--- title: "Adiabatic saturation and wet bulb temperature" subject: "Applied Thermodynamics" chapter: 4 key: "semester_3/applied_thermodynamics:adiabatic-saturation-wet-bulb" source: https://www.bctengineer.com/q/semester_3/applied_thermodynamics/adiabatic-saturation-wet-bulb site: BCT Engineering --- # Adiabatic saturation and wet bulb temperature Applied Thermodynamics · Chapter 4: Air-Conditioning - Asked in: year not given, 5 marks - Video: [Adiabatic Saturation Temperature](https://www.youtube.com/watch?v=QLPUOC3P7A0) — TutorialsPoint - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/applied_thermodynamics/adiabatic-saturation-wet-bulb - Whole chapter: https://www.bctengineer.com/md/semester_3/applied_thermodynamics/chapter-4 ## Question Explain the adiabatic saturation process and the thermodynamic wet bulb temperature. How is the wet bulb temperature measured, and what is a sling psychrometer? ## Short answer Adiabatic saturation: air flows over water in an insulated duct, exiting saturated at temperature t*. Energy balance h₁ + (ω*−ω₁)h_f* = h* gives the thermodynamic wet bulb temperature. WBT is measured with a wet-wick thermometer in moving air. A sling psychrometer whips dry- and wet-bulb thermometers to obtain the required air velocity. ## Answer #### Adiabatic saturation Unsaturated air at state 1 ($t_1, \omega_1$) flows through a long, insulated duct over a water surface. Water evaporates and the air becomes more humid. If the duct is long enough, the air leaves saturated at temperature $t^*$ and the water remains at $t^*$. Make-up water is supplied at $t^*$. ```text air in ---> [ insulated duct, water pool at t* ] ---> saturated air out t1, w1 t*, w* ^ make-up water at t* ``` **Energy balance** (steady flow, no heat exchange): $$ h_1 + (\omega^* - \omega_1)\,h_{f}^{*} = h^* $$ which gives $$ \omega_1 = \frac{(2501 - 2.381\,t^*)\,\omega^* - 1.006\,(t_1 - t^*)}{2501 + 1.805\,t_1 - 4.186\,t^*} $$ The temperature $t^*$ is the **thermodynamic wet bulb temperature** (adiabatic saturation temperature). The air gives up sensible heat, which supplies the latent heat of evaporation. #### Measurement of WBT A thermometer bulb is covered with a wet wick and exposed to air moving at 2.5 to 5 m/s. Evaporation cools the bulb until heat gained by convection equals the heat lost by evaporation. For air-water vapour mixtures at normal pressures the reading is very nearly equal to the thermodynamic WBT. A **sling psychrometer** has a dry bulb and a wet bulb thermometer mounted on a frame that is whirled by hand to give the required air velocity. The DBT and WBT read are used with the psychrometric chart to find RH, DPT and $\omega$. An aspirated (Assmann) psychrometer uses a small fan. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the adiabatic saturation process, what is the condition of air at the exit of the long insulated duct? A. Saturated at temperature t* B. Unchanged at t1 C. Dry with omega = 0 D. At the dew point of the inlet air with no added water Answer: A. A long enough duct lets air pick up vapour until saturated; the water stays at t*, the adiabatic saturation temperature. (Key point: Adiabatic saturation set-up) 2. Where does the latent heat for evaporation come from in the adiabatic saturator? A. An external heater B. Sensible heat given up by the air C. The make-up water supply being hot D. The duct walls Answer: B. The duct is insulated, so air cools and its sensible heat supplies the latent heat of evaporation. (Key point: Energy source of evaporation) 3. Why must the wet bulb be exposed to air moving at about 2.5 to 5 m/s? A. To keep the wick dry B. So the reading equals the thermodynamic WBT, with heat gain by convection balancing evaporative loss C. To raise DBT D. To measure dew point directly Answer: B. Adequate air speed makes convective gain equal evaporative loss at a steady value very close to the thermodynamic WBT. (Key point: Measuring WBT) 4. What is a sling psychrometer? A. A dry bulb and wet bulb thermometer on a frame whirled by hand for air velocity B. A device with a fan and no thermometers C. A hair hygrometer D. A single thermometer in a closed duct Answer: A. Whirling gives the needed air flow; the DBT and WBT readings then give RH, DPT and omega from the chart. (Key point: Sling psychrometer) ## Related in Applied Thermodynamics - [Adiabatic mixing of two air streams](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Amixing-of-air-streams-numerical) — Applied Thermodynamics, chapter 4 - [Definitions of psychrometric terms](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Apsychrometric-terms-definitions) — Applied Thermodynamics, chapter 4 - [Properties of moist air numerical](https://www.bctengineer.com/question.md?key=semester_3%2Fapplied_thermodynamics%3Apsychrometric-properties-numerical) — Applied Thermodynamics, chapter 4 ## Related in other subjects - [Adiabatic diffuser: exit temperature and pressure](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aadiabatic-diffuser-air-exit-temp-pressure) — Fundamental of Thermodynamics & Heat Transfer, chapter 4 - [Thermodynamic process; constant pressure heating diagrams](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aprocess-sketch-constant-pressure-heating) — Fundamental of Thermodynamics & Heat Transfer, chapter 1 - [Thermodynamic process; isothermal expansion diagrams](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aprocess-sketch-isothermal-expansion) — Fundamental of Thermodynamics & Heat Transfer, chapter 1 - [Wet process of cement manufacture](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Acement-wet-process-flow-diagram) — Civil Engineering Materials, chapter 5, asked 4×