High-efficiency Mist cooling system for power plants

High-efficiency Mist cooling system for power plants
Power plants always face challenge to keep constant cold water temperature from cooling tower to maintain efficiency of power plant at desired level. The ultimate Mist creation technology is the best alternative to conventional cooling towers.  Here is the review of basic cooling systems used in power plants and other industriesIn the steam cycle of a power plant, low-pressure water condensed in the 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 the steam expands to low pressure, providing the energy to drive a generator. This low-pressure steam has to be condensed in a condenser in order to complete the steam cycle.Similarly, in process and chemical plants, product vapour generated in the process is condensed in a heat exchanger and recovered back.
The condensation of steam and vapour requires a cooling medium. In early days, this was achieved by using water from a river, a pond or sea. The cold water is pumped through a heat exchanger and the warm water is discharged back to the water source. This is called “once through” cooling system.A “once through” system is an open loop system. The necessity to reduce the huge amount of water generates the idea of closed loop system. Thus the “wet cooling” system came into effect.
In a “wet cooling” system, water is circulated to condense the steam in the same type of heat exchanger that is used in the “once through” cooling. The warm water, instead of being returned to the water source, 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 by makeup water. Thus requirement of water quantity is vastly reduced.
Wet cooling systemsWet cooling tower systemThe wet cooling tower system is based on the principle of evaporation. The heated water coming out of the surface condenser is cooled as it flows through a cooling tower, where air is forced through the tower by either mechanical or natural draft. Nowadays, mostly, all wet cooling towers are mechanical draft cooling towers where the air flow is accomplished by fans.
In case of a power plant, the steam turbine is not directly connected to the cooling system, so this is, in fact, an indirect cooling system. The steam from the steam turbine is condensed at the outside of the surface condenser tubes, using cold water coming from the cooling tower. Part of the cooling water is evaporated in the cooling tower, and a continuous source of fresh water (makeup water) is required to operate a wet cooling tower. Makeup requirements for a cooling tower consists of the summation of evaporation loss, drift loss and blowdown.
The principle cooling devices used in an induced or forced draft cooling tower are fans which run at the top of cooling tower (CT). Air enters through side louvers and escapes from the top. Water enters at the top and trickles down while getting cooled by air draft.A correctly designed induced draft CT can give an approach of 4-6 C to wet bulb temperature with a temperature drop of 10 C. Even a very highly efficient CT cannot give an approach less than 4 C to WBT. Moreover, if ambient temperature or humidity levels rise, efficiency of CT reduces.
Let’s consider this through an example. In a power plant having 6 MW condensing turbine, about 25 TPH steam is condensed in condenser. Cooling towers are designed for a ∆T of 8 C considering a wet bulb temperature of 28 C, and design cold water temperature of 32 C, with approach of 4 C. Total circulation water quantity is about 2,000 cubic metre/hour.
In peak summer or monsoon, when humidity levels reaches more than 90 per cent with ambient temperature of above 40 C, CT approach goes up from 4 C to 8 C. Thus cold water temperature increases from design 32 C to above 35 C.
This increase in temperature directly increases consumption of steam or reduces power output. Hence, all power plants normally operate with low efficiency or higher steam consumption in summer and monsoon.
Similarly, if we consider case of a petrochemical or refinery plant in peak summer when WBT reaches around 29/30 C, CT gives an approach of 5 C to 6 C. Thus due to this rise in cold water temperature, these industries always experience loss in production by at least 5 to 7 per cent. These losses do not occur in winter. This means that the plant will operate at a reduced efficiency for almost 6 to 8 months in a year.
Also, due to use of fans, CT consumes a lot of power. It is observed that the efficiency of CT reduces over a period of time due to wear and tear of moving parts, fills, fins, etc. which invite heavy maintenance.
Hence there is an urgent demand from the industry for a water-cooling system, which will operate with high efficiency even in adverse climatic and maintain cold water temperature in closed vicinity to WBT.
Mist cooling systemMREPL has come out with a solution by designing high-efficient mist cooling system, which ensures an approach of 1C to prevailing wet bulb temperature with a temperature drop of 12 to 15 C even in adverse climatic conditions.
In tropical conditions, worst wet bulb temperature even at coastal applications is maximum 30.5 C. Hence MCS will always maintain cold water of around 31 C plus 1 C throughout the year. No other cooling system can operate with such efficiency, making cooling tower and spray pond systems obsolete.
Salient features of Mist cooling systemCold water temperatureMist cooling system can ensure an approach of 0 to 1 C to WBT with a  temperature drop of 12 C to 5 C.
Energy savingsDue to such high temperature drop obtained, water quantity required at the process side is much less. MCS requires water pressure equivalent to the height of cooling tower. Hence, considerable amount of energy is saved on circulation water pumping. Also, MCS does not require any fans for cooling. Thus, a huge amount of energy is saved on circulation and cooling.

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