Transcript MECHANICAL AND ELECTRICAL EQUIPMENTS

ME 222
MECHANICAL AND ELECTRICAL
EQUIPMENTS
COURSE SYLLABUS
Practical application of mechanical and electrical system design,
operation and maintenance principles pertinent to commercial
buildings and emphasizing a designer’s perspective on mechanical
and electrical power equipment and distribution systems, energy
management, fire protection, communication, control and signal
systems, lighting, and security systems.
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INDOOR AIR QUALITY
HEAT FLOW
HVAC FOR SMALLER BUILDINGS
LARGER BUILDINGS HVAC SYSTEMS
ELECTRIC LIGHTING APPLICATION
WATER SUPPLY
LIQUID WASTE
SOLID WASTE
FIRE PROTECTION
ELECTRICAL SYSTEMS AND MATERIALS
PHOTOVOLTAIC SYSTEMS
FACILITY TRANSPORTATION
INDOOR AIR QUALITY
WHY IAQ (Indoor Air Quality)
- increasingly large percentage of people’s time is spent indoors.
-oil embargo (1973), raised world’s consciousness regarding finite
energy sources (energy conserving designs).
-proliferations of chemical in our environment has produced a vast
array of potential air pollutants synthetic products permanently
installed within the building, from equipment used indoors, and
from cleaning fluids used in maintenance.
With less fresh air and surrounded by more pollution sources,
increasing number of buildings have experience sick building
syndrome (SBS).
SICK BUILDING SYNDROME (SBS)
Situation wherein more than 20% of
the occupants complain of symptoms
associated with SBS- such as
headaches, upper respiratory irritations,
and irritations of the eye, among others.
ACCEPTABLE INDOOR AIR
QUALITY (ASHRAE 2004)
-Air
in which there are no known
contaminants at harmful concentrations
as determined by cognizant authorities
and with which a substantial majority
(80% or more) of the people exposed
do not express dissatisfaction
IAQ DEPENDS UPON
1.
2.
3.
4.
Limiting pollution at the source (choosing materials and
equipment carefully).
Isolating unavoidable sources of pollution.
Providing for an adequate supply and filtering of fresh
air (and recirculated air).
Maintaining a building and its equipment in a clean
condition.
POLLUTION SOURCES AND IMPACTS
Contaminants – gaseous, organic, particulates
 Effects – odors, irritants, toxic substances
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ODORS
- most immediate indicators of IAQ problem
- perceived most strongly on initial encounters
- our reactions are positive, negative, or neutral
IRRITANTS
- often imperceptible at first but cause increasing distress over time.
- volatile organic compounds (VOC), chemicals containing carbon
molecules that are volatile (i.e. methane, CFC’s, HCFC, formaldehyde)
TOXIC PARTICULATES SUBSTANCES
- asbestos
BIOLOGICAL CONTAMINANTS
-bacteria, fungi
RADON AND SOIL GASES
- radioactive gas that decays rapidly releasing radiation at each stage
- can cause lung cancer
-other soil gases include methane, pesticides that volatize and enter the
buildings with soil gases
PREDICTING INDOOR AIR QUALITY
Assuming that pollutant sources have been minimized, we need to
know how much indoor air and what extent of filtering will
produce acceptable IAQ.
Ventilation rate
- the most common remedy for SBS is to increase the rate of outdoor air
ventilation. Very small amount of outdoor air will provide sufficient oxygen,
and although human odor control is usually achievable at a rate of from 6 to 9
cfm (3 to 4.5 L/s) of outdoor air per occupant, outdoor air has more to do than
provide oxygen and control odors.
Comfort formula:
G
Q  10
ci  c o
Where:
Q = ventilation rate, L/s
G = total pollution sources, 0lf
Ci = perceived indoor air quality, decipol
Co = perceived outdoor air quality, decipol
Olf is a unit of pollution (1 olf = the bioeffluents produced
by the average person)
Decipol = a unit of perceived air quality
Ci is recommended to be set at 1.4 decipol, which
represents an expectation of 80% of occupants satisfied
with IAQ.
The concept of replacance affects the design of
ventilation systems. Based on studies, the rate of
1 air change per hour (ACH) of outdoor air, an
indoor space would have only 63% “new air”
after 1 hour; about 8 hours at this rate is required
for all the “old” air to be exhausted. The
difference between the ACH and replacance is the
fraction of air molecules at one specified times
that was not in the indoor space at an earlier
reference time.
ZONING FOR IAQ
After pollution control had been implemented at the source, remaining unavoidable
pollutant sources should be identified. More sensitive areas of the building
should be isolated from the key contaminants.
Differential air pressures are often maintained to discourage air flow from dirty to
clean zones – with higher pressure in clean areas, lower pressure in dirty areas.
Lower pressures can be created by exhaust air from such spaces, as well as by
limiting the volume of supply air.
Higher pressure areas can be created by installing make-up air equipment, as well
as increasing the volume of supply air from the HVAC system.
PASSIVE AND LOW-ENERGY APPROACHES
TO VENTILATION
“the solution to pollution is dilution”
a)
WINDOWS
b)
STACK EFFECT
c)
UNDERSLAB VENTILATION
d)
PREHEATING VENTILATION AIR
EQUIPMENT FOR CONTROL OF IAQ
EXHAUST FANS
- remove air that is odorous and/or excessively humid before it can spread
beyond bathrooms, kitchens, or process areas, creating a negatively pressured
area that further limits the spread of undesirable air. ANSI/ASHRAE requires
exhaust fan of at least 50 cfm (25 L/s) capacity for bathrooms and 100 cfm (50
L/s) for kitchen.
HEATING/COOLING OFMAKEUP AIR
- where climates are mild and/or energy is inexpensive, special equipment
other than heat exchangers can be used to heat and/or cool a particularly large
quantity of makeup air. These simple devices often supplement the building’s
main heating/cooling system, which deals primarily with heat gains/losses
through the building skin. Even in hot, humid climates, indirect evaporative
cooling can help lower the temperature of makeup air.
HEAT EXCHANGER
- as the tightness of construction increases and fewer air changes per hour
(ACH) occur from infiltration (unintended air leaks), forced ventilation
becomes more attractive as a means of reducing indoor air pollution. When
heat exchanger is used, it is possible to maintain an adequate supply of fresh air
without severe energy consumption consequences. Some commercially
available heat exchangers are capable of extracting 70% or more of the heat
from exhaust air. The lower the volume of air flow the higher the efficiency.
DESICCANT COOLING
- another rotating wheel process, they are attractive because they use no
refrigerants, and they lower humidity without having to overcooling the air.
The desiccants (such as silica gel, activated alumina, or synthetic polymers) in
an active system must be heated to drive out the moisture they remove from the
incoming air
TASK DEHUMIDIFICATION AND HUMIDIFICATION
- for spaces that need only dehumidification rather than mechanical cooling,
refrigerant dehumidifiers are commercially available. Their advantage over
desiccant dehumidifiers is that the air temperature remains essentially
unchanged during dehumidification (desiccant dehumidifiers raise the
temperature of the dried air). Accumulated water must periodically be removed
from these units and, if untended, could become a source of disease. Task
humidifiers are available and often used to relieve symptoms of respiratory
illnesses.
FILTERS
Particulate filters
Panel filters – furnished with HVAC equipment and functions mainly to
protect the fans from large particles of lint or dust. They
are crude and are not really considered to be air-cleaning
equipment.
Media filters – are much finer, using highly efficient pleated filter paper
within a frame. The larger particles are strained out by the
closely spaced filter fibers, while some of the smaller
particles that would other wise pass through are pushed
into the fibers due to air turbulence.
Adsorption filters – are for gaseous contaminant removal and vary
according to the pollutant in question. Activatedcharcoal filters are the most common of these type,
absorbing materials with high molecular weights but
allowing those of lower weights to pass.
Air washers – sometimes used to control humidity and bacterial growth.
The moisture involved can pose a threat if these devices are
not well maintained.
Electronic air cleaners – advantage of demanding less maintenance, but
can pose a different threat due to ozone production. Static
electricity is produced in the self-charging mechanical
filters by air rushing through it; larger particles thus cling to
the filter. The more humid and/or higher the air velocity,
the lower the filtering efficiency.
Locating air-cleaning equipment
- before the advent of IAQ, buildings were designed with rather crude
panel filters located only at the HVAC equipment; they were primarily
intended to intercept materials that might adversely affect combustion or
heat exchange. In building requiring high IAQ, a combination of highefficiency particle filters and adsorption filters are required. Panel filters
are usually located upstream from the fan unit. High efficiency particle
and adsorption filtering systems should be located downstream from the
HVAC cooling coils and drain pans to ensure that any microbiological
contaminants from those wet surfaces are removed rather than being
distributed throughout the building.
Ultraviolet Radiation (UV)
- has been used to kill harmful microorganisms, but under tightly
controlled conditions. Now there are UV lamp units that work within
HVAC systems, promising to control fungi, prevent the development and
spread of bacteria, and reduce the spread of viruses. An additional
benefit, cooling coils and drain pans stay cleaner.
Individual space air cleaning
- energy conservation have reduced the air circulation rate in many central air
handling systems moving less air reduces the energy used by fans. One result
can be low distribution efficiency, which causes poorly mixed air within the
occupied spaces. With local (individual space) air filtering equipment, both a
high circulation rate and proper air mixing are achievable. Each unit has its
own fan that can operate either with or without the central HVAC fan.
Controls for IAQ
- a large number of air quality monitoring devices are available, some of which
can control the operation of IAQ related equipment.
HEAT FLOW
Heat flow, also known as heat transfer, heat exchange, or simply heat, is the
transfer of thermal energy from one region of matter or a physical system
to another. When an object is at a different temperature from its
surroundings, heat transfer occurs so that the body and the surroundings
reach the same temperature at thermal equilibrium. Such spontaneous
heat transfer always occurs from a region of high temperature to another
region of lower temperature, as required by the second law of
thermodynamics.
In engineering, energy transfer by heat between objects is classified as
occurring by heat conduction, also called diffusion, of two objects in
contact; fluid convection, which is the mixing of hot and
cold fluid regions; or thermal radiation, the transmission
of electromagnetic radiationdescribed by black body theory. Engineers
also consider the transfer of mass of differing chemical species, either cold
or hot, to achieve heat transfer.
'Skin' is treated in a broad way. It is regarded as
the non-structural covering to a framed
structure. Structure supported mainly by a
skeleton, or frame, of wood, steel, or reinforced
concrete rather than by load-bearing walls.
Called by their familiar name, the basic
components of a building envelope (skin)
includes windows, doors, floors, walls and
roofs.
The flow of heat through a building envelope
varies both by season and by the path of the
heat (through the materials of a building’s skin
or by way of outdoor air entering the interior
through intentional and unintentional
pathways.)
Heat flows are to two forms:
Sensible heat – results in change in
temperature
Latent heat – results in change in moisture
content
- Total heat flow is the summation of sensible
heat and latent heat.
- Materials react differently to sensible and latent
heat flows.
Heat flow processess
- buildings like bodies, experience sensible heat
loss to, and gain from, the environment in three
principal ways:
Convection – heat is exchanged between a fluid
(typically air) and a solid, with motion of the fluid due
to heating or cooling playing a critical role in the extent
of heat transfer.
Conduction – heat is transferred directly from molecule
to molecule, within or between materials, with
proximity of molecules (material density) playing a
critical role in the extent of heat transfer.
Emittance – radiation heat transfer is highly influenced
by surface characteristics. Shiny materials are much
less able to radiate than common rough building
materials.
Thermal classification of materials
- Materials generally interact with heat either as
insulators that retard heat or conductors that
encourages heat flow. It is common that a
combination of insulators and conductors can
be found in construction.
- INSULATIONS
1. Inorganic fibrous or cellular products (such as
glass, rock wool, slag wool, perlite, or vemiculite.)
2. Organic fibrous or cellular products (such as
cotton, synthetic fibers, cork, foamed rubber, or
polystyrene)
3. Metallic or metalized organic reflective
membranes (which must face an air space to
be effective)
Insulating materials are available in:
Loose fill
Insulating cement
Formed-in-place
Flexible, semirigid insulation
Rigid insulation
Reflective material –available in sheets and rolls
either single or multiple layers.
CONDUCTORS
- Materials used as conductors are typically
dense, durable, and diffuse heat readily.
AIR FILMS and AIR SPACES
- Actually void of materials, they have potentially
useful thermal properties and contribute
substantially to the insulating capabilities of
some construction materials.
Composite thermal performance:
1
U 
R
U = overall coefficient of thermal resistance
(BTU/h-ft2; W/m2-K)
R = resistance
HVAC FOR SMALLER BUILDINGS
Heating, Ventilating, and Air conditioning (HVAC):Typical design
processes
PRELIMINAR DESIGN
Most general combination of comfort needs and climate
characteristics are considered:
- Activity comfort needs are listed
- An activity schedule is developed
- Site energy resources are analyzed
- Climate design strategies are listed
- Building form alternatives are considered
- combinations of passive and active systems are
considered
- One or several alternatives are sized by general design
guidelines
DESIGN DEVELOPMENT PHASE
1.
Establishes design conditions,
A. By activity, lists the range of acceptable air and surface
temperatures, air motions, relative humidities, lighting
levels, and background noise levels.
B. Establishes the schedule of operation.
II.
Determines the HVAC zones, considering:
A. Activities.
B. Schedule.
C. Orientation.
D. Internal heat gains
III.
Estimates the thermal loads on each zone:
A. For worst winter conditions.
B. For worst summer conditions.
C. For the average condition or conditions that represent the great majority
of the building’s operating hours.
D. Frequently, an estimate of annual energy consumption is made.
IV.
V.
VI.
VII.
Selects the HVAC systems. Often, several systems will be
used within one large building because of orientation, activity,
or scheduling differences may dictate different mechanical
solutions. Especially common is one system for the all-interior
zones of large buildings and another system for the perimeter
zones.
Identifies the HVAC components and their locations.
A. Mechanical rooms.
B. Distribution trees – vertical chases, horizontal runs.
C. Typical in-space components, such as under-window fancoil units, air grilles, and so on.
Sizes the components.
Lays out the system. At this stage, conflicts with other
systems (structure, plumbing, fire safety, circulations, etc.) are
most likely to become evident.
Design Finalizing Phase
- At this stage, the HVAC system designer verifies the match
between the loads on each component and the component’s
capacity to meet the load. Final layout drawings then are
completed.
Equipment location and service distribution
- smaller buildings are typically skin-load dominated: that is, for
them, the climate dictates whether heating or cooling is the
major concern. In some climates, only heating systems are
needed; the building can “keep itself cool” during hot weather
without mechanical assistance. In other climates, only cooling
is needed. In still others, both heating and cooling is required.
a)
b)
Central or Local
- a skin dominated building may have such differing but
simultaneous needs that a room-by-room solution (“local”) for
heating, ventilating, and conditioning is desirable. The
advantage of local systems is their ability to respond quickly
to individual rooms’ needs.
- the central system also has advantages: the equipment
is contained within its own space rather than taking up space
within each room, and maintenance can be carried out
without disrupting activities within those rooms.
Central heating or Cooling equipment
- in the early stages of design, determining an approximate
size for the largest equipment is sometimes useful. Once the
heating or cooling capacities are known, manufacturers’
catalogs can be consulted for the dimensions of the heating
and cooling equipment.
Design temperature – critical decision in sizing the heating
equipment. What is the lowest reasonable outdoor
temperature for which a heating device can be sized if the
desired interior temperature is to be maintained?
Design t = inside temperature – outside temperature
Required capacity of a building’s heating equipment:
Btu/h heating capacity = ((Btu/DD)ft2/24 h)(t)(ft2 floor area)
Cooling size (mechanical refrigeration)
Sensible Btu/h cooling capacity = [approx. heat gain(Btu/h
ft2)][floor area(ft2)]
Required capacity in tons:
Tons of cooling = heat gain, Btu/h/12,000
1 ton = 12,000 Btu/h (3516 W)
c)
Distribution trees
- Central heating and cooling systems produce
heating and cooling in one place, then distribute
them to other building spaces according to their
respective needs. The distribution tree is the means
for delivering heating and cooling: the “roots” are the
machines that provide heat and cold, the “trunk” is
the main duct or pipe from the mechanical
equipment to the zone to be served, and the
“branches” are the many smaller ducts or pipes that
lead to the individual spaces. The “leaves” are the
point of interchange between the piped or ducted
heating or cooling and the spaces served.
Controls for Smaller Building Systems
- Older buildings had mechanical equipment for heating or
cooling only, thermostats were simple on-off devices; when the
temperature rise or fall below a set point, the heat or cooling
load was turned on.
- Building management system (BMS) are now capable of
regulating far more than temperature. They are capable of
remote control, allowing systems to be activated in advance of
the occupants’ arrival.
- Future developments include neural networks, where
automation systems are capable of learning while being used.
They thus predict usage pattern, adjusting in advance without
needing specific commands from occupants.
REFRIGERATION CYCLE
Compressive refrigeration:
- A scheme for transferring heat from one circulated
water system (chilled water) to another (condenser
water). This is done by the liquefaction and
evaporation of a refrigerant, during which processes it
gives off and takes on heat, respectively. The heat it
gives off must be disposed of (except in the heat
pump), but the heat it acquires is drawn out of the
circulated water known as the chilled water, which is
the medium for subsequent cooling processes.
Absorption Refrigeration cycle
- No CFC’s or HCFCs are used here; the process uses distilled
water as the refrigerant and lithium bromide (salt solution). In
order to remove heat from chilled water, this cycle uses still
more heat in regenerating the salt solution. Typically, it is less
efficient than the simple compressive cycle and needs about
twice the capacity for rejecting heat. Because the high-grade
energy (electricity) needed to run a compressor is replaced by
the lower-grade heat needed to run the generator, the
absorption cycle can enjoy an energy advantage over the
compressive cycle, even though it is less efficient.
COOLING-ONLY SYSTEMS
a)
b)
c)
d)
e)
Fans
Unit air conditioners
Evaporative cooling: Misting
Evaporative cooling: roof spray
Evaporative coolers
- affectionately termed swamp coolers or desert coolers
and are familiar devices in hot, arid climates. They are used in
other climates for special high-heat applications such as
restaurant kitchens. They require a small amount of electricity
to run a fan and some water to increase the RH of the air they
supply to the building,
In a typical indirect evaporative air cooler, the essential element is a heat exchanger in
which dry air contacts heat-exchange surfaces whose other sides are cooled
evaporatively.
HEATING/COOLING SYSTEMS
A)
Cooling coils added to warm air furnaces
This common system utilizes a rather simple arrangement of the refrigeration cycle. The
condenser heat is carried away by water and the evaporation process draws heat out of water
in another circuit to produce chilled water. The heat is moved to a heat rejection location
outdoors. Another type of cooling is air to air refrigeration device. Air instead of water can be
used to cool the condenser, and indoor air can be cooled directly by being passed over the
evaporator coil in which the refrigerant is expanding from a liquid to a gas. Heat is removed
from the indoor air to the outdoor air by the step-up action or heat-pumping nature of the
refrigeration cycle. When indoor air is cooled by this manner, by expanding the refrigerant , the
process is usually known as direct expansion. The cooling coils are referred to as DX coils.
B)
Hydronic and coils
This system combines a perimeter hot water heating pipe with an overhead air-handling
systems. A boiler with a tankless coil supplies domestic hot water. The boiler’s heat output
supplies both the perimeter loop and a coil in the air-handling unit of the duct system. The total
heating load is met by the combination of radiant heat generated by the perimeter loop and
heated air from the overhead air-handling system.
Air-Air heat pumps – these use the refrigeration cycle in both
heating and cooling, thus eliminating the use of boilers and
cooling coils. Heat pumps can transfer heat air-air, air-water,
water-water.
Single or Split-type system
In a single package (also called unitary) system only one piece of
equipment is involved. A single-package air-air heat pump
moves heat between an outdoor air stream and an indoor air
stream; although kept separate, both streams pass through a
single outdoor unit. A system with both outside and inside
components is called a split system. A split-system air-air heat
pump moves heat via a refrigerant loop between the outdoor
unit (which also contains the compressor), through which
outdoor air passes, and the indoor unit (which usually contains
backup heating coils) for the treatment and circulation of
indoor air.
Split systems are popular because the noise of the compressor
and the outdoor air fan are removed from the interior, and the
size of the indoor unit can be quite small. This indoor element
is often mounted either high on the wall or on the ceiling. Such
an indoor unit is available with automatically changing louvers;
when it is in the cooling mode, it delivers cool air along the
ceiling, from where it sinks to the level of occupancy; cold air
blowing directly on people is avoided. In the heating mode, the
louvers shift to direct hot air steeply downward. The greater the
distance between the indoor and outdoor units, the greater the
strain on the refrigerant loop.
Heat pumps have a high initial cost, and they have in the past
shown a relatively high frequency of compressor failure. Noise
from the compressor and the outdoor air fan may affect site
planning, especially for residences.
One of the primary attractions of the heat pump is that in its
heating mode it can deliver more energy than it consumes
(electrically). Although energy (usually electricity) is required to
run the cycle, the pump draws “free” heat from a source such
as outdoor air. The total heat delivered to the building is more
than the heat (electricity) required to run the cycle. The
measure of this heat advantage is called the coefficient of
performance (COP), defined as:
Ground source heat pump
Ground-air heat pumps, also called geothermal heat pumps or geoexchange
systems. They often provide domestic hot water in addition to heating and
cooling. An environmentally safe refrigerant is circulated through a loop
installed underground (or in a pond or lake), taking heat from the soil in
winter and dischargingheat to the soil in summer. The loop is often highdensity polyethylene (HDPE). Below the surface, soil temperatures are more
stable year-round than outdoor air temperatures, thus raising the COP
relative to that of air-air heat pumps. This system is almost completely out of
sight, with no maintenance or weathering of exterior equipment. Noise is
confined to the compressor in a small indoor mechanical room.
Some common configurations are shown. In the closed systems (a–c) the flow
rate is typically 2 to 3 gpm/ton of refrigeration (0.3 to 0.5 mL/J), with lower
flows in the open loop systems.
Horizontal ground source closed loop heat pump
Trenches requires 1 – 2 m deep
-
400 to 650 ft (120 to 200 m) of pipe are installed per ton (12,000 Btu/h or 3.5 kW)
-
of heating and cooling capacity
a “slinky” coiled pipe is sometimes used. The trenches can be placed below
parking lots or lawns and gardens.
Vertical ground source closed loop heat pump
- particularly applicable where the site area is limited.
Vertical holes are bored from 150 to 450 ft (46 to 137 m) deep.
Each hole contains a single full-depth loop and is backfilled (or grouted)
after the loop is installed.
Since summer temperature is much lower at greater depths, less pipe
length is required than for horizontal loops.
The distance between bore holes varies from a minimum of 15 ft (4.6 m)
with high water table and low building cooling loads to as much as 25 ft (7.6
m) for buildings with high cooling loads. A minimum distance of 20 ft (6 m)
is usually recommended.
Pond or lake closed loop heat pump
sometimes used when a building is close to an adequately large body of
water. The loop is submerged, and the surrounding water conducts heat far
more rapidly than does soil. The resulting shorter length required, and the
low cost of placing the coils in water, can make this attractive.
the water level in the pond should never drop below a minimum of 8 ft (2.5
m) and must have sufficient surface area for heat exchange.
Groundwater-source open loop heat pump
(“pump and dump”) is suitable only where groundwater is plentiful, and may
be prohibited by local codes and environmental regulators.
One variant of this system (Fig. d) takes water from one well, through a heat
exchanger within the building, then discharges it to a second well.
Another variant (Fig. e) takes water from the bottom and discharges back
into the top of the same (standing) well, typically 6 in. (150 mm) in diameter
and as deep as 1500 ft (460 m).
Ground source heat pumps are often used in retrofits, especially in schools
where site areas are plentiful, or historic structures where small-size interior
mechanical equipment is highly desirable.
LARGE BUILDING HVAC SYSTEM
At the onset of the twenty-first century, large building HVAC is showing several
trends. One is the increasing willingness to let mechanical equipment share
its tasks with natural ventilation and daylighting. Building automation has
made this easier to manage. Another trend is toward an underfloor plenum
air supply (related to, but not identical to, displacement ventilation
approaches commonly used in critical-environment facilities) rather than
using ducts connected to diffusers and return grilles, both on the ceiling.
Concern about air quality indoors and the environment outdoors is
producing a variety of approaches to increased ventilation effectiveness.
Refrigerants containing CFCs and HCFCs are being avoided. Fuel cells and
photovoltaics are promising increased energy autonomy to larger buildings.
BASIC HVAC SYSTEMS: TASKS AND COMPONENTS
HVAC SYSTEMS TYPES
Four main systems classification:
 Direct refrigerant systems
 All-air systems
 Air and water systems
 All-water systems
(A) DIRECT REFRIGERANT SYSTEMS
These systems nearly eliminate the distribution
trees of air or water, relying instead on a
heating/cooling device adjacent to or within the
space to be served. Thus, they are prevalent in
skinload- dominated buildings with extensive
perimeter zones; these tend to be smaller
buildings.
(B) ALL-AIR SYSTEMS
Fig. 10.12 (a–f) Schematic diagrams of all-air HVAC systems.
An underfloor air supply is shown here to simplify the
diagram, but a ceiling supply is much more common.
The more common variations on all-air systems are shown in Fig.
10.12. Because air is the only heat transfer medium used
between the mechanical room (central station) and the zones it
serves, and because air holds much less heat per unit volume
than water, the distribution trees for this class are quite thick.
Sometimes, to reduce duct sizes, higher velocities are used for
supply air. This generates more noise and higher friction,
resulting in more energy used by fans; higher velocity should be
used only sparingly, where space limitations are extreme. For
comfort, however, these systems are, over all, the best. The
quantities of air moved through the central station(s) are
heated or cooled, humidity controlled, filtered, and freshened
with outdoor air—all under controlled conditions. Within the
zones, supply diffusers/registers and return grilles allow a wellplanned stream of conditioned air to thoroughly permeate all
work areas.
(C) AIR AND WATER SYSTEMS
Fig. 10.13 (a–c) Schematic diagrams of air and water HVAC systems.
An underfloor air supply is shown here to simplify the diagram,
but a ceiling supply is more common. In (b) the supplementary air is
often delivered directly to the fan-coil unit.
Most of the heating and cooling of each zone is accomplished via the water
distribution tree, which is much thinner than the tree needed by air. For air
quality—filtering, humidity, freshness—a small, centrally conditioned
airstream, equal to the total fresh air required, is provided. Thus, several
distribution trees are involved, yet the total space they require is almost
always less than that required by all-air systems. Exhaust air may be
gathered in a return air duct system, making heat recovery possible. Or (as
a cheaper alternative) air can be exhausted locally to avoid the construction
of yet another distribution tree. If the water distribution provides either
heating or cooling only, it is called a two-pipe system (shown throughout Fig.
10.13). If it provides simultaneous heating and cooling, it is a four-pipe
system. (Three-pipe systems are a lower-first-cost alternative allowing
simultaneous heating and cooling [from two supply pipes with a single
return pipe], but they waste energy by mixing hot return and cold return
water flows in one return pipe. They are no longer permitted in most
locales.)
(D) ALL-WATER SYSTEM
Fig. 10.14 Schematic diagram of all-water HVAC systems. Fourpipe distribution trees require smaller volumes than do those for
air systems; however, less thorough provision of outdoor air is a
potential concern.
The more simple-appearing all-water systems are shown in Fig. 10.14. These
systems only heat and cool; the distribution trees are indeed slim. Air quality
is dealt with elsewhere—either locally, by means of infiltration or windows; or
by a separate fresh air supply system; or simply by fresh air from an
adjacent system, such as a ventilated interior zone. This ambiguity about
fresh air leads to similar ambiguities about whether a system is air-andwater or all-water. A fan-coil terminal is often employed so that air motion
occurs along with heating or cooling. (Sometimes the fan-coil unit is located
against the exterior wall so that fresh air may be brought in and mixed with
the room air through the fan.) Both baseboard and valence (above-window)
units are also commonly available. Because air is handled so locally, there
is very little mixing of air from one zone to another, making this attractive
where potential air contamination (or smoke from a fire) is a special
concern. It is also an easy system to retrofit. However, maintenance is high;
filters in each fan-coil must be cleaned, and drain pans are potentially
problematic.
(e) Equipment Space Allocations
An important early design decision is whether to integrate or separate the
heating/cooling equipment and the air-handling equipment (see Fig. 10.2).
If they are integrated, one or a few central mechanical room(s) can serve
many floors, and each mechanical room will need area and height sufficient
for both heating/cooling and air-handling equipment. If separated, one (or
several, in tall buildings) large space for heating/cooling equipment is
typically located in the basement or the penthouse, with a smaller fan room
on each floor. Each mechanical room should have both a central location
relative to the area it serves and direct access to the outside—contradictory
requirements in many cases. Central locations within the area served
minimize the distribution tree size; access to the outdoors facilitates the use
of outdoor air as a heat source (winter) or sink (summer) and allows
equipment to be installed or removed in later remodeling. Mechanical
rooms serving both heating/cooling and air-handling equipment need
relatively high ceilings; 12-ft (3.7-m) clear is a typical minimum, 20-ft (6-m)
clear a typical maximum.
Fig. 10.15 Some basic components of HVAC central equipment. (a) A
simplified diagram of a cooling cycle, in which chilled water is
circulated to air-handling coils and heat is disposed of through a
cooling tower. (b) Schematic diagram of major components of
central equipment for both heating and cooling.
AIR DISTRIBUTION WITHIN SPACES
When large office buildings had far greater interior (core) areas than perimeter areas
and were filled with less-efficient lighting at high luminance levels, they were
considered to always need cooling. Because cool air supplied at the ceiling would
naturally fall toward the level of occupants, and because suspended ceilings were
ubiquitous, supply air from the ceiling was almost universal. The suspended ceiling
was also tempting for the return air provisions, whether as a plenum or for another
ducted system. With both supply and return air at the ceiling, the danger of shortcircuiting arose: supply air heading quickly for the return opening, with resulting
shortages of both cooling and IAQ. In this section, we first look at approximate duct
sizing and then consider three air distribution systems for multistory office buildings.
(a) Air Ducts
Duct sizes (in cross section) are frequently of interest early in the design process. Duct
depths can help determine floor heights; duct cross sections influence the sizes and
shapes of the vertical cores that serve multistory buildings. An approximation of duct
size can be obtained as follows:
1. Determine the quantity of air to be distributed through the largest duct, using
Table 10.5.
2. If necessary, convert cfh to cfm:
cfh 1 h/ 60 min = cfm
3. Find the maximum velocity of this air within the duct from Table 9.4, expressed in feet per minute
(fpm).
4. The approximate minimum required cross-sectional area of duct A is then:
A in.2 = [volume of air (cfm)/ velocity (fpm)] × 144 in.2/ft2 × friction allowance
where the friction allowances are
• round ducts = 1.0 (may be neglected)
• nearly square ducts (ratio of width to depth, 1:1)
small (<1000 cfm) = 1.10
large (>1000 cfm) = 1.05
• thin rectangular ducts (ratio of width to depth, 1:5) = 1.25
Then check this against the recommended duct cross-sectional area from Table 10.4.
Remember that the minimum duct area will carry a penalty of increased noise and friction.