There is an important need for clean, pure drinking water in many developing countries. Often water sources are brackish (i.e. contain dissolved salts) and/or contain harmful bacteria and therefore cannot be used for drinking. In addition, there are many coastal locations where seawater is abundant but potable water is not available. Pure water is also useful for batteries and in hospitals or schools. Distillation is one of many processes that can be used for water purification. This requires an energy input, as heat, solar radiation can be the source of energy. In this process, water is evaporated, thus separating water vapour from dissolved matter, which is condensed as pure water. For people concerned about the quality of their municipally-supplied drinking water and
unhappy with other methods of additional purification available to them, solar distillation of
tap water or brackish groundwater can be a pleasant, energy-efficient option.This water can be used for many purposes.one of it’s industrial application is,”“Distilled water is used as a feed stock for Industrial Boilers.
Introduction
There is an important need for clean, pure drinking water in many developing countries. Often water sources are brackish (i.e. contain dissolved salts) and/or contain harmful bacteria and therefore cannot be used for drinking. In addition, there are many coastal locations where seawater is abundant but potable water is not available. Pure water is also useful for batteries and in hospitals or schools.
Distillation is one of many processes that can be used for water purification. This requires an energy input, as heat, solar radiation can be the source of energy. In this process, water is evaporated, thus separating water vapour from dissolved matter, which is condensed as pure water.
Solar water distillation is a solar technology with a very long history and installations were built over 2000 years ago, although to produce salt rather than drinking water. Documented use of solar stills began in the sixteenth century. An early large-scale solar still was built in 1872 to supply a mining community in Chile with drinking water. Mass production occurred for the first time during the Second World War when 200,000 inflatable plastic stills were made to be kept in life-crafts for the US Navy.
There are a number of other approaches to water purification and desalination, such as photovoltaic powered reverse-osmosis, for which small-scale commercially available equipment is available. These are not considered here.
In addition, if treatment of polluted water is required rather than desalination, slow sand filtration is a good option.
The purpose of this technical brief is to provide basic information and direct the reader to other, more detailed sources.
Energy requirements for water distillation
The energy required to evaporate water is the latent heat of vaporisation of water. This has a value of 2260 kilojoules per kilogram (kJ/kg). This means that to produce 1 litre (i.e. 1kg since the density of water is 1kg/litre) of pure water by distilling brackish water requires a heat input of 2260kJ. This does not allow for the efficiency of the heating method, which will be less than 100%, or for any recovery of latent heat that is rejected when the water vapour is condensed.
It should be noted that, although 2260kJ/kg is required to evaporate water, to pump a kg of water through 20m head requires only 0.2kJ/kg. Distillation is therefore normally considered only where there is no local source of fresh water that can be easily pumped or lifted.
How a simple solar still operates
Figure 1 shows a single-basin still. The main features of operation are the same for all solar stills. The incident solar radiation is transmitted through the glass cover and is absorbed as heat by a black surface in contact with the water to be distilled. The water is thus heated and gives off water vapour. The vapour condenses on the glass cover, which is at a lower temperature because it is in contact with the ambient air, and runs down into a gutter from where it is fed to a storage tank.
Design objectives for an efficient solar still
For high efficiency the solar still should maintain:
• a high feed (undistilled) water temperature
• a large temperature difference between feed water and condensing surface
• lowvapour leakage.
A high feed water temperature can be achieved if:
• a high proportion of incoming radiation is absorbed by the feed water as heat. Hence low absorption glazing and a good radiation absorbing surface are required
• heat losses from the floor and walls are kept low
• the water is shallow so there is not so much to heat.
A large temperature difference can be achieved if:
• the condensing surface absorbs little or none of the incoming radiation
• condensing water dissipates heat which must be removed rapidly from the condensing surface by, for example, a second flow of water or air, or by condensing at night.
Design types and their performance
Single-basin stills have been much studied and their behaviour is well understood. Efficiencies of 25% are typical. Daily output as a function of solar irradiation is greatest in the early evening when the feed water is still hot but when outside temperatures are falling.
Material selection is very important. The cover can be either glass or plastic. Glass is considered to be best for most long-term applications, whereas a plastic (such as polyethylene) can be used for short-term use.
Sand concrete or waterproofed concrete are considered best for the basin of a long-life still if it is to be manufactured on-site, but for factory-manufactured stills, prefabricated ferro-concrete is a suitable material.
Multiple-effect basin stills have two or more compartments. The condensing surface of the lower compartment is the floor of the upper compartment. The heat given off by the condensing vapour provides energy to vaporize the feed water above. Efficiency is therefore greater than for a single-basin still typically being 35% or more but the cost and complexity are correspondingly higher.
Solar distillation Practical Action 3
Wick stills - In a wick still, the feed water flows slowly through a porous, radiation-absorbing pad (the wick). Two advantages are claimed over basin stills. First, the wick can be tilted so that the feed water presents a better angle to the sun (reducing reflection and presenting a large effective area). Second, less feed water is in the still at any time and so the water is heated more quickly and to a higher temperature.
Simple wick stills are more efficient than basin stills and some designs are claimed to cost less than a basin still of the same output.
Emergency still - To provide emergency drinking water on land, a very simple still can be made. It makes use of the moisture in the earth. All that is required is a plastic cover, a bowl or bucket, and a pebble.
Hybrid designs - There are a number of ways in which solar stills can usefully be combined with another function of technology. Three examples are given:
• Rainwater collection. By adding an external gutter, the still cover can be used for rainwater collection to supplement the solar still output.
• Greenhouse-solar still. The roof of a greenhouse can be used as the cover of a still.
• Supplementary heating. Waste heat from an engine or the condenser of a refrigerator can be used as an additional energy input.
Output of a solar still
An approximate method of estimating the output of a solar still is given by:
Q = E x G x A
2.3
where:
Q = daily output of distilled water (litres/day)
E = overall efficiency
G = daily global solar irradiation (MJ/m²)
A = aperture area of the still ie, the plan areas for a simple basin still (²)
Solar distillation Practical Action 4
In a typical country the average, daily, global solar irradiation is typically 18.0 MJ/m² (5 kWh/m²). A simple basin still operates at an overall efficiency of about 30%. Hence the output per square metre of area is:
daily output = 0.30 x 18.0 x 1
2.3
= 2.3 litres (per square metre)
The yearly output of a solar still is often therefore referred to as approximately one cubic metre per square metre.
Would a solar still suit your needs?
Human beings need 1 or 2 litres of water a day to live. The minimum requirement for normal life in developing countries (which includes cooking, cleaning and washing clothes) is 20 litres per day (in the industrialised world 200 to 400 litres per day is typical). Yet some functions can be performed with salty water and a typical requirement for distilled water is 5 litres per person per day. Therefore 2m² of still are needed for each person served.
Solar stills should normally only be considered for removal of dissolved salts from water. If there is a choice between brackish ground water or polluted surface water, it will usually be cheaper to use a slow sand filter or other treatment device. If there is no fresh water then the main alternatives are desalination, transportation and rainwater collection.
Unlike other techniques of desalination, solar stills are more attractive, the smaller the required output. The initial capital cost of stills is roughly proportional to capacity, whereas other methods have significant economies of scale. For the individual household, therefore, the solar still is most economic.
For outputs of 1m³/day or more, reverse osmosis or electrodialysis should be considered as an alternative to solar stills. Much will depend on the availability and price of electrical power.
Solar distillation Practical Action 5
For outputs of 200m³/day or more, vapour compression or flash evaporation will normally be least cost. The latter technology can have part of its energy requirement met by solar water heaters.
Design Specifications
The Distiller is made of the following parts:
1. Tempered Glass Plate
Glass has the property of selectively allowing only the higher energy radiation to
pass through and blocking the longer wavelengths. This particular property aids
in the distiller as it captures most of the incoming higher energy radiation but
does not allow it to radiate back. This also serves as a condensing surface being
open to atmosphere it will always be at a lower temperature than the water
inside. It is made slanting so that any water droplets that are formed finally move
along the gradient where they finally deposit the condensate into collector.
2. Top water reservoir
Water is stored on top just under the glass plate. This water needs to be
recharged everyday. The floor of the container is painted black to maximize the
irradiation capture. The paint needs to be not water soluble and dried in sun
In many parts of the world, fresh water is transported from another region or location by boat, train, truck or pipeline. The cost of water transported by vehicles is typically of the same order of magnitude as that produced by solar stills. A pipeline may be less expensive for very large quantities.
Rainwater collection is an even simpler technique than solar distillation in areas where rain is not scarce, but requires a greater area and usually a larger storage tank. If ready-made collection surfaces exist (such as house roofs) these may provide a less expensive source for obtaining clean water.
For the purpose of design we will assume a very low conversion efficiency of around 20%.
Given the highly erratic supply of sunlight which depends greatly on weather conditions we
have to over design it for high factor of safety – in this case 2. In real life we expect the
efficiency to be higher than 40%.
The first step in design is to calculate the aperture area.
Aperture Area = Energy required for distillation of 30 liters of water / Solar energy
available per m2 * conversion efficiency
= (30 kg/day * 4.2kJ/kg oC * (60-30) oC)/(1 kW/m2 * 3600 s/hour *6
hours/day)*(0.2)
= 0.8 m2
So we need total area of 0.8 m2 for the distillation of 30 liters of water daily.
INTRODUCTION TO BOILERS |
A boiler is an enclosed vessel that provides a means for combustion heat to be transferred into
water until it becomes heated water or a gas (steam). The steam
or hot water under pressure is then usable for transferring the heat
to a process. Water is a useful and cheap medium for transferring
heat to a process. When water is boiled into steam its volume
increases about 1,600 times, producing a force that is almost as
explosive as gunpowder. This causes the boiler to be an
extremely dangerous item that must be treated with utmost
respect.
Boilers were used in crude fashions for several centuries but
development was slow because construction techniques were
crude and the operation was extremely dangerous. But by the
industrial revolution of the mid 1800’s boilers had become the
main source of energy to power industrial operations and
transportation. The use of water as a heat transfer medium has
many advantages. Water is relatively cheap, it can be easily
controlled, the gas in invisible, odorless, and extremely high purity.
The process of heating a liquid until it reaches it's gaseous state is called evaporation. Heat is
transferred from one body to another by means of (1) radiation, which is the transfer of heat
from a hot body to a cold body through a conveying medium without physical contact, (2)
convection, the transfer of heat by a conveying medium, such as air or water and (3)
conduction, transfer of heat by actual physical contact, molecule to molecule. The heating
surfaceis any part of the boiler metal that has hot gases of combustion on one side and water
on the other. Any part of the boiler metal that actually contributes to making steam is heating
surface. The amount of heating surface a boiler has is expressed in square feet. The larger the
amount of heating surface a boiler has the more efficient it becomes. The measurement of the
steam produced is generally in pounds of water evaporated to steam per hour.
Gallons of water evaporated x 8.3 pounds/gallon water = Pounds of steam
In firetube boilers the term boiler horsepower is often used. A boiler horsepower is 34.5
pounds of steam. This term was coined by James Watt a Scottish inventor. The measurement
of heat is in British Thermal Units (Btu’s). A Btu is the amount of heat required to raise the
temperature of one pound of water one degree Fahrenheit. When water is at 32 oF it is
assumed that its heat value is zero.
SENSIBLE HEAT:-
The heat required to change the temperature of a substance is
called its sensible heat. In the teapot illustration to the left the 70
oF water contains 38 Btu’s and by adding 142 Btu’s the water is
brought to boiling point.
In the illustration to the left, to
change the liquid (water) to itsgaseous state (steam) an
LATENT HEAT
additional 970 Btu’s would be
required. This quantity of heat
required to change a chemical
from the liquid to the gaseous
state is called latent heat.
The saturation temperature or boiling point is a function of
pressure and rises when pressure increases. When water
under pressure is heated its saturation temperature rises
above 212 oF. This occurs in the boiler. In the example below
the boiler is operating at a pressure of 100 psig which gives a
steam temperature of 338 oF or 1185 Btu’s.
When heat is added to saturated steam out of contact with
liquid, its temperature is said to be superheated. The
temperature of superheated steam, expressed as degrees above saturation, is referred to as the
degrees of superheat.
BOILER TYPES:
There are virtually infinite numbers of boiler designs but generally
they fit into one of two categories: (1) Firetube or as an easy way
to remember "fire in tube" boilers, contain long steel tubes through
which the hot gasses from a furnace pass and around which the
water to be changed to steam circulates, and (2) Watertube or
"water in tube" boilers in which the conditions are reversed with the
water passing through the tubes and the furnace for the hot gasses
is made up of the water tubes. In a firetube boiler the heat (gasses)
from the combustion of the fuel passes through tubes and is
transferred to the water which is in a large cylindrical storage
area. Common types of firetube boilers are scotch marine, firebox,
HRT or horizontal return tube. Firetube boilers typically have a
lower initial cost, are more fuel efficient and easier to operate but
they are limited generally to capacities of 50,000pph and pressures of 250 psig. The more
common types of watertube boilers are "D" type, "A" type, "O" type, bent tube, and cast-iron
sectional. All firetube boilers and most watertube boilers are packaged boilers in that they can
be transported by truck, rail or barge. Large watertube boilers used in industries with large
steam demands and in utilities must be completely assembled and constructed in the field and
are called field erected boilers.
Vertical tubeless boilers are used for small loads but really do not fit into either category as they
do not have tubes.
Boilers and pressure vessels are built under requirements of the American Society of
Mechanical Engineers or ASME referred to as the "ASME Code." High pressure boilers are
fired vessels for an operation greater than 15 psig and 160oF and are built in accordance with
Section I of the ASME Code with the ASME S stamp. Vessels with design pressures below 15…
The water supplied to the boiler that is converted into steam is called feedwater. The two
sources of feedwater are: (1) Condensate. or condensed steam returned from the processes
and (2) Makeup water (usually city water) which must come from outside the boiler room and
plant processes. For higher boiler efficiencies the feedwater can be heated, usually by
economizers.
MAKEUP WATER
A. WATER SOFTENERS:
Water as it passes over the ground, through caves and springs picks up some of the elements
from the limestone and other elements of nature which dissolved and remain. These elements
collectively are called hardness. Grandma's tea kettle, used as an example in Chapter One,
always seemed to have a "build up" in the bottom which she removed periodically usually with
vinegar. This "build up" is called hardness. In a heavy use industrial steam boiler the water is
could be completely replaced as often as once each hour. Obviously at higher turnover,
temperatures and pressures than the tea kettle the boiler would quickly have scale from this
hardness that would reduce and ultimately prevent water circulation and heat transfer which will
destroy the boiler. The higher the operating pressure of the boiler the more critical the removal
of foreign items from the feedwater becomes. Large utility boilers operating at 3,000 psig + may
actually use distilled water for ultimate purity.
The purpose of a water softener is primarily for the removal of hardness from the boiler makeup
water. Makeup water is the water supplied from the municipal water system, well water, or
other source for the addition of new water to the boiler system necessary to replace the water
evaporated. Some filtering of the water may occur in the water softener but that is not the
purpose of its design and too much of other pollutants in the water could actually foul the water
softener affecting its operation. Hardness is composed primarily of calcium (Ca) and
magnesium (Mg) but also to lesser amounts sodium (Na), potassium (P), and several other
metals. Hardness is measured in grains with one grain of hardness in the water being 17.1 ppm
of these elements. The purpose of using hardness as the unit of measure is that tests to
measure in parts per million (ppm) are much more difficult and expensive to use. Hardness
varies from area to area. Usually near salt water the hardness is very low as the limestone is
virtually non existent and in mountainous areas where limestone is everywhere hardness is
usually very high.
All softeners soften or remove the hardness from the water. The primary minerals in the water
that make "hard" water are Calcium (Ca++) and Magnesium (Mg++). They form a curd with
soap and scale in piping, water heaters and whatever the hard water contacts. Hardness is
removed from the water by a process known as positive ion exchange. This process could also
be known as "ion substitution", for substitution is what occurs. Sodium (Na+) ions, which are
"soft" are substituted or exchanged for the Calcium and Magnesium as the water passes through
the softener tank.
The softening media is commonly called resin or Zeolite. The proper name for it is polystyrene
resin. The resin has the ability to attract positive charges to itself. The reason it does so is
because in its manufacture it inherits a negative charge. It is a law of nature that opposite
charges attract, i.e., a negative will attract a positive and vice versa. A softener tank contains
hundreds of thousands of Zeolite beads. Each bead is a negative in nature and can be charged
or regenerated with positive ions. In a softener, the Zeolite is charged with positive, "soft" sodium
ions.
As "hard" water passes through the Zeolite, the Calcium and Magnesium ions are strongly
attracted to the beads. As the "hard" ions attach to the Zeolite bead, they displace the "soft"
Sodium ions that are already attached to the bead. In effect, the Sodium is "exchanged" for the Calcium and Magnesium in the water supply with the Calcium and Magnesium remaining on the
Zeolite beads and the Sodium ions taking their place in the water flowing through the softener
tank. The result of this "exchange" process is soft water flowing out of the tank.
It can now be readily understood that a softener will continue to produce "soft" water only as
long as there are Sodium ions remaining on the Zeolite beads to "exchange" with the Calcium
and Magnesium ions in the "hard" water. When the supply of Sodium ions has been depleted,
the Zeolite beads must be "regenerated" with a new supply of Sodium ions. The regeneration of
the Zeolite beads is accomplished by a three step process.
SOFTENER DESIGN:
Water softeners come as single mineral tank units (simplex), double mineral tank units (duplex)
and multiple mineral tank units. Since regeneration cycles can take approximately one hour
simplex units are used only when this interruption can be tolerated. To avoid interruption duplex
units are used so that the regeneration of one unit can be accomplished while the second unit is
on line. Triplex or other multiplex units usually are the result of need for increased capacity and
units can be added to keep soft water available. The reliability of new electronic
metering/controls for regeneration have allowed users to depend on smaller units with more
frequent regeneration.
REGENERATION PROCESS
BACKWASH:
The flow of water through the mineral bed is reversed. The mineral bed is loosened and
accumulated sediment is washed to the drain by the upward flow of the water. An automatic
backwash flow controller maintains the proper flow rate to prevent the loss of resin.
BRINE DRAW AND SLOW RINSE:
Ordinary salt has the capability to restore the exchange capacity of the mineral. A given amount
of salt-brine is rinsed slowly through the mineral bed. After the salt-brine is drawn, the unit will
continue to rinse slowly with water to remove all of the salt-brine from the media bed.
FAST RINSE:
A high down flow of water repacks the mineral bed. Any trace of brine not removed in slow rinse
is flushed to the drain.
The unit is then returned to SERVICE the brine maker is refilled with fresh water to form salt
brine for the next regeneration. The total regeneration time is approximately 60-90 minutes
SOFTENER SIZING FORMULA:
C = M * T * H /R
C = Capacity of softener in cubic feet of resin
M = Makeup water volume per hour in gallons; the volume needed to be softened (8.34 pounds
per gallon)
T = Time in hours desired between regeneration cycles
H = Hardness of water in grains (17.1 ppm per grain hardness)
R = Resin Capacity per cubic foot (this is virtually always 30,000 grains
Distillation is one of many processes that can be used for water purification. This requires an energy input, as heat, solar radiation can be the source of energy. boiler installation Birmingham
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