BOMI: AHU, Water Treatment, and Plumbing
Questions And Answers.
What is the relationship between thermal comfort and metabolic heat? - answer:-
Thermal comfort exists when the temperature and relative humidity of a person's
environment permit the person to lose, without conscious effort, metabolic heat at the
same rate he or she produces it. Because temperature and humidity combine to make
humans feel comfortable in their surroundings, these characteristics must be adjusted
by the AC system in order to enhance human comfort.
What occupant activity level is chosen when engineers design an air-conditioning
system? - answer:-Office building air-conditioning systems are typically designed to
accommodate people engaged in sedentary activities, including tasks done while
seated or standing. Sedentary activity is the most important condition because when
people are sedentary, they think about their comfort or discomfort.
What is the rate of exchange of fresh air for return air based on in an AC system? -
answer:-The rate of exchange of fresh air for returned air is based on the quantity of
fresh air required for each building occupant over the course of time.
Define heat - answer:-Heat is energy that flows from one object to another. Heat always
travels from an object with more heat to an object with less heat until both bodies are
the same temperature.
What happens to the rate of heat transfer if the difference in temperature increases? -
answer:-The rate at which heat transfers increases as the difference in temperature
between the two objects increases.
What are the three methods of heat transfer? - answer:-Conduction, Convection, and
Radiation
Define conduction - answer:-Conduction is the direct transfer of heat from one body to
another. The substances have to be in physical contact with each other, but there is no
movement of the material itself.
Define latent heat - answer:-Latent heat is the energy that is used or released during a
change of state. Latent heat cannot be felt or measured with a thermometer because it
is hidden within a substance.
Define relative humidity - answer:-RH (relative humidity) describes the amount of water
vapor held in the air.
Does moist air have more or less density than dry air? - answer:-Moist air has less
mass than dry air and is less dense than dry air.
, BOMI: AHU, Water Treatment, and Plumbing
Questions And Answers.
What is the atmospheric pressure of air at sea level? - answer:-At sea level,
atmospheric pressure exerts an average force of 14.7 pounds on each square inch of
surface (14.7 psi).
Explain Charles' Law. - answer:-Charles' law explains why gases expand when heated
and contract when cooled. Given a certain quantity of gas, as temperature increases,
volume increases. This is because the molecules have the opportunity to move more
and spread out. As pressure increases and the volume decreases, the temperature will
increase because compressing the gas causes the molecules to
vibrate faster and collide into each other more frequently, causing friction heat.
Explain Boyle's Law. - answer:-Boyle's law states that at a constant temperature, as the
volume increases, the pressure decreases. If the pressure on a
certain amount of gas is doubled, the volume of space it requires will be cut in half. As
pressure goes up, volume goes down.
How does the human body regulate its temperature? - answer:-The body dissipates
heat by sending blood from internal organs to the skin where the heat is thrown off into
the
surroundings by radiant heat loss, convective heat loss, or evaporation. Perspiring and
losing the heat through
evaporation is an evaporative cooling process and is used in air-conditioning systems
as well as human bodies.
What system produces the most heat in a building? - answer:-Building lights are the
most significant internal heat source. The heat given off by lights depends mainly on the
wattage rating of the lighting elements. Lighting systems produce heat loads of up to
four watts per square foot. The heat gain from one watt of electrical energy is equivalent
to 3.4 Btu per hour.
How have office building AC systems been designed to compensate for heat gain from
office equipment? - answer:-To compensate for this heat gain from office equipment in
modern office buildings, air-conditioning systems have been designed to offset one to
three watts per square foot.
What is pull-down load? - answer:-Heat left over from the day before must be removed
the following morning when the air-conditioning system is restarted. The leftover heat is
called the pull-down load and must be added to the instantaneous solar heat gain
during the morning hours to
determine the cooling load for the air-conditioning system.
, BOMI: AHU, Water Treatment, and Plumbing
Questions And Answers.
How can the problem of air and water vapor infiltration into a building be solved? -
answer:-To reduce these infiltration problems, ventilation air should be introduced
through the air-conditioning system in constant quantities to maintain a slight positive
pressure within the building. This pressure causes an outward escape of air through
cracks, windows, and doors, limiting infiltration. In this way, the only outside air admitted
into the building is that which has
been cooled, dehumidified, and cleaned before being introduced into the occupied
spaces.
Define absolute pressure - answer:-Gauge pressure plus standard pressure gives psia
(pounds per square inch absolute), or absolute pressure.
Define static pressure - answer:-Static pressure is generally used to describe duct
pressure. Static pressure is the pressure of the air being pushed against
the inside walls of a closed duct by the fan.
How is velocity pressure calculated? - answer:-To measure static pressure, a pitot tube
partially filled with fluid is attached at a right angle to a long, straight length of duct. The
air entering the tube from the duct will push the fluid inside the tube against the outside
air that is used as a reference. The outside tubing allows a measure of the static
pressure; the velocity pressure is eliminated from the reading. Velocity pressure is
derived by subtracting static pressure from total pressure.
What is pressure drop? - answer:-The pressure drop is measured as the difference in
pressure between where air enters a device and where it comes out of the device.
What is friction loss? - answer:-Friction loss is caused by air molecules slowing down by
banging against the surface of the duct interior.
When designing a duct system, what is important to remember about directional
changes? - answer:-The more changes of direction there are in the duct run, the less
direct the airflow will be. Directional changes increase
friction and reduce efficiency.
What is a problem with using fibrous glass wool as a ductwork material? - answer:-The
World Health Organization has declared fibrous glass wool as "[possibly] carcinogenic
to humans."
Which type of duct uses fiberglass insulation? - answer:-Fiberglass insulation is the
primary material used to insulate flex duct.
Questions And Answers.
What is the relationship between thermal comfort and metabolic heat? - answer:-
Thermal comfort exists when the temperature and relative humidity of a person's
environment permit the person to lose, without conscious effort, metabolic heat at the
same rate he or she produces it. Because temperature and humidity combine to make
humans feel comfortable in their surroundings, these characteristics must be adjusted
by the AC system in order to enhance human comfort.
What occupant activity level is chosen when engineers design an air-conditioning
system? - answer:-Office building air-conditioning systems are typically designed to
accommodate people engaged in sedentary activities, including tasks done while
seated or standing. Sedentary activity is the most important condition because when
people are sedentary, they think about their comfort or discomfort.
What is the rate of exchange of fresh air for return air based on in an AC system? -
answer:-The rate of exchange of fresh air for returned air is based on the quantity of
fresh air required for each building occupant over the course of time.
Define heat - answer:-Heat is energy that flows from one object to another. Heat always
travels from an object with more heat to an object with less heat until both bodies are
the same temperature.
What happens to the rate of heat transfer if the difference in temperature increases? -
answer:-The rate at which heat transfers increases as the difference in temperature
between the two objects increases.
What are the three methods of heat transfer? - answer:-Conduction, Convection, and
Radiation
Define conduction - answer:-Conduction is the direct transfer of heat from one body to
another. The substances have to be in physical contact with each other, but there is no
movement of the material itself.
Define latent heat - answer:-Latent heat is the energy that is used or released during a
change of state. Latent heat cannot be felt or measured with a thermometer because it
is hidden within a substance.
Define relative humidity - answer:-RH (relative humidity) describes the amount of water
vapor held in the air.
Does moist air have more or less density than dry air? - answer:-Moist air has less
mass than dry air and is less dense than dry air.
, BOMI: AHU, Water Treatment, and Plumbing
Questions And Answers.
What is the atmospheric pressure of air at sea level? - answer:-At sea level,
atmospheric pressure exerts an average force of 14.7 pounds on each square inch of
surface (14.7 psi).
Explain Charles' Law. - answer:-Charles' law explains why gases expand when heated
and contract when cooled. Given a certain quantity of gas, as temperature increases,
volume increases. This is because the molecules have the opportunity to move more
and spread out. As pressure increases and the volume decreases, the temperature will
increase because compressing the gas causes the molecules to
vibrate faster and collide into each other more frequently, causing friction heat.
Explain Boyle's Law. - answer:-Boyle's law states that at a constant temperature, as the
volume increases, the pressure decreases. If the pressure on a
certain amount of gas is doubled, the volume of space it requires will be cut in half. As
pressure goes up, volume goes down.
How does the human body regulate its temperature? - answer:-The body dissipates
heat by sending blood from internal organs to the skin where the heat is thrown off into
the
surroundings by radiant heat loss, convective heat loss, or evaporation. Perspiring and
losing the heat through
evaporation is an evaporative cooling process and is used in air-conditioning systems
as well as human bodies.
What system produces the most heat in a building? - answer:-Building lights are the
most significant internal heat source. The heat given off by lights depends mainly on the
wattage rating of the lighting elements. Lighting systems produce heat loads of up to
four watts per square foot. The heat gain from one watt of electrical energy is equivalent
to 3.4 Btu per hour.
How have office building AC systems been designed to compensate for heat gain from
office equipment? - answer:-To compensate for this heat gain from office equipment in
modern office buildings, air-conditioning systems have been designed to offset one to
three watts per square foot.
What is pull-down load? - answer:-Heat left over from the day before must be removed
the following morning when the air-conditioning system is restarted. The leftover heat is
called the pull-down load and must be added to the instantaneous solar heat gain
during the morning hours to
determine the cooling load for the air-conditioning system.
, BOMI: AHU, Water Treatment, and Plumbing
Questions And Answers.
How can the problem of air and water vapor infiltration into a building be solved? -
answer:-To reduce these infiltration problems, ventilation air should be introduced
through the air-conditioning system in constant quantities to maintain a slight positive
pressure within the building. This pressure causes an outward escape of air through
cracks, windows, and doors, limiting infiltration. In this way, the only outside air admitted
into the building is that which has
been cooled, dehumidified, and cleaned before being introduced into the occupied
spaces.
Define absolute pressure - answer:-Gauge pressure plus standard pressure gives psia
(pounds per square inch absolute), or absolute pressure.
Define static pressure - answer:-Static pressure is generally used to describe duct
pressure. Static pressure is the pressure of the air being pushed against
the inside walls of a closed duct by the fan.
How is velocity pressure calculated? - answer:-To measure static pressure, a pitot tube
partially filled with fluid is attached at a right angle to a long, straight length of duct. The
air entering the tube from the duct will push the fluid inside the tube against the outside
air that is used as a reference. The outside tubing allows a measure of the static
pressure; the velocity pressure is eliminated from the reading. Velocity pressure is
derived by subtracting static pressure from total pressure.
What is pressure drop? - answer:-The pressure drop is measured as the difference in
pressure between where air enters a device and where it comes out of the device.
What is friction loss? - answer:-Friction loss is caused by air molecules slowing down by
banging against the surface of the duct interior.
When designing a duct system, what is important to remember about directional
changes? - answer:-The more changes of direction there are in the duct run, the less
direct the airflow will be. Directional changes increase
friction and reduce efficiency.
What is a problem with using fibrous glass wool as a ductwork material? - answer:-The
World Health Organization has declared fibrous glass wool as "[possibly] carcinogenic
to humans."
Which type of duct uses fiberglass insulation? - answer:-Fiberglass insulation is the
primary material used to insulate flex duct.
