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Unit 3 — Refrigeration System Fundamentals & Maintenance
Section 2 — Phase Transition of Water

2.3 Evaporation, Condensation & Dew Point

Evaporation and condensation happen constantly in HVAC/R systems. Understanding dew point, surface condensation, and evaporative cooling explains how air conditioners remove moisture and how cooling towers reject heat.

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💨Evaporation vs. Boiling 🌫️Dew Point ❄️Coil Condensation 🏭Cooling Towers

2.3.1 — Evaporation vs. Boiling

Both evaporation and boiling convert liquid water into vapour, but they are fundamentally different processes. Confusing them leads to mistakes when diagnosing why moisture leaves a surface or why a refrigerant behaves the way it does at a given pressure.

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Evaporation

Occurs only at the surface of a liquid, at any temperature above freezing. High-energy molecules near the surface escape into the air as vapour.

  • Slow and continuous process
  • No bubbles form in the liquid
  • Can occur well below the boiling point
  • Cools the remaining liquid (evaporative cooling)
  • Rate increases with temperature, airflow, and surface area
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Boiling

Occurs throughout the entire volume of the liquid, only when the liquid reaches its boiling point at the current pressure.

  • Rapid and vigorous process
  • Vapour bubbles form and rise through the liquid
  • Requires continuous heat input to sustain
  • Temperature stays constant during boiling
  • Boiling point changes with pressure
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Example — Evaporative Cooling in Everyday Life

When you step out of a swimming pool on a warm day, your skin feels cold even though the air temperature may be 90°F. The water is evaporating from your skin — not boiling. As each water molecule escapes, it carries away latent heat (970 BTU/lb), cooling the surface it leaves behind.

  • Sweat evaporation — the body’s natural cooling system works on exactly this principle.
  • Wet coil surfaces — condensate on an evaporator coil can re-evaporate if airflow over it slows down, reducing moisture removal efficiency.
  • Wet-bulb temperature — measured by a thermometer with a wet wick; evaporation cools it below the dry-bulb (air) temperature. The difference indicates humidity level.

2.3.2 — Dew Point

The dew point is the temperature at which water vapour in the air begins to condense into liquid water. When a surface cools below the dew point of the surrounding air, moisture forms on that surface — just like a cold glass sweating on a humid day.

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Dew Point and Relative Humidity

At the dew point, the relative humidity = 100%. The air is fully saturated and can no longer hold more moisture in vapour form. Any further cooling causes condensation.

Dry Air — Low Dew Point
  • Air temp: 75°F
  • Dew point: 45°F
  • Relative humidity: ~34%
  • Feels comfortable and dry
Humid Air — High Dew Point
  • Air temp: 75°F
  • Dew point: 70°F
  • Relative humidity: ~84%
  • Feels sticky and uncomfortable
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Dew Point in HVAC Practice

  • The evaporator coil must be colder than the air’s dew point to remove moisture from the airstream. Typical coil temperatures are 35–45°F for comfort cooling.
  • A dew point above 60°F (15.5°C) indicates uncomfortable indoor humidity — the system has a latent load problem.
  • Technicians use a psychrometric chart or digital hygrometer to read dew point and diagnose humidity complaints.
  • Condensation on cold ductwork, walls, or windows always means a surface has dropped below the air’s dew point — a moisture damage risk.

2.3.3 — Condensation on Evaporator Coils

An air conditioner’s evaporator coil does two things at once: it cools the air (sensible cooling) and it removes moisture from it (latent cooling). Condensation on the coil is what makes dehumidification possible.

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What Happens at the Coil — Step by Step

  1. Warm, humid room air passes over the cold evaporator coil.
  2. The air temperature drops as it gives up sensible heat to the cold refrigerant inside the coil.
  3. When the air temperature falls to its dew point, water vapour begins to condense on the coil surface as liquid droplets.
  4. Each drop of condensate releases its latent heat of vaporization at the coil surface (970 BTU/lb for water; similar values for refrigerant).
  5. The liquid water drains away through the condensate pan and drain line — this is the moisture physically removed from the building.
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Why Humid Days Are Harder on the System

On a humid day, a larger fraction of the system’s total capacity is used for latent cooling (dehumidification). Less capacity is left for sensible cooling, which is why the space can feel warm and clammy even when the system is running at full load.

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Airflow Speed and Moisture Removal

The speed of airflow across the coil directly affects how much moisture is removed:

Slower Airflow
  • Air spends more time on the coil
  • Coil gets colder (more subcooling)
  • More moisture condensed out
  • Better dehumidification
Faster Airflow
  • Air moves through quickly
  • Coil stays warmer
  • Less moisture removed
  • Adequate if latent load is low

Oversized systems that short-cycle are notorious for poor dehumidification — they cool the air temperature quickly and shut off before the coil has had time to condense enough moisture out of the air.

2.3.4 — Cooling Towers & Evaporative Cooling

A cooling tower uses deliberate evaporation to remove heat from condenser water. It is the “outdoor” side of a large chiller plant, rejecting the heat that the chiller collected from the building.

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How a Cooling Tower Works

  1. Hot condenser water (typically 95°F / 35°C) is pumped to the top of the tower and sprayed over fill media to create a large water surface area.
  2. A fan draws ambient air upward through the falling water.
  3. A small fraction of the water evaporates into the moving airstream — typically 1–2% of total flow.
  4. Each pound of water that evaporates absorbs ~970 BTU of latent heat from the remaining water, cooling it significantly.
  5. Cooled water (typically 85°F / 29°C) collects in the cold-water basin and is pumped back to the chiller condenser.
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Why Evaporative Cooling Is So Effective

About 75–95% of a cooling tower’s heat rejection comes from evaporation — not from direct air contact cooling. This is because the latent heat of vaporization (970 BTU/lb) is so large that even a tiny amount of evaporation moves a lot of heat.

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Evaporative Coolers (Swamp Coolers)

Residential and light commercial units that cool supply air directly by evaporating water into it. Work best in dry climates (low dew point). In humid climates, the air is already near saturation and little evaporation — and therefore little cooling — can occur.

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Evaporative Condensers

Commercial refrigeration units that combine an air-cooled condenser with a water spray. Water evaporates around the refrigerant coil, removing heat more efficiently than air alone — especially in hot weather.

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