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HVAC - Air Conditioning Systems I
- Unit 28 - Chapter 132 - B2 -
Terms in this set (64)
What are the seven main functions of an air conditioning system?
What are the three main categories of air conditioning systems?
What is the purpose of an air conditioning system?
-to maintain appropriate conditions for the intended function of the building
What is the main purpose of heating?
-to offset heat lost to the atmosphere
What is the main purpose of cooling?
-to offset heating from internal and external sources
What are the main purposes of ventilation?
-provide makeup air to maintain an acceptable air balance
What is the main purpose of humidification?
-to increase relative humidity to an acceptable level for comfort
-minimize the effect of static electricity
What is the main purpose of dehumidification?
-acceptable air for the operation of sensitive equipment
-(may also be used in humid areas like swimming pools during winter to prevent frost formation on exterior walls though this is rarely done)
What are the main purposes of air circulation?
-assist in dissipation of body heat
-prevent a buildup of odors
What is the purpose of filtration?
-remove contaminants from recirculated and outside air
What factors dictate the requirements of an air conditioning system?
-geographical location of the building
-type of occupancy
-building layout and construction
What is a unitary air conditioning system?
-each room or zone has it's own air conditioning unit
What is a central air conditioning system?
-AHU(s) in a central mechanical room
-conditioned air distributed via ducting to various parts of building
-mainly controlled from mechanical room, though may have supplementary thermostats in individual zones
What is a combined air conditioning system?
-superior zone temp control
-central AHU(s) in mechanical room for ventilation and cooling
-supplementary equipment in each zone to provide heating and additional cooling
What are the basic components of an AHU?
-source of heating and cooling
-motorized dampers for air flow control
What are the 4 types of intake arrangements for AHUs?
-fixed percentage of outside air
-100% outside air
What is the most commonly used intake arrangements in central and combined air conditioning systems?
What is the desired temp of supply air?
Why are the dampers in mixed air systems sized for maximum airflow?
-so they can operate on full outdoor air, full recirculated air or a mixture of outdoor and return air
What is the approximate percentage of outdoor air needed for a 12.5*C mixed air supply air temp during the winter?
-(100% as outdoor temp increases to 12.5*C)
What is free cooling?
-mixed air system, dampers controlled automatically
-when outdoor air is already at 12.5*C
-dampers open to 100% outdoor air
-results in increased efficiency because no mechanical cooling is needed
What is changeover?
-mixed air system, dampers auto
-outdoor air temp exceeds 24*C
-would require cooling to meet desired 24*C return air temp
-more efficient to run on recirculation
-dampers adjust to 10-15% outdoor air position
-occurs when outdoor air enthalpy exceeds return air enthalpy
What is a unitary air conditioning system?
-utilizes self-contained unit installed in or adjacent to the zone being conditioned
-usually packaged with all necessary components
-little or no ducting
-controlled by space thermostat or unit mounted one
-most common in residential and commercial buildings
What are the advantages and disadvantages of a unitary air conditioning system?
-low initial cost
-limited scope of environmental control
What are four common unitary air conditioning systems?
How does a window air conditioner provide cooling and dehumidification?
-DX refrigeration system
How many compartments us a window air conditioner divided into?
-the supply plenum containing; low efficiency air filter, evaporator coil, fan, and adjustable directional louvres
-the condenser compartment containing; the compressor, air cooled condensing coil, and the condenser fan
How is control of the window air conditioner achieved?
-thermostat, with sensing bulb located in the return airﬂow to the evaporator coil, maintains room temperature by stopping and starting the compressor
-manually controlled damper allows minimum ﬂow of outdoor air into room for ventilation if desired
How is water condensed from the discharge air in the window air conditioner without having accumulations?
-condensate from the evaporator coil drained back to a pan located below the condenser fan
-evaporated when thrown against the condensing coil by a "slinger ring" on the fan
What, generally, is the capacity range of window air conditioners?
- 0.3-1.8 tonnes(1/3-2 tons) of refridgeration
What are the advantages/disadvantages of using a window air conditioner?
-low initial cost
-well suited to local cooling in residential and commercial buildings
-wide swings in temp due to on-off control
-no direct control of dehumidification (occurs as byproduct of cooling coil operation)
What is the difference between window air conditioners and packaged air conditioning units?
Window Air Conditioners
-supply conditioned air directly to the space without duct-work
-normally installed in a window or exterior wall opening.
Packaged Air Conditioning Units;
-normally ﬂoor mounted in a vertical position
-may also be mounted horizontally on ﬂoors or suspended in false ceiling spaces
-single zone, but can handle ,uch larger areas
Where can the heat of refrigeration from a packaged air conditioner be dissipated?
-city water cooled condenser
-remote, outdoor air cooled condensing unit on the roof
What is the typical range of capacity of a packaged air conditioner?
-2.7-13.6 tonnes ( 3 to 15 ton)
What kind of packaged air conditioners have stepped capacity control?
-larger units with dual compressors
Where are packaged air conditioners well suited?
-restaurants and commercial or retail spaces as well as laboratories
-well suited for areas where supplemental cooling may be required due to heat released from equipment
What kind of variations are found on packaged air conditioners used to cool computer rooms?
-variation of the standard packaged air conditioning unit
-more complex design
-larger capacity fans
-high efficiency filter
-dual refrigeration circuits
-higher capacity humidifiers
-sophisticated electronic controls
-specifically designed for applications where room conditions must be maintained within a limited range at all times
What are the advantages/disadvantages of unit ventilators?
-room temp controlled either by a space or return air thermostat which operates a control valve in the water supply to the coil
-provides acceptable room temp control, particularly if modulating valve used for supply water coil
-requires minimal space in the room
-no humidification provided
-outdoor air intake may be subject to freezing problems
during winter in colder climates
-location of the air intakes must be carefully considered, particularly at ground level where vehicle exhaust fumes may be present
-condensate drain piping required for each cooling coil which may be diﬃcult to install.
What is the main purpose of the unit ventilator air conditioning system?
Where are they mounted?
-to provide individual room temp control in buildings with many zones and limited floor space available for installations
Describe the basic operating principle of the unit ventilator air conditioning system.
-room air and outdoor air are mixed, filtered and blown through water coils
-temp is controlled in room by a space or return air thermostat which operates a control valve in the water supply of the coil
What is a rooftop air conditioning unit?
-self contained AHUs for larger applications
-manufactured in sections on a modular basis
have weather proof casing enclosure for exposed installations on roofs
A rooftop unit may consist of what basic components?
-plenum section w/ motorized dampers
What is the general range of cooling capacity of a rooftop unit?
-2.7-68 tonnes (3-75 tons)
Where are rooftop units commonly used?
What are they not suited to?
-single zone commercial buildings, such as; department stores, restaurants, food stores, shopping centers, warehouses, light manufacturing plats, recreational facilities, gymnasiums (single story buildings with large floor areas)
-not well suited to multi-zone controls
What are the advantages/disadvantages of roof top air conditioning units?
-very flexible in design
-can provide only the basic heating and ventilation requirements, or provide full environmental control
-very competitive initial cost due to factory assembly
-do not require floor space or building enclosure for installation
-exposed location=more difficult/frequent maintenance
-depending on application, may need to be carefully designed to prevent noise transmission
-not well suited to multi-zone control
What are the main advantages/disadvantages of central air conditioning systems over unitary ones?
-easier to operate and maintain since major components in mechanical room
-can be inspected without disrupting occupied areas
-noise and vibration can be more easily and effectively controlled when not immediately adjacent to occupied areas
-large components with higher efficiencies and lower initial cost per unit output can be used
-space required for mechanical room
-cost and distribution of extended ductwork
-breakdown of one key component may result in shutdown of the entire system
What is the most common central air system and Where is it usually used?
-the forced air system
Describe a forced air central air conditioning system.
-furnace in basement provides conditioned at through ducts to various rooms in the home
-outdoor duct usually connected to return duct to provide a fixed amount of outside air
What are the drawbacks of the forced air central air conditioning system.
-supply registers at floor make people complain about drafts during cooling in summer
How are single, constant air systems different from root top air conditioning systems?
-rather then being mounted on the roof they are mounted in a mechanical room
Describe the operation of a terminal reheat-low velocity air conditioning system.
-similar to the single zone, constant air volume system
-zone reheat coils added to allow multi-zone temp
-located in a central mechanical room
-however, temp of supply air doesn't vary with space load requirements
-instead, constant supply air discharge temperature of approximately 12.5°C maintained by air mixing dampers
-supplemental heat during winters and cooling during summers
-conditioned air distributed by low velocity ductwork
-each zone is fed by a separate branch duct
-temp of zone controlled by thermostat in zone which controls
steam or hot water reheat coil installed in branch duct
Why is the terminal reheat-low velocity air conditioning system rarely used today?
Where is it still used?
-high energy consumption because it is designed to satisfy max annual cooling load in each zone and so, cannot take advantage of the diversity of loads
-specific applications such as labs and hospitals where a constant room air circulation rate and fine temp control are required
How is a terminal reheat-low velocity AC system different from a terminal reheat-high velocity system?
-variation of terminal reheat-low velocity system
-size of supply ductwork reduced by utilizing higher air velocities
- terminal reheat box required in the branch duct to each zone to reduce the air ﬂow to low velocity and to maintain a constant supply air volume in spite of air pressure fluctuations in the high velocity ducts
-boxes consist of a volume regulator, reheat coil and acoustic insulation
-each box rated for maximum capacity
-actual air ﬂow can be set by manually adjusting a volume controller on the exterior of the box.
What are the advantages/disadvantages of a terminal reheat-high velocity system?
-higher air velocities reduce ductwork costs and space requirements
-provides effective control of room conditions
-used in large buildings where the added cost of high velocity components and terminal boxes oﬀset by
savings from reduced building space requirements.
- well suited to retroﬁt projects where building that has limited available existing space for ductwork
- more powerful supply fans required to develop higher supply air pressures
-may be more difficult to maintain than the low velocity system, due to noise and vibration from high velocity
Describe the operation of a multi-zone central AC system.
-constant volume system
-discharge of AHU modified to provide multi-zone control
-typical arrangement; mixed air dampers, filters, humidifier and fan
-cooling and heating coils located downstream from
fan and are oﬀ set rather than in-line.
-supply air may flow into two separate discharge compartments, a "hot deck" or a "cold deck".
-two to twelve or more pairs of zone mixing dampers located at discharge of the unit
-each pair connected by separate duct to each zone
-space thermostat in the zone modulates the dampers and adjusts the mixture of hot and cold air by varying supply air temp
-dampers linked together but 90° out of phase so full opening of the cold deck results in full closing of the hot deck.
-ensures that constant volume of air is supplied to each zone and a constant total volume of air is handled by the supply fan
-cold deck supplies air at 12.5°C on a year round basis and utilizes free cooling whenever possible
-output of hot deck varies in accordance with heating requirements
-hot water coils used, water temp is normally reset on an outdoor air temperature schedule to save energy
-preheat coil may be included in colder climates for heating of mixed air to 12.5°C if required, and a supplementary humidifier can be installed in the hot deck for winter
How is a dual duct system different from the multi-zone system?
-both mix hot and cold air to vary supply air temp
-rather than mixing at the discharge, hot and cold ducted separately
-blended in a mixing box in each zone
-thermostat in each zone controls the temp of the air entering the zone by controlling dampers in the mixing box
True OR False?
In a multi-zone system, when the hot deck is fully opened, the colde deck is fully closed.
What is the unitary air conditioning system which uses a DX refrigeration system to provide cooling and dehumidification in a local area without ductwork?
-the window air conditioner
What is the supply temperature of the cold deck air in a dual duct system?
When air handling units use hot water heating coils, how can energy be saved?
-by adjusting the water temp setpoint according to the outside air temp
What is the most common system, installed in most residences, where the furnace is in the basement and provides conditioned air via ducts to each room?
-forced air system
Which of the following air handling systems is not appropriate for a single zone application?
a) A packaged air conditioner
b) A dual duct system
c) A unit ventilator
d) A rooftop unit
e)A single zone constant air volume system
True OR False?
A dual duct system provides excellent humidity control.
-false, because hot and cold air are ducted separately and mixed just before discharge to the conditioned space making it difficult to control humidification
"Free cooling", with regard to HVAC systems, refers to ____________________________________.
-manipulating mixed air dampers on cooler days to eliminate the need for mechanical cooling
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