Cooling Tower Designing

Introduction

You know a typical refinery uses 25 barrels of water for every barrel of oil being processed. 80 to 90% of all the water is used as a coolant to absorb heat from the processed fluid.

Since it is not viable to draw new water every time and dispose of it to the environment. Because it would be very costly and would cause serious environmental pollution.

Therefore, the best method is to cool the hot water that absorbs heat from the process and reuse it. Refineries and other process industries use cooling towers for this purpose.

The molecules in hot water have energy stored in them which causes them to move. The molecules with higher heat energy move fast enough to break away from the water body and mix with air to escape.

In this way, they take that high heat energy along with them while leaving the rest of the molecules at a lower level of heat energy. This results in decreasing the temperature of the remaining water.

This is called partial evaporation (evaporative cooling). Cooling towers work on the principle of evaporative cooling. However, in the detailed design of the cooling tower, it is observed that conduction and convection do happen in the process as cool air from the surrounding encounters with the hot water.

Reference: API Cooling Tower Operation

Factors Affecting Evaporative Cooling

Surface-to-air contact

To achieve maximum evaporation large number of water molecules must encounter the surrounding air. The cooling tower designing is carried out in a way to provide a large surface-to-air contact.

Atmospheric pressure

Evaporation would be much higher at low atmospheric pressures since it exerts pressure on the body of water.

Dry and Wet Bulb Temperatures

A dry bulb temperature is often regarded as the ambient air temperature. However, if the thermometer is encased in a wetted wick, it reads wet-bulb temperature. As the surrounding air encounters the wetted wick, partial evaporation of the water vapours from the wick occurs, thus reducing the reading of the thermometer.

Mercury is cooled due to the escape of water molecules (with high heat energy) taking place. More evaporation will be caused if the surrounding air is dry, ultimately increasing the difference in the dry bulb and wet bulb temperatures. So, a greater difference in the reading shows that air has less relative humidity.

We can find the actual percentage of the relative humidity by using a psychometric graph, plotted between dry bulb temperature, wet-bulb temperature, and % relative humidity.

Low wet-bulb temperatures (i.e., low relative humidity) are more favourable for effective cooling in the cooling towers.

Relative Humidity

Air has a certain amount of moisture stored in it that depends on the ambient conditions, which is called its relative humidity.

In other words, “Relative humidity is the ratio of moisture in the air to the maximum moisture which the air could hold at that temperature”. Relative humidity of 50% would mean that air is holding half of the moisture that it would exactly hold at that temperature.

As the air absorbs more water vapours, its relative humidity increases. If it gets saturated with water vapours, it will no longer absorb more of them.

So, for a better rate of evaporative cooling air must have lower relative humidity values. For a better understanding of why the term relative humidity is used, we must define the actual or absolute humidity which is, “the mass of water vapours associated with a unit mass of dry air” i.e.,

          H = M(vap) /M(dry air)

Since finding out the actual or absolute humidity would mean taking out a volume of air, extracting all the moisture out of it and then weighing it.

So, to minimize such an effort, the term relative humidity is defined, which is, “the ratio of the partial pressure of water vapour in the mixture to the equilibrium vapour pressure of water (over a flat surface of pure water) at a given temperature”.

                        Relative Humidity = pH2O/p*H2O

Reference (Relative Humidity): Wikipedia

L/G Ratio

The L/G ratio of a cooling tower is generally derived while calculating the height of the cooling tower. This is the most crucial step in doing mass transfer calculations across the cooling tower.

Since to the energy conservation principle, the amount of heat rejected by the water must be taken (same amount) by the air.

A high L/G ratio means a low airflow rate. However, to further decrease the water temperature (because of more heat and mass transfer) the airflow must be increased. But the driving force, in this case, would be the enthalpy difference of both the fluids.

L/G ratio has a great effect on the approach of the cooling tower.

Types of Cooling Towers

Atmospheric Cooling towers

Atmospheric cooling towers are those which require no mechanical devices such as fans or blowers to flow air through them.

Their effectiveness entirely depends upon the wind speed and relative humidity of the air. However, greater wind speeds may also result in excessive drift loss as well which is compensated by adding more make-up water.

Atmospheric Spray Tower

It has the simplest configuration as water is sprayed downwards, meanwhile, air flows across it. They are used in small processes, and adverse wind can greatly affect their operations.

Atmospheric Spray Type Cooling Tower
Figure Reference; Cooling Tower Fundamentals by SPX Cooling Technologies, USA

 Natural Draft Cooling Tower

One of the added benefits of forced draft towers is, that they use centrifugal blowers instead of propellers. Despite having high electricity consumption, they can operate against high static pressures (ductwork). This makes them suitable for being used in closed indoors which have separated inlets and exits, thus minimizing recirculation.

They are one of the widely used cooling towers in large industrial processes especially those in power generation sectors. Natural draft cooling towers are typically very large in size ranging from 250,000 GMP to greater.

 As we know, hot gases are less dense than cooler ones. They tend to rise upward and escape into the atmosphere hence drawing in the cooler air.

The hyperbolic geometry of the natural draft cooling tower defines the principle of the chimney effect (natural pressure difference).

It serves two purposes, (i) the hyperbolic shape helps in accelerating the hot gases which causes a drop in pressure inside the tower, thus drawing cooler air from the atmosphere into the tower (ii) it helps in providing structural strength with minimum use of construction material.

Despite the fact stated above, they are still very expensive and are used for large industrial processes e.g., power plants and in areas with higher relative humidity.

Irrespective of their initial cost, they don’t require a mechanical fan for air circulation, and the recirculation of hot air doesn’t occur since the stack outlet is located high up.

Mechanical Draft Cooling Towers

Mechanical draft cooling towers use one or more fans to circulate a known volume of air. Hence, we can achieve more stabilized thermal performance which is also affected by fewer other psychometric variables, compared to the natural draft.

The fans also provide regulation of air which could compensate for the fluctuating heat loads.

Forced Draft (Mechanical Towers)

Forced draft mechanical cooling towers have a fan that is located at the inlet of ambient air and blows it through.

They have high inlet and low exit velocities which makes them likely of recirculating the saturated stream of exit air, ultimately decreasing their efficiency compared to the induced draft.

Moreover, the location of the fans at the inlet of cool air makes them susceptible to icing in cold weather.

Induced Draft (Mechanical Towers)

Induced draft mechanical cooling towers have fans located at the exiting stream that draw air through the tower.

The exiting velocity of air in the induced draft is 3-4 times greater than that of entering velocity, so a reduced pressure zone is not created by the fan itself. Therefore, recirculation may depend upon the ambient conditions.

Mechanism of Air Flow

Cooling towers can also be classified on the mechanism of water-air flow. The two basic types of flows are discussed briefly.

Counter Flow Towers

In Counter Flow Towers, air moves vertically up through the fill while the water is sprayed downwards by pressurized nozzles.

The operating cost is higher due to the use of pumps to develop a large head for creating a pressurized spray of water. However, the breakup of water in spray increases the effective heat transfer area thus maximizing cooling.

Cross Flow Towers

In Crossflow Towers, air enters perpendicular to the falling water through the fill and exits at the top. The water flows down through the fill under the effect of gravity. It has a low operating cost as compared to the counter flow.

Reference (Types of Cooling Tower): Cooling tower Fundamentals by SPX Cooling Technologies, USA

Factors Affecting the Efficiency of Cooling Towers Design

Following are some of the important parameters that can affect the performance, design, and operational costs of a cooling tower.

Range and Approach

The range is basically the difference in temperatures of water entering the cooling tower and water leaving the cooling tower. Hence,

Range = Cooling Water Inlet Temperature – Cooling Water Outlet Temperature

While the approach is:

Approach = Cooling Water Outlet Temperature – Wet Bulb Temperature

The combined effect of range, and approach on the design of the cooling tower will be discussed after describing the heat load of the cooling tower.

Heat Load

The heat load on the cooling tower in BTU/min can be calculated by using the following relation:

Heat Load = gpm x 8.33 x 60 x R  = BTU/hr…     (1)          Since Cp of Water is 1 BTU/lb˚F

Where: gpm = the rate of circulating water in gallons per minute

8.33 = pounds per gallon of water

60 = conversion factor for minutes to hours

R = Cooling Range, which we’ve discussed above

Range and Approach of Cooling Tower
Figure reference: Cooling Tower Fundamentals by SPX Cooling Technologies, USA

If we look at the above graphical representation of range and approach, it is evident that a high range would require a tall cooling tower. But as per equation (1), if the range is fixed, the approach would be fixed by sizing the cooling tower and determining its optimum efficiency.

So, we can conclude that a tall cooling tower doesn’t necessarily guarantee that cold water from the tower (cooling water outlet temperature) would be close enough to the wet-bulb temperature. To achieve better results in this regard high efficiency is always required.

Efficiency

Cooling Tower’s efficiency relies on the range and approach directly as:

Efficiency = (Range/Range + Approach) x 100

There are several factors that can affect the efficiency of a cooling tower.  For example, high dry bulb temperature of the inlet air due to the location of heat sources upstream of the air, L/G ratio, recirculation of the exit air of the cooling tower, wind direction, air restrictions, type of the cooling tower, type of flow etc.

The engineers must manipulate these factors in a way to get the optimum efficiency.

The details of these factors are beyond the scope of this discussion. However, one can study the document “Cooling Tower Fundamentals by SPX Cooling Technologies, USA” to acquire a deeper understanding.

Maintaining the Quality of Water

Water comes from natural resources like rivers, lakes, and wells. It has some quantity of dissolved as well as suspended particles. Since it circulates many times in the system.

It picks up more solid particles and when it gets evaporated in the cooling towers, those solid particles don’t escape with it. So, their concentration in the remaining water gets increased.

The suspended particles are responsible for fouling the equipment related to water circulation. This ultimately decreases heat transfer rates in exchangers and increases pressure drops in the water circuit.

Dissolved solids form scales on the equipment walls and can also lead to corrosion (e.g., Sodium Chloride increases the electrical conductivity of water thus increasing its corrosivity).

Calcium and Magnesium Carbonates are less soluble in hot water. Therefore, when water with a high concentration of these salts is boiled in vessels or passed through heat transfer equipment, these salts get deposited on the walls of that equipment.

To decrease the concentration of solids impurities, blowdown along with some chemical treatment is done.

Blowdown

The optimum way to control the total dissolved solids (TDS) concentration is blowdown. During blowdown, some of the circulating water is continuously wasted and the same amount of makeup (freshwater) water which has lower concentrations of dissolved solids is introduced into the system. The rate of blowdown can be estimated by using the following formula:

 B = [E – {(C-1) x D}]/(C-1)

Where E, C, and D, are evaporation losses, cycles of concentration and drift losses respectively.

Evaporation Loss

We can calculate the evaporation loss by multiplying the total amount of water circulating (gpm) times 0.0008 times the cooling range (in ˚F).

Cycles of Concentration

Cycles of concentration are the ratio of dissolved solids in circulating water to that in makeup water.

It can lie between 3-7 cycles depending upon the processing conditions and amount of TDS. High COC means more TDS in the circulating water which ultimately reduces the requirement for makeup water. But at the same time increases the likelihood of more fouling and scaling.

Suppose we want to keep the levels of chloride to 700 ppm in the circulating water and its level in the makeup water is 175 ppm. Therefore, the allowable COC would be 700/175 = 4. Since COC is known so we use this value to calculate the blowdown.

Drift Loss

These are the losses in the form of water droplets that get suspended in the air and are carried away with it. Drift losses are always provided by the manufacturer.

However, one can estimate them by multiplying the circulating water rate with a factor of 0.002. Since a percent of 0.2 is estimated to be lost by the drift. To minimize the drift losses, cooling towers are provided with drift eliminators.

Chemical Treatment

Corrosion and Scaling

Corrosion can be resulted either from the dissolved solids as we’ve discussed above or due to dissolved gases like carbon dioxide. Carbon dioxide after reacting with water forms carbonic acid, ultimately decreasing the pH of water.

Some other gases like, oxygen and hydrogen sulfide may also be dissolved in water decreasing its pH. Strong alkalis are added to increase the pH of water in this case.

However, on the other hand, scaling results from a high pH. So, it is important to keep the pH of water in range. To decrease the pH, acids can be added.

Scale inhibitors are also added which form a layer on the solid surfaces to avoid the dissolved deposition of dissolved solids.

Every facility has its own set of chemical treatment methods depending upon the quality of water in that vicinity and type of operation.

Biocides

Biocides are often used to control the growth of microorganisms such as algae or bacteria. These microorganisms require air and sunlight for growth.

Thus a cooling tower can be a good breeding ground for them. They not only cause fouling but also release oxygen which can lead to corrosion.

One of the most widely used biocides is chlorine.

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3 Responses

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