Evaporative Cooling: Principle, Efficiency, Psychrometric Chart

Evaporative cooling evaporates water into an air stream: evaporation removes heat and the air cools down, and it does so with no compressor, no refrigerant and far lower energy use than conventional air conditioning. Thermodynamically it is an adiabatic process: the total enthalpy does not change, only the split between sensible heat (temperature falls) and latent heat (humidity rises). On the psychrometric chart it therefore runs along a line of constant enthalpy, and its natural limit is the wet-bulb temperature of the inlet air.

The physical principle: evaporation and latent heat

When water evaporates into air, it consumes the latent heat of vaporization — about 1,075 Btu/lb at 32°F, or 1,050 Btu/lb at 77°F. This energy comes from nowhere other than the heat content of the air itself. The sensible heat the air releases as its temperature falls is exactly the heat that evaporation consumes, so the total enthalpy stays (approximately) the same.

Where the process runs on the psychrometric chart

The initial state is point A (for example 95°F / φ = 20%). The air passes through a humidifier and advances approximately along a constant-enthalpy line (more precisely along a line of constant wet-bulb temperature) toward a lower temperature — up and to the left on the Carrier chart. The process ends either at the required point or, at the latest, on the saturation curve ($\varphi = 100\%$) — this is the theoretical maximum, the so-called adiabatic saturation temperature, which corresponds to the wet-bulb temperature of the inlet air. Real equipment reaches 70–95% of this maximum, depending on the adiabatic effectiveness $\eta_{a}$ (saturation efficiency, $\varepsilon$ in ASHRAE).

Calculating the saturation efficiency

The adiabatic effectiveness $\eta_{a}$ (saturation efficiency) is the basic parameter of every evaporative cooler:

$$\eta_{a} = \frac{t_{\text{in}} - t_{\text{out}}}{t_{\text{in}} - t_{wb}}$$

where $t_{wb}$ is the wet-bulb temperature of the inlet air.

Example: inlet air 95°F / φ = 15%, wet-bulb temperature 63.0°F, a cooler with $\eta_{a} = 0.85$:

$$t_{\text{out}} = 95 - 0.85 \cdot (95 - 63.0) = 95 - 27.2 = 67.8\ ^{\circ}\text{F}$$

Cooling by more than 27°F without a single watt from a compressor.

Direct vs. indirect evaporative cooling

Direct evaporative cooling

The air comes into direct contact with water or a water mist — the familiar swamp cooler. Simple, cheap, very effective in a dry climate. The drawback: the air entering the room is more humid. Along the Gulf Coast or through the Southeast, where the inlet φ is usually 50–60% at design conditions, there is less room for cooling and the resulting indoor humidity can be uncomfortable.

Indirect evaporative cooling

Evaporation cools a secondary air stream, whose heat is passed to the primary stream through an exchanger without the two mixing. The primary air cools without picking up moisture — on the chart it moves horizontally to the left (sensible cooling). The most advanced variant is the Maisotsenko cycle (M-cycle), which can bring the primary-air outlet temperature below the wet-bulb temperature. From a hygiene standpoint, indirect cooling is a common way to bring partial air cooling into a project at low capital cost.

The limits of evaporative cooling

The fundamental limit is clear: cooling works better the drier the inlet air is. Once the inlet φ exceeds 60–70%, the room for cooling drops dramatically. This is why the technology is a mainstay across the arid Southwest and the Mountain West — Phoenix, Albuquerque, Denver — and a marginal proposition in Houston or Atlanta, where a humid summer afternoon leaves little wet-bulb depression to work with; the indirect variants stay competitive even there. Another limit is water quality: hard water fouls the systems with scale, soft water can cause corrosion. A further drawback is the hygiene risk from Legionella, which is kept in check by adequate water treatment, including blowdown and, where appropriate, UV disinfection. On direct systems this contamination control is mandatory. Direct evaporative cooling is not suitable for supplying spaces with elevated cleanliness requirements. Water consumption during operation is a further constraint.

Comparison with vapor-compression cooling

An evaporative cooler typically uses 10–25% of the energy of a vapor-compression system of the same capacity. The ratio of cooling output to the fan and pump power input (a simplified “COP” in this sense, not the thermodynamic COP of a refrigeration cycle) is on the order of 10–40 for evaporative systems, whereas the actual COP of a split air conditioner is usually 2.5 to 5. The trade-off is the dependence on the humidity of the inlet air and the inability to guarantee precise conditions without a supplementary system. For data centers, industrial buildings and outdoor cooling in a dry climate, evaporative cooling is the first choice; where humidity has to be held to a tolerance, more of a supplement.

Frequently asked questions

Why can’t an evaporative cooler cool the air below the wet-bulb temperature? Because the process runs along a line of constant enthalpy, and the wet-bulb temperature is the point where this line crosses the saturation curve. Beyond it, the air cannot be cooled further, because it is saturated. The exception is advanced indirect cycles (the M-cycle).

Where in the US does evaporative cooling pay off? Anywhere the design wet-bulb sits well below the dry-bulb: the Southwest, the Great Basin and the high plains. In the humid Southeast, direct cooling has limited potential, but the indirect variants remain energy-efficient.

How does the saturation efficiency relate to design? It determines how close to the wet-bulb temperature the outlet gets. From $\eta_{a}$ and the inlet conditions you compute the outlet temperature directly, which can be verified on the psychrometric chart.