Unit 42
What is the velocity of the air in duct with a total pressure of 0.15 in wc and a static pressure of 0.1 in wc
.05
What is the airflow in CFM through a 10in x 20in duct with a total pressure of .15 in wc and a static pressure of 0.1 in wc?
.05 inches wc velocity pressure 896 CFM Rounded
How much does perfectly dry air weigh?
0.075lb/ft3
Infiltration
Air comes from outside to inside
What tools are used to measure air velocity?
Anemometers
What units are used to measure air volume?
Cubic feet per minute CFM
What is velocity pressure?
Difference between the total and static pressure from the total pressure.
In general, which is heavier: humid air or dry air?
Dry air
What units are used to measure air velocity?
Feet per minute, FPM
Why does the air's relative humidity affect comfort?
High humidity, people will feel warmer in the air. Low humidity, people will feel cooler in the air.
What instruments are used for measuring duct pressure?
Inclined meter, digital manometer or magnehelic gauge.
Ventilation
Introduction of outdoor air into a building by mechanical means.
What is a psychrometric chart?
Is a graphical representation of air properties.
What is a wet bulb temperature reading?
Is measured by placing a wick soaked with water around the thermometer bulb and moving it rapidly through the air.
What is enthalpy?
Measures the total heat content of the air in BTU per pound of air.
How does flowhood measure airflow?
Measures the velocity the known area of the flowhood and displays the airflow in CFM
What is the Mixed air temperature equation?
Mixed air = makeup air temperature x percent outside air + return air temperature x percent return air
What 3 environmental conditions have the most effect on comfort?
Temperature, humidity and air movement
What is the relationship between air volume and air weight (density)?
They vary with temp., pressure and humidity.
What 2 pressures are created by air moving through the duct system?
Velocity and static pressure
Difference between wet bulb and dry bulb is what?
Wet bulb depression
What pressure units are used for measuring duct pressures?
inches of water column
How is relative humidity measured?
psychrometer
List four ways the human body expels heat.
1. Conduction 2. Convection of cooling air 3. Radiation 4. Evaporation
What are 4 measurements necessary to calculate the actual cooling capacity of an air conditioning?
1. Wet bulb temperature of the entering ai 2. Wt bulb temperature of the leaving air 3. Dry bulb temperature of the leaving air 4. Airflow volume in CFM 5. Entering air dry bulb
What is the airflow through a register with a free area of 72 in2 if the air velocity is 300 FPM?
150 CFM
What is the typical airflow per ton residential air conditioning equipment?
400 CFM per ton
What is the Formula to convert velocity pressure to velocity?
4500square root velocity pressure
What would the mixed air temperature be for a system with 20% outdoor air if the outdoor temperature is 10 degrees F and the return air temperature is 75 degrees F?
62 degrees F mixed air
What is the density for water?
62.4lb/ft3
What does air consist?
78% nitrogen, 21% oxygen, 1% other gases
body temperature
98.6
How is the air volume affected by temperature?
Air volume increases as the air temperature increases
What are grains, and what air property is measured in grains?
Grain is a very small unit of weight, 7,000 grains in 1 pound. Specific humidity
Formula for mixed air grains:
Mixed air grains= outdoor air grains x percent outdoor air + return air grains x percent return air
Explain how to take a static pressure reading on a duct system.
Place probe where it will not pick up any velocity pressure. Placing probe perpendicular to the airflow is the most common method.
What is pitot tube?
Single probe that can be inserted in the duct and connected to the manometer for reading velocity pressure.
What is the difference between specific humidity and relative humidity?
Specific humidity measures the weight of water vapor in the air grains. Relative humidity expresses the percentage of water vapor in the air compared to the amount of water vapor the air can hold.
What is the dew point temperature?
The temperature at which the water vapor can condense out of the air.
