Cooling Tower Experiment and Lab Report
Objective of the experiment - To study the cooling tower apparatus and to apply mass balance & energy balances
Cooling tower lab experiment is one of the starting lab experiment in chemical engineering, It covers the basics of mass and energy balance theories applied in chemical engineering.
Introduction to cooling tower
Cooling towers can be defined as one of the heat removal devices used to transfer process waste heat to the atmosphere. Many industrial plants used water as a coolant medium. If the cooling water is to be recycled in the plant, its temperature has to be lowered before re-entry into the plant.
If a plant had no cooling tower and used once-through cooling water, it would require a large amount of water, and that amount of water would have to be continuously returned to the ocean, lake, or river from which it was obtained and continuously re-supplied to the plant. Furthermore, discharging large amounts of hot water may raise the temperature of the body to an unacceptable level. A cooling tower serves to dissipate the heat into the atmospheric air instead and wind and air diffusion spread the heat over a much larger area in a short time period than hot water can distribute heat in a body of water.
Cooling towers operate on the principle of evaporation. Some coolers operate by heat transfer through a surface that separates the working fluid from ambient air.In the process of the cooling process, the warm water falls straight from the top of the cooling tower & contact with atmospheric air forcibly sent by a blower from the bottom of the cooling tower.
In the process of the cooling process, the warm water falls straight from the top of the cooling tower & contact with atmospheric air forcibly sent by a blower from the bottom of the cooling tower.
Procedure of conducting cooling tower experiment in lab
- Identify all the flow streams and vessels of the experimental set up. you can use the manual of cooling tower available with technical
officers. Draw a schematic diagram to show all-important parts of the cooling tower.
- Fill the water to the hot water reservoir (the process). Operating knobs of WATER TEMP CONTROL (coarse) and AIR TEMP CONTROL (fine)
knobs should be located on the lowest number prior to experiment. Also confirm to switch off all POWER SUPPLY, WATER HEATING, WATER PUMP and
AIR HEATING. Put the switches ON in the following order,
- STEP 1 - POWER SUPPLY, BLOWER, WATER HEATING.
- STEP 2 - Turn the knobs of WATER TEMP CONTROL on the desired number.
- STEP 3 - Switch WATER PUMP on.
- Adjust water flow rate and airflow rate by using "water flow rate control valve" and "blower diaphragm" respectively. Maintain a constant water level in the hot water reservoir by supplying make-up water.
After the steady state is reached, record the followings.
- Ambient temperature
- Dry bulb temperature
- Wet bulb temperature
- Cooling tower air inlet temperature
- Dry bulb temperature
- Wet bulb temperature
- Cooling tower air outlet temperature
- Dry bulb temperature
- Wet bulb temperature
- Water inlet temperature to cooling tower
- Water outlet temperature from cooling tower
- Water flow meter reading
Data collected after steady state is reached
After the steady state is reached, following values were obtained for relevant parameters.
- Specific heat capacity of water: 4200 J/kg 0C
Notations used in calculations of cooling tower experiment
Several more data are obtained from the reading of psychrometric chart.
Calculations of cooling tower experiment
Following parameters are calculated during calculations.
- Air flow rate
- Amount of water evaporated within the cooling tower
- Make-up water as a percentage of cooling water flow rate
Calculation of air flow rate
Calculation of amount of water evaporated within the cooling tower
Amount of water evaporated in water stream per second (Q?.) is calculated using dry air flow rate (m'da) and humidity difference (δH) of air stream as below.
Calculation of Make-up water as a percentage of cooling water flow rate
Basis : Per second
Assumptions:
- No losses
- No mass generation
- No consumption
- No accumulation
- Steady state operation
Apply Mass balance (for water) as below.
Discussion - Discuss different types of cooling towers available for industrial applications.
In industrial processes and machines, there is a very large amount of heat generation. Cooling towers are used to remove this heat in an efficient way. They are usually used in industries like power plants, petroleum refineries, and various manufacturing facilities where hot water is recycled back to the process. Along with the development of the technology, cooling towers were also developed and now they are available in different types and sizes according to the requirement.
Cooling towers are generally classified considering their built, heat transfer method, or airflow generation methods. Depending on how the water stream or air stream passes through the cooling tower, cooling towers can be divided into three types as below.
Natural draft cooling towers
These towers have used the design and shape of the tower itself to move up the air naturally using fans. The law of different densities between ambient air and the warm air in the tower is used to operate the cooling tower. Hence, these towers are tall to induce airflow and are shaped like a "hyperbole". They are typically located outside the buildings to allow for airflow.
Mechanical draft cooling towers
An electrically powered mechanical fan is used to force the air to flow through the cooling tower. Generally, a centrifugal fan or propeller is used to circulate air inside the cooling tower, and deciding fan type and power of the fan is done according to the defined values of process parameters such as the output temperature of the water stream. Because mechanical fans are used, a pressure difference can be generated to suck the air in an efficient way. Therefore, mechanical draft cooling towers are much smaller in structure and size than natural draft cooling towers.
Capacity control is easy in these types of towers since the speed of the fan can be controlled by using a variable frequency drive. Because pressure difference can be generated anywhere by the fan unlike natural draft towers, mechanical draft cooling towers can be located anywhere inside the process area.
Cross flow cooling towers
This type of cooling tower is structured to allow air to flow horizontally while the water flows down vertically. This is done through open-through systems in the fan deck, fitted with nozzles. Since the airflow and water stream contact time is very short, more air is required to remove the heat from the hot water stream.
Disadvantages to cross-flow cooling towers
- Higher power consumption due to the higher airflow required
- Maintenance is time-consuming
- Susceptible to scaling and clogging of openings.
Counter flow cooling towers
Counter flow cooling towers use hot water that enters at the top, while the air is introduced at the bottom and exits at the top. Both forced and induced draft fans are used to make the airflow. Contact time between hot water stream and the air is higher than in cross-flow cooling towers. The distribution is done through a channel with lateral pipes, fitted with splash spray nozzles.
The growth of algae is highly restricted, as the lateral pipes are closed units and not located in direct sunlight. Counter flow cooling tower uses less power than cross-flow units and offers an economical advantage and easy maintenance is one more added advantage.
Questions
