Psychrometric Processes in HVAC Systems

Psychrometric Processes in HVAC Systems

Every HVAC system performs psychrometric processes to condition air for thermal comfort and process requirements. Understanding these processes enables equipment selection, energy analysis, and system troubleshooting. This guide examines the six fundamental processes: sensible heating, sensible cooling, cooling with dehumidification, humidification, chemical dehumidification, and air mixing.

Process Representation on Psychrometric Chart

Psychrometric processes appear as paths on the psychrometric chart connecting initial and final state points. The path geometry indicates the type of process:

• Horizontal line (constant W): Sensible heating or cooling
• Vertical line (constant T): Pure humidification or dehumidification
• Diagonal line toward saturation: Cooling with dehumidification
• Diagonal line away from saturation: Heating with humidification
• Straight line between points: Air mixing
• Constant WBT line: Adiabatic saturation (evaporative cooling)

Sensible Heating Process

Sensible heating adds heat without changing moisture content. Humidity ratio remains constant while dry bulb temperature increases.

Process Characteristics

On Psychrometric Chart:

• Horizontal line moving right
• Constant humidity ratio (W₁ = W₂)
• Decreasing relative humidity
• Increasing dry bulb temperature
• Increasing enthalpy

Physical Explanation: Heat transfer to air increases molecular kinetic energy, raising temperature. No phase change occurs; water vapor mass remains constant. As temperature rises, saturation pressure increases, reducing relative humidity even though absolute moisture stays constant.

Governing Equations

Energy balance for sensible heating:

$$Q_s = \dot{m}_a c_p (T_2 - T_1)$$

Where:

• $Q_s$ = sensible heat transfer rate (Btu/hr)
• $\dot{m}_a$ = mass flow rate of dry air (lb_da/hr)
• $c_p$ = specific heat of air = 0.24 Btu/(lb·°F)
• $T_2$ = final temperature (°F)
• $T_1$ = initial temperature (°F)

Volumetric Form:

$$Q_s = 1.08 \times CFM \times \Delta T$$

Where:

• CFM = volumetric flow rate (cubic feet per minute)
• ΔT = temperature rise (°F)
• 1.08 = 60 min/hr × 0.075 lb/ft³ × 0.24 Btu/(lb·°F)

Derivation of 1.08 Constant:

$$Q = \frac{CFM \times 60 \text{ min/hr} \times 0.075 \text{ lb/ft}^3 \times 0.24 \text{ Btu/(lb·°F)} \times \Delta T}{1}$$

$$Q = 1.08 \times CFM \times \Delta T$$

This assumes standard air density (0.075 lb/ft³). For other densities:

$$Q_s = 60 \times CFM \times \rho \times c_p \times \Delta T$$

Applications

Preheat Coils: Protect downstream coils from freezing by warming outdoor air above 35-40°F before mixing or further cooling.

Reheat Coils: Raise supply air temperature after cooling/dehumidification to prevent overcooling spaces or achieve specific supply temperatures for VAV systems.

Furnaces and Unit Heaters: Provide space heating through direct sensible heat addition.

Sensible Cooling Process

Sensible cooling removes heat without moisture condensation. Like sensible heating, humidity ratio remains constant.

Process Characteristics

On Psychrometric Chart:

• Horizontal line moving left
• Constant humidity ratio
• Increasing relative humidity
• Decreasing dry bulb temperature
• Decreasing enthalpy

Cooling Limit: Process continues until saturation (100% RH). Further cooling causes condensation.

Governing Equations

$$Q_s = 1.08 \times CFM \times (T_1 - T_2)$$

Where $T_1 > T_2$ for cooling.

Temperature drop from heat removal:

$$\Delta T = \frac{Q_s}{1.08 \times CFM}$$

Cooling with Dehumidification

Most cooling coils simultaneously reduce temperature and remove moisture. The process line crosses the saturation curve, causing condensation.

Apparatus Dew Point (ADP)

The apparatus dew point represents the effective coil surface temperature for heat and mass transfer. Air leaving a perfect coil would be saturated at the ADP temperature.

Bypass Factor (BF): Real coils don’t achieve perfect contact. Some air “bypasses” the coil:

$$BF = \frac{T_{leaving} - T_{ADP}}{T_{entering} - T_{ADP}}$$

Where:

• BF = bypass factor (0 to 1.0)
• Typical values: 0.05 to 0.30
• Lower BF = better coil performance

Contact Factor:

$$CF = 1 - BF$$

Sensible Heat Ratio (SHR)

SHR indicates the proportion of sensible versus total cooling:

$$SHR = \frac{Q_{sensible}}{Q_{total}} = \frac{1.08 \times CFM \times \Delta T}{4.5 \times CFM \times \Delta h}$$

Simplified:

$$SHR = \frac{0.24 \Delta T}{\Delta h}$$

Typical SHR Values:

• Office spaces: 0.70 to 0.85
• Retail: 0.75 to 0.80
• Laboratories: 0.50 to 0.70
• Indoor pools: 0.30 to 0.50

Governing Equations

Total Cooling:

$$Q_t = 4.5 \times CFM \times \Delta h$$

Where:

• 4.5 = 60 min/hr × 0.075 lb/ft³
• Δh = enthalpy difference (Btu/lb_da)

Sensible Cooling:

$$Q_s = 1.08 \times CFM \times \Delta T$$

Latent Cooling:

$$Q_l = Q_t - Q_s$$

Or directly:

$$Q_l = 4840 \times CFM \times \Delta W$$

Where:

• 4840 = 60 min/hr × 0.075 lb/ft³ × 1076 Btu/lb (approximate $h_{fg}$)
• ΔW = humidity ratio reduction (lb_w/lb_da)

Condensate Removal Rate:

$$\dot{m}_{condensate} = 4.5 \times CFM \times (W_1 - W_2) \text{ lb/hr}$$

Convert to gallons per hour: divide by 8.33 lb/gal

Humidification Processes

Humidification adds moisture to air. Two methods exist: steam injection and water evaporation.

Steam Humidification

Steam injection adds both moisture and heat. The process line moves nearly vertically upward and slightly to the right.

Mass Balance:

$$\dot{m}_{steam} = \dot{m}_a (W_2 - W_1)$$

In terms of CFM:

$$\dot{m}_{steam} = 4.5 \times CFM \times (W_2 - W_1) \text{ lb/hr}$$

Energy Balance:

Temperature rise from steam heat:

$$\Delta T = \frac{(W_2 - W_1) \times h_{steam}}{c_p + W_{avg} c_{pw}}$$

For low-pressure steam ($h_{steam} \approx 1150$ Btu/lb):

$$\Delta T \approx \frac{(W_2 - W_1) \times 1150}{0.24}$$

Typically 2-5°F temperature rise per 0.001 lb/lb humidity increase.

Evaporative Humidification

Water spray or wetted media humidification follows an adiabatic process (constant enthalpy). The process line follows a constant wet bulb temperature line.

Process Characteristics:

• Enthalpy constant: $h_1 = h_2$
• Temperature decreases
• Humidity ratio increases
• Follows constant WBT line

Evaporative Cooling Effectiveness:

$$\epsilon = \frac{T_1 - T_2}{T_1 - T_{wb,1}}$$

Where ε ranges from 0.60 to 0.95 depending on design.

Air Mixing Process

Mixing two airstreams creates a final state point lying on a straight line between the two initial state points.

Governing Equations

Mass Balance:

$$\dot{m}_3 = \dot{m}_1 + \dot{m}_2$$

Energy Balance:

$$\dot{m}_3 h_3 = \dot{m}_1 h_1 + \dot{m}_2 h_2$$

Moisture Balance:

$$\dot{m}_3 W_3 = \dot{m}_1 W_1 + \dot{m}_2 W_2$$

Final State Properties:

$$h_3 = \frac{\dot{m}_1 h_1 + \dot{m}_2 h_2}{\dot{m}_1 + \dot{m}_2}$$

$$W_3 = \frac{\dot{m}_1 W_1 + \dot{m}_2 W_2}{\dot{m}_1 + \dot{m}_2}$$

Volumetric Basis (approximation):

For small temperature differences (< 30°F), mass flow ratios approximately equal volume ratios:

$$\frac{\dot{m}_1}{\dot{m}_2} \approx \frac{CFM_1}{CFM_2}$$

Graphical Method:

Mixed state point divides the line between state 1 and state 2 proportionally to mass flow rates:

$$\frac{distance(1-3)}{distance(1-2)} = \frac{\dot{m}_2}{\dot{m}_1 + \dot{m}_2}$$

Combined Processes: Air Handler Analysis

Real HVAC systems perform multiple processes in sequence.

Typical Air Handler Sequence:

1. Mixing (return + outdoor air)
2. Filtering (minimal psychrometric change)
3. Cooling/dehumidification
4. Reheat (if required)
5. Supply fan (small temperature rise from fan heat)

Fan Heat Addition:

$$\Delta T_{fan} = \frac{BHP \times 2545}{1.08 \times CFM \times \eta_{motor}}$$

Where:

• BHP = brake horsepower
• η = motor efficiency
• Typical ΔT = 1-3°F

Summary Table of Process Characteristics

Conclusion

Mastery of psychrometric processes enables proper equipment selection and system design. Each process follows thermodynamic principles visible on the psychrometric chart. Combined processes require sequential analysis, with outlet conditions from one process becoming inlet conditions for the next.

Engineers must understand sensible heat ratios, bypass factors, and apparatus dew points for accurate cooling coil selection. Mixing processes determine economizer operation and outdoor air introduction strategies.

Related Technical Guides:

• Psychrometric Fundamentals
• Cooling Load Calculations
• Heating Load Calculations

References:

• ASHRAE Handbook of Fundamentals, Chapter 1: Psychrometrics
• ASHRAE HVAC Systems and Equipment Handbook, Chapter 4: Air Handling and Distribution
• Stoecker, W.F. and Jones, J.W. Refrigeration and Air Conditioning