Unit 10 HVAC fundamentals For Facility Operators Flashcards
Ventilation process
Supplies air or removes air from a space.
Filtration process
Removes particles from the air
Humidification process
Controls the humidity of the air
Air conditioning
Collectively; Heating, cooling, filtration, ventilation, and humidification
Exhausting air is important because
Air may contain odours, excess humidity, CO, and other contaminants which may affect people’s health
Building changes per hour
Each building is required to change the air in the building. Measured in m3/hour
To increase building efficiencies, a percentage of building air is
Exhausted to the outside, and a percentage is recirculated back to the building
HVAC systems usually include;
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Fans, filters, ducting, heating coils, cooling coils, humidity control
The purpose of a HVAC is to
Improve the comfort of the people occupying the building space.
Factors that influence human comfort
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Temperature, humidity, air quality, air movement
Air quality
Odours, particulate, and other conditions that affect human health
Air movement
Refers to the velocity, noise, drafts, and air circulation
Fixed louvres
Used to redirect airflow, causes rain or snow to fall out of air suspension
Dampers
Used to control air flow
Finned heating coils
They use steam to heat the air. Steam circulates through the coil and heats the air that travels around the coil. Steam condenses inside the coil, and returns back to the boiler
Finned cooling coils
May use chilled water to cool the air. Water circulates through the coil, and removes heat from the air that travels around the coil. Warm water leaves the coil and returns back to chiller
Humidifiers
Add water to the air by pumping water and injecting it into the air flow using sprayer nozzles
Air diffusers
Directs the flow of air into the space without creating draft
Fans
Mechanical devices that cause air flow or movement
Return air
Percentage of exhaust air that returns from the building back to the HVAC system, recirculated to increase efficiency
Outside air
Also known as make-up air. Required to help maintain human health conditions
Mixed air
Mixture of return air and make-up air. Flows to HVAC system to be treated
Supply Air
Air treated by the HVAC system that will be supplied to the building
Human health is the _______ for the design of an HVAC system
Key criterion
High humidity increases the chance for
Mold to form inside the building
Effective temperature (ET)
Measure of comfort which involves the dry bulb temperature, relative humidity, and air movement.
In °C/°F, but not an actual temperature and cannot be measured on a thermometer
During summer and winter, relative humidity should be kept between
30% and 70%.
Above 70% the air will feel muggy, below 30% the air becomes too dry and effects nasal membrane
An increased airflow increases the heat loss of the body by
Conduction, convection, and evaporation. Helps the body remain comfortable even when room temperature is higher than normal
An air flow of _____ is considered to be relatively still air
Airflow of _____ would be considered a draft by most people
- 6 - 7.6m/min
19. 8m/min
Tests show that 2% of the people will not
Be satisfied no matter how the conditions are changed.
Ventilation
Provision of outside air into the building is required to replenish the oxygen supply for the occupants. Outdoor air is necessary to dilute odours
When the relative humidity of the outdoor air is low,
Moisture must be added within the building to maintain acceptable conditions for human comfort.
Moisture minimizes the effects of
Static electricity, which may be important for operation of sensitive equipment
________ is a method of removing excessive moisture from the building.
Dehumidification
Air movement is required in each space to assist in
Dissipation of body heat from occupants and prevent a buildup of odours
Air filters are required to remove
Contaminants such as dust, dirt, smoke, pollen, lint, from recirculated air and from outside air before it is introduced to the building
Three categories of air conditioning systems
Unitary systems, Central systems, combined systems
Unitary systems
Each individual room or zone is provided with a self-contained air conditioning unit
Central systems
An air conditioned air handling unit located in a central mechanical room. Conditioned air is distributed through ductwork to all zones. Mainly controlled in mechanical room, but may have supplementary thermostats
Combined systems
Air is conditioned in an air handling unit in the central mechanical room and is ducted to all areas of the building. Provides only ventilation, pre Heating and Cooling while supplementary equipment in each Zone provides Heating and additional cooling if required. More complex and costly then Central systems, but provide Superior Zone control
Air handling unit consists of;
Fan, filters, humidifier, source of heating and cooling, motorized dampers for air flow control
Possibly a humidifier or dehumidifier
Medium to large systems utilizing return air in mixed air may require;
Exhaust, or return, fan to overcome friction losses
Fixed outdoor air percentage system
Supply air is drawn through non modulating dampers. Dampers are ffixed to permit 10 to 30% outdoor air. Cheaper than variable percentages, because they do not use control systems to modulate dampers. Do not respond well to weather changes
Variable mixed percentage air handling system
Varies the ratio of makeup to recirculated air based on climate conditions. More costly, but balanced human comfort. Installed in residential, commercial, and industrial buildings
Completely pure and dry air does not
Exist in nature.
Moisture in the air is not in the form of droplets,
Rather it is water that has been evaporated into vapour, which has mixed with the air.
Water vapour in the air is
Low pressure, low temperature steam
Absolute or specific humidity
Actual mass of moisture present in a specific amount of air.
Expressed as kg per kg of dry air
Relative humidity (RH)
Ratio of the mass of moisture in a given amount of air to the maximum amount of moisture the air can contain at that temperature.
Expressed as a percentage
Relative humidity and temperature relation
Inversely
Dew point
The temperature at which air subjected to cooling becomes saturated, and below this temperature water vapour will condense from the air
Dry bulb temperature
The temperature of the air measured with an ordinary thermometer
Wet-bulb temperature
Temperature indicated by a thermometer having its bulb covered by a water wetted Wick, which is exposed to a stream of rapidly moving air
A wet bulb thermometer measures
The dry bulb temperature and subtracts the cooling effect of evaporation
The rate of evaporation of the moisture in the wick depends on
The humidity in the air
Wet-bulb depression
Difference between the dry bulb temperature and the wet-bulb temperature
The purpose of humidification is to increase
The relative humidity in the air
The higher the temperature of air, the _____ moisture it can absorb
The lower the temperature of air, the _____ moisture it will hold
More
Less
In order to maintain comfortable conditions inside a building during the heating season,
It is necessary to add humidity to the air.
From a health standpoint, relative humidity between ______ is desirable
35% and 50%
When slight condensation appearing on the corners of the window panes,
The relative humidity inside the building has reached its maximum permissible value with relation to the outside temperature.
Three types of humidifiers used for residential purposes
Pan, wetted element, and atomizing
Pan type humidifier with plates
Shallow pan or tray partially filled with water. Water is drawn to the plates by a hygroscopic action
Wetted element humidifier
Water-saturated evaporation pad through which warm air is forced. Water is fed by gravity or by spray
Atomizing humidifier
Water is broken up into small particles by a Spinning Disk or a cone, or spray nozzle. Usually used as a portable or stationary room unit
In a humidifier, proper evaporation is prevented when clogging occurs due to
Dissolved mineral salts in the water, once evaporated, stay behind in the evaporation plates and pads
Atomizing humidifier disadvantage
Water is sprayed into the air and the salts in solution in the water appear as a fine white dust, which settles in ducts and rooms
Three types of Industrial and Commercial humidifiers used in HVAC systems
Air washers, steam grid humidifiers, pan type humidifier
Air washers
One of the most versatile pieces of equipment in HVAC systems. Main purpose is humidification of air as well as;
Heat, cool, dehumidify, clean the air
Eliminators are installed at the outlet of the washer to
Prevent water droplets from being Carried Away by the air. Designed so they change direction of airflow several times causing droplets to fall
Are washers are not effective in the removal of
Greasy particles, soot or tobacco smoke
An air washer should be installed on the
Suction side of the fan where it can be maintained at pressure slightly below atmosphere to avoid leakage of water.
When the air washer is used, the power engineer can have considerable control over
Temperature and humidity
Steam grid humidifier
When steam is available, humidification is achieved by injecting steam into the supply Airstream
With the steam grid humidifier, to achieve satisfactory results, steam pressure should be in the range of
35 kPa to 70 kPa to prevent splashing of condensate and a drastic increase in temperature of supply air.
Steam used for humidification can cause health problems, if it contains
Chemicals such as amines. Steam must have high purity
Clean Steam humidification
Uses a heat exchanger that is supplied with steam as the heat source. Heat will be added to demineralized water and caused it to boil into low pressure clean Steam. Will have no chemicals and minerals, will leave no mineral dust or chemicals in the air
Clean steam humidification is relatively expensive because
Requires a heat exchanger to be used in conjunction with the steam boiler.
Applications for clean Steam humidification include;
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Hospitals, archives, pharmaceutical industry, museums
Pan type humidifier
Offers a broad range of capacity. Installed on the inside casing of air handling apparatus, inside the supply duct, or attached to the underside of a duct.
Pan type humidifier can be ________ heated or _______ heated
Electrically
Steam
Static pressure
Not caused by air movement. Exerted by the air on duct walls. Can be read by connecting the manometer at right angles to the duct.
Static pressure of the air overcomes
The resistance of ducts and filters
Velocity pressure
Caused by moving air. It is the pressure caused by the impact of the air flowing through a duct, only detected when it strikes a surface.
Velocity pressure will be in addition to the
Static pressure in the system.
Total pressure
Sum of static pressure and velocity pressure. Can be measured With a manometer
Total pressure formula
Total pressure =
static pressure + velocity pressure
Static pressure can be either ____ or ____
Positive, above atmospheric
Negative, below atmospheric
Conversion of pressure and velocity is never 100% efficient, due to
The formation of Eddy currents
Two general groups of fans
Centrifugal and axial flow
Centrifugal fans
Air flows radially (perpendicular) through the impeller. Classified according to the shape and design of the blades
Axial flow fans
Air flows axially (parallel) through the impeller. Classified on the principles are designed for
Another common name for a centrifugal fan is
Squirrel cage
Classes of centrifugal fans
Class 1
Class 2
Class 3
Class 4
Classes of centrifugal fans
Class 1
Class 2
Class 3
Class 4
Fans with backward inclined blades require
Less power to deliver a specified Air volume, making them more efficient than forward curved fans. Higher efficiencies result in quieter operation — for medium to large fans
Backward inclined airfoil fan is mostly used for
High capacity, high pressure, applications where power savings outweigh the higher cost
Radial blade fan
Lower initial cost, but less efficient and noisier
Three types of axial fans
Propeller fan, tube axial fan, vane axial fan
Propeller fan
Driven by motor, directly, or V belt. Higher moving capacity, but unable to produce High pressures. Cannot be used in ducted air distribution, but can be used in air-cooled condensers, unit heaters and exhaust fans for ventilation
The propeller fan is the most economical type to produce
Large air movements at low pressures
Tube axial fan
Heavy-duty suited for duct mounting. Discharges are in a spiral motion, resulting in higher friction losses in the Ducts. Noisy in operation, used in industrial air conditioning applications, where noise is less objectionable
Vane axial fan
Tube axial fan with guide vanes behind the fan wheel to straighten out the Spiral Flow of air. Results in reduced turbulence and friction with improved efficiency. Relatively noisy, needs sound attenuators at strategic locations
Fan performance curves are obtained from
A series of laboratory tests conducted on a fan with various restrictions at the end of the test duct.
The purpose of fan performance curves are to
Allow the designers to determine the correct fan selection for the system and to help operators troubleshoot fan performance issues
Backward inclined fan curve
Considered non overloading fan. Will not change significantly with changing air flow conditions
Forward curved fan curve
Considered overloading. At constant speed, With changes in airflow the power of a power requirements will increase causing the fan and motor to overload
2 fan drives
Direct drives or V-belt drives
Direct drives
Normally used on smaller-sized air moving equipment
V belt drives
Used on most Air Conditioning, Heating, and ventilation applications.
Belt guards
Required for safety on all V belt drives.
Coupling guards
Required for direct-drive equipment
Considerable economy and power consumption can be obtained on large air conditioning and ventilation systems by reducing the
Air volume handled by the fan during periods of partial loads, such as evenings or weekends
Four methods for volume control
Variable speed motor control, fluid Drive, Outlet damper control, variable Inlet vane control
Variable speed motor control
Most efficient, but most expensive
Fluid Drive
Used on Industrial applications, efficient but expensive. Simplified startup and de-clutching feature, relieves driven fan of shock loading during startup.
Outlet damper control
Used on fans with constant speed motors. Least expensive and least efficient
Variable Inlet vane control
Radial vanes installed in the fan Inlet. Reduces energy consumption by controlling amount of air spin at the fan Inlet.
Ventilation rate
Air changes per hour
Minimum number of air changes per hour for buildings is determined by
Number of occupants, activities, type of contaminants in area
Ventilation must be operated in such a way that there is a balance between
Energy Efficiency and human comfort
Two methods of ventilation
Natural and mechanical
Natural ventilation
Air movement is caused by the temperature difference or wind. Not used in many buildings, except small buildings with low occupancy rates. Not very effective, difficult to maintain minimum air changes per hour.
Mechanical ventilation divided into three groups according to the way the fan is used to move the air.
Includes;
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Natural air intake, mechanical exhaust
Mechanical air intake, natural exhaust
Mechanical air intake, mechanical exhaust
Natural air intake, mechanical exhaust
Stale air is drawn out by a motor-driven fan, and replaced by fresh air through windows or in intakes. Fresh air drawn into the building is neither filtered or heated. Low cost but low efficiency
Mechanical air intake, natural exhaust
Fresh air forced into building by a fan or blower, moving the stale air out through the Windows or Outlets.
Mechanical air intake, mechanical exhaust
In larger buildings, fans or blowers Supply fresh air into and remove stale air from the building. Designed to maximize the movement of air to increase human comfort.
If people are having trouble opening or closing exterior doors, one possible cause is
Incorrect building static pressure.
One disadvantage of the mechanical system is the
Greater costs for maintenance and operations
Contaminants suspended in the air
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Solid particles of visible size, solid particles of microscopic size, liquid impurities, vapour and gases, living organisms
The greatest cost of a filter is usually not the filter itself, rather
The energy cost the filter causes, increase the energy requirements to operate the fan
Mechanical filters
Throw away or cleanable / renewable filters.
Dry filters
Efficient when the screening dust, but dust holding capacity is small. Best filters for lint removal, but ineffective for removing smoke and fine particles.
Large air conditioning systems, dry filter is sometimes used in rotating air filter, also called
A a roller or curtain filter
Dry filter replaceable bag type
High dust removal efficiency and low air resistance
Try filter wedge type
Large filtering area, large-capacity, and a longer service life.
To protect the more expensive filter, it is important to
Provide pre filtering, will result in having to change expensive filters less
Viscous impingement filters
Most common filter used in ordinary air conditioning systems. Coated with viscous, sticky fluid.
High-efficiency particle air (HEPA) filter
Filters much finer particles than dry or viscous. Very efficient, but very expensive. 99% efficiency, but require a pre-filter for larger particles.
When a HEPA filter is used, ______ should be used
Multi-stage filtering.
HEPA filtration systems are commonly found in
Hospitals, Laboratories
Electronic air cleaners, two classes
Electrostatic precipitators and charged media electronic air cleaners
Electrostatic precipitators
Positively charged particles, flow through a bank of metal collector plates arranged parallel to air flow. Plates are negatively charged with 6000v DC. If power supply fails, filtration ceases. Common to use throw-away type-ahead of it
Electrostatic filter is the best filter available for
Removal of fine dust, smoke and fumes. Used when are Supply must be as clean as possible. Most expensive
Charged media electronic air cleaner
V-shaped bank across the air duct and mounted in non-conductive frames. Has charge members held at a potential of 12,000v DC. If power supply fails, filter media will still act as an impingement filter and clean the air.
Electronic air filters are expensive and require
Maintenance by a qualified person. Electrician may be required
Air washers
May be used for air cleaning. Low deficiency in dust removal, practically negligible and removal of small particles, especially greasy ones.
The most common and successful way of removing odours and Vapors in an air conditioning system is by means of an ______.
Odour absorber.
In-duct air sterilizers
Circular channels in the direction of airflow, containing a tubular ultraviolet lamp. Airflow is subjected to ultraviolet radiation.
Common way to classify duct systems is by _____ and _____
Pressure
Velocity
Low pressure duct system
less than 11m/s
Less than 0.5Kpa
Medium pressure duct system
11m/s to 25m/s
Up to 1.5kpa
High pressure duct system
Greater than 25m/s
1.5kpa to 2.5kpa
Low pressure system is more conventional for
Small and medium-sized buildings. Silent air movement, lower costs.
Losses that are functions of velocity
Noise, vibration, and friction
Materials used in sheet metal ducts are
Galvanized or black steel, aluminum, stainless steel and copper
Galvanized steel
Corrosion resistance but with a higher cost
Black steel heavy gauge sheets are used for
Kitchen exhaust ducts, ducting with waterproof Coatings, drain pans. Durable and High strength
Aluminum, stainless steel, or copper is used where
The cost is justified by the need for maximum resistance to moisture.
In order to maintain duct pressure and velocity, reducers are used to
Compensate for the reduction of air volume
SMACNA
Sheet metal and air conditioning contractors National Association standards
Elephant trunk style air duct
May lead to a high pressure drop in the system, lowering minimum air changes
Air duct leakage is a
Transmission loss
Are leakages from ducts vary from
5% to 30%
Effects of air leaks in supply ducts differ from those in
Return air ducts
Air leaks from supply ducts create loss of
Heating and Cooling
The direction of airflow has little or no effect on
The transmission of noise
To restrict spread of smoke and fire through duct systems and to minimize ignition sources, the duct lining material must satisfy
Local building codes and the requirements of the National Fire Protection Association standard 90A
Manual dampers
Used in smaller systems to close the outdoor air intake, to prevent freezing when the system is off
Modulating dampers
Produces changes in the volume of air proportional to a control signal
Two types of modulating dampers
Mixing boxes, face and bypass dampers
Face and bypass dampers
Used to modulate the volume of air passing through a heating or cooling coil
2 position dampers
Either fully open or fully closed
Relief dampers
Similar to a check valve, allow the air to flow in One Direction only.
Single blade dampers
Restricted to small sizes because of the difficulty to secure proper operation with high velocity air
Parallel blade damper
Best suited for 2 position damper operation, where no throttling of airflow is needed
Opposed blade damper
Used for modulating air flow.
Fire dampers
Installed to close in the same direction as the moving airstreams, so the air pressure will close them, but not open them.
National Building Code requires Supply and return ducts of air conditioning systems to be equipped with _______ at the point where they pass from one firezone, to another
Fire dampers
A smoke damper is a device to resist
The passage of smoke. Operates automatically and controlled by a smoke detector.
Stationary air louvres are used in fresh air intake openings to
Minimize the amount of water and snow entering an air handling system
Terminal devices four main functions
Distribute are to spaces
provide uniforms space temperatures
control effects of conduction, radiation, and convection
Provide adequate air motion
Terminal distribution devices use
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Diffusion, induction, entertainment and aspiration
Diffuser
Device used to reduce the velocity of air and introducing air to an occupied space. Create turbulence for conditioned air, reduces its velocity, causes rapid mixing with room air
One of the most effective locations for sound absorbing lining is the
Suction and discharge plenum of the fan
Where available duct is insufficient to provide the required attenuation, prefabricated in duct
Sound absorbers or duct Splitters can be used.
Resistance to airflow will increase operating cost
Three main media used to supply heat to a heating coil
Steam, hot water, glycol
Steam or hot water systems do not require the installation of
An additional heating circuit, as with glycol systems
Steam and hot water posed no
Environmental concerns, as does glycol.
Using glycol eliminates concerns surrounding
Coil freeze up
To reduce the risk of freeze up in Steam and hot water heating coils, important to consider;
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Size of heating coil
Configuration of heating coils
The way the coil operates
Steam coils are sized according to tube length.
Short length coils are _____
Long tube coils are _____
1.1m or less
Greater than 1.1m
Methods to minimize the hazard of freeze up while controlling the output of steam heating coils
Use a preheat coil, use a preheat coil with a reheat coil
Preheat coils
Provides Heat to the outdoor air to prevent freezing Downstream equipment. Used when there is 35% to 100% fresh air makeup. Used to increase air temperature to no more than 18°C.
When an increase in temperature is greater than 18°C, preheat coils should be used with
A reheat coil to minimize chances of freeze up.
GE control system must delay the opening of outdoor air dampers for at least
10 minutes at the start-up of the air handling unit and after steam is turned on
It is good design practice to select steam coils so that at least ____ of the coil capacity is used when the air temperature entering the coil is at ____
40%
-1°C
Common practice to select a control valve for a pressure drop corresponding to
50% of its initial steam pressure
A vacuum develops in a coil when
The steam flow control valve fills the coil with steam, and then shuts off. As it gives off heat, initial volume of steam in the coil condenses to 1/1600 of its original volume
Vacuum breaker
Prevents possible freeze ups caused by a vacuum in the coil. Installed after the steam flow control valve and near as possible to the inlet connection to the coil
A steam trap will not function satisfactorily without a
Vacuum breaker
It is preferable to use a _______ arrangement for hot water coils use to heat sub-freezing air
Parallel flow
Glycol coils
Use of ethylene glycol solution will result in the following
Lower heat transfer rate, increased friction losses, larger pumps due to higher power consumption
A glycol velocity of _____ is recommended to ensure a turbulent flow and reasonable heat transfer efficiency
1.2 to 1.8m/s
Whenever possible, steam coils should be installed with
Tubes in a vertical position
Support the piping ______ of the
Independently
Available pressure
Pressure over and above the pressure in the condensate return main
