Thermodynamics, Heat Transfer & Fluid–Gas Dynamics – Comprehensive RT Exam Notes

Heat Transfer & Thermodynamics Fundamentals

• Four classic modes of heat transfer
  • Conduction
  • Convection
  • Radiation
  • Vaporization (4th mode emphasised in lecture)
• Thermodynamics (clinical context)
  • Study of how matter behaves at different temperatures
  • Clinical oxygen systems, humidification devices, ventilator circuits, etc., all rely on these principles

Hospital Liquid-Oxygen System (Real-World Example)

• Large insulated outdoor silo/tank stores liquid O$_2$ at \approx -180^{\circ}C (exact value not memorised in lecture but very cold)
• Piping route
  • Small outlet at tank base ⟶ curved pipe ⟶ underground main ⟶ bedside wall outlets
• Vaporisation step
  • A tiny engineered hole in the pipe briefly exposes flowing liquid O$_2$ to ambient air
  • Even a “minuscule” temperature rise is enough to convert liquid O$_2$ to gas (dry, anhydrous)
  • Down-stream piping therefore carries gaseous O$_2$ only; humidification is later added at bedside
• Visual clue: section of pipe exiting tank is ice-covered even on hot summer days ➜ testimony to cryogenic temperature

Vaporisation vs. Evaporation

• Vaporisation = phase change liquid → gas irrespective of mechanism
• Evaporation = special case of vaporisation that:
  • Occurs below the boiling point, at surface only
  • Requires heat to be taken from surrounding environment (cooling effect)
  • Examples
  • Human sweating: warm air ⟶ holds more moisture ⟶ sweat evaporates ⟶ skin cools (evaporative heat loss)
  • Glass of water on nightstand: water line drops over days because random surface molecules gain kinetic energy (from light/room heat) and escape into air

Boiling Point & Critical Temperature

• Boiling point (BP)
  • Instant just before first steam bubbles / visible vapor appear
  • Internal vapor pressure of liquid = external barometric pressure
• Critical temperature (TC)
  • The split-second after BP when molecules escape as visible vapor/steam
• Altitude effect (Denver example)
  • Lower \bar P_{atm} (≈ 633\,\text{mmHg} vs. 760\,\text{mmHg} at sea level)
  • T_{BP} reached at lower absolute temperature ➜ foods cook longer even though water “boils sooner” (e.g.
  • 8\,\text{min} sea-level egg ≈ 11\,\text{min} in Denver)
• General rule
  P{vapor}=P{atm}\;\Rightarrow\;\text{boiling begins}

Temperature Scales & Conversions

• Kelvin (K)
  • Absolute scale, 0\,\text K = -273^{\circ}C (absolute zero)
  • Used in research/teaching hospitals (UCLA, UCI etc.)
• Clinical practice alternates between °F & °C
  • Key conversions
  • ^{\circ}C = (^{\circ}F - 32)/1.8
  • ^{\circ}F = (^{\circ}C \times 1.8) + 32
  • Examples
  • Normal body temp 98.6^{\circ}F \Rightarrow 37^{\circ}C
  • Drop of 1^{\circ}C represents larger thermal change than drop of 1^{\circ}F

Condensation & Humidity Basics

• Condensation = gas cools ⟶ water droplets form (reverse of evaporation)
• Seen on windshields, iced drink glasses, ventilator circuits when heater too low
• To minimise circuit rainout ➜ increase humidifier temperature closer to 37^{\circ}C

Pascal, Buoyancy & Archimedes (Liquids)

• Pascal’s Principle
  • P = \rho g h ; pressure exerted by a liquid depends on density and depth
• Archimedes / Buoyancy
  • Upward buoyant force B = \rho{water} \times V{displaced}
  • Object appears lighter in water; force under object > atmospheric force above
  • Clinical imaging (e.g., pleural effusion) & rehab pools rely on buoyancy

Bernoulli, Venturi & Coanda (Flow Dynamics)

• Bernoulli’s Principle (applies to liquids and gases)
  • In a narrowing tube: pressure lateral to flow decreases, velocity & momentum increase ➜ transition from turbulent (large lumen) to laminar (small lumen)
  • Explains airway & vascular branching: trachea/aorta turbulent ➜ bronchioles/capillaries laminar
• Venturi Effect (gas-specific Bernoulli)
  • High-velocity jet through narrow orifice entrains ambient air via side ports
  • Basis of air-entrainment masks, nebulisers, certain suction systems
• Coanda Effect
  • Fluid tends to follow contour of adjacent surface
  • Ventilator circuits: condensate will run along tubing walls; patient rollover can cause unintended tracheal lavage if tubing suddenly straightens

Viscosity & Surface Tension

• Viscosity = internal friction / opposition to flow
  • Water vs. honey demo; blood ≈ 5× more viscous than water
  • Elevated red cell count = polycythaemia ➜ ↑ viscosity ➜ clot risk
• Surface Tension (γ)
  • "Force per unit length among like molecules at liquid surface"
  • Water: 2nd only to mercury in γ
  • Leads to spherical raindrops; belly-flop pain illustrates large force required to disrupt surface
• Laplace’s Law (spheres)
  • P = \dfrac{2\gamma}{r}
  • Smaller radius ⇒ higher collapsing pressure; explains tendency of tiny balloons or alveoli to empty into larger ones
  • Pulmonary surfactant lowers γ, stabilising small alveoli and counteracting Laplace

Gas Properties & Respiratory Relevance

• Compressibility
  • Gases compress easily (contrast liquids)
• Kinetic Theory
  • ↑Temp ⇒ ↑molecular velocity ⇒ ↑ moisture-holding capacity
• BTPS vs. ATPS
  • BTPS (Body Temperature & Pressure Saturated): 37^{\circ}C, 47\,\text{mmHg H}_2\text O, fully saturated (moisture content 43.8\,\text{mg L}^{-1})
  • ATPS (Ambient Temperature & Pressure Saturated): ambient T, 100 % RH; used in blood-gas reporting adjustments
• Humidification
  • Dry gas causes tracheal epithelial damage within minutes ➜ nasal passages normally warm/humidify; artificial airways require active humidification
• Helium & Heliox
  • Helium = very low density; mixture with O$2$ (e.g., 80 % He / 20 % O$2$) = Heliox
  • Used acutely to reduce work of breathing in upper-airway obstruction (peanut aspiration, tumours, etc.) by decreasing resistance (Reynolds number ↓)

Gas Laws Mentioned

• Dalton’s Law: total pressure of gas mixture = sum of partial pressures
• Avogadro’s Number: 6.023\times10^{23} molecules in 1 mol of any substance
• Alveolar Air Equation (clinical recall) P{A\,O2} = (P{B} - 47)F{i\,O2} - \dfrac{P{a\,CO_2}}{R}
  • Example Denver calc: PB = 633\,\text{mmHg}, F{iO2}=0.21, P{aCO2}=42, R=0.8 ⇒ P{AO2}\approx70\,\text{mmHg} (low ➜ raise FiO$2$)

Clinical Pearls & Miscellaneous Examples

• Altitude cooking
  • Higher altitude = lower barometric pressure ➜ water boils at lower T ➜ longer cook times
• Kelvin note: not tested but may appear in research units; keep 0\,\text K = -273^{\circ}C in mind
• Emergency O$_2$ outlets in most modern hospital rooms are piped from central LOX; simulation labs may have non-functional look-alikes
• Ventilator circuit management
  • Keep heater at 35–37^{\circ}C
  • Drain or wick condensate frequently; avoid bolus lavage
• Evaporative cooling & hydration
  • Encourage fluid intake during heat exposure; evaporation requires body water to dissipate heat

Quick Reference Numbers

• LOX storage temperature ≈ -180^{\circ}C
• Water vapor pressure in alveoli =47\,\text{mmHg}
• Moisture content at full saturation (BTPS) =43.8\,\text{mg L}^{-1}
• Sea-level barometric pressure =760\,\text{mmHg}
• Denver barometric pressure ≈633\,\text{mmHg}
• Normal body temperature =37^{\circ}C=98.6^{\circ}F
• Absolute zero =0\,\text K = -273^{\circ}C

Vocabulary & Abbreviations

• BP – Boiling Point
• TC – Critical Temperature
• BTPS / ATPS – Body / Ambient Temperature & Pressure Saturated
• Heliox – He/O$_2$ therapeutic blend
• Polycythaemia – high red-cell count (↑ viscosity)
• Lavage – "to wash"; unintentional tracheal lavage = accidental water dump into ETT
• Laminar vs. Turbulent – orderly vs. chaotic flow

Ethical / Practical Implications

• Patient safety
  • Prevent cold, dry gas injury by proper humidification
  • Avoid inadvertent lavage by monitoring condensate
• Resource awareness
  • Understanding central LOX systems critical during hospital O$_2$ shortages
• Education
  • RTs must translate complex thermodynamic principles into bedside practice (e.g., adjusting FiO$_2$, heater settings, air-entrainment ratios)