid to be cooled flows first through the dry finned coil and then through the prime surface evaporative coil, where the cooled fluid exits the unit. Spray water is drawn from the cold water basin and pumped to the water distribution system above the prime surface coil. Wetting the prime surface coil allows evaporative cooling to occur. The spray water falls from the prime surface coil over the fill surface, enhancing the evaporative heat transfer by sub-cooling the spray water. Air is drawn through both the prime surface coil and through the fill where it is saturated and picks up heat. The air is, however, still cold enough to achieve significant cooling within the finned coil, which is installed at the discharge above the fan(s).
In the dry/wet mode, both sensible and evaporative heat transfer are used. Compared to a conventional evaporative unit, the potential for plume is substantially reduced and significant water savings can be obtained, even at peak design conditions. At reduced heat load and/or ambient temperatures, the evaporative cooling portion, and hence water usage, is further reduced as the flow through the evaporative coil is gradually decreased. This is accomplished by a modulating flow control valve arrangement, which controls the outlet fluid temperature. This control arrangement automatically assures maximum use of sensible cooling in the finned coil and minimum use of evaporative cooling in the prime surface coil. The heat transfer method and flow control are arranged to achieve maximum water savings in the dry/wet mode. Plume is minimized by reducing the amount of evaporated water and the heating of the entire discharge air with the dry finned coil.
The adiabatic mode occurs when the fluid to be cooled completely bypasses the evaporative prime surface coil. No heat is rejected from this coil and the recirculating spray water merely serves to saturate and adiabatically pre-cool the incoming outside air. In most climates, the ambient air still has considerable potential for absorbing moisture.
Thus adiabatic cooling of the incoming air results in significantly lower air temperatures, which greatly increases the rate of sensible heat transfer. Compared to conventional evaporative cooling equipment, visible plume and water consumption are greatly reduced while maintaining the low fluid design temperatures required to maximize system efficiency.
During the dry operation mode the spray water system is turned off, saving on pump energy. The fluid to be cooled is fed from the finned coil to the prime surface coil. The modulating flow control valve remains fully open to ensure both coils receive the full fluid flow in series; hence the maximum heat transfer surface is available. In this mode no water consumption occurs, and plume is completely eliminated. HXV units can be economically selected for dry bulb switchover points of 50°F (10°C) to 60°F (15°C) or higher, depending on the specific needs of the project. When the equipment operates in the dry mode for prolonged periods, draining the cold water basin is recommended, eliminating the need for freeze protection and water treatment.
Heat rejection equipment must be selected for the maximum heat load at summer peak air temperatures. In most climates peak wet-bulb temperatures are significantly lower than peak dry-bulb temperatures. Evaporative cooling equipment based on the ambient air wet-bulb therefore has a greater temperature driving force, thus allowing the use of lower system temperatures. This greater driving force also allows the use of less and thus more cost-effective heat transfer surface area. Since the HXV utilizes evaporative cooling during peak load operation it inherently benefits from this advantage. Evaporative cooled units such as the HXV have a plan area and fan horsepower advantage over the typical air-cooled arrangement, saving on support structures and electrical hook-ups. The HXV design also avoids the corrosion and scaling that can be associated with spraying of standard air-cooled equipment on design days for additional capacity. The lower process fluid temperatures that can be achieved compared to air-cooled systems and the greatly reduced fouling factors of closed loop cooling result in lower first cost of process equipment such as chillers or refrigeration compressors. Lastly, the costs associated with plume abatement are eliminated, as the design is inherently plume-free.
Due to its water saving concept and combined flow design, the HXV offers significant operating cost benefits. Water consumption is minimized throughout the year. During peak summer operation a large amount of heat load is already transferred by the finned coil. As the ambient temperature and/or heat load drops, the amount of evaporative heat transfer is further reduced by controlling the flow through the wet coil. This reduces the evaporation loss and blow-down as well as water treatment requirements compared to conventional evaporative cooling equipment. In the “adiabatic” mode only a small amount of water is needed to saturate the air and the amount of blow-down is reduced even further. Finally in the “dry” mode no water is used at all (while saving the energy associated with running the spray pump). With HXV hybrid units water savings up to 70% or more are possible. Depending on local water costs and availability, this advantage alone can pay for the equipment in as little as two years through cost savings in water use, water treatment chemicals, and higher system efficiencies. In addition, fouling potential associated with open circuit cooling towers is eliminated through both the closed loop cooling system and the Combined Flow Technology design of the HXV, assuring peak efficiency and energy savings over time. Finally, the induced draft propeller fan design results in low fan energy requirements compared to centrifugal fan units.