Quiz 4

Question 1

Comfort for Occupants:

Answer 1

● Room temperature (also called dry-bulb temperature)

● Airflow (rate of air motion)

● Humidity or moisture content of the room

● Radiation or surface temperature of surrounding surfaces in the rooms

● Indoor air quality

Question 2

Room temperature:

Answer 2

72 to 78 F (22.2C to 25.5C) in the winter.

In the summer: 72 - 76F (22.2 to 24.4C)

Question 3

Humidity: moisture content

Answer 3

A high humidity (often occurring in the summer) will cause the surrounding
air to absorb less heat from the body, making the occupant feel warmer.

A low humidity (often occurring in the winter) allows the air to absorb
greater quantities of body heat, making the occupant feel cooler.

• To lower the summer humidity
• To increase the winter humidity

Question 4

Airflow: rate of air motion

Answer 4

In the summer:
increased air motion increases the evaporation rate of heat
from the body to help keep it cool.
Buildings without air conditioners use fans to increase the
movement of the air on hot days.
In the winter:
a slower rate of air motion is desired, or the occupants will
feel cool due to evaporative heat loss.
Even in the winter it is desirable to have a flow of air to keep
the air from becoming stagnant.

When the air velocity is less than 10 feet per minutes the space is considered to
be stuffy.

Air velocities more than 50 feet per minute will seem drafty.

Question 5

Radiation:

Answer 5

heat flows from a hot to a cold surface.
Windows and exterior walls will probably be cooler in the winter than
interior walls and the furniture.

The temperatures of the floors and ceilings will depend on the air
temperature on the other side of the construction.

Comfort can be increased (in the winter) by directing the flow of warm
air over the colder surfaces.
This is why most heating elements are located under a window.

Question 6

HVAC: Heating, Ventilation, Air Conditioning

Answer 6

These systems are designed to provide
* Thermal conditions
* Adequate amounts of outdoor air

Question 7

Absolute Humidity (humidity ratio, specific humidity, W):

Answer 7

Ratio of the mass of water vapor (mw) to the mass of dry
air (ma):

Question 8

Types of air

Answer 8

Dry air: air that contains no water vapor.

Moist air: air that contains water vapor.

Saturated air: air that contains the maximum amount of moisture it can hold at that temperature.

Question 9

Relative Humidity (RH):

Answer 9

Ratio of the actual water content to the maximum
amount of moisture it could hold at the same temperature
and pressure (when fully saturated).
It is expressed in percent.

Relative humidity is measured by an instrument called hydrometer.

Question 10

Dew point (or saturation) temperature (DPT):

Answer 10

The temperature of the air/water-vapor mixture when it is fully saturated (100% relative humidity).

At this temperature moisture will begin to condense (water vapor from
the air begins to form water droplets on surfaces that are colder than the
dew point of the air).

Question 11

Frost point temperature:

Answer 11

Dew point temperature below freezing. In this case, the water vapor creates frost instead of dew.

Question 12

Effect of Temperature on Humidity

Answer 12

● The moisture holding capacity of the air depends on the air temperature.
● Warm air holds more moisture than cold air.

Question 13

Dry bulb temperature (DBT)

Answer 13

The temperature of the mixture of air and moisture
at rest.
It is measured with a common thermometer.

Question 14

Wet-bulb Temperature:

Answer 14

Temperature measured by a thermometer with a
wetted cloth sock covering its bulb as fast-moving air
passes across it.

Question 15

Specific volume (v):

Answer 15

Ratio of unit volume of dry air per unit weight of dry air.
It is expressed in cubic feet of air per pound of dry air (ft3/lb)
or cubic meter of air per kilogram of dry air (m3/kg).
It is the inverse of density.

Question 16

Enthalpy (h):

Answer 16

Quantity of heat contained in air under specific conditions

Question 17

Psychrometric chart:

Answer 17

Graphical representation of the thermodynamic properties
of moist air.
It describes the relationships between psychrometric variables

Question 18

Sensible heat:

Answer 18

Heat associated with change in temperature.
As heat is added to the air the temperature increases.
When heat is removed the temperature of the air decreases.

Question 19

Air contains thermal energy in two forms:

Answer 19

● temperature
● water vapor (humidity)

Question 20

Latent heat:

Answer 20

Release or storage of heat associated with change in phase of
a substance, without a change in the substance’s temperature.
Heat is required to cause the liquid water to change from a liquid to a
gas.
The heat absorbed by the water that causes this phase change is
latent heat.

Question 21

Sensible heating (cooling):

Answer 21

Occurs when the temperature of the air-water mixture is
raised (lowered) but the absolute moisture remains the
same.
On the psychrometric chart, it is represented by a
horizontal movement.

Question 22

Cooling and Dehumidification:

Answer 22

Process of lowering both the dry bulb
temperature and the humidity ratio of the moist air.
On the psychrometric chart, it is represented by a diagonal
movement down to the left.

Question 23

Heating and Humidification:

Answer 23

Process of simultaneously increasing both the dry
bulb temperature and humidity ratio of the air.
On the psychrometric chart, it is represented by a diagonal
movement up and to the right.

Question 24

Evaporating Cooling:

Answer 24

Process that sensible heat in the air is converted to latent
heat in the added vapor.
On the psychrometric chart, it is represented by a diagonal
movement up and to the left along the wet bulb
temperature line.

Question 25

Desiccant Dehumidification:

Answer 25

Involves the use of chemical or physical absorption of
water vapor to dehumidify air.
On the psychrometric chart, it is represented by a diagonal
movement down and to the left.

Question 26

HVAC systems use fluids to transport heat and cold.

Answer 26

● air
● water
● steam
● refrigerant

HVAC systems are designed to satisfy heat loads by using transport fluids.

Question 27

Selecting Fluid Flow Rates for HVAC

Answer 27

1. Estimate building heat loads

Heating loads = how much heat is lost and must be made up by the system

Cooling loads = how much heat is gained and must be removed

2. Decide the proper combination of flow and conditions for fluids used to transfer heat and compensate for loads.

Question 28

Heating loads
Cooling loads

Answer 28

how much heat is lost and must be made up by the system

how much heat is gained and must be removed

Question 29

Critical Conditions for Design HVAC

Answer 29

• Outside weather
• Solar effects
• Inside temperature
• Humidity
• Status of building operations

Question 30

For heating:

Answer 30

The critical design condition will occur during cold weather
Important decision:
The selection of an appropriate cold outside air temperature

Question 31

For cooling:

Answer 31

The critical design condition is:

The peak occurrence of heat, humidity, solar effects and internal heat
gains from equipment, lights, and people.

Question 32

R =

Answer 32

resistance to heat transfer

Question 33

Infiltration loads depend on

Answer 33

● The amount of outside air leakage
● The difference in conditions between the outside and inside air.