In this article, we are going to learn about the most important topics related to the cooling tower. We have covered every essential topic related to the cooling tower study for engineers. So let's get started.
Cooling water is one of the most important utilities in any chemical processing plant. Cooling water is most widely used in heat exchangers to cool liquid or gas or to condense vapors.
Now, as cooling water exchanges heat with other fluids, it needs to cool down before reusing it. So to cool down this cooling water, cooling towers are used.
So, Cooling Towers are a very important part of any chemical processing plant. Designing and operating cooling towers efficiently and economically is an important area for chemical engineering study.
What is a cooling tower?
Definition: Cooling tower is the equipment used to reduce the temperature of cooling water so it can be used back for cooling purposes inside a chemical processing plant.
A cooling tower is an equipment used to reject the heat of a cooling stream (generally cooling water) to the atmosphere.
The main aim of the cooling tower is to cool down the cooling stream so it can be used again in the plant. Cooling water has a box-type structure that has water sprays on top and may or may not have a fan on top of the bottom. Both mass and heat Transfer simultaneously takes place inside the cooling tower.
Cooling towers are built in hyperboloid (generally natural draft cooling towers are hyperboloid) or rectangular (generally forced draft cooling towers are rectangular) shape.
Parts of cooling tower
Parts of the cooling tower are different for various types of the cooling tower. But there are some common parts that major type of cooling towers has are:
- Cooling water inlet/outlet
- Water distributor: To spray water overfilling material.
- Filling material: To provide contact between air and cooling water.
- Drift eliminator: To reduce water loss caused due to drift and evaporation
- Water basin: To store for cooled water
- Treatment tank: Small part of water basin to dose chemicals to cooling water.
- Fan (optional): To improve air circulation
Working of cooling tower
Principle: Cooling Towers works on principles of convective heat transfer and evaporative cooling.
Let's start by taking a look, how convective Mass transfer takes place in the cooling tower.
In a cooling tower, the cooling water is sprayed from the top by distributors. This sprayed water in the form of drops that falls on the fill material. Air is circulated naturally or mechanically inside the cooling tower. Both air and water come into contact with each other and heat transfer take place between air and water. Air being colder than water gets heat from it and cools it down. But this is not the only phenomenon that cools down the water. Another and the more important phenomenon is evaporative cooling.
In evaporative cooling, As water drop comes into contact with unsaturated air (water vapors partial pressure is less than the vapor pressure of water) results in evaporation of water. This evaporation will result in taking heat from the air or from the water drop itself. The heat transfer coefficient between air and water is low so major heat is taken from the drop itself, which results in the cooling of water.
One limitation of cooling towers is that the minimum temperature attained by water in a cooling tower is wet bulb temperature. So to cool water below its wet-bulb temperature, chilling water systems are employed.
Applications of cooling tower
Cooling towers are most widely used in thermal and nuclear power stations, oil refineries, petrochemical and chemical processing plants, and heating, ventilation, and air conditioning (HVAC) systems.
Important terms related to cooling tower
Dry-bulb temperature
- A dry bulb is a temperature measured by the ordinary thermometer of air whose bulb is dry. So this represents the actual temperature of the air.
Wet-bulb temperature
- Wet-bulb temperature is a steady-state temperature attained by a small amount of water that is evaporated by taking heat from the air.
- Wet-bulb temperature is the minimum attainable temperature by evaporative cooling. So cooling towers can only cool up to wet bulb temperature theoretically (practically it is very difficult to attain wet bulb temperature in cooling towers.
- Wet-bulb temperature is tricky to understand to so let's learn it from example,
- Now, consider you have a thermometer and you are measuring the temperature of the air. if bulb of the thermometer which is in contact with air is dry. It will measure the actual temperature of the air. This is what we called dry bulb temperature. Now, if we wet the surface of the thermometer and pass the air on it. If the air is unsaturated, a small amount of water will evaporate due to the concentration driving force. Now, the heat required for evaporation can be taken from air or from the water itself. Taking heat of air is difficult due to the low heat transfer coefficient. So evaporation of water will cool down water near to the wetted surface. So the thermometer will indicate a lower value than the actual temperature. This value indicated by the thermometer is called wet bulb temperature at that pressure and air humidity.
- As air humidity and atmospheric pressure change, wet bulb temperature also changes.
Dew point temperature
- When we cool humid air, after some temperature, dews (small water droplets) start to form. The temperature at which this dew formation starts is called the dew point for air for that specified pressure and humidity.
The cycle of Concentration (COC)
The cycle of concentration is a representation of accumulated dissolved minerals in cooling water circulation. As cooling water is reused many times, the minerals keep increasing in concentration due to drift and evaporation losses.
So the cycle of concentration is an indirect measure of the concentration of minerals inside water. Water can only dissolve minerals up to their saturation concentration. As mineral concentration increase in the cooling water then solubility, minerals start to precipitates, and this causes scaling and fouling in the equipment which hinders the performance of that equipment. So to decrease these salt concentration levels, some amount of water is discarded as blowdown, and freshwater is added as makeup. This results in a decrease in the concentration of salts in the basin water (system water).
The cycle of concentration is calculated for many minerals like silica, chlorine, salts, etc. The formulas of the cycle of concentration are:
The cycle of concentration = Mass fraction of minerals in the basin (system) water/Mass fraction of minerals in makeup water
The cycle of concentration = Conductivity of water in basin (system) water/Conductivity of water in makeup water
The cycle of concentration = Mass fraction of chlorine in the basin (system) water/Mass fraction of chlorine in makeup water
From the formula, you can understand that cycle of concentration is a dimensionless quantity. The value of the cycle of concentration is generally 3 to 7 for efficient operation.
The capacity of Cooling Tower
The capacity of a cooling tower is the maximum amount of working fluid cooling towers can handle per hour. As the capacity of the cooling tower increase, the efficiency of the cooling tower decrease.
Performance of Cooling Towers
Range of Cooling Tower
Cooling tower range is the difference between the temperature of water at the inlet to the temperature of water at the outlet. The range gives us an idea about the reduction of temperature, the cooling tower is performing. A higher value of range signifies an efficient cooling tower.
Range = Inlet temp. of cooling water (Ti) - Outlet temp. of cooling water (To) = Ti - To
The approach of Cooling Tower
The cooling tower approach is defined as the difference between the temperature of the water going out from the cooling tower to the wet-bulb temperature of the air which is used for cooling purposes. As we discussed, wet bulb temperature is the minimum temperature a cooling tower can attain (theoretically). So approach gives us an idea about how near cooling tower is cooling the water to wet bulb temperature. Lowered value of the approach signifies an efficient cooling tower.
Approach = Outlet temp. of cooling water (To) - Wet-bulb temperature (Twb) = To - Twb
Using less moist air (low moisture content) in the cooling tower will help us increase the range and decrease the approach which is an indication of the efficient cooling tower.
The efficiency of Cooling Tower
The efficiency of the cooling tower is measured by comparing the difference between inlet and out the temperature of cooling water with the difference between inlet of cooling water and wet-bulb temperature of the air. The wet-bulb temperature of the water is the minimum temperature that can be attainable by cooling water theoretically. Equation of efficiency of cooling water is:
Efficiency (μ) = [(Ti - To)*100]/(Ti - Twb)
Here,
- Ti = Inlet temperature of cooling water
- To = Outlet temperature of cooling water
- Two = Wet-bulb temperature of the air
The common range of cooling tower efficiency is 70-75 %.
Water losses from cooling towers
Drift Loss (Windage)
Drift is water molecules that get carried away with flowing air from the cooling tower. Drift eliminators are generally used in the cooling tower to reduce this drift loss from the cooling tower. 0.3 to 1% of circulating water is lost as drift loss in natural draft cooling towers (without drift eliminators). 0.1 to 0.2% of circulating water is lost in the induced draft cooling towers. Only 0.0005 to 0.001 % of circulating water is lost as drift in the cooling towers with drift eliminators. Which means,
Drift Loss (D) = 0.3 to 1 % of Circulating Water (Natural draft cooling tower)
Drift Loss (D) = 0.1 to 0.2 % of Circulating Water (Induced draft cooling tower)
Drift Loss (D) = 0.0005 to 0.001 % of Circulating Water (Cooling tower with drift eliminators)
Evaporation Loss
As we learned that cooling towers work on the evaporative cooling principle, Some amount of water gets lost due to evaporation. There is a rule of thumb that says that for every 5.5 °C of cooling water temperature change in the cooling tower, 1% of the total amount of cooling water is lost in the evaporation. Another equation that can be used for evaporation loss calculations are:
Evaporation loss (E) = [C *Cp *(Ti - To)]/ λ
Evaporation loss (E) = [0.00085 *C*(Ti - To)]
Here,
- C = Circulating water amount
- Cp = Specific heat of water
- Ti = Inlet temperature of the water
- To = Outlet temperature of the water
- λ = Latest heat of vaporization of water
Blowdown
Some amount of water is drawn off to maintain the concentration of hardness causing salts in the cooling water. The concentration of these salts increases due to evaporation and drift losses, so some amount of water is dumped and some fresh make-up water is added to compassionate that water loss. The amount of water is dumped as blowdown is calculated by the equation:
Blowdown (B) = [E - (COC-1)*D]/(COC-1)
Here,
- E = Evaporation loss
- D = Drift Loss
- COC = Cycle of concentration (ratio of chloride content in circulation water and makeup water)
Amount of make-up water required to compassionate all these changes are some of these amounts,
Makeup water (M) = Blowdown (B) + Drift Loss (D) + Evaporation loss (E)
Cooling Water Temperature Diagram
Types of cooling tower
Wet cooling towers
- Wet cooling towers work on the principles of evaporative cooling. This type of cooling tower is majorly used to cool down cooling water.
- A small amount of cooling tower evaporates and it provides a cooling effect to the remaining water as we discussed in the working of the cooling tower.
Closed-circuit cooling tower
- In closed circuit cooling towers, both working fluid and coolant come in indirect contact with each other. Working fluid is passed through a heat exchanger where cooling by water spray and airflow by a fan is provided.
- First of all, water is sprayed on the packing material, by which air is passed or directly sprayed over the air. This reduces the temperature of the air and this air is used to cool down the working fluid.
Dry cooling towers
- In dry cooling towers, there is indirect contact between coolant and working fluid like a radiator, and cooling is provided by only heat transfer. Convective heat transfer plays important role in these types of cooling towers.
- Evaporation of working fluid doesn't take place in this type of cooling tower.
- Air is majorly used as a coolant in this type of cooling tower.
Hybrid cooling towers
- Hybrid cooling towers can work as both, dry and wet types of cooling towers. This operational flexibility makes these types of cooling towers efficient in a variety of weather conditions.
Natural draft cooling towers
- In natural circulation cooling towers, the flow of air is induced by the buoyancy effect due to temperature differences. Relatively large size of cooling tower is built and a hyperboloid type of shape made up of concrete is provided to enhance the natural flow of air.
- The power required to operate this type of cooling tower is less than forced draft cooling towers. But initial capital investment is higher in this type of cooling tower.
- These types of cooling towers have higher capacity than forced draft cooling towers. These types of cooling towers are majorly used in thermal or nuclear power plants.
Mechanical draft cooling towers
- In mechanical draft cooling towers, the flow of air induced by an external fan can be forced or induced draft. The fan is operated mostly by electric motors.
- The relatively small size of cooling tower is required then natural draft cooling towers which result in low initial capital investments. But due to the addition of a fan, operating cost is higher in mechanical draft cooling towers.
- The capacity of mechanical draft cooling towers is lower than natural draft cooling towers.
- These types of cooling towers are majorly used in oil refineries, chemical, or petrochemical processing industries.
- Now there are two major types of cooling towers are available under the category of mechanical draft cooling towers:
- Induced draft cooling towers
- Forced draft cooling towers
Induced draft cooling towers
- In induced draft cooling towers, Fan is provided at the top of the cooling tower which takes out air from the cooling towers. This creates suction for air that is surrounding cooling towers and air flows from outside comes in contact with working fluid and then leaves from the top.
- Entering velocity is lower than exiting velocity for these types of cooling towers. Air recirculation inside the cooling tower is lower than a forced draft cooling tower.
- Also, power requirement in these type of cooling towers which makes them the most efficient cooling towers.
- These types of cooling towers can be cross-flow or counter-flow types of cooling towers.
Forced draft cooling towers
- In forced draft cooling towers, Fan is provided at the bottom of the cooling tower which forces the air into cooling towers. Air enters forcefully from the bottom at higher velocity and leaves from the top at low velocity.
- Power requirements in this type of cooling tower are high so they are less efficient than induced draft cooling towers.
Crossflow Cooling Tower
- In crossflow cooling towers, both working fluid and airflow are perpendicular to each other. Generally, working fluid flows down the cooling tower and coolant (generally air) flows from outside to inside the cooling tower in the perpendicular direction of the working fluid movement.
- This type of flow pattern is not as efficient as counterflow cooling towers.
Counterflow Cooling Tower
- In counterflow cooling towers, both working fluid and airflow are in opposite directions. Coolant flows from the bottom to the top and working fluids flow from top to bottom.
- These types of cooling towers are more efficient than crossflow cooling towers.
Cooling water treatment in cooling tower
Cooling water is treated with chemicals in the cooling tower to prevent scaling and fouling in equipment where this water is about to use.
- Water is filtered to remove any size of suspended solids from it.
- Then, Water should be treated with chlorine to prevent the growth of biocides (microorganisms) and algaecides (algae) in the water.
- Some amount of corrosion inhibitors and scale inhibitors can also be added to prevent corrosion and scale formation in important parts of the equipment.
Environmental aspects of cooling tower study
- Blowdown of water from the cooling tower into water bodies increases the local temperatures and harms local ecosystems. These elevated temperatures can be harmful to aquatic organisms inside those water bodies.
- Air coming out from the cooling tower are at higher temperatures which increases the local atmospheric temperature. These higher temperatures can be harmful to live animals near to site.
- Some refineries near to sea use seawater for cooling purposes, due to drift losses, these salts levels increase in the land near the site. This affects the agriculture in that land. Also, this salt content sometimes suspends in the atmosphere which can cause health effects in workers of the plant.
References
- Cooling Tower - Wikipedia
- Cooling Tower Operation - ChemEngOnline
- How Do You Calculate Water Loss In A Cooling Tower? - Delta Cooling
- Cooling Tower Efficiency - Engineering Toolbox