1、CHAPTER 1CONTROL FUNDAMENTALS(2)HVAC SYSTEM CHARACTERISTICSGeneral An HVAC system is designed according to capacity requirements,an acceptable combination of first cost and operating costs,system reliability,and available equipment space.Figure 1 shows how an HVAC system may be distributed in a smal
2、l commercial building.The system control panel,boilers,motors,pumps,and chillers are often located on the lower level.The cooling tower is typically located onthe roof.Throughout the building are ductwork,fans,dampers,coils,air filters,heating units,and variable air volume(VAV)units and diffusers.La
3、rger buildings often have separate systems for groups of floors or areas of the building.The control system for a commercial building comprises many control loops and can be divided into central system and local-or zone-control loops.For maximum comfort and efficiency,all control loops should be tie
4、d together to share information and system commands using a building management system.(Refer to the Building Management System Fundamentals 77-1224).The basic control loops in a central air handling system can be classified as shown in Table 1.Depending on the system,other controls may be required
5、for optimum performance.Local or zone controls required depend on the type of terminal units used.HeatingGeneral Building heat loss occurs mainly through trans-mission,Infiltration/exfiltration,and ventilation(Fig.2).The heating capacity required for a building depends on the design temperature,the
6、quantity of outdoor air used,and the physical activity of the occupants.Prevailing winds affect the rate of heat loss and the degree of infiltration.The heating system must be sized to heat the building at the coldest outdoor temperature the building is likely to experience(outdoor design temperatur
7、e).Transmission is the process by which energy enters or leaves a space through exterior surfaces.The rate of energy transmission is calculated by subtracting the outdoor temperature from theindoor temperature and multiplying the result by the heat transfer coefficient of the surface materials.The r
8、ate of transmission varies with the thickness and construction of the exterior surfaces but is calculated the same way for all exterior surfaces:Energy Transmission per Unit Area and Unit Time=(Tin-Tout)x HTCWhere:Tin=indoor temperature Tout=outdoor temperature HTC=heat transfer coefficient Infiltra
9、tion is the process by which outdoor air enters a building through walls,cracks around doors and windows,and open doors due to the difference between indoor and outdoor air pressures.The pressure differential is the result of temperature difference and air intake or exhaust caused by fan operation.H
10、eat loss due to infiltration is a function of temperature difference and volume of air moved.Exfiltration is the process by which air leaves a building(e.g.,through walls and cracks around doors and windows)and carries heat with it.Infiltration and exfiltration can occur at the same time.Ventilation
11、 brings in fresh outdoor air that may require heating.As with heat loss from infiltration and exfiltration,heat loss from ventilation is a function of the temperature difference and the volume of air brought into the building or exhausted.Heating Equipment Selecting the proper heating equipment depe
12、nds on many factors,including cost and availability of fuels,building size and use,climate,and initial and operating cost trade-offs.Primary sources of heat include gas,oil,wood,coal,electrical(radiant heat and heat pump),and solar energy.Sometimes a combination of sources is most economical.Boilers
13、 are typically fueled by gas and may have the option of switching to oil during periods of high demand.Solar heat can be used as an alternate or supplementary source with any type of fuel.Figure 3 shows an air handling system with a hot water coil.A similar control scheme would apply to a steam coil
14、.If steam or hot water is chosen to distribute the heat energy,high-efficiency boilers may be used to reduce life-cycle cost.Water is generally used more often than steam to transmit heat energy from the boiler to the coils or terminal units,because water requires fewer safety measures due to lower
15、pressure and is typically more efficient,especially in mild climates.An air handling system provides heat by moving an air stream across a coil containing a heating medium,across an electric heating coil,or through a furnace.Unit heaters(Fig.4)are typically used in shops,storage areas,stairwells,and
16、 docks.Panel heaters(Fig.5)are typically used for heating floors and are usually installed in a slab or floor structure,but may be installed in a wall or ceiling.Unit ventilators(Fig.6)are used in classrooms and may include both a heating and a cooling coil.Convection heaters(Fig.7)are used for peri
17、meter heating and in entries and corridors.Infrared heaters(Fig.8)are typically used for spot heating in large areas(e.g.,aircraft hangers,stadiums).In mild climates,heat can be provided by a coil in the central air handling system or by a heat pump.Heat pumps have the advantage of switching between
18、 heating and cooling modes as required.Rooftop units provide packaged heating and cooling.Heating in a rooftop unit is usually by a gas-or oil-fired furnace or an electric heat coil.Steam and hot water coils are available as well.Perimeter heat is often required in colder climates,particularly under
19、 large windows.A heat pump uses standard refrigeration components and a reversing valve to provide both heating and cooling within the same unit.In the heating mode,the flow of refrigerant through the coils is reversed to deliver heat from a heat source to the conditioned space.When a heat pump is u
20、sed to exchange heat from the interior of a building to the perimeter,no additional heat source is needed.A heat-recovery system is often used in buildings where a significant quantity of outdoor air is used.Several types of heat-recoverysystems are available including heat pumps,runaround systems,r
21、otary heat exchangers,and heat pipes.Controls for the air handling system and zones are specifically designed for a building by the architect,engineer,or team who designs the building.The controls are usually installed at the job site.Terminal unit controls are typically factory installed.Boilers,he
22、at pumps,and rooftop units are usually sold with a factory-installed control package specifically designed for that unit.CoolingGeneral Both sensible and latent heat contribute to the cooling load of a building.Heat gain is sensible when heat is added to the conditioned space.Heat gain is latent whe
23、n moisture is added to the space(e.g.,by vapor emitted by occupants and other sources).To maintain a constant humidity ratio in the space,water vapor must be removed at a rate equal to its rate of addition into the space.Convection is the process by which heat movesbetween spaces with unequal space
24、temperatures.Heat may move through exterior walls and roof,or through floors,walls,or ceilings.Solar radiation heats surfaces which then transfer the heat to the surrounding air.Internal heat gain is generated by occupants,lighting,and equipment.Warm air entering a building by infiltration and throu
25、gh ventilation also contributes to heat gain.Building orientation,interior and exterior shading,the angle of the sun,and prevailing winds affect the amount of solar heat gain,which can be a major source of heat.Occupants,lighting,equipment,and outdoor air ventilation and infiltration requirements co
26、ntribute to internal heat gain.For example,an adult sitting at a desk produces about 117W.Incandescent lighting produces more heat than fluorescent lighting.Copiers,computers,and other office machines also contribute significantly to internal heat gain.Cooling Equipment An air handling system cools
27、by moving air across a coil containing a cooling medium(e.g.,chilled water or a refrigerant).Figures 10 and 11 show air handling systems that use a chilled water coil and a refrigeration evaporator(direct expansion)coil,respectively.Chilled water control is usually proportional,whereas control of an
28、 evaporator coil is two-position.In direct expansion systems having more than one coil,a thermostat controls a solenoid valve for each coil and the compressor is cycled by a refrigerant pressure control.This type of system is called a pump down system.Pump down may be used for systems having only on
29、e coil,but more often the compressor iscontrolled directly by the thermostat.Two basic types of cooling systems are available:chillers,typically used in larger systems,and direct expansion(DX)coils,typically used in smaller systems.In a chiller,the refrigeration system cools water which is then pump
30、ed to coils in the central air handling system or to the coils of fan coil units,a zone system,or other type of cooling system.In a DX system,the DX coil of the refrigeration system is located in the duct of the air handling system.Condenser cooling for chillers may be air or water(using a cooling t
31、ower),while DX systems are typically air cooled.Because water cooling is more efficient than air cooling,large chillers are always water cooled.Compressors for chilled water systems are usually centrifugal,reciprocating,or screw type.The capacities of centrifugal and screw-type compressors can be co
32、ntrolled by varying the volume of refrigerant or controlling the compre-ssor speed.DX system compressors are usually reciprocating and,in some systems,capacity can be controlled by unloading cylinders.Absorption refrigeration systems,which use heat energy directly to produce chilled water,are someti
33、mes used for large chilled water systems.Dehumidification Air that is too humid can cause problems such as condensation and physical discomfort.Dehumi-dification methods circulate moist air through cooling coils or sorption units.in many parts of the country,dehumidification is required only during
34、the cooling season.In those applications,the cooling system can be designed to provide dehumidification as well as cooling.For dehumidification,a cooling coil must have a capacity and surface temperature sufficient to cool the air below its dew point.Cooling the air condenses water,which is then col
35、lected and drained away.When humidity is critical and the cooling system is used for dehumidification,the dehumidified air may be re-heated to maintain the desired space temperature.When cooling coils cannot reduce moisture content sufficiently,sorption units are installed.A sorption unit uses eithe
36、r a rotating granular bed of silica gel,activated alumina or hygroscopic salts(Fig.11),or a spray of lithium chloride brine or glycol solution(Fig.12).In both types,thesorbent material absorbs moisture from the air and then the saturated sorbent material passes through a separate section of the unit
37、 that applies heat to remove moisture.The sorbent material gives up moisture to a stream of scavenger air,which is then exhausted.Scavenger air is often exhaust air or could be outdoor air.Humidification Low humidity can cause problems such as respiratory discomfort and static electricity.Humidifier
38、s can humidify a space either directly or through an air handling system.For satisfactoryenvironmental conditions,the relative humidity of the air should be 30 to 60 percent.In critical areas where explosive gases are present,50 percent minimum is recommended.Humidification isusually required only d
39、uring the heating season except in extremely dry climates.Humidifiers in air handling systems typically inject steam directly into the air stream(steam injection),spray atomized water into the air stream(atomizing),or evaporate heated waterfrom a pan in the duct into the air stream passing through t
40、he duct(pan humidification).Other types of humidifiers are a water spray and sprayed coil.In spray systems,the water can be heated for better vaporization or cooled for dehumidification.Ventilation Ventilation introduces outdoor air to replenish the oxygen supply and rid building spaces of odors and
41、 toxic gases.Ventilation can also be used to pressurize a building to reduce infiltra-tion.While ventilation is required in nearly all buildings,the design of a ventilation system must consider the cost of heating and cooling the ventilation air.Ventilation air must be kept at the minimum required l
42、evel except when used for free cooling(refer to ASHRAE Standard 62,Ventilation for Acceptable Indoor Air Quality).To ensure high-quality ventilation air and minimize the amount required,the outdoor air intakes must be located to avoid building exhausts,vehicle emissions,and other sources of pollutan
43、ts.Indoor exhaust systems should collect odors or contaminants at their source.The amount of ventilation a building requires may be reduced with air washers,high efficiency filters,absorption chemicals(e.g.,activated charcoal),or odor modification systems.Figure 13 shows a ventilation system that su
44、pplies 100 percent outdoor air.This type ofventilation system is typically used where odors or contaminants originate in the conditioned space(e.g.,a laboratory where exhaust hoods and fans remove fumes).Such applications require make-up air that is conditioned to provide an acceptable environment.I
45、n many applications,energy costs make 100 percent outdoor air constant volume systems uneconomical.For that reason,other means of controlling internal contaminants are available,such as variable volume fume hood controls,space pressurization controls,and air cleaning systems.A ventilation system tha
46、t uses return air(Fig.14)is more common than the 100 percent outdoor air system.The return-air ventilation system recirculates most of the return air from the system and adds outdoor air for ventilation.The return-air system may have a separate fan to overcome duct pressure losses.The exhaust-air sy
47、stem may be incorporated into the air conditioning unit,or it may be a separate remote exhaust.Supply air is heated or cooled,humidified or dehumidified,and discharged into the space.Basic ventilation systems controls attempt a constant mixed air temperature.More sophisticated systems go beyond basi
48、c ventilation by control dampers to maintain proper building pressurization.Filtration Air filtration is an important part of the central air handling system and is usually considered part of the ventilation system.Two basic types of filters are available:mechanical filters and electrostatic precipi
49、tation filters(also called electronic air cleaners).Mechanical filters are subdivided into standard and highefficiency.Filters are selected according to the degree of cleanliness required,the amount and size of particles to be removed,and acceptable maintenance requirements.High-efficiency particula
50、te air(HEPA)mechanical filters(Fig.15)do not release the collected particles and therefore can be used for clean rooms and areas where toxic particles are released.HEPA filters significantly increase system pressure drop,which must be considered when selecting the fan.Figure 16 shows other mechanica
