HVAC Psychrometric Chart Practice Questions ASHRAE Fundamentals Key Points

ASHRAE Handbook Fundamentals - Critical Points

1. Thermal Comfort Standards (ASHRAE 55)

Parameter	Winter Range	Summer Range
Operative Temp (°F)	68-74	73-79
Relative Humidity (%)	≥30	≤60
Air Speed (fpm)	<50	<160

PMV-PPD Model: Acceptable PPD < 10% (PMV ±0.5)

2. Design Conditions

Condition	Dry Bulb (°F)	Wet Bulb (°F)	Application
1% Cooling DB	Location-specific	-	Peak load calc
0.4% Cooling DB/MCWB	Higher than 1%	Mean coincident	Equipment sizing
99% Heating DB	Location-specific	-	Heating systems

3. Heat Transfer Fundamentals

Q = U × A × ΔT

Material	U-value (Btu/h·ft²·°F)	R-value (ft²·°F·h/Btu)
Single pane glass	1.10	0.91
Insulated wall (R-13)	0.07	13.0
Roof (R-30)	0.03	30.0

4. Ventilation Requirements (ASHRAE 62.1)

Space Type	Outdoor Air (cfm/person)	Air Changes (ACH)
Office	5-17	0.3-0.6
Classroom	10-15	0.6-1.2
Retail	7.5-20	0.2-0.4

Ventilation Rate Procedure: V_ot = V_oz × P_z + V_bz × A_z

5. Moisture Control Principles

Dew point must remain below surface temperatures
Vapor retarder placement: Warm side in cold climates
Permeance limits: <1 perm for vapor barriers
Condensation risk when: T_surface ≤ T_dewpoint

Grains difference = 0.018 × (P_i - P_o) × R_v

6. Load Calculation Essentials

Component	Formula	CLTD Values
Conduction	Q = U × A × CLTD	Table 18-12
Solar Gain	Q = A × SC × SHGF × CLF	Tables 15-17
Internal Loads	Q = People + Lights + Equipment	Tables 6-8

7. Psychrometric Chart Key Points

Sensible heat ratio (SHR) = Q_sen/Q_total
Apparatus dew point (ADP) determines coil performance
Bypass factor: 0.1-0.3 for typical coils
Standard air: 68°F DB, 50% RH, 0.075 lb/ft³