Cooling seawater

There has been a recent shift in the Middle East towards more environmentally friendly seawater cooling towers.

Seawater cooling is essential for the Gulf’s power plants, but which technology is best? Aslan Al-Barazi, consultant and specialist in cooling towers and seawater cooling tech,  at IMEC Electro Mechanical Engineering, examines the evidence

The Gulf region has traditionally relied on seawater cooling for the power plant process, and the location of a power plant would determine the type of cooling process used.

For areas requiring power located further away from the sea, air coolers, air cooled condensers, or air cooled heat exchangers would typically be used. The region however has typically relied on direct seawater cooling, also known as ‘once through cooling’ which can damage the marine ecosystem with thermal pollution.

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Recently there has been a noticeable shift to move to the more environmentally friendly seawater cooling towers system for this, as well as additional reasons advantageous in a cooling tower system.

These include making minimum use of a limited sea bank (due to small bleed rate discharge back to the sea), where in many cases competition for land is typically between the industrial zone and the tourist hotels and residential resorts, and the ability with the seawater cooling towers to meet the thermal challenge of delivering cold water even during the summer when the seawater temperatures are high.

The aim here is to compare both the direct once-through seawater cooling system with a seawater cooling tower system from a technical, commercial and environmental perspective. To begin with it would be a good idea to explain how both systems work.

A once-through seawater cooling system takes cold water from the sea, cools the turbine condenser heat exchanger, and discharges the hot or warm seawater (from the heat rejection of the turbine) back to the sea. It is called once-through seawater cooling because the cold seawater is used once and then discharged back to the sea.

A cooling tower on the other hand relies on re-cooling technology. The hot water leaving the turbine circuit enters the cooling tower and is sprayed as fine water droplets, falling to the PVC fill before going into the cold water basin. It is then directed back through the circuit towards the turbine condenser, from where the loop is continuously repeated.

As the water droplets are falling into the cooling tower, ambient air is entering the cooling tower and going upwards thereby removing heat from the water droplets. This will account for the heat transfer evaporation process, whereby one per cent of the water is evaporated, cooling the remaining 99 per cent of the water in the system in approximate proportionate measures.

Heat transfer
Another important heat transfer mechanism occurs when the water droplets fall onto the PVC fill (or splash bar fill in the case of industrial or high-concentration particulate water).

The PVC fill impedes the water’s fall, allowing more time for the heat transfer process to occur while stretching the cross sectional area of the water droplet surface against the PVC fill wall layer, allowing forced convection to occur on the water droplets, further removing heat from the water.

It can be said that from the total heat transfer mechanism in the cooling tower, around 70 to 90 per cent is related to the evaporation process while 10 to 30 per cent is due to the forced convection heat transfer process.

There are several different ways cooling towers can be designed to achieve this.
Typical towers used in our region include induced draught counterflow, induced draught crossflow and forced draught cooling towers.

Natural draught cooling towers would not work in this region because outside ambient temperatures are not cool enough, air density variance is minimal between certain heights and there is not enough natural draught in the air (particularly in hot weather) for this type of tower to efficiently work in the GCC.

Induced draught simply means that the fan is positioned on top of the tower, and induces the air from the top where the fan is located thereby allowing the air to enter from the sides or from the bottom part of the tower, and flow upwards to the tower fan discharge area.

A forced draught cooling tower simply means that the fan of the cooling tower is situated on the lower bottom end of the cooling tower and forces the air upwards against static pressure in the way, such as the PVC (heat transfer) fill, drift eliminators and fan stack external static pressure.

An induced draught counter flow simply means that when the hot water enters into the cooling tower it is sprayed through the water distribution pipes or nozzles. As it falls downwards towards the PVC fill, it continues to the cold water basin.

Meanwhile the ambient air is entering the cooling tower, at a 180 degree angle to the falling water.

This is the most efficient mode of heat transfer as the force per unit area between the water droplets falling and the air countering it is at the maximum 180 degree angle.
Consequently, the minimum fan power is consumed and for industrial range towers this is the optimum design selection.

An induced draught cross flow cooling tower varies from a counterflow in that while the water droplets in the tower are falling, the air crosses the water from the side.

This tower is typically used when height is a problem, and for smaller commercial range tower capacities when maintenance accessibility becomes a problem in counterflow towers. The disadvantage on the crossflow tower is the higher energy consumption in comparison with a counter flow design.

Finally, the forced draught cooling tower is typically used for low-noise applications and length and width space savings. The disadvantages include high initial cost, and high energy consumption.

For the seawater intake station of the cooling system, this may be classified into three different types depending on the capacity usage and whether a once through seawater cooling or a sea water cooling tower system is required in the design.

Small capacities belong to the passive screen, medium capacities to the travelling band screen while larger capacities to the drum screen. The passive screen goes up in capacities to 15,000 gallons per minute (GPM) per unit.

The travelling band screen is typically available up to 230,000 GPM, while the drum screen goes up in capacity up to 630,000 GPM. The passive screen is typically a fish protection system as well, as fish cannot enter due to low velocity suction and small clearance intake protective slits at the entry.

Travelling band screens as well as the drum screens typically have variations in design including fish protection systems, stop gates (for seawater intake station building process and maintenance shutdown), bar screens (to clear any major obstacles in the way such as tree logs, obstructive trash, seaweeds) and raking systems if needed, to remove obstructive obstacles.

Smaller-size filtration systems can also be included downstream into the intake system before reaching the seawater condenser, allowing the removal of particles from 1mm in size down to 30 microns.

It is worth noting that a seawater cooling tower system, due to its low water requirements, would typically require a seawater intake station a fraction of the cost and size of the once through direct seawater cooling system, varying with the former between a small passive screen to a medium capacity travelling band screen at its maximum flow rate capacity.