3.2 Psychrometric Chart Navigation, Air Properties & Core HVAC Processes

Psychrometric Chart Navigation, Air Properties & Core HVAC Processes

Psychrometrics is the study of thermodynamic properties of gas-vapor mixtures, specifically moist atmospheric air (a mixture of dry air and water vapor). For HVAC contractors, the psychrometric chart is an indispensable diagnostic and engineering tool used to evaluate cooling coil performance, dehumidification capability, economizer operation, and building ventilation mixing.

---

1. The Seven Fundamental Properties of Moist Air

At standard atmospheric pressure (29.921 in. Hg / 14.696 psia at sea level), the state of moist air is fully defined by identifying any two independent thermodynamic properties. Once that "state point" is located on the chart, the remaining five properties can be read directly.

+-----------------------------------------------------------------------------------------+
|                         THE SEVEN MOIST AIR PROPERTIES SUMMARY                          |
|                                                                                         |
|   1. DRY-BULB TEMPERATURE (T_db)    --> True kinetic temperature; vertical lines (°F)   |
|   2. WET-BULB TEMPERATURE (T_wb)    --> Evaporative cooling limit; slanted lines (°F)   |
|   3. DEW-POINT TEMPERATURE (T_dp)   --> Condensation threshold; horizontal lines (°F)   |
|   4. RELATIVE HUMIDITY (% RH)       --> Moisture saturation fraction; curved lines (%)  |
|   5. HUMIDITY RATIO (W)             --> Absolute moisture mass; grains/lb or lb/lb      |
|   6. SPECIFIC VOLUME (v)            --> Inverse density; steep diagonal lines (ft³/lb)  |
|   7. ENTHALPY (h)                   --> Total thermal energy; outer diagonal scale (BTU/lb) |
+-----------------------------------------------------------------------------------------+

Detailed Engineering Analysis of Properties

1. Dry-Bulb Temperature (T_db):

   • The sensible temperature of the air measured by an ordinary thermometer shielded from direct thermal radiation.
   • Chart Representation: Uniform vertical lines extending from the bottom horizontal axis (°F).

2. Wet-Bulb Temperature (T_wb):

   • The lowest temperature air can reach through adiabatic evaporation of water into the airstream. Measured by a thermometer whose bulb is covered in a water-saturated cotton wick exposed to rapid air velocity (~900 ft/min).
   • Chart Representation: Diagonally sloping lines running from upper-left (saturation curve) downward to the bottom right.

3. Dew-Point Temperature (T_dp):

   • The temperature to which air must be cooled at constant barometric pressure and moisture content to become completely saturated (100% RH), initiating water vapor condensation (dew formation).
   • Chart Representation: Horizontal lines extending left to the saturation curve.

4. Relative Humidity (% RH):

   • The ratio of the actual partial water vapor pressure (p_w) in the air to the saturation vapor pressure (p_ws) of water at the same dry-bulb temperature, expressed as a percentage: RH = (p_w / p_ws) × 100%.
   • Chart Representation: Distinct curved lines radiating from lower-left to upper-right. The outermost curved boundary represents 100% RH (the Saturation Curve), where T_db = T_wb = T_dp.

5. Humidity Ratio / Specific Humidity (W):

   • The actual mass of water vapor present per unit mass of bone-dry air. Expressed in grains of moisture per pound of dry air (7,000 grains = 1.0 lb) or pounds of water per pound of dry air.
   • Chart Representation: Horizontal lines read on the far-right vertical axis.

6. Specific Volume (v):

   • The volume occupied by one pound of dry air plus its associated water vapor. Specific volume is the reciprocal of air density (v = 1 / ρ).
   • Chart Representation: Steeply angled lines (steeper than wet-bulb lines), typically ranging from 12.5 to 15.0 ft³/lb dry air.

7. Enthalpy (h):

   • The total thermal energy content of moist air per unit mass, combining sensible heat (air and vapor temperature) and latent heat (vaporization energy). Expressed in BTUs per pound of dry air relative to a 0°F dry air baseline.
   • Chart Representation: Read on the diagonal scale positioned outside the saturation curve, aligning closely with wet-bulb lines.

---

2. The Core Air Conditioning Processes

Thermodynamic processes alter the temperature and moisture state of air. On a psychrometric chart, each process forms a distinct vector path:

+-----------------------------------------------------------------------------------------+
|                        PSYCHROMETRIC PROCESS DIRECTIONAL VECTOR MAP                     | 
|                                                                                         |
|                                    Humidification                                       |
|                                          ^                                              |
|                                          |                                              |
|                    Evaporative Cooling   |   Heating & Humidification                   |
|                    (Constant Enthalpy)   |                                              |
|                             \            |            /                                 |
|                              \           |           /                                  |
|        Sensible Cooling <----------------+----------------> Sensible Heating            |
|        (Constant Moisture)               |                  (Constant Moisture)         |
|                              /           |           \                                  |
|                             /            |            \                                 |
|                     Cooling &            |   Chemical Dehumidification                  |
|                     Dehumidification     |   (Desiccant)                                |
|                                          v                                              |
|                                   Dehumidification                                      |
+-----------------------------------------------------------------------------------------+

1. Sensible Heating (Horizontal Right Vector)

• Mechanism: Air passes across a hot heating coil or furnace heat exchanger with no moisture added.
• Thermodynamic Impact: T_db increases, Enthalpy (h) increases, Humidity Ratio (W) remains constant, Dew-Point (T_dp) remains constant, Relative Humidity (% RH) decreases.

2. Sensible Cooling (Horizontal Left Vector)

• Mechanism: Air passes across a cooling surface whose surface temperature is above the dew-point temperature of the entering air (no condensation occurs).
• Thermodynamic Impact: T_db decreases, Enthalpy (h) decreases, Humidity Ratio (W) remains constant, Dew-Point (T_dp) remains constant, Relative Humidity (% RH) increases.

3. Cooling with Dehumidification (Down-and-Left Vector)

• Mechanism: Standard direct-expansion (DX) or chilled water cooling coil operation. Air passes across a coil whose surface temperature (Apparatus Dew Point / ADP) is below the dew point of entering air. Water vapor condenses out of the airstream onto the coil fins and drains away.
• Thermodynamic Impact: T_db decreases, T_wb decreases, T_dp decreases, Humidity Ratio (W) decreases, Enthalpy (h) decreases, Relative Humidity increases (typically leaving at 90%–95% RH).
• Coil Bypass Factor (BF): The percentage of air that passes through the coil fins without contacting the cold surface. Modern residential coils have a bypass factor between 0.05 and 0.15 (5% to 15%).

4. Humidification (Vertical Upward Vector)

• Mechanism: Direct steam injection into the airstream.
• Thermodynamic Impact: Humidity Ratio (W) increases, Dew-Point (T_dp) increases, Relative Humidity increases, T_db remains nearly constant.

5. Heating and Humidification (Up-and-Right Vector)

• Mechanism: Winter conditioned air passing through a warm air furnace and an active steam/bypass humidifier.
• Thermodynamic Impact: T_db increases, W increases, Enthalpy (h) increases significantly.

6. Evaporative Cooling / Adiabatic Saturation (Up-and-Left Along Constant WB)

• Mechanism: Direct evaporative cooler (swamp cooler). Water evaporates into unsaturated air with no external heat added or removed (adiabatic).
• Thermodynamic Impact: Sensible heat transforms into latent heat of vaporization. T_db drops significantly, Humidity Ratio (W) rises, % RH rises, while Wet-Bulb Temperature (T_wb) and Enthalpy (h) remain constant.

---

3. Mixed Air Calculations (Ventilation & Economizers)

Commercial and residential systems frequently mix two airstreams: Return Air (RA) from the conditioned building and Outdoor Air (OA) for ventilation or free economizer cooling.

+-----------------------------------------------------------------------------------------+
|                            AIR MIXING CONSERVATION DIAGRAM                              |
|                                                                                         |
|   Return Air (RA):   CFM_RA, T_RA, W_RA, h_RA                                           |
|   ===============================================\                                      |
|                                                   +===> Mixed Air (MA):                 |
|   ===============================================/      CFM_total, T_MA, W_MA, h_MA     |
|   Outdoor Air (OA):  CFM_OA, T_OA, W_OA, h_OA                                           |
+-----------------------------------------------------------------------------------------+

Governing Mathematical Formulas for Air Mixing

Based on the conservation of mass and energy:

Total Airflow: CFM_total = CFM_RA + CFM_OA

Mixed Air Dry-Bulb Temperature (T_MA):
T_MA = ( (CFM_RA × T_RA) + (CFM_OA × T_OA) ) / CFM_total
T_MA = T_RA + [ (CFM_OA / CFM_total) × (T_OA - T_RA) ]

Mixed Air Enthalpy (h_MA):
h_MA = ( (CFM_RA × h_RA) + (CFM_OA × h_OA) ) / CFM_total

Mixed Air Humidity Ratio (W_MA):
W_MA = ( (CFM_RA × W_RA) + (CFM_OA × W_OA) ) / CFM_total

Percentage Outdoor Air (% OA):
% OA = ( (T_MA - T_RA) / (T_OA - T_RA) ) × 100%

---

4. Step-by-Step Worked Air Mixing Calculation Example

Scenario: A rooftop commercial packaged unit in Louisville, KY operates with an outdoor air economizer delivering a total airflow of 4,000 CFM. The system settings and psychrometric readings are:

• Return Air (RA): 3,000 CFM (75% of total airflow), 76°F DB, 63°F WB, Enthalpy h_RA = 28.5 BTU/lb, Moisture W_RA = 65 grains/lb.
• Outdoor Air (OA): 1,000 CFM (25% of total airflow), 92°F DB, 76°F WB, Enthalpy h_OA = 39.5 BTU/lb, Moisture W_OA = 118 grains/lb.

Step 1: Calculate Mixed Air Dry-Bulb Temperature (T_MA)
T_MA = [ (3,000 CFM × 76°F) + (1,000 CFM × 92°F) ] / 4,000 CFM
T_MA = [ 228,000 + 92,000 ] / 4,000
T_MA = 320,000 / 4,000 = 80.0°F DB

Step 2: Calculate Mixed Air Enthalpy (h_MA)
h_MA = [ (3,000 CFM × 28.5 BTU/lb) + (1,000 CFM × 39.5 BTU/lb) ] / 4,000 CFM
h_MA = [ 85,500 + 39,500 ] / 4,000
h_MA = 125,000 / 4,000 = 31.25 BTU/lb

Step 3: Calculate Mixed Air Humidity Ratio (W_MA)
W_MA = [ (3,000 CFM × 65 gr/lb) + (1,000 CFM × 118 gr/lb) ] / 4,000 CFM
W_MA = [ 195,000 + 118,000 ] / 4,000
W_MA = 313,000 / 4,000 = 78.25 grains/lb dry air

Step 4: Calculate Total Required Coil Cooling Load
If the evaporator coil cools this mixed airstream down to 55°F DB leaving condition with enthalpy h_leaving = 23.0 BTU/lb:
Δh_coil = 31.25 BTU/lb - 23.0 BTU/lb = 8.25 BTU/lb
Qt = 4.5 × CFM_total × Δh_coil
Qt = 4.5 × 4,000 CFM × 8.25 BTU/lb = 148,500 BTU/hr
Total Tonnage = 148,500 / 12,000 = 12.375 Tons