Measurement of Flow of Fluids

In a milk product manufacturing process it is often required to measure the proportion of materials introduced in to a process and the amount of materials produced. For this purpose it is essential to measure the flow rate and flow quantities of fluids.Also the flow measurement is required for the purpose of cost accounting for the services like steam and water.

i. Types of meters

The flow of fluids in closed pipes can be measured by many methods, each working on separate principle of operation. These are:

a) Head meter: These meters operate by measuring the pressure differential across a suitable restriction to flow. For example, orifice meters, the venturi tube, weirs etc. These instruments found almost no application in milk processing industries.

b) Area meter: They operate on the principle of variation in area of a flow stream.For example, rotameter. These are very commonly employed in dairy industry.

c) Quantity meter: They measure the time integral of flow rate. That is, the

quantity of fluid passed at a given point. For example domestic water meter.In this section, the description of flow meter shall be restricted to the brief study of construction and working of a rotameter.

ii. Rotameter

Principle of operation: Rotameter is a type of area flow meter. It operates on the principle that the variation in area of the flow stream required to produce a constant pressure differential at a restriction of flow is proportional to the flow rate.

Construction: It consists of a tapered metering tube and a float which is free to move up or down within the tube. The metering tube is
mounted vertically with the smaller end at the bottom. The fluid whose flow rate is to be measured enters the tube at the bottom, passes around the float and moves out of the tube at the top.

When there is no flow through the rotameter, the float rests at the bottom of the tube. At the bottom of the tube the diameter of the float is approximately same as that of the tube. Thus the area of float nearly equals the area of the tube and there is a very small annular opening between the float and the tube. When the fluid enters the tube, the pressure drop across the float increases and it raises the float.This upward movement of the float increases the area between the float and the tube until the upward hydraulic forces acting on the float are balanced by its weight. The metering float now floats in the fluid stream.The float moves up or down in the tube in proportion to the fluid flow rate and the annular area between the float and the tube. It reaches a stable position in the tube .when the forces are in equilibrium. Every float position corresponds to one particular flow rate for a fluid of a given density and viscosity. A calibration scale is provided on the tube and flow rate can be determined by direct observation of the position of the float in the metering tube.

Materials: The tapered tube is made up of Pyrex glass. Metal tapering tubes are used in applications where glass could not be used. In case of metal tubes float position is determined indirectly. This is done by magnetic or electrical techniques.The use of indirect float sensors is better than direct visual indication. The float is made up of dense material such as silver or tantalum. The shape of the float is of bob-shape or inverted cone shape so to provide constant viscous drag at all flow rates.

Installation: For proper functioning and accurate results the float must centreitself in the fluid stream. To achieve this, the tube must be installed vertically. It must be plumb to within about two geometrical degrees.

Measuring Pressure of Fluids

The measurement of pressure and vacuum has always been important in dairy processing industry. Such measurements are generally made continuously with standard industrial appliances. Before we study these pressure measuring devices,let us first understand the concept of pressure and vacuum.

i. Concept of Pressure and Vacuum

The pressure exerted by fluid is given in terms of force per unit area; the force exerted in a direction perpendicular to the surface of unit area. Following terms are generally associated with pressure and its measurement.

Atmospheric Pressure (P at ): This is the pressure exerted by the envelope of air surrounding the earth’s surface. Atmospheric pressure is usually determined by a mercury-column barometer. Atmospheric pressure varies with altitude. At sea level,the value of atmospheric pressure is close to 1.013 Kg/cm 2 or 1.01325 bar or 760 mm of mercury column.

Absolute Pressure (P abs ): Pressure has been defined as the force per unit area due to interaction of fluid particles among themselves. Zero pressure intensity will occur when molecular momentum is zero. Such a situation may occur only when there is perfect vacuum, that is, a vanishingly small population of gas molecules or of molecular velocity. Pressure intensity measured from this state of vacuum or zero pressure is called absolute pressure.

Gauge Pressure (P g ) and gauge vacuum (P vac ): Instruments and gauges used to measure fluid pressure generally measure the difference between the unknown pressure ‘P’ and the existing atmospheric pressure P at . When the unknown pressure is more than the atmospheric pressure, the pressure recorded by the instrument is called gauge pressure. A pressure reading below the atmospheric pressure is known as vacuum or negative pressure. Actual absolute pressure is then the algebraic sum of the gauge indication and the atmospheric pressure.

P abs = P at + P g

P abs = P at - P vac

Units of Pressure: The unit used for expressing pressure is ‘Atmosphere’ abbreviated as ‘atm’. One ‘atm’ is simply the pressure in Kg/cm 2 exerted by the atmosphere. Pressure can be expressed in multiples of ‘atm’.The unit of pressure in M.K.S. system is Kg/cm 2 , in S.I. units N/m 2 and in F.P.S.units is pound per sq. inch (psi). One atm is equal to 1.013 Kg/cm 2 It can also be expressed as the height of mercury column. One atm is equal to 76 cm of Hg column at 0°C.

ii. Pressure Gauge

Bourdon tube type pressure gauge is the most common for industrial use. It employs a Bourdon tube element for direct indication of pressure. The bourdon tube element can be of ‘C’ type, spiral type or helical type. Due to its simplicity and ruggedness the ‘C’ type element is commonly employed.

Principle of operation: One end of the ‘C’ type bourdon tube element is sealed at its tip, while the other end is connected to the process pressure which is to be measured. Because of its ‘C’ type shape, there is a difference between inside and outside radii of the tube and the bourdon tube presents different areas to pressure.This causes the tube to tend to straighten up when the pressure is applied to it and results in a motion of the sealed tip end of the tube. The extent of movement of the tip of tube depends upon the amount of applied pressure. The movement of sealed end of the bourdon tube is thus an indicator of the applied pressure or vacuum.

Construction: Construction of a ‘C’ bourdon tube as used in a direct indicating gauge. The ‘C’ tube usually has an arc of 250°. The process pressure is connected to the fixed socket end of the tube while the tip end is sealed.As the pressure is applied there is a movement of the sealed end of the tube. This tip motion is non-linear because less motion results from each increment of additional pressure. This non-linear motion has to be converted into linear rotational pointer response. This is done mechanically by means of a geared sector and pinion movement. The tip motion is transferred to the tail of the movement sector by the connector link. The sector tail is called the ‘traveling angle’. This angle changes with the tip movement in a non-linear fashion and so the movement of the pinion and therefore pointer is linear. This type of pressure gauge is used in all industries and may be obtained in sizes from 5cm diameter up to 35 cm diameter and in many different indicating styles.

Material: The metallic materials used for construction of bourdon tube include brass, bronze, phosphor bronze, beryllium-copper alloy, alloy steel or stainless steel.The non-metallic materials are leather, neoprene and rubber.

Installation: In nearly all pressure gauges, the fluid in which pressure is measured is conducted to the inside of the pressure measuring element and is in direct contact with the element. This creates a problem of handling high temperature, corrosive, sludgy or semisolid materials. Some pressure- gauge elements can be protected by copper-plated, nickel-plated or tinned plated surfaces. But this is always possible especially on the inside of the bourdon tubes. Therefore some other method of excluding the measured fluid must be employed.

One of the effective methods of protecting a pressure gauge element is siphon arrangement. A single coil siphon arrangement  This is very effective in protecting the pressure-gauge element from the high temperature of steam. The brass coil traps condensate steam and limits the temperature rise in the gauge. A siphon is necessary on all steam pressure gauges.Another arrangement is diaphragm seal as shown in Fig. 12.6. The unit is usually made up of bronze with a neoprene or thin metal diaphragm. The system is solidly filled with a liquid such as glycerin or oil. The diaphragm is quite flexible, so that the pressure on both sides of the diaphragm is equal. This way the problem of corrosive material can be tackled, as such material would not be in contact with the tube element. The line leading to pressure gauge is always filled with clean oil.

iii. Manometers

One of the oldest means of measurement of pressure is liquid column manometer.It is the simplest, most direct and most accurate of all pressure measuring means.This is a fundamental instrument for detecting the pressure and is used for calibration of other sensors. There are no moving parts, no friction or inertia involved in the measurement and therefore, its accuracy is limited only by the scale visibility.

Principle of operation: The pressure exerted by a column of liquid of height ‘h’ and density ‘r’ at the base of the cylinder containing it, is equal to ‘hrg’, where ‘g’ is the acceleration due to gravity. A manometer is shown in Fig. 12.5 below.

Tube Manometer It has two vertical tubes known as legs, which are connected at the base. The assembly is partly filled with manometer liquid, which may be water, oil or mercury.When the legs of the manometer are vertical, then:

The pressure at the bottom of the right leg = P 1 + h 1 rg

The pressure at the bottom of the left leg = P 2 + h 2 rg

When the fluid is under static balance condition, then

P 1 + h 1 rg = P2 + h 2 rg

Therefore P 1 -P 2 = (h2-h 1 ) rg

A manometer can thus be used for measurement of pressure differential, that is,the difference in pressures P 1 and P 2 . Further, if one of the legs of the manometer is connected to the vessel in which the fluid pressure or vacuum is to be measured and other leg is kept open to atmosphere then this manometer would be able to read the pressure difference between the vessel and the atmosphere.

Construction: Liquid manometers are the simplest differential pressure or vacuum detectors. A simple U-tube Manometer  It has two vertical tubes known as legs which are connected at the base through a metal fitting. The assembly is partly filled with manometer liquid, which may be water, oil or mercury.An easier-to-read scale can be is attached to the manometer. In industrial installations,the use of glass tube manometers is limited to locations where tube breakage will not create hazardous conditions for the operator.

Materials: The performance of any manometer is largely a function of the indicating fluid selected. Amongst various manometer liquids, most commonly employed are water, oil or mercury. The filling fluid has to be chemically inert and compatible with the process media and produce a clear, visible interface. The fluid should not coat the glass tube and should not be corrosive to standard material such as copper,aluminum and steel. The fluid should not freeze due to low ambient temperature be capable to maintain its density unaffected by temperature.

Installation: Glass manometer is usually mounted on the equipment or pressure vessel at that place where the operator could easily read the manometer scale. In industrial installations, the use of glass tube manometers is limited to locations where tube breakage will not create hazardous conditions for the operator. At critical locations the gauge glass tube is provided with a thick guard glass cover to protect against flying glass pieces in case of accidental breakage of tube.

Measuring Temperature of Fluids

Temperature is the most important variable in milk processing. It is the thermal state of the product that determines whether the physical conditions of the manufactured product are correct or whether the desired chemical reaction will take place or not.

i. Temperature scales

A temperature scale represents the temperature of a body quantitatively. Designating two thermal equilibrium points by numbers forms the temperature scale. The equilibrium point of ice and water at standard pressure is called ice-point. The equilibrium point between water and steam at standard pressure is called steam point. There are a number of temperature scales each assigning a different unit values to these points.

The Fahrenheit Scale (abbreviated as °F) – This scale assigns 32° F to the ice point and 212 F° to the steam point.The Centigrade Scale (abbreviated as °C) - Also known as Celsius Scale. This scale assigns 0° C to the ice point and 100°C to the steam point. This scale is commonly used in scientific calculations.

The Kelvin scale (abbreviated as °K) - Also known as Absolute Celsius Scale.This scale assigns 273°K to the ice point and 373°K to the steam point. This scale is widely used in technical calculations.

The interrelations between these scales can be worked out as below:

The equation relating Kelvin and Celsius scale is:

°K = °C + 273

The relationship between Celsius Temperature and Fahrenheit Temperature is:

°C = (5/9) (F- 32)

ii. Indicating Thermometers

Glass Thermometer: The mercury in glass thermometer is one of the simplest temperature measuring devices.

Principle of operation: It utilizes the volumetric expansion of mercury with temperature as a mean of indicating temperature.

Construction: Construction of a thermometer

It has a bulb formed by a glass envelope. This bulb contains mercury. Bulb is attached to the stem, which contains a fine capillary tube in it. Bulb of the thermometer is inserted into the hot medium whose temperature is to be measured. As the heat is transferred from hot medium into the mercury in the bulb, the mercury expands. This expansion pushes a thread of mercury into the capillary. The glass of the thermometer is generally shaped as to magnify the apparent width of the thread of mercury. A temperature scale is put on the stem of thermometer, which indicates the temperature being measured.

Installation: The industrial thermometer is installed in such a way that it is protected from damage or breakage. The bulb of thermometer is inserted in a metal thermal well. The heat is transferred through the metal well into the bulb. A metal scale is mounted behind the upper end of thermometer and glass cover is provided over the scale. This type of installation provides a complete protection to the thermometer and makes it strong and rugged. Thermal well is made up of brass, steel or aluminum. Space between bulb and the well is filled up with a conducting medium such as mercury or oil, to increase the rate of heat transfer. The thermometer bulb is installed in such a manner that it is immersed to a sufficient length in the medium.This is to attain temperature equilibrium with the medium whose temperature is to be measured.

Pressure-Spring Thermometer: Based on the principle of operation and the material used this thermometer could be of any of the following types:

Principle of Operation

a) Liquid Expansion Thermometer: - It utilizes the volumetric expansion of liquid caused by temperature changes to measure the temperature.

b) Gas Expansion Thermometer: - It operates on the principle that the pressure of gas varies directly as the temperature, if the volume is kept constant.

c) Vapour Pressure Thermometer: - It operates on the principle that the ‘vapour pressure’ of a liquid increases with temperature.

Expansion Material: The commonly used expanding material in the liquid expansion type thermometer is ethyl alcohol. In gas expansion thermometer it is nitrogen gas and in vapour pressure thermometer it is ethyl ether. Of all the above thermometers the vapour pressure thermometer is widely used because it is less costly and simpler to maintain.

Construction: The construction of all of the above mentioned pressure spring thermometers are similar. The basic construction of these thermometers. It consists of a bulb which contain either a liquid or gas or liquid-vapour fluid. A metal capillary is connected to the bulb on one end and to a receiving element at the other end. The receiving element is usually a bourdon tube or pressure spring. A pointer is attached to the pressure spring through appropriate linkage. The whole system containing bulb, capillary and pressure spring is a sealed unit.

Working: The bulb of thermometer is inserted into the medium whose temperature is to be measured. The bulb comes in thermal equilibrium with the medium and transfer heat to the fluid inside the bulb. With this heat a pressure is developed in the fluid and the capillary connected to the bulb transfers this pressure to the receiving element, that is, the pressure spring. The pressure spring converts this pressure into a motion that moves the pointer on a scale to indicate the temperature.

Installation: The bulb of pressure spring thermometer is installed with a thermal well. The bulb and the capillary are made up of stainless steel or copper. For protection against damage the capillary is enclosed in another tubing or protective covering.

iii. Electric Temperature Indicators

Amongst the electrical temperature indicators the electrical resistance thermometer is being widely used in recent years in industries because of its accuracy and simplicity. It also makes it possible to detect very small increments of temperature to be detected. An electrical resistance thermometer is useful in wide range of temperature limits, that is, -180° to 650° C.

Principle of operation: The resistance thermometer is based on the principle of change in electrical resistance of a substance with temperature. In metals, the electrical resistance increases with the increase in temperature, where as, in semiconductor materials their resistance decreases with increase in temperature.

Construction: An electrical resistance thermometer bulb could be made up in many forms. The element may be made up of a strip of very thin foil or a coil of very fine wire wound on a frame. The industrial resistance thermometer is of a probe type . The connection to the wires of the resistance bulb is carefully made.

Material: Usually the material of a resistance thermometer is metal but non-metallic material may also be used. The industrial resistance thermometer employs platinum, nickel or copper. In processing industries the platinum-resistance element is used.The accuracy of a resistance thermometer is better than the expansion thermometers.Measurement of temperature with resistance thermometer reduces to the measurement of electric resistance and the techniques of resistance measurement are well advanced.

Purpose of Measurements

i. Improvement in the Quality of Product

The fundamental purpose of measurement of process variables is to improve the quality of product by processing it under the optimum processing conditions. The operating conditions of a process, that is, processing temperature, steam pressure, flow rates of various streams, etc., must be controlled with in specific limits. If all these process variables are maintained at their optimum values, the milk products produced would be of the best quality.

ii. Enhancing the Production Capacity and Efficiency of the Machine 

All the milk processing equipment are designed to operate under given set of machine parameters, such as speed, pressure or vacuum, flow rates and heat exchanger temperature, etc. The efficiency of these equipment or machines would be maximum, if they are operated under the parameters for which they have been designed. Their production capacity and the economy of operation would also be more at the optimum machine parameters. Thus the continuous monitoring of these machine parameters through accurate measurement devices is essential for operating the machine at its optimum parameters to achieve the higher production capacity and efficiency of the machine.

iii. Control of Engineering Services

In a dairy plant there is requirement of several services, such as, steam, chilled water, fresh water, refrigerant, electricity, etc. to carry out the processing operations like pasteurization, homogenization, cream separation, evaporation, drying, freezing and the washing of equipment. There are cold storages, boiler, electricity generators, and waste treatment units to provide engineering services in a dairy plant. In all the above services, variables like temperature, pressure, flow rates etc. are needed to be controlled. In order that these process variables may be controlled, the prerequisite is that they can be measured at the desired location in the individual equipment.

iv. Cost Appraisal

Measurement of a variable is also required to compute the cost of the commodity used. Water, electric energy meters and weighing balances are installed to measure the quantity of these commodities being used to work out the cost factor.All of the above functions require measurements. This is because proper and economical design, operation and maintenance of different processes, plant and machinery require a feedback of information. This information is supplied by making suitable measurements.

Instruments for Measurement of Process Parameters

We have already learnt in earlier units the concept, principles and construction of various milk processing equipments. We have also studied the working of steam boilers, generation and utilization of steam in a milk processing plant. During the operation of these equipment and machinery the plant operator has to keep a constant watch on various machine parameters and process variables, such as, temperature, pressure, flow, etc. For indicating these variables and parameters several devices and instruments are installed on the equipment. The satisfactory operation of the equipment depends upon the reliability of these instruments. It is thus of importance to understand the working of the devices used to measure the important process variables.Now we shall develop the understanding of basic concept of some important process variables. This will be followed by the description of measurement instruments for these variables, which are commonly used in a milk processing plant.

Energy Conservation Accessories in a Steam Boiler

The design of water-tube and the fire-tube boilers have been perfected with time and suitably modified for the increasing industrial applications. Considering the rapidly decreasing fossil fuel availability, increased emphasis is progressively laid onincreasing the heat utilization factor in the boiler. Several energy conservation devices and accessories have been evolved for the boilers to achieve the economy of heat utilization. Before studying these devices let us first discuss different types of heat losses from the boilers which could cause the decrease in thermal efficiency of the boiler and during the transmission of steam from one location to another.

i. Heat losses

These losses occur on account of many factors, some of which have been listed below.

i) Loss of unburnt or partially burnt fuel in the fuel bed or in the exhaust gases from the chimney.

ii) Poor heat transfer from hot flue gases to water due to formation of soot later on the flue gas side and water scale on the waterside.

iii) Inadequacy of insulation on steam pipe lines and removal of condensate from the pipe lines resulting in loss of heat energy during transmission of steam

iv) Heat energy losses with stack gases due to excessive temperature difference between steam and the stack gases temperature.

The greatest amount of heat loss in a boiler is the heat carried away by the hot flue gases up the chimney. A certain amount of heat loss is unavoidable as the hot gases are required to be hotter than the water in the boiler. However some of the heat being carried away by the hot flue gases can be recovered and sent back to the boiler, if the energy conservation accessories like economiser, air pre-heater and steam-superheater are installed.

ii. Economiser

It is an accessory installed with the boiler to utilize a portion of heat from the flue gases for preheating the feed water to the boiler. It consists of a series of vertical tubes through which feed water passes on its way from feed water pump to the boiler water space. These tubes are placed in the path of hot flue gases after the combustion chamber. The waste flue gases flow outside the economiser tubes. The feed water flowing into the boiler while passing through these tubes gets heated up after getting heat from the flue gases. This way a portion of heat is recovered from the flue gases, which would have otherwise gone waste to the atmosphere. The outside surface of these tubes is always in contact of hot flue gases and is prone to deposition of soot. Hence the outer surface is kept clean and free from soot by means of mechanized scrapers.

The advantages gained by installing an economiser are:

i) Fuel economy

ii) Long life of the boiler

iii) Increase in steaming efficiency

The saving of fuel affected by an economizer is proportional to the amount of heat recovered in the feed water. The average percent saving is approximately 1% for every 5.5° C increase in feed water temperature.

iii. Air Pre-heater

The function of an air pre-heater is to extract heat from the flue gases and transfer it to air entering the boiler furnace. It is installed between the economizer and the chimney. The commonly available design of air pre-heater is tubular type, in which the tubes are so placed that the flue gases pass through them. Air being heated is made to make a number of passes across and around the outside of these tubes.Designs are also available in which the hot gases flows outside the tubes and air inside.

The installation of air pre-heater increases the overall efficiency of the boiler plant.This increase in efficiency varies between 2 to 10 per cent. In addition of above advantage of increase in plant efficiency, the pre-heating of air also have additional benefits of improving the heat generation and transfer in the furnace. Some of these are:

i) It creates higher furnace temperatures.

ii) It accelerates combustion and increases the percentage of CO 2 in the furnace gases by ensuring the complete combustion.

iii) The pre-heating of air facilitates the burning of poor grade fuel.

iv. Steam Superheater

We have already read that if the saturated steam is separated from the contact of water and further heat is provided to it, keeping the steam pressure same, then the heat content of steam increases. Such steam is known as superheated steam. Due to higher heat contents the superheated steam effects the improvements and economy in the following ways:

i) Reduces the steam consumption for a given process.

ii) Reduces the condensate losses in steam pipe lines.

iii) Increases the capacity of the plant.

iv) Eliminates friction in steam lines.

Steam Superheater is the most important accessory of a boiler. It is a device in which steam is superheated. A steam superheater is a set of pipe line coils through which steam is passed after it is separated from contact of water. These coils are placed in the path of flue gases so that a part of heat in the flue gases is utilized to superheat the steam. The superheater tubes are usually 5 cm in diameter and are generally made up of carbon steel or chrome nickel alloy to withstand high temperatures. Saving in steam consumption by use of superheated steam is about 1.5 to 2% for each 5° C of superheat.

V. Soot Blowers

We have read in this unit that the accumulation of soot layer on the flue gas side of the boiler tubes reduces the heat transfer from hot gases to water. This is because the soot is bad conductor of heat. It is thus of utmost importance that the external surfaces of the tubes be kept free from soot by brushing or by mechanical blowing. The mechanical device used for removing soot is known as soot blower.In economizer the soot is removed by scrapers, which travel slowly and continuously up and down the tubes Sometimes special free flowing powders are used which when introduced in the furnace get Vapourized. These powders contain a catalyst, which reduces the ignition temperature of the adhering carbon in the soot, and an oxidizing agent in the powder provides combustible oxygen to burn this carbon. This way the deposition of soot on the surface is reduced.

Energy Conservation Principles

Energy is a critical component of processing industries. There is a total dependence on fossil fuels for this energy supply. It is also a know fact that our fossil fuel reserves are finite and we should utilize these resources judiciously. Efforts are also being made in several sectors to tap and develop new energy sources as an alternative to the fossil fuels. But until the alternative sources are put to commercial use we have to rely on the available means. Due to increased prices of fossil fuels,the cost of energy sources has increased tremendously. This has forced the industries to take the possible energy conservation measures. The concept of energy conservation is based on following general principles:

i) Save Energy: Economize the use of commercial energy. The amount of energy saved in a process can be equated as equivalent amount of energy generated.The wasteful expenditure of energy should be avoided. Minimize the losses of energy during its use.

ii) Enhance Equipment Efficiency: The energy utilization efficiency of equipments should be enhanced by incorporating regenerator, economizer, pre-heater, etc. and also by improvements in processing technologies.

iii) Heat Recovery from Discharge Streams: Hot condensate and exhaust air which are discharged in to atmosphere contain considerable amount of heat energy. Appropriate means and devices should be adopted recover this.

iv) Alternate Energy sources: Wherever possible, encourage the use of renewable energy sources, such as, solar energy, wind energy, bio gas etc. to ease the load on costly fossil fuel energy.

Dairy processing involves essentially the heating and cooling of milk and milk products. The related equipments are required to perform these operations efficiently and in doing so, the operation consumes a large amount of energy. Savings in this energy expenditure is possible if we apply the conservation measures. Energy conservation should receive attention of both the equipment manufacturer and the entrepreneurs of the processing plants, especially in view of the declining energy supply and rising energy cost.

For the milk processing plants the energy use profile is dependent on the product mix. The two major energy inputs during processing are the electricity and industrial oil fuel. There is a significant potential for energy conservation in the above two inputs with the improved engineering services and processing technologies in a dairy plant. The analysis in this section identifies areas for improvements in energy resource utilization. Following text describes some of the guidelines for conservation of energy in a dairy plant.

i) Look into the possibility to reduce peak load demand of steam and refrigeration by spreading the actual requirement over the day. This would not save energy but would redistribute the energy requirement from an expensive peak demand period to a period with low loading.

ii) Rationalize the use of cold storage rooms and shutting down rooms not required. Ensure the minimum ingress of heat into cool rooms through open doors and poor insulation. Cooled produce should be immediately moved to cold storage and the doors of the cold room be kept closed at all possible times.

iii) Production processes which involve pre-cooling with unreferigerated water should be controlled in such a way that operators will not short circuit the use of water and switch to refrigeration to meet a too rigid time schedule.

iv) The thermal efficiency of steam boilers operating in a dairy industry varies between 55 to 80%. It is not unusual for boilers to have an exhaust gas temperature of 220 to 250° C. It is worthwhile to consider the installation of feed water pre-heater or an air pre-heater which utilizes the heat in the exhaust gases down to 159 – 160 °C.

v) Insulate storage tanks, cold-water tank, chilled water pipes and steam pipe lines. Avoid water leaks through unattended losses and pipe connections.

vi) Regenerative application of heat exchangers is an effective means of conserving energy.

vii) Vapour recompression in multi-effect evaporators reduces the steam consumption and energy input. Use thermo-compression or mechanical vapour recompression in conjunction with multiple evaporators.

viii) Multistage spray drying plants equipped with heat and mass recovery system are successful in saving energy in spray drying of milk.

ix) Dairy industry mainly has small and medium sized motors with a low power factor or a high consumption of reactive power. Fitting a power factor correction system and ensuring that electric motor operate at high loads can reduce electric cost.

x) Replacing the traditional double wash procedure by a single wash procedure can reduce the energy cost in CIP-cleaning.

xi) Greater use of microprocessor based automatic controls improves the energy efficiency in food processing.

Care and Maintenance of Steam Lines

It is often assumed that piping system, once installed and connected never requires much attention there after. This is not so. It must be noted that pipe line system comprises of several component/parts, such as regulating valves, pressure gauges,expansion bends and joints, steam traps, steam strainers, etc. All of them are essential for proper functioning, efficient performance and safety of the plant.Without timely inspection and proper maintenance their performance and useful life are decreased. Therefore, while attending to the maintenance of pipelines due care must be given to all these component/parts of the system. An overall care and maintenance programme for pipelines must include the following points:

i) Drain all condensate pockets by ensuring there are no pipe sag and all the steam traps are working properly. Check the system to make sure that low points are raised. Sags caused by misalignment can be rectified by support adjustment.

ii) Attend to the cause of water hammer immediately. Inspect the pipe guides and anchor points for damage, if any.

iii) Check each hanger or other support of pipeline to ensure that it carries its share of load. Inspect that the anchors are holding and showing no signs of breaking or slipping.

iv) Supports for a horizontal pipe, far from an anchor point must allow ample slide or swing in the direction of the pipe expansion, which arises due to heat of steam inside the pipe.

v) Do not force flanges into the pipeline. It may lead to poor alignment. A poor alignment would result in leakage at pipeline joints, steam traps, steam strainer, steam valves and gauges.

vi) General routine inspection should be made to discover leaks and signs of corrosion and weakness.

vii) Steam traps should be tested on a regular schedule basis. They should be opened at least twice a year, cleaned thoroughly and if any valve or seat is found worn out it should be immediately replaced.

viii) Check that the pipeline insulation is securely bound to the pipe and the joints are closely fitted. Ensure the intactness of water proofing on the outdoor pipelines.

ix) Steam pressure gauges should be routinely tested for their accuracy, properly serviced and calibrated.

x) Maintain compete drawings of the pipeline system. All changes and corresponding dates should be indicated. Record the date of installation on all the piping elements. This will help in organizing a care and maintenance procedure.

Steam Pipe Line Insulation

To prevent heat losses from the steam pipe lines, the pipes are insulated. The pipe insulation cover is also known as lagging. The common insulating materials used are Asbestos sponge felt, Magnesia or asbestos hair and glass wool. The insulating efficiency of these materials is about 85%.The thickness of insulation varies from 2.5 cm to 7.5 cm depending upon the temperature of steam in the pipe. The higher the temperature, the larger is the thickness of insulating material. The insulation material should be able to withstand the pipe temperature and stand ordinary handling.

The insulation is securely bound to the pipe and the joints are closely fitted. The lagging over the pipelines is covered with a protective covering made up of aluminum or galvanized iron sheet. The pipe lines which are laid outdoor are required to be provided with water proofing.

Steam Strainer

Sometimes steam carries along with it some floating or suspended materials. Such materials results from the corrosion of steam pipe lines or the pipe or joint rust. This type of floating or suspended material is ultimately drained out with the steam condensate. The condensate will lead them to the steam trap. The steam trap will get clogged and its working will be hampered. For smooth and continuous working of steam traps such materials must be removed from the steam condensate. Installing a steam strainer, between the steam line and the steam trap inlet, does this.

Steam strainer. The steam condensate enters the strainer at the inlet ‘1’ and passes through the strainer mesh ‘3’ and eventually passes out from the outlet at ‘2’. The solid particles are retained inside the wire mesh and a clear condensate passes out to the out let of the strainer. The mesh is removable type and can be taken out for cleaning. This is periodically done by opening the valve screw at ‘4’. When the valve is opened the grits are blown out with the help of pressure of steam from the inlet. There is no requirement of dismantling the steam line to clear the strainer. The above arrangement eliminates the stoppage of steam to remove the strainer mesh for cleaning. Usually it is desirable to keep the spare strainer mesh in the store so that it could be replaced easily as and when required.

Steam Traps

Steam trap is a device which is used to remove condensate from steam. It is installed at a position and in such a manner that the condensate flow freely into it.Selecting a Steam Trap: It is necessary to have the knowledge of following factors while selecting the suitable steam trap for a given steam line installation
 

a) Rate of condensate discharge.

b) Method of disposal of condensate, whether the condensate is to be reused in the boiler or it is to be discharged free into atmosphere.

c) Pressure and temperature of steam in the pipe line.

Construction of Steam Trap: Depending upon the principle of operation and construction, stem traps can be grouped into various categories. The most commonly used steam traps are the open bucket type steam trap and the ball float type steam trap.

i) Open-Bucket Type Steam Trap: An open bucket type steam trap . The steam trap body has an open bucket inside it. When the condensate enters the steam trap, the bucket inside it rises with the rise in water level. This causes the closure of opening at the outlet valve. With more and more condensate entering the steam trap, the trap body and the bucket inside it get filled with water. As soon as the bucket is full of water, it sinks in the condensate and moves to the bottom of the trap body. The downward movement of bucket causes the outlet valve to open and the steam condensate is discharged out of the steam trap. This action of upward and downward movement of bucket repeats again and again with the fresh flow of condensate in to the steam trap. The trap body is provided with an air vent at the top to release air from the system in case there is an air lock.

ii) Ball-Float Type Steam Trap:  It consists of a trap body having a float ball in it. When the water level inside the trap body is low,the ball positions itself in front of the outlet orifice. Due to pressure of steam the ball is pressed against the orifice and seals it. As the condensate level in the trap body increases the float ball rises and changes its position relative to the outlet opening. The outlet opens and the condensate is discharged out. This discharge continues till the condensate is flushed out. As soon as the water level inside the trap body drops, the float ball again positions itself against the outlet orifice, thereby closing it. This operation repeats again and again and the condensate is discharged periodically.

Installation of steam trap: Steam trap is installed at a position and in such a manner that the condensate flows freely into it. Wherever physically possible,steam trap is installed below the equipment to be drained, so that condensate can flow by gravity. The trap should be accessible for servicing. Trap leakage represents a common cause of trouble. Other trap troubles include rusting and sticking of the mechanism, float leaks etc. Steam traps should be tested on a regular schedule basis. They should be opened at least twice a year, cleaned thoroughly and if any valve or seat is found worn out it should be immediately replaced.

Steam Line Expansion Bends and Joints

Steam pipes carries steam at high temperature. Due to this high temperature because of the heat of steam, the steam pipe lines are subjected to thermal expansion.If the provision for this expansion in pipe lines is not made then it develops thermal stresses and eventually damages the pipe line installation.

The thermal expansion in steam lines is taken care of by installing suitable expansion bends or joints in the pipe lines. A few different types of expansion bends. Expansion pipe bends are commonly used in the steam pipe lines for higher pressures. With the expansion bends the forces to be handled by the anchors are substantial. A well lubricated sliding expansion joint may reduce the resulting forces but it requires increased care and maintenance.

Steam Line System in a Dairy Plant

Transfer of steam from boiler to the point of steam use is the most common need in the dairy plant. Steam is transported in thicker walled conduits known as pipes.

Steam pipelines are different from sanitary pipes used in dairy plant. The sanitary pipe and fittings refers to the stainless steel tubing used for transportation of milk and milk products. These sanitary pipe fittings have sanitary design features, such as, ease in assembling and dismantling, smooth polish finish both inside and outside and hygienic characteristics to avoid product contamination. These types of pipelines are discussed separately

The material of construction of steam pipes is mild steel (MS). Mild steel is iron with low carbon content. Steam lines are subjected to pressure as well as thermal stresses and require extra care. The most important factors, which govern the design and installation of steam lines, are : pipe size, pipeline support system,alignment, drainage of condensate from steam lines and adequate insulation cover.

a) Pipe size: a pressure drop accompanies Flow of steam through a pipe. Steam pipeline must be of proper size to carry the steam load requirement without undue pressure drop.

b) Pipeline supports: Various types of pipe support systems are used in a dairy plant.  The pipes may be supported from the ceiling for single, double or multiple lines. It may be supported from the walls by means of brackets. Whether the support is from ceiling or from walls, flange type plates are fixed firmly into ceiling or the walls. The tubes,which are used to support the structure, are welded to these plates and the pipe lines are placed over the support structure. Pipe supports should be spaced closely enough to prevent undue sag in the span.

c) Alignment: Poor alignment of piping is the frequent cause of leaks in pipe line joints. Leaks are also there if the joints are not designed or supported properly.Hence the pipe line is precisely aligned with due respect to all the bends and joints at the time of installation. Proper support is provided at the joints. Piping is always supported on both sides of every large valve. Screwed or the expanded type flanges are used at the joints on steam pipe lines for pressures up to 3000kPa and temperatures up to 672 K. For higher pressures and temperatures the welded type flanges are used.

d) Drainage of condensate in the pipe line: As the steam flows through the pipes some condensation usually occurs. If the steam condensate is not drained out of the steam pipe it accumulates at the pockets in the pipe line. Whenever the valve in the line is opened, this accumulated water moves in a column. The sudden movement and stoppage of this water column in the pipe line results in
water hammer. If a steam pipe is constantly hammered it is damaged. Proper drainage of condensate from the steam pipe line is thus of utmost importance.For self draining of condensate a slope of 0.25 to 0.3 % is given to the steam pipe line in the direction of steam flow. A full bore Tee (Equal Tee) is used for trapping the condensate from the line and lead it to the traps located at vulnerable points. Any sag in the pipe line is undesirable as it would result in condensate pocket. The sag in the pipe is removed by proper spacing between pipeline supports.

e) Pipeline insulation: To prevent heat losses from the steam pipe lines, the pipes are insulated. The pipe insulation cover is also known as lagging. The common insulating materials used are Asbestos sponge felt, Magnesia or asbestos hair or glass wool.

An ideal piping system is that in which each part is self supported and imposes no stress on the other part. All elements in the system hold their correct relative positions and alignment despite thermal expansion and contraction. Anchors are used to fix certain points of the system, and expansion bends, joints and supports are provided for free movement for all the rest of the piping in the system. Anchors are designed to securely lock the anchored points in the piping to heavy steel or concrete work. Steam line is properly pitched for condensate drainage. It is ensured that sag in the pipe, if there is any, does not result in the centre point of span between two supports below its lower support, and otherwise it would form a condensation pocket that may lead t water hammer.

Steam Supply Line Accessories and Energy Conservation

We have learnt in the earlier units that from amongst the several alternative-heating arrangements, heating of milk in the vessel with the help of steam are the most convenient and appropriate. Steam can be transported through pipe lines from its source of generation to all the places of utilization very conveniently. It is the most economical and common source heat energy and has been adapted by all the milk processing plants. We have learnt the principles of generating steam, understood the general constructional features of a boiler vessel in which the steam is generated,and are now familiar with the component units or devices, which are mounted on the body of the boiler for the safety of boiler and control of its operation.Knowledge of steam pipeline and fittings is of great value to the dairyman because of universal use of pipes for conveying fluids. The safety and satisfactory working of steam utilization system in a milk processing plant largely depends upon the reliability and the effectiveness of the steam distribution lines. Now we shall discuss in detail the steam supply pipeline and the accessories attached to them in a dairy plant.

Boiler Safety Mountings

i. Safety Valve

As you know steam is generated in the boiler at a particular pressure. There are possibilities that the steam pressure in the boiler continues to rise and may exceed the value for which the boiler is designed. This may happen due to many reasons,such as, if the water in the boiler shell decreases or the feed water pump fails or the burner continues to supply heat even if the required steam pressure is reached or the steam demand is decreased.

If such thing happens the boiler shell will get overheated and may explode and result in great damage. To avoid this thing to happen, the boiler is provided with a safety device, which is known as the Safety Valve.Function of Safety Valve: The function of a safety valve is to safeguard the boiler from the hazard of pressures higher than the design value. It prevents the steam pressure in a boiler from exceeding a prefixed value. If the inside pressure exceeds the maximum working pressure, this valve opens and automatically discharges the steam from the boiler.

When once the excess steam is vented out and the pressure in the boiler is reduced to the working pressure the safety valve resets to its closed position automatically.The safety valve is located at the highest point in the steam space.

Types of Safety Valves and their construction

The safety valves may be classified into two groups:

a) Weight loaded safety valves

b) Spring loaded safety valves

Dead Weight Safety Valve: The weight loaded safety valve is commonly known as Dead weight safety valve.In this valve the weight, in the form of cylindrical cast iron discs, is placed directly on the valve. The valve is made of gunmetal and rests on a gunmetal seat. It is secured on the top of a vertical cast steel pipe. The pipe is bolted to the mounting block which is riveted to the top of the shell. The valve is threaded into a large cast iron casting which is like a vertical pipe like cover. This cover pipe carries the cylindrical disc shaped weights in it. Thus the total load on the valve is the sum total of weight of the valve itself, the weight of cover pipe and the weights placed on the cover pipe. When this load is greater than the force due to steam pressure acting on the valve, the steam will not escape. When the force due to steam pressure exceeds the load on the valve the valve is lifted up from its seat and the steam will escape. The lift of valve is controlled by set screws.Control and Safety Devices for Steam Boilers

Spring Loaded Safety Valve: If the valve in a safety valve is loaded with a spring instead of dead weights, then it is known as a spring loaded safety valve. A spring loaded safety valve .The spring used in these valves is in the helical form with round or square wires.The wires are made of steel. The load due to steam pressure acts along the axis of the spring. In the tensile load designs the spring of the safety valve is elongated when the excess steam pressure acts on the valve. Whereas, in the compressive load designs the spring of the valve is compressed when the excess steam pressure acts on it. As soon as the excess steam is released, and the pressure reduces below the working pressure, the spring brings back the valve to the valve seat and closes the valve.

The spring-loaded safety valves possess many advantages over the dead weight safety valves. These could be listed as below:

i) A spring safety valve eliminates the need of heavy weights.

ii) A spring safety valve offers easy examination of it and is easy to maintain.

iii) A spring-loaded safety valve is not affected due to jerks or movement.
 

ii. Fusible Plug

Function of Fusible Plug: Each furnace of the boiler is required to be fitted with a fusible plug. It is used to protect the boiler against the damage due to overheating for the reasons of low water levels. Its function is to extinguish the fire in the furnace of a boiler when the water level in the boiler falls to an unsafe extent and prevent the explosion which may take place due to overheating of the furnace plate. The plug is made of a fusible alloy. It melts away at high temperatures and releases the steam over the grate, thus quenching the fire.There are many designs of fusible plug.

iii. Water level Indicator

Function: The function of water level indicator in a boiler is to show the level of water in the boiler. If the water level in the boiler falls below a minimum level, the operator should make arrangements either to supply more water or to stop firing the fuel. There are two such indicators mounted on the boiler to ascertain the level of water in it. They are also called as water gauges.

Boiler Mountings and Accessories

All boilers are fitted with mountings or accessories for the safety of the boiler and for complete control of process of steam generation. These components or elements function with the boiler as a whole and contribute their individual share in the task of steam generation.

i. Boiler Mountings

One category of such component units, primarily intended for safety of boiler, which are installed or mounted on the body of the boiler itself are known as boiler mountings. The safety and satisfactory working of boiler largely depends upon the reliability of various boiler mountings. In accordance with the Indian Boiler Regulations, the following mountings must be fitted to the boilers: two safety valves,a fusible plug, two water level indicators, a pressure gauge, a steam stop valve, a feed check valve, a blow off cock, an attachment for inspector’s test gauge, a man hole and a mud hole or sight hole.

ii. Boiler Accessories

Another category of components or elements are the auxiliary units, which are installed with the boilers. These auxiliary units are known as boiler accessories.They either help the boilers in their working or increase the efficiency of the boilers. The accessories attached to a modern boiler are: feed pump, feed water heater, air pre-heater, steam super heater and the draught equipment.

Control and Safety Devices for Steam Boilers

We have learnt in the previous unit that steam is the most economical and convenient source of heat energy and has been adapted by all the milk processing plants. You have also known about the principles of generating steam, different types of steam and their heat contents and how the necessary calculations are made to estimate the amount of steam required for a given milk processing operation. We haveunderstood the general constructional features of a boiler, in which the steam is g enerated. The Boiler occupies such an important place in dairy plant that much of plant’s successful operation depends upon its unfaltering performance The safety and satisfactory working of a boiler largely depends upon the reliability of several component units, which are mounted on the body of the boiler. These devices are primarily intended for the safety of boiler and control of its operation.In the foregoing no discussion has been made about these devices, so in this unit we shall discuss various essential boiler mountings in detail.

Operating a Steam Boiler

In the foregoing sections we have outlined the basic principles of steam generation and the component parts of a steam boiler. Therefore now we may find it convenient and helpful to refer to the earlier portions of this unit to understand the operation of a boiler while reading this section.

i. Feeding Water to the Boiler
For operating a boiler continuous supply of feed water is required. This is to maintain a constant water level in the boiler in relation to generated steam. Water is provided to a boiler by a feed pump. This pump is meant to force water into the boiler. Since the inside pressure of the boiler is high, the water needs pumping to a considerable pressure above that of boiler. Both reciprocating and rotary pumps could be used and generally pumps are installed in duplicate for the safety of boiler operation. Feed water is supplied to the boiler through a special one-way valve. This valve does not allow the back flow of water from the boiler due to higher pressure inside the boiler.

The water supply to a boiler is very important. In most localities the natural water contains impurities like suspended solids or the dissolved chemicals. Suspended matter can usually be handled by proper filtration. The dissolved impurities in water are due to compounds of calcium and magnesium. The water containing such impurities is known as hard water. If this hardness of water is not removed, these impurities result in deposition in the form of scale on the heat exchange surfaces in the boiler. The deposition of scale over the tubes in the boiler reduces the transfer of heat from the hot gases to water and decreases the steam raising efficiency of the boiler. It may also lead to corrosion of tubes and damage to the boiler. To avoid this problem, the feed water to the boiler is treated to remove its hardness. Specialized equipment is used for this treatment of water and it is known as water softening plant. It is advised to install a water softening unit with every boiler to assure a continuous supply of soft water, otherwise the boiler has to blown down and cleaned frequently and regularly.

ii. Air Circulation & Combustion of Fuel

Fans are used to provide adequate and continuous supply of air for combustion of fuel, circulating the hot gases in the boiler and to discharge burnt flue gases into the atmosphere. A small pressure difference is kept between the point of entry of air at furnace and the exit of burnt gases at the chimney. This pressure difference helps in the flow of air throughout the system. This pressure difference is termed as boiler draught. The amount of required draught depends upon the type of fuel and the rate of burning. A high chimney helps in creating the natural draught in the boiler without any fan. A chimney also facilitates the safe discharge of burnt gases to the atmosphere. But the amount of this natural draught is low and is used only in boilers of low capacity. For larger capacity boilers the requirement of combustion air is more and hence a fan is installed at the inlet to the furnace to provide positive pressure. This is called forced drought.

iii. Care and Maintenance

The time that should be taken in raising steam from cold water depends upon the type of boiler and the degree of urgency. In every case steam should be raised as slowly as possible. Similarly rapid cooling of boiler is harmful as it puts excessive strains on the joints. Periodic blow down is necessary with all types of boilers, no matter whether treated or untreated feed water is used. This removes all the solid impurities, if any, present in the boiler. Boiler undoubtedly deteriorates more rapidly when allowed to remain idle than during long periods of normal work. If it is required to remain idle for longer periods most satisfactory method is to remove all doors and covers and thoroughly clean and dry all internal and external surfaces and give a coating of a preservative paint.

Boiler occupies such an important place in dairy plant that much of plant’s successful operation depends upon its unfaltering performance. The life of any steam generating plant is dependent upon the amount of care and attention it receives while under steam and during idle periods. Indian Boiler Act-1923 prescribes that only a Certified Boiler Attendant is authorized to operate a boiler. Yet it is worth while for all the dairy workers to become acquainted with general construction of the boiler, for overall safety and precautions.

Different Types of Steam Boilers

In order to have an efficient heat transfer from flue gases to water in the boiler,the heat transfer surface should be made as large as possible. To achieve this two methods could be thought of and based on this the boilers are categorized into two categories.

i. Fire-Tube Boilers
First method could be that the water to be evaporated is contained in a vessel,which is provided with large number of tubes. If the hot gases from the furnace are made to flow through the tubes, then a large amount of heating surface of these tubes will be exposed to water to heat the same. Thus considerable amount of water particles will be able to come directly in contact with heating surface. The boilers designed on this principle are known as fire tube boilers. In the fire-tube boilers, the hot flue gases from the furnace are made to pass through the tubes and water surrounds these tubes. The number of tubes varies as per the capacity and design of the boiler. A boiler with large number of tubes is known as multi-tubular boiler. These tubes are placed in the boiler shell.Water in the shell is heated by the conduction of heat from hot gases through the walls of the tubes. Since the shell holds a large amount of water, the evaporation process is slow and it takes more time to raise steam in such boilers. This type of boilers cannot be used to work under very high pressure and are usually made for smaller capacity.

ii. Water-Tube Boilers

In the second case, a chamber consisting of a furnace at the bottom is provided with a number of tubes .Here the hot gases travel from the furnace to the chimney through the space outside the tubes in the chamber, while water, which is to be heated, is circulated through the tubes. The boilers designed on this principle are known as water tube boilers. Thus, in the water-tube boilers, the water is contained inside the tubes and hot flue gases from outside surround these tubes. Since the amount of water contained inside the tubes is small the heating is efficient and the evaporation is faster. This helps in increasing the capacity and the working pressure of the boiler.However, these boilers require more attention and the cleaning of tubes is difficult.Commercial Boilers Amongst the commercially available fire tube boilers are : Cornish boiler, Lancashire boiler and the Locomotive boilers. Various commercially available water tube boilers are : Babcock & Wilcox boiler, Sterling boilers, etc.

iii. Additional classification of boilers

In addition to the above classification of boilers into fire tube or water tube boilers,further classification of conventional type of boilers may be done in many more different ways according to their construction and service conditions. Boilers are classified according to their use as Stationary, Portable and Locomotive. According to position of furnace inside the shell or outside the shell they are termed as internally fired or externally-fire boilers. Depending upon the circulation of water in the boiler they are classified as Natural Convection Circulation type or Forced Pump Circulation type boilers. A boiler is termed as a High Pressure Boiler if the working pressure is over 80 kg/cm 2 and as a Low Pressure Boiler if the working pressure is less than that.

Steam Boiler

Evaporating the water at appropriate temperature and pressure does the generation of steam. It might look that to perform this function is an easy task. But if large quantities of steam at high pressure are to be produced rapidly and with economy then a considerable amount of skill and specialized equipments are required. The production of steam at high pressure in large quantities would not be possible and economical if the water is heated in ordinary way in a vessel with furnace at its bottom. The heat will go to waste in large quantities by radiation. There will not be an efficient heat transfer to water and evaporation of water may not be controlled.

In order to overcome these defects water is evaporated in a closed vessel, in which heat transfer and the evaporation process is controlled. This equipment is known as steam boiler.Boiler is thus, a vessel in which steam is generated. Heat is produced in the boiler by burning of fuel. This heat is transferred to the water contained in the boiler and the water evaporates to form steam. As has been discussed earlier that steam is generated at a desired saturation pressure, the boiler thus also maintains the required pressure in it. A boiler is therefore also known as a pressure vessel. The construction and appearance of a boiler depends upon the arrangements made for burning the fuel and the mode of transfer of this heat to water.

i. Components of a Steam Boiler
Every boiler consists of three distinct regions, such as, the space for burning of fuel and flue gases, the space for water and the space for steam .The Fuel Space: It consists of the furnace chamber and the passage of hot gases as they flow through the boiler. The fuel is burned in the furnace chamber or the fire box. If the fuel is a solid fuel, such as, coal it is burnt by placing it over a grate.The grate consists of cast iron bars with spacing between the bars for the flow of air. If the fuel is oil or gas it is burned with help of a specially designed oil or gas burner. The combustion of fuel is maintained by a steady supply of air to the furnace. The waste furnace gases escape the boiler through a high chimney. A high chimney helps in safe discharge of waste gases in the atmosphere and also creates a necessary pressure differential for flow of air and gases in the boiler.

The Water Space and Steam Space: The water and steam in the boiler is contained in metallic water drums and tubes. The water space is the volume of the drum or shell that is occupied by water, and steam space is the volume of the entire shell not occupied by water. The level at which the water stands in the boiler shell is known as water level and it is indicated by a water level indicator. The level of water fluctuates in the boiler at times but for the best operation the variation should be small. The heating surface is the surface of shell exposed to the fire or the hot gases from the fire.

The general constructional features of a boiler are illustrated in fig. 9.1. In its simplest form, the boiler shell is kept vertical. It consists of a large cylindrical shell.A firebox is provided in the shell by suitable positioning it with metal plates. The firebox carries a grate at the bottom. The water space in the shell surrounds the firebox from all sides.

The Uptake and Chimney: A tube called the uptake starting from the top of firebox passes through the shell and connects the base of the chimney. The chimney is placed at the top of the uptake. One or more cross tubes are is fitted across the firebox to increase the heating surface area and to ensure better circulation of water. A fire-hole is provided in front of the boiler slightly above the grate level.A manhole and a hand hole are provided in the shell to have an access for cleaning.To drain the mud that settles down a mud hole is provided at the bottom. All the openings are closed with suitable covers.

ii. Functioning of a Boiler 

The coal burns in the firebox over the grate. Heat produced by burning fuel in the firebox is radiated to the water in the boiler shell. The hot gases while moving upward come in contact with cross tubes and then pass through the uptake tube.

During this contact, they give part of their heat to the metal of the tubes and this heat is transferred to water. The water gets heated up and evaporates. The steam produced by evaporation of water gets accumulated in the boiler shell, in the space above the surface of water. The steam generated in the boiler is then tapped off through a suitable valve fitted in the steam portion of the shell.

A boiler also has component units which are primarily intended for the safety of boiler. These components are mounted on the body of the boiler and are known as Boiler mountings. Various boiler mountings include: pressure gauge, safety valve,water-level indicator, fusible plug, the safety and satisfactory working of the boiler largely depends upon the reliability of these mountings. We shall discuss various essential boiler mountings in detail in Unit-10.

In the smaller boilers the firebox forms an integral part of the boiler shell. But in large boilers it is separately constructed of brickwork and is known as settings. The brickwork forms the walls of furnace and combustion chamber. It confines the heat to the boiler and makes passage through which the hot gases pass. It also provides support to the boiler shell.

Steam is generated in the boiler under the set of conditions of inlet water and exit steam, while a certain rate of fuel is being consumed. It is neither practical nor expected that whole of the heat produced by the combustion of fuel in a boiler will be transferred to water for generation of steam. Certain amount of heat will go to waste through radiation and through flue gases, etc. Nevertheless, a good boiler is one which gives the economy of steam generation and provides high performance efficiency.

iii. Requisites of a Good Boiler

We would now specify and lay down the necessary requisites of a good boiler.

Requisites of a good boiler could be listed as follows;

a) It should be capable of producing maximum amount of steam with minimum cost.

b) It should be capable of quick starting and should be able to meet the variations in steam requirements.

c) All parts and components should be easily accessible for inspection and repair.

d) In order to make the best use of heat supplied, the boiler should have proper arrangement of circulation of water and hot gases.

e) It should be safe in working.

There are many factors for selection of a boiler for a given situation. The most important ones are: the required steam capacity, working pressure in the boiler and the availability of fuel. Based on these the size and the type of boiler are selected.We therefore now need to know about various types of boilers and how these are classified.