# 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.
