Theory Flashcards
What are physical properties that space conditioning is trying to control?
Temperature
Humidity
Freshness
What types of spaces are classified in HVAC design?
Residential buildings
Commercial buildings (e.g., office, retail)
Places of assembly (e.g., school, arena, hospital)
Specialty area (e.g., car, cargo truck, aircraft)
Enthalpy is most relevant to…
A) Temperature
B) Flowrate (velocity, m/s)
C) Pressure
D) Elevation
What is convection?
Energy transferred between a surface and fluid in contact with the surface.
A combination of heat conducted at the fluid-surface interface, conduction and mixing within the moving fluid, and thermal energy carried by the flow.
What are the two duties of the mechanical engineer?
Determine the capacity of the equipment that is to be installed
Determine the annual energy consumption of the equipment and the cost of operating the HVAC system
What is the influence from the weather?
Design conditions: specified extreme weather conditions 8==> equipment sizing
Track the instantaneous system performance hourly 8==> summing up for the annual performance
Does feeling equal satisfaction?
No
If they feel it, doesn’t mean they’re satisfied. Story of my life..
What is Sol-air temperature (Tsa)?
A “hypothetical” outside temperature that considers the radiation effect
It’s not air temperature (TA)
What does one MET equal?
Average person at rest
Which decision factor has a stronger influence to conduction gain and solar gain, respectively?
Conduction = Construction aka window panes
Solar Gain = surface treatment
What are the three types of VAV boxes?
Throttling Box
Induction Box
Fan-Powered Box
What are the design variables in duct distribution?
Amount of outside air Inlet temperature Duct sizes Pressure drop of different sections Fan types and sizes Volume flow rate Air velocity
How do you determine the friction rate?
Low friction rate: if the cost of energy is high and duckwork is inexpensive (bigger duct)
High friction rate: if the cost of energy is low and duckwork costs are high
Define static regain method..
“With this procedure approximately the same static pressure exists at the entrance to each branch, which simplifies terminal unit selection and aids system balancing.”*
What are the two characteristics of static regain method?
Need lower velocity to regain the static pressure
Resulting in relative large duct size
What is the trend of duct size if we look for a longer term of LCC?
BIGGER DUCTS
WBVD
Air Diffusion Performance Index (ADPI)
ADPI is a measure of the effectiveness of the air distribution system in distributing conditioned air throughout the zone
An ADPI close to 100 % is desirable
Jet throw
The distance that the air jet from the outlet travels before the velocity drops below a minimum value (50 fpm or 0.25m/s)
What are the possible applictions of psychrometrics?
Plenum chamber for mixing of air streams Heating or cooling coil Cooling tower Evaporative cooler Desiccant dehumidifier Humidifier Air washer
Evaporative coolers are best used in what climate?
Hot and dry climates
Air stream in evaporative cooler is cooled by
A matrix that is continuously wetted by water
Why are evaporative coolers constant wet bulb?
This is because there is no change in the amount of energy in the air. The energy is merely converted from sensible energy to latent energy. The moisture content of the air increases as the water is evaporated which results in an increase in relative humidity along a line of constant wet bulb temperature.
How is efficiency affected by NTU?
Higher NTU means Higher efficiency
How is efficiency affected by C*?
Lower C* means Higher efficiency
