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Air Conditioning Clinic V A V Systems One of the Systems Se ries TRG-TRC014-EN.
V A V Systems One of the Systems Se ries A publication of The T rane Company.
Preface © 2001 American Stand ard In c. All ri ghts res erved TRG-TRC01 4-EN ii The T rane Company beli eves t hat it is inc umbent on m anufact urers to s erve the industry by regularly dis seminating i nformation gathe red through lab oratory research, tes ting programs, and field experien ce.
TRG-TR C0 14-EN iii Contents period one What Is Variabl e Air Volume? ........ ...... ...... ...... 1 Why VAV? . ....... ...... ...... ...... ...... .......... ...... ...... ...... ... 7 period two Components of a VAV System ... ......... ...... .... 12 period three System Configurations .
iv TRG-TRC01 4-EN.
TRG-TR C0 14-EN 1 note s peri od o ne What Is V ariable Air V olume? A variable-a ir -volume (V A V) air -conditioning sys tem varies th e volume of constant- temperature air th at is suppl ied to meet th e changing load conditio ns of t he spac e.
2 TRG-TRC01 4-EN note s peri od one What Is V ariable Air V olume? This equation des cribes the performance of a constant-volum e, variable- temperature s ystem at v arious load c onditions. With these design con ditions: space sen s ible heat g ain = 40,00 0 Btu/hr [11,72 4 W] space d ry-bulb (DB) te mperature = 75 °F [23.
TRG-TR C0 14-EN 3 peri od one What Is V ariable Air V olume? note s Rearranging th e equati on and using the constant supply ai rflow of 1,840 c fm [0.
4 TRG-TRC01 4-EN note s peri od one What Is V ariable Air V olume? However , because this type of syst em can res pond to the d emands of o nly one thermostat, it can serve only tho se building spaces with similar cooling requirem ents. If a bu ilding has m any space s with div e rse cooling needs, each must be serv ed by i ts own sy ste m.
TRG-TR C0 14-EN 5 peri od one What Is V ariable Air V olume? note s The termina l reheat system u ses a centra l air han dler and co oling coil to deliver cool primary a ir to all the spaces. Ea ch space has its own heat ing coil to temper the air t o satisfy the space loa d.
6 TRG-TRC01 4-EN note s peri od one What Is V ariable Air V olume? In contras t, a variable-air -volume (V A V) system delivers the primary ai r at a constant temperature and vari es the airflow to maintain the required space temperature a t all load c onditions.
TRG-TR C0 14-EN 7 peri od one What Is V ariable Air V olume? note s However , at part load, th e V A V system pro duces a balance by m odulating the volume of c onstant 55 °F [12.8°C] su pply air . In this ex ample, the a ir volume required to bala nce the lesser 20,00 0 Btu/hr [5,862 W] s pace sen sible load is: That is, 9 20 cfm [0.
8 TRG-TRC01 4-EN note s peri od one What Is V ariable Air V olume? the air volume reduc tion creates an opportunity to reduce the fan ene rgy required to move this air . The amount of energy saved depends on the method used to m odulate th e capacity of t he fan (to be discus sed in Perio d Four).
TRG-TR C0 14-EN 9 peri od one What Is V ariable Air V olume? note s Contrasting th e three systems di scussed in this exa mple, the si mple, constant- volume, v ariable-tempera ture system consumes const ant fan energy while providing refrige ration ene rgy savings a t part load.
10 TRG-TRC01 4-EN note s peri od one What Is V ariable Air V olume? There a re some bas ic building charac teristics tha t favor the applicatio n of V A V syste ms. We will d iscus s three : n V ariabl e thermal load profiles in the spac es. n The need for multiple , independen tly-controlled spaces.
TRG-TR C0 14-EN 11 peri od one What Is V ariable Air V olume? note s Another feat ure of the V A V system is the ability to efficiently sa tisfy the comfort requirements of many diffe rent spaces withi n the building .
12 TRG-TRC01 4-EN note s Next, w e will examine the individual comp onents th at compose a V A V system. A simple V A V system in cludes the followin g: n Central air ha ndler wit h a variable-v olume.
TRG-TR C0 14-EN 13 period two Components of a V A V System note s V A V T erminal Un its A V A V terminal unit is a shee t-metal ass embly insta lled upstrea m of its respectiv e space diffus ers.
14 TRG-TRC01 4-EN note s period two Components of a V A V System The simpl est of all V A V terminal units is the single-duct, cooling-only terminal unit. It consis ts of an airflow modul ation device with control s package d in a she et-metal enc losure.
TRG-TR C0 14-EN 15 period two Components of a V A V System note s determined by the design cooling loa d of the space and the minimum settin g is normally dete rmined by the spac e ventilation requ irement or minimum airfl ow for proper diffus er selection.
16 TRG-TRC01 4-EN note s period two Components of a V A V System VAV r e h e a t terminal units provide sup ply-air tempering or space heating by reheating th e cool primary air . This is accomp lished by adding an electric, or hot-water , heating coil to the discharg e of a cooli ng-only unit.
TRG-TR C0 14-EN 17 period two Components of a V A V System note s During the heating mode, the primary airflow mu st often be greater than the minimum s etting of the un it for the cooli ng mode.
18 TRG-TRC01 4-EN note s period two Components of a V A V System A parallel, fan-powered term inal unit con s ists of a primary ai rflow modulation device and a sma ll, integral, c onstant-volum e fan pack aged to provide para llel airflow p aths. In the coo ling mode, the unit is controlled in the same m anner as the cooling- only unit.
TRG-TR C0 14-EN 19 period two Components of a V A V System note s When in co oling mode, th e supply a irflow to the s pace equa ls the pri mary airflow . When in tempering and heating modes, the space receives a c onstant supply a irflow equal to the minimum primary airflow plus the plenum airflow delivered by the terminal fan.
20 TRG-TRC01 4-EN note s period two Components of a V A V System In the coo ling mode, the primary a irflow is redu ced as the cooling loa d in the space d ecreases. The total supply airflow to the spac e remains c onstant, a combinati on of cool prima ry air and warm p lenum air .
TRG-TR C0 14-EN 21 period two Components of a V A V System note s Most designe rs also believe tha t series uni ts offer improved acous tical conditions in the spa ce.
22 TRG-TRC01 4-EN note s period two Components of a V A V System A dual-duct terminal unit consists of two airflow mod ulation d evices with control s packaged in a sheet-m etal enclos ure. One co ntrols the cool primary air and the oth er controls the warm primary air .
TRG-TR C0 14-EN 23 period two Components of a V A V System note s As before, this graph indica tes how the a ir supplied to t he space by the termin al unit varies as the sp ace loads change. Th e vertical axis indi cates the total airflow (co ol primary air plus wa rm primary a ir) supplied to the space .
24 TRG-TRC01 4-EN note s period two Components of a V A V System T o deliver a constant v olume of sup ply air to the space in a two-fan, dual-duct system as the cool ing load in the spac e decre ases.
TRG-TR C0 14-EN 25 period two Components of a V A V System note s the respectiv e duct system to the modula tion devi ces in the dual -duct V A V terminal uni ts. While this single-fan configuratio n requires onl y one air han dler , it is very complica ted to control efficiently .
26 TRG-TRC01 4-EN note s period two Components of a V A V System V A V T erminal Unit Controls Next we will explor e the control o f V A V terminal un its. Before dis cussing the commonly -available c ontrol techno logies, it is imp ortant to first un derstand a fundamenta l control concept: press ure-dependent versus pressure- independent control.
TRG-TR C0 14-EN 27 period two Components of a V A V System note s duct stati c pressure, airflow to the spa ce can v ary even when th e immediate modulat ion device does not c hange po sition. A pressure-dependent V A V control sche me uses the space temper ature sensor to directly con trol the positio n of the modul ating dev ice.
28 TRG-TRC01 4-EN note s period two Components of a V A V System Accura te measurem ent of prima ry airflow is requi red to enable pressure - independe nt control. Mos t terminal units accompl ish this w ith a multipoin t airflow se nsor mounted on the inlet.
TRG-TR C0 14-EN 29 period two Components of a V A V System note s A pneumatically-controlled te rminal unit has an airflo w-modul ation device operated by a pneumatic actuator co nsisting of an inflatable bladder and a return spring.
30 TRG-TRC01 4-EN note s period two Components of a V A V System An electronically-contr olled termina l unit has a n airflow-modul ation devic e operated by an electronic actuator that c an drive the modula tion device op en or closed.
TRG-TR C0 14-EN 31 period two Components of a V A V System note s A digitally-controlled termina l unit has a n airflow- modulation de vice operate d with an el ectric actu ator in the sa me mann er as the ele ctronic con trol option.
32 TRG-TRC01 4-EN note s period two Components of a V A V System Similar t o electronic con trol, an electronic pressu re transdu cer enables pressure-indepe ndent control of the V A V terminal u n it. The digita l controller positions the modulatio n device and operate s the fan and the heat sourc e.
TRG-TR C0 14-EN 33 period two Components of a V A V System note s Diffuse rs Each V A V terminal unit is commonly c onnected to a downstre am, sheet-m etal duct that i s then conn ected to the remotely-loca ted diffusers by flexible duc ts. Diffuse rs distribute th e supply a ir effectively to the condi tioned space.
34 TRG-TRC01 4-EN note s period two Components of a V A V System Linear slo t diffusers are general ly preferred for V A V air distribution. They use a principle k nown as the Coanda effe ct to distribute a ir to the condition e d space.
TRG-TR C0 14-EN 35 period two Components of a V A V System note s Because variable -air -volume systems are insta lled in, and distribute air along the surfac es of, ceilings , it is ess ential that th e diffusers be comp atible with th e ceiling d esign.
36 TRG-TRC01 4-EN note s period two Components of a V A V System The linear s lot diffuser is placed on th e structura l T -bars with the mounting fe et resting on t he flanges. The cros s member fits in a slot in the diffuse r air opening, a ligning th e unit with th e ceiling tile pa ttern.
TRG-TR C0 14-EN 37 period two Components of a V A V System note s Supply Duct System The supply duct system transports the primary air from the air handler to the V A V terminal u nits and the n on to the s pace diffus ers.
38 TRG-TRC01 4-EN note s period two Components of a V A V System Designers commonl y use two meth ods to engineer the s upply duct sys tem: n Equal friction n Stat ic reg ain Equal friction duct syste ms are designed for an equal static-pre ssure drop per foot of d uct.
TRG-TR C0 14-EN 39 period two Components of a V A V System note s Other publi c ations c ontain more c omplete detai ls relate d to duct design , but here are a few gene ral recommen dations: n Keep t.
40 TRG-TRC01 4-EN note s period th ree System Configurations T o permit a comple te understanding of the V A V system, we must look at it from a syste m level. In this se ction we w ill explore a few commo n space types and how V A V systems address the cooling and heati ng needs ass ociated with them.
TRG-TR C0 14-EN 41 period thre e System Configurations note s and equipment. Interior spaces on th e top floor of a build ing often nee d to be treated as a perime ter space du e to the heat gain/los s from the r oof. Each sp ace is typically s erved by a n individual V A V terminal unit, all owing independe nt cooling an d heating c ontrol.
42 TRG-TRC01 4-EN note s period thre e System Configurations If the heat loss of the peri meter space exc eeds 450 Btu/hr per linear foot [430 W per linear me ter] of outside wall, an u nder -the-window , or bas eboard, heati ng system is typically used.
TRG-TR C0 14-EN 43 period thre e System Configurations note s If the heat loss of the perimete r space is less than 450 Btu/hr per linear foot [430 W per linear m eter] of outside wall, downdrafts are less proble matic and h eated air supplie d through cei ling diffusers ca n provide a satisfactory solution.
44 TRG-TRC01 4-EN note s period thre e System Configurations When over head heati ng is accepta ble, V A V reheat, fan-pow ered, or d ual-duct terminal uni ts can be used to p rovide both pe rimeter cooling and heating.
TRG-TR C0 14-EN 45 period thre e System Configurations note s Similarly , fan-pow ered terminal units can be u sed in perime ter spaces tha t require sea sonal cool ing and heati ng. In the hea ting mode, the parallel, fan-powered unit turns on a small fan as the first stage of heating.
46 TRG-TRC01 4-EN note s period thre e System Configurations Interior Spaces Because an interior space is surrounde d by spac es at the s ame temperatu re, it does not e xperience t he same heat ga in and l oss fluctuation s as a peri meter space.
TRG-TR C0 14-EN 47 period thre e System Configurations note s T ypica lly , either V A V reheat or fa n-powere d terminal uni ts are us ed to provide the temperin g needed to balance the reduced cooling loa d. Changeove r/Bypass V A V System Many sma ller buildings cannot afford c ommercial, or applie d, equipment.
48 TRG-TRC01 4-EN note s period thre e System Configurations Any unn eeded air is diverted to the return air stream , allowing individual comf ort co ntro l of t he sp aces .
TRG-TR C0 14-EN 49 period thre e System Configurations note s In a typic al commerc ial building, the occupied mo de occurs during day time operation. T he building must be v e ntilated and the comfort cooling or h eating temperature s et points m ust be mainta ined in all o ccupied zone s.
50 TRG-TRC01 4-EN note s period thre e System Configurations In a typic al commerc ial facility , the unoccupied mode oc curs at nig ht. The building do es not require ventilati on because it is n ot occupied , and the temperature i n the perimete r spaces m ust be pr evented from g etting too col d (per haps 60 ° F [15.
TRG-TR C0 14-EN 51 period thre e System Configurations note s The morning warm-up/cool-down mode typical ly occurs as a transiti on from the unocc upied mode to the occupied mode .
52 TRG-TRC01 4-EN note s peri od f our Fan Modulation T o accommod ate the variable -volume require ments imposed by a V A V system , the sys tem supply fan must be selec ted and con trolled in such a ma nner that it is capable of modulating over the req uired airflow range wit hout entering an unstable area of operation.
TRG-TR C0 14-EN 53 peri od four Fan Modulation note s plot performan ce on the ba sis of the st atic press ure com ponent only . Performance curves pub lished f or some pac kaged air ha ndlers will include the effects o f the dam pers, filters, and coil s that are in the unit.
54 TRG-TRC01 4-EN note s peri od four Fan Modulation When the s ystem re sistance curve is s uperimpos ed on the fa n curve, the intersecti on predicts th e airflow an d static pressure at which the s ystem wi ll balance (ope rating point “ A ”).
TRG-TR C0 14-EN 55 peri od four Fan Modulation note s This “ridin g the fan curve” meth od of modulation can b e used wi th any centrifugal fan. Howeve r , it is most energy efficient wh en applied to forward- curved (FC) fans.
56 TRG-TRC01 4-EN note s peri od four Fan Modulation Fan Modulation Curve The fan modul ation curve illustrates the static pres sure requirem ent of the V A V syste m over the rang e of airflows . This require ment consists of a fixed componen t and a v ariable comp onent.
TRG-TR C0 14-EN 57 peri od four Fan Modulation note s T o achiev e this bala nce, a sim ple con trol loop is us ed. First, the static pres sure is sensed from a particula r location in the system. Second, a c ontroller compares this static pressure rea ding and the set point of th e system.
58 TRG-TRC01 4-EN note s peri od four Fan Modulation The sys tem stat ic pressure controlle r senses th is higher pres sure an d sends a signal to the supply fa n to reduce c apacity .
TRG-TR C0 14-EN 59 peri od four Fan Modulation note s As the V A V terminal units m odulate shu t, the system resistanc e curve s hifts upward s. The fan begins to “ ride up ” the constant-s peed performan ce curve, from the design operating point ( A ), trying to reach the balance po int with this new sys tem resista nce curve .
60 TRG-TRC01 4-EN note s peri od four Fan Modulation Inlet vanes modu late the ca pacity of a fan by “ p re-swirli ng ” the air in the directi on of fan whe el rotation bef o re it enters the wheel. By changi ng the angle of air entry into the fa n, the modula ting inlet va nes lessen the ability of the fan wheel to “ bite ” the a ir .
TRG-TR C0 14-EN 61 peri od four Fan Modulation note s Fan-speed control modulate s fan capa city by varying the speed of rotation. This is c ommonly ac compli shed using a variable-s peed drive o n the fan moto r .
62 TRG-TRC01 4-EN note s peri od four Fan Modulation Finally , t he capaci ty of vaneax ial fans ca n be mo dulated by a djusting the p itch angle of the actual fan blades. Th is diagram s hows a s et of fan blade s that can swivel to modulate the capacity of th e fan.
TRG-TR C0 14-EN 63 peri od four Fan Modulation note s In response to reduced airflo w and rising syste m static pre ssure, the varia ble- pitch vane axial fan res ponds simila rly to a centrifuga l fan with eith er inlet-vane or variable-s peed capaci ty control.
64 TRG-TRC01 4-EN note s peri od four Fan Modulation System Static-Pressure Control Each of th ese fan-capacity c ontrol me thods requires a signal f r om a con troller that monitors static pres sure, using a sensor located som e where in the supply- duct sys tem.
TRG-TR C0 14-EN 65 peri od four Fan Modulation note s included in the supply duct, this method ensu res that th e sensor is on the fan side of the damper so t hat the duct is protec ted from hi gh pressures. Als o, depending on the layout of the du ct system, this method may eliminate the need for mul tiple duct-mounted se nsors.
66 TRG-TRC01 4-EN note s peri od four Fan Modulation The optimized static-pres sure control method co mbines the location - related benefits of f an outlet con trol with operating c ost savin gs that e xceed those of s upply duct static-p ressure con trol.
TRG-TR C0 14-EN 67 peri od four Fan Modulation note s A comparis on of these st atic-pressure c ontrol methods demonstrate s the energy sa vings poten tial.
68 TRG-TRC01 4-EN note s peri od fi ve Application Considerations When desi gning a V A V system for a building, th ere are s everal cons ideration s to address: n System-lev el ventilatio n n Free ze.
TRG-TR C0 14-EN 69 period five Application Considerations note s System-Level V entilation During operatio n, a typical V A V system delivers a mixture of outdoor air and recirculate d air to the mu ltiple spac es it serves. The volume o f supply air that enters a space is c ontrolled by a temperature s ensor in that space .
70 TRG-TRC01 4-EN note s period five Application Considerations With all three compon ents, DDC controls o n the V A V terminal un its and air handler , a method to me asure outdoo r airflow a t the a.
TRG-TR C0 14-EN 71 period five Application Considerations note s Freeze Protect ion for Coils As mentione d, proper V A V system venti lation often le ads to a hig h percentage of outdoor air at part-load conditions .
72 TRG-TRC01 4-EN note s period five Application Considerations Coils can be protec ted from freez ing in many ways, su ch as: n Draining the c oils d uring cold we ather n Keeping the water in moti o.
TRG-TR C0 14-EN 73 period five Application Considerations note s Minimum s pace ventil ation requireme nts often dic tate minimum primary airflow se ttings on V A V terminal un its.
74 TRG-TRC01 4-EN note s period five Application Considerations Building Pressure Control Maintaining a slightly positive pressure in side buildin gs, espec ially in hum id climates , increase s comfort and helps a void indoor a ir quality (IAQ) problems related to m icrobial gr owth and outd oor contamina nt sources.
TRG-TR C0 14-EN 75 period five Application Considerations note s A sensitive s tatic-pre ssure control ler dir ectly comp ares the varia tions between the spac e static pressure a nd the ou tdoor reference sta tic pressure.
76 TRG-TRC01 4-EN note s Now , we will review som e of the main concepts from this clinic on v a riable- air-volu me (V A V) sys tem s. A variable-a ir -volume (V A V) air -conditioning sys tem varies th e volume of constant- temperature prima ry air to me et changing load condit ions in the space.
TRG-TR C0 14-EN 77 peri od six Review note s A simple V A V system is comprised of th e following compone nts: n Central air ha ndler n Supply duct n V A V terminal u nit with thermosta ts and supply .
78 TRG-TRC01 4-EN note s peri od six Review A simple bu ilding has t w o types o f spaces: perimeter a nd interior . These two space types require differe nt types of V A V terminal units to satisfy thei r requirements .
TRG-TR C0 14-EN 79 peri od six Review note s Fina lly, when app lyin g a V A V syst em t o a bu ildi ng, ther e a re sev eral conside rations that mu st be addres sed.
80 TRG-TRC01 4-EN note s peri od six Review V isit the AS HRAE Booksto re at www .ashrae .org. For inform ation on addi tional educa tional ma terials ava ilable from T rane, contact y our local T rane sales of fice (reques t a copy of th e Educationa l Materials price sheet— T rane order numbe r EM-ADV1) or v isit our online bookstore at www .
TRG-TR C0 14-EN 83 Questions for Period 1 1 Given a space s ensible load of 3 0 ,000 Btu/hr [8 ,793 W], a s pace dry-bu lb tempera ture of 78 °F[25.6°C], and a primary air dry-bulb temper ature of 58°F [14 .
84 TRG-TRC01 4-EN note s Quiz 11 What method of static-pres sure control provi des the grea test power - savings potential for a V A V system? Questions for Period 5 12 What indus try standard ou tlin.
TRG-TR C0 14-EN 85.
86 TRG-TRC01 4-EN 1a 1,382 c fm [0.65 m 3 /s] 1b 63°F [17 .2° C] 1c 829 cfm [0.39 m 3 /s] 2 The redu ced airflow ac ross the c ooling coil a t part load causes the refrigeration s ystem to thrott le back in order to st abilize the su pply air temperature c ondition.
TRG-TR C0 14-EN 87 chan geover/byp ass V A V syst em A system t hat allows var iable su pply air to the spac es but uses a constant-v olume central supply fan. Th is is acc omplished by using a large d amper to b ypass the exc ess air . This is c ommon in sm aller buildings r equiring in dividual sp ace comfort control.
88 TRG-TRC01 4-EN Glossary fan spee d control Method of supply-fan modulat ion that affe cts the capa city of a fan by varying the speed of rotation—commo nly accom plished us ing a variable-s peed drive on the fan motor .
TRG-TR C0 14-EN 89 Glossary constant- volume fan, p ackaged s o that the ai rflow path s are in series. This provid es a constant vol ume of suppl y air to the space w hen operating .
The T ran e Compa ny Wo rldwi de Applie d Systems Gro up 3600 Pammel C reek Road La Crosse, WI 54601 -7599 www .trane.com An American Standard C ompany Literature Or der Number TRG-TRC 014-EN (May 200.
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