2.6 Applied Thermodynamics

2.6 Applied Thermodynamics

1. HVAC Systems

Definition

• Heating, Ventilation, and Air Conditioning systems control indoor environmental conditions.

• Maintains temperature, humidity, air quality, and air circulation.

Main Components

1. Heating System

   • Furnaces, boilers, heat pumps

   • Transfers heat to indoor air

2. Cooling System

   • Air conditioners, chillers

   • Removes heat from indoor air

3. Ventilation System

   • Exchanges indoor and outdoor air

   • Maintains air quality, removes contaminants

4. Air Distribution System

   • Ductwork, fans, dampers, diffusers

   • Circulates conditioned air

Types of HVAC Systems

1. Split Systems: Separate indoor and outdoor units

2. Packaged Systems: All components in single outdoor unit

3. Central Systems: Large systems for buildings

4. Heat Pumps: Can provide both heating and cooling

2. Boilers

Definition

• Closed vessel where water is heated to generate steam or hot water.

Classification

1. By Tube Content:

   • Fire-tube boilers: Hot gases pass through tubes surrounded by water

   • Water-tube boilers: Water passes through tubes surrounded by hot gases

2. By Pressure:

   • Low-pressure (< 15 psi steam)

   • High-pressure (> 15 psi steam)

3. By Fuel:

   • Coal, oil, gas, electric, biomass

Key Components

• Burner: Mixes fuel and air for combustion

• Combustion chamber: Where fuel burns

• Heat exchanger: Transfers heat from gases to water

• Controls: Regulate temperature, pressure, safety

Efficiency Terms

• Boiler Efficiency: $\eta_{boiler} = \frac{\text{Heat output to water/steam}}{\text{Heat input from fuel}} \times 100\%$

• Blowdown: Removal of concentrated boiler water to control impurities

3. Compressors

Function

• Increases pressure of gas by reducing its volume.

• Main component in refrigeration, air conditioning, and gas compression systems.

Types of Compressors

1. Positive Displacement Compressors

• Trap gas in a volume and reduce that volume.

• Reciprocating Compressor:

  • Piston-cylinder arrangement

  • Common in small to medium capacity applications

  • Intercooling between stages improves efficiency

• Rotary Compressor:

  • Rotary vane: Sliding vanes in rotor

  • Screw compressor: Two meshing screws

  • Scroll compressor: Two interleaving scrolls

2. Dynamic Compressors

• Impart velocity to gas then convert velocity to pressure.

• Centrifugal Compressor:

  • Uses rotating impeller to accelerate gas

  • Diffuser converts kinetic energy to pressure

  • High capacity, smooth flow

• Axial Compressor:

  • Gas flows parallel to axis of rotation

  • Multiple stages of rotating and stationary blades

  • Very high efficiency, used in jet engines

Performance Parameters

• Volumetric Efficiency: $\eta_v = \frac{\text{Actual volume flow rate}}{\text{Piston displacement volume}}$

• Isentropic Efficiency: $\eta_{isentropic} = \frac{\text{Isentropic work}}{\text{Actual work}}$

• Compression Ratio: $r_p = \frac{P_2}{P_1}$

4. Refrigerants and Their Properties

Definition

• Working fluid in refrigeration cycle that absorbs heat at low temperature and pressure, releases heat at high temperature and pressure.

Desirable Properties

1. Thermodynamic:

   • High latent heat of vaporization

   • Moderate pressure at evaporation and condensation temperatures

   • Critical temperature well above condensing temperature

2. Physical/Chemical:

   • Low viscosity for good heat transfer

   • High thermal conductivity

   • Chemical stability

   • Non-flammable, non-explosive

   • Non-toxic

   • Non-corrosive to materials

3. Environmental:

   • Low Ozone Depletion Potential (ODP)

   • Low Global Warming Potential (GWP)

Classification

1. Halocarbons (CFCs, HCFCs, HFCs):

   • R-12 (CFC), R-22 (HCFC), R-134a (HFC)

   • Phase-out due to ODP and GWP

2. Natural Refrigerants:

   • R-717 (Ammonia): High efficiency, toxic, flammable

   • R-744 (Carbon dioxide): Low GWP, high operating pressure

   • R-290 (Propane): Flammable, low GWP

   • R-600a (Isobutane): Flammable, low GWP

3. Hydrofluoroolefins (HFOs):

   • R-1234yf, R-1234ze

   • Very low GWP, emerging replacements

Nomenclature

• R-xyz: where x+1 = number of C atoms, y-1 = number of H atoms, z = number of F atoms

• Remaining bonds filled with chlorine atoms

5. Psychrometrics

Definition

• Study of moist air properties and processes.

Key Properties of Moist Air

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

• Temperature measured by ordinary thermometer.

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

• Temperature measured by thermometer with wet wick around bulb.

• Indicates moisture content through evaporative cooling.

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

• Temperature at which condensation begins when air is cooled at constant pressure.

• $T_{dp} \le T_{wb} \le T_{db}$

4. Relative Humidity ($\phi$)

• Ratio of actual water vapor pressure to saturation pressure at DBT. $\phi = \frac{P_v}{P_g} \times 100\%$ where $P_v$ is vapor pressure, $P_g$ is saturation pressure at DBT.

5. Specific Humidity or Humidity Ratio ($\omega$)

• Mass of water vapor per unit mass of dry air. $\omega = \frac{m_v}{m_a} = 0.622 \frac{P_v}{P - P_v}$ where $P$ is total pressure.

6. Enthalpy of Moist Air ($h$)

• Sum of enthalpies of dry air and water vapor. $h = h_a + \omega h_v$

• Approximate formula (kJ/kg dry air): $h \approx 1.005 T_{db} + \omega (2501 + 1.86 T_{db})$

7. Specific Volume ($v$)

• Volume per unit mass of dry air.

Psychrometric Chart

• Graphical representation of moist air properties.

• Coordinates typically: DBT (x-axis) vs Humidity Ratio (y-axis).

• Contains lines of constant:

  • Relative humidity

  • Wet bulb temperature

  • Enthalpy

  • Specific volume

Psychrometric Processes

1. Sensible Heating/Cooling

• Change in DBT without change in moisture content.

• Horizontal movement on psychrometric chart.

2. Humidification/Dehumidification

• Change in moisture content at constant DBT.

• Vertical movement on psychrometric chart.

3. Cooling with Dehumidification

• Air cooled below its dew point.

• Moisture condenses out.

• Follows saturation curve.

4. Evaporative Cooling

• Air contacted with water, adiabatic saturation occurs.

• Follows constant wet bulb temperature line.

5. Adiabatic Mixing

• Mixing of two air streams without heat transfer.

• Resulting state lies on straight line connecting two states.

Air Conditioning Processes

• Summer AC: Cooling and dehumidification

• Winter AC: Heating and humidification

• By-pass factor: Fraction of air that bypasses coil surface

Useful Formulas

• Mass balance for water: $m_{a1} \omega_1 + m_w = m_{a2} \omega_2$

• Energy balance for adiabatic mixing: $m_{a1} h_1 + m_{a2} h_2 = (m_{a1} + m_{a2}) h_3$ $m_{a1} \omega_1 + m_{a2} \omega_2 = (m_{a1} + m_{a2}) \omega_3$

• Sensible Heat Factor (SHF): $SHF = \frac{\text{Sensible heat}}{\text{Total heat}} = \frac{Q_s}{Q_s + Q_l}$ where $Q_s$ is sensible heat, $Q_l$ is latent heat.