HVAC II Final Review Flashcards
What is Approach Temperature?
Delta T between evaporator coil temp and chiller water leaving temperature
What is the purpose of an Expansion Tank?
Located above boiler to give space for air initially trapped in system and for expanded water when heated
What type of pump is used for a circulator?
Centrifugal Pumps
What are the advantages of a HEPA Filter?
Very high-eff (0.3um and lower), remove 99.99%
How is a Purge Unit connected to a system?
It is connected to the top of the condenser to collect and remove the air
What is Ventilation?
Process of supplying and removing air by natural or mechanical means to and from a space
Where is a Balancing Valve installed on a heating terminal unit?
Installed on the return side of each heating unit
What are the types of Cooling Tower Air Flow?
Cross-Flow (air in thru side, pulls air out thru top) and Counter-Flow (air in thru btm, up thru top)
What is the purpose of the Load Limiter?
To sense the motor amperage and partially close the guide vanes to limit the compressor amperage to the full-load value
How is capacity control done on a Rotary Screw Compressor?
Slide valve that blocks suction gas before entering. Typically by differential pressure in the system
What does a normal gas flame look like?
Blue with slight orange tips
What are the types of Fan Switches?
Temperature (in the air stream near Heat Ex) and Timer (non-adjustable)
What is the delta T between condenser refrig temp and ambient temp for a High Eff system?
20 degrees F
How does a Thermocouple work?
Two dissimilar metals welded at one end (hot junction). When heated, generates small V (15mV loaded (connected to coil) 30mV unloaded (w/o coil). Voltage travels to cold junction at gas valve
What is the ideal delta T between the evap refrig coil temp and the air entering the evap coil?
10-20 degrees F
What is the purpose of Guide Vanes?
At entrance of impeller eye to allow or reduce flow by pneumatic or electronic motors to open or close vanes. Also, guide refrig to impeller eye.
What is the condition of refrig entering the compressor?
Superheated Vapor
What should the delta T be between the condenser coil and the ambient air in a standard efficiency system?
30 degrees F
What conditions cause Centrifugal Compressors to surge?
Too high head pressure or evaporator pressure too low that the compressor can’t overcome the pressure difference
Why do we use Crank Case Heaters?
To prevent refrigeration migration and to vaporize refrig that migrated to the compressor
Name the types of Evaporator Defrost methods:
Random or Off-Cycle, Planned (forcing comp off for short periods), Hot Gas, Electric
What is the purpose of a Gas Regulator?
Drops NG to usuable pressure (3.5” WC) and meters constant flow
Name the types of Low Ambient Head Pressure Control:
- fan cycling using pressure control 2. fan speed control 3. air volume control using shutters and fan cycling 4. condenser flooding devices
Describe a flame with Primary Air Restriction:
Yellow
What is a Sling Psychrometer used for?
To obtain Wet-bulb and Dry-bulb temperatures
What are the basic furnace styles?
Horizontal, Uplow, Downflow, Low Boy, Multipoise
What is the minimum slope of horizontal flue pipe?
1/4” per foot
STUDY P/T CHARTS
Study it
What is the purpose of a Draft Diverter?
Prevent pilot blowout and main burner flame from being altered
What are the characteristics of Pictoral Drawings?
Shows location of each component as it actually appears to the person installing or servicing equipment
Under what conditions are people at rest most comfortable?
75 degrees F, 50% Humidity
Define Humidity:
Moisture in the air
Define Dew Point:
The exact temperature at which moisture begins to form
If we know two point on Psychrometric Chart, how many conditions can be determined?
Dry/Wet Bulb give 5 (dew point, total heat (enthalpy), moisture content, RH, specific volume of air)
Which is Heavier, NG or air?
Air 1.00, NG 0.60 specific gravity
Name the types of burners:
Drilled Port, Slotted Port, Ribbon Burner, Single Port (inshot or upshot)
What is the normal pressure of a NG Furnace?
3.5” WC (1psi = 27.7” WC)
Where is the High Side Float at in the system (Floats are only on Chiller units)?
In Liquid Line entering the evaporator
What happens to down stream temperature if there is a restriction in the Liquid Line?
Pressure will drop, temp will drop, capacity will drop (becomes vapor sooner)
How close can cooling tower water be cooled to the Wet Bulb Temp?
Within 7 degrees of Wet-Bulb Temperature
What material is used to make Electric Heat Elements?
Nichrome (Nickel-Chromium)
What is the purpose of Auxilary Heat in a Heat Pump System?
Supplimental heat, emergency heat
How does a Limit Switch function in an electric heating system?
Shuts off unit if high temperature occurs
In what units is gas pressure measured?
inches WC
Where is the Oriface in a gas system?
Spud
Name the two basic chiller types:
Compression Cycle Chiller (Compression AC sys) and Absorption Chiller (sodium solution)
How is flue gas removed from a conventional furnace?
Natural Draft
To what is the Rupture Disc attached?
On evaporator section or suction line, breaks at 15psig
How does a Limit Switch function on a Hydronic Boiler?
Shuts off unit if high temperature occurs. Immersed into boiler water or placed in a dry well into boiler. Home and light commercial low pressure at less than 30 psig set for 200 degrees F off
