Psychrometric chart reference: The “alignment circle” commonly marked on the comfort psychrometric chart corresponds to which dry-bulb temperature (DBT) and relative humidity (RH)?

Introduction / Context:
Many psychrometric charts used in HVAC (e.g., comfort charts) include an “alignment circle” or reference point to help align scales or to denote a representative comfort condition. Knowing this reference aids quick estimations and chart navigation.

Given Data / Assumptions:

• Standard comfort psychrometric charts used in HVAC education and practice.
• Alignment circle denotes a nominal comfort state near mid-range humidity.
• Units in SI (°C, % RH).

Concept / Approach:
Common charts mark a reference near 26 °C DBT and 50% RH (approximately 80 °F and 50% RH). This lies centrally in the comfort region and provides convenient alignment for scales like enthalpy or specific volume lines used in quick calculations.

Step-by-Step Solution:
Identify typical comfort zone: DBT roughly 22–26 °C and RH 40–60%.Select the central reference often used by chart publishers: 26 °C and 50% RH.Confirm that this point falls close to mid-range properties, simplifying alignment tasks.

Verification / Alternative check:
Cross-checking standard HVAC psychrometric charts shows a reference/marker near 26 °C, 50% RH; while exact labeling may vary by publisher, 26 °C and 50% RH is a widely used reference.

Why Other Options Are Wrong:

• 20 °C combinations: Cooler than typical alignment reference used for comfort chart alignment.
• 26 °C at 60% RH: In comfort zone but not the commonly marked alignment circle.
• 24 °C, 50% RH: Reasonable comfort point but not the standard alignment marker.

Common Pitfalls:
Assuming all charts use identical markers; some charts are customized. For general exam purposes, 26 °C and 50% RH is the accepted reference.

Final Answer:
26 °C DBT and 50% RH