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Adiabatic saturation and wet bulb temperature

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.

Explain the adiabatic saturation process and the thermodynamic wet bulb temperature. How is the wet bulb temperature measured, and what is a sling psychrometer?

Adiabatic saturation

Unsaturated air at state 1 (t1,ω1t_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∗t^* and the water remains at t∗t^*. Make-up water is supplied at t∗t^*.

  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):

h1+(ω∗−ω1) hf∗=h∗h_1 + (\omega^* - \omega_1)\,h_{f}^{*} = h^*

which gives

ω1=(2501−2.381 t∗) ω∗−1.006 (t1−t∗)2501+1.805 t1−4.186 t∗\omega_1 = \frac{(2501 - 2.381\,t^*)\,\omega^* - 1.006\,(t_1 - t^*)}{2501 + 1.805\,t_1 - 4.186\,t^*}

The temperature t∗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 ω\omega. An aspirated (Assmann) psychrometer uses a small fan.

✓ Answer checked by Claude Opus 5.5 (high effort)

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