Log Mean Temperature Difference (LMTD) Method with Examples

Logarithmic Mean temperature Difference is used to measure the Heat transfer rate in double pipe heat exchanger. The rate of heat transfer is got from the formula -

Q = U.A.LMTD .

U= Heat Transfer Co-efficient
A= Heat Transfer Surface Area
LMTD=  Mean Temperature Difference

LMTD is the logarithmic temperature difference between the hot and cold fluids passing through the each ends of heat exchanger pipes.

Formula for LMTD calculation - 

LMTD or Logarithmic Mean Temperature Difference  is calculated by keeping some factors in mind 

• The type of the heat exchanger 
• temperature of the fluids at the four end positions 

These are - 

• Inlet temperature of the hot fluid 
• Inlet temperature of the cold fluid 
• Outlet temperature of the cold fluid
• Outlet temperature of the hot fluid 

LMTD =  dT1 - dT2 /ln (dT1/dT2)  

For parallel flow - 

dT1 = Temperature of the hot fluid entering - Temperature of the cold fluid entering 

dT2 = Temperature of the hot fluid exit - Temperature of the cold fluid exit 

For counter flow - 

dT1 = Temperature of the hot fluid entering - Temperature of the cold fluid exiting

dT2 = Temperature of the hot fluid exit - Temperature of the cold fluid entering  

Mathematical Example : 

1. For a Parallel Flow Heat Exchanger,  Hot fluid entering at temperature 100 degree Celsius and exiting at 90 degree Celsius . Cold fluid Entering at 30 degree Celsius and exiting at 50 degree Celsius. Find the LMTD . 

For Parallel or co - current flow , 

According to the above mentioned formulas - 

LMTD = ((100 - 30)-(90-50)) / ln (100-30/90-50) = 53.6 degree Celsius . 

2. For a Counter Flow Heat Exchanger for same data calculate the LMTD . 

For Counter Current HE , 

LMTD = ((100-50)-(90-30))/ln(100-50/90-30) = 54.85 degree Celsius . 

So Heat transfer rate for Counter Flow is higher . 

Read full post »

Biomass-Advantages and Disadvantages

Biomass - Important facts 

Biomass advantages and disadvantages are associated with some very important facts. Day by day the conventional energy sources like fossil fuels is reducing. Finding sources for conventional energy is a very complicated and lengthy process. But with the increasing population, the need for new energy sources is also rapidly increasing in developing countries and in rural areas. Sometimes the grid power is not economically feasible to expand in those areas. In many south Asian countries like India, Bangladesh and Pakistan biomass is the first choice when it comes to the production of heat for cooking. Here you can find the impact of renewable energy on the global warming in India. The most important biogas plant found in India is Fixed Dome Type Janata Model. 

image courtesy: http://renewableenergyhub.com/

Definition of biomass fuel?

Biomass is naturally produced from plants and animals. It is natural and organic. Biomass is one of the most cheapest non-conventional energy sources because it is produced from micro organisms. Biomass stores energy from sunlight by the photosynthesis process. Though it is cheap and clean it is not a popular form of energy source like fossil fuels. Especially in the developed countries biomass has lost its popularity. But in the rural areas biomass is widely used in small industries and for cooking purposes. Some examples of biomass fuels are wood, manure, animal and human wastes, crops, manure etc. In the last two decades biomass has become very common alternative energy. 

Please read : 
Renewable energy the great green hope ! 
Renewable energy sources - graphical representation  

Classifications of biomass 

Biomass is generally classified in two big classes 

1. Solid biomass  like - weeds, agricultural residues, coconut shells, cotton stock con cob etc. 
2. Powdery biomass like - rice husk, ground, cofee husk, sugarcane smashes parts etc. 

Biomass advantages and disadvantages 

If you talk about the advantages and disadvantages of biomass energy then we will see that there are very few cons. There are different techniques for the energy harnessing. One of the biggest disadvantage of biomass energy is that sometimes some biomass fuels are needed direct burning. Wood or dried cow dung cakes are some of the sources which need direct burning. And this can cause some pollution. So if not taken care of, biomass can produce severe environmental problems as it releases a lot of carbon. As a result the balance in the ecosystem can be hampered. 

We cannot deny the fact that biomass can be a good alternative fuel to fossil fuels. Biomass can be collected from various sources. So if this energy is harvested in a way that creates negligible harm then it can dispel some of the energy crisis of the world. So for the proper use of the biomass energy , appropriate policy should be made. Awareness about the potential biomass energy advantages and disadvantages should be created. All the energy sources have their own advantages and disadvantages. With proper guidelines and policy biomass can be a great sustainable and low cost clean energy. Like any other energy sources there are advantages and disadvantages of using biomass energy. Let's discuss about them: 

Advantages

• Biomass energy is renewable or alternative. 
• Biomass is carbon neutral. 
• It is  inexhaustible fuel source. 
• Biomass produces very low amount of  carbon compared to fossil fuel energy;
• The environmental impact is minimum if the direct burning burning is avoided. Instead of that fermentation or pyrolysis can be used 
• Alcohol and other forms of fuels produced from biomass is very clean burning and environment friendly. 

Disadvantages

• Fossil fuels are more efficient than biomass fuels. 

• Sometimes biofuel production can be proved a little expensive. 
• Some biomass production plants need a lot of space to grow the raw materials of the plant (crops and plants)
• Direct burning can produce pollution by particulate emission. 

Conclusion

In developing and rural areas biomass is still used in heating purposes and also for cooking. Using biomass farmers can do their agricultural activities. These days almost 16% of the world's total energy supply from biomass. In developing countries 40% of the rural supply come from the biomass energy. A number of environmental groups are strongly opposing the wide use of forest biomass use because of high carbon emission. For the better realization of biomass advantages and disadvantages, I think this post will be very helpful. Biomass fuels can be great option to replace the fossil fuels as a source of  power generation in rural, developing as well as developed countries. 

Please Read :

• Pros and Cons of Geothermal Energy 
• Solar energy pros and cons 
• Different renewable energy sources - graphical presentation 
• What is the cheapest renewable energy source - biomass 
• Solar pond pump or solar fountain pump advantages and disadvantages 
• Wave energy pros and cons 

Read full post »

Renewable Energy Sources: The Great Green Hope for clean environment

Fossil fuel use is the prime cause of global warming and ocean acidification. Then the obvious solution to that problem is that:

• Either some way is found out to use fossil fuels, but not let the resulting CO₂ escape to the atmosphere, in other words to sequester (catch/lock up) the CO₂ , or
• To find other fuels which do not release CO₂ (or other greenhouse gases) 

Renewable Energy Sources: The Great Green Hope 

The world is trying both the options. The sequestration of CO₂ is very much possible theoretically, it is as of now uneconomical. Moreover, quantities of CO₂ that must be sequestered is so huge that there is no way to store it without serious risks to environment and public safety.

Realizing the limitations of sequestering fossil fuel CO_2, great hope has been pinned on the renewable energy sources. The year 2008 has seen more funds to be allocated globally to renewable – based power generation than ever before. The year also witnessed renewables getting higher share of funds than conventional energy sources for the first time ever.

How tough it is to find replacement which will be as efficient as petrol and diesel, and yet be non-polluting, can be gauged from figure below. It show s that the volumetric energy density, in other words energy packed in each liter of diesel is as much as double of that of ethanol and three times higher than liquid hydrogen. Petrol is more energy-dense than bio-diesel; also ethanol, hydrogen, etc. This means that if vehicles are to be run on liquid hydrogen- which is ultimate aim of most of the R&D- they have to have their tanks three times larger than what they were for the diesel engine. To put in other words, even if we succeed in finding fuels which are cleaner than petrol and diesel, we have little hope of finding fuels which are as efficient as the twoRenewable Energy Sources: The Great Green Hope

Fossil fuel use is the prime cause of global warming and ocean acidification. Then the obvious solution to that problem is that:

Either some way is found out to use fossil fuels, but not let the resulting CO₂ escape to the atmosphere, in other words to sequester (catch/lock up) the CO₂ , or

To find other fuels which do not release CO₂ (or other greenhouse gases) 

The world is trying both the options. The sequestration of CO₂ is very much possible theoretically, it is as of now uneconomical. Moreover, quantities of CO₂ that must be sequestered is so huge that there is no way to store it without serious risks to environment and public safety.

Realizing the limitations of sequestering fossil fuel CO_2, great hope has been pinned on the renewable energy sources. The year 2008 has seen more funds to be allocated globally to renewable – based power generation than ever before. The year also witnessed renewables getting higher share of funds than conventional energy sources for the first time ever.

How tough it is to find replacement which will be as efficient as petrol and diesel, and yet be non-polluting, can be gauged from figure below. It show s that the volumetric energy density, in other words energy packed in each liter of diesel is as much as double of that of ethanol and three times higher than liquid hydrogen. Petrol is more energy-dense than bio-diesel; also ethanol, hydrogen, etc. This means that if vehicles are to be run on liquid hydrogen- which is ultimate aim of most of the R&D- they have to have their tanks three times larger than what they were for the diesel engine. To put in other words, even if we succeed in finding fuels which are cleaner than petrol and diesel, we have little hope of finding fuels which are as efficient as the two. 

Figure: Fuel and Power Sources

Please Read the following articles : 

• Solar Pond fountain Pump - Features 
• Geothermal Energy Pros and Cons 
• Biogas Plant construction 
• Cheapest Renewable Energy-Biomass  
• Flash Steam Type Geothermal energy plant 
• Different Types of Nuclear Reactors 
• Advantages and Disadvantages of Hydroelectric Power 
• Biomass - pros and cons 

Read full post »

Solar Air Heater Classification and Advantages

This post provides the description and analysis of different types of solar air heaters used in space heating and drying purposes. The Solar air heaters have the following advantages over other solar heat collectors.

Solar Air Heater Advantages 

• The need to transfer heat from working fluids to another fluid is eliminated as air is being used directly as the working substance. The system is compact and less complicated.
•  Corrosion is a great problem in solar water heater. And this problem is not experienced in solar air heaters.
•  Leakage of air from the duct does not create any problem.
• Freezing of working fluid virtually does not exist.
• The pressure inside the collector does not become very high. 
• Thus air heater can be designed using cheaper as well as lesser amount of material and it is simpler to use than the solar water heaters.

Solar Air Heater Disadvantages 

• Air heaters have certain disadvantages also the first and foremost is the poor heat transfer properties of air. Special care is required to improve the heat transfer. 
• Another disadvantage is the need for handling large volume of air due to its low density. 
• Air cannot be used as a storage fluid because of its low thermal capacity. 
• In the absence of proper design the cost of solar air heaters can be very high. 

Solar Air Heater Applications  

The applicability of the solar air heater depends on various factors like high efficiency, low fabrication cost, low installation and operational cost and some other specific factors regarding specific uses. Extensive work in solar air heaters has been done. Various geometries have been proposed and their theoretical investigation is carried out. But it needs commercial exploitation.
 

Solar Air Heater Classification

A conventional solar air heater is essentially a flat plate collector with absorber plate, a transparent cover system at the top and insulation at the bottom and on the sides. The whole assembly is encased in a sheet metal container. The working fluid is air, though the passage for its flow varies according to the type of air heater.
Material for construction of air heaters are similar to those of liquid flat plate collectors. The transmission of solar radiation through the cover system and its subsequent absorption in the absorber plate can be given by expressions identical to that of liquid flat plate collectors. Selective coating on the absorber plate can be used to improve the collection efficiency but cost effectiveness criterion should be kept in mind.
 

Non-porous Type solar air heater

In non-porous type, air stream does not flow through below the absorber plate but air may flow above and/or behind the plate.

In first type, no separate passage is required and the air can flows between the transparent cover system and the absorber plate. ( see the figure). In this heater as the hot air flows above the absorber, the cover receives much of the heat and in turn, loses it to the ambient. Thus a substantial amount of heat is lost to the ambient and hence this air heater is not recommended.
The non-porous type with air passage below the absorber is most commonly used. A plate parallel to the absorber plate is provided in between the absorber and the insulation, thus forming a passage of high aspect ratio.
In another variety of non-porous type air heater, the absorber plate is cooled by air stream flowing on both sides of the plate.  

Depending on the type of the absorber plate, the air heater can be non-porous and porous. Figure below shows the non-porous absorber type air heaters.

It may be noted that the heat transfer between the absorber plate and the flowing air being low, the efficiency of air heaters is less. The performance, however, can be improved by roughening the absorber surface or by using a vee-corrugated plate as the absorber plate. Turbulence induced to the air flow helps increase the convective heat transfer.
The radiative loss from the absorber plate are significant, unless selective coatings are used, decreasing the collector efficiency. Also, the uses of fin may result in a prohibitive pressure drop, thus limiting the applicability of non-porous type.
 

Porous type solar air heater    

The second type of air heaters has porous absorber which may include slit and expanded metal, overlapped glass plat absorber and transpired honeycomb.

The sir heater with porous type of absorber has the following advantages: 

Advantages of porous solar air heater 

• Solar radiation penetrates to a great depth and is absorbed along its path. Thus the radiation loss decreases. Air stream heats up as it passes through the matrix. 

• The pressure drop is usually lower than the non-porous type. 

It may be noted however, that an improper choice of matrix porosity and thickness may cause reduction in efficiencies as beyond an optimum thickness, matrix may not be hot enough to transfer the heat to air stream.

Wire mesh porous bed formed by broken bottles and overlapped glass plate are some examples of porous type absorbers used in Solar air heaters. 

Read these articles for further studies:

• Solar Pond fountain Pump - Features 
• Geothermal Energy Pros and Cons 
• Biogas Plant construction 
• Cheapest Renewable Energy-Biomass  
• Flash Steam Type Geothermal energy plant 

Read full post »

Fixed Dome type – Janata Model Biogas Plant Construction

Biogas can be a great alternative of fossil fuels. It is already in use in many rural areas. Biogas plants use animal waste, plant waste and human waste. All this wastes have great combustible property. Biogas is an excellent renewable energy source. It is produced by the digestion of waste materials by the means of anaerobic reaction. Anaerobic means the absence of Oxygen. In most of the rural areas cow and buffalo dungs is used as biomass fuel for producing gas. The typical composition of biogas is

Methane - CH₄  (55 - 65 %)
Carbon dioxide CO₂  (30-40%) 

H_{2 ,} H₂S , N₂      (< 10%) 

Biogas Technology involves the bacterial breakdown of the waste materials to produce Methane, Carbon Dioxide and Water . The process involves the following three steps - 

Hydrolysis 

Organics materials contains mainly carbohydrate mainly in the form of cellulose, hemicellulose and lignin. These have very complex structure which is not suitable for absorption. So these matters are converted into simple soluble materials by the action of celluolytic or hydrolytic bacteria. Concentration of bacteria in the organic materials, temperature and pH controls the rate of hydrolysis. pH between 6 to 7 and temperature between 30-40 degree Celsius is good for bacteria to work. 

Acid Formation 

Simple organic materials are turned into acid by acetogenic bacteria. 

Methane Formation 

Methanogenic bacteria turns the acid into methane, carbon dioxide, hydrogen, nitrogen and oxygen. The methane content is 60%. It has high calorific value. Very good for combustion and producing energy. 

Biogas plants 

Biogas plant converts the organic wastes like dung, human waste and plant wastes into a inflammable and it also produces a high quality organic manure as a by product. Most popular two designs of biogas plant is 

1. Fixed Dome Type Biogas plant (Janata Model) (Operates in constant volume)

2. Floating Drum type Biogas plant. (Operates in constant pressure)

In this article I will discuss about the first one - The Fixed Dome type or Janata Model biogas plant. 

Fixed Dome Type Biogas Plant - Janata Model 

This type of biogas plant is very economical is design. It works with the constant volume principle. The main structure is made up of brick and cement masonry. This type of plant doesn't have any moving parts so it is safe from wear and tear. The operating pressure varies from 0 to 100 cm of water column. It is also known as Janata model.  

Read full post »