Psychrometric chart reading: On a standard psychrometric chart, the constant relative humidity lines are:
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AVertical and uniformly spaced
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BHorizontal and uniformly spaced
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CHorizontal and non-uniformly spaced
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DCurved lines (approximately concave to the humidity ratio axis)
Answer
Correct Answer: Curved lines (approximately concave to the humidity ratio axis)
Explanation
Introduction / Context:Accurate reading of a psychrometric chart is fundamental for HVAC design, cooling tower analysis, and dehumidification calculations. Each family of lines corresponds to a different air property.
Given Data / Assumptions:
- Standard psychrometric chart at 1 atm.
- Dry-bulb temperature plotted on the abscissa; humidity ratio on the ordinate.
- Other property lines (wet-bulb, RH, enthalpy, specific volume) are overlaid.
Concept / Approach:Constant relative humidity (RH) lines are curved, intersecting dry-bulb verticals and trending toward the saturation curve (100% RH). Their curvature reflects Clausius–Clapeyron behaviour of water vapour pressure versus temperature.
Step-by-Step Solution:
Identify saturation curve (100% RH) as the left boundary of the chart’s state region.Trace RH = 10%, 30%, 50%, 70%, 90% lines: they bow and converge toward the saturation curve.Contrast with other families: dry-bulb lines are vertical; humidity ratio lines are horizontal; wet-bulb lines are quasi-straight slanted.Thus, RH lines are definitively curved.Verification / Alternative check:Consult any standard chart; the geometry is consistent across references though exact spacing is non-uniform due to property equations.
Why Other Options Are Wrong:
- Vertical/horizontal descriptors apply to DBT and humidity ratio, not RH.
- “Horizontal non-uniform” describes enthalpy or wet-bulb trends poorly; RH is not horizontal.
Common Pitfalls:Mistaking wet-bulb lines (near straight slanted) for RH; always locate the saturation curve first to orient the chart.
Final Answer:Curved lines (approximately concave to the humidity ratio axis)