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
26、eted brazing technology have made the plate-type heat exchanger increasingly practical.In HVAC applications,large heat exchangers of this type are called plate-and-frame;when used in open loops,these heat exchangers are normally of the gasket type to allow periodic disassembly,cleaning,and inspectio
27、n.There are many types of permanently bonded plate heat exchangers,such as dip-brazed,vacuum-brazed,and welded plate varieties,and they are often specified for closed-loop applications such as refrigeration.Plate heat exchangers also differ in the types of plates that are used,and in the configurati
28、ons of those plates.Some plates may be stamped with chevron,dimpled,or other patterns,where others may have machined fins and/or grooves.TextPlate and shell heat exchangerA third type of heat exchanger is a plate and shell heat exchanger,which combines plate heat exchanger with shell and tube heat e
29、xchanger technologies4.The heart of the heat exchanger contains a fully welded circular plate pack made by pressing and cutting round plates and welding them together.Nozzles carry flow in and out of the platepack(the Plate side flowpath).The fully welded platepack is assembled into an outer shell t
30、hat creates a second flowpath(the Shell side).Plate and shell technology offers high heat transfer,high pressure,high operating temperature,compact size,low fouling and close approach temperature.In particular,it does completely without gaskets,which provides security against leakage at high pressur
31、es and temperatures.Adiabatic wheel heat exchangerA fourth type of heat exchanger uses an intermediate fluid or solid store to hold heat,which is then moved to the other side of the heat exchanger to be released.Two examples of this are adiabatic wheels,which consist of a large wheel with fine threa
32、ds rotating through the hot and cold fluids,and fluid heat exchangers.TextHeat DistributionAn HVAC centrifugal pump is a electrical power-driven machine that is used to overcome system resistance and produce required water flow.As the pump impeller is rotated,centrifugal force throws the water outwa
33、rd from the impeller.The centrifugal force and other design characteristics reduce the pressure(a partial vacuum is created)at the inlet of the impeller and allow more water to be forced in through the pump suction opening by atmospheric or external pressure.This makes the pumps discharge pressure h
34、igher than the pumps suction pressure.After the water enters the pumps suction opening,theres a further reduction of pressure between this opening and the inlet of the impeller.The lowest pressure in the system is at the pump inlet.The water leaves the impeller at a relatively high velocity.Then,in
35、the pump casing,the velocity(velocity pressure)is reduced and converted into static pressure.The size of the pump impeller and its rotational speed determines the static head pressure developed by the pump.A typical HVAC centrifugal pump will have a volute(spiral)casing with one or more closed backw
36、ard curved radial flow impellers.If the pump has one impeller,it is a single stage pump.If it has two or more impellers in series on a common shaft it is a multistage pump.The inlet to the pump may be on just one side.TextThis is a single inlet pump.If the inlet to the pump is on both sides of the p
37、ump it is a double suction inlet pump.The suction pipe may be the same size or larger than the discharge pipe.Most HVAC water pumps are constant volume machines and are coupled directly(direct drive)to a constant speed motor.Some direct drive pumps are driven by a variable frequency-variable speed m
38、otor.Varying the speed of the motor changes the speed of the pump.Varying the pump speed makes the pump variable volume.However,some HVAC pumps are belt driven.These pumps may be either constant speed and volume or variable speed and volume.Hydronics is the process of heating or cooling with water.S
39、ome of the classifications for hydronic piping systems are open,closed,series loop,one-pipe,two-pipe,three-pipe,four-pipe,direct-return,reverse return and combination.Hydronic systems are generally two-or four-pipe heating and cooling circuits.These piping circuits provide heated or chilled water to
40、 coils in central air handling units,fan-coil units,ductwork,and terminal boxes.Hydronic coils also are found in unit heaters,valence units,and in fin-tube radiation.Water flow is controlled through the use of various types of valves.Automatic and manual flow control valves and manual balancing valv
41、es are used to regulate flow rate.Service valves are used to isolate part or all of the system.TextCheck valves are used to limit the direction of flow.There are three basic types of manual valves:flow control or balancing valves(The types of flow control or balancing valves include:ball valves,butt
42、erfly valves,globe valves,combination valves,plug valves,and calibrated balancing valves.),service valves,and check valves.Specific types are described in the following sections.Automatic temperature control valves(ATCV)can be classified according to type of construction Two-way valves:single seated
43、 and double seated;and three-way valves:single-seated mixing valves and double-seated diverting valves5.Flow meters such as annular,orifice plate,venturi,and calibrated balancing valve are permanently installed devices used for flow measurements of pumps,primary heat exchange equipment,distribution
44、pipes and terminals.For flow meters to give accurate,reliable readings they should be installed far enough away from any source of flow disturbance to allow the turbulence to subside and the water flow to regain uniformity.The manufacturers of flow meters usually specify the lengths of straight pipe
45、 upstream and downstream of the meter needed to get good readings.Straight pipe lengths vary with the type and size of flow meter but typical specifications are between 5 to 25 pipe diameters upstream and 2 to 5 pipe diameters downstream of the flow meter.TextAfter the water system is constructed it
46、 is filled with water through the city supply main or other appropriate source.The pressure reducing valve(PRV)is adjusted and the system is tested.Water expands when heated and contracts when cooled.When the boiler is started(fired)the water is heated and begins to expand.If the expanding water has
47、 nowhere to go,the increased pressure in the system could break a pipe or damage other equipment.Water expansion tanks are used to keep this from happening.These tanks maintain the proper pressure on the system and accommodate the fluctuations in water expansion and contraction while controlling pre
48、ssure changes in the system.Expansion tanks are used in open systems.Compression tanks are use in closed systems.However,in most cases compression tanks are called expansion tanks.An expansion tank is simply an open tank used in an open water system to compensate for the normal expansion and contrac
49、tion of the water.As the water temperature increases the water volume in the system increases and the water in the expansion tank rises.Corrosion problems are associated with open expansion tanks as a result of the exposure to the air and evaporation and/or boiling of the water.Because of this,expan
50、sion tanks are limited to installations having operating water temperatures of 180F or less.hearthn.炉膛，炉缸，壁炉地面earthenwaren.陶器，瓦器combustionv.燃烧，氧化bookkeepingv.记账，簿记penaltyn.不利后果；劣势；苦难petrochemicaln.石油化学产品；石油化学的configurationv.构造；形状，外貌，轮廓；配置nozzlen.管嘴，喷嘴adiabaticv.绝热的；不传热的impeller n.（离心泵、压缩机等里的）叶轮，转子ca
