1060 unit 1 (CH42)

What would the mixed air temperature be for a system with 20% outdoor air if the outdoor temperature is 90°F and the return air temperature is 80°F?

0.2 × 90°F =18°F; 0.8 × 80°F = 64°F; 18°F + 64°F = 82°F mixed air temperature

What would the mixed air temperature be for a system with 30 percent outdoor air if the outdoor temperature is 20°F and the return air temperature is 70°F?

0.3 × 20°F = 6°F; 0.7 × 70°F = 49°F; 6°F + 49°F = 55°F mixed air temperature

What is the airflow through a 2 ft by 3 ft hole if the air velocity is 100 ft per minute?

2 × 3 = 6 sq feet; 6 × 100 FPM = 600 CFM

What is the industry standard rate of through-the-evaporator airflow in CFM per ton for most air-conditioning equipment?

400 CFM per ton

What type of measurement is a grain?

A grain is a small measurement of weight.

A graphical representation of air properties is:

A psychrometric chart.

A process that moves horizontally from left to right across a psychrometric chart is:

A sensible process.

What is a wet-bulb temperature reading?

A temperature measured with a water-soaked wick around the thermometer bulb

How is the air volume affected by temperature?

Air volume increases as the air temperature increases.

In the heating season, the comfort level can be improved at lower temperatures:

By adding humidity to the air.

In the cooling season, the comfort level can be improved at higher temperatures:

By removing humidity and increasing air velocity.

The relationship of CFM and FPM can be summarized as:

CFM = FPM × area

The formula for calculating CFM is:

CFM = area × velocity.

The body cooling mechanisms include:

Conduction, convection, radiation, and evaporation.

Being cooled by a fan moving room-temperature air across the body is an example of cooling by

Convection

if a technician wanted to accurately measure static pressure in a duct system, how would she place the sensing probe of the magnehelic gauge?

Correct Perpendicular to airflow

Total pressure readings are taken on a duct system by:

Correct Pointing the pressure-sensing tube into the airstream.

The quantity of air a blower is moving is measured in:

Cubic feet per minute - CFM.

What do technicians call the temperature at which water vapor begins to condense out of the air?

Dew point

The temperature where water vapor can begin condensing out of the air is the:

Dew point.

In general, which is denser: humid air or dry air?

Dry air

________ is the measurement of the total heat content of the air expressed in BTU per pound of air.

Enthalpy

How does the human body expel heat if all the surrounding environment is above the body temperature?

Evaporation

Sweating when we work is an example of cooling by:

Evaporation.

The speed of air is measured in:

Feet per minute - FPM.

What air property is measured in grains?

Grains are used to measure the amount of water in the air.

What pressure units are used for measuring duct pressures?

Inches water column

A process that moves vertically up a psychrometric chart:

Is a latent process.

Physically active people feel more comfortable at:

Lower temperatures.

Static pressure readings are taken on a duct system by:

Pointing the pressure-sensing tube perpendicular to the air stream.

People maintain their steady internal body temperature by:

Producing more internal heat than needed and expelling the extra heat.

Getting a chill when standing near a cold wall is an example of cooling by:

Radiation.

________ represents the amount of water vapor in the air compared to the amount of water vapor the air can hold.

Relative humidity

Velocity pressure readings are taken on a duct system by:

Subtracting the static pressure from the total pressure.

What three environmental conditions have the most effect on comfort?

Temperature, humidity, and air movement

Why does the air's relative humidity affect comfort?

The relative humidity affects how rapidly perspiration can evaporate.

What is specific humidity?

The weight of water in each pound of dry air, measured in grains

The performance of an air-conditioning system can be determined by measuring the airflow and:

The wet-bulb temperature difference across the evaporator coil.

What two pressures are created by air moving through the duct system?

Velocity pressure and static pressure

Relative humidity is measured using:

Wet-bulb and dry-bulb temperature readings.

The enthalpy lines are nearly parallel to the lines for:

Wet-bulb temperature.