HVAC Flashcards
(16 cards)
What is the purpose of an HVAC system?
Heat
- overcome heat losses
Ventilation
- remove toxins, CO2, odors
Air Conditioning
- control temperature AND humidity
What is the First Law of Thermodynamics?
How is it applies to HVAC?
- Energy cannot be created nor destroyed
- We can use heat to do work or use work to transfer heat
What are sources of heat gain in a building?
- People
- Equipment (appliances, computer, plug loads)
- Lighting
- Solar
What are the units of energy?
Thermal energy: Btu
- British thermal unit, amount of energy required to raise 1lb of water 1 degree F
- Equivalent: Joule (J)
Electrical Energy: kWh or kWxHour
Describe the basic refrigeration cycle, including all key pieces of equipment in the process
- Steps move in a cycle
- Condenser
- Refrigerant condenses
- High pressure, hot gases, refrigerant rejects energy to warm air environment and turns back to liquid
- Heat output here - Expansion Valve
- Refrigerant expands
- Hot, liquid refrigerant drops pressure and temperature
- like the reset - Evaporator
- Refrigerant evaporates
- Low pressure, cold liquid/gas mix, pulls energy from the environment changing from mixture to gas state
- Heat input here - Compressor
- Refrigerant compresses
- Work added to the system to compress the gas refrigerant
How do you calculate refrigeration cycle efficiency?
Coefficient of performance (COP) = Q_L / W_in
Q_L = heat we want to/can absorb, desired output
W_in = work to move energy, required input
Calculate heat transfer, CFM, or a temperature using the heat transfer equation
**Describe the operation and components of a heat pump
- Heat goes in at the evaporator
- Heat goes out at the condenser
Explain common air system and water system components
Air Systems:
- Mixing box (where outside and return air mix)
- Air filter
- Heating element (heating coil)
- Cooling element (cooling coil)
- Fan (sends supply air out)
Water Systems:
- Pipes (carry water through system)
- Valves (control water flow and shutoff)
- Pumps (move water through hydronic system)
- Chillers (chill water through refrigeration cycle)
- Cooling tower (rejects heat to atmosphere)
- Boiler (heats water)
- Terminal units (radiators, chilled beams etc)
What is the difference between CAV and VAV systems?
CAV: Constant Air Volume
- CFM_SA remains constant (fan speed remains constant)
- T_SA changes with load
- Zone thermostat controls T_SA
- For large open areas, computer rooms, labs, hospitals, processes
VAV: Variable Air Volume
- T_SA remains constant
- CFM_SA changes with load
- Zone thermostat controls CFM_SA
- Can have VAV with reheat
- For larger facilities, perimeter zones, commercial offices, classrooms
What is the difference between centralized and decentralized systems?
Decentralized: on system per zone
Centralized: HVAC equipment serves multiple zones
What are the units of Power?
Btu/hr (energy rate)
Electrical power: kW (kilowatt)
What is the unit of a “ton”? When do you use it?
1 ton = 12,000 Btu/hr
Used only for cooling
What is the Second Law of Thermodynamics?
How can it be applied to HVAC?
- Heat transfers from a hot to cold environment
- We can input work to move heat in the opposite direction
- We need HVAC to control the heat gains and losses experienced in the summer/winter
What are sources of heat loss in a building?
- envelope losses
- slab losses
- infiltration
- ventilation
What is the Energy Efficiency Ratio (EEF)?
How is it different from COP?
What is kW/ton?
EER is used for heat pumps
COP is used for heat pumps & decentralized units
kW/ton used to describe efficiency of chillers
