7.2 Psychrometric Air Conditioning Processes: Cooling, Heating, Humidification & Dehumidification
Key Takeaways
- The four cardinal thermodynamic directions on the psychrometric chart are sensible heating (due east/right), sensible cooling (due west/left), humidification (due north/up), and dehumidification (due south/down).
- Cooling and dehumidification proceeds along a diagonal path down and to the left directed toward the Apparatus Dew Point (ADP), which represents the effective coil surface saturation temperature.
- Coil Bypass Factor (BF = [T_leaving - ADP] / [T_entering - ADP]) quantifies the percentage of air passing through a coil without contacting fin surfaces; Contact Factor (CF = 1 - BF) defines coil thermal effectiveness.
- Adiabatic humidification (direct evaporative cooling) follows a line of constant wet-bulb temperature and constant enthalpy upward and to the left, converting sensible heat into latent heat with zero net total heat transfer (Q_t = 0).
- Mixed air calculations use mass-weighted conservation of energy: Mixed Air Temperature (MAT) = (RAT × %RA) + (OAT × %OA), with identical weighted relationships governing mixed air enthalpy (h_m) and humidity ratio (W_m).
7.2 Psychrometric Air Conditioning Processes: Cooling, Heating, Humidification & Dehumidification
Thermodynamic air conditioning processes represent continuous physical changes in air temperature, moisture content, and energy as air traverses fans, heating exchangers, DX evaporator coils, humidifiers, and mixing plenums.
Mapping these processes on the psychrometric chart allows HVAC engineers and contractors to visualize conditioning pathways, evaluate coil performance, calculate moisture removal rates, and size multi-zone ventilation systems.
1. The Cardinal Psychrometric Conditioning Vectors
Every environmental conditioning process is a directional vector on the psychrometric chart composed of sensible (horizontal) and latent (vertical) components:
CARDINAL PSYCHROMETRIC VECTORS
Humidification Only
(Constant T_db) ▲ [North]
│
Evaporative Cooling │ Chemical Dehumidification
(Adiabatic, Constant h/T_wb) │ (Sensible Heat Addition)
▲ │ ▲
\ │ /
◄──────────────────────┼──────────────────────►
Sensible Cooling │ Sensible Heating
(Constant W) [West] │ (Constant W) [East]
│
│
▼ [South]
Dehumidification Only
(Constant T_db)
Thermodynamic Vector Summary Table
| Process Vector | Chart Direction | Dry-Bulb (T_DB) | Humidity Ratio (W) | Relative Humidity (% RH) | Enthalpy (h) |
|---|---|---|---|---|---|
| Sensible Heating (Electric strip, gas furnace) | Due East (Right) | Increases (ΔT > 0) | Constant (ΔW = 0) | Decreases | Increases |
| Sensible Cooling (Dry coil, above T_DP) | Due West (Left) | Decreases (ΔT < 0) | Constant (ΔW = 0) | Increases | Decreases |
| Pure Humidification (Steam injection) | Due North (Up) | Constant (ΔT = 0) | Increases (ΔW > 0) | Increases | Increases |
| Pure Dehumidification (Solid desiccant wheel) | Due South (Down) | Constant (ΔT = 0) | Decreases (ΔW < 0) | Decreases | Decreases |
| Cooling & Dehumidifying (DX cooling coil) | Southwest (Down-Left) | Decreases | Decreases | Increases (approaches 100%) | Decreases |
| Adiabatic Evaporative Cooling (Swamp cooler) | Northwest (Up-Left) | Decreases | Increases | Increases | Constant (Δh = 0) |
2. Cooling and Dehumidification & Apparatus Dew Point (ADP)
When air enters a direct-expansion (DX) evaporator coil or chilled water coil, moisture removal (dehumidification) occurs if and only if the coil surface temperature is below the entering air's dew-point temperature (T_coil surface < T_DP,entering).
COOLING & DEHUMIDIFICATION PROCESS CURVE
Saturation Curve (100% RH)
┌───────────────────────────────────────────────┐
│ │
│ [Apparatus Dew Point - ADP] │
│ (50°F, 100% RH) │
│ * │
│ \ │
│ \ Leaving Air Point │
│ * (55°F DB / 53°F WB) │
│ \ │
│ \ │
│ * Entering Air Point │
│ (80°F DB / 67°F WB) │
└───────────────────────────────────────────────┘
Apparatus Dew Point (ADP)
The Apparatus Dew Point (ADP) is the effective coil surface temperature. It is graphically determined by extending the straight process line connecting the entering air state point and the leaving air state point until it intersects the 100% RH saturation curve.
Coil Bypass Factor (BF) and Contact Factor (CF)
In real-world HVAC equipment, not all air entering the coil comes into direct physical contact with the cold fins and tubes. A portion of the air stream passes through unobstructed open spaces between fins without undergoing heat transfer:
- Bypass Factor (BF): The fraction of total airflow that completely bypasses the cooling surfaces unchanged.
- Contact Factor (CF): The fraction of total airflow that makes perfect thermodynamic contact with the coil surfaces (CF = 1.0 - BF).
Mathematical Formulation of Bypass Factor:
BF = (T_DB,leaving - ADP) / (T_DB,entering - ADP) = (W_leaving - W_ADP) / (W_entering - W_ADP) = (h_leaving - h_ADP) / (h_entering - h_ADP)
Rearranging to solve for the leaving air dry-bulb temperature:
T_DB,leaving = ADP + BF × (T_DB,entering - ADP)
Typical Equipment Bypass Factors:
- Residential DX split-system evaporator coils: BF = 0.10 to 0.20 (10% to 20% bypass)
- Commercial multi-row chilled water coils (6 to 8 rows): BF = 0.03 to 0.08 (3% to 8% bypass)
Worked Example: Leaving Air Temperature Calculation
Problem: Return air enters a residential cooling coil at 80.0°F dry-bulb. The coil operates at an Apparatus Dew Point (ADP) of 48.0°F with an engineered Bypass Factor (BF) of 0.15. What is the leaving supply air dry-bulb temperature?
- Apply the bypass equation:
T_DB,leaving = 48.0°F + [0.15 × (80.0°F - 48.0°F)] - Calculate temperature difference across coil potential:
80.0°F - 48.0°F = 32.0°F - Solve:
T_DB,leaving = 48.0°F + (0.15 × 32.0°F) = 48.0°F + 4.8°F = 52.8°F
Result: The leaving air dry-bulb temperature is 52.8°F.
3. Adiabatic Evaporative Cooling (Constant Wet-Bulb Process)
Adiabatic evaporative cooling is the thermodynamic process where liquid water evaporates into an unsaturated air stream with zero external heat added or removed (Q_total = 0).
ADIABATIC SATURATION THERMODYNAMICS
Dry Air Stream ──► [Wetted Media Pad] ──► Cool, Humid Air
• High Dry-Bulb • Low Dry-Bulb
• Low Moisture (Grains) • High Moisture (Grains)
• Constant Enthalpy (h) ◄────────────────► • Constant Enthalpy (h)
Thermodynamic Mechanism
- As air passes through saturated media pads, sensible heat from the dry air is absorbed to supply the latent heat of vaporization (h_fg ≈ 1,061 BTU/lb) needed to evaporate the liquid water into vapor.
- Sensible dry-bulb temperature drops significantly, while humidity ratio (W) increases proportionally.
- Because no heat enters or leaves the system boundary, the total energy content is conserved: Enthalpy (h) and Wet-Bulb Temperature (T_WB) remain constant throughout the entire direct evaporative process.
- The process line slopes upward and to the left along the constant wet-bulb coordinate line toward the saturation curve.
4. Mixed Air Calculations (Return Air + Outdoor Air)
Commercial packaged rooftop units (RTUs) and residential systems equipped with mechanical fresh air ventilation mix two distinct air streams prior to entering the conditioning coil: Return Air (RA) from the conditioned space and Outdoor Air (OA) for code-mandated ventilation (ASHRAE 62.1/62.2).
AIR STREAM MIXING PLENUM SCHEMATIC
Outdoor Air (OA) ──────┐
(e.g., 105°F, 20% CFM) │
├──► [MIXING PLENUM] ──► Mixed Air to Coil (MAT)
Return Air (RA) ──────┘
(e.g., 75°F, 80% CFM)
Mass and Energy Balance Formulas
Because mass and energy are strictly conserved, the thermodynamic properties of the Mixed Air (MA) stream are calculated as the volumetric-weighted averages of the constituent streams:
MAT = (RAT × %RA) + (OAT × %OA)
MAT = RAT + [(CFM_OA / CFM_Total) × (OAT - RAT)]
Enthalpy: h_mixed = (h_RA × %RA) + (h_OA × %OA)
Humidity: W_mixed = (W_RA × %RA) + (W_OA × %OA)
Comprehensive Mixed Air Worked Example
Scenario: A light commercial RTU serving a retail space in Scottsdale, Arizona circulates a total volumetric airflow of 4,000 CFM. The economizer minimum ventilation damper introduces 15% Outdoor Air (600 CFM), while the remaining 85% (3,400 CFM) is Return Air.
Given Psychrometric Conditions:
- Outdoor Air (OA): 105.0°F DB, Enthalpy h_OA = 36.0 BTU/lb, Moisture W_OA = 85.0 gr/lb
- Return Air (RA): 75.0°F DB, Enthalpy h_RA = 28.0 BTU/lb, Moisture W_RA = 60.0 gr/lb
Calculations:
-
Mixed Air Temperature (MAT):
- MAT = (75.0°F × 0.85) + (105.0°F × 0.15) = 63.75°F + 15.75°F = 79.5°F
-
Mixed Air Enthalpy (h_mixed):
- h_mixed = (28.0 BTU/lb × 0.85) + (36.0 BTU/lb × 0.15) = 23.8 + 5.4 = 29.2 BTU/lb
-
Mixed Air Humidity Ratio (W_mixed):
- W_mixed = (60.0 gr/lb × 0.85) + (85.0 gr/lb × 0.15) = 51.0 + 12.75 = 63.75 gr/lb
[!IMPORTANT] Diagnostic Standard: The mixed air temperature entering the cooling coil in this desert application is 79.5°F rather than the room temperature of 75.0°F. When performing capacity calculations or verifying coil delta-T, technicians must always measure entering dry-bulb downstream of the mixing box (entering coil face) rather than at a room return grille.
Which of the following describes the thermodynamic behavior of air during an ideal direct evaporative cooling process?
A commercial air handler circulates 10,000 CFM total. The system mixes 2,000 CFM of outdoor air at 100°F DB with 8,000 CFM of return air at 75°F DB. What is the resulting Mixed Air Temperature (MAT)?
Return air at 80°F DB enters a DX cooling coil having an Apparatus Dew Point (ADP) of 50°F. If the coil has a Bypass Factor (BF) of 0.20, what is the dry-bulb temperature of the air leaving the coil?