ASHRAE Psychrometrics and Moist Air Properties 3

Question 1

Which ASHRAE Handbook volume contains the primary psychrometric data, charts, and moist air property equations?

Accepted Answer

ASHRAE Handbook — Fundamentals

Answer

ASHRAE Handbook — HVAC Systems and Equipment

Answer

ASHRAE Handbook — Refrigeration

Answer

ASHRAE Handbook — HVAC Applications

Question 2

How do psychrometric properties change at higher altitudes compared to sea level for the same dry-bulb temperature and relative humidity?

Accepted Answer

Humidity ratio and enthalpy values increase because lower atmospheric pressure allows more moisture per unit mass of dry air

Answer

The saturation curve shifts to higher dry-bulb temperatures

Answer

The humidity ratio and enthalpy values remain essentially unchanged

Answer

The dry-bulb temperature scale compresses proportionally with altitude

Question 3

What is the approximate specific volume of standard air at 70°F (21°C) and standard sea-level pressure?

Accepted Answer

13.5 ft³/lb (0.843 m³/kg)

Answer

11.0 ft³/lb (0.687 m³/kg)

Answer

15.0 ft³/lb (0.937 m³/kg)

Answer

12.4 ft³/lb (0.775 m³/kg)

Question 4

What distinguishes the room sensible heat ratio (RSHR) from the effective sensible heat ratio (ESHR) in cooling system design?

Accepted Answer

RSHR applies only to the room thermal load; ESHR accounts for outdoor ventilation air mixed into the supply airstream

Answer

RSHR includes ventilation air loads while ESHR excludes them

Answer

RSHR measures latent heat loads; ESHR measures only sensible heat loads

Answer

RSHR is used for heating design; ESHR is used for cooling design only

Question 5

What is the primary engineering purpose of the psychrometric chart in HVAC system design?

Accepted Answer

To graphically represent thermodynamic properties of moist air and visualize HVAC processes for load calculations

Answer

To determine structural and seismic loads on ductwork and air handling units

Answer

To size refrigerant piping and select expansion valves for DX systems

Answer

To calculate static pressure losses and friction factors in duct distribution systems

Question 6

For air-water vapor mixtures at typical HVAC conditions, what is the relationship between wet-bulb temperature and thermodynamic adiabatic saturation temperature?

Accepted Answer

They are approximately equal for air-water systems due to the Lewis number being close to unity

Answer

They are always mathematically identical by thermodynamic definition

Answer

Wet-bulb temperature is always measurably lower than adiabatic saturation temperature

Answer

They are fundamentally unrelated properties with different physical meanings

Question 7

In English unit HVAC calculations, what is a 'grain' of moisture as used to express humidity ratio?

Accepted Answer

A unit of mass in the English system where 7,000 grains equals one pound, used to express humidity ratio in gr/lb_da

Answer

A unit of pressure equal to 0.001 psi used for vapor pressure calculations

Answer

A dimensionless ratio of water vapor mass to total moist air mass

Answer

A unit of specific enthalpy for water vapor at standard conditions