Embed Size (px)
Citation preview
htt
p:\
\ww
w.In
dia
nJo
urn
als.
com
A p
rod
uct
of
Div
an E
nte
rpri
ses
Do
wn
load
ed F
rom
IP -
203
.199
.182
.67
on
dat
ed 2
1-D
ec-2
009
The Bulletin on Energy Efficiency June 2005 Vol 5 Issue 6 12
IntroductionInsulation is defined as those materialsor combinations of materials, whichretard the flow of heat energy byperforming one or more of the followingfunctions:� Conserve energy by reducing heat
loss or gain.� Control surface temperatures for
personnel protection and comfort.� Facilitate temperature control of a
process.� Prevent vapor flow and water
condensation on cold surfaces.� Increase operating efficiency of
hea t ing /ven t i l a t i ng /coo l ing ,plumbing, steam, process andpower systems found in commercialand industrial installations.
� Prevent or reduce damage toequipment from exposure to fire orcorrosive atmospheres.
� Assist mechanical systems inmeeting food and drugs standardscriteria in food and cosmetic plants.
Resistance to heat is only one ofthe several criteria to be consideredwhile selecting an insulation material.The choice is generally a compromisebetween the total cost of insulation andits installation. However, due toincreasing cost of energy a newparameter is gaining importance inmaking a decision. This is called life-cycle cost based on the initial cost plusmaintenance and operating cost for areasonable life expectancy rather thaninitial low cost.
This article deals with makingdecisions on selection of insulatingmaterial based on various criteria.Before considering the economiccriteria, the insulation material mustpass through the technical criteria.
Technical Criteria for InsulationMaterialThe basic technical criteria in selectinga suitable insulating material for aparticular service are to meet minimum
Insulation helps in efficient use of energysafety and process conditions. To meetthe criteria material of a particular type,form and properties of the material isselected.
Types
Fibrous InsulationThis insulation is composed of smalldiameter fibers, which finely divide theair space. The fibers may beperpendicular or horizontal to thesurface being insulated and they mayor may not be bonded together. Silica,rock wool, slag wool and alumina silicafibers are used. The most widely usedinsulations of this type are glass fiberand mineral wool.
Cellular InsulationThis insulation is composed of smallindividual cells separated from eachother. The cellular material may beglass or foamed plastic such aspolystyrene (closed cell), polyurethane,polyisocyanurate, polyolefin, andelastomers.
Granular InsulationThis insulation is composed of smallnodules, which contain voids or hollowspaces. It is not considered a truecellular material since gas/air can betransferred between the individualspaces. This type may be produced asa loose or pourable material, orcombined with a binder and fibers tomake a rigid insulation. Examples ofthese insulations are calcium silicate,expanded vermiculite, perlite, cellulose,diatomaceous earth, and expandedpolystyrene.
This insulation is produced in avariety of forms suitable for specificfunctions and applications. Thecombined form and type of insulationdetermine its proper method ofinstallation. The forms most widelyused are:� Rigid boards, blocks, sheets, and
pre-formed shapes such as pipe
insulation, curved segment, lagging,etc: Cellular, granular, and fibrousinsulations are produced.
� Flexible sheets and pre-formedshapes: Cellular and fibrousinsulations are produced.
� Flexible blankets: Fibrousinsulations are produced.
� Cements (insulating and finishing):Produced from fibrous and granularinsulations and cement, they maybe of the hydraulic setting or air-drying type.
� Foam: Poured or froth foam used tofill irregular areas and voids. Sprayused for flat surfaces.
Properties� Conductivity: The most importantproperty of an insulation – lower thevalue of conductivity less will be thethickness of the insulation. In manyapplications, thermal conductivity ofmass insulation is a combination of thefollowing heat transfer mechanisms:thermal conductivity of air, thermalconductivity of the solid structure ofinsulating material, convective heattransfer within the pore structure, theradiation heat transfer within thestructure and interaction of thesemechanisms. Other derived propertybased on conductivity is conductance,which is a measure of rate of heat flowfor a particular thickness of insulationmaterial.� Temperature limitations: Insulating
The basic technical criteriain selecting a suitable
insulating material for aparticular service are to
meet minimum safety andprocess conditions. To
meet the criteria materialof a particular type, formand properties of
the material isselected
TECHNOLOGY – I
htt
p:\
\ww
w.In
dia
nJo
urn
als.
com
A p
rod
uct
of
Div
an E
nte
rpri
ses
Do
wn
load
ed F
rom
IP -
203
.199
.182
.67
on
dat
ed 2
1-D
ec-2
009
The Bulletin on Energy Efficiency June 2005 Vo l 5 Issue 6 13
materials are normally classifiedaccording to service-temperatureranges they are suitable for. However,at many times they tend to be arbitrary.When operating temperatures reach acertain upper limit, the materials maybecome structurally unstable orbecome uncompetitive because of arelatively high conductivity. Within eachtemperature range choice amongmaterial available is generally based onother properties and cost. Normallytemperature ranges and types ofinsulation materials used are as follows:Cryogenic range (–270 0C to –75 0C):In this range, insulations fall within twotypes, vacuum and massive, the latterconsisting of one or more solid phasedistributed with a gas like dry air.Low temperature range (–75 0C to100 0C): At the lower end of the range,foam insulations are used more. Themain problems are moisturepermeation and fire hazards. Since thecost of refrigeration is greater than thecost of heating, more insulation is oftenjustified. At the upper end of the range,insulation is normally used for safetyand personal protection.Intermediate range (100 0C to550 0C): This is normally encounteredin most chemical process industriesand steam systems. The mostimportant type of insulation in thisrange include:� Calcium silicates� Diatomaceous silica� Cellular glass (up to 450 0C)� Glass fibers bonded with high
temperature binders� Magnesium carbonate with
asbestos or other fibers and binders(up to about 300 0 C)
� Rock wool or mineral derived fibers� Expanded perlite with binders
Selection of insulation material inthis temperature range is stronglydictated by value of thermalconductivity, mechanical properties,forms available, and the cost ofinstallation.High temperature range (550 0C andabove): This range approaches therefractory range of materials. The
materials used in this range are:� Mineral fiber: 550 to 1,000 0C� Calcium silicate: 650 to 1,100 0C� Ceramic fibers based on Al2O3-SiO2
systems: 850 to 1,450 0C� Castable ceramic insulating
refractories: 1,000 to 1,650 0C� Oxide fibers primarily Al2O3 or ZrO2
:
1,500 to 1,650 0C� Rigid ceramic insulating brick: 1,000
to 1,750 0C.� Carbon fibers: up to 2,000 0 C.
� Thermal-shock resistance: Theinsulation material should resist theattack of heat particularly at hightemperatures.� Coefficient of expansion: This isrequired to be considered whilecreating the design and spacing ofexpansion/contraction joints and/or theuse of multiple layer insulationapplications.
chemicals are present. Corrosionresistance must also be considered.Particularly chloride concentration ofinsulating material has to be below theacceptable limit in case of use in SSpiping or vessels.� Environmental resistance: Thisfactor is significant when theatmosphere is salty or chemical laden.� Alkalinity (pH or acidity): It isimportant when corrosive atmospheresare present. Also, insulation must notcontribute to corrosion of the theItsystem.� Density and strength: In a numberof applications, densities of aninsulating material containing solidstructure can be modified to get asuitable apparent conductivity (“ka”) bycreating dead air spaces (porosity). Butmore porosity reduces strength of theinsulating material such as insulatingfirebricks. Therefore, a carefulassessment of strength and apparent“ka” required is done with data providedby insulation manufacturers.� Appearance: It is important inexposed areas and for codingpurposes.� Capillarity: It must be consideredwhen material may be in contact withliquids.� Fire retardancy: Flame spread andsmoke developed ratings of thematerial should be considered.� Combustibility: One of the measuresof a material’s contribution to a firehazard has to be taken into account.� Dimensional stability: It hassignificance when the material isexposed to atmospheric andmechanical abuse such as twisting orvibration from thermally expandingpipe.� Hygroscopicity: Tendency of amaterial to absorb water vapor from theair needs to be considered.� Resistance to ultraviolet light: It isimportant if application is outdoors.� Resistance to fungal or bacterialgrowth: It is necessary in food orcosmetic process areas.� Shrinkage: Its significance inapplications involving cements andmastics.
In a number ofapplications, densities of
an insulating materialcontaining solid structurecan be modified to get a
suitable apparentconductivity (“ka”) by
creating dead air spaces(porosity). But more
porosity reduces strengthof the insulatingmaterial such as
insulating firebricks
� Abrasion and erosion resistance: Incase of refractory materials erosionoccurs more rapidly in turbulent zonesrather than non-turbulent zones.� Compressibility: it is Important if theinsulation must support a load orwithstand mechanical abuse withoutcrushing. If, however, cushioning orfilling in space is needed as inexpansion/contraction joints, lowcompressive strength materials arespecified.� Chemical resistance: Leakagesfrom fluid should not affect theinsulating material. Also, potential firehazards exist in areas where volatile
TECHNOLOGY – I
htt
p:\
\ww
w.In
dia
nJo
urn
als.
com
A p
rod
uct
of
Div
an E
nte
rpri
ses
Do
wn
load
ed F
rom
IP -
203
.199
.182
.67
on
dat
ed 2
1-D
ec-2
009
The Bulletin on Energy Efficiency June 2005 Vol 5 Issue 6 14
� Sound absorption coefficient: Itmust be considered when soundattenuation is required, as it is in radiostations, some hospital areas, etc.� Sound transmission loss value: It isimportant when constructing a soundbarrier.� Toxicity: It must be considered infood processing plants and potentialfire hazard areas.� Ease of application: Installation ofinsulating material on valves, pipe-fittings should be as easy as possible.This is important in case of thermalinsulation.
Not all properties are significant forall materials or applications. Therefore,many are not included in themanufacturer’s published literature. Insome applications, however, omittedproperties may assume extremeimportance (i.e. when insulations mustbe compatible with chemicallycorrosive atmospheres.)
If the property is significant for anapplication and the measure of thatproperty cannot be found in themanufacturer’s literature, effort shouldbe made to obtain the informationdirectly from the manufacturer, testinglaboratory, or insulation contractorsassociation.
While selecting insulations ontechnical grounds, it is possible tomake use of combinations – layers ofdifferent insulating materials to getoptimum size, weight and damageresistance. Some insulations areavailable in a prefabricated form forpiping, valves and fittings. It is possibleto reduce application/installation costof insulation by procuring prefabricatedforms.
In case of thermal insulations, thebarrier system must be effective. Itshould allow little or no water leakage.Any penetration of water or for thatmatter any other liquid seriouslydiminishes its insulating properties tillit dries out. In case of steam turbines,sometimes due to an ineffect ivebarrier system, insulation material onsteam lines get soaked in oil due toleakage in lube oil system and posefire hazards. Jacketing of insulating
material protects the insulat ionagainst mechanical abuse as well asagainst weather and other damage.Consider corrosion resistance asimportant in selection of the jacketingmaterial.
Economic CriteriaIt is based on calculation of the amountof insulation that would save enoughenergy over a set period of time tojustify the investment. Economicthickness calculations based onequations first published in 1926 havebeen refined over the years taking intoconsideration new inputs,particularly with respect to methodsfor developing investment returnanalyses. Interest in economicthickness increased more with therise in energy costs. The earliercookbook approach based on somedesign standards was prevalent.However, insulation costs today arenot considered as expenses but asinvestment. Therefore, methods usedin calculating the economic feasibilityof a project are also used indetermining the economic thickness ofthe insulation.
Basic Model and Theory inEconomic Insulation CalculationThe most basic model for insulation ona pipe is shown in the figure above (1).R1 and R2 show the inside and outsideradius of the pipe, respectively. R3shows the radius of the insulation.Typically when dealing with insulations,engineers must look at linear heat lossor heat loss per unit length.
Generally, the heat transfercoefficient of ambient air is about 40W/m2 K. This coefficient can of courseincrease with wind velocity if the pipeis outside. The total heat loss per unitlength can then be calculated by:
Q = 2πR3U∆T (2)Lwhere ∆T = Tinsidepipe - Tambient
SafetyPipes that are readily accessible byworkers are subject to safetyconstraints. The recommended safe“touch” temperature range is from 550C to 65 0C. Insulation calculationsshould aim to keep the outsidetemperature of the insulation around
U = 1 (1)R3 +R3 LN (R2/R1)+R3 LN (R3/R2)+ 1R1 hi kpipe kinsulation ho
U = 1R4 + R4 LN (R2/R1) + R4 LN (R3/R2) + R4 LN (R4/R3) + 1R1 hi kpipe kinsulation kahminum ho
and Equation 2 becomes:
Q = 2πR4 U ∆T (3)Lwhere R4 is the radius of the pipe, insulation, and aluminum cover combined
TECHNOLOGY – I
htt
p:\
\ww
w.In
dia
nJo
urn
als.
com
A p
rod
uct
of
Div
an E
nte
rpri
ses
Do
wn
load
ed F
rom
IP -
203
.199
.182
.67
on
dat
ed 2
1-D
ec-2
009
The Bulletin on Energy Efficiency June 2005 Vo l 5 Issue 6 15
Since the heat loss is constant foreach layer, use Equation 4 to calculateQ from the bare pipe, then solveEquation 6 for T4 (surfacetemperature). Use the economicthickness of your insulation as a basisfor your calculation, after all, if the mostaffordable layer of insulation is safe,that’s the one you’d want to use. If theeconomic thickness results in too higha surface temperature, repeat thecalculation by increasing the insulationthickness by half an inch each time untila safe touch temperature is reached.
As can be seen, using heat balanceequations is certainly a valid means ofestimating surface temperatures, but itmay not always be the fastest. Chartsare available that utilize a characteristiccalled “equivalent thickness” to simplifythe heat balance equations. Thiscorrelation also uses the surface resis-tance of the outer covering of the pipe
Economic thickness of insulation isa well-documented calculationprocedure. The calculations typicallytake in the entire scope of theinstallation including plant depreciationto wind speed. Data charts forcalculating the economic thickness ofinsulation are widely available.
Economic thickness for sameservice will differ due to a number offactors, which may be consideredduring evaluation. These factors are:� Type and cost of fuel� Heating value of fuel� Expected annual fuel price increase� Efficiency of conversion of fuel to
heat� Capital investment for heat plant� Cost of investment to finance the
plant� Heating equipment depreciation
period� Expected annual heat production� Income tax rate� Insulation depreciation period� Insulating material� Jacket emmisivity� Surface resistance� Installed insulation prices/rates� Pipe size� Piping complexity factor� Average ambient temperature to be
Q = T1-T2 = T2-T3 = T3-T4 (R2-R1)/(kpipeALMpipe) (R3-R2)/(kinsALMins) (R4-R3)/(kinscoverALMinscover)
where:ALMpipe = (2π R2 L) - (2π R1 L),
LN 2π R2 L2π R1 L
ALMins = (2π R3 L) - (2π R2 L),
LN 2π R3 L2π R2 L
ALMinscover = (2π R4 L) - (2π R3 L),
LN 2π R4 L 2π R3 L
Rearranging Equation 4 by solving the three expressions for the temperaturedifference yields:
Q = T1-T4
(R2-R1) + (R3-R2) + (R4-R3) (kpipeALMpipe) (kinsALMins) (kinscoverALMinscover)
{ }
{ }
{ }
{ { {} } }
(4)
(5)
Each term in the denominator ofEquation 5 is referred to as the“resistance” of each layer. This can bedefined as Rs and rewrite the equationas:
Q = T1-T4 (Rspipe + Rsins + Rsinscover
(6)
60 0C. An additional tool employed tohelp meet this goal is aluminumcovering wrapped around the outsideof the insulation. Aluminum’s thermalconductivity of 209 W/m K does notoffer much resistance to heat transfer,but it does act as another resistancewhile also holding the insulation inplace. The typical thickness ofaluminum used for this purpose rangesfrom 0.2 mm to 0.4 mm. The additionof aluminum adds another resistanceterm to Equation 1 when calculating thetotal heat loss.
However, when considering safety,
engineers need a quick way tocalculate the surface temperature thatwill come into contact with the workers.This can be done with equations or theuse of charts. This can be done bylooking at another diagram (see above):
TECHNOLOGY – I
htt
p:\
\ww
w.In
dia
nJo
urn
als.
com
A p
rod
uct
of
Div
an E
nte
rpri
ses
Do
wn
load
ed F
rom
IP -
203
.199
.182
.67
on
dat
ed 2
1-D
ec-2
009
The Bulletin on Energy Efficiency June 2005 Vol 5 Issue 6 16
considered in calculating economicthickness
� Process temperature� Annual operating hours� Previous thickness if specified
To calculate the economicinsulation thickness considering all theabove factors, computer models inExcel spreadsheets are used. It ispossible to simplify computer modelsignoring factors having less impact.Some simple methods with an exampleare presented below.
For the sake of demonstratingapplications of different methods, asystem where present investment oninsulation based on standards forsafety and process requirements istaken at Rs 200,000. By increasingthickness in steps to arrive at theeconomic thickness, the investment isRs 250,000. Annual fuel requirements-existing at Rs 40,000 and for economicthickness at Rs 30,000 are considered.The life of insulation is taken as 20years. The cost of capital investmentin insulation is considered as 10percent.
Payback period: It is essentially thetime required to repay the capitalinvestment with savings accruedthrough operation.
The above method does not takeinto account the time value of money.Also it does not take into account cashinflows beyond the payback period.
Investment is done first and benefitsaccrued later over a number of years.Therefore, methods based on timevalue of money are more appropriatelyused in calculating the economicinsulation. Minimum annual cost andmaximum net present value aremethods that are normally used.
Minimum annual cost: In this methoddiscounted (at an assigned rate)annualized cost of insulation and thecosts of heat lost through the insulationare considered to arrive at the minimumannual cost. See tables for example:
Basis� Investment for standard insulation
thickness: Rs. 200,000� Investment for economic insulation
thickness: rs. 250,000� Life of insulation: 20 years
ConclusionThe Net present value of cost ateconomic insulation thickness is lessthan at standard thickness. Therefore,it is the better of the two investmentoptions.
It may be noted that in the annuliseddiscounted cost and NPV method,depreciation is not considered as it isnot cash outflow.
There are computer programs
Thickness Based Economic Differenceon Current Standard Thickness (Incremental)
Insulation investment, Rs. 200,000 250,000 50,000Annual fuel cost, Rs. 40,000 30,000 10,000
Simple payback period will be 5 years (50,000/10,000).
Item @std. Current @Economic CommentsThickness Thickness
Annual energy cost 40,000 30,000Insulation depreciation 10,000 12,500 20,000/20 and250,000/20Annual cash cost 30,000 17,500 (1) – (2)Capital recovery factor for 0.1175 0.1175 1/ 8.51420 years* @10%Equivalent annual insulation 23,491 29,363 200,000 x 0.1175cost and
250,000 x.1175Total Annual cost 53,491 46,863 (3) plus (5)
Conclusion: The lowest annual cost (Rs 46,863) is better of the two investment options.
Maximum Net present value
@Std. @ Economic CommentsThickness Ins.
Thickness
Annual energy cost 40,000 30,000Annual Ins. depreciation 10,000 12,500Annual cash cost 30,000 17,500 (1) – (2)Net present value factor for 20 years 8.514 8.514Present value of annual cash cost 255,420 148,995Initial cost of investment 200,000 250,000Net present value of cost 455,420 398,995 (5) plus
(6)
available based on this method. To findout economic insulation thickness, theprogram increases the insulationthickness from the current thicknessand finishes the finding when theinsulation thickness reaches themaximum thickness of the last layer.
During the thickness increase theselected economic insulation thicknessis the thickness of one step beforewhere the annual total cost changesfrom decrease to increase.
Courtesy: Ramchandra V. Nesari, Dy.General Manager (Chemicals),Rashtriya Chemicals & Fertilizers LtdE-mail: [email protected]
TECHNOLOGY – I
