2.2 Temperature, Pressure, Enthalpy, and the Pressure-Temperature Relationship

Accurate measurement and diagnostic interpretation of temperature, pressure, and enthalpy are the foundation of HVAC service and installation. A contractor must understand how ambient dry-bulb, wet-bulb, and dew point temperatures interact, how pressure scales convert, and how refrigerant state changes are evaluated using pressure-temperature ($P-T$) relationships and Mollier Pressure-Enthalpy ($P-h$) diagrams.

---

Temperature Definitions and Psychrometric Properties

In HVAC engineering, three primary temperature parameters describe atmospheric air conditions:

                      [ DRY-BULB TEMP ]
               Standard Ambient Air Temperature
                               |
            +------------------+------------------+
            |                                     |
            v                                     v
   [ WET-BULB TEMP ]                      [ DEW POINT TEMP ]
Reflects Evaporative Cooling          100% Relative Humidity
 (Moisture Content & Enthalpy)        Moisture Condensation Point

1. Dry-Bulb Temperature ($T_{db}$)

Dry-bulb temperature is the ambient air temperature measured by a standard thermometer exposed to the air stream while shielded from direct radiation and moisture. It measures purely sensible heat content.

2. Wet-Bulb Temperature ($T_{wb}$)

Wet-bulb temperature is measured by a thermometer whose sensing bulb is covered by a clean, water-saturated cloth wick over which air is passed at high velocity ($>500 \text{ ft/min}$). As water evaporates from the wick, it absorbs latent heat, lowering the temperature reading. The rate of evaporation depends on the relative humidity of the surrounding air:

• In dry air, evaporation is rapid, resulting in a large wet-bulb depression ($T_{db} - T_{wb}$).
• In 100% humid air (saturated air), no net evaporation can occur; thus, dry-bulb, wet-bulb, and dew point temperatures are all identical ($T_{db} = T_{wb} = T_{dp}$).

Wet-bulb temperature directly reflects the total heat content (enthalpy) of moist air and is critical when charging fixed-orifice air conditioners using superheat charts.

3. Dew Point Temperature ($T_{dp}$)

Dew point temperature is the exact temperature to which air must be cooled at constant pressure to reach $100%$ relative humidity (saturation). At the dew point, air can no longer hold water in gaseous form, and water vapor begins condensing into liquid droplets. In Texas air conditioning design, keeping indoor cooling coil surface temperatures below the room dew point is necessary to achieve effective moisture removal.

---

Pressure Scales and Measurement Fundamentals

Pressure ($P$) is defined as force per unit area ($P = F / A$). In HVAC systems, pressure is measured in multiple units depending on the application.

Atmospheric Pressure Standards

At standard sea level ($59^\circ\text{F}$ or $15^\circ\text{C}$), standard atmospheric pressure created by the weight of the earth's atmosphere is defined as: $\text{Standard Atmosphere} = 14.696 \text{ PSIA} \approx 14.7 \text{ PSIA} = 0 \text{ PSIG} = 29.92 \text{ in. Hg} = 760 \text{ mm Hg} = 101.325 \text{ kPa}$

To convert between gauge pressure (PSIG) and absolute pressure (PSIA): $\text{PSIA} = \text{PSIG} + 14.7$ $\text{PSIG} = \text{PSIA} - 14.7$

Note for Texas Contractors: High-altitude regions such as Amarillo or El Paso ($3,700-4,000 \text{ ft}$ elevation) experience lower atmospheric pressure (approx. $12.7 \text{ PSIA}$). Gauge conversions at high altitudes must adjust atmospheric constants accordingly.

---

Saturation Temperature and Pressure ($P-T$ Relationship)

A pure refrigerant in a closed system containing both liquid and vapor exists at a saturated state. At saturation, there is a fixed, direct mathematical relationship between pressure and temperature: for every specific pressure, there is exactly one saturation temperature at which liquid boils into vapor or vapor condenses into liquid.

• Boiling Point Elevation: Increasing system pressure raises the saturation boiling temperature.
• Condensing Point Depression: Lowering system pressure reduces the saturation condensing temperature.

Refrigerant blends (such as R-454B or R-407C) exhibit temperature glide, boiling across a range of temperatures between the Bubble Point (100% liquid saturation used for subcooling calculations) and the Dew Point (100% vapor saturation used for superheat calculations).

---

Superheat and Subcooling Diagnostic Calculations

Superheat and subcooling are the two key diagnostic metrics used by technicians to evaluate refrigerant charge, metering device performance, and coil airflow.

                               SUPERHEAT CALCULATION
 [ Actual Suction Line Temp ]  -  [ Evaporator Saturation Temp (from PSIG) ]  =  SUPERHEAT

                               SUBCOOLING CALCULATION
 [ Condenser Saturation Temp (from PSIG) ]  -  [ Actual Liquid Line Temp ]  =  SUBCOOLING

Superheat Calculation

Superheat is the sensible heat added to 100% saturated refrigerant vapor after it has completely boiled in the evaporator coil, raising its temperature above the saturation boiling point.

$\text{Superheat} = T_{\text{actual suction line}} - T_{\text{evaporator saturation}}$

Why Superheat Matters:

1. Compressor Protection: Ensures 100% pure vapor enters the compressor suction port. Liquid refrigerant is incompressible; liquid floodback will shatter compressor valves, score scrolls, and wash out motor oil.
2. Coil Capacity: Low superheat ($5^\circ\text{F}-8^\circ\text{F}$) indicates a flooded coil maximizing surface area usage, while high superheat ($>25^\circ\text{F}$) indicates a starved coil with low capacity.

Subcooling Calculation

Subcooling is sensible heat removed from 100% saturated liquid refrigerant in the condenser coil, cooling it below its saturation condensing temperature.

$\text{Subcooling} = T_{\text{condenser saturation}} - T_{\text{actual liquid line}}$

Why Subcooling Matters:

1. Metering Device Seal: Guarantees a solid column of liquid reaches the expansion valve inlet without flash gas bubbles.
2. Refrigerant Charge: Subcooling is the primary method for charging systems equipped with Thermostatic Expansion Valves (TXV) or Electronic Expansion Valves (EEV).

Field Diagnostic Matrix

---

Pressure-Enthalpy ($P-h$) Diagram Analysis

The Pressure-Enthalpy ($P-h$) diagram (Mollier Chart) visually maps the thermodynamic states of a refrigerant across the four stages of the mechanical vapor-compression cycle.

  Pressure (P)
       ^
       |               /--- SATURATED LIQUID LINE (left boundary, through 3 and 4)
       |              /
       |   LIQUID    /   VAPOR DOME   VAPOR
       |   REGION   /   (2-PHASE)    REGION
       |           /                  \
  P_c  +----------3--------------------2  <-- Condensing Pressure
       |          |                    |
       |          |                    |
  P_e  +----------4--------------------1  <-- Evaporating Pressure
       |           \                  /
       |            \                /
       |                             \--- SATURATED VAPOR LINE (right boundary, through 2 and 1)
       +---------------------------------------------> Enthalpy (h)

The Four Phases of the Vapor-Compression Cycle

1. Compression (Point 1 to Point 2):
   • Low-pressure, superheated vapor entering compressor port (1) is compressed isentropically (constant entropy) to high-pressure, superheated vapor (2).
   • Vertical rise in pressure; increase in enthalpy due to mechanical Heat of Compression (HOC).
2. Condensing (Point 2 to Point 3):
   • High-pressure vapor releases superheat (desuperheating), releases latent heat at constant condensing pressure/temperature across the vapor dome, and cools further into subcooled liquid (3).
   • Total heat rejected equals Total Heat of Rejection (THOR) ($h_2 - h_3$).
3. Expansion / Throttling (Point 3 to Point 4):
   • Subcooled liquid expands through TXV/EEV metering device, dropping rapidly to evaporator pressure (4).
   • This is an isenthalpic (constant enthalpy) process ($h_3 = h_4$). A portion of liquid flashes into vapor ("flash gas") to cool the remaining liquid down to evaporator temperature.
4. Evaporation (Point 4 to Point 1):
   • Low-pressure 2-phase mixture absorbs heat from indoor air, boiling liquid into vapor across the dome and adding superheat (1).
   • Enthalpy gained ($h_1 - h_4$) represents the Net Refrigerating Effect (NRE).

Key Refrigeration Cycle Equations from $P-h$ Chart

$\text{Net Refrigerating Effect (NRE)} = h_1 - h_4 \quad (\text{BTU/lb})$ $\text{Heat of Compression (HOC)} = h_2 - h_1 \quad (\text{BTU/lb})$ $\text{Total Heat of Rejection (THOR)} = h_2 - h_3 \quad (\text{BTU/lb})$ $\text{Coefficient of Performance (COP)} = \frac{\text{NRE}}{\text{HOC}} = \frac{h_1 - h_4}{h_2 - h_1}$ $\text{Refrigerant Mass Flow Rate} = \frac{\text{Required System Cooling Capacity (BTU/hr)}}{\text{NRE (BTU/lb)}} \quad (\text{lb/hr})$