On a standard psychrometric chart, which of the following statements about dew-point temperature (DPT) lines is correct?

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:

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