Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ASME SI-4-1976 — SI Units in Thermodynamics (First Edition)
🔥 ASME SI-4-1976 is the fourth booklet in the ASME International System (SI) of Units series, dedicated to SI units in thermodynamics, first published in November 1976. It was prepared by James W. Murdock and Leo T. Smith of the Mechanical Engineering and Mechanics Department at Drexel University. The booklet serves students, teachers, and engineers in practice, and its application areas include internal combustion engine cycles, gas turbines, jet propulsion, compressors, steam turbines, refrigeration, and air conditioning.
💡 The booklet points out that many U.S. thermodynamics texts mix U.S. Customary, metric (cgs), and, occasionally, SI units, and that SI property data were almost completely unavailable in the United States at the time — the gap this booklet helps to close. A true SI capability requires SI property data first.
1. Overview and Scope 📌
Thermodynamics is the branch of physics that deals with the various phenomena of energy transfer. From a few basic postulates it derives relationships between properties of matter, especially those affected by changes in temperature and pressure, and it studies the laws of transformation of heat into other forms of energy and vice versa. Classical thermodynamics takes the macroscopic view of matter, whereas statistical thermodynamics takes the microscopic view; only classical thermodynamics is treated in this booklet.
Solving thermodynamic problems requires tables, charts, and graphs of physical properties. Regrettably, SI-unit data of this kind were almost entirely unavailable in the United States at publication time, a situation that had to be corrected before a true SI capability could develop. Even the steam tables reprinted in recent texts — abridged from Keenan, Keyes, Hill, and Moore (International Edition, Metric Units) — although metric, are not SI and must be converted before use (SI-10 “ASME Steam Charts” provides steam properties in SI graphical form). Because a full SI property database could not be included, every problem statement in Sections 4 and 5 supplies all the data necessary for an SI solution.
⚠️ Key warning: the Keenan–Keyes–Hill–Moore “International Edition” steam tables use metric values, but they are not an SI unit system (calories, kgf, dyne, and cgs quantities may appear). Every value must be checked and converted before entering an SI calculation — one of the most easily overlooked traps in engineering thermodynamics.
2. Principal Quantities and Unit Systems 🔧
The principal quantities encountered in thermodynamics, in both U.S. Customary and SI units, are tabulated below:
| Quantity | U.S. Customary Unit (Symbol) | SI Unit (Symbol) |
|---|---|---|
| Energy / work / heat | Btu (IT) | joule (J) |
| Power / heat rate | Btu/h, horsepower (hp) | watt (W) |
| Pressure | lbf/in² (psi), atmosphere (atm) | pascal (Pa), kPa, MPa |
| Temperature | °F, °R | °C, kelvin (K) |
| Mass | lbm, slug | kilogram (kg) |
| Gas constant | Btu/(lbm·°R), ft·lbf/(lbm·°R) | J/(kg·K) |
| Specific enthalpy | Btu/lbm | J/kg (commonly kJ/kg) |
| Specific entropy | Btu/(lbm·°R) | J/(kg·K), kJ/(kg·K) |
| Specific heat | Btu/(lbm·°R) | J/(kg·K) |
| Specific internal energy | Btu/lbm | J/kg |
| Specific volume | ft³/lbm | m³/kg |
| Density | lbm/ft³ | kg/m³ |
| Mass flow rate | lbm/s, lbm/min, lbm/h | kg/s |
| Volume flow rate | ft³/min, ft³/s, US gal/min | m³/s |
The booklet also tabulates SI unit prefixes (Table 5): T (10¹²), G (10⁹), M (10⁶), k (10³), h (10²), da (10¹), d (10⁻¹), c (10⁻²), m (10⁻³), µ (10⁻⁶), n (10⁻⁹), and p (10⁻¹²), promoting a standardized way to handle orders of magnitude.
🚨 Temperature is the single most error-prone quantity in thermodynamics: do not confuse the Celsius scale (°C, for everyday statements) with the kelvin scale (K, the absolute temperature used in the gas laws, entropy, and temperature ratios). A temperature interval ΔT has the same numerical value on both scales, but whenever T enters a ratio such as p₁/T₁ = p₂/T₂ or the Carnot efficiency 1 − T₂/T₁, it must be in kelvins!
3. Conversion Factors and Conversion Techniques ⚖️
Sections 2 and 3 provide conversion factor tables and a dedicated “Conversion Techniques” treatment showing how to convert non-SI data systems into SI. Representative conversions:
| Quantity | Conversion | Factor |
|---|---|---|
| Energy / work / heat | 1 Btu (IT) to J | 1.055056 × 10³ |
| Power | 1 hp (550 ft·lbf/s) to W | 7.456999 × 10² |
| Gas constant | 1 Btu/(lbm·°R) to J/(kg·K) | 4.1868 × 10³ |
| Gas constant | 1 ft·lbf/(lbm·°R) to J/(kg·K) | 5.380320 |
| Specific enthalpy / internal energy | 1 Btu/lbm to J/kg | 2.326 × 10³ |
| Pressure | 1 atm to Pa | 1.01325 × 10⁵ |
| Pressure | 1 bar to Pa | 1.0 × 10⁵ |
| Mass flow rate | 1 lbm/h to kg/s | 1.259979 × 10⁻⁴ |
| Volume flow rate | 1 ft³/min to m³/s | 4.719474 × 10⁻⁴ |
| Volume flow rate | 1 US gal/min to m³/s | 6.309020 × 10⁻⁵ |
| Density | 1 lbm/ft³ to kg/m³ | 1.601846 × 10¹ |
| Force | 1 lbf to N | 4.448222 |
The heart of the conversion technique is dimensional decomposition: for example, 1 Btu/lbm = (1055.056 J)/(0.45359237 kg) = 2326 J/kg = 2.326 kJ/kg. The booklet emphasizes deriving conversions from dimensions rather than memorizing factors.
💡 Air values for quick engineering estimates: gas constant R ≈ 287 J/(kg·K), constant-pressure specific heat cp ≈ 1.005 kJ/(kg·K), constant-volume specific heat cv ≈ 0.718 kJ/(kg·K), specific heat ratio k ≈ 1.40; in the ideal gas law pv = RT use p in Pa, v in m³/kg, T in K, and R = 287 J/(kg·K).
4. Solved Examples and Practice (SI) ✏️
Section 4 provides worked examples and Section 5 practice problems with answers; both state all data directly in SI units. Typical problem types include:
- Ideal gas law: pV = mRT, with p in Pa, V in m³, m in kg, and T in K.
- First law of thermodynamics: for a closed system Q − W = ΔU, with Q and W in J (or kJ) and U in J.
- Steady-flow energy equation: h₁ + v₁²/2 + gz₁ + q = h₂ + v₂²/2 + gz₂ + w, with specific enthalpy in kJ/kg and velocities in m/s.
- Cycle efficiency: thermal efficiency η = Wnet/Qin, with work and heat input expressed in the same energy unit (J or kJ).
⚠️ Both the examples and practice problems state every needed property directly in SI units where the tables in Section 2 do not cover them (e.g., specific heats or steam state properties), so the reader is never forced to dig through non-SI property tables. Follow the same principle when composing your own exercises.
5. Engineering Practice Essentials 🛠️
- 🔍 Unify units before calculating: convert all non-SI inputs to a consistent SI set (Pa/kPa, J/kJ, K, kg, m³) before starting; avoid converting “along the way”.
- 🔧 Steam property lookup: pair SI thermodynamic calculations with SI-10 “ASME Steam Charts”; convert legacy metric-but-not-SI steam tables first.
- 📊 Temperature scale selection: use kelvins for gas laws, entropy changes, and efficiency formulas; Celsius is fine for reporting, but always substitute K into equations.
- 📋 Engineering cycles: evaluate gas turbine, steam turbine, and refrigeration cycle performance in SI to interoperate directly with manufacturer data (mostly SI).
✅ Dimensional check: after solving, run the result and the intermediate quantities through a dimensional analysis — work must reduce to kg·m²/s² (J), and an enthalpy drop multiplied by a mass flow rate must yield a power (W). Only dimensionally consistent results may be delivered.
6. Frequently Asked Questions ❓
❓ Q: Why is a “metric steam table” not the same as an “SI steam table”?
A: SI is a precisely defined system built on the base units m, kg, s, K, A, mol, and cd with derived units J, Pa, W, etc. “International Edition metric” steam tables often retain non-SI quantities such as calories, kcal, kgf, and dyne, or even cgs quantities, so every value must be converted before entering an SI calculation.
A: SI is a precisely defined system built on the base units m, kg, s, K, A, mol, and cd with derived units J, Pa, W, etc. “International Edition metric” steam tables often retain non-SI quantities such as calories, kcal, kgf, and dyne, or even cgs quantities, so every value must be converted before entering an SI calculation.
❓ Q: When must kelvins and degrees Celsius be distinguished?
A: For a temperature interval ΔT the two scales are interchangeable (1 K = 1 °C). But whenever the absolute temperature T enters a ratio — p₁/T₁ = p₂/T₂ or the Carnot efficiency 1 − T₂/T₁ — it must be in kelvins. Add 273.15 to a Celsius value to obtain kelvins.
A: For a temperature interval ΔT the two scales are interchangeable (1 K = 1 °C). But whenever the absolute temperature T enters a ratio — p₁/T₁ = p₂/T₂ or the Carnot efficiency 1 − T₂/T₁ — it must be in kelvins. Add 273.15 to a Celsius value to obtain kelvins.
❓ Q: Should I use kJ/kg or J/kg in engineering calculations?
A: Specific enthalpy and specific heats are commonly expressed in kJ/kg and kJ/(kg·K) for convenient magnitudes. When multiplying by a mass flow rate (kg/s), remember that kJ/s = kW — do not mistake kJ for J, which introduces a factor-of-1000 error.
A: Specific enthalpy and specific heats are commonly expressed in kJ/kg and kJ/(kg·K) for convenient magnitudes. When multiplying by a mass flow rate (kg/s), remember that kJ/s = kW — do not mistake kJ for J, which introduces a factor-of-1000 error.
🔍 Q: Which engineering scenarios is this booklet used for?
A: Power cycles (steam Rankine, gas Brayton), refrigeration and air conditioning, compressor and turbine thermodynamic design, and chemical-process energy balances — any teaching or engineering situation involving energy and state-parameter calculations, always in conjunction with the SI-1 guide.
A: Power cycles (steam Rankine, gas Brayton), refrigeration and air conditioning, compressor and turbine thermodynamic design, and chemical-process energy balances — any teaching or engineering situation involving energy and state-parameter calculations, always in conjunction with the SI-1 guide.
