Evaporative Cooling Towers (part 4)

The amount of evaporated water in the surface portion dA can be expressed through the relationship:

dL = k_v (p_sat – p_v) dA, where k_v is the evaporation function index.

The following expression describes the amount of heat (Q_D) removed from water during evaporation:

dQ_D = r dL    (3), where r is the heat of vaporization.

In equilibrium conditions, there is a balance between the amount of heat loss due to fluid evaporation and to  the quantity of heat  (Q_c)  transferred to it by conduction: dQ_D = dQ_c 

which written in terms of temperature leads to the following expression:

r dL = G c_p dt

while in terms of heat exchange surface, we have:

r [k_v (p_sat – p_v)] dA = α (t_G,DB- t_L1) dA    (4)

Instead of  the p_sat and p_v pressure functions, it's possible to calculate water quantity (dL) as a function of water contained in air or specific humidity (x); this gives an immediate idea of the amount of water vapor that is transferred to the air.

If p_A and p_V represent the partial pressures due to the above-mentioned components, the total pressure of the air is p_T = p_A + p_V  (Dalton); since the steam is overheated and its behavior is very close to that of a perfect gas, it's possible to apply the law PV = RT; meaning that for the two components, after the appropriate steps, it's possible to describe water content in saturated air (x_sat) as x.

PMV = molecular weight of water vapor = 18

PMG = molecular weight of the dry air »29

It's thus possible to obtain the values of saturation and water vapor pressure, respectively.

The simplified expressions have been written taking into account that generally, and especially, in the temperature range where cooling towers operate, the values p_v and p_sat are small compared to the value of the total pressure, where the constant c is a function of the total pressure and of the molecular weights of the components.

All this allows rewriting equation (4), which after appropriate simplification becomes:

r [c k_v (x _sat - x)] = α (t_G,DB- t_L)

i.e., introducing the overall coefficient of mass transfer K = c (kV) in relation to the water content:

r (x _sat - x) = (α / K) (t_G,DB- t_L)    (5)

then,

((t_G,DB-t_G,DB1) c_p = (x₁ - x) r        (6)

If we consider a channel of infinite length, we must attend a full compensation between water and air to the complete saturation, i.e., for which continues to be valid equation (6), hence:

(t-θ_e) c’_p = (X’’_e - X) r    (7)

when the temperature (θ_e ) and the relative water content at saturation level (X’’_e) are at fixed values, i.e., values that are known and do not vary can be considered both the specific heat of air (c’_p). The evaporation heat (r): equation (7) shows that the relationship between temperature and water content in air is linear.

The temperature measured in air-saturated conditions, also called wet-bulb temperature or adiabatic saturation (t_WB), is the limit temperature of water cooling.

The above content wants to illustrate that the cooling water temperature for cooling towers cannot be lower than the wet-bulb temperature. Therefore, the greater the difference in temperature between cooling water and wet-bulb temperature (approach), determines a smaller cooling tower.

The importance of water in the cooling tower industry - Water (part 6)

SYSTEMS THAT COOL WATER IN AN EVAPORATIVE WAY: WHERE THEY ARE USED

A hint is given by knowing how refrigerators function in terms of "transfer of energy-heat". Although this topic is very interesting, we will not linger on the quality of energy.

We only need to know that not all energy is equal. There is no difference between the physical and mathematical way.

In practice, from an economic point of view, it is very important to know how to take advantage of the energy that is available.

We must say that the waste heat (energy that cannot be used) from plants, unfortunately, can only be used in a few plants. This is because their natural use in "cascade" presupposes that the plant being served needs to use the same amount of energy at the same time, and this is what makes more difficult. Let us recall that it's very difficult to store energy in an economically way.

Now we will discuss refrigerators

Contrary to what is known, refrigerators "do not produce cold." Cold cannot be produced, or make!

Cold is something you “feel", it exists because “it lacks” heat; in other words, we do not produce cold but we remove heat, hence, we have cold.

Refrigerating machines do the following: remove heat, or better carries heat from one system component (called evaporator) to another component (called condenser).

For example, to learn how much heat a refrigerator carries, it's enough to know the power of the engine required to make the refrigerator function. In practice, usually, 1 kW is required to "carry" about 2,500-3,000 kCal / h.

What happens if the cooling tower is not working properly and the plant is blocked? First thing: do not panic!

What happens if the cooling tower is not working properly and the plant is blocked?


First thing: do not panic! 

Many things could go wrong!  We might not even be aware of the many inconveniences that can occur to an industrial plant when the water cooling tower is not working properly.

Some of the causes are:

• malfunctioning of one or more components that make up the tower (breakage, wear, fluid hammer, high temperature)
• encrustation, algae, and clogging of the water or air side passages
• bad design

We shall focus on the last point, i.e., bad design.

Bad design is due to:

• thermal load assessment error
• water tower temperature outlet assessed with great accuracy
• wrong design assumption for the wet-bulb air temperature

What will happen in the various plants?

Usually, the tower used in industry serves:

• steelworks
• refineries
• plants for the production of chemical products
• others

For each different plant, the damage caused by a non-functioning tower can be calculated in terms of production lost.

In the next posts, we will analyze the risk and try to resolve bad accuracy in the design work with the help of our followers.

A cooling system is essential for the operation of any modern geothermal power plant

Cooling Tower System: Converting Geothermal Energy into Electricity

Example of flash power plant producing electricity

Heat emanates from the earth's interior and crust generates magma (molten rock). Because magma is less dense than surrounding rock, it rises but generally does not reach the surface, heating the water contained in rock pores and fractures. Wells are drilled into this natural collection of hot water or steam, called a geothermal reservoir, in order to bring it to the surface and use it for electricity production.

The whole process of turning hydro-thermal resources into electricity is based on conversion technologies. That is, there are three basic types of geothermal electrical generation facilities:

• binary (it function as closed loop systems that make use of resource temperatures as low as (74°C),
• steam (it makes use of a direct flow of geothermal steam), and
• flash (uses a mixture of liquid water and steam).

Flash power plant is the most common and it uses a mixture of liquid water and steam.

The type depends on reservoir temperatures and pressures. Each type produces somewhat different environmental impacts.

Example of flash power plant producing electricity

The most common type of power plant to date is a flash power plant (flash steam is the condensation caused by reducing pressure) with a water cooling system, where a mixture of water and steam is produced from the wells. The steam is separated in a surface vessel (steam separator) and delivered to the turbine, and the turbine powers a generator.

A cooling system is essential for the operation of any modern geothermal power plant, because cooling towers prevent turbines from overheating and prolong facility life. Most power plants, including most geothermal plants, use water cooling systems.

Water cooled systems generally require less land than air cooled systems, and are considered overall to be effective and efficient cooling systems. The evaporative cooling used in water cooled systems, however, requires a continuous supply of cooling water and creates vapor plumes. Usually, some of the spent steam from the turbine (for flash- and steam-type plants) can be condensed for this purpose.

Reliability of Geothermal Power Generation

The source of geothermal energy, heat from the earth, is available 24 hours a day, 365 days a year. Solar and wind energy sources, in contrast, are dependent upon a number of factors, including daily and seasonal fluctuations and weather variations. For these reasons, electricity from geothermal energy is more consistently available, once the resource is tapped, than many other forms of electricity.

Examples of Power Plant Size and Applications

Though the size of a power plant is determined primarily by resource characteristics, these are not the only determining factors. Factors that favor the development of larger geothermal plants include things such as cost decreases when larger quantities of materials, including steel, concrete, oil, and fuel, are purchased at one time.

Cooling System

Most power plants, including most geothermal plants, use water-cooled systems – typically in cooling towers.

References/Sources - Idaho National Lab (INL) - Wikipedia - Geothermal Energy Association - U.S. Department of Energy

 

********** 

Software Calculator for cooling tower design and maintenance and TURBOsplash PAC ™ for filling material.

**********

The function of Cooling Towers with Geothermal Energy

Use of Cooling Towers with Geothermal Energy

1. What is geothermal energy?

Geothermal energy derives from the heat of the earth’s core. Here we will refer to energy deriving from the core of the earth. Based on new research, the earth’s core temperature is believed to be anywhere between 6000°C and 6500°C. This intense heat is absorbed by the different layers of the earth and, consequently, this heats our planet.

This geothermal energy can be used to generate geothermal power and is the source of our hot springs, volcanoes and geysers.

2. How is geothermal energy harnessed?

Geothermal energy is heat that is extracted from the earth. Pressurized hot water and steam, is produced when groundwater meets with the molten magma ascending from the earth’s core. Hot water flows to the surface through wells. Once pressure is released, the water flashes to steam. Deep wells, a mile or more deep, can tap reservoirs of steam or very hot water that can be used to drive turbines which power electricity generators.

3. How are cooling towers used with geothermal energy?

Let's suppose steam is separated from the water and this steam is used to drive a turbine generator. Conventional cooling towers are used to condense steam on the low-pressure side of the turbine to maximize electrical generation efficiency. Either direct condensers or surface heat exchanger condensers are utilized. In most cases, the condensate is used as makeup for the cooling towers. There is excess condensate available and this means that cooling towers run at low cycles. Low cycle operation results in excessive cooling tower treatment costs unless programs can be employed that are effective at low dosage rates.

4. Neri Calculator for cooling tower design and maintenance and TURBOsplash PAC ™ for filling material.

Geothermal power plants are designed with corrosion resistant materials of construction such as stainless steel in order to withstand the trace contaminants that enter the cooling systems with the steam. 

Cooling towers, if properly sized and filled with high efficiency fills, will yield optimal performance.

**************

References
1. University of Florida
2. International Geothermal Association
3. Wikipedia

The importance of water in the cooling tower industry - Water (part 10)

WHAT ARE THE CONSEQUENCES OF A COOLING TOWER OPERATING AT LOW EFFICIENCY?

Cooling tower detailed calculations

A low efficient cooling tower brings very serious consequences.

Bruno Neri

The tower is part of a system designed to dispose residual waste heat from the production plant or other primary system. The lower efficiency of the tower affects the performance of the primary plant with considerable waste of energy and, most of all, reduction or lack of production.

CONSTRUCTION OF A COOLING TOWER: TIPS ON ITS COMPONENTS AND THEIR USE.

As we saw, the cooling tower is an essential part of the plant system and, generally, it is separated from the primary plant system to which it drives.

The cooling tower is normally ignored until it goes into failure. Hence, the choice of components is of utmost importance.

Choose a tower made of stainless steel. Towers that need to be installed in heavy environments, such as chemical industries, choose polyester reinforced with glass fiber.

We have witnessed towers that have been operating for over 30 years, in these heavy environments, in perfectly stable structure.

Other useful tips and / or necessary will be exposed our next documentation work for publishing:

"A practical guide for the design of components that impact cooling tower thermal efficiency"

The importance of water in the cooling tower industry - Water (part 7)

Process plants that can be cooled with evaporation systems.
The importance of disposing quantity of energy.
Efficiency of the cooling system.

Image from previous post "NERI Calculator". Click on the above image to learn more about the calculator. 

From what has been written on this subject, the disposal of heat from evaporative cooling towers use something in particular, that is, disposing large amounts of heat from the water by means of the air: two natural elements.

Heat disposal has, therefore, a relatively low cost.

To name a few examples where towers are used:

(a) disposing heat from the various refrigeration groups and city building air conditioning condensers.

(b) in industries such as oil refineries, chemical plants,

(c) in industrial process plants for the production of food products,

(d) in thermoelectric power plants,

(e) in geothermal systems.

(f) …

Obviously each type of installation has different requirements for heat disposal (amount of cooled water). Temperatures and their range must be designed appropriately and carefully, and, most of all, the air thermal characteristic data to be considered for cooling tower design.

Air data has to include temperature, humidity and altitude related to the location of the tower. These three values (temperature, humidity, altitude) are very important.

If the data is not chosen correctly, this can lead to the wrong tower sizing, up to three or four times higher, or lower, than the actual needs of the system.

If the tower sizing is higher than the correct value, this will lead to waste of material and, hence, higher cost of the system. On the other hand, if faulty sizing will lead to equipment that is not adequate to the system, hence, useless!

→ See NERI Calculator ←

The importance of water in the cooling tower industry - Water (part 6)

SYSTEMS THAT COOL WATER IN AN EVAPORATIVE WAY: WHERE THEY ARE USED

A hint is given by knowing how refrigerators function in terms of "transfer of energy-heat". Although this topic is very interesting, we will not linger on the quality of energy.

We only need to know that not all energy is equal. There is no difference between the physical and mathematical way.

In practice, from an economic point of view, it is very important to know how to take advantage of the energy that is available.

We must say that the waste heat (energy that cannot be used) from plants, unfortunately, can only be used in few plants. This is because their natural use in "cascade" presupposes that the plant being served needs to use the same amount of energy at the same time, and this is what makes more difficult. Let us recall that it's very difficult to store energy in an economically way.

Now we will discuss about refrigerators

Contrary to what is known, refrigerators "do not produce cold." Cold cannot be produced, or make!

Cold is something you “feel", it exists because “it lacks” heat; in other words, we do not produce cold but we remove heat, hence, we have cold.

Refrigerating machines do the following: remove heat, or better carries heat from one system component (called evaporator) to another component (called condenser).

For example, to learn how much heat a refrigerator carries, it's enough to know the power of the engine required to make the refrigerator function. In practice, usually, 1 kW is required to "carry" about 2,500-3,000 kCal / h.

Evaporative Cooling Towers (part 4)

Evaporation

The amount of evaporated water in the surface portion dA can be expressed through the relationship:

dL = kv (psat – pv) dA

where kv is the evaporation function index.

 The following expression describes the amount of heat (QD) removed from water during evaporation:

dQD = r dL    (3)

where r is the heat of vaporization.

In equilibrium conditions, there is a balance between the amount of heat lost due to fluid evaporation and to  the quantity of heat  (Qc)  transferred to it by conduction:

dQD = dQc

which written in terms of temperature leads to the following expression:

r dL = G cp dt

while in terms of heat exchange surface, we have:

r [kv (psat – pv)] dA = α (tG,DB- tL1) dA    (4)

Instead of  the psat and pv pressure functions, it's possible to calculate water quantity (dL) as a function of water contained in air or specific humidity (x); this gives an immediate idea of the amount of water vapor that is transferred to the air.

If pA and pV represent the partial pressures due to the above mentioned components, the total pressure of the air is pT = pA + pV  (Dalton); since the steam is overheated and its behavior is very close to that of a perfect gas, it's possible to apply the law PV = RT; meaning that for the two components, after the appropriate steps, it's possible to describe water content in saturated air (xsat) as x.

PMV = molecular weight of water vapor = 18

PMG = molecular weight of the dry air »29

It's thus possible to obtain the values of saturation and water vapor pressure, respectively.

The simplified expressions have been written taking into account that generally, and especially, in the temperature range where cooling towers operate, the values pv and psat are small compared to the value of the total pressure, where the constant c is a function of the total pressure and of the molecular weights of the components.

All this allows to rewrite equation (4), which after appropriate simplification, becomes:

r [c kv (x sat - x)] = α (tG,DB- tL)

i.e., introducing the overall coefficient of mass transfer K = c (kV) in relation to the water content:

r (x sat - x) = (α / K) (tG,DB- tL)    (5)

then,

((tG,DB-tG,DB1) cp = (x1 - x) r        (6)

If we consider a channel of infinite length, we must attend a full compensation between water and air to the complete saturation, i.e., for which continues to be valid equation (6), hence:

(t-θe) c’p = (X’’e - X) r    (7)

when the temperature (θe ) and the relative water content at saturation level (X’’e) are at fixed values, i.e., values that are known and do not vary can be considered both the specific heat of air (c’p). The evaporation heat (r): equation (7) shows that the relationship between temperature and water content in air is linear.

The temperature measured in air saturated conditions, also called wet-bulb temperature or adiabatic saturation (tWB), is the limit temperature of water cooling.

The above content wants to illustrate that the cooling water temperature for cooling towers cannot be lower than the wet-bulb temperature. Therefore, the greater the difference of temperature between cooling water and wet-bulb temperature (approach), determines a smaller cooling tower.

A cooling system is essential for the operation of any modern geothermal power plant

Cooling Tower System: Converting Geothermal Energy into Electricity

Example of flash power plant producing electricity

Heat emanates from the earth's interior and crust generates magma (molten rock). Because magma is less dense than surrounding rock, it rises but generally does not reach the surface, heating the water contained in rock pores and fractures. Wells are drilled into this natural collection of hot water or steam, called a geothermal reservoir, in order to bring it to the surface and use it for electricity production.

The whole process of turning hydro-thermal resources into electricity is based on conversion technologies. That is, there are three basic types of geothermal electrical generation facilities:

• binary (it function as closed loop systems that make use of resource temperatures as low as (74°C),
• steam (it makes use of a direct flow of geothermal steam), and
• flash (uses a mixture of liquid water and steam).

Flash power plant is the most common and it uses a mixture of liquid water and steam.

The type depends on reservoir temperatures and pressures. Each type produces somewhat different environmental impacts.

Example of flash power plant producing electricity

The most common type of power plant to date is a flash power plant (flash steam is the condensation caused by reducing pressure) with a water cooling system, where a mixture of water and steam is produced from the wells. The steam is separated in a surface vessel (steam separator) and delivered to the turbine, and the turbine powers a generator.

A cooling system is essential for the operation of any modern geothermal power plant, because cooling towers prevent turbines from overheating and prolong facility life. Most power plants, including most geothermal plants, use water cooling systems.

Water cooled systems generally require less land than air cooled systems, and are considered overall to be effective and efficient cooling systems. The evaporative cooling used in water cooled systems, however, requires a continuous supply of cooling water and creates vapor plumes. Usually, some of the spent steam from the turbine (for flash- and steam-type plants) can be condensed for this purpose.

Reliability of Geothermal Power Generation

The source of geothermal energy, heat from the earth, is available 24 hours a day, 365 days a year. Solar and wind energy sources, in contrast, are dependent upon a number of factors, including daily and seasonal fluctuations and weather variations. For these reasons, electricity from geothermal energy is more consistently available, once the resource is tapped, than many other forms of electricity.

Examples of Power Plant Size and Applications

Though the size of a power plant is determined primarily by resource characteristics, these are not the only determining factors. Factors that favor the development of larger geothermal plants include things such as cost decreases when larger quantities of materials, including steel, concrete, oil, and fuel, are purchased at one time.

Cooling System

Most power plants, including most geothermal plants, use water-cooled systems – typically in cooling towers.

References/Sources - Idaho National Lab (INL) - Wikipedia - Geothermal Energy Association - U.S. Department of Energy

 

********** 

Software Calculator for cooling tower design and maintenance and TURBOsplash PAC ™ for filling material.

**********

The function of Cooling Towers with Geothermal Energy

Use of Cooling Towers with Geothermal Energy

1. What is geothermal energy?

Geothermal energy derives from the heat of the earth’s core. Here we will refer to energy deriving from the core of the earth. Based on new research, the earth’s core temperature is believed to be anywhere between 6000°C and 6500°C. This intense heat is absorbed by the different layers of the earth and, consequently, this heats our planet.

This geothermal energy can be used to generate geothermal power and is the source of our hot springs, volcanoes and geysers.

2. How is geothermal energy harnessed?

Geothermal energy is heat that is extracted from the earth. Pressurized hot water and steam, is produced when groundwater meets with the molten magma ascending from the earth’s core. Hot water flows to the surface through wells. Once pressure is released, the water flashes to steam. Deep wells, a mile or more deep, can tap reservoirs of steam or very hot water that can be used to drive turbines which power electricity generators.

3. How are cooling towers used with geothermal energy?

Let's suppose steam is separated from the water and this steam is used to drive a turbine generator. Conventional cooling towers are used to condense steam on the low-pressure side of the turbine to maximize electrical generation efficiency. Either direct condensers or surface heat exchanger condensers are utilized. In most cases, the condensate is used as makeup for the cooling towers. There is excess condensate available and this means that cooling towers run at low cycles. Low cycle operation results in excessive cooling tower treatment costs unless programs can be employed that are effective at low dosage rates.

4. Neri Calculator for cooling tower design and maintenance and TURBOsplash PAC ™ for filling material.

Geothermal power plants are designed with corrosion resistant materials of construction such as stainless steel in order to withstand the trace contaminants that enter the cooling systems with the steam. 

Cooling towers, if properly sized and filled with high efficiency fills, will yield optimal performance.

**************

References
1. University of Florida
2. International Geothermal Association
3. Wikipedia

The importance of water in the cooling tower industry - Water (part 10)

WHAT ARE THE CONSEQUENCES OF A COOLING TOWER OPERATING AT LOW EFFICIENCY?

Cooling tower detailed calculations

A low efficient cooling tower brings very serious consequences.

 

Bruno Neri

The tower is part of a system designed to dispose residual waste heat from the production plant or other primary system. The lower efficiency of the tower affects the performance of the primary plant with considerable waste of energy and, most of all, reduction or lack of production.

CONSTRUCTION OF A COOLING TOWER: TIPS ON ITS COMPONENTS AND THEIR USE.

As we saw, the cooling tower is an essential part of the plant system and, generally, it is separated from the primary plant system to which it drives.

The cooling tower is normally ignored until it goes into failure. Hence, the choice of components is of utmost importance.

Choose a tower made of stainless steel. Towers that need to be installed in heavy environments, such as chemical industries, choose polyester reinforced with glass fiber.

 

We have witnessed towers that have been operating for over 30 years, in these heavy environments, in perfectly stable structure.

 

Other useful tips and / or necessary will be exposed our next documentation work for publishing:

"A practical guide for the design of components that impact cooling tower thermal efficiency"

The importance of water in the cooling tower industry - Water (part 7)

Process plants that can be cooled with evaporation systems.
The importance of disposing quantity of energy.
Efficiency of the cooling system.

Image from previous post "NERI Calculator". Click on the above image to learn more about the calculator. 

From what has been written on this subject, the disposal of heat from evaporative cooling towers use something in particular, that is, disposing large amounts of heat from the water by means of the air: two natural elements.

Heat disposal has, therefore, a relatively low cost.

To name a few examples where towers are used:

(a) disposing heat from the various refrigeration groups and city building air conditioning condensers.

(b) in industries such as oil refineries, chemical plants,

(c) in industrial process plants for the production of food products,

(d) in thermoelectric power plants,

(e) in geothermal systems.

(f) …

Obviously each type of installation has different requirements for heat disposal (amount of cooled water). Temperatures and their range must be designed appropriately and carefully, and, most of all, the air thermal characteristic data to be considered for cooling tower design.

Air data has to include temperature, humidity and altitude related to the location of the tower. These three values (temperature, humidity, altitude) are very important.

If the data is not chosen correctly, this can lead to the wrong tower sizing, up to three or four times higher, or lower, than the actual needs of the system.

If the tower sizing is higher than the correct value, this will lead to waste of material and, hence, higher cost of the system. On the other hand, if faulty sizing will lead to equipment that is not adequate to the system, hence, useless!

→ See NERI Calculator ←

The importance of water in the cooling tower industry - Water (part 6)

SYSTEMS THAT COOL WATER IN AN EVAPORATIVE WAY: WHERE THEY ARE USED

A hint is given by knowing how refrigerators function in terms of "transfer of energy-heat". Although this topic is very interesting, we will not linger on the quality of energy.

 

We only need to know that not all energy is equal. There is no difference between the physical and mathematical way.

 

In practice, from an economic point of view, it is very important to know how to take advantage of the energy that is available.

 

We must say that the waste heat (energy that cannot be used) from plants, unfortunately, can only be used in few plants. This is because their natural use in "cascade" presupposes that the plant being served needs to use the same amount of energy at the same time, and this is what makes more difficult. Let us recall that it's very difficult to store energy in an economically way.

 

Now we will discuss about refrigerators

 

Contrary to what is known, refrigerators "do not produce cold." Cold cannot be produced, or make!

 

Cold is something you “feel", it exists because “it lacks” heat; in other words, we do not produce cold but we remove heat, hence, we have cold.

 

Refrigerating machines do the following: remove heat, or better carries heat from one system component (called evaporator) to another component (called condenser).

 

For example, to learn how much heat a refrigerator carries, it's enough to know the power of the engine required to make the refrigerator function. In practice, usually, 1 kW is required to "carry" about 2,500-3,000 kCal / h.