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ASME SI-7-1977 — SI Units in Heat Transfer (First Edition)
🌡️ ASME SI-7-1977 is the seventh booklet in the ASME International System (SI) of Units series, dedicated to SI units in heat transfer, first published in March 1977 and prepared by James W. Murdock, Fellow ASME, of Drexel University. The booklet serves students, teachers, and engineers in practice, covering applications such as the design and operation of steam generators, furnaces, preheaters, exchangers, coolers, evaporators, and condensers, and builds a working capability in SI units for the heat-transfer discipline.
💡 As in thermodynamics, SI-unit transport and thermodynamic property data were almost completely unavailable in the United States at publication time, and textbook data were a hodgepodge of U.S. Customary, metric (cgs), and occasional SI units. This booklet provides selected SI data in an appendix and teaches conversion techniques to handle the rest.
1. Overview and Scope 📌
Heat transfer is the branch of theoretical and applied physics that seeks to predict the rate of energy transfer resulting from a difference in temperature between two or more bodies. The three modes of heat transfer are conduction, convection, and radiation. Applications include steam generators, furnaces, preheaters, heat exchangers, coolers, evaporators, and condensers — equipment used in every major industry.
Solving heat-transfer problems requires tables, charts, and graphs of the thermodynamic and transport properties of substances. The appendix (Selected Data) supplies property data in SI units, including the properties of air at one atmosphere, saturated water, and 316 stainless steel; wherever the appendix does not apply, the problem statements in Sections 4 and 5 give all necessary data directly in SI units.
⚠️ As in the thermodynamics booklet, legacy data sources mix U.S. Customary, cgs, and SI units. Normalize everything with the Section 2 conversion factors and Section 3 conversion techniques before calculating — never substitute mixed-unit textbook data directly.
2. Principal Quantities and Unit Systems 🔧
The principal quantities encountered in heat transfer and their SI derived units are tabulated below:
| Quantity | Derived Unit Name | Symbol | In Terms of SI Base Units |
|---|---|---|---|
| Energy (heat) | joule | J | m²·kg·s⁻² |
| Specific energy (enthalpy, internal energy) | joule per kilogram | J/kg | m²·s⁻² |
| Force | newton | N | m·kg·s⁻² |
| Frequency | hertz | Hz | s⁻¹ |
| Gas constant / specific heat | joule per kilogram kelvin | J/(kg·K) | m²·s⁻²·K⁻¹ |
| Power (heat rate) | watt | W | m²·kg·s⁻³ |
| Pressure / stress | pascal | Pa | m⁻¹·kg·s⁻² |
| Surface tension | newton per meter | N/m | kg·s⁻² |
| Dynamic viscosity | pascal-second | Pa·s | m⁻¹·kg·s⁻¹ |
| Thermal conductivity | watt per meter kelvin | W/(m·K) | m·kg·s⁻³·K⁻¹ |
| Heat transfer coefficient | watt per square meter kelvin | W/(m²·K) | kg·s⁻³·K⁻¹ |
| Heat flux | watt per square meter | W/m² | kg·s⁻³ |
In Fourier’s law Q̇ = −kA(dT/dx), use Q̇ in W, k in W/(m·K), A in m², and dT/dx in K/m; in Newton’s law of cooling Q̇ = hAΔT, use h in W/(m²·K). In the Stefan–Boltzmann radiation law Q̇ = εσA(T₁⁴ − T₂⁴), σ ≈ 5.67 × 10⁻⁸ W/(m²·K⁴) and the temperatures must be in kelvins.
🚨 Radiation heat transfer is the most frequent site of temperature-scale errors: the T⁴ terms require absolute temperature in kelvins — substituting degrees Celsius gives absurd results. Likewise, any temperature ratio in gas laws or efficiency formulas must use K.
3. Dimensionless Numbers and Conversion Factors ⚖️
The booklet lists the principal dimensionless numbers used in heat transfer (Nusselt number Nu, Reynolds number Re, Prandtl number Pr, Grashof number Gr, etc.) with their SI unit combinations, plus commonly used conversion factors (including coefficient of thermal expansion 1/°F to 1/K × 1.8, density lbm/ft³ to kg/m³ × 1.601846×10¹, etc.):
| Quantity | Conversion | Factor |
|---|---|---|
| Linear acceleration | ft/s² to m/s² | 3.048 × 10⁻¹ |
| Area | in² to m² | 6.4516 × 10⁻⁴ |
| Area | ft² to m² | 9.290304 × 10⁻² |
| Coefficient of thermal expansion | 1/°F to 1/K | 1.8 |
| Density | lbm/ft³ to kg/m³ | 1.601846 × 10¹ |
| Energy / work / heat | 1 Btu (IT) to J | 1.055056 × 10³ |
| Thermal conductivity | Btu/(h·ft·°F) to W/(m·K) | 1.730735 |
| Heat transfer coefficient | Btu/(h·ft²·°F) to W/(m²·K) | 5.678263 |
| Pressure | 1 lbf/in² to Pa | 6.894757 × 10³ |
| Dynamic viscosity | 1 lbm/(ft·s) to Pa·s | 1.488164 |
| Specific heat | 1 Btu/(lbm·°F) to J/(kg·K) | 4.1868 × 10³ |
Two of the most used conversions: 1 Btu/(h·ft²·°F) ≈ 5.678 W/(m²·K) for heat transfer coefficients, and 1 Btu/(h·ft·°F) ≈ 1.731 W/(m·K) for thermal conductivity.
💡 Quick engineering magnitudes: water at room temperature has k ≈ 0.6 W/(m·K), air ≈ 0.026 W/(m·K), 316 stainless steel ≈ 16 W/(m·K); forced-convection water-side coefficients range from about 1000 to 5000 W/(m²·K), natural-convection air-side from about 5 to 50 W/(m²·K) — use these to locate the dominant thermal resistance in a heat exchanger.
4. Worked Examples and the SI Approach ✏️
The solved and practice problems in Sections 4 and 5 are stated directly in SI units and cover:
- Plane wall and cylindrical conduction: Q̇ = kAΔT/L or the log-mean-temperature-difference formula for cylinders, with k in W/(m·K), dimensions in m, and ΔT in K.
- Composite walls and resistance networks: each layer’s resistance R = L/(kA) in K/W, summed in series for the total heat rate.
- Convection: Q̇ = hAΔT, paired with the Nusselt number Nu = hL/k and the Reynolds number Re to identify the flow regime and the appropriate empirical correlation.
- Radiation: Q̇ = εσA(T₁⁴ − T₂⁴), with temperatures in kelvins.
⚠️ A temperature interval ΔT has the same numerical value in K and °C and may be interchanged; but T⁴ in radiation and the T in gas-law relations must be in kelvins. Also, although both names contain “coefficient,” thermal conductivity (W/(m·K)) and heat transfer coefficient (W/(m²·K)) differ by one length dimension — never mix them.
5. Engineering Practice Essentials 🛠️
- 🔍 Heat exchanger thermal design: use consistent SI inputs — flow rates (kg/s), specific heats (kJ/(kg·K)), temperature differences (K) — so the duty Q̇ = ṁ·c·ΔT comes out directly in kW.
- 🔧 Insulation design: insulation conductivity in W/(m·K); economic-thickness calculations become self-consistent in SI.
- 📊 Surface temperature and heat-flux measurement: report heat flux in W/m² and surface coefficients in W/(m²·K), matching sensor calibrations (mostly SI).
- 📋 Teaching practice: reconvert U.S. Customary exercises (Btu/h, Btu/(h·ft²·°F)) to SI and re-solve, mastering the key factors 5.678 and 1.731.
✅ Dimensional check: a heat-transfer power must reduce to W = J/s; expanding the conduction formula gives W/(m·K) × m² × K/m = W. Only dimensionally self-consistent results should be delivered.
6. Frequently Asked Questions ❓
❓ Q: What is the difference between thermal conductivity and heat transfer coefficient?
A: Thermal conductivity k characterizes conduction within a material, unit W/(m·K), used in Fourier’s law and related to “per meter of thickness”; the heat transfer coefficient h characterizes convection between a solid surface and a fluid, unit W/(m²·K), used in Newton’s law of cooling and related to “per square meter of surface”. They differ by one length dimension.
A: Thermal conductivity k characterizes conduction within a material, unit W/(m·K), used in Fourier’s law and related to “per meter of thickness”; the heat transfer coefficient h characterizes convection between a solid surface and a fluid, unit W/(m²·K), used in Newton’s law of cooling and related to “per square meter of surface”. They differ by one length dimension.
❓ Q: How do I convert Btu/(h·ft²·°F) to W/(m²·K)?
A: Multiply by 5.678263: 1 Btu/(h·ft²·°F) ≈ 5.678 W/(m²·K); conversely, 1 W/(m²·K) ≈ 0.1761 Btu/(h·ft²·°F). This is one of the most common relations in heat-exchanger design and performance conversion.
A: Multiply by 5.678263: 1 Btu/(h·ft²·°F) ≈ 5.678 W/(m²·K); conversely, 1 W/(m²·K) ≈ 0.1761 Btu/(h·ft²·°F). This is one of the most common relations in heat-exchanger design and performance conversion.
❓ Q: What property data does the appendix provide?
A: Selected SI data include the properties of air at one atmosphere, saturated water, and 316 stainless steel (thermal conductivity, density, specific heat, viscosity, etc., as functions of temperature), convenient for quick heat-transfer calculations.
A: Selected SI data include the properties of air at one atmosphere, saturated water, and 316 stainless steel (thermal conductivity, density, specific heat, viscosity, etc., as functions of temperature), convenient for quick heat-transfer calculations.
🔍 Q: How does this booklet coordinate with the other SI booklets?
A: Start with SI-1 (the orientation guide). Heat-transfer analysis frequently couples with thermodynamics (SI-4, for state equations and steam properties) and fluid mechanics (SI-5, for convection regimes and pressure drop); keep unit conventions consistent across booklets and, in radiation problems, always use kelvins.
A: Start with SI-1 (the orientation guide). Heat-transfer analysis frequently couples with thermodynamics (SI-4, for state equations and steam properties) and fluid mechanics (SI-5, for convection regimes and pressure drop); keep unit conventions consistent across booklets and, in radiation problems, always use kelvins.
