On a standard psychrometric chart, which of the following statements about dew-point temperature (DPT) lines is correct?
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ADPT lines are horizontal and uniformly spaced
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BDPT lines are horizontal and non-uniformly spaced
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CDPT lines are inclined and converge at the saturation curve
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DDPT lines are vertical and uniformly spaced
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EDPT lines coincide with enthalpy lines
Answer
Correct Answer: DPT lines are horizontal and uniformly spaced
Explanation
Introduction / Context:The psychrometric chart is a graphical tool representing moist-air properties. Correctly identifying families of lines (dry-bulb, wet-bulb, humidity ratio, dew point) is essential for tracing HVAC processes like cooling, heating, humidification, and dehumidification.
Given Data / Assumptions:
- Standard sea-level psychrometric chart representation.
- Dew-point temperature at a given pressure is a direct function of humidity ratio.
Concept / Approach:Dew-point temperature corresponds to the temperature at which moist air becomes saturated at its prevailing water vapor partial pressure. On common charts, humidity ratio w is plotted on the vertical axis, and lines of constant w are horizontal and equally spaced. Because DPT is a one-to-one function of w at fixed pressure, constant DPT lines coincide with constant humidity-ratio lines—hence horizontal and evenly spaced.
Step-by-Step Solution:
1) Recognize constant w lines are horizontal, equally spaced.2) For fixed barometric pressure, DPT is a function of w alone.3) Therefore, constant DPT lines align with constant w lines → horizontal and uniformly spaced.Verification / Alternative check:Inspect any standard chart: DPT scale marks align with constant humidity-ratio horizontal grids; spacing is uniform on most charts for the typical w-axis.
Why Other Options Are Wrong:
- (b) Non-uniform spacing is not typical for w/DPT families.
- (c) Enthalpy lines (often slanted) differ from horizontal DPT/w lines.
- (d) Vertical lines correspond to constant dry-bulb temperature, not DPT.
- (e) DPT lines do not coincide with enthalpy lines.
Common Pitfalls:Confusing wet-bulb/enthalpy lines (slanted) with humidity-ratio/dew-point lines (horizontal).
Final Answer:DPT lines are horizontal and uniformly spaced