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Technology | February 2013

Mist Cooling System for Power Plants

The challenge being faced by power plant is to keep constant cold water temperature from cooling tower to maintain efficiency of power plant at desired level. In monsoon and summer when humidity is high cooling tower fails to achieve desired cold water temperature which results in lower efficiency of power plant, in turn resu­lting in loss of valuable power or higher consumption of fuel. Now it is time to find new solution to get desired cold water temperature throughout year for TG condenser.

The ultimate mist creation technology is the best alternative to conventional cooling towers. This advanced mist creation system can achieve an approach of 1 to 2°C to WBT as against 5 to 6°C approach for conventional cooling tower. Hence guaranteed cold water temperature of around 30°C could be obtained throughout the year in our tropical climate. Also the spraying head is equivalent to the height of cooling towers thus requiring same pumping power. As mist creation system does not require fans for cooling it saves huge amount of power. Also, as there are no moving parts involved in Mist creation system the maintenance cost is negligible and system runs trouble free. 

Let us review the basic types of cooling systems that are used in power plants and other industries to clarify why the new advanced mist creation sys­tem is superior as compared to a conventional coo­ling tower.

In 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/Chemical Plants, product vapour generated in the process is condensed in a Heat exchanger and is recovered back. The condensation of steam/vapour requires a cooling medium. In early days, this was achieved by using water from a river, a pond or seawater. 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 gave birth to 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 blow-down needs to be replenished by make-up water. Thus requirement of water quantity is vastly reduced.
 
WET COOLING SYSTEMS
1. Wet Cooling Tower System Circulation Water Cycle in Cooling Tower Plan A
The 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. Now a days, 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 blow-down. The principle cooling devices, used in an induced/forced draft cooling tower, are the 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 to 6°C to wet bulb temperature with a temperature drop of 10°C. Even a very highly efficient CT can not give an approach less than 4°C to WBT. Moreover, if ambient temperature or humidity levels rise, efficiency of CT reduces.

For 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 (CW) temperature of 32°C, with approach of 4°C. Total circulation water quantity is about 2000 M³/Hr.

In peak summer or monsoon, when humidity levels reaches 90per cent plus with ambient temperature of 40°C plus, CT approach goes up from 4°C to 8°C. Thus Cold Water temperature increases from design 32°C to 35°C plus.

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 the case of a petrochemical / refinery plant, in peak summer when WBT reaches around 29/30°C, CT gives an approach of 5 to 6°C. Thus due to this rise in cold water temperature, these industries always experience loss in production by at least 5 to 7per cent. These losses do not occur in winter months. 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 ware and tear of moving parts, Fills, Fins etc. which invites 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 conditions and maintain cold water temperature in closed vicinity to WBT.
 
2.Mist Cooling System
MREPL has come out with a solution by designing Mist Cooling System which is a high efficiency system, which ensures an approach of 1°C to prevailing wet bulb temperature with a Temp. drop of 12 to 15°C even in adverse climatic conditions.

Circulation Water Cycle in MCS Plan B  
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 + 1°C throughout the year. No other cooling system can operate with such efficiency and it makes cooling tower/spray pond systems obsolete.
 
SALIENT FEATURES OF MIST COOLING SYSTEM  
Cold Water Temperature  
Mist Cooling System can ensure an approach of 0 to 1°C to WBT with a temperature drop of 12°C to 15°C.

Energy Savings  

Due 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.

Process Benefits  
Due to lower cold water temperature obtained from MCS, designed vacuum is maintained at the exhaust of the TG throughout the year, thus ensuring desired energy output at lowest possible steam consumption.

Maintenance  
MCS has no moving parts. Also the material used in the mist cooling system is special grade saran polymer, a highly non-corrosive material having a life of more than 10-15years. This makes MCS absolutely maintenance free. As against this, cooling towers require a heavy maintenance in form of replacement of louvers, fan blades, clamps etc. every year.
MCS operates with a choke less design. Size of smallest opening in MCS is more than one inch (25 mm).Hence Chances of particles choking the system are minimum.
 
VARIOUS DESIGNS OF MCS TO SUIT SITE CONDITIONS

A) Open Pond MCS
Here, MCS ensures an approach of 0 to 1°C to WBT with a ?T of 12 to 15°C. Water loss due to drift is 0.1 to 0.25per cent depending on wind load
B) Closed Pond MCS
Here MCS pond is closed from sides, up to a height of 5 to 6 mtr by louver type cover sheeting. MCS ensures an approach of 2.5°C to WBT with a ?T of 12 to 15°C. Drift loss comes down to less than 0.02per cent and also space requirement reduces considerably.
    
All Power Plants are using this deign
C)    Table Top Design To Prevent Algae Formation:
    Latest table top design of MCS pond does not allow water to form column inside and all water passes to suction pit is covered from top thus minimizing chances of algae formation.
 
D)    Working Of MCS In Dusty Envir­onment:
Unique suction pit design does not allow dust to pass to the inlet of circulation pumps. Dust s drained from drain valve while only clear water circulation water pumps. 

Water Quality: MCS ens­ures efficient atomization and the consequent abso­rption & retention of air by water particles thus causing aeration of water, showing better BOD & COD values. System Flexibility MCS is the only system, which gives you such a high flexibility in operation.
 
HYDRO- BALANCE VALVE  

a) Sub cooling in winter:-
MCS is provided with an inbuilt hydro-balance system (HBV) for temperature control. HBV can be set at a specific pressure. To avoid sub cooling in winter, HBV can be set at a lower pressure than the system operating pressure. Hence it will by-pass some fixed amount of hot water ( as per setting) from distribution header directly into MCS pond so as to obtain desired C.W. temperature. This H.B.V. is normally manual controlled. Butcan be offered with actuator operation if desired.

 b) Hydro-balance system also helps to release the excess pressure which may develop on the system at times.

Chemical Treatment : Chemical dosing requirements are similar to that of cooling tower as same hold up of water is maintained in suction pit.

Make-Up Water Requirement: Due to latest closed pond design, drift loss through MCS is reduced to less than 0.02per cent while maintaining an approach of around 2°C to wet bulb temperature. Hence, Overall make-up water quantity required is approximately same as compared to cooling towers.

MCS MATCHES THE DESIGN AS PER NEED
MCS can be put to use in Open Pond or Closed Pond designs to suit the need. Open pond ensures an approach of 1 deg.C to WBT while closed pond ensures an approach of 2.5 deg.C to WBT. Space requirement of closed pond is only 65 to 70per cent of open pond. Also , there is an option of Advance MCS best suitable for plants where there is space limitations.

Considering the need for high efficiency system required by the various industries, MCS surely meets the demand at a extremely affordable price.
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