Overall Heat Transfer Coefficient

Overall Heat Transfer Coefficient

The overall heat transfer coefficient is one of the most important concepts in thermal engineering. Whether you’re designing heat exchangers, analyzing building insulation, or optimizing cooling systems, understanding the heat transfer coefficient is essential for success. This comprehensive guide explains everything about the overall heat transfer coefficient in simple terms. First, we’ll cover the overall heat transfer coefficient definition and heat transfer coefficient units. Then, we’ll explore the overall heat transfer coefficient formula for different applications including pipes, walls, and heat exchangers.

Moreover, you’ll learn how to calculate heat transfer coefficient values for real systems. We’ll provide overall heat transfer coefficient tables with typical values for water, air, and other fluids. Additionally, we’ll show practical examples and common mistakes to avoid.

At CFDLand, we specialize in Heat Transfer CFD Simulation that helps engineers analyze and optimize thermal performance. Furthermore, this guide connects theory with practical applications, including how to use overall heat transfer coefficient in ANSYS Fluent.

By the end of this article, you’ll master the overall heat transfer coefficient concept and apply it confidently in your projects. Let’s start with the basics!

 

Figure 1: Some CFD Heat Transfer Analysis done by CFDLAND using ANSYS Fluent

What is Overall Heat Transfer Coefficient?

The overall heat transfer coefficient is a fundamental parameter that measures how well heat transfers through a system. Simply put, the overall heat transfer coefficient (commonly written as U value or just U) tells us the total heat transfer rate through materials and fluids.

Furthermore, the overall heat transfer coefficient definition includes all resistances to heat flow. These resistances come from:

  • Convection on the hot side
  • Conduction through solid materials
  • Convection on the cold side

What is the overall heat transfer coefficient

Figure 2: Thermal resistance in a medium

Why is Heat Transfer Coefficient Important?

The heat transfer coefficient helps engineers design efficient thermal systems. Moreover, understanding the overall heat transfer coefficient is crucial for:

  • Sizing heat exchangers correctly
  • Calculating heat loss in buildings
  • Designing cooling systems
  • Optimizing industrial processes

In heat exchangers, the overall heat transfer coefficient combines the convection heat transfer coefficient of both fluids plus the conduction resistance of the pipe wall. Additionally, any fouling or scaling affects the U value heat transfer significantly.

The Physical Meaning

Think of the overall heat transfer coefficient like this: it’s similar to electrical conductance. In other words, a higher heat transfer coefficient means heat flows more easily (like high electrical conductance). Conversely, a lower U value means more resistance to heat flow (better insulation).

The overall heat transfer coefficient answers this question: How much heat transfers per square meter for each degree of temperature difference?

Overall Heat Transfer Coefficient Formula and Calculation

The overall heat transfer coefficient formula is the foundation for all thermal calculations. Simply put, this formula helps us calculate how much heat transfers through any system.

Basic Heat Transfer Equation

The fundamental overall heat transfer coefficient formula is:

Q = U × A × ΔT

Where:

  • Q = Heat transfer rate (Watts)
  • U = Overall heat transfer coefficient (W/m²·K)
  • A = Heat transfer area (m²)
  • ΔT = Temperature difference (K or °C)

Moreover, this simple equation shows that heat transfer depends on three factors: the heat transfer coefficient, the area, and the temperature difference. Double the U value and you double the heat transfer rate!

How to Calculate Overall Heat Transfer Coefficient

For a flat wall with convection on both sides, the overall heat transfer coefficient calculation uses this formula:

1/U = 1/h₁ + L/k + 1/h₂

Where:

  • h₁ = Convection heat transfer coefficient on hot side (W/m²·K)
  • h₂ = Convection heat transfer coefficient on cold side (W/m²·K)
  • L = Wall thickness (m)
  • k = Thermal conductivity of wall material (W/m·K)

Furthermore, each term represents a resistance to heat flow:

  • 1/h₁ = Convection resistance on side 1
  • L/k = Conduction resistance through wall
  • 1/h₂ = Convection resistance on side 2

Thermal Equivalent Circuit

The thermal resistance concept makes heat transfer coefficient calculations much easier to understand. Simply put, heat flow is like electrical current, and temperature difference is like voltage.

 

Thermal equivalent circuit

Figure 3. Heat transfer between two fluids through a composite wall. Note that A1 and A4 are equal, and L2 and L3 are equal.

The thermal equivalent circuit of the system is in Figure 2

Figure 4. The thermal equivalent circuit of the system is in Figure 2.

Just like electrical circuits, thermal resistances add up:

  • R_conv1 = 1/(h₁A) – Convection resistance on hot side
  • R_cond = L/(kA) – Conduction resistance through wall
  • R_conv2 = 1/(h₂A) – Convection resistance on cold side

Therefore, If thermal resistances are in series, the total thermal resistance is:

For Series (most common):

R_total = R₁ + R₂ + R₃ + …

For Parallel paths:

1/R_total = 1/R₁ + 1/R₂ + 1/R₃ + …

Moreover, parallel heat paths occur in:

  • Composite walls with different materials side by side
  • Finned surfaces where heat flows through fins AND base
  • Windows with frame and glass areas

The overall heat transfer coefficient relates to total resistance:

U = 1/(A × R_total)

Calculating Overall Heat Transfer Coefficient for Parallel Paths

When heat paths are parallel, the overall heat transfer coefficient calculation becomes:

U_total × A_total = U₁ × A₁ + U₂ × A₂ + …

For example, a wall with a window:

  • Wall area: A₁ with U₁ = 2 W/m²·K
  • Window area: A₂ with U₂ = 5 W/m²·K

The average overall heat transfer coefficient is: U_avg = (U₁ × A₁ + U₂ × A₂)/(A₁ + A₂)

Remember: parallel paths increase the overall heat transfer coefficient because heat finds the easiest route.

Practical Example

Let’s calculate the overall heat transfer coefficient for a simple wall:

  • Hot side: h₁ = 100 W/m²·K (convection heat transfer coefficient for air)
  • Wall: L = 0.1 m, k = 1 W/m·K (brick)
  • Cold side: h₂ = 25 W/m²·K (natural convection)

Using the overall heat transfer coefficient formula: 1/U = 1/100 + 0.1/1 + 1/25 = 0.01 + 0.1 + 0.04 = 0.15

Therefore: U = 6.67 W/m²·K

Additionally, this shows that the wall conduction (0.1) is the biggest resistance. The heat transfer coefficient is limited by the largest thermal resistance.

Overall Heat Transfer Coefficient for Different Geometries

The overall heat transfer coefficient changes with geometry. Moreover, cylindrical shapes like pipes and tubes require different formulas than flat walls. Let’s explore the most common geometries in detail.

Overall Heat Transfer Coefficient: Cylindrical Tubes

Pipes and tubes are everywhere in heat exchangers. Furthermore, the overall heat transfer coefficient cylindrical formula accounts for the curved surface area that changes with radius.

For cylindrical tubes, the heat transfer coefficient calculation becomes:

1/U = 1/h_i + (r_o/r_i) × (1/h_i) + (r_o × ln(r_o/r_i))/k + 1/h_o

Where:

  • h_i = Inner convection heat transfer coefficient (W/m²·K)
  • h_o = Outer convection heat transfer coefficient (W/m²·K)
  • r_i = Inner radius (m)
  • r_o = Outer radius (m)
  • k = Thermal conductivity of pipe material (W/m·K)

At CFDLand, we’ve conducted a comprehensive Flow Over Heated Cylinder CFD Simulation + Analytical Solution Validation that demonstrates:

  • ANSYS Fluent prediction of heat transfer coefficient around cylinder circumference
  • Comparison between CFD results and analytical correlations
  • Local Nusselt number distribution showing maximum heat transfer at stagnation point
  • Validation of overall heat transfer coefficient calculations

CFD Fluent

Figure 5: Flow over a tube CFD analysis – Validation against analytical solution by means of ANSYS Fluent

Moreover, this study shows how CFD simulation captures the complex heat transfer patterns that simple correlations miss. The heat transfer coefficient varies significantly around the cylinder, being highest at the stagnation point and lowest in the wake region.

Simplified Formula for Thin Tubes

For thin-walled tubes where the thickness is much smaller than the radius, we can simplify the overall heat transfer coefficient formula:

1/U ≈ 1/h_i + t/k + 1/h_o

Where t = tube wall thickness (m)

This simplification works when (r_o – r_i) << r_i. Additionally, this formula looks similar to the flat wall formula, making calculations easier.

Heat Exchanger Geometries

Heat exchangers use various geometries. Moreover, each geometry affects the overall heat transfer coefficient differently:

Shell and Tube Heat Exchangers

The heat exchanger overall heat transfer coefficient depends on:

  • Tube arrangement (triangular, square)
  • Number of tube passes
  • Baffle design
  • Flow patterns (counterflow, parallel flow)

For shell and tube designs, the overall heat transfer coefficient typically ranges:

  • U = 150-1200 W/m²·K for liquid-liquid
  • U = 15-70 W/m²·K for gas-gas
  • U = 150-500 W/m²·K for gas-liquid

Plate Heat Exchangers

Plate heat exchangers achieve higher heat transfer coefficient values due to:

  • Thinner walls
  • Corrugated surfaces creating turbulence
  • Larger surface area per volume

Typical overall heat transfer coefficient for plate exchangers:

  • U = 3000-7000 W/m²·K for water-water
  • U = 500-2000 W/m²·K for oil-water

Finned Surfaces

Fins increase the overall heat transfer coefficient by adding surface area. Furthermore, the fin efficiency must be considered:

1/U_finned = 1/(η_f × h_f × A_f/A_total) + R_wall + 1/h_other

Where:

  • η_f = Fin efficiency (0 to 1)
  • A_f = Fin area
  • A_total = Total area (fins + base)

Remember: fins help most when one side has a low convection heat transfer coefficient, like air cooling.

 

Overall heat transfer coefficient units

The units of overall heat transfer coefficient expressed in various systems:

  • International System of Unit (SI): Watts per square meter per Kelvin (W/(m2.K))
  • Imperial Units (British Engineering Unit): BTU per hour per square foot per degree Fahrenheit (BTU/(h.ft2.°F))
  • Metric Unit: Joule per second per square meter per Kelvin (J/(s.m2.K)) or Watt per square meter per degree Celsius (W/(m2.°C))
  • International Unit: Kilocalorie (IT) per hour per square meter per degree Celsius (kcal/(h.m2.°C))

 

Heat Transfer Coefficient Values and Tables

Here are typical U values for common applications:

TABLE 2 Representative Values of the overall heat transfer coefficient, from fundamentals of heat and mass transfer by Frank P. Incropera et al.

Application Fluids Overall Heat Transfer Coefficient (W/m²·K) U Value (BTU/hr·ft²·°F)
Condensers Steam to water 1000-6000 175-1050
Heat Exchangers Water to water 800-1500 140-265
Heat Exchangers Oil to water 100-350 20-60
Air Coolers Water to air 10-40 2-7
Boilers Hot gases to water 10-50 2-9
Building Walls Air to air 1-5 0.2-0.9

Typical values of the overall heat-transfer coefficient for various types of heat exchanger are given in Table 3. The reference values of Table 3 have been taken from Coulson & Richardson’s Chemical Engineering Vol. 6, 2nd Edition & 3rd Edition, R K Sinnot.

Table 3. Typical overall heat transfer coefficients

Shell and tube exchangers

Hot fluid Cold fluid U (W/(m2.K))
Heat exchangers

Water

Organic solvents

Light oils

Heavy oils

Gases

Coolers

Organic solvents

Light oils

Heavy oils

Gases

Organic solvents

Water

Gases

Heaters

Steam

Steam

Steam

Steam

Steam

Dowtherm

Dowtherm

Flue gases

Flue

Condensers

Aqueous vapours

Organic vapours

Organics (some non-condensables)

Vacuum condensers

Vaporisers

Steam

Steam

Steam

 

Water

Organic solvents

Light oils

Heavy oils

Gases

 

Water

Water

Water

Water

Brine

Brine

Brine

 

Water

Organic solvents

Light oils

Heavy oils

Gases

Heavy oils

Gases

Steam

Hydrocarbon vapours

 

Water

Water

Water

Water

 

Aqueous solutions

Light organics

Heavy organics

 

800-1500

100-300

100-400

50-300

10-50

 

250-750

350_ 900

60-300

20-300

150-500

600-1200

15-250

 

1500-4000

500-1000

300_ 900

60-450

30-300

50-300

20-200

30-100

30-100

 

1000-1500

700-1000

500-700

200-500

 

1000-1500

900-1200

600_ 900

Air-cooled exchangers

Process fluid
Water

Light organics

Heavy organics

Gases, 5-10 bar

10-30 bar

Condensing hydrocarbons

300—450

300-700

50-150

50-100

100-300

300-600

Immersed coils

Coil Pool
Natural circulation Steam

Steam

Steam

Water

Water

Agitated

Steam

Steam

Steam

Water

Water

 

Dilute aqueous solutions

Light oils

Heavy oils

Aqueous solutions

Light oils

Dilute aqueous solutions

Light oils

Heavy oils

Aqueous solutions

Light oils

 

500-1000

200-300

70-150

200-500

100-150

800-1500

300-500

200-400

400-700

200-300

Jacketed vessels

Jacket Vessel
Steam

Steam

Water

Water

Dilute aqueous solutions

Light organics

Dilute aqueous solutions

Light organics

500-700

250-500

200-500

200-300

Gasketed-plate exchangers

Hot fluid Cold fluid
    Light organic

Light organic

Viscous organic

Light organic

Viscous organic

Light organic

Viscous organic

Condensing steam

Condensing steam

Process water

Process water

Dilute aqueous solutions

Condensing steam

Light organic

Viscous organic

Viscous organic

Process water

Process water

Cooling water

Cooling water

Light organic

Viscous organic

Process water

Cooling water

Cooling water

Process water

2500-5000

250-500

100-200

2500-3500

250-500

2000-4500

250-450

2500-3500

250-500

5000-7500

5000-7000

5000-7000

3500-4500

Heat Transfer Coefficient of Water

Water is the most common fluid in heat transfer. Furthermore, the heat transfer coefficient of water varies significantly with flow conditions:

Natural Convection in Water:

  • Horizontal plates: h = 100-600 W/m²·K
  • Vertical plates: h = 200-1000 W/m²·K

Forced Convection in Water:

  • Low velocity (0.1 m/s): h = 500-1500 W/m²·K
  • Medium velocity (1 m/s): h = 3000-6000 W/m²·K
  • High velocity (3 m/s): h = 7000-11000 W/m²·K

The convection heat transfer coefficient of water increases dramatically with velocity. Additionally, turbulent flow gives much higher values than laminar flow.

Heat Transfer Coefficient for Common Fluids

Different fluids have vastly different heat transfer coefficient values:

Air (Forced Convection):

  • Low velocity: h = 10-30 W/m²·K
  • High velocity: h = 50-250 W/m²·K

Oil:

  • Natural convection: h = 50-150 W/m²·K
  • Forced convection: h = 200-700 W/m²·K

Steam:

  • Condensing: h = 5000-25000 W/m²·K
  • Superheated: h = 30-300 W/m²·K

Factors Affecting Heat Transfer Coefficient Values

The overall heat transfer coefficient depends on many factors. Moreover, understanding these helps predict U value changes:

1. Fluid Properties

  • Thermal conductivity: Higher k means higher heat transfer coefficient
  • Viscosity: Lower viscosity increases convection coefficient
  • Specific heat: Affects heat capacity but not directly the h value

2. Flow Conditions

  • Velocity: Faster flow = higher heat transfer coefficient
  • Turbulence: Turbulent flow dramatically increases h values
  • Flow pattern: Counter-flow better than parallel flow

3. Surface Conditions

  • Roughness: Can increase turbulence and heat transfer coefficient
  • Fouling: Deposits reduce overall heat transfer coefficient over time
  • Corrosion: Creates additional thermal resistance

 

Overall Heat Transfer Coefficient in ANSYS Fluent CFD Simulation

ANSYS Fluent is the industry-leading CFD software for calculating heat transfer coefficient values. Moreover, CFD simulation provides accurate overall heat transfer coefficient predictions for complex geometries that manual calculations cannot handle.

Why Use ANSYS Fluent for Heat Transfer Coefficient?

ANSYS Fluent CFD calculates the overall heat transfer coefficient with unprecedented accuracy. Furthermore, CFD heat transfer simulation offers advantages that traditional methods cannot match:

  • 3D visualization of heat transfer coefficient distribution
  • Local h values at every point on the surface
  • Conjugate heat transfer between fluids and solids
  • Turbulence effects on convection heat transfer coefficient
  • Complex geometry analysis for real heat exchangers

How ANSYS Fluent Calculates Heat Transfer Coefficient

ANSYS Fluent solves fundamental equations to find the heat transfer coefficient:

  1. Energy Equation: Calculates temperature distribution
  2. Navier-Stokes Equations: Determines flow field affecting convection coefficient
  3. Turbulence Models: Predicts mixing that enhances heat transfer
  4. Wall Functions: Computes near-wall heat transfer coefficient

Additionally, Fluent CFD automatically calculates:

  • Local heat transfer coefficient: h = q/(T_wall – T_fluid)
  • Average heat transfer coefficient over surfaces
  • Overall heat transfer coefficient for heat exchangers

Step-by-Step Guide: Overall Heat Transfer Coefficient in Fluent

In a heat transfer simulation using ANSYS Fluent, you do not need to manually enter the value of the overall heat transfer coefficient. The software calculates heat transfer and mass transfer directly based on the defined boundary conditions, material properties, and the governing equations.

After running an ANSYS Fluent simulation, the overall heat transfer coefficient value can indeed be extracted as an output from the software. However, the software takes the reference temperature (T) from a predefined value, which is sometimes unsuitable and needs to be changed. Therefore, it is often necessary to set the reference value manually. By extracting heat fluxes and temperature differences and calculating the heat transfer coefficient outside the software, using equation 2, sometimes a more accurate overall heat transfer coefficient may be achieved.

Overall heat transfer coefficient in ANSYS Fluent

Figure 6. Reference values in ANSYS Fluent

At CFDLand, we offer comprehensive Heat Exchanger CFD Simulation tutorials that teach you how to extract desired variables out of the CFD simulation regarding overall heat transfer coefficient, nusselt number, etc.

Conclusion: Mastering Overall Heat Transfer Coefficient

The overall heat transfer coefficient is essential for successful thermal engineering design. Moreover, understanding the heat transfer coefficient formula Q = U × A × ΔT and thermal resistance calculations helps you design better heat exchangers and thermal systems. Whether working with flat walls, cylindrical tubes, or complex geometries, the overall heat transfer coefficient varies significantly based on fluid properties, flow conditions, and surface characteristics. From building insulation with U values of 1-5 W/m²·K to steam condensation exceeding 10,000 W/m²·K, knowing typical heat transfer coefficient ranges validates your calculations.

ANSYS Fluent CFD simulation revolutionizes heat transfer coefficient analysis for complex applications. Furthermore, CFD provides accurate overall heat transfer coefficient predictions that manual calculations cannot achieve, especially for real-world geometries with entrance effects and turbulence. At CFDLand, our Heat Exchanger CFD Simulation tutorials teach you to master ANSYS Fluent for calculating heat transfer coefficient values, modeling complex heat exchangers, and optimizing thermal performance. Ready to excel in thermal engineering? Explore our comprehensive Heat Transfer CFD Simulation tutorials and master overall heat transfer coefficient analysis today!

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