313D Psychrometrics and Humidity Control 3

Question 1

What is the Apparatus Dew Point (ADP) of a cooling coil?

Accepted Answer

The effective average surface temperature of the coil at which moisture condenses

Answer

ADP is the effective mean surface temperature of a cooling coil — the theoretical temperature at which the coil would saturate leaving air if bypass factor were zero.

Question 2

How does dehumidification occur in a typical residential split-system air conditioner?

Accepted Answer

Warm humid air is chilled below its dew point over the evaporator coil, causing moisture to condense on the coil

Answer

When warm humid air contacts the cold evaporator coil (below the air's dew point), water vapor condenses on the coil surface and drains away, reducing indoor humidity.

Question 3

Under what condition are dry-bulb and wet-bulb temperatures equal?

Accepted Answer

When air is at 100% relative humidity (fully saturated)

Answer

At 100% relative humidity no evaporation occurs from a wet wick, so the wet-bulb thermometer reads the same temperature as the dry-bulb thermometer.

Question 4

What is humidity ratio (specific humidity) as used in HVAC psychrometrics?

Accepted Answer

The mass of water vapor per unit mass of dry air, expressed in lbs/lb or grains/lb

Answer

Humidity ratio (W) is the actual mass of water vapor per unit mass of dry air — an absolute measure of moisture content independent of temperature.

Question 5

A large wet-bulb depression (big difference between dry-bulb and wet-bulb temperatures) indicates what about the air?

Accepted Answer

The air is relatively dry with low relative humidity

Answer

A large wet-bulb depression means significant evaporation is occurring from the wet wick, which only happens when the surrounding air has a low relative humidity.

(313D) Residential Air Conditioning Systems Mechanic Certification Practice Test

(313D) Residential Air Conditioning Systems Mechanic Certification Practice Test

313D Psychrometrics and Humidity Control 3