chapter 06 - electricity for heating

8/11/2019 Chapter 06 - Electricity for Heating

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ELECTRICITY FORELECTRICITY FOR

HEATINGHEATINGbyby

EngrEngr. Alexis T.. Alexis T. BelonioBelonioDepartment of Agricultural Engineering andDepartment of Agricultural Engineering and

Environmental ManagementEnvironmental Management

College of AgricultureCollege of AgricultureCentral Philippine UniversityCentral Philippine University

Iloilo CityIloilo City

[email protected]@yahoo.com


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IntroductionIntroduction

Heat is a form of energy. It is the energyHeat is a form of energy. It is the energy

possessed by a body due to the internalpossessed by a body due to the internalmotion of the bodys molecules.motion of the bodys molecules.

TemperatureTemperatureit is the measure of theit is the measure of the

intensity of heat not the quantity of heat orintensity of heat not the quantity of heat orthe speed of the bodys molecules. It isthe speed of the bodys molecules. It is

measured in degrees Fahrenheit,measured in degrees Fahrenheit,

Centigrade,Centigrade, RankineRankine, or Kelvin, or Kelvin


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MolecularMolecularmotion at highermotion at higher

temperature istemperature is

faster than atfaster than atlowerlower

temperaturetemperature


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Heated medium isHeated medium islighter than coldlighter than coldmediummedium

Heat moves fromHeat moves fromhigher temperature tohigher temperature tolower temperaturelower temperature


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Temperature ConversionTemperature Conversion

FromFrom ToTo UseUse

FahrenheitFahrenheit CentigradeCentigrade C = [5/9] FC = [5/9] F3232

CentigradeCentigrade FahrenheitFahrenheit F = [9/5] C + 32F = [9/5] C + 32

CentigradeCentigrade KelvinKelvin K = CK = C

CentigradeCentigrade RankineRankine R = [4/5] CR = [4/5] C

FahrenheitFahrenheit KelvinKelvin K = 1.8 FK = 1.8 F

FahrenheitFahrenheit RankineRankine R = [4/9] FR = [4/9] F -- 3232


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Quantity of HeatQuantity of Heat

The quantity of heat is measured in BritishThe quantity of heat is measured in British

thermal Unit (BTU)thermal Unit (BTU) BTU is the quantity or heat required toBTU is the quantity or heat required to

raise the temperature of one pound ofraise the temperature of one pound ofwater one degree Fahrenheit. In metricwater one degree Fahrenheit. In metric

system it is 1 kilo calories per kilogramsystem it is 1 kilo calories per kilogram

degree Centigrade.degree Centigrade.


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Heat TransferHeat Transfer

ConvectionConvection heat isheat istransferred by thetransferred by themovement of the heatedmovement of the heated

medium.medium.

ConductionConduction heat isheat is

transferred by means oftransferred by means ofmolecular vibration.molecular vibration.

RadiationRadiation heatheatmovement from one placemovement from one placeto another isto another isaccomplished by meansaccomplished by means

of waves in the mediumof waves in the mediumwhich fills all the space.which fills all the space.


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Advantages of ElectricAdvantages of Electric

HeatingHeating Low initial cost for equipmentLow initial cost for equipment

Easy to install the equipmentEasy to install the equipment

Infrequent and inexpensive repairsInfrequent and inexpensive repairs

Healthy type of heatHealthy type of heat

SafetySafety

CleanlinessCleanliness

QuitenessQuiteness

Compactness of equipmentCompactness of equipment

PortabilityPortability

Absence of residueAbsence of residue

Accurate and remote controlAccurate and remote control

Automatic regulation by a variety of controlsAutomatic regulation by a variety of controls


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Disadvantages of ElectricDisadvantages of Electric

HeatingHeating High operating costHigh operating cost

Loss of heat in case of a power failureLoss of heat in case of a power failure


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Electric HeatingElectric Heating

ElementsElements Open, Immersion, and Strip HeatersOpen, Immersion, and Strip Heaters

Open type heating element is better adaptedOpen type heating element is better adaptedto room and smallto room and small--work space heating than towork space heating than togeneral farm use.general farm use. NichromeNichrome wire or ribbonwire or ribbon

which has the higher resistance per foot ofwhich has the higher resistance per foot oflength is used for the heat source.length is used for the heat source.

Immersion heating elements are designed forImmersion heating elements are designed for

heating liquids. Theheating liquids. The nichromenichrome wire enclosedwire enclosedin a seamless tube is used.in a seamless tube is used.

Strip heater are used or certain farmStrip heater are used or certain farm

application such as drying and curing. Theapplication such as drying and curing. Theelement are available from 150 to 1350 watts.element are available from 150 to 1350 watts.


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Infrared LampsInfrared Lamps

Infrared lamps radiate heat in theInfrared lamps radiate heat in theform of infrared energy. They areform of infrared energy. They aredesigned to fit the standard lampdesigned to fit the standard lamp

socket.socket. Infrared lamps are available in 125Infrared lamps are available in 125--,,

250250--, 375, 375--, and 500, and 500--watt sizes.watt sizes.

Infrared energy does not warm the airInfrared energy does not warm the airbut warms the objects which absorbbut warms the objects which absorbthe radiated waves.the radiated waves.

It is a quick source of heat and isIt is a quick source of heat and isused for many applications such asused for many applications such asdrying paint, warming hands, tools,drying paint, warming hands, tools,

and machinery parts, brooding, andand machinery parts, brooding, andthawing water pipes.thawing water pipes.


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Heating CableHeating Cable

Some of the most popular used of heatingSome of the most popular used of heating

cables includes hotbeds, brooders, preventingcables includes hotbeds, brooders, preventing

the freezing of water pipes, melting ice, coldthe freezing of water pipes, melting ice, coldframes and moisture removal.frames and moisture removal.

Heating cables available in sizes ranging fromHeating cables available in sizes ranging from

109 ft (300 watts) to 1090 ft (3,000 watts).109 ft (300 watts) to 1090 ft (3,000 watts).

Maximum surface temperature ranges fromMaximum surface temperature ranges from

160 to 190 F.160 to 190 F.


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Band HeaterBand Heater

Band heater areBand heater are

designed for specializeddesigned for specialized

constructions, and areconstructions, and are

ideal for every type ofideal for every type of

plastics processingplastics processing

equipment.equipment. Typical watt densitiesTypical watt densities

ranged from 8.5 to 35.7ranged from 8.5 to 35.7

Watts/cmWatts/cm22 and maximumand maximumoperating temperature ofoperating temperature of

480 to 760 C.480 to 760 C.


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Cable HeaterCable Heater

The cable heater can beThe cable heater can beformed into a variety offormed into a variety ofshapes as dictated by itsshapes as dictated by itsmany applications. Thesemany applications. Thesesmall diameter, highsmall diameter, highperformance units are fullyperformance units are fully

annealed and readily bentannealed and readily bentto the desiredto the desiredconfiguration.configuration.

Typical density is 4.6Typical density is 4.6watt/cmwatt/cm22 with maximumwith maximumoperating temperature ofoperating temperature of650 C.650 C.

Applications includes foodApplications includes foodprocessing and airprocessing and airheating.heating.

C i H tC i H t


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Ceramics HeaterCeramics Heater

It integrate a high temperatureIt integrate a high temperatureironiron--chromechrome--aluminum (ICA)aluminum (ICA)heating element wire withheating element wire withceramic fiber insulation. Itsceramic fiber insulation. Its

insulation isolates the highinsulation isolates the hightemperatures inside the heatedtemperatures inside the heatedchamber from the outside. Thechamber from the outside. Theheaters are low mass, fastheaters are low mass, fastheating, with high insulationheating, with high insulationvalues and selfvalues and self--supportedsupportedheating elements that offerheating elements that offer

some of the highestsome of the highesttemperature heatingtemperature heatingcapabilities.capabilities.

Typical watt density is 1.5Typical watt density is 1.5

watt/cmwatt/cm22 at max temperature ofat max temperature of1205 C.1205 C.


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Flexible HeaterFlexible Heater

Flexible heaters are thin,Flexible heaters are thin,

bendable and shaped to fitbendable and shaped to fit

equipment. It can beequipment. It can be

applied in most complexapplied in most complex

shapes geometries withoutshapes geometries without

sacrificing efficiency orsacrificing efficiency ordependability.dependability.

Typical density ofTypical density of

application ranged fromapplication ranged from1.7 to 17.0 watts/cm1.7 to 17.0 watts/cm22 withwith

maximum operatingmaximum operating

temperature of 120temperature of 120--595 C.595 C.


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MultiMulti--Coil HeaterCoil Heater It combines multipleIt combines multiple

coils andcoils andthermocouples, creatingthermocouples, creating

a tubular heater witha tubular heater withprecise temperatureprecise temperaturemeasurement and quickmeasurement and quickinstallation. This typeinstallation. This type

can can be formed intocan can be formed intomost configurations tomost configurations tofit your heatingfit your heating

requirement.requirement. Sheath temperatures canSheath temperatures can

vary up to 648Cvary up to 648C(1200F) with maximum(1200F) with maximum

watt densities up to 9.3watt densities up to 9.3W/cmW/cm22 (60 W/in(60 W/in22).).


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Strip HeaterStrip Heater Strip heaters are theStrip heaters are the

versatile solution for aversatile solution for anumber of applications.number of applications.They can be bolted orThey can be bolted orclamped to a solid surfaceclamped to a solid surfacefor freeze and moisturefor freeze and moisture

protection, food warmingprotection, food warmingand other applications.and other applications.They are commonly usedThey are commonly usedfor air heating, dryingfor air heating, drying

ovens, and space heaters.ovens, and space heaters. They are available from 7.8They are available from 7.8

to 15.5 Watts/cmto 15.5 Watts/cm22 withwith

maximum operatingmaximum operatingtemperature of 480temperature of 480--760 C760 C


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Tubular HeaterTubular Heater

Tubular heater elements areTubular heater elements aredesigned primarily for directdesigned primarily for directimmersion in liquids such asimmersion in liquids such aswater, oils, solvents and processwater, oils, solvents and processsolutions, molten materials assolutions, molten materials aswell as air and gases. Bywell as air and gases. Bygenerating all the heat within thegenerating all the heat within the

liquid or process, these heatersliquid or process, these heatersare virtually 100 percent energyare virtually 100 percent energyefficient. These versatile heatersefficient. These versatile heaterscan also be formed and shapedcan also be formed and shaped

into various geometries forinto various geometries forradiant heating and contactradiant heating and contactsurface heating applications.surface heating applications.

Typical watt density is 18.6Typical watt density is 18.6W/cm with sheath temperatureW/cm with sheath temperatureof 400of 400--815 C.815 C.

A li i f El i H i


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Applications of Electric HeatingApplications of Electric Heating

ElementsElements Animal BroodingAnimal Brooding

such as chicks, litter,such as chicks, litter,etc.etc.

Poultry IncubationPoultry Incubation

for hatching eggsfor hatching eggs


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Electric FurnaceElectric Furnace

Electric DryerElectric Dryer


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Water HeatingWater Heating

Space HeatingSpace Heating


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Wattage Requirement for Farm HeatingWattage Requirement for Farm Heating

ApplicationApplication

ApplicationApplication Outside TemperatureOutside Temperatureabove 30 Fabove 30 F

Outside TemperatureOutside Temperaturebelow 30 Fbelow 30 F

Brooding chicks,Brooding chicks,

infraredinfrared2.52.5--3 watts per chick3 watts per chick 33--4 watts per chick4 watts per chick

Brooding chicks,Brooding chicks,

underheatunderheat1.51.5--2 watts per chick2 watts per chick 22--2.5 watts per chick2.5 watts per chick

Pig BrooderPig Brooder 250 watts per litter250 watts per litter 250 watts per litter250 watts per litter

HotbedsHotbeds 99--10 watts per sq ft10 watts per sq ft 99--10 watts per sq ft10 watts per sq ft

Stock wateringStock watering 1 watt per gal1 watt per gal


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Control for Electric HeatingControl for Electric Heating

ThermostatThermostatIt is a device that isIt is a device that is

automatically switch off the heatingautomatically switch off the heatingelement after a certain temperature level iselement after a certain temperature level is

reached.reached.

Bellow-Type Wafer-Type


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Sample Installation of Thermostat


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Heat ConversionHeat Conversion

1 kW = 3412.2 BTU/hr

= 859.87 Kcal/hr

1 kW = 3600 kJ/hr

= kJ/sec

1 BTU = 251.98 Cal

= 1.055 kJ

1 kCal = 4.1668 kJ

1 W = 860 Cal/hr

1 BTU/hr-ft-F = 1.488 kCal/hr-m-C= 0.0173 Watts/cm-C

A l t i d i h t f 5 000 BTU/h


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An electric dryer requires a heat energy of 5,000 BTU/hr.If the drying system efficiency is 46%, what is electrical

energy to be supplied in the dryer?

Given: Heat required- 5,000 BTU/hr

Drying system efficiency 46%Required: Electrical heat energy required

Solution: Qe = Qh / d

= [5,000 BTU/hr]/0.46

= 10,869 BTU/hr /3412.2 BTU/hr-kw

= 3.186 kW

Heat Energy Required to RaiseHeat Energy Required to Raise


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Heat Energy Required to RaiseHeat Energy Required to Raise

the Temperature of a Liquidthe Temperature of a Liquid

Qs = m cp [Tf Ti] / twhere:

Qs - sensible heat, kcal/hrm - mass of liquid, kg

cp - specific heat of a liquid, kcal/kg-C

Tf - final temperature of liquid, C

Ti - initial temperature of liquid, C

t - time, hr

Ten kg of water initially from 27 C will be boiled


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Ten kg of water initially from 27 C will be boiledto 100 C in 10 minutes. What is the electrical

energy needed in watts to raise the temperatureof water.

Given: weight of water - 10 kg

initial water temperature - 27 C

final weight of water temperature - 100 C

time to heat water - 10 min

Required: Electric energy requirement

Solution: Qh = 10 kg[1kCal/kg-C][100C-27C]/10min

= 73 kCal/min x 60min/hr

= 4380 kCal/hr

Qe = [4380 kCal/hr] / [859.87 Kcal/hrkW]

= 5.09Kw

Heat Energy Required toHeat Energy Required to


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Heat Energy Required toHeat Energy Required to

Vaporize LiquidVaporize Liquid

Qe = Wf x Hfg

where:

Qe - heat energy to vaporize liquid, kCal/hr

Wf - weight of fluid, kg/hr

Hfg - heat of vaporization of liquid, kCal/kg

Hfg for water= 540 kCal/kg

Five kilograms of water is to be boiled from 27 C to 100 C in


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Five kilograms of water is to be boiled from 27 C to 100 C in10 minutes. Of the total volume of water 60% will besuccessively evaporated in 20 minutes. What is the

electrical heat energy in watts required to raise thetemperature successively evaporate the water?

Given: Weight of water- 5 kg Weight to evaporate - 60%

Initial temp - 27 C Time to evaporate - 20 min

Final temp - 100 C

Time to boil - 10 min

Required:Electrical heat energy to boil and evaporate the waterSolution:

Qb = [(5kg)(1kCal/kg-C)(100C-27C)]/

[10 min(60min/hr) 859.87 Kcal/hr-kW]= 2.547 kW

Qe = [5 kg (0.6)540 kCal/kg (60 min/hr)/[10 min

(859.87 kCal/hr-kW)]

= 11.30 kW

Determine the electrical wattage requirement of an incubatorwhose dimension is 2 ft W x 4 ft L x 1.5 ft H. The incubator


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whose dimension is 2 ft W x 4 ft L x 1.5 ft H. The incubatoruses a 10 mm thick plywood (K=0.129 kCal/hr-m-C) Assume20% heat loss due to infiltration on walls and doors of the

incubator.Given: Dimension - 2 ft w x 4 ft L x 1.5 ft H

Wall thickness - 10 mm

Material - plywood with K = 0.129 kCal/hr-m-C

Infiltration - 20%

Incubator Temp - 37.5 C

Ambient Temp - 28 C

Required: Electrical wattage requirement

Solution:

Qk = A K [Ti To] / x

A = 2[2x4]+2[1.5x4]+2[1.5x2] = [16ft2+12ft2+6ft2

= 34 ft2x [3.28 ft/m]2 = 3.16 m2

Qk = 3.16 m2x 0.129 kCal/hr-m-C(37.5C-28C)/0.01 m

= 387.258 kCal/hr

Qt = 387.258 kCal/hr (1.20) = 464.71 kCal/hr

Electrical Wattage = 464.71 kCal/hr/859.87 kCal/hr-kW

= 0.540 kilowatts

R fR f


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References:References:

Brown, R. 1956. Farm Electrification. McGrawBrown, R. 1956. Farm Electrification. McGraw--HillHill

Book Company. New York. 367pp.Book Company. New York. 367pp.

JakobJakob, M and G. A. Hawkins. 1956. Elements of Heat, M and G. A. Hawkins. 1956. Elements of Heat

Transfer. 3Transfer. 3rdrd Edition. John Wiley and Sons, Inc. NewEdition. John Wiley and Sons, Inc. New

York. 317pp.York. 317pp.

chapter 06 - electricity for heating

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