Cooling with Mist
Mist cooling can provide a high-efficiency, energy-saving alternative to closed-loop cooling towers. In the steam cycle of a power plant and other industrial operations, low-pressure water from a steam condenser is pumped to high pressure before it enters the boiler or heat recovery steam generator (HRSG), where superheated steam is produced. The superheated steam is sent to the steam turbine, where it expands to low pressure and provides the energy required to drive the generator. This low-pressure steam must be condensed again to complete the cycle. The condensation of the steam requires a cooling medium. In conventional once-through systems, cooling is achieved by using water from a river, pond or other nearby body of water. The cold water is pumped through a heat exchanger, and the warm water is discharged back to the water source in an open-loop design.
The need to reduce the amount of water used in the cooling process gave rise to the idea of a closed-loop system known as wet cooling. In a wet cooling system, water is circulated to condense the steam in the same type of heat exchanger used in once-through cooling. However, instead of being returned to the water source, the warm water is cooled in a cooling tower using air as the cooling medium. Only the water carried away due to evaporation, drift and blowdown needs to be replenished. Such systems substantially reduce water consumption compared to open-loop designs. Modern closed-loop cooling towers also are designed to be energy-efficient, which is increasingly important with today’s rising energy costs.
However, another type of cooling system provides energy and water savings with a higher level of cooling efficiency. Called mist cooling, the technology is capable of maintaining water temperatures of around 88°F ±2°F (31°C ±1°C) throughout the year, regardless of climate conditions, with minimal power consumption and low maintenance requirements.
Wet Cooling Systems
To understand the benefits of mist cooling, it can be helpful to review how wet cooling systems work. Wet cooling towers are based on the principle of evaporation (figure 1). The heated water coming out of the surface condenser is cooled by air as it flows through a cooling tower. The air is circulated through the tower by either natural or mechanical draft.
Natural-draft towers, also sometimes called hyperbolic towers because of their shape, have been used at nuclear plants and large coal-fired power plants. However, they operate with low efficiency. The efficiency of a cooling tower usually is described in terms of its approach to wet-bulb temperature (WBT), or the difference between the cooled-water temperature and the entering-air wet-bulb temperature. In a natural-draft tower, the approach to WBT is about 11 to 14°F (6 to 8°C), with a temperature drop of 14°F (8°C). As a result, natural draft towers are used only in applications where a low level of cooling is required.
Most of the wet cooling towers in use today have a mechanical-draft or induced-draft design, in which the airflow is achieved with fans. Air enters through side louvers and escapes through the top of the tower. Water enters at the top and is cooled by the air draft as it trickles down through the system.
A correctly designed induced-draft cooling tower can give an approach to WBT of 7 to 11°F (4 to 6°C) with a temperature drop of 18°F (10°C). However, even a highly efficient cooling tower cannot give an approach to WBT of less than 7°F. Moreover, if the ambient temperature or outdoor humidity levels rise, the cooling tower efficiency is reduced. Cooling tower efficiency also drops over time due to the wear and tear on moving parts, fins and fills. Cooling towers also require regular maintenance, and they consume a lot of energy to operate the fans.
Case in Point: Two Cooling Tower Applications
In a power plant with a 6 MW condensing turbine, about 25 tons per hour of steam is condensed in condenser. The cooling towers are designed for a ΔT of 14°F, assuming a wet bulb temperature of 82°F (28°C) and a cold water temperature of 90°F (32°C), with approach of 7°F (4°C). Approximately 528 gal/hr of water circulates through the system.
In the peak summer months when humidity levels are at 90 percent or higher and ambient temperatures average 104°F (40°C), the cooling tower approach increases from 7 to 14°F (4 to 8°C), and the cold water temperature rises from 90 to 95°F (32 to 35°C) or higher. This rise in temperature increases the consumption of steam or reduces power output. Hence, all power plants normally operate with lower efficiency or higher steam consumption in the summer.
A similar situation occurs in a petrochemical or refinery plant. When the WBT reaches around 84 to 86°F (29 to 30°C) in the summer, the cooling tower gives an approach of 9 to 11°F (5 to 6°C). These industries experience a 5 to 7 percent drop in production due to this rise in the cold water temperature. In fact, the only time the cooling tower operates at its optimum approach temperature is during the winter months. This means that the plant operates at a reduced efficiency for six to eight months of every year.
Clearly there is a need for a water cooling system that will operate with high efficiency and maintain cold water temperatures closer to the WBT even in adverse climate conditions.
Mist cooling provides an efficient alternative to cooling towers. The technology uses recirculation pumps to draw water from a shallow pond (approximately 3' deep) and propel it through nozzles at high velocities (figure 2). The intensely atomized particles (subdivided to around 5 µm) rise about 25' above the nozzles to create a cooling mist. As they rise, the water particles develop a resonance that allows them to repel other water particles and prevents them from coalescing. The surface evaporation occurs quickly - faster than the water can reach equilibrium. As a result (table 1), mist cooling is able to provide an approach to WBT of 0 to 2°F (0 to 1°C) with a temperature drop of 22 to 27°F (12 to 15°C).
The ability of a mist cooling system to supply cold water with an approach to WBT of 0 to 2°F (compared to the 7 to 11°F approach of an induced-draft cooling tower) reduces the product vapor losses in shell-and-tube heat exchangers. It also allows plants to operate at optimum efficiency levels throughout the year, regardless of climate conditions. For example, in tropical conditions, the worst wet bulb temperature even in coastal applications is a maximum of 86.9°F (30.5°C). In this climate, mist cooling will maintain cold water of around 88°F ±2°F (31°C ±1°C) throughout the year. (A built-in hydro-balance system releases any excess pressure that might develop and prevents sub-cooling in the winter in colder climates.)
The high temperature drop of a mist cooling system reduces the amount of water required on the process side by approximately 35 percent compared to the amount required for a closed-loop cooling tower. Additionally, the technology does not require energy-intensive fans; instead, it relies on the water pressure available at the return line of recirculation pumps to create the cooling mist, thereby reducing energy consumption compared to induced-draft cooling towers.
Another benefit of mist cooling is reduced maintenance requirements. While cooling towers use louvers, fan blades, clamps and other components that must be replaced, mist cooling systems do not have any moving parts and therefore require little maintenance. Shallow mist cooling ponds also are easier to clean than the deeper ponds required for cooling towers, and ponds can be designed with two or three compartments to provide additional maintenance flexibility.
For plants with space constraints, a closed pond can be used. The approach to WBT increases to 4.5°F (2.5°C) with a closed-pond design, but a closed pond is 30 to 35 percent smaller than an open pond.
Chemical dosing, makeup water and blowdown requirements are similar to what is required with cooling towers. However, the atomization in mist cooling, along with the related absorption and retention of air by the water particles, allows the water to have better biochemical oxygen demand (BOD) and chemical oxygen demand (COD) values than the water in cooling towers.
Most plants that have installed mist cooling systems have seen a return on their investment in less than one year due to the water, energy and maintenance savings provided by the technology. As companies search for ways to improve production efficiency and reduce energy consumption, mist cooling presents an option.