In process or 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 basin 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 need 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 makeup water. Thus, requirement of water quantity is vastly reduced.
Wet Cooling Systems
Wet Cooling Tower System
We will first consider the Wet Cooling Tower System. 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. Nowadays, mostly, all wet cooling towers are mechanical draft cooling towers, where the air flow is accomplished by fans.
The principle cooling devices used in an Induced or forced draft cooling tower are fans, which run at the top of the Cooling Tower (CT). Air enters through side louvers and escapes from the top. H2O enters at the top and trickles down while getting cooled by the air draft.
A correctly designed induced draft CT can give an approach of 4 to 6OC to wet bulb temperature with a temperature drop of 10OC. Even a very highly efficient CT cannot give an approach less than 4OC to WBT. Moreover, if ambient temperature or humidity levels rise, efficiency of CT reduces.
Let’s consider this through in example
For a chemical plant, an induced draft cooling tower is designed to maintain cold water temperature of 32OC at a WBT of 28OC with an approach of 4OC. The CT performs as desired during winter and the early summer months. But during peak summer or monsoon, efficiency of the cooling tower reduces as humidity rises and its approach to WBT reaches beyond 6OC from designed 4OC. Thus, due to this rise in cold water temperature, these industries always experience loss in production by at least 5 to 7%. These losses do not occur in winter months. This means that the plant will operate at a reduced efficiency for almost 5 to 6 months in a year (Please refer Graphs A & B).
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.
Mist Cooling System
MREPL has come out with a solution by designing a MIST COOLING SYSTEM, which is a high efficiency system and ensures an approach of 1OC to prevailing wet bulb temperature – with a temperature drop of 12 to 15OC even in adverse climatic conditions.
In tropical conditions, worst wet bulb temperature – even at coastal applications – is maximum 30.5OC. Hence, MCS will always maintain cold water of around 31OC±1OC throughout the year. No other cooling system can operate with such efficiency, and it makes the CT or spray pond systems obsolete.
Salient Features of Mist Cooling System (MCS)
Cold Water Temperature
Mist Cooling System ensures an approach of 1OC to WBT with a temperature drop of 12OC to 15OC.
Energy Savings (Please refer diagrams Plan A & B)
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 (as shown in the diagrams). 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
Mist Cooling System will supply cold water at a temperature very close to WBT (Approach of 1OC) as against an approach of 4 to 5OC in CT. This will reduce the product vapour losses in shell & tube type heat exchangers. Also, this will make sure that your plant operates at an enhanced yield in summer and monsoon – giving stable throughout through out the year.
Maintenance
MCS has no moving parts. It does not use any fills and fins for cooling. Also material used in the MCS is special grade saran polymer, a highly non-corrosive material having a life of more than 10-15 years.
Chokeless Design of Nozzles
MCS operates with a chokeless design. Size of smallest opening in MCS is more than one inch (25 MM) in diameter. Hence, chances of particles choking the system are remote. This makes MCS absolutely maintenance free.
Various Designs of MCS to Suit Site Conditions
- Open Type MCS: Here, MCS ensures an approach of 1OC to WBT with a ΔT of 12 to 15OC. Water loss due to drift is 0.1 to 0.25% depending on wind load.
- Louver Type MCS: Here, the MCS basin is closed from all sides, up to a height of six mtrs. by louver type cover sheeting. MCS ensures an approach of 2OC to WBT with a ΔT of 12 to 15OC. Drift loss can be limited up to 0.002% – and also space requirement reduces considerably.
- Table Top Design to Prevent Algae Formation: The latest table top design does not allow formation of water level inside the basin – and all water passes to suction pit that is covered from top, thus minimising chances of algae formation.
- MCS Design for Working in Dusty Environment: The unique 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 passes to the circulation water pumps.
System Flexibility (Capacity Turn Down Ratio)
MCS is offered with individual line isolation valve. It is the only system, which gives such a high flexibility in operation.
Hydro-Balance Valve
A Hydro-Balance Valve (HBV) is provided to take care of sub-cooling, which may happen in winter season, and also is helpful to release excess pressure _ which may develop on system at times.
Chemical Treatment
Chemical dosing requirements are similar to that of the cooling tower _ as the same hold up of water is maintained in suction pit of table top MCS.
Make-up Water Requirement
Due to the latest “Louver Type” design, drift loss through MCS can be limited to less than 0.002%. Hence, Overall make-up water quantity required is approximately the same as compared to the cooling towers.
Pay Back Period
The Pay Back period of the MCS, in most of the cases, will be obtained in less than ONE year only.
MCS Matches the Design as per need
MCS can be put to use in Open Type or Louver Type MCS designs to suit the need. Open Type design ensures an approach of 1 deg.C to WBT while Louver Type MCS design ensures an approach of 2.5 deg.C to WBT. Space requirement of Louver Type design is only 65 to 70% of Open Type design. 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 an extremely affordable price.
