Commercial Air Conditioning: Exam 2 Flashcards
What is tempering air?
bringing it to room temp (preheating)
Why is negative pressure in a building bad?
It will pull moisture from outside into your building because the humidity outside is higher
What is better; slight positive pressure in building or slight negative?
slight positive pressure
Air Conditioning can be accomplished by using either single zone or _____-zone control
single or multi-zone
Four types of HVAC systems;
- All ___
- Air-________
- All-_________
4.
- All air
- Air hydronic
- All hydronic
- Unitary (rooftop all in one)
What is a double split unit referring to?
compressor sits inside condenser unit
What is a triple split unit referring to?
compressor sits outside condenser unit
Think of a zone as the area controlled by one ________
thermostat
How to calibrate pocket thermometer?
cup of ice water
Word for heat sitting on top of cooled air?
Stratification
Zone control is accomplished by individual systems for each space or by supplying many spaces and regulating;
- Air temperaturę with constant volume of air to each space - constant volume systems (__)
- Air volume with constant temp air to each space - variable air volume (___)
- Air volume and temp to each space - variable volume and temperature (___)
- Each of these may have system combinations which add heat or cooling energy to control the comfort level in __________ zones
- (CV)
- (VAV)
- (VVT)
- individual
CV dual duct supplies two ducts to each space, one ___ and one ____
hot, cold
VAV has ___ supply duct to each space. Air volume varies based on thermostat in each zone. Supply temperature is ______
one, constant
VAV dual duct has two supply ducts to each space. One ___, one ____ VAV terminal mixed as required, varying the volume
hot, cold
VVT has one supply duct to each space. Air volume varies based on _________ in each zone. The temperature varies
thermostat
VAV fan powered mixing box
The VAV terminal box may be equipped with a ___ to use the return air heat to assist in maintaining required air temp.
fan
VAV with Heat
One supply duct to each space. Air volume varies based on thermostat in each zone. Temp is ________ when minimum air volume will overcool.
modulated
Air - Hydronic
Supplies air to conditioned spaces or zones as well as ______ to conditioned space or zone (perimeter heating) fan coil units. Duct system can be;
- single zone
- multi zone
- dual duct
- terminal reheat
- constant volume
- variable volume
water
indirect cooling tower is water being sprayed over condenser water _____ to provide cooling
tubes
Direct cooling tower water is sprayed and falls into ____ then flows back to condenser and recirculates
sump
All-Water Systems: Duct vs Pipe; Remember
5/8” OD pipe: 2.4 gpm @ 6 fps = 1 ton of cooling
8” round duct: 400 cfm @ 1200 fpm = 1 ton of cooling
5” round duct 400 cfm @ 3000 fpm = 1 ton of cooling
yes
All - hydronic (when duct is not available)
Supplies _____ only to conditioned spaces or zones by the use of special terminal units; ex. Fan coil, unit ventilators, valance units
water
Unitary systems are ____-contained packaged equipment normally an All air system
self
Air flow can be;
Low velocity - ____FPM
High velocity - _____-_____FPM
Low velocity = 2000fpm
high velocity = 2001-6000fpm
Air flow entering a zone should be between 20fpm - ___fpm
20-65fpm
Less than 20FPM you get _________, greater than 65FPM you get _____
stratification, draft
An Impeller is the part that moves the air and must be rotated by a prime mover (normally an ________ motor). The fan is classified according to the direction of airflow through the impeller.
electric
Air Flow
- Flow perpendicular from axis of impeller referred to as _________ flow
- Flow parallel to the axis of impeller referred to as _____ flow
- centrifugal flow
2. axial flow
Types of Impellers: P_______;
Fan disk (axial flow) Tube axial (spiral discharge) Vane axial (laminar discharge) Laminar = straight flow
propeller
Types of Impellers: (no answer just remember) Fan Wheel (centrifugal) 1. 2. 3. 4.
- backward curve
- forward curve
- inclined
- radial
Fan Performance:
The performance of a fan is its ability to generate the required ______ pressure for the system, yet maintain the required velocity pressure to ensure delivery of the design CFM.
static
Fan selection is based on the following factors;
- system ____
- ___ required
- Total _______ (static pressure req’d)
- _____ availability
- system size
- cfm required
- resistance
- space
Fan selection
Fan selection is based on the ability to overcome system ________ and create enough velocity for proper air movement.
resistance
System Resistance components; name 7
- filter
- coil
- duct elbows
- supply duct
- supply diffuser
- return grille
- return duct
Terminology:
- Overloading Type Fan; on a sudden decrease in ______ the prime mover will develop excessive HP and overload
- Non - Overloading Type Fan; On a sudden decrease in static the prime mover will ___ develop excessive hp thus will not overload
- STATIC
2. NOT
Fan Arrangements:
- Draw thru; The fan is located at ______ of main plenum
- Blow thru: The fan is located at _____ of main plenum
- outlet
2. inlet
Two fan systems have _______ fans for supply and returns
One fan system uses ___ fan for return and supply air
separate, one
Forward curve fan:
Used in most packaged equipment. Usually smaller, lighter, and runs slower. Remove resistance and air quantity would increase but so would HP. If allowed to continue to rise the motor would over ____
over amp
Backward curve fan:
Remove the resistance and air quantity would increase. HP would increase to a point then decrease, therefore known as non-____________.
Non-overloading
State the main difference between forward vs backward curved fans.
Backward curved fans are non-overloading
Propeller Fan
Used in applications where large quantity required with little resistance to air flow. Resistance increase, CFM decreases, and HP _______
increases
FAN CHARACTERISTICS: remember
- Forward curve: 20,000 CFM @ max 5” of static (overloading)
- Backward curve: 20,000 CFM @ 3’’ of static (non-overloading)
- Axial: 50,000CFM+ @ 3” of static+
yes
When most fans approach the block tight static operating condition, instability is encountered. This condition is known as _____
surge
Surge occurs when the air volume moved by the fan falls below that necessary to sustain the existing static _______ difference between the inlet and outlet sides of the fan. When this occurs, air ______ back through the fan wheel, momentarily reducing the duct static pressure
pressure, surges
How many cfm per ton?
400CFM
The surge of air enables the fan to again establish the original static pressure. The resulting fluctuation in cfm and static pressure within the fan and ductwork results in _____ and _________
noise and vibration
What is static pressure?
resistance to airflow in ductwork
System Resistance
With each cfm flow rate, an air distribution system imposes a certain resistance to the passage of air. The resistance is the ___ of all of the pressure losses experienced as air passes through ductwork, diffusers, return air grills, dampers, filters, coils, etc.
sum
True or False?
The unit measure of resistance, or pressure loss, is in inches of water column, static pressure
True
It is not practical to test a fan at every speed at which it may be applied.
Fortunately, by the series of equations commonly referred to as “___ ____, it is possible to predict with good accuracy the performance of a fn at conditions other than those of the original rating.
fan laws
Fan Laws:
The performance of all fans is governed by certain rules of physics called fan laws. The variables are; 1. 2. 3. 4.
- CFM - cubic feet per minute
- RPM - revolutions per minute
- SP - static pressure
- HP - horse power
The three main fan laws: The most commonly used fan laws in simplified form are: TRUE OR FALSE
- CFM varies directly with RPM
- Ps varies with the square of RPM
- bhp varies with the CUBE or the RPM
True
Duct system pressures
External Static Pressure (ESP)
- is the term used by the OEM to express the total energy available to overcome the __________ of the ductwork and accessories in the system.
- accessories include coils, fans, grills
resistance
Duct system pressures
Pressure drop (PD) 1. Is the reduction in total \_\_\_\_\_\_\_ caused by components added to a duct system
- Pressure drop is the difference in ______ pressure from one side of the component to the other.
- pressure
2. static
Duct system pressures:
Static pressure (SP) 1. Is the pressure pushing \_\_\_\_\_\_ or inward on the walls of the duct
outward
Duct system pressures:
Velocity Pressure (VP) 1. Is the pressure exerted by the \_\_\_\_\_ of air along the duct
- flow
Duct system pressures:
Total pressure (TP) 1. Is the sum of \_\_\_\_\_\_ pressure and \_\_\_\_\_\_ pressure
- Total pressure is consumed by the resistance of the duct system and its _________.
- Total pressure will decreases the further you are from the ___
- static pressure and velocity pressure
- components or accessories
- fan
Pressure Measurement:
Pressures in the duct system are measured in inches of water or in. wg, read with a _________
manometer
In. wg is an accurate way to read very small pressures above and below __________ found in a duct system
atmospheric
Pressure Measurement:
- one inch of water gauge is equal to ____psi
- 0.036psi
Air Measuring Devices:
Manometers (oil filled)
-U tube and _______ tube
inclined
Air Measuring Devices:
Magnehelic (non oil filled)
- used for _______ measurement of pressure drop across filter banks
direct
Air Measuring Devices:
Anemometer or Velocity meter:
1. Used for a direct measurement of ________ in FPM
velocity
Air Measuring Devices:
- Using a PITOT tube allows the measurement of total and static pressures at the _____ time
- Only ___ hole is required to be drilled into the duct
- The pitot tube must be facing directly into the ___ stream for proper measurement
- same
- one
- air
Duct Traverse:
- For round duct drill only _ holes.
- Take readings and average for most accurate figure. Drill holes about __ duct diameters from any elbow. If this is not possible then take more traverse readings for greater accuracy.
- 2
2. 10 duct diamters
Duct Traverse:
- If only a ball park figure is required, take reading from centre or duct and multiply by __%
- ___ feet of straight duct before and after measuring point is required if this method is used.
- 90%
2. 10 feet
Free Area:
- When taking velocity measurements at supply or return air grills, the “____ area” must be factored into the calculation.
- Free area is total area of the grill itself less the area taken up by the “hardware”
- Normally found in the _____________ data, usually a percent factor.
- free area
3. manufacturers
Overall Requirement for A/C systems are:
- _______ control
- ___________ control
- Ventilation
- Filtration
- humidity control
- temperature control
- ventilation
- Filtration
Ductwork;
Insulation:
1. Outside - used if ambient air surrounding the ducting is _______ or colder than conditioned air
- Inside - used for noise control and to protect from weather
- hotter
Air Flow Control:
Louvers;
1. Louvers are _____ or manually adjusted.
- Normally equipped with animal or ___ screens
- fixed
2. bug
Lovers:
Used for;
- Fresh air intake (min __% fresh air)
- _______ air
- Zone supply
- min 20%
2. exhaust
Dampers and Air flow control:
- Dampers can also be called ______ dampers
- Used for controlling airflow rate
- in high pressure air systems they are called pressure reducing valves
- volume dampers
Air flow control:
There are two types of multiple blade dampers: name them;
- ________ blade
- _______ blade
- Parallel and opposed refer to the movement of the adjacent blades
- parallel
2. opposed
Air flow control: Parallel blade;
- When this damper opens and closes, the air has a tendency to be deflected to ___ side of the duct
- one
Air flow control: Opposed blade;
- When this damper opens and closes, the air is not deflected to one side of the duct and is therefore more efficient, causing less ___________
- turbulence
