Overall Heat Transfer Coefficient PDF

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4/23/2021

Overall Heat Transfer Coefficient

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Overall Heat Transfer Coefficient Calculate overall heat transfer coefficients for walls or heat exchangers

Heat transfer through a surface like a wall can be calculated as q = U A dT

(1)

where q = heat transfer (W (J/s), Btu/h) U = overall heat transfer coefficient (W/(m2K), Btu/(ft2 h oF)) A = wall area (m2, ft2) dT = (t1 - t2) = temperature difference over wall (oC, oF) The overall heat transfer coefficient for a multi-layered wall, pipe or heat exchanger - with fluid flow on each side of the wall - can be calculated as 1 / U A = 1 / hci Ai + Σ (sn / kn An) + 1 / hco Ao

(2)

where https://www.engineeringtoolbox.com/overall-heat-transfer-coefficient-d_434.html

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U = the overall heat transfer coefficient (W/(m2 K), Btu/(ft2 h oF)) kn = thermal conductivity

of material in layer n (W/(m K), Btu/(hr ft °F))

hc i,o = inside or outside wall individual fluid convection heat transfer

coefficient (W/(m2

K), Btu/(ft2 h oF)) sn = thickness of layer n (m, ft) A plane wall with equal area in all layers - can be simplified to 1 / U = 1 / hci + Σ (sn / kn) + 1 / hco Thermal conductivity - k - for some typical materials

(3) (not that conductivity is a property that may

vary with temperature) Polypropylene PP : 0.1 - 0.22 W/(m K) Stainless steel : 16 - 24 W/(m K) Aluminum : 205 - 250 W/(m K) Convert between Metric and Imperial Units 1 W/(m K) = 0.5779 Btu/(ft h oF) 1 W/(m2 K) = 0.85984 kcal/(h m2 oC) = 0.1761 Btu/(ft2 h oF) Conductive heat transfer Thermal conductivity of commonly used materials The convection heat transfer coefficient - h - depends on type of fluid - if its gas or liquid flow properties like velocity other flow and temperature dependent properties Convective heat transfer coefficient for some common fluids: Air - 10 to 100 W/m2K Water - 500 to 10 000 W/m2K

Multi-layered Walls - Heat Transfer Calculator

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This calculator can be use to calculate the overall heat transfer coefficient and the heat transfer through a multi-layered wall. The calculator is generic and can be used for metric or imperial units as long as the use of units is consistent. 2

A - area (m2, ft2)

100

t1 - temperature 1 (oC, oF)

20

t2 - temperature 2 (oC, oF)

50

hci - convective heat transfer coefficient inside wall (W/(m2 K), Btu/(ft2 h oF))

0.0001

s1 - thickness 1 (m, ft)

0.1

k1 - thermal conductivity 1 (W/(m K), Btu/(hr ft °F))

0

s2 - thickness 2 (m, ft)

1

k2 - thermal conductivity 2 (W/(m K), Btu/(hr ft °F))

0

s3 - thickness 3 (m, ft)

1

k3 - thermal conductivity 3 (W/(m K), Btu/(hr ft °F))

50

hco - convective heat transfer coefficient outside wall (W/(m2 K), Btu/(ft2 h oF))

Calculate!

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Heat Transfer Thermal Resistance

Heat transfer resistance can be expressed as R=1/U

(4)

where R = heat transfer resistance (m2K/W, ft2 h°F/ Btu) The wall is split in sections of thermal resistance where the heat transfer between the fluid and the wall is one resistance the wall it self is one resistance the transfer between the wall and the second fluid is a thermal resistance Surface coatings or layers of "burned" product adds extra thermal resistance to the wall decreasing the overall heat transfer coefficient. Some typical heat transfer resistances static layer of air, 40 mm (1.57 in) : R = 0.18 m2K/W inside heat transfer resistance, horizontal current : R = 0.13 m2K/W outside heat transfer resistance, horizontal current : R = 0.04 m2K/W inside heat transfer resistance, heat current from down upwards : R = 0.10 m2K/W outside heat transfer resistance, heat current from above downwards : R = 0.17 m2K/W

Example - Heat Transfer in Air to Air Heat Exchanger An air to air plate exchanger with area 2 m2 and wall thickness 0.1 mm can be made in polypropylene PP, aluminum or stainless steel. The heat transfer convection coefficient for air is 50 W/m2K. Inside temperature in the exchanger is 100 oC and outside temperature is 20 oC. The overall heat transfer coefficient U per unit area can be calculated by modifying (3) to U = 1 / (1 / hci + s / k + 1 / hco)

(3b)

The overall heat transfer coefficient for heat exchanger in

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polypropylene with thermal conductivity 0.1 W/mK is UPP = 1 / (1 / (50 W/m2K) + (0.1 mm) (10-3 m/mm)/ (0.1 W/mK) + 1 / (50 W/m2K)) = 24.4 W /m2K The heat transfer is q = (24.4 W /m2K) (2 m2) ((100 oC) - (20 oC)) = 3904 W = 3.9 kW stainless steel with thermal conductivity 16 W/mK: USS = 1 / (1 / (50 W/m2K) + (0.1 mm) (10-3 m/mm)/ (16 W/mK) + 1 / (50 W/m2K)) = 25 W /m2K The heat transfer is q = (25 W /m2K) (2 m2) ((100 oC) - (20 oC)) = 4000 W = 4 kW aluminum with thermal conductivity 205 W/mK: UAl = 1 / (1 / (50 W/m2K) + (0.1 mm) (10-3 m/mm)/ (205 W/mK) + 1 / (50 W/m2K)) = 25 W /m2K The heat transfer is q = (25 W /m2K) (2 m2) ((100 oC) - (20 oC)) = 4000 W = 4 kW 1 W/(m2 K) = 0.85984 kcal/(h m2 oC) = 0.1761 Btu/(ft2 h oF)

Typical Overall Heat-Transfer Coefficients Free Convection Gas - Free Convection Gas : U = 1 - 2 W/m2K (typical window, room to outside air through glass) Free Convection Gas - Forced liquid (flowing) water : U = 5 - 15 W/m2K (typical radiator central heating) https://www.engineeringtoolbox.com/overall-heat-transfer-coefficient-d_434.html

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Free Convection Gas - Condensing Vapor Water : U = 5 - 20 W/m2K (typical steam radiators) Forced Convection (flowing) Gas - Free Convection Gas : U = 3 - 10 W/m2K (superheaters) Forced Convection (flowing) Gas - Forced Convection Gas : U = 10 - 30 W/m2K (heat exchanger gases) Forced Convection (flowing) Gas - Forced liquid (flowing) water : U = 10 - 50 W/m2K (gas coolers) Forced Convection (flowing) Gas - Condensing Vapor Water : U = 10 - 50 W/m2K (air heaters) Liquid Free Convection - Forced Convection Gas : U = 10 - 50 W/m2K (gas boiler) Liquid Free Convection - Free Convection Liquid : U = 25 - 500 W/m2K (oil bath for heating) Liquid Free Convection - Forced Liquid flowing (Water) : U = 50 - 100 W/m2K (heating coil in vessel water, water without steering), 500 - 2000 W/m2K (heating coil in vessel water, water with steering) Liquid Free Convection - Condensing vapor water : U = 300 - 1000 W/m2K (steam jackets around vessels with stirrers, water), 150 - 500 W/m2K (other liquids) Forced liquid (flowing) water - Free Convection Gas : U = 10 - 40 W/m2K (combustion chamber + radiation) Forced liquid (flowing) water - Free Convection Liquid : U = 500 - 1500 W/m2K (cooling coil stirred) Forced liquid (flowing) water - Forced liquid (flowing) water : U = 900 - 2500 W/m2K (heat exchanger water/water) Forced liquid (flowing) water - Condensing vapor water : U = 1000 - 4000 W/m2K (condensers steam water) Boiling liquid water - Free Convection Gas : U = 10 - 40 W/m2K (steam boiler + radiation) Boiling liquid water - Forced Liquid flowing (Water) : U = 300 - 1000 W/m2K (evaporation of refrigerators or brine coolers) Boiling liquid water - Condensing vapor water : U = 1500 - 6000 W/m2K (evaporators steam/water)

Related Topics Heat Loss and Insulation - Heat loss from pipes, tubes and tanks - with and without insulation - foam, fiberglass, rockwool and more Insulation - Heat transfer and heat loss from buildings and technical applications - heat transfer coefficients and insulation methods and to reduce energy consumption Steam - Heat Loss and Insulation - Steam and condensate pipes - heat loss uninsulated and insulated pipes, insulation thickness and more Thermodynamics - Effects of work, heat and energy on systems

Related Documents Arithmetic and Logarithmic Mean Temperature Differences in Heat Exchangers Arithmetic Mean Temperature Difference - AMTD - and Logarithmic Mean Temperature Difference - LMTD - formulas with examples - Online Mean Temperature Calculator Conductive Heat Transfer - Heat transfer takes place as conduction in a solid if there is a temperature gradient Convective Air Flow from Typical Heat Sources - Convective air flow from typical heat sources - like people, computers, radiators and more https://www.engineeringtoolbox.com/overall-heat-transfer-coefficient-d_434.html

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Convective Heat Transfer - Heat transfer between a solid and a moving fluid is called convection. This is a short tutorial about convective heat transfer Emissivity Coefficients common Materials - Radiation emissivities of some common materials like water, ice, snow, grass and more Fluid Heat Transfer Coefficients - Heat Exchanger Surface Combinations - Average overall heat transmission coefficients for some common fluids and surface combinations as Water to Air, Water to Water, Air to Air, Steam to Water and more Fouling and Reduced Heat Transfer in Heat Exchangers - The heat-transfer in a heat exchanger is reduced by fouling Heat Exchanger Heat Transfer Coefficients - Overall heat transfer coefficients in heat exchanger constructions - tubular, plate or spiral Radiation Heat Transfer - Heat transfer due to emission of electromagnetic waves is known as thermal radiation Steam Condensation and Heat Transfer - Heat transfer when condensing steam Submerged Coils - Heat Transfer Coefficients - Heat transfer coefficients for steam and hot water coils submerged in oil tanks Thermal Conductivities of Heat Exchanger Materials - Typical heat exchanger materials and their thermal conductivities Thermal Conductivity of some selected Materials and Gases - Thermal conductivity of some selected gases, insulation products, aluminum, asphalt, brass, copper, steel and other common materials Thermal Conductivity Online Converter - Convert thermal conductivities Thermal Transmittance and Thermal Resistance - Thermal transmittance U and thermal resistance R Water - Thermal Conductivity - Figures and tables showing thermal conductivity of water (liquid and gas phase) with varying temperature and pressure, SI and Imperial units Windows - Inside Condensation - Outside temperature, inside humidity and water condensation on the inside of glass windows surfaces

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Citation This page can be cited as Engineering ToolBox, (2003). Overall Heat Transfer Coefficient. [online] Available at: https://www.engineeringtoolbox.com/overall-heat-transfer-coefficient-d_434.html [Accessed Day Mo. Year]. Modify access date.

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