Mist Cooling System (Best Alternative to IDCT)

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 CoolingMist 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 re-circulation 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.

TYPE OF COOLING TOWER

Cooling towers are a very important part of many chemical plants. The primary task of a cooling tower is to reject heat into the atmosphere. They represent a relatively inexpensive and dependable means of removing low-grade heat from cooling water. The make-up water source is used to replenish water lost to evaporation. Hot water from heat exchangers is sent to the cooling tower. The water exits the cooling tower and is sent back to the exchangers or to other units for further cooling. Typical closed loop cooling tower system is shown in Figure 1.1.

Figure 1.1 Cooling Tower Type

Cooling Tower Types :- Cooling towers fall into two main categories: Natural draft and Mechanical draft.

Natural draft cooling towers use very large concrete chimneys to introduce air through the media. Due to the large size of these towers, they are generally used for water flow rates above 45,000 m3 /hr. These types of towers are used only by utility power stations.

Mechanical draft cooling towers utilize large fans to force or suck air through circulated water. The water falls downward over fill surfaces, which help increase the contact time between the water and the air - this helps maximize heat transfer between the two. 

Cooling rates of Mechanical draft towers depend upon their fan diameter and speed of operation. Since, the mechanical draft cooling towers are much more widely used, the focus is on them.

Mechanical draft towers are available in the following airflow arrangements:

1. Counter flows induced draft.

2. Counter flow forced draft.

3. Cross flow induced draft.

In the counter flow induced draft design, hot water enters at the top, while the air is introduced at the bottom and exits at the top. Both forced and induced draft fans are used.

In cross flow induced draft towers, the water enters at the top and passes over the fill. The air, however, is introduced at the side either on one side (single-flow tower) or opposite sides (double-flow tower). An induced draft fan draws the air across the wetted fill and expels it through the top of the structure.

The Figure 1.2 illustrates various cooling tower types. Mechanical draft towers are available in a large range of capacities. Normal capacities range from approximately 10 tons, 2.5 m3 /hr flow to several thousand tons and m3 /hr. Towers can be either factory built or field erected - for example concrete towers are only field erected.

Many towers are constructed so that they can be grouped together to achieve the desired capacity. Thus, many cooling towers are assemblies of two or more individual cooling towers or "cells." The number of cells they have, e.g., an eight-cell tower, often refers to such towers. Multiple-cell towers can be lineal, square, or round depending upon the shape of the individual cells and whether the air inlets are located on the sides or bottoms of the cells.

Components of Cooling Tower
The basic components of an evaporation tower are: Frame and casing, fill, cold water basin, drift eliminators, air inlet, louvers, nozzles and fans.

Frame and casing: Most towers have structural frames that support the exterior enclosures (casings), motors, fans, and other components. With some smaller designs, such as some glass fiber units, the casing may essentially be the frame.

Fill: Most towers employ fills (made of plastic or wood) to facilitate heat transfer by maximizing water and air contact. Fill can either be splash or film type.

With splash fill, water falls over successive layers of horizontal splash bars, continuously breaking into smaller droplets, while also wetting the fill surface. Plastic splash fill promotes better heat transfer than the wood splash fill.

Film fill consists of thin, closely spaced plastic surfaces over which the water spreads, forming a thin film in contact with the air. These surfaces may be flat, corrugated, honeycombed, or other patterns. The film type of fill is the more efficient and provides same heat transfer in a smaller volume than the splash fill.

Cold water basin: The cold water basin, located at or near the bottom of the tower, receives the cooled water that flows down through the tower and fill. The basin usually has a sump or low point for the cold water discharge connection. In many tower designs, the cold water basin is beneath the entire fill.

Figure 1.2 Cooling Tower Type

In some forced draft counter flow design, however, the water at the bottom of the fill is channeled to a perimeter trough that functions as the cold water basin. Propeller fans are mount-ed beneath the fill to blow the air up through the tower. With this design, the tower is mounted on legs, providing easy access to the fans and their motors.

Drift eliminators: These capture water droplets entrapped in the air stream that otherwise would be lost to the atmosphere.

Air inlet: This is the point of entry for the air entering a tower. The inlet may take up an entire side of a tower–cross flow design– or be located low on the side or the bottom of counter flow designs.


Louvers: Generally, cross-flow towers have inlet louvers. The purpose of louvers is to equalize air flow into the fill and retain the water within the tower. Many counter flow tower designs do not require louvers.

Nozzles: These provide the water sprays to wet the fill. Uniform water distribution at the top of the fill is essential to achieve proper wetting of the entire fill surface. Nozzles can either be fixed in place and have either round or square spray patterns or can be part of a rotating assembly as found in some circular cross-section towers.

Fans: Both axial (propeller type) and centrifugal fans are used in towers. Generally, propeller fans are used in induced draft towers and both propeller and centrifugal fans are found in forced draft towers. Depending upon their size, propeller fans can either be fixed or variable pitch.
A fan having non-automatic adjustable pitch blades permits the same fan to be used over a wide range of kW with the fan adjusted to deliver the desired air flow at the lowest power consumption. Automatic variable pitch blades can vary air flow in response to changing load conditions.


Tower Materials
In the early days of cooling tower manufacture, towers were constructed primarily of wood. Wooden components included the frame, casing, louvers, fill, and often the cold water basin. If the basin was not of wood, it likely was of concrete.

Today, tower manufacturers fabricate towers and tower components from a variety of mate-rials. Often several materials are used to enhance corrosion resistance, reduce maintenance, and promote reliability and long service life. Galvanized steel, various grades of stainless steel, glass fiber, and concrete are widely used in tower construction as well as aluminum and various types of plastics for some components.

Wood towers are still available, but they have glass fiber rather than wood panels (casing) over the wood framework. The inlet air louvers may be glass fiber, the fill may be plastic, and the cold water basin may be steel.

Larger towers sometimes are made of concrete. Many towers–casings and basins–are constructed of galvanized steel or, where a corrosive atmosphere is a problem, stainless steel. Sometimes a galvanized tower has a stainless steel basin. Glass fiber is also widely used for cooling tower casings and basins, giving long life and protection from the harmful effects of many chemicals.

Plastics are widely used for fill, including PVC, polypropylene, and other polymers. Treated wood splash fill is still specified for wood towers, but plastic splash fill is also widely used when water conditions mandate the use of splash fill. Film fill, because it offers greater heat transfer efficiency, is the fill of choice for applications where the circulating water is generally free of debris that could plug the fill passageways.

Plastics also find wide use as nozzle materials. Many nozzles are being made of PVC, ABS, polypropylene, and glass-filled nylon. Aluminum, glass fiber, and hot-dipped galvanized steel are commonly used fan materials. Centrifugal fans are often fabricated from galvanized steel. Propeller fans are fabricated from galvanized, aluminum, or molded glass fiber reinforced plastic.

COOLING TOWER PERFORMANCE

Figure 1.3 Cooling Tower Range and Approach

The important parameters, from the point of determining the performance of cooling towers, are:

i) "Range" is the difference between the cooling tower water inlet and outlet temperature. (See Figure 1.3).

ii) "Approach" is the difference between the cooling tower outlet cold water temperature and ambient wet bulb temperature. Although, both range and approach should be monitored, the 'Approach' is a better indicator of cooling tower performance. (see Figure 1.3).

iii) Cooling tower effectiveness (in percentage) is the ratio of range, to the ideal range, i.e., difference between cooling water inlet temperature and ambient wet bulb temperature, or in other words it is = Range / (Range + Approach).

iv) Cooling capacity is the heat rejected in kCal/hr or TR, given as product of mass flow rate of water, specific heat and temperature difference.

v) Evaporation loss is the water quantity evaporated for cooling duty and, theoretically, for every 10,00,000 kCal heat rejected, evaporation quantity works out to 1.8 m3. An empirical relation used often is:

*Evaporation Loss (m3/hr) = 0.00085 x 1.8 x circulation rate (m3/hr) x (T1-T2)

T1-T2 = Temp. difference between inlet and outlet water.

*Source: Perry’s Chemical Engineers Handbook (Page: 12-17)

vi) Cycles of concentration (C.O.C) is the ratio of dissolved solids in circulating water to the dissolved solids in make up water.

vii) Blow down losses depend upon cycles of concentration and the evaporation losses and is given by relation:  Blow Down = Evaporation Loss / (C.O.C. – 1)

viii) Liquid/Gas (L/G) ratio, of a cooling tower is the ratio between the water and the air mass flow rates. Against design values, seasonal variations require adjustment and tuning of water and air flow rates to get the best cooling tower effectiveness through measures like water box loading changes, blade angle adjustments.

Thermodynamics also dictate that the heat removed from the water must be equal to the heat absorbed by the surrounding air:

L(T1 –T2) = G(h2 – h1)

L = h2 – h1
G   T1– T2

where:

L/G = liquid to gas mass flow ratio (kg/kg)

T1 = hot water temperature (°C)

T2 = cold water temperature (°C)

h2 = Enthalpy of air-water vapor mixture at exhaust wet-bulb temperature
        (same units as above)

h1 = Enthalpy of air-water vapor mixture at inlet wet-bulb temperature
        (same units as above)

INDUSTRIAL COOLING TOWER

Cooling Tower

Industrial cooling towers might use river water, coastal water (seawater), or ground water as their supply of contemporary cooling water. the massive mechanical induced-draft or forced-draft cooling towers in industrial plants continuously circulating cooling water through heat exchangers and alternative equipment wherever the water absorbs heat. That heat is then rejected to the air by the partial evaporation of the water in cooling towers wherever up flowing air is contacted with the circulate down flow of water. The loss of gaseous water into the air exhausted to the air is replaced by "make-up" recent stream water or fresh cooling water. Since the vaporization of pure water is replace by make-up water contain carbonates and alternative dissolved salts, some of the current water is additionally continuously discarded as "blow-down" water to stop the excessive build-up of salts within the circulating water.

The Indian Energy Center. Over a billion fish eggs and larvae are killed in its cooling system every year.

Cooling water intake of a nuclear energy plant
On terribly large rivers, however more usually at coastal and estuarine sites, "direct cooled" systems are normally used, instead. These industrial plants don't use cooling towers and also the atmosphere as a heat sink, however put the waste heat to the watercourse or coastal water instead. These unlisted systems so depend upon an honest provide of stream water or water for his or her cooling desires. several facilities, particularly power plants, use lots of gallons of water per day for cooling. Such facilities were built with intake structures designed to pump in massive volumes of water at a high rate of flow. These structures tend to additionally pull in massive numbers of fish and alternative aquatic organisms, that are killed or scraped on the intake screens.

The warm water is came back on to the aquatic surroundings, typically at temperatures considerably (to aquatic life) higher than the ambient receiving water. pollution of rivers, estuaries and coastal waters may be a thought once siting such plants.

High-grade industrial water (produced by reverse osmosis) and potable water are generally utilized in industrial plants requiring high-purity cooling water.

Some nuclear reactors use heavy water as cooling. heavy water is used in nuclear reactors as a result of it's a weaker neutron absorbent. this permits for the utilization of less enriched fuel. For the most cooling system, traditional water is ideally used through the utilization of a heat exchanger, as heavy water is far costlier. Reactors that use alternative materials for moderation (graphite) can also use traditional water for cooling.

WATER COOLING

Water cooling is a technique of heat removal from parts and industrial component. As against air cooling, water is used because of the heat conductor. Water cooling is often used for cooling automobile combustion engines and huge industrial facilities like steam power plants, electricity generators, oil refineries and chemical plants. Alternative uses consist of cooling the barrels of machine guns, cooling of lubricator oil in pumps; for cooling functions in heat exchanger; cooling product from tanks or columns, and recently, cooling of varied major parts within high-end personal computers. the most mechanism for water cooling is convective heat transfer.

Cooling water is that the water removing heat from a machine or system. Cooling water could also be recycled through a recirculating system or utilized in one pass once-through cooling (OTC) system. Recirculating systems could also be open if they depend on cooling towers or cooling ponds to get rid of heat or closed if heat removal is accomplished with negligible phase change loss of cooling water. A heat exchanger or condenser might separate non-contact cooling water from a fluid being cool or contact cooling water might directly strike things like saw blades wherever phase distinction permits simple separation. Environmental rules emphasize the reduced concentrations of waste product in non-contact cooling water.

Advantages

Water is cheap and non-toxic. the benefits of using water cooling over air cooling involve water's higher specific heat capacity, density, and thermal conduction. this enables water to transmit heat over larger distances with much less volumetrical flow and reduced temperature distinction. For cooling electronic hardware cores in computing equipment, the first advantage of water cooling is that its enormously inflated ability to move heat far from supply to a secondary cooling surface permits for large, more optimally designed radiators instead of little, inefficient fins mounted directly on the heat supply. The vessel around an engine is also very effective at deadening mechanical noises, that makes the engine quieter. However, the disadvantage is that it prices considerably over an air-cooled engine system.