Design of a Low-cost, Active Evaporation System

We wanted a preliminary conceptual design for a low energy, low cost, high volume fresh water evaporation system. 

Figure 1 illustrates the proposed system.  A high pressure pump forces water through an atomizing nozzle.  At the same time, a low-speed fan induces a stream of ambient air into the tank. The air exits the tank through a duct configured to form a vortex inertial separator.  A portion of the atomized water evaporates into the ambient air stream through the tank, while the remainder falls back to the reservoir or is collected in the inertial separator and directed back to the reservoir.

Figure 1.  Diagram of conceptual design

Performance

   The liquid water storage capacity depends only on the design choice of reservoir size.  The evaporation rate is also a design variable dependent on energy usage.  The table below shows two preliminary estimates of evaporation performance and energy usage detailed by fan and pump power for just one possible system configuration.  These are only very rough estimates and have not been experimentally validated.

evaporation capacity	pump power (est.)	fan power (est.)
3 liter/hr	1 W	21 W
1 liter/hr	0.37 W	8.2 W

Advantages

    The proposed system offers a number of significant advantages over conventional evaporation approaches: 

           Design freedom

The combination of a high pressure atomizer with an inertial separator allows great flexibility in system design by permitting trade-offs in individual component specifications.  In general, higher pressure water supply will result in smaller droplet size distributions from atomizing nozzles, however this costs more in pumping power and pump cost.  More expensive nozzles will produce smaller droplets and narrower droplet size distributions. Larger droplets can be produced less expensively in terms of nozzle and pumping costs, but they will require greater residence time for evaporation.  Higher air flow rates and higher air stream turbulence levels increase evaporation rates but cost more in fan power.  Trade-offs between various component costs (nozzle, pump, fan, inertial separator) and between capital and operating (energy) costs can be optimized to give excellent performance at minimum overall cost.  The inertial separator at the exit will insure that liquid water is contained so that wide variations in performance of individual upstream components can be tolerated while still resulting in an acceptable design.

Off–design operation

The same flexibility that allows for tremendous design freedom also permits the design of a very robust overall system.  Many factors including nozzle wear, off-design environmental conditions, water contamination, operator interference, control system failure, etc., can result in system operation at conditions outside of the original design parameters.  Ideally, a system will continue to give acceptable performance away from a single operating specification.  The flexibility inherent in this concept will easily allow for such a robust design. 

Evaporative cooling

In general, the evaporative cooling effect will result in a cool and moist air stream exiting the system.   This may be a primary aim or a desirable side effect.  This system maintains the principal advantage of a conventional evaporative cooling system—low operating cost--while avoiding the disadvantages inherent in maintaining a wetted media for evaporation – mold/fungus growth, higher pressure drop for the air system, and maintenance/replacement of the media.