1、Unit35Mechanical Engineering Design ProcessTextThe decisions made by the designers of the buildings heating,ventilating,and air-conditioning(HVAC)system are crucial in determining thermal comfort,the quality of the indoor air,and the efficiency of energy use by the building.Air exchange rates affect
2、 the amount of energy used to heat or cool fresh air,and the energy lost when used air is exhausted.The American Society of Heating,Refrigerating,and Air-Conditioning Engineers(ASHRAE)requirements for ventilation include minimum rates for replacing previously circulated air in the building with fres
3、h air.Energy costs can be reduced or eliminated by improving building insulation,lighting design,and the efficiency of HVAC and other building equipment.Buildings that allow natural ventilation,and those that employ such techniques as heat reclamation,thermal-storage systems,and flexible air handlin
4、g and chiller units lower energy use and reduce costs.The architect and engineers usually make the decisions on what systems to employ,but the responsibility for finding appropriate solutions depends on creativity and integrated efforts of the entire design team,in which the interior designer should
5、 play a significant role.TextThe mechanical systems of the building have their own model codes,which are geared toward professional mechanical engineers and installers.They are based on three model codes:the Building Officials Code Administrators International(BOCA)National Mechanical Code(NMC),the
6、Standard National Mechanical Code(SMC),and the Uniform National Mechanical Code(UMC).These model codes are revised every three years.As an interior designer,you will rarely need to refer to the mechanical codes,but you should be familiar with some of their general requirements and terms,especially t
7、hose affecting energy conservation requirements.In buildings where there is a minimum of mechanical work,the mechanical engineer or contractor will work directly off the interior designers drawings.For example,the interior design drawings may be the source for information in a renovation project whe
8、re a few supply diffusers or return grilles are being added to an existing system.In any event,you may need to coordinate your preliminary design with the mechanical engineer or contractor to make sure you leave enough room for clearances around HVAC equipment.TextThe mechanical engineer,like the in
9、terior designer,is trying to achieve an environment where people are comfortable,and to meet the requirements of applicable codes.By calculating how much heating or cooling is needed to achieve comfort,the engineer is able to develop design strategies that affect both the architecture and the mechan
10、ical systems of the building,such as the optimal size of windows,or the relative amounts of insulation or thermal mass.The engineer will figure out how big the UVAC system components should be to provide enough heating and/or cooling for the most extreme conditions the building is likely to experien
11、ce.The engineer will calculate the amount of energy used for normal conditions in a typical season and adjust the design to reduce long-term energy use.The number of people using the building both seasonally and hourly is also taken into account.The amount of heat gained or lost from the outside env
12、ironment will be considered.The materials,areas,and rates of heat flow through the buildings envelope affect this calculation.The amount of fresh air introduced into the system also influences these calculations,so the engineer will look at the volumes of the spaces in the building and the rates of
13、fresh air exchange.The engineer will suggest window locations and other design elements that minimize the heat gain within the building.TextPHASES OF THE DESIGN PROCESSThe phases of the engineering design process are similar to those of architects and interior designers:preliminary design,design dev
14、elopment,design finalization and specification,and the construction phase.During the preliminary design phase,the engineer considers the most general combinations of comfort requirements and climate characteristics.The schedule of activities that will take place in the space is listed,along with the
15、 conditions required for comfort during performance.The engineer analyzes the sites energy resources and lists strategies to design with the climate.Building form alternatives are considered and discussed with the architect.Available systems are reviewed,including both passive(nonmechanical)and acti
16、ve alternatives.Then the engineer figures out the size of one or more alternative systems using general design guidelines.In smaller buildings,the architect may do the system design.For larger,more complex buildings,the mechanical engineer will work as a team with architects,landscape architects,and
17、 the interior designer.The team approach helps to assess the value of a variety of design alternatives arising from different perspectives.When mutual goals are agreed upon early in the design process,this team approach can lead to creative innovations.TextThe more that the siting,layout,and orienta
18、tion of the building reduces heat loss,the less energy the healing and cooling equipment consumes.Opportunities may arise for the design of the HVAC system to be expressed in the form of the building.Creative teamwork can lead to new designs that offer better environments with less energy use,and th
19、at can be applied to many other buildings later on.During the design development phase,one alternative is usually chosen as presenting the best combination of aesthetic,social,and technical solutions for the buildings program.The engineer is given the latest set of drawings and programming informati
20、on for the building.The architect and engineer then establish the design conditions by listing the range of acceptable air and surface temperatures,air motions,relative humidities,lighting levels,and background noise levels for each activity to take place in the building.A schedule of operations for
21、 each activity is also developed.By considering these activities and their schedule,the amount of heat that will be generated by the activities,and the buildings orientation,the engineer then determines the HVAC zones for the building.Each of these zones has its own set of functional,scheduling,and
22、orientation concerns that determine when and how much heating,cooling,or ventilation is needed.TextFor each zone,the engineer establishes the thermal load(the amount of heat gained or lost)for the worst winter and summer conditions,and for average conditions during the majority of the buildings oper
23、ating hours.An estimate of the buildings annual energy consumption may also be made at this time.With all this detailed information in place,the engineer next selects the HVAC systems.More than one system may be used to meet different conditions in a large building.For example,one system may serve z
24、ones that are completely within the interior of the building,with a separate system for perimeter zones.Next,the engineer identifies the components of the HVAC system,and locates them within the building.Mechanical rooms,distribution trees(vertical chases and horizontal runs of ductwork),and compone
25、nts like fan-coil units(FCUs)under windows and air grilles within specific spaces all have to be selected and located.Sizes for these components are also specified.Once the engineer lays out the system,it is time to coordinate conflicts with other building systems,such as the structure,plumbing,fire
26、 safety,and circulation.TextBy drawing sections through the building,architects and engineers can identify clearance problems and see opportunities to coordinate the HVAC system with other building systems.The process of design finalization involves the de-signer of the HVAC system verifying the loa
27、d on each component and the components ability to meet this load.Then the final drawings and specifications are completed.During construction,the engineer may visit the site to assure that work is proceeding according to design,and to deal with unanticipated site conditions.TextTHERMAL COMFORT ZONES
28、The way zones for heating and cooling are set up by the mechanical engineer has implications for the architecture and interior design of the space.Zones may occupy horizontal areas of a single floor,or may be vertically connected between floors.The function of a space affects both its vertical and h
29、orizontal zoning(Fig.1).Some functions may tolerate higher temperatures than others.Some functions require daylight,which may add heat to the space,while others are better off away from the buildings perimeter.In some areas,such as laboratories,air quality and isolation is a major concern.The input
30、of the interior designer can be an important component in making sure that the clients needs are met.Fig.35.1Thermal zones based on activity patterns.TextZones also take into consideration the schedule of use of the space.Spaces that gain heat from daylight or electric lighting during the day only m
31、ay be able to flush that heat to another space for use at night.An isolated activity with a different schedule from the rest of the building may need a separate mechanical system.The buildings orientation will also affect the HVAC zones.Exposure to daylight direct sun,and wind all create specific he
32、ating and cooling requirements.Perimeter spaces have different needs from interior spaces.In multistory buildings,interior spaces on inter-mediate floorsthose spaces not at the buildings perimeter or on the top or ground floorsmay be able to use ventilation air as their only heating load source.Thes
33、e areas are so well shielded from the buildings exterior that they may not need additional heal,and can be served only by cooling.The amount of electrically generated heat,plus that produced by human activity and other heat-generating sources,usually outweigh the cooling effect of the amount of outd
34、oor air supplied by minimal ventilation,even in winter weather.In the summer,most of the interior cooling loads are generated inside the building.The perimeter areas of the building are much more weather sensitive.TextHEATING AND COOLING LOADSHeating and cooling loads are the amounts of energy requi
35、red to make up for heat loss and heat gain in the building(Fig.2).The rate of flow of hot or cold air coming into the building from ventilation and infiltration influences the amount of heating or cooling load.It is also dependent upon the difference in temperature and humidity between the inside an
36、d outside air.The amount of outside air coming in is expressed in liters per second(cubic feet per minute,or cfm).Fig 35.2Heat gains and losses in buildings.TextHeat Loss and Heating LoadsA heating load is created when a building loses heat through the building envelope.Cold outside air entering a b
37、uilding through ventilation,such as an open window,or as a result of infiltration,as when air leaks through cracks in the building envelope,also add to the heating load.Convection,radiation,or conduction of heat through the buildings exterior walls,windows,and roof assemblies and the floors of unhea
38、ted spaces are the main sources of heal loss in cold weather.Wind passing the building both draws warm air out and forces cold air in.Infiltration of cold air through cracks in the exterior construction,especially around doors and windows contributes significant heat loss.This heat loss places a hea
39、ting load on the buildings mechanical system,which must make up heat in spaces that lose it through cracks and poorly insulated areas.Energy auditors use equipment to locate air leaks and areas with inadequate insulation.They know what to look for in new and older buildings,and the cost of an energy
40、 audit is a good investment for a building owner.Some utilities will supply basic energy audits for free.Trained experts,sometimes called house doctors or home performance contractors,look at the building as a system and evaluate safety,comfort,energy efficiency,and indoor air quality.You can get a
41、listing of qualified,trained energy auditors from your stale energy office or cooperative extension service.TextThe most common sources of air leaks are where plumbing,wiring,or a chimney penetrates through an insulated floor or ceiling,or along the sill plate or band joist on top of the buildings f
42、oundation wall,fireplace dampers and attic access hatches are other likely suspects.Anywhere that walls and ceilings or floors meet or where openings pierce the buildings exterior is an opportunity for air to infiltrate.Air can leak where the tops of interior partition walls intersect with the attic
43、 space and through recessed lights and fans in insulated ceilings.Missing plaster allows air to pass through a wall,as do electrical outlets and switches on exterior walls.Window,door,and baseboard moldings can leak air,as can dropped ceilings above bathtubs and cabinets.Air can also leak at low wal
44、ls along the exterior in finished attics,especially at access doors,and at built-in cabinets and bureaus.Gaps under 4 mm(1/4 in.)wide can be scaled with caulk,which is available in a variety of types for different materials.Specify caulks with 20-year flexibility lifetimes and select either colored
45、or paintable caulk for visible use.Avoid using the cheapest caulks,as they dont hold up well.Larger cracks and holes that are protected from the sun and moisture can be filled with expanding one-part polyurethane foam sealant.Look for a safe-for-ozone label for foam sealants without chlorofluorocarb
46、ons(CFCs).TextFor even larger cracks and for backing in deep cracks,specify backer rod or crack filler,usually in the form of a round 4-to 25-mm(1/4-1-in.)diameter coil made of a flexible foam material.The crack is then scaled with caulk.Rigid foam insulation or fiberglass insulation wrapped in plas
47、tic can be used for very large openings like plumbing chases and attic hatch covers.Avoid using plastic in places with high temperatures,as it may melt.Metal flashing with high temperature silicone sealants may be permitted around chimney by some building codes.Heat Gains and Cooling LoadsBuildings
48、gain heat from occupants and their activities.Cooling loads are defined as the hourly rate of heat gain in an enclosed space,and are expressed in Btu per hour.Cooling loads are used as the basis for selecting an air-conditioning unit or a cooling system.Cooling loads represent the energy needed to o
49、ffset the heat gained through the building envelope in hot weather or from hot air entering by infiltration or ventilation.Peoples body heal,showering,cooking,lighting,and appliances and equipment use also create cooling loads.TextThe heat generated by lighting is often the greatest part of the tota
50、l cooling load in a building.All types of electric lighting convert electrical power into light plus heat.Eventually,the light is also converted to heat within the space(think about a lamp shining on a desk and the desk becoming warmer).All the electrical power that enters a lighting fixture ends up
