Moist Air Processes: Heat, Cool, Humidify, Dry

The psychrometric chart shows every state of moist air as a point and every thermodynamic process as a line. There are four basic processes that form the building blocks of any air handling system — heating, cooling, humidification and drying. Each looks different on the chart, changes different quantities and has different energy demands. Their combinations describe even the most complex air conditioning schemes.

Heating: the simplest process on the chart

Heating with a heating coil (an electric coil, a hot-water or steam exchanger) is a horizontal line to the right on the psychrometric chart. Temperature rises, enthalpy rises, but the humidity ratio $W$ stays constant — no water is added to or removed from the air. The result is a drop in relative humidity, because the air’s maximum capacity for vapor grows with temperature and the air moves away from the saturation curve. Which type of heating you choose follows from the heat sources available.

Coil capacity:

$$Q_s = 1.08 \cdot \dot{V} \cdot \Delta t \quad \text{[Btu/h]}$$

with $\dot{V}$ in CFM and $\Delta t$ in °F. The 1.08 bundles 60 min/h, the standard air density 0.075 lb/ft³ and $c_{pa} = 0.240$ Btu/(lb·°F) — recompute it if the density is far from standard. More precisely, the capacity corresponds to the enthalpy difference, $Q = 4.5 \cdot \dot{V} \cdot \Delta h$.

Typical applications: preheating the incoming outdoor air in winter, reheating downstream of a cooling coil in summer.

Cooling: it depends on where you start

Cooling is not simply the reverse of heating — the result depends on whether the temperature falls above or below the dew point.

Cooling above the dew point (sensible cooling)

If the temperature does not drop below the dew point, this is sensible cooling. On the chart it is a horizontal line to the left: temperature falls, $W$ is constant, relative humidity rises (we approach the saturation curve). No condensation, no moisture removal.

Cooling below the dew point (cooling with dehumidification)

As soon as the temperature reaches the dew point, condensation begins. The air cools further along the saturation curve ($\varphi = 100\%$) — both the temperature and the humidity ratio $W$ fall, so the path runs down and to the left. Condensate drips off the coil as water. This is exactly how standard air conditioning works in summer.

The cooling coil capacity includes both a sensible and a latent component, which is why the correct quantity is enthalpy, not temperature:

$$Q_{total} = 4.5 \cdot \dot{V} \cdot (h_1 - h_2) \quad \text{[Btu/h]}$$

Humidification: two paths to the same result

Isothermal humidification (steam humidifier)

Supplying water vapor at constant temperature: on the chart a vertical line upward: $W$ rises, the temperature stays roughly constant, enthalpy rises. Electric or gas steam humidifiers are more energy-intensive, but precisely controllable. They suit facilities with higher demands on cleanliness, hygiene and precision.

Adiabatic humidification (water humidifiers)

Evaporating liquid water into the air with no external heat input: the air moves along a line of constant enthalpy, up and to the left: temperature falls, humidity rises, enthalpy stays almost constant. This is how evaporative cooling and an ultrasonic atomizer work. The energy for evaporation comes from the heat of the air itself, so adiabatic humidification requires no additional energy — unlike the isothermal kind. A comparison of the methods is in the article Types of air humidification. It is used most often in industry, wherever the boundary conditions allow it: where neither the product nor the space is troubled by the higher relative humidity of the supply air.

Drying: removing moisture from the air

Condensation drying (cooling below the dew point)

The most widespread method: the air is cooled below the dew point, moisture condenses out and drains away, after which the air is usually reheated to the required temperature. The result is air at the required temperature but with a lower humidity ratio (and therefore lower relative humidity). It is used mostly where the demands on dehumidification capacity are modest.

Desiccant drying (with a sorbent)

The air passes through a sorption material (silica gel, zeolites) that binds water vapor. On the chart the air moves down and to the right: humidity falls, but temperature rises (sorption releases heat). It can reach very low dew points (even below −40°F) that condensation drying cannot handle economically. For the details, see the article Desiccant dehumidification. It finds use in food processing, pharmaceuticals and specialized manufacturing or storage processes.

Air mixing: a combined process

In practice, two streams are almost always mixed, for example fresh outdoor air with recirculated indoor air. The resulting mixed state lies on the straight line connecting the two inlet states, at the position set by the mass ratio of the streams. Mixing ⅓ outdoor and ⅔ indoor air places the resulting point 1/3 of the way from the indoor state toward the outdoor one.

Combined processes: a real air conditioning scheme

Summer air conditioning of an office looks roughly like this on the chart:

1. Mixing — outdoor air (90°F / 50% RH) mixes with recirculated air (79°F / 55% RH).
2. Cooling with dehumidification — the mixture is cooled below the dew point, for example to 54–57°F / 95% RH.
3. Reheat — the air is brought back up to the supply temperature (61–64°F) so as not to create a draft.
4. Supply to the room — the air gradually absorbs the thermal and moisture load and shifts its state back toward the design conditions.

The whole cycle is a sequence of lines connecting the state points. The design procedure is covered in more detail in the article Air conditioning design on the psychrometric chart.

Frequently asked questions

Why does relative humidity fall during heating when no water is added? Heating increases the air’s capacity for water vapor. The water content (the humidity ratio) stays the same, but the ratio to the higher capacity — the relative humidity — falls.

What is the difference between isothermal and adiabatic humidification? Isothermal (steam) humidification adds vapor at constant temperature and uses energy to generate the steam. Adiabatic (water) humidification evaporates liquid water along a line of constant enthalpy — the air cools in the process and takes the energy for evaporation from its own heat.