Final Exam
Why does radiation have a time delay?
-(Step 1:) Solar radiation coming directly into the space through openings heats the floor, interior walls, and furniture -(Step 2:) Energy is then transferred to the air through convection
How does energy transfer in an un-shaded window?
-About 8% of radiant energy is typically reflected back outdoors -80% is transmitted directly indoors -Remaining 12% is absorbed into glass (~4% radiates indoors, 8% radiates outdoors)
What are fenestrations?
-Any glazed aperture in a building envelope (windows/doors/skylights designed to permit the passage of light) -Heat gains associated with fenestrations are quite difference than those of the non-transmitting parts of the building envelope
What are the LST's for the time zones?
-Eastern Standard Time (EST) = 75° W longitude -Central Standard Time (CST) = 90° W longitude -Mountain Standard Time (MST) = 105° W longitude -Pacific Standard Time (PST) = 120° W longitude
What does the Heat Balance (HB) method do?
-Ensures that all energy flow in each zone is balanced. -Involves the solution of a set of energy balance equations for the zone air and the interior and exterior surfaces of each wall, roof, and floor
What is local civil time (LCT)?
-LCT takes Daylight Savings Time (DST) into account -If DST is in effect, LST = DST - 1 hr
What is Local Solar Time (LST)?
-LST is determined by a sundial (sun's position in the sky) -15 degrees longitude = 1 hr solar time
What are the causes of heat gain?
-Solar radiation through openings (summer) -Heat conduction through building envelope -Sensible heat convection and radiation from internal objects (summer) -Ventilation (outside air) and infiltration air -Latent heat gains generated within the space
What is the definition of heat gain?
-The instantaneous rate of energy transferred to (or generated inside) a space -Heat gain is not the same as cooling load!
What are the assumptions to be made about the latent and sensible loads due to occupancy?
1. Assume latent load to be removed instantaneously 2. Assume 30% convective heat load and 70% radiative heat load
Which cooling load predictions use non-heat-balance methods?
1. Cooling Load Factor (CLF) 2. Time Average Method (TA)
How should one apply the values in Table 5.2 for sensible and latent heat to all male occupants?
1. Find the ratio of sensible and latent heat gain to the total adjusted heat gain 2. Multiply this ratio by the total male heat gain, giving you the total sensible and total latent heat gain per male 3. Finally, multiply these numbers by the number of males in the space
What are the five components of fenestrations?
1. Glazing material 2. Framing mullions 3. External shading devices 4. Internal shading devices 5. Integral (between glass shading systems)
Which cooling load predictions use heat balance methods?
1. Heat Balance Method (HB) 2. Radiant Time Series Method (RTS)
What are the four approaches for predicting cooling load?
1. Heat Balance Method (HB) 2. Radiant Time Series Method (RTS) 3. Cooling Load Factor Method (CLF) 4. Time Average Method (TA)
What are the four distinct processes of the Heat Balance (HB) Method?
1. Outside-face heat balance 2. Wall conduction process 3. Inside-face heat balance 4. Air heat balance
What are the three modes of internal heat load gains?
1. People 2. Lighting 3. Equipment
How should total heat gain in a space be broken up?
1. Total sensible (radiative and convective components) 2. Total latent
Whenever a designer performs a load calculation, what three questions can arise?
1. What dimensions should be used for walls, roofs, etc. (inside or outside)? 2. How much infiltration will there be? 3. What are the internal lighting loads going to be?
What is the difference in heat transfer for a recessed light fixture vs. a hanging light fixture?
1. With a recessed light fixture, heat transfer goes to the plenum, surrounding structure, and space 2. With a hanging light fixture, all heat goes directly into the space
What is thermal storage?
A physical phenomenon causing delay of the heat load
What is the general equation for cooling load?
Cooling Load = Convective Heat Gain +/- Thermal Lag
What does peak condition mean?
Cooling load at the worst case scenario
What date holds the longest day and shortest night along the Tropic of Capricorn?
December 21st
What date holds the shortest day and longest night along the Tropic of Cancer?
December 21st
What are thermally heavy buildings?
Heavy mass can delay the cooling or heating load for several hours (Example: thick concrete building)
What date holds the longest day and shortest night along the Tropic of Cancer?
June 21st
What date holds the shortest day and longest night along the Tropic of Capricorn?
June 21st
What are thermally light buildings?
Light mass will cause thermal load to rise more quickly on a hot clear day, but also decline more rapidly as outdoor temperature falls (Example: steel/wood/glass)
What is latent load?
Load component directly related to occupancy, water usage, and mechanical equipment
What is the stack effect?
Pressure difference (phenomenon) related to the height of the building and bouyancy
Why does cooling load generally differ from heat gain?
Radiation from the inside surface of walls and interior objects
How do occupants add sensible and latent heat gain to a space?
Sensible and latent heat gain is dependent on activity level of the occupants.
In what cases is RTS method applicable and not applicable?
The RTS method is suitable for peak design load calculations, but it should not be used for annual energy simulations due to its simplified assumption such as steady-periodic conditions
What happens when lights are turned off?
The cooling load decreases, but does not go to zero immediately
What determines thermal lag?
The heat storage and heat transfer characteristics of the structure and interior objects of a space
What is the definition of cooling load?
The rate at which energy must be removed from a space to maintain the temperature and relative humidity at the design indoor condition
What would happen if lights were to be left on for 24 hours a day?
The space would approach an equilibrium condition, where the cooling load equals the power input (Fu=1)
What is transmission heat loss?
Thermal losses due to heat transfer through walls, floors, etc.
What is the general equation for total heat gain?
Total heat gain = solar heat gain + conduction heat gain