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Enhancing Efficiency Of ACHEs

Globally, there is a growing need for both electric power and water. Due to large scarcity of water, this may lead to conflict between water for power plant cooling requirements and that for other agricultural, residential, or environmental uses. Hence, there is a huge inclination towards use of dry cooling (air cooling), which is supposed to be an attractive alternative to the use of wet cooling – basically on the grounds of water conservation.

It is well known that dry cooling on one hand substantially reduces the need of water in process/power plants, on the other hand – it may cause reductions in plant efficiency and output during hottest periods of the year.

Mist Ressonance Engg. Pvt. Ltd. has come out with a solution by designing Mist Cooling System for air cooled heat exchangers/ condensers – where air temperatures are reduced by 8 to 10°C during hot period of the year. Water quantity required for doing this is approx 1/10th of what a water cooled cycle would require, and hence it provides a perfect way of running your process /power plants at its designed efficiency throughout the year without any water/ environmental issue.

Mist Cooling System (MCS)

MCS induces water to intensive atomisation – i.e., water particles are sub-divided to around five microns. The atomised particles shoot out of ‘mistcreator nozzles’ at immense speed and form mist.

This ensures extensively large surface area for a longer interval – and at high velocity providing a mist formation. Surface evaporation is very fast – i.e., even faster than the time needed for reaching equilibrium. This is similar to the phenomenon of formation of hailstone, when rainwater reaches temperatures much lower than wet-bulb. MCS simulates the nature and temperature to as low as wet-bulb temperature. This is just not possible in conventional spray or cooling towers – where ultimate temperatures are 5 to 7OC higher than prevailing wet-bulb temp.

Purpose of installation of MCS for ACHE

ACHE (Air Cooled Heat Exchanger) is designed to cool / condense the product fluid / steam from process plant by air at certain maximum ambient temperature.

Normally, in peak summer, the air temperature exceeds the designed temperature mainly in day time. This reduces the efficiency of ACHE by a considerable amount. Hence, for running the ACHE with the same efficiency in summer as in winter months, air has to be cooled to the designed temperature by some external medium. We propose to do this by passing this inlet air through a water / mist curtain before entering ACHE.

The detailed scheme

As explained earlier while describing the technology, mist creator nozzles induce water to intensive atomisation, thus subdivides it to around five Microns. As shown in the graphic above, a bank of ‘mist creator nozzles’ are installed at an elevation of +3 metres above ground level or RCC floor all along four sides of the Air Cooled Condenser.

A collection pond has to be constructed at ground level for collection and circulation of water. Depth of the pond could be kept as 1 to 2 feet only. Circulation water pumps are installed at the cold water suction pit of depth 1 to 2 metres created in the MCS pond.

The pump delivery line brings pond water to MCS header at desired elevation. Water pressure is maintained at inlet of ‘mist creator nozzles’ at 3 Kg/cm2. Circulation pumps are designed accordingly. ‘Mist creator nozzles’ are installed in such a manner that they spray water downwards. A mist curtain is prepared all along the Air Cooled Heat Exchanger periphery. Any air entering the ACC passes through this mist curtain, thus getting cooled to the designed air inlet temperature or even lower. A demister pad is placed between the mist curtain and the fan deck in order to arrest fine droplets from going to tube bundle. The Demister Pad is designed in such a fashion that, it offers minimum pressure drop for air flow. Water is collected on the RCC floor /suction pit of MCS pond & the cycle continues.

Nozzle arrangement or design

The degree, at which water sprayed in to inlet air stream evaporates, raises a number of important questions. Incomplete evaporation introduces several problems. Un-evaporated droplets contributes nothing to cooling effects but adds to the plant water use – if they cannot be collected and recycled. If they are carried into the ACHE’S finned tube bundles, there is a danger of scaling and corrosion of surfaces. Extremely fine spray can approach complete evaporation, but results in high pressure. Low flow design of nozzles shall also require a strong back up filtration system. MREPL has developed ‘mist creator nozzles’ with a bore size of 8 to 10 mm that can produce mist of size up to five microns. Due to such high bore size, it operates with a choke-less design. Also, nozzles are arranged just below fans of ACHE in such a way that they spray water downwards (graphic above).


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