Topic 4.1 - Principles of Environmental Control (Psychrometry and Air Conditioning)

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Institution: CSUCEGS CSUDABE College of Engineering Agricultural and Biosystems Engineering and GEosciences
Subjects Covered:
- Psychrometry and Air Conditioning
- Insulation and Vapor Barriers
- Energy Considerations

The atmosphere in buildings primarily contains air and water vapor.
High moisture levels combined with high temperatures affect:
- Comfort of humans
- Comfort of animals
- Production of livestock
Controlling moisture levels is crucial for maintaining a desirable environment in farm buildings.

Definition: Psychrometry is the study of the properties of moist air and its behavior under varying conditions.
Key Properties of Moist Air include:
- Dry-Bulb Temperature (DBT)
- Wet-Bulb Temperature (WBT)
- Humidity
- Relative Humidity (RH)

Dry-Bulb Temperature (DBT):
- The temperature of air measured by a thermometer; denoted as T or Tdb.
Wet-Bulb Temperature (WBT):
- The temperature recorded when the bulb of the thermometer is covered with a wet wick and air flows over it at 2.5 to 10 m/s.
Dew Point Temperature (DPT):
- The temperature at which moisture in the air starts to condense; denoted as Tdp or DPT.
- For dehumidification, cooling coils should be below the DPT to condense moisture.
- DPT can be found using steam tables corresponding to the partial pressure of water vapor (Pv).

Pressure:
- Refers to the mixture of water vapor and remaining gases in air.
- Based on Dalton's Law of Partial Pressure:
  - Total Pressure of Air = Partial Pressure of Dry Air + Partial Pressure of Water Vapor.
- Carrier’s Equation is used to find the partial pressure of water vapor.
Specific Humidity or Humidity Ratio (w):
- Ratio of mass of water vapor to mass of dry air contained in the air sample.
- Expressed in kg/kg of dry air:
  - w = mass of water vapor / mass of dry air.
Relative Humidity (RH):
- Ratio of the mass of water vapor in the air volume at specific conditions to the maximum water vapor that volume can hold at the same temperature.
- Defined as:
  - RH = vapor pressure of water vapor / vapor pressure of saturated air at the same temperature.
- Measured in percentage.
Enthalpy:
- Total heat content of air, determined by adding dry air's enthalpy and water vapor's enthalpy.
- For perfect gas: ha = cpaTdb kJ/kg.
Specific Volume:
- The volume of air per unit mass of dry air, expressed in m³/kg.
Degree of Saturation:
- Ratio of mass of water vapor in the air sample to the mass of water vapor in saturated air at the same temperature.
  - Degree of Saturation = 0 when air is totally dry.

A psychrometric chart graphically represents the thermodynamic properties of moist air and varies with both atmospheric pressure and altitude.
Chart Variables:
- DBT, WBT, Relative Humidity, Total Heat, Vapor Pressure, Actual Moisture Content.
Layout:
- Dry-bulb temperature (x-axis) and mass of water vapor/kg of dry air (ordinate).
different lines for DBT, WBT, Specific Humidity, and others to understand the relationships between air properties.

Sensible Heating:
- Adding heat to air without changing moisture; occurs over hot surfaces.
Sensible Cooling:
- Removing heat while keeping moisture constant; occurs when hot air passes over the cooling coils.
Cooling and Dehumidification:
- Air is cooled sufficiently to separate moisture; requires the Cool Coils' surface temperature to be below the Dew Point Temperature (DPT).
Heating and Humidification:
- Heat and water vapor are added to air, increasing various humidity and temperature parameters.

Designed for installation in or near conditioned space.
Factory-assembled containing all necessary components.
Typically includes basic refrigeration components without complex heating systems.

Used for multiple zones or buildings.
Greater flexibility in designing and controlling temperature across larger areas.

Body heat loss is critical and influenced by Air Temperature, Air Humidity, Motion, and surrounding Object Temperature.
Effective Temperature (ET) is a defining parameter that represents the comfort level felt under different humidity and temperature conditions.
Maintaining good Indoor Air Quality involves controlling pollutants through proper ventilation and filtration.

Activity: Microclimate Analysis
- Identify ideal microclimates for five plants and livestock based on factors such as Temperature, Relative Humidity, Wind Velocity, Light, Precipitation, Soil Moisture, Topography.
- Use A4 bond paper and limit to 6-10 printed pages.

Review the impact of moisture levels, temperature, and air ventilation on agriculture and comfort in indoor environments.
Next topic will cover Insulation, Vapor Barriers, and Energy Considerations.