《专业英语》课件Unit_25.pptx
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1、Unit25Heating EngineeringTextFor over 10,000 years,man has used fire to warm himself.In the beginning,interior heating was just an open fire,but comfort and health was greatly improved by finding a cave with a hole at the top.Later,fires were contained in hearths or sunken beneath the floor.Eventual
2、ly,chimneys were added which made for better heating,comfort,health,and safety and also allowed individuals to have private rooms.Next,came stoves usually made of brick,earthenware,or tile.In the 1700s,Benjamin Franklin improved the stove,the first steam heating system was developed,and a furnace fo
3、r warm-air heating used a system of pipes and flues and heated the spaces by gravity flow.In the 1800s,high speed centrifugal fans and axial flow fans with small,alternating current electric motors became available and high-pressure steam heating systems were first used1.The 1900s brought the Scotch
4、 marine boiler and positive-pressure hydronic circulating pumps that forced hot water through the heating system.The heating terminals were hot water radiators,which were long,low,and narrow,as compared to steam radiators,and allowed for inconspicuous heating.Centrifugal fans were added to furnaces
5、in the 1900s to make forced-air heating systems.The heating plants(furnaces and boiler)discussed in this context are often called the primary systems.Systems intended to distribute heat produced by the primary systems are called secondary systems and include ducts and pipes,fans and pumps,terminal d
6、evices,and auxiliary components.The terms primary and secondary are equivalent to the terms plant and system used by some building analysts and HVAC system modelers.TextThe goal of this context is to have the reader understand the operation of various heat generation or transfer systems and their pe
7、rformance.Furnaces and BoilersFurnaces are used to heat air streams that are in turn used for heating the interior of buildings.A boiler is a device made from copper,steel,or cast iron to transfer heat from a combustion chamber(or electric resistance coil)to water in either the liquid phase,vapor ph
8、ase,or both.Boilers are pressure vessels used to transfer heat,produced by burning a fuel,to a fluid.Boilers are classified both by the fuel used and by the operating pressure.The most common heat transfer fluid used for this purpose in buildings is water,in the form of either liquid or vapor.Pressu
9、re classifications for boilers for buildings are:Low Pressure:Steam boilers with operating pressures below 15 psig(100 kPa).Hot water boilers with pressures below 150 psig(1000 kPa);temperatures are limited to 250F(120C).High Pressure:Steam boilers with operating pressures above 15 psig(100 kPa).Hot
10、 water boilers with pressures above 150 psig(1000 kPa);temperatures are above 250F(120C).TextThe key distinction between furnaces and boilers is that air is heated in the former and water is heated in the latter.The fuels used for producing heat in boilers and furnaces include natural gas(i.e.,metha
11、ne),propane,fuel oil,wood,coal,and other fuels including refuse-derived fuels.Since boilers and furnaces operate at elevated temperatures(and pressures for boilers),they are hazardous devices.As a result,a body of standards has been developed to assure the safe operation of this equipment.Heat pumps
12、A heat pump extracts heat from environmental or other medium temperature sources(such as the ground,groundwater,or building heat recovery systems),raises its temperature sufficiently to be of value in meeting space heating or other loads,and delivers it to the load2.Heat pumps are available in sizes
13、 ranging from small residential units(10 kW)to large central systems(up to 15 MW)for commercial buildings.Large systems produce heated water at temperatures up to 220F(105C).Central systems can use both environmental and internal building heat sources.In many practical circumstances the heat gains i
14、n the core zones of a commercial building could satisfy the perimeter heat losses in winter.TextA heat pump could be used to efficiently condition both types of zones simultaneously.Heat pumps require a compressor and two heat exchangers.In the energy bookkeeping that one does for heat pumps,the pow
15、er input to the compressor is added to the heat removed from the low temperature heat source to find the heat delivered to the space to be heated.Increased heating capacity at low air source temperatures can be achieved by oversizing the compressor.To avoid part load penalties in moderate weather,a
16、variable speed compressor drive can be used.Figure 25-1Air-to-air heat pump diagram.A reciprocating compressor is used.This design allows operation as a heat pump or an air conditioner by reversing the refrigerant flow.TextHeat pump efficiency is greater if lower high side temperatures can be used.I
17、n order to produce adequate space heat in such conditions,a larger coil may be needed in the air stream.However,if the coil is sized for the cooling load,it will nearly always have adequate capacity for heating.In such a case,adequate space heat can be provided at relatively low air temperatures,951
18、10F(3543C).Table 25-1 summarizes advantages and disadvantages of air and water source heat pumps.Table 25-1The advantages and disadvantages of air and water source heat pumps.TypeAdvantagesDisadvantagesAir sourceIndoor distribution permits air conditioning and humidity controlOutdoor air source read
19、ily availableSimple installationLeast expensiveEstablished commercial technologylDefrost requiredlLow capacity at cold outdoor temperaturelLower efficiency because of large evaporator T 30FlIndoor air distribution temperature must be high for comfort reasonslReliability at low temperature is only fa
20、ir,due to frosting effectslMust keep evaporator clear of leaves,dirt,etc.Water sourceMultiple family and commercial installation as central systemIn commercial installations,good coupling to cooling towersNo refrigerant reversal needed;reverse water flow insteadlNeeds water source at useful temperat
21、urelEfficiency penalty due to space heat exchanger TTextHeat exchangersA heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another.The media may be separated by a solid wall,so that they never mix,or they may be in direct contact.They are widely used in spac
22、e heating,refrigeration,air conditioning,power plants,chemical plants,petrochemical plants,petroleum refineries,natural gas processing,and sewage treatment.The types of heat exchangers are as follows:Shell and tube heat exchangerShell and tube heat exchangers consist of a series of tubes.One set of
23、these tubes contains the fluid that must be either heated or cooled.The second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required.A set of tubes is called the tube bundle and can be made up of several types of tubes:plain,long
24、itudinally finned,etc.Shell and tube heat exchangers are typically used for high-pressure applications(with pressures greater than 30 bar and temperatures greater than 260C).This is because the shell and tube heat exchangers are robust due to their shape.TextPlate heat exchangerAnother type of heat
25、exchanger is the plate heat exchanger.One is composed of multiple,thin,slightly separated plates that have very large surface areas and fluid flow passages for heat transfer3.This stacked-plate arrangement can be more effective,in a given space,than the shell and tube heat exchanger.Advances in gask
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