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.
Process benefitsDue 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.
Hence installation of MCS will ensure large commercial benefits to you in form of additional product obtained at a higher yield.MaintenanceMCS 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 10-15 years. This makes MCS absolutely maintenance free. As against this, cooling towers require a heavy maintenance with the 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 conditionsOpen pond MCSHere, MCS ensures an approach of 0 to 1C to WBT with a ΔT of 12 to 15 C. Water loss due to drift is 0.1 to 0.25 per cent depending on wind load.
Closed pond MCSHere MCS pond is closed from sides, up to a height of 5 to 6 metres by louver type cover sheeting. MCS ensures an approach of 2.5 C to WBT with a ΔT of 12 to 15 C. Drift loss comes down to less than 0.02 per cent and also space requirement reduces considerably. All power plants are using this design.
Tabletop design to prevent algae formationLatest tabletop design of MCS pond does not allow water to form column inside, and all water passes to suction pit is covered from top, minimising chances of algae formation.
Working of MCS in dusty environmentUnique suction pit design does not allow dust to pass to the inlet of circulation pumps. Dust is drained from drain valve while only clear water circulation water pumps.
Water quality: MCS ensures efficient atomisation and the consequent absorption and retention of air by water particles, causing aeration of water, showing better BOD and COD values.
System flexibility (capacity turn down ratio): MCS with individual line isolation valve is the only system which gives such a high flexibility in operation.
Hydro-balance valveMCS 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 bypass some fixed amount of hot water (as per setting) from distribution header directly into MCS pond so as to obtain desired CW temperature.
This HBV is normally manual controlled, but can be offered with actuator operation if desired.
Hydro-balance system also helps to release the excess pressure which may develop on the system at times.
Chemical treatmentChemical dosing requirements are similar to that of cooling tower as same hold up of water is maintained in suction pit.Makeup water requirementDue to latest closed pond design, drift loss through MCS is reduced to less than 0.02 per cent while maintaining an approach of around 2 C to wet bulb temperature. Hence, overall makeup water quantity required is approximately same as compared to cooling towers.
Payback period of the system will be less than one year only
MCS matches the design as per needMCS can be used in open pond or closed pond designs to suit the need. Open pond ensures an approach of 1 C to WBT while closed pond ensures an approach of 2.5 C to WBT. Space requirement of closed pond is only 65 to 70 per cent of open pond. Also, there is an option of advanced MCS best suitable for plants where space is limited.
Considering the need for high-efficiency system required by the various industries, MCS surely meets the demand at an affordable price.
Authored by -Makarand A. Chitale,Technical Director,Mist Ressonance Engineering Pvt. Ltd.
High-efficiency Mist cooling system for power plants