Comercial HVAC Systems Flashcards
How could a constant volume system best be described in relationship to air flow and temperature?
Supply air flow rate is constant but the supply air temperature is varied to meet the thermal loads of a space.
Provide some applications for a constant air volume system.
Restaurants, offices, apartment buildings, supermarkets, plants, laboratories and schools.
What are some system variations for constant air volume systems?
CAV with reheat, CAV multi zone, CAV primary-secondary.
Why are constant air volume systems often the system of choice?
Their simplicity, low cost and reliability.
What does the acronym VAV mean?
Variable Air Volume.
What are the two main styles of VAV systems?
Regular VAV system and true VAV system.
How does a regular vav system control its temperature
Regular VAV system uses dampers to throttle or bypass the air to control its quantity.
What reduces energy consumption by fans in a VAV system?
Fan capacity control.
How many air inlets are there on a single supply VAV box?
One.
How does a single duct VAV system differ from a dual duct VAV system?
A single duct VAV uses a volume control damper to control how much cool air enters the zone whereas a dual duct system has 2 supplies, one for heating and one for cooling (hot deck and cold deck).
Where do the two air streams mix in a dual duct VAV system?
Mixing Box.
Describe how a dual duct system can provide better air flow to the conditioned space than a single duct system.
A dual duct system provides thermal efficiency while maintaining higher airflows, resulting in better circulation of air in the room when heating is required.
How does the air volume vary in a bypass system?
By cycling unwanted conditioned air back to the central unit without allowing it to pass through the space.
Reheat coils are either hot water or electric coils and are usually located in or close to the zone they serve. True or False?
True.
What controls the amount of air and its temperature delivered to each zone in a multi-zone system?
A fan blows streams of supply air over both a heating coil and a cooling coil in a parallel configuration where they can be mixed to the required temperature before being distributed to the zone.
For higher velocity air systems, how is air velocity slowed down before entering the conditioned space?
Air velocity is slowed down by the attenuator box.
Where are Induction Reheat Systems often found?
Beneath windows.
Why do most building exhaust air systems require make-up air systems?
Because if the air is not replaced at the same rate it is expelled, the building can experience a negative pressure which can cause issues.
List the two types of gas-fired make-up air units.
Direct gas fired and Indirect gas fired.
How do Indirect and direct gas fired MUA systems differ?
A direct gas fired MUA has a gas burner directly installed in the outside makeup air stream whereas the Indirect gas fired MUA has a sealed combustion chamber where all products of combustion are discharged through a flue.
What type of air system is usually paired with most gas-fired make-up air units?
An exhaust system.
What are three main applications for direct fired make-up air units?
Make up air heating and cooling, space heating, space pressure control.
Flue gases from combustion are discharged outdoors through the flue in direct-fired make-up air units. True or False?
False.
Must the exhaust air and makeup air systems be interlocked?
Yes.
List 4 types of vav systems
Single duct
Dual duct
Bypass
True vav
Vav systems are preferred because of their energy saving capabilities
True
Single duct VAV systems adjust the volume of the air entering space
T OR F
True
Dual duct vav systems blend the cooled and heated air at the air handling unit and then distribute it through the ductwork to the terminal units
T OR F
False
A dual duct system maintains the temperature in the space by ?
Increasing or decreasing the quantity of heated or cooled air
How does a bypass vav system vary the volume of air being delivered to a space
By cycling uneeded conditioned air back to the central unit without allowing it to pass through the spaces
The bypass vav system uses________ to control the space temperature?
The opening or closing of a volume damper
A true Vav system varies the__________to control the temperature in the space?
Air volume
What is the air temperature that is delivered from the central air handling unit in a CVS
55 degrees F
List three applications of direct fired MUA systems
MUA heating/cooling
Space heating
Space pressure control
What does the acronym CV-VT stand for ?
Constant Volume - Varying Temperature
What temperature is air deliverd at in the CVS single zone reheat system?
55 degrees F
What is the primary use of a MUA unit
Replaces air that is expelled by the buildings exhaust system
A direct fired makeup air unit that can utilize return is called a___________?
Recirculating direct fired makeup air unit
How does a true vav system control its temperature
The true VAV system changes the speed of the fan to control the quantity of airflow
