A Psychrometric chart can graphically represents the thermodynamic properties of air-water vapor mixture. Standard psychrometric charts are bounded by the dry-bulb temperature line (X – axis) and the vapor pressure or humidity ratio (Y – axis). The Left Hand Side of the psychrometric chart is bounded by saturation line.
Below image shows the schematic diagram of a psychrometric chart.
Image source : ArthurOgawa( WikiMedia Commons)
- Vertical lines with uniform spacing on psychrometric chart are called as dry bulb temperature (DBT) lines.
- Inclined lines sloping downwards to right with uniform spacing starting from Relative Humidity (RH) = 100 % lines are called on the psychrometric chart parallel to R.H 1005 line are called Relative humidity (pie) lines.
- When constant specific humidity lines meet RH = 100 % are called Dew Point Temperature (DPT).
- For any condition of air on constant specific humidity lines, DPT is same.
- Enthalpy scale is provided on chart above RH = 100 % line.
- Constant enthalpy lines lines are inclined lines sloping downwards to right starting from RH = 100 % curve.
- For all practical purposes constant enthalpy lines and Wet Bulb Temperature (WBT) lines are parallel.
- Technically speaking there is slight deviation between constant enthalpy lines and WBT lines. As deviation is appreciable we consider them as parallel for all practical purposes.
- Constant specific volumes lines are inclined (more than WBT) lines.
- Sensible Heat Factor (SHF) lines are inclined lines sloping downwards to left.
- Horizontal lines with non uniform spacing on the psychrometric chart are called as constant vapor pressure lines.
- The state having 25ºC DBT and 50 % RH is the comfort zone. If conditions of air is in that zone human feels maximum comfort.
Applications of Psychrometry:
- HVAC systems, animal, plant and human comfort.
- Air-conditioning devices.
- Cooling towers.
- Industrial processes requiring close control of the vapour content in air.
- Food science and engineering.
- Animal housing.
- Plant systems (Greenhouse gases, growth chambers, plant based bio-degenerative life support systems etc.).
- Evaporative coolers.
- Mold problems & health related issues (Dew & Condensation).
- Water harvesting in arid lands.