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Read this essay to learn about the various sectors of industries in which significant energy can be saved.
Energy Saving Sectors of Industries
Essay Contents:
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- Essay on Boiler
- Essay on Furnaces
- Essay on Air Compressors
- Essay on Refrigeration Systems
- Essay on Heat Exchanger
- Essay on Heat Pumps
- Essay on Turbines
- Essay on Electrical Drives
- Essay on Pumps
- Essay on Cooling Towers
- Essay on Fans and Blowers
1. Essay on Boiler:
Boilers are integral part of any process industry and used for generating steam which is required in various processes for heating purposes mainly and in some other applications such as flushing out of various piping and its associated equipments at the end of the process or before the start of process.
Boilers used large amount of fuel cost. Therefore an efficient operation of boiler is must for overall energy efficiency of the utility. Various performance tests for boilers are necessary to judge its performance efficiency.
Performance of the boiler, like efficiency and evaporation ratio reduces with time, due to poor combustion, heat transfer fouling and poor operation and maintenance. Deterioration of fuel quality and water quality also leads to poor performance of boiler. Efficiency testing helps us to find out how far the boiler efficiency drifts away from the best efficiency.
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Any observed abnormal deviations could therefore be investigated to pinpoint the problem area for necessary corrective action. Hence it is necessary to find out the current level of efficiency for performance evaluation, which is a pre requisite for energy conservation efforts in any process industry using boilers. The terms associated with performance of boiler are its efficiency (boiler efficiency) and evaporation ratio.
These are defined as:
Boiler efficiency:
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Boiler efficiency is defined as ratio of heat output to the heat input in a boiler expressed in percentage and which is given as;
(Heat Output/Heat Input) × 100
where heat output is the heat of steam at the output in kilo calories and heat input is the heat of fuel input in kilo calories.
The various efficiency terms related to boiler are as follows:
(1) Thermal efficiency:
Thermal efficiency reflects how well the boiler vessel transfers heat. The figure usually excludes radiation and convection losses.
(2) Combustion efficiency:
Combustion efficiency typically indicates the ability of the burner to use fuel completely without generating carbon monoxide or leaving hydrocarbons unburned.
(3) Boiler efficiency:
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Boiler efficiency mean any fuel-use figure by comparing energy put into the boiler with energy coming out.
Evaporation ratio:
Evaporation ratio is defined as ratio of quantity of steam generation to the quantity of fuel consumption.
Factors affecting the efficiency calculation of boiler system:
Following factors affects the calculation of efficiency of a boiler system:
(1) Boiler stack temperature:
Boiler stack temperature is the temperature of the combustion gases leaving the boiler. This temperature represents the major portion of the energy not converted to usable output. The higher the temperature, the less energy transferred to output and the lower the boiler efficiency.
When stack temperature is evaluated, it is important to determine if the value is proven. For example, if a boiler runs on natural gas with a stack temperature of 350°F, the maximum theoretical efficiency of the unit is 83.5%. For the boiler to operate at 84% efficiency, the stack temperature must be less than 350°F.
(2) Heat content of fuel:
The efficiency calculation requires knowledge of the calorific value of the fuel (heat content), its carbon to hydrogen ratio, and whether the water produced is lost as steam or is condensed, and whether the latent heat (heat required to turn water into steam) is recovered.
(3) Fuel specification:
The fuel specified has a dramatic effect on efficiency. With gaseous fuels having higher the hydrogen content, the more water vapor is formed during combustion. The result is energy loss as the vapor absorbs energy in the boiler and lowers the efficiency of the equipment.
The specification used to calculate efficiency must be based on the fuel to be used at the installation. As a rule, typical natural gas has a hydrogen/-carbon (H/C) ratio of 0.31. If an H/C ratio of 0.25 is used for calculating efficiency, the value increases from 82.5% to 83.8%.
(4) Excess air levels:
Excess air is supplied to the boiler beyond what is required for complete combustion primarily to ensure complete combustion and to allow for normal variations in combustion. A certain amount of excess air is provided to the burner as a safety factor for sufficient combustion air.
(5) Ambient air temperature and relative humidity:
Ambient conditions have a dramatic effect on boiler efficiency. Most efficiency calculations use an ambient temperature of 80°F and a relative humidity of 30%. Efficiency changes more than 0.5% for every 20°F change in ambient temperature. Changes in air humidity would have similar effects; the more the humidity, the lower will be the efficiency.
Reference standards for determining boiler efficiency:
The British and Indian standards related to boiler efficiency calculations are as follows:
a. British standards, BS845:1987:
The British Standard BS845:1987 describes the methods and conditions under which a boiler should be tested to determine its efficiency. For the testing to be done, the boiler should be operated under steady load conditions (generally full load) for a period of one hour after which readings would be taken during the next hour of steady operation to enable the efficiency to be calculated.
The efficiency of a boiler is quoted as the % of useful heat available, expressed as a percentage of the total energy potentially available by burning the fuel. This is expressed on the basis of gross calorific value (GCV).
This deals with the complete heat balance and it has two parts: Part One deals with standard boilers, where the indirect method is specified whereas Part Two deals with complex plant where there are many channels of heat flow. In this Case, both the direct and indirect methods are applicable, in whole or in part.
b. IS 8753: Indian standard for boiler efficiency testing:
Most standards for computation of boiler efficiency, including IS 8753 and BS845 are designed for spot measurement of boiler efficiency. Invariably, all these standards do not include blow down as a loss in the efficiency determination process.
Boiler efficiency can be tested by the following methods:
(1) The direct method:
Where the energy gain of the working fluid (water and steam) is compared with the energy content of the boiler fuel.
(2) The indirect method:
Where the efficiency is the difference between the losses and the energy input.
Boiler efficiency evaluation:
For calculating the efficiency of boilers, direct and indirect testing methods are used to calculate the various losses in boiler operation, which are then used in calculating the efficiency of boilers.
These methods are as follows:
(i) Direct method testing:
This is also known as ‘input-output method’ due to the fact that it needs only the useful output (steam) and the heat input (i.e. fuel) for evaluating the efficiency.
This efficiency can be evaluated using the relation as:
Efficiency = (Heat Output/Heat Input) × 100
Boiler efficiency can also be given as:
For calculating the efficiency, the accurate measurement of heat input to boiler and heat output from the boiler is required.
These measurements are done as follows:
a. Heat input:
Both heat input and heat output must be measured. The measurement of heat input requires knowledge of the calorific value of the fuel and its flow rate in terms of mass or volume, according to the nature of the fuel.
For gaseous fuel:
A gas meter of the approved type can be used and the measured volume should be corrected for temperature and pressure. A sample of gas can be collected for calorific value determination, but it is usually acceptable to use the calorific value declared by the gas suppliers.
For liquid fuel:
Heavy fuel oil is very viscous, and this property varies sharply with temperature. The meter, which is usually installed on the combustion appliance, should be regarded as a rough indicator only and, for test purposes, a meter calibrated for the particular oil is to be used and over a realistic range of temperature should be installed. Even better is the use of an accurately calibrated day tank.
For solid fuel:
The accurate measurement of the flow of coal or other solid fuel is very difficult. The measurement must be based on mass, which means that bulky apparatus must be set up on the boiler-house floor. Samples must be taken and bagged throughout the test, the bags sealed and sent to a laboratory for analysis and calorific value calculation. In some more recent boiler houses, the problem has been alleviated by mounting the hoppers over the boilers on calibrated load cells.
b. Heat output:
There are several methods, which can be used for measuring heat output. With steam boilers, an installed steam meter can be used to measure flow rate, but this must be corrected for temperature and pressure. In earlier years, this approach was not favoured due to the change in accuracy of orifice or venturi meters with flow rate. It is now more viable with modern flow meters of the variable-orifice or vortex-shedding types.
The alternative with small boilers is to measure feed water, and this can be done by previously calibrating the feed tank and noting down the levels of water during the beginning and end of the trial. Care should be taken not to pump water during this period. Heat addition for conversion of feed water at inlet temperature to steam, is considered for heat output.
In case of boilers with intermittent blowdown, blowdown should be avoided during the trial period. In case of boilers with continuous blowdown, the heat loss due to blowdown should be calculated and added to the heat in steam.
Using the direct method, the efficiency of the boiler in percentage is calculated as:
Calculation:
where Q = Quantity of steam generated per hour (kg/hr)
q = Quantity of fuel used per hour (kg/hr)
GCV = Gross calorific value of the fuel (kCal/kg)
H = Enthalpy of steam (kCal/kg)
h = Enthalpy of feed water (kCal/kg)
Advantages and disadvantages of direct method:
Advantages:
(i) Plant people can evaluate quickly the efficiency of boilers
(ii) Requires few parameters for computation
(iii) Needs few instruments for monitoring.
Disadvantages:
(i) This method does not give clues to the operator as to why efficiency of system is lower.
(ii) This method does not calculate various losses accountable for various efficiency levels.
(iii) Evaporation ratio and efficiency may mislead, if the steam is highly wet due to water carryover.
(ii) Indirect method:
The efficiency of the boiler can also be measured easily by measuring all the losses occurring in the boilers. The limitations of the direct method can be overcome by indirect method, which calculates the various heat losses associated with boiler. The efficiency can be arrived at, by subtracting the heat loss fractions from 100. An important advantage of this method is that the errors in measurement do not make significant change in efficiency.
The various heat losses in the boiler can be represented as:
The losses in the boiler are:
(1) Loss due to dry flue gas (sensible heat)
(2) Loss due to hydrogen in fuel (H2)
(3) Loss due to moisture in fuel (H2O)
(4) Loss due to moisture in air (H2O)
(5) Loss due to carbon monoxide (CO)
(6) Loss due to surface radiation, convection and other unaccounted
(7) Fly ash loss
(8) Bottom ash loss
The losses due to fly ash are applicable to solid fuel type boilers only.
All the losses in the boilers are calculated and the efficiency of boiler is calculated by subtracting these losses which are in percentage from the 100.
Data can be tabulated to calculate the efficiency as:
Measurements required for performance assessment testing:
The following parameters need to be measured, as applicable for the computation of boiler efficiency and performance:
(a) Flue gas analysis:
1. Percentage of CO2 or O2 in flue gas
2. Percentage of CO in flue gas
3. Temperature of flue gas
(b) Flow meter measurements:
1. Fuel
2. Steam
3. Feed water
4. Condensate water
5. Combustion air
(c) Temperature measurements:
1. Flue gas
2. Steam
3. Makeup water
4. Condensate return
5. Combustion air
6. Fuel
7. Boiler feed water
(d) Pressure measurements:
1. Steam
2. Fuel
3. Combustion air, both primary and secondary
4. Draft
(e) Water condition:
1. Total dissolved solids (TDS)
2. pH
3. Blow down rate and quantity
From the measurement data and calculated efficiency of the boiler, steps for energy efficiency improvement can be carried out to keep these parameters under specified permissible limits so as to get the maximum boiler efficiency for optimum performance of the boiler.
Factors affecting boiler performance:
The various factors affecting the boiler performance are listed below:
(i) Periodical cleaning of boilers.
(ii) Periodical soot blowing.
(iii) Proper water treatment programme and blow down control.
(iv) Draft control.
(v) Excess air control.
(vi) Percentage loading of boiler.
(vii) Steam generation pressure and temperature.
(viii) Boiler insulation.
(ix) Quality of fuel.
(x) Ambient Temperature.
All these above mentioned factors individually/combined, contribute to the performance of the boiler and reflected either in boiler efficiency or evaporation ratio. Based on the results obtained from the testing further improvements have to be carried out for maximizing the performance.
The test can be repeated after modification, or rectification of the problems and compared with standard norms. Energy auditor should carry out this test as a routine manner once in six months and report to the management for necessary action.
2. Essay on Furnaces:
Furnaces in industry are used to heat the metals to melt them for castings or heat materials for change of shape. In other applications, furnaces are also used as hot air generators for producing high temperature air called hot air in various drying processes in industries. Furnaces are also used to heat the houses to maintain the cozy temperature inside the house situated mostly in colder region of the world.
Process furnaces, dryers and kilns are used in such diverse applications as melting metal, drying wood, evaporating water, and manufacturing lime, bricks and ceramics. Some facilities are constructed and operated solely for the purpose of a single heating manufacturing process.
Consequently, the furnace could be the single largest consumer of fuel energy. All non-electric furnaces utilize a burner to deliver a mixture of fuel and air for the combustion process that produces heat, which is subsequently transferred to the product either directly (within the combustion chamber) or indirectly (through a heat exchanger). There may also be electrical energy input to operate auxiliary equipment such as blowers and draft fans.
As with boilers, it is important to evaluate furnace performance and efficiency over the range of actual or partial loads. Unlike boilers, there is usually no large heating distribution system with its accompanying losses (i.e. the end-use of the heat is within the furnace).
Maintaining the optimum ratio of fuel to air is critical to efficient operation of fuel-burning furnaces. A lack of air leads to incomplete combustion, resulting in losses of combustibles in the flue gases (smoky flame). Excess air needlessly increases the dry flue gas losses, as indicated by higher flue gas temperatures.
In addition, the excess air entering the furnace must be heated, increasing energy losses. The temperature of the flue gas also depends on the effectiveness of heat transfer to the product being processed and is a good indicator of the condition of internal heat transfer surfaces. In some cases a large amount of excess air is required to maintain product quality. In that case heat recovery should be considered.
The portable combustion analyser is a useful tool for gauging the combustion efficiency of process furnaces, dryers and kilns. Furnaces consumes reasonable amount of energy for producing the heat. Therefore, for proper distribution of heat and its optimized use, proper design and operation of the furnace system is essential for energy efficient operation. Furnaces heat air and distribute the heated air through the house using ducts.
Furnaces consume large amount of energy in the form of fuel consumption. Therefore an efficient operation of furnaces is necessary for getting the optimal energy efficiency of furnaces.
Types of furnaces:
Furnaces are classified according to the type of fuel used such as oil fired type etc. Electrical furnaces are also available which involves the neat & clean process. Classification of furnaces is also done on the basis of modes of heat transfer in the furnaces, modes of charging and modes of heat recovery in the furnaces.
Energy losses in the furnaces:
To determine the efficiency of the furnaces, first of all the losses in the furnaces needs to be identified and after calculating these losses, the efficiency of furnace is calculated knowing the amount of fuel input used.
The various losses in furnaces are represented as per the following diagram:
The efficiency of the furnace is calculated using direct method. In this method, all the losses in the furnaces are calculated and quantified. The output is then calculated after subtracting these losses from the total fuel input applied and then total efficiency in %age is evaluated.
The various losses involves in furnaces are:
i. Flue gas losses (waste gas loss).
ii. Wall losses (insulation leakage).
iii. Opening loss (radiation loss).
iv. Stored heat loss.
v. Improper Heat distribution.
vi. Improper flame loss.
vii. Fuel leakage loss.
viii. Improper burning of fuel.
Measures to reduce heat losses in furnaces:
Losses in the furnaces can be significantly reduced by adopting the following measures:
i. Complete combustion of fuel with min. excess air.
ii. Proper heat distribution.
iii. Operating the furnace at desired temperature.
iv. Using high quality liners for furnace walls to reduce wall losses.
v. Maintaining correct amount of furnace draught.
vi. Using waste heat from flue gases for preheating.
vii. Using ceramic coatings.
viii. Regular maintenance.
Maintaining furnaces:
Proper care and maintenance can improve and maintain the efficiency of furnace system.
The following maintenance should be done routinely for efficient operation of furnace system:
i. Check the condition of your vent connection pipe and chimney. Parts of the venting system may have deteriorated over time. Chimney problems can be expensive to repair, and may help justify installing new heating equipment that won’t use the existing chimney.
ii. Adjust the controls on the furnace to provide optimum air temperature settings for both efficiency and comfort.
iii. Perform a combustion-efficiency test.
iv. Check the combustion chamber for cracks.
v. Test for carbon monoxide (CO) and rectify if found.
vi. Adjust blower control and supply-air temperature.
vii. Clean and oil the blower.
viii. Remove dirt, soot, or corrosion from the furnace.
ix. Check fuel input and flame characteristics, and adjust if necessary.
x. Seal connections between the furnace and main ducts.
xi. Check for chimney blockage for proper exhaust of combustion gases.
xii. Check the condition of chimney liners.
3. Essay on Air Compressors:
Air compressors are widely used in industries to produced compressed air which is used in various industrial processes and other applications. Air compressors utilizes fair amount of electricity in the industries especially in process industries such as cement manufacturing units. Routine checks and periodic audits are necessary to judge the optimal performance of air compressors in terms of their energy consumption.
Energy audits of air compressors is done to achieve the following objectives:
i. Increase productivity.
ii. Reduce capital costs.
iii. Reduce compressor maintenance costs.
iv. Reduce energy costs.
v. Improve quality.
Due to the complexity of the air compressor system, the type of the energy audit to be carried out is important. There are two types of audits namely supply side audit and demand side audit which are suitable for air compressor systems.
(i) Supply side audit:
A supply side audit studies how an air compressor generates air and how it is treated. A supply side audit will help identify ways to reduce compressor maintenance controls, improve air quality, and energy cost savings. The supply side audit is limited to the compressor room and doesn’t focus on your process. A supply side audit’s recommendations usually require the highest capital costs with the least amount of savings.
(ii) Demand side audit:
A demand side audit studies how an air compressor utilizes air. A demand side audit will identify the most potential savings with the least amount of capital investment. A demand side audit will help improve process, reduce energy costs, reduce compressor maintenance costs, and increase productivity.
Before carrying out the audit, feasibility study is of entire air compressor system and facility is necessary. The objective of the feasibility study is to make recommendations on how to improve the system and help decide if a more extensive audit is required.
Tips for improving energy efficiency in compressed air systems:
Pressured air from compressors is widely used but it is energy intensive. Compressors are operated in very different conditions and annual running hours vary a lot. There is space for energy efficiency improvements in compressors and energy savings up to 15-30% of the total energy consumption of compressors can be achieved. The energy conservation steps include awareness, better control and use of energy efficient devices to drive the compressors.
Replacing simple on-and-off control with a sophisticated variable speed drive (VSD) based control system keeps the pressure in air pipes stable and supplies the required air flow. It also reduces compressor motor starts, service requirements and extends the motor’s life time. It even supports leakage detection.
Other option for energy savings is the use of energy efficient motor to drive the compressor shaft. Energy efficient motors have higher efficiency than ordinary motors and consume less energy as compared to other motors of same ratings.
Understand the air compressor system first:
Before implementing energy reduction strategies, be familiar with all aspects of your compressed air system.
(i) System supply:
Analyse the supply side of your compressed air system for the types of compressors used and the type, suitability and settings of capacity controls and other operating conditions. Understand the basic capabilities of the system and its various modes of operation.
Verify that air compressors are not too big for end uses. For example, an air compressor is oversized if the end use only requires air pressure that is 50 % of the pressure that the compressor is capable of producing. Once the big picture is in view, supply side operating conditions can be modified, within the constraints of the compressed air unit, to better match the demand side uses of compressed air.
(ii) System demand:
Identify all the uses of compressed air in the plant. Quantify the volume of air used in each application and generate a demand profile, quantity of air used as a function of time, for the compressor. Equipment specifications for operations that use air are good resources for obtaining data on air volume use rates. The profile highlights peak and low demand. A general assessment of compressed air use will help identify inappropriate uses of air.
(iii) System diagram:
Develop a sketch of your compressed air system—including compressors, air supply lines with dimensions, and compressed air end uses—to provide an overall view of the entire compressed air process.
(iv) Distribution system:
Investigate the distribution system for any problems related to line size, pressure loss, air storage capacity, air leaks and condensation drains. Verify that all condensation drains are operating properly because inadequate drainage can increase pressure drop across the distribution system.
(v) Maintenance:
Evaluate maintenance procedures, records and training. Ensure that procedures are in place for operating and maintaining the compressed air system, and that employees are trained in these procedures.
Energy conservation strategies for air- compressors:
Identify easy to implement energy conservation opportunities in your compressed air system by conducting a walk-through assessment. Simple conservation opportunities can result in savings up to 25% of the current cost to run the compressed air system.
(a) Leaks:
Routinely check your system for leaks. A distribution system under 100 pounds-per-square-inch gauged (psig) of pressure, running 40 hours per week, with the equivalent of\a quarter-inch diameter leak will lose compressed air at a rate of over 100 cfm. In noisy environments an ultrasonic detector may be needed to locate leaks.
(b) Compressor pressure:
The compressor must produce air at a pressure high enough to overcome pressure losses in the supply system and still meet the minimum operating pressure of the end use equipment. Pressure loss in a properly designed system will be less than 10% of the compressor’s discharge pressure—found on a gage on the outlet of the compressor.
If pressure loss is greater than 10%, evaluate your distribution system and identify areas causing excessive pressure drops. Every two pounds-per- square-inch decrease in compressor pressure will reduce your operating costs 1.5%.
(c) Identify artificial demands:
Artificial demand is created when an end use is supplied air pressure higher than required for the application. If an application requires 50 psi but is supplied 90 psi, excess compressed air is used. Use pressure regulators at the end use to minimize artificial demand.
(d) Inappropriate use of compressed air:
Look for inappropriate uses of compressed air at your facility. Instead of using compressed air, use air conditioning or fans to cool electrical cabinets; use blowers to agitate, aspirate, cool, mix, and inflate packaging; and use low-pressure air for blow guns and air lances. Disconnect the compressed air source from unused equipment.
(e) Heat recovery:
As much as 80 to 90% of the electrical energy used by an air compressor is converted to heat. A properly designed heat recovery unit can recover 50 to 90% of this heat for heating air or water. Approximately 50,000 British thermal units (Btus) per hour is available per 100 cfm of compressor capacity when running at full load.
(f) Inlet air filters:
Maintain inlet air filters to prevent dirt from causing pressure drops by restricting the flow of air to the compressor. Retrofit the compressor with large-area air intake filters to help reduce pressure drop.
(g) Compressor size:
If your compressor is oversized add a smaller compressor and sequence-controls to make its operation more efficient when partially loaded. Sequence-controls can regulate a number of compressors to match compressed air needs, as they vary throughout the day.
(h) Air receiver/surge tank:
If your compressed air system does not have an air receiver tank, add one to buffer short-term demand changes and reduce on/off cycling of the compressor. The tank is sized to the power of the compressor. For example, a 50 hp air compressor needs approximately a 50-gallon air receiver tank.
(i) Cooler intake air:
When in taking cooler air, which is more dense, compressors use less energy to produce the required pressure. For example, if 90o F intake air is tempered with cooler air from another source to 70o F, the 20o F temperature drop will lower operating costs by almost 3.8%.
(j) V-Belts:
Routinely check the compressor’s v-belts for proper tightness. Loose belts slip more frequently which reduces compressor efficiency.
4. Essay on Refrigeration System:
Refrigeration systems are used in many different environments – residential, commercial and industrial. All these systems are designed for one basic purpose to move heat from a lower temperature (heat source) to a higher temperature (heat sink) medium, using a transfer fluid (refrigerant). Since this is the reverse of the natural direction of heat flow, energy input is required, usually in the form of electricity. Depending on the amount of heat to be moved, the cost of refrigeration can be significant.
Refrigeration systems are relatively complex, and their efficiency is affected by the operating conditions. While a system is typically rated for a particular maximum or design cooling load, it usually operates for most of its life at some fraction of that output, or at partial load.
The efficiency of a cooling system can vary significantly with load, depending on the capacity control method employed. Consequently, it is important to evaluate system performance and efficiency over the range of actual loads.
The energy required to run a cooling system is proportional to the temperature difference between the heat source and heat sink. Therefore reducing the temperature difference between the cooled medium (e.g., refrigerated storage) and the condensing (e.g. cooling tower) temperature has a substantial effect on the energy input to the system. Various measuring devices such as a wattmeter, thermometer, psychrometer or pressure gauge can be useful in evaluating the cooling efficiency of refrigeration systems.
The various losses in refrigeration system are:
i. Primary conversion losses- Electric motor and shaft losses between motor and compressor, pump, fan etc.
ii. Losses in auxiliary equipment such as fan, pump etc.
iii. Control losses – due to load variation.
iv. Losses due to poor insulation and leaks.
v. Cooling losses after evaporator.
Energy conservation steps in refrigeration systems:
Efficiency of the refrigeration system can be improved by minimising the losses as mentioned above in the refrigeration system.
The various steps of energy conservation efforts which can be applied in refrigeration system to improve its efficiency are as follows:
(i) Make a profile of load and temperature requirements as per the season and occupancy.
(ii) Avoid simultaneous heating and cooling preferably.
(iii) Calibrate the control and set the temp, at highest acceptable level.
(iv) Ensure appropriate capacity control of refrigeration system.
(v) Review the settings of defrost control regularly.
(vi) Ensure that all the heat exchange surfaces are cleaned and maintained regularly.
(vii) Lower condensing temperature by ensuring free circulation of air around condensing units and cooling tower.
(viii) Inspect and maintain the cooling tower regularly to achieve lowest temperature.
(ix) Use compressors with highest efficiency (COP).
(x) Optimize the operation of cooling system.
(xi) Utilize de-super-heaters to recover heat rejected from the condenser.
These measures when adopted, can significant improve the efficiency of refrigeration system and can save or conserve significant amount of useful energy.
5. Essay on Heat Exchanger:
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall to prevent mixing or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries, natural gas processing, and sewage treatment.
The classic example of a heat exchanger is found in an internal combustion engine in which a circulating fluid known as engine coolant flows through radiator coils and air flows past the coils, which cools the coolant and heats the incoming air.
There are three primary classifications of heat exchangers according to their flow arrangement. In parallel-flow heat exchangers, the two fluids enter the exchanger at the same end, and travel in parallel to one another to the other side. In counter-flow heat exchangers the fluids enter the exchanger from opposite ends.
The counter current design is the most efficient, in that it can transfer the most heat from the heat (transfer) medium due to the fact that the average temperature difference along any unit length is greater.
For efficiency, heat exchangers are designed to maximize the surface area of the wall between the two fluids, while minimizing resistance to fluid flow through the exchanger. The exchanger’s performance can also be affected by the addition of fins or corrugations in one or both directions, which increase surface area and may channel fluid flow or induce turbulence.
Fouling in heat exchangers:
A heat exchanger in a steam power station contaminated with macro fouling. Fouling occurs when impurities deposit on the heat exchange surface.
Deposition of these impurities can decrease heat transfer effectiveness significantly over time and are caused by:
i. Low wall shear stress
ii. Low fluid velocities
iii. High fluid velocities
iv. Reaction product solid precipitation
v. Precipitation of dissolved impurities due to elevated wall temperatures
The rate of heat exchanger fouling is determined by the rate of particle deposition less re-entrainment/suppression.
Monitoring and maintenance of heat exchangers:
Online monitoring of commercial heat exchangers is done by tracking the overall heat transfer coefficient. By periodically calculating the overall heat transfer coefficient from exchanger flow rates and temperatures, the owner of the heat exchanger can estimate when cleaning the heat exchanger is economically attractive.
Integrity inspection of plate and tubular heat exchanger can be tested in situ by the conductivity or helium gas methods. These methods confirm the integrity of the plates or tubes to prevent any cross contamination and the condition of the gaskets.
6. Essay on Heat Pumps:
A heat pump is a device that transfers heat energy from a heat source to a heat sink against a temperature gradient. Heat pumps are designed to move thermal energy opposite the direction of spontaneous heat flow. A heat pump uses some amount of external high-grade energy to accomplish the desired transfer of thermal energy from heat source to heat sink.
While compressor-driven air conditioners and freezers are familiar examples of heat pumps, the term ‘heat pump’ is more general and applies to HVAC devices used for space heating or space cooling. When a heat pump is used for heating, it employs the same basic refrigeration-type cycle used by an air conditioner or a refrigerator but in the opposite direction, releasing heat into the conditioned-space rather than the surrounding environment. In this use, heat pumps generally draw heat from the cooler external air or from the ground.
In heating and air conditioning (HVAC) applications, the term heat pump usually refers to easily reversible vapor-compression refrigeration devices optimized for high efficiency in both directions of thermal energy transfer.
Heat spontaneously flows from warmer places to colder spaces. A heat pump can absorb heat from a cold space and release it to a warmer one, and vice-versa. ‘Heat’ is not conserved in this process, which requires some amount of external high grade (i.e., low-entropy) energy to be expended.
Heat pumps are used to provide heating because less high-grade energy is required for their operation than appears in the released heat. Most of the energy for heating comes from the external environment, and only a fraction comes from electricity (or some other high-grade energy source required running a compressor).
In electrically powered heat pumps, the heat transferred can be three or four times larger than the electrical power consumed, giving the system a Coefficient of Performance (COP) of 3 or 4, as opposed to a COP of 1 of a conventional electrical resistance heater, in which all heat is produced from input electrical energy.
Heat pumps use a refrigerant as an intermediate fluid to absorb heat where it vaporizes, in the evaporator, and then to release heat where the refrigerant condenses, in the condenser. The refrigerant flows through insulated pipes between the evaporator and the condenser, allowing for efficient thermal energy transfer at relatively long distances.
Reversible heat pumps:
Reversible heat pumps work in either thermal direction to provide heating or cooling to the internal space. They employ a reversing valve to reverse the flow of refrigerant from the compressor through the condenser and evaporation coils.
i. In heating mode, the outdoor coil is an evaporator, while the indoor is a condenser. The refrigerant flowing from the evaporator (outdoor coil) carries the thermal energy from outside air (or soil) indoors, after the fluid’s temperature has been augmented by compressing it. The indoor coil then transfers thermal energy (including energy from the compression) to the indoor air, which is then moved around the inside of the building by an air handler.
Alternatively, thermal energy is transferred to water, which is then used to heat the building via radiators or under-floor heating. The heated water may also be used for domestic hot water consumption. The refrigerant is then allowed to expand, cool, and absorb heat to reheat to the outdoor temperature in the outside evaporator, and the cycle repeats.
This is a standard refrigeration cycle, save that the ‘cold’ side of the refrigerator (the evaporator coil) is positioned so it is outdoors where the environment is colder.
ii. In cooling mode the cycle is similar, but the outdoor coil is now the condenser and the indoor coil (which reaches a lower temperature) is the evaporator. This is the familiar mode in which air conditioners operate.
iii. In inverse mode the cycle operates with evaporator in the lower pressure zone and condenser in the higher pressure zone.
Operating principle of heat pumps:
Heat pumps use waste heat that would otherwise be rejected to the environment; they increase air temperature to a more effective level. Heat pumps can deliver heat for less money than the cost of fuel.
Heat pumps operate on a thermodynamic principle known as the Carnot Cycle. Degrading high-grade thermal energy into lower-grade thermal energy creates shaft work, or power, in the Rankine Cycle. In a steam turbine, this is accomplished by supplying high-pressure steam and exhausting lower-pressure steam.
In contrast, mechanical heat pumps operate in the opposite manner. They convert lower temperature waste heat into useful, higher-temperature heat, while consuming shaft work.
The work required to drive a heat pump depends on how much the temperature of the waste heat is increased; in contrast, a steam turbine produces increasing amounts of work as the pressure range over which it operates increases.
Heat pumps consume energy to increase the temperature of waste heat and ultimately reduce the use of purchased steam or fuel. Consequently, the economic value of purchasing a heat pump depends on the relative costs of the energy types that are consumed and saved.
Several types of heat pumps exist, but all heat pumps perform the same three basic functions:
i. Receipt of heat from the waste-heat source.
i. Increase of the waste-heat temperature.
iii. Delivery of the useful heat at the elevated temperature.
Mechanical heat pumps exploit the physical properties of a volatile evaporating and condensing fluid known as a refrigerant. The heat pump compresses the refrigerant to make it hotter on the side to be warmed, and releases the pressure at the side where heat is absorbed.
Vapour-compression refrigeration cycle of heat pump as shown in the figure has following elements:
(1) Condenser,
(2) Expansion valve,
(3) Evaporator, and
(4) Compressor.
The working fluid, in its gaseous state, is pressurized and circulated through the system by a compressor. On the discharge side of the compressor, the now hot and highly pressurized vapour is cooled in a heat exchanger, called a condenser, until it condenses into a high pressure, moderate temperature liquid.
The condensed refrigerant then passes through a pressure-lowering device also called a metering device. This may be an expansion valve, capillary tube, or possibly a work-extracting device such as a turbine. The low pressure liquid refrigerant then enters another heat exchanger, the evaporator, in which the fluid absorbs heat and boils.
The refrigerant then returns to the compressor and the cycle is repeated. It is essential that the refrigerant reaches a sufficiently high temperature, when compressed, to release heat through the “hot” heat exchanger (the condenser).
Similarly, the fluid must reach a sufficiently low temperature when allowed to expand, or else heat cannot flow from the ambient cold region into the fluid in the cold heat exchanger (the evaporator). In particular, the pressure difference must be great enough for the fluid to condense at the hot side and still evaporate in the lower pressure region at the cold side.
The greater the temperature difference, the greater the required pressure difference, and consequently the more energy needed to compress the fluid. Thus, as with all heat pumps, the Coefficient of Performance (amount of thermal energy moved per unit of input work required) decreases with increasing temperature difference.
Insulation is used to reduce the work and energy required to achieve a low enough temperature in the space to be cooled.
Heat transport:
Heat is typically transported through engineered heating or cooling systems by using a flowing gas or liquid. Air is sometimes used, but quickly becomes impractical under many circumstances because it requires large ducts to transfer relatively small amounts of heat.
In systems using refrigerant, this working fluid can also be used to transport heat a considerable distance, though this can become impractical because of increased risk of expensive refrigerant leakage. When large amounts of heat are to be transported, water is typically used, often supplemented with antifreeze, corrosion inhibitors, and other additives.
Heat sources/sinks:
A common source or sink for heat in smaller installations is the outside air, as used by an air-source heat pump. A fan is needed to improve heat exchange efficiency.
Larger installations handling more heat, or in tight physical spaces, often use water-source heat pumps. The heat is sourced or rejected in water flow, which can carry much larger amounts of heat through a given pipe or duct cross-section than air flow can carry.
The water may be heated at a remote location by boilers, solar energy, or other means. Alternatively when needed, the water may be cooled by using a cooling tower, or discharged into a large body of water, such as a lake or stream.
Geothermal heat pumps or ground-source heat pumps use shallow underground heat exchangers as a heat source or sink, and water as the heat transport medium. This is possible because below ground level, the temperature is relatively constant across the seasons, and the earth can provide or absorb a large amount of heat.
Ground source heat pumps work in the same way as air-source heat pumps, but exchange heat with the ground via water pumped through pipes in the ground. Ground source heat pumps are simpler and more reliable than air source heat pumps – they do not need fan systems or defrosting systems and can be housed inside – but the need for a ground heat exchanger requires a higher initial capital cost in exchange for lower annual running costs as well-designed ground source heat pump systems enjoy a more efficient operation.
Heat pump installations may be installed alongside an auxiliary conventional heat source such as electrical resistance heaters, or oil or gas combustion. The auxiliary source is installed to meet peak heating loads, or to provide a back-up system.
Efficiency of heat pump & energy saving:
When comparing the performance of heat pumps, it is best to avoid the word “efficiency” which has a very specific thermodynamic definition. The term coefficient of performance (COP) is used to describe the ratio of useful heat movement per work input.
Most vapour-compression heat pumps use electrically powered motors for their work input. However, in many vehicle applications, mechanical energy from an internal combustion engine provides the needed work.
When used for heating a building on a mild day, for example 10 °C, a typical air-source heat pump (ASHP) has a COP of 3 to 4, whereas an electrical resistance heater has a COP of 1.0. That is, one joule of electrical energy will cause a resistance heater to produce only one joule of useful heat, while under ideal conditions, one joule of electrical energy can cause a heat pump to move much more than one joule of heat from a cooler place to a warmer place.
Note that an air source heat pump is more efficient in hotter climates than cooler ones, so when the weather is much warmer the unit will perform with a higher COP (as it has less work to do). Conversely in extreme cold weather the COP approaches 1. Thus when there is a wide temperature differential between the hot and cold reservoirs, the COP is lower (worse).
Under the right circumstances, a heat pump can reduce energy costs and provide an attractive cost-reduction project, particularly when:
i. The heat output is at a temperature where it can replace purchased energy such as boiler steam or gas firing.
ii. The cost of energy to operate the heat pump is less than the value of the energy saved.
iii. The net operating cost savings (reduction in purchased energy minus operating cost) is sufficient to pay back the capital investment in an acceptable time period.
7. Essay on Turbines:
Turbines have been the world’s energy workhorses for generations and their history can be traced back to primitive devices such as waterwheels used about 2,000 years ago and windmills over 1,000 years old. Today, turbines not only power aircraft and vehicles of all sorts, they are the heart of almost all of the world’s electric generating systems. Power generation from fossil energy relies upon gas turbines and steam turbines and in combination as combined-cycle units.
A turbine is a machine that extracts energy from the flow of a fluid. The fluid could be air (in the case of wind turbines), steam (as in coal -powered turbines and turbines in nuclear plants), water (as in hydraulic turbines), or combustion products (as in natural gas turbines or aerospace jet engines).
Turbines are used in energy conversion system and about 97% of the total electricity generation in the world is achieved using different types of turbines. The steam turbines have major share among various types of turbines used in energy conversion systems. However, most of the energy is lost in conversion process.
Because conversion losses are so large, relatively small reductions in losses (or improvements in efficiency) can yield substantial increases in the energy output. In other words, for every one per cent improvement in efficiency, the available energy is increased by 2.5%.
Since turbines make up nearly all of the world’s electric generation capacity, incremental turbine efficiency improvements can have extremely large impacts on the bottom line of energy production and fuel consumption. The modern power-generating turbine has been undergoing efficiency improvements since its first production.
Combined cycle plants, in which exhaust heat from a gas turbine is used to make steam to power a separate turbine also producing electricity, are the most efficient and have been in use since 1968. Many of today’s operating power-generating turbines were designed based on technology that is 10 to 50 years old.
The latest technologies would make them run cleaner and more efficiently. A typical large older gas turbine used in combined cycle applications produces 100 to 250 megawatts of power at 48-52% thermal efficiency; new combined-cycle power plants (designed within the last ten years) can see efficiencies as high as 59%. When efficiency is increased, less fuel is required for the same amount of energy produced and less CO2 emissions are generated for the same amount of energy produced because less fuel is used.
Latest trends in turbine technologies:
With the help of advanced technological development in turbine systems, modern gas turbines could operate at temperatures in excess of 2600 degrees F (300 degrees hotter than conventional turbines) and achieve efficiencies above 60 percent. At the same time, new combustion techniques were developed to limit the formation of nitrogen oxide (NOx) emissions (the principal air pollutant released by gas turbines).
As a result, high-efficiency natural gas turbines continue to be among the cleanest ways to generate electricity from fossil fuels. Further, if the waste heat is captured from these systems for heating or industrial purposes, the overall energy cycle efficiency could approach 80 percent.
The use of gases produced from coal as gas turbine fuel offers an attractive means for efficiently generating electric power from our fossil fuel resource. New turbine technologies enable advanced turbines to operate cleanly and efficiently when fuelled with coal derived synthesis gas and hydrogen fuels.
Developing this turbine technology is critical to the creation of near-zero emission power generation technologies. This will assist with the deployment of future plants that may couple production of hydrogen and electricity from coal with sequestration of the carbon dioxide that is produced.
8. Essay on Electrical Drives:
Electrical drives consume about 60% of all electricity used in industry. There is great potential for improved efficiencies in such drives.
Electrical drive systems consist of the following units:
i. The electric motor, which converts electric power into mechanical power,
ii. A frequency converter, which converts the electrical power of the mains in a controlled form (electronic speed control),
iii. And the gearbox, which adjusts the mechanical power of the motor to the working point of the driven machine (reducing speed and increasing torque).
Consider the lifetime of an electric motor, the costs associated with the consumption of electricity account for up to 96% of the total cost. Therefore, when purchasing a motor, it is important to bear in mind its expected electricity consumption as this is a considerably greater factor than the initial purchase cost.
Great savings potential in electrical drive systems lies in the use of energy-saving motors. These energy-optimised motors convert electrical into mechanical energy with the fewest possible losses whilst maintaining the required technical properties.
In industry, three-phase asynchronous motors are widely used as standard drives. The vast majority of three-phase motors used today are asynchronous machines because they are good value for money and require very little maintenance.
In terms of energy efficiency, however, they cannot compete with other types of motor. However, strong efforts have been made in the past years to reduce the energy losses of such asynchronous machines substantially.
Higher efficiency levels may be obtained when using special motor types such as synchronous motors or EC motors:
i. Synchronous motors have a very high electrical efficiency, even during partial load operation. Precise regulation of frequency converters is possible.
ii. Electronically commutated (EC) motors, also known as brushless DC motors (BLDCs), supplement the positive attributes of synchronous machines by being able to adjust to their load. They are highly efficient, even when working with partial loads, have a high power spectrum and are easily regulated.
At first glance, simply replacing an old motor with an energy efficient motor is the simplest way to improve energy efficiency. However, when assessing the economic efficiency of an electrical drive, it is not primarily the motor that determines the optimal efficiency but rather the way in which the motor or machine speed is controlled.
The savings potential of electronic speed control is four to five times greater than that of energy efficient motors. Electronic speed control can save between 20% and 70% of the energy costs of conventional mechanical methods such as throttle valves. Taking life cycle costs into consideration, investments in energy saving can often be amortised within just a few months.
i. Optimise operating voltage level of motor for lightly loaded motors
ii. Replace eddy current controls with variable frequency drives for varying speed driven equipment.
iii. Provide interlock for electric motor to avoid idle running.
iv. Replace motor generating sets with thyristor drives.
v. Avoid frequent rewinding of motors. Greater the number of rewind, lesser the efficiency.
vi. Carry out preventive maintenance and condition monitoring schedule regularly.
Advantages of energy efficient motors:
i. Reduced operating costs.
ii. Less heat losses.
iii. Extended winding lifespan.
iv. Extended lubricating grease service life.
v. Lower noise levels than other motors.
vi. Reduced energy costs. The higher purchase price investment pays off.
vii. Reduce emission of CO2 and NOx greenhouse gasses from power stations for positive environmental effect.
9. Essay on Pumps:
Pump systems presently account for a good 25% of the industrial electricity consumed worldwide. It is believed that approximately 40% of this energy could be saved. Centrifugal and displacement pumps occupy a large market share, with centrifugal pumps accounting for 73%. Centrifugal pumps, in particular, represent great potential for energy savings because approximately 75% of these pumps are oversized, frequently by more than 20%.
Pumps in industrial process consume about 30 % of the total electricity consumption. In addition to comprehensive system optimisation, the use of efficient high-tech products and highly developed controls are the two main ways to increase energy efficiency in pumps.
The following steps can significantly reduce the energy consumption in pumps:
i. Replace oversized pumps with smaller pumps that have highly efficient motors;
ii. Use highly efficient pumps;
iii. Better maintenance strategy
iv. Use frequency converters for variable-speed operations,
v. Equip pumps with proportional control,
vi. Optimise downstream heat exchangers.
vii. Select a pump of the right capacity in accordance with the irrigation requirement. Improper selection of pump can lead to large wastage of energy. A pump with 85% efficiency at rated flow may have only 65% efficiency at half the flow.
viii. Matching of the motor with the appropriate-sized pump.
ix. Using throttling valves instead of variable speed drives to change flow of fluids is a wasteful practice. Throttling can cause wastage of power to the tune of 50% to 60%.
x. It is advisable to use a number of pumps in series and parallel to cope with variations in operating conditions by switching on or off pumps rather than running one large pump with partial load.
xi. Void valves in the pipe line throttle wastes energy. A positive displacement pump with variable speed drive is recommended.
xii. Proper installation of the pump system, including shaft alignment, coupling of motor and pump is a must. Drive transmission between pumps and motors is very important. Loose belts can cause energy loss up to 15% to 20%.
xiii. Use efficient transmission system. Maintain right tension and alignment of transmission belts.
xiv. Use of modern synthetic flat belts in place of conventional V belts can save five per cent to 10 per cent of energy.
xv. Properly organised maintenance is very important. Efficiency of worn out pumps can drop by 10% to 15%, unless maintained properly.
xvi. Use low friction rigid PVC pipes and foot valves.
xvii. Avoid use of unnecessary bends and throttle valves.
xviii. Use bends in place of elbows.
xix. The suction depth of six meters is recommended as optimum for centrifugal pumps. The delivery line should be kept at the minimum required height in keeping with requirements.
xx. Periodically check pump system and carry out corrective measures such as lubrication, alignment, tuning of engines and replacement of worn-out parts.
xxi. Over irrigation can harm the crops and waste vital water resource. Irrigate according to established norms for different crop.
xxii. Use drip irrigation for specific crops like vegetable, fruits, tobacco, etc. Drip systems can conserve up to 80% water and reduce pumping energy requirement.
10. Essay on Cooling Towers:
i. Replacement of inefficient aluminium or fabricated steel fans with moulded FRP fans with aerofoil designs results in electricity savings in the range of 15% to 40% in cooling towers.
ii. A study on a typical 20 feet diameter fan revealed that replacing the wooden blade drift eliminators with newly developed cellular PVC drift eliminators reduces the drift losses from 0.01% to 0.02% with a fan power energy saving of 10% in cooling towers.
iii. Install automatic ON-OFF switches on cooling tower fans and save up to 40% on electricity costs.
iv. Use of PVC fills in place of wooden bars results in a saving in pumping power of up to 20%.
11. Essay on Fans and Blowers:
Fans and blowers are the main equipment in process industry. The performance of the fans and blowers is governed by their system characteristics. Characteristics like system resistance influences the performance of fans and blowers.
Before highlighting the energy conservation opportunities in fans and blowers, let us briefly study the working principle and types of fans and blowers.
Most manufacturing plants use fans and blowers for ventilation and for industrial processes that need an air flow. Fan systems are essential to keep manufacturing processes working and consist of a fan, an electric motor, a drive system, ducts or piping, flow control devices and air conditioning equipment (filters, cooling coils, heat exchangers, etc.).
Fans, blowers and compressors are differentiated by the method used to move the air and by the system pressure they must operate against.
The American Society of Mechanical Engineers (ASME) uses the specific ratio, which is the ratio of the discharge pressure over the suction pressure, to define fans, blowers and compressors (see Table below):
System resistance affects the performance of fans and blowers. The term ‘system resistance’ is used when referring to the static pressure. The system resistance is the sum of static pressure losses in the system. The system resistance is a function of the configuration of ducts, pickups, elbows and the pressure drops across equipment, for example bag filter or cyclone.
The system resistance varies with the square of the volume of air flowing through the system. For a given volume of air, the fan in a system with narrow ducts and multiple short radius elbows is going to have to work harder to overcome a greater system resistance than it would in a system with larger ducts and a minimum number of long radius turns.
Long narrow ducts with many bends and twists will require more energy to pull the air through them. Consequently, for a given fan speed, the fan will be able to pull less air through this system than through a short system with no elbows. Thus, the system resistance increases substantially as the volume of air flowing through the system increases; square of air flow.
Conversely, resistance decreases as flow decreases. To determine what volume the fan will produce, it is therefore necessary to know the system resistance characteristics.
Apart from system resistance, performance of the fans is also depicted by fan characteristics. Fan characteristics can be represented in form of fan curve(s). The fan curve is a performance curve for the particular fan under a specific set of conditions.
The fan curve is a graphical representation of a number of inter-related parameters. Typically a curve will be developed for a given set of conditions usually including: fan volume, system static pressure, fan speed, and brake horsepower required to drive the fan under the stated conditions.
Fan laws:
The fans operate under a predictable set of laws concerning speed, power and pressure. A change in speed (revolutions per minute or RPM) of any fan will predictably change the pressure rise and power necessary to operate it at the new RPM. This is shown in Figure 5.13.
Type of fans and blowers:
Types of fans:
There exist two main fan types. Centrifugal fans used a rotating impeller to move the air stream. Axial fans move the air stream along the axis of the fan.
(i) Centrifugal fans:
Centrifugal fans (Figure 5.14) increase the speed of an air stream with a rotating impeller. The speed increases as the reaches the ends of the blades and is then converted to pressure. These fans are able to produce high pressures, which makes them suitable for harsh operating conditions, such as systems with high temperatures, moist or dirty air streams and material handling. Centrifugal fans are categorized by their blade shapes as summarized in Table 5.3.
(ii) Axial fans:
Axial fans (Figure 5.15a) move an air stream along the axis of the fan. The way these fans work can be compared to a propeller on an airplane: the fan blades generate an aerodynamic lift that pressurizes the air. They are popular with industry because they are inexpensive, compact and light. The main types of axial flow fans (propeller, tube-axial and vane-axial) are summarized in Table 5.4.
Types of blowers:
Blowers can achieve much higher pressures than fans, as high as 1.20 kg/cm. They are also used to produce negative pressures for industrial vacuum systems.
The centrifugal blower and the positive displacement blower are two main types of blowers, which are described below.
(i) Centrifugal blowers:
Centrifugal blowers look more like centrifugal pumps than fans. The impeller is typically gear-driven and rotates as fast as 15,000 rpm. In multi-stage blowers, air is accelerated as it passes through each impeller. In single-stage blower, air does not take many turns, and hence it is more efficient.
Centrifugal blowers typically operate against pressures of 0.35 to 0.70 kg/cm, but can achieve higher pressures. One characteristic is that airflow tends to drop drastically as system pressure increases, which can be a disadvantage in material conveying systems that depend on a steady air volume. Because of this, they are most often used in applications that are not prone to clogging.
(ii) Positive-displacement blowers:
Positive displacement blowers have rotors, which ‘trap’ air and push it through housing. These blowers provide a constant volume of air even if the system pressure varies. They are especially suitable for applications prone to clogging, since they can produce enough pressure (typically up to 1.25 kg/cm) to blow clogged materials free. They turn much slower than centrifugal blowers (e.g., 3,600 rpm) and are often belt driven to facilitate speed changes.
Assessment of fans and blowers:
Fan efficiency/performance:
Fan efficiency is the ratio between the power transferred to the air stream and the power delivered by the motor to the fan. The power of the airflow is the product of the pressure and the flow, corrected for unit consistency.
Another term for efficiency that is often used with fans is static efficiency, which uses static pressure instead of total pressure in estimating the efficiency. When evaluating fan performance, it is important to know which efficiency term is being used.
The fan efficiency depends on the type of fan and impeller. As the flow rate increases, the efficiency increases to certain height (“peak efficiency”) and then decreases with further increasing flow rate (see Figure 5.17). The peak efficiency ranges for different types of centrifugal and axial fans are given in Table 5.5.
Fan performance is typically estimated by using a graph that shows the different pressures developed by the fan and the corresponding required power. It is essential in designing, sourcing and operating a fan system and is the key to optimum fan selection.
Energy efficiency opportunities in fans and blowers:
Most important energy efficiency opportunities for fans and blowers are as follows:
(i) Choose the right fan:
Important considerations when selecting a fan are (US DOE, 1989):
i. Noise
ii. Rotational speed
iii. Air stream characteristics
iv. Temperature range
v. Variations in operating conditions
vi. Space constraints and system layout
vii. Purchase costs, operating costs (determined by efficiency and maintenance) and operating life.
But as a general rule it is important to know that to effectively improve the performance of fan systems, designers and operators must understand how other system components function as well. The ‘systems approach’ requires knowing the interaction between fans, the equipment that supports fan operation and the components that are served by fans. The use of a ‘systems approach’ in the fan selection process will result in a quieter, more efficient and more reliable system.
A common problem is that companies purchase oversized fans for their service requirements. They will not operate at their Best Efficiency Point (BEP) and in extreme cases these fans may operate in an unstable manner because of the point of operation on the fan airflow pressure curve.
Oversized fans generate excess flow energy, resulting in high airflow noise and increased stress on the fan and the system. Consequently, oversized fans not only cost more to purchase and to operate, they create avoidable system performance problems. Possible solutions include, amongst other replacing the fan, replacing the motor, or introducing a variable speed drive motor.
(ii) Reduce the system resistance:
The system resistance has a major role in determining the performance and efficiency of a fan. The system resistance also changes depending on the process. For example, the formation of the coatings/ erosion of the lining in the ducts, changes the system resistance marginally.
In some cases, the change of equipment, duct modifications, drastically shift the operating point, resulting in lower efficiency. In such cases, to maintain the efficiency as before, the fan has to be changed. Hence, the system resistance has to be periodically checked, more so when modifications are introduced and action taken accordingly, for efficient operation of the fan.
(iii) Operate close to BEP:
It is earlier described that the fan efficiency increases as the flow increases to certain point and thereafter it decreases. The point at which maximum efficiency is obtained is called the peak efficiency or ‘Best Efficiency Point’ (BEP). Normally it is closer to the rated capacity of the fan at a particular designed speed and system resistance. Deviation from the BEP will result in increased loss and inefficiency.
(iv) Maintain fans regularly:
Regular maintenance of fans is important to maintain their performance levels.
Maintenance activities include:
i. Periodic inspection of all system components.
ii. Bearing lubrication and replacement.
iii. Belt tightening and replacement.
iv. Motor repair or replacement.
v. Fan cleaning.
(v) Control the fan airflow:
Normally, an installed fan operates at a constant speed. But some situations may require a speed change, for example more airflow may be needed from the fan when a new run of duct is added, or less airflow may be needed if the fan is oversized. There are several ways to reduce or control the airflow of fans.
Summary of energy conservation options in fans and blowers:
i. Use smooth, well-rounded air inlet cones for fan air intake.
ii. Avoid poor flow distribution at the fan inlet.
iii. Minimize fan inlet and outlet obstructions.
iv. Clean screens, filters and fan blades regularly.
v. Minimize fan speed.
vi. Use low slip or flat belts for power transmission.
vii. Check belt tension regularly.
viii. Eliminate variable pitch pulleys.
ix. Use variable speed drives for large variable fan loads.
x. Use energy-efficient motors for continuous or near continuous operation.
xi. Eliminate leaks in duct works.
xii. Minimize bends in duct works.
xiii. Turn fans and blowers off when not needed.
xiv. Reduce the fan speed by pulley diameter modifications in-case of oversized motors.
xv. Adopt inlet guide vanes in place of discharge damper control.
xvi. Change metallic / Glass Reinforced Plastic (GRP) impeller by more energy efficient hollow FRP impeller with aerofoil design.
xvii. Try to operate the fan near its Best Operating Point (BEP).
xviii. Reduce transmission losses by using energy efficient flat belts or cogged raw-edged V-belts instead of conventional V-belt systems.
xix. Minimizing system resistance and pressure drops by improving the duct system.
xx. Ensure proper alignment between drive and driven system.
xxi. Ensure proper power supply quality to the motor drive.
xxii. Regularly check for vibration trend to predict any incipient failures like bearing damage, misalignments, unbalance, foundation looseness etc.