Psychrometric processes: On a standard chart, adiabatic humidification (evaporative cooling of unsaturated air) is represented by which type of line?
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AHorizontal line (constant humidity ratio)
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BVertical line (constant dry-bulb temperature)
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CInclined line (approximately straight along near-constant wet-bulb/enthalpy)
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DCurved line (toward saturation at constant dew point)
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ERandom trajectory depending only on pressure
Answer
Correct Answer: Inclined line (approximately straight along near-constant wet-bulb/enthalpy)
Explanation
Introduction / Context:Understanding how processes plot on the psychrometric chart is crucial for sizing evaporative coolers, spray washers, and air-handling equipment. Adiabatic humidification adds moisture while air gives up sensible heat, moving toward saturation.
Given Data / Assumptions:
- Atmospheric pressure is near 1 atm.
- Water temperature approximates the entering air wet-bulb temperature.
- Heat exchange with surroundings is negligible (adiabatic).
Concept / Approach:For adiabatic humidification, the air enthalpy remains nearly constant while humidity ratio increases and dry-bulb temperature decreases. On a psychrometric chart, constant enthalpy and constant wet-bulb lines are nearly straight slanted lines; the process thus follows an inclined path up and to the left toward the saturation curve.
Step-by-Step Solution:
Start at the initial state point (DBT high, moderate humidity ratio).As water evaporates, latent heat is supplied by air's sensible heat, so DBT drops while humidity ratio rises.Because the process is adiabatic, enthalpy stays approximately constant.Plotting on the chart gives an inclined line toward the saturation curve.Verification / Alternative check:Compare with measured psychrometer data: states with equal wet-bulb temperature lie nearly on straight lines; evaporative cooler performance follows those lines.
Why Other Options Are Wrong:
- Horizontal implies constant humidity ratio (no moisture addition), which contradicts humidification.
- Vertical implies constant dry-bulb (pure steam injection at matched sensible heat), not adiabatic evaporative cooling.
- Curved constant-dew-point is not the evaporative path.
Common Pitfalls:Confusing steam injection (often increases DBT) with evaporative cooling (decreases DBT); and misreading wet-bulb vs dew point lines.
Final Answer:Inclined line (approximately straight along near-constant wet-bulb/enthalpy)