INFLUENCE OF WIND VELOCITY TO SUPPLY WATER TEMPERATURE IN HOUSE HEATING INSTALLATION AND HOT-WATER DISTRICT HEATING SYSTEM

Transcription

1 Dr. Branislav Zivkvic, B. Eng. Faculy f Mechanical Engineering, Belgrade Universiy Predrag Zeknja, B. Eng. Belgrade Municipal DH Cmpany Angelina Kacar, B. Eng. Faculy f Agriculure, Belgrade Universiy INFLUENCE OF WIND VELOCITY TO SUPPLY WATER TEMPERATURE IN HOUSE HEATING INSTALLATION AND HOT-WATER DISTRICT HEATING SYSTEM INTRODUCTION A huse h-waer heaing insallains, qualiaive regulain is in cmmn use. Regarding variain in he uside emperaure, varied is supply waer emperaure while, waer flw hrugh hea emiing unis is kep cnsan. Prfessinal lieraure deals wih equains relaing change in characerisic waer emperaures in huse insallain vs. he uside air emperaure, i.e., udr/indr rese schedule [1]: - supply waer emperaure " " 1 / m 1 " s = mn Z + N Z+ 2 i (1) - reurn waer emperaure " " 1/ m 1 " r = mn Z N Z+ 2 i (2) where: i Z = (3) i N Equains (1) - (2) are made cnsidering predicin ha building hea lsses are linear funcin f he insananeus uside air emperaure; ha building venilain hea lsses depend nly n he uside emperaure, n n he wind velciy; and ha building heaing sysem regulain is ideal, prviding ha hea emiing uni upu is equal he insananeus rm hea lsses. IMPACT OF WIND VELOCITY ON HEAT LOSSES During heaing seasn, her han air emperaure, wind velciy is als variable. I predminanly influences building venilain hea lsses. In fac, here als exiss he impac f wind velciy n ransmissin hea lsses, because he film cefficien n he uside walls and windws depends n he air velciy, hus he verall hea ransfer cefficien hrugh he building envelpe is als he funcin f he wind velciy. Hwever, his impac n al building hea lsses is relaively small, hus i is negleced in his paper. Observed is he impac f wind velciy nly n venilain (infilrain) hea lsses.

2 = When he insananeus wind velciy is lwer han he design ne, bh venilain hea lsses, as well as al hea nes are lwer. Accrding German sandard DIN 4701, a calculaing hea demands, impac f wind velciy n venilain hea lsses is aken indirecly hrugh building characerisic H. Each H -value crrespnds cerain design wind velciy. Saring frm he fac ha wind is effecive nly venilain hea lsses, inrduced is facr Y which shws relaive change f al hea lsses a wind velciy lwer han he design ne: Y Q TN H QVN = + (4) Q H Q N N N Using general equains fr h waer emperaure (changed by he udr/indr rese schedule r he s-called sliding diagram as waer emperaure slides accrding uside emperaure) (1) - (2) and previus equain fr relaive change f al rm hea lsses, equains fr calculaing necessary waer emperaure in h waer huse heaing insallain may be frmed, due he uside emperaure and wind velciy variain: - supply waer emperaure ( ) " " 1 / m 1 " s = mn Y Z + N Y Z+ 2 i (5) - reurn waer emperaure ( ) " " 1 / m 1 " r = mn Y Z N Y Z+ 2 i (6) I is pssible use mre sphisicaed venilain mdel fr he purpse f analyzing air infilrain in he single rm r in a grup f similar rm mdels. Bu, in case f buildings wih large number f rms, differen in size, shape, windw ype and area, air ighness, ec., all parameers in any sphisicaed venilain mdel have be averaged (because hey differ in real buildings), On averaging all hese parameers, final resul is alms he same as wih simple venilain mdel. The her reasn use simple venilain mdel adped in DIN sandard is ha, in engineering pracice, we calculae venilain hea demands (and apprpriae radiar surface) accrding DIN 4701, fr nminal (design) wind velciy. I is reasnable, and echnically accepable calculae venilain hea lsses fr he her wind velciies using he same, ye n quie precise mehdlgy. The uside design emperaure fr heaing is BSNB = -15P PC fr Belgrade, and he average inside air emperaure is BiB +20P PC. Regarding he cmmnly used h-waer heaing sysem f 90/70P PC in huse insallain under he design cndiins, wih cas-irn radiars aken as hea emiing unis (expnen m=4/3), equains fr characerisic waer emperaures are in frms as fllws: " s " r 20 = Y 35 = Y / Y / 10 Y (7) (8) Rai beween venilain and ransmissin hea lsses under he design cndiins differs frm ne rm anher. Therefre, his rai is aken as he variable. The fllwing rais QBTNB:QBVNB = :20, 70:30, :, 50:50, and : are analyzed. Higher raes crrespnd frmer way f

3 building cnsrucin (exerir walls were n hermally insulaed), while lwer raes represen curren way f cnsrucin in ur cunry. Research regarding rai beween ransmissin and venilain hea lsses in large number f recenly buil buildings in Belgrade has shwn ha he average rai fr he whle building is 50:50±10 % [3]. Regarding single rms in a building, his rai has shwn bigger deviains: 50:50±20 %. The maer f analysis is a single building wih pen psiin, lcaed in windy area. The building s 2/3 3 characerisic H is 4.47 Wh PaP P/mP PK, which crrespnds wind velciy f 8 m/s. Wind velciy is varied frm he design velciy zer, wih sep 2 m/s. Prediced is ha venilain hea lsses are resul f wind impac nly, n f graviy frce ( chimney effec ) which, physically, crrespnd less high building (up 10 m). Tgeher wih he saed hypheses, calculaed are supply waer emperaures in funcin f he udr emperaure and he insananeus wind velciy. In rder reduce he paper rainal scpe, nly w sliding diagrams f supply waer emperaure in huse insallain fr he ms cmmn rai QBTNB:QBVNB in Belgrade s aparmens are given. Figure 1 shws he supply waer variain fr he rai QBTNB : QBVNB = 50 : 50 which crrespnds he currenly buil dwellings. Figure 2 is drawn fr he rai QBTNB : QBVNB = 70 : 30, relaing frmerly buil dwellings (up 10 years ag) represening he basis f al dwellings in Belgrade. Values f crrespnding waer emperaures fr her analyzed raes QBTNB:QBVNB have similar characer. The lwer rai is, he bigger impac f wind velciy n supply and reurn waer emperaure is niced. 90 W = 14 m/s W = 12 m/s Supply waer emperaure s [ C] W = 6 m/s Q TN :Q VN = 0.5:0.5 W * = 4 m/s W = 2 m/s W = 0 m/s W = 10 m/s W N = 8 m/s 30 Ouside air emperaure [ C] Figure 1. "Sliding diagram" f supply waer emperaure in huse insallain, in dependence n he uside air emperaure and wind velciy, fr rai: QBTNB : QBVNB = 0.5 : 0.5

4 Necessary supply waer emperaures fr wind velciies higher han nminal, bu cmbined wih air emperaures higher han design nes, are als drawn in diagrams 1 and 2. Higher wind velciies are aken in cnsiderain because, in winer ime, a emperaures arund zer, suh-eas wind is very srng. The nly limiain is ha supply waer emperaure may n vercme he design value, i.e. 90P PC. Thicker line n diagrams 1 and 2 crrespnds supply waer emperaure under nminal wind velciy. 90 W = 14 m/s W = 12 m/s Supply waer emperaure s W = 6 m/s Q TN :Q VN = 0.7:0.3 W * = 4 m/s W = 2 m/s W = 10 m/s W = 0 m/s W N = 8 m/s 30 Ouside air emperaure Figure 2. "Sliding diagram" f supply waer emperaure in huse insallain, in dependence n he uside air emperaure and wind velciy, fr rai: QBTNB : QBVNB = 0.7 : 0.3 Due hea accumulain wihin he building envelpe, change f he uside emperaure influences hea lsses wih he ime lag (6 36 hurs fr ypical buildings). Infilrain f he udr air causes insananeus hea lsses (r alms insananeus, as hea accumulain exers cerain influence even he said hea lsses). The fac is ha wind prfiles cninuusly change. In ur mdel we have used he average wind velciy, averaged ver shr perid (an hur, r cuple f hurs), n ver he lnger ne! Our pinin is ha he infilrain rae, calculaed by wind velciy averaged per hur, and caused by real wind velciy (which prfile scillaes in ha perid) des n differ significanly. Even if he impac f wind velciy scillain in he shrer ime perid (wind blws frm ime ime, in frm f guss) was f grea imprance fr venilain hea lsses, hydrnic heaing sysem is n able respnd and prmply fllw he changes. Hydrnic heaing sysem has be regulaed accrding mean hurly wind velciy. Change f he infilraed air quaniy in shrer ime inervals, caused by wind velciy prfiles scillain, wuld cause scillain f he indr air

5 wp wp wp P = P = P = emperaure; he effec wuld be reduced by hea ransfer by radiain and cnvecin frm he building envelpe inernal surfaces. BOUNDARY WIND VELOCITY A wind velciy lwer han he design ne, lwer are al rm hea lsses, hus lwer emperaure f supply heaing waer is needed. Bundary case, fr he wind velciy equal zer, invlves waer emperaure required cmpensae nly ransmissin hea lsses. Hwever, fr hygienic reasns, adequae rm venilain is necessary, i.e. cerain number f air change raes per hur is bliged. When here is n wind causing air infilrain, rm venilain is made hrugh pen windws r in anher way, which invlves cerain hea quaniy fr cmpensain f venilain hea lsses. One f he aims f his paper is he deerminain f bundary wind speed w*, a which hea lsses, caused by he infilrain, are equal hea quaniy necessary fr heaing f 0,5 air change per hur. Analyzed are ver hiry rm mdels, ypical fr curren way f building cnsrucin in Belgrade, a which variables are: rm s size, windw s number, size and air ighness. This research is sill under way, hus nly preliminary resuls are given. The average windw area in recenly buil buildings in Belgrade is beween 1/7 and 1/5 f he flr area. Average bundary wind velciies fr differen windw air ighness, calculaed fr he rm mdels, as well as checked in ciu, are given in able 1. Table 1 Average bundary wind velciy Air ighness Bundary wind velciy 3 2/3 [mp P/mh PaP P] [m/s] a = 0,3 * 5,2 ± 0,6 a = 0,4 * 4,2 ± 0,5 a = 0,6 * 3,1 ± 0,3 Bundary wind velciy depends n he windw ype (air ighness and crack lengh) and has values in range frm 2,8 5,8 m/s. Averaging hese values, and aking in accun predminan windw ype in Belgrade s dwellings, i is fund ha he average bundary wind velciy is 4 m/s; fr ha value, supply waer emperaures in diagrams 1 and 2 are marked wih dashed line. When wind velciy is lwer han he design ne, and higher han he bundary velciy, supply waer emperaure in huse insallain shuld be changed accrding he apprpriae curve (diagram 1 r 2), fr cerain wind velciy. Bu, when wind velciy is lwer han he bundary ne, heaing sysem shuld perae accrding he curve calculaed fr w*. Air quaniy which des n ener he rm by air infilrain hrugh dr and windw cracks, will ener hrugh rm venilain. Fr his reasn, i is necessary prvide enugh hea hea emiing uni, fr he purpse f air heaing up he rm emperaure. Figure 3 shws supply waer emperaures, calculaed fr criical (bundary) wind velciy (minimum supply waer emperaure) fr differen raes beween ransmissin and venilain rm

6 hea lsses under design cndiins. I is adped ha venilain hea lsses are cnsan (calculaed fr w*), and ha al hea lsses vary because f differen building hermal insulain (differen ransmissin hea lsses). As already menined, a wind velciy lwer han he design ne, he bigger rai QBTNB : QBVN Bis, he higher supply waer emperaure is necessary (fig. 3) : :0.3 w * = 4 m/s Suplly waer emperaure s : : : Ouside air emperaure Figure 3. "Sliding diagram" f supply waer emperaure in huse insallain, in dependence n he uside air emperaure and rai QBTNB : QBVNB fr bundary wind velciy w*=4m/s DISTRICT HEATING SYSTEM When huse heaing insallain is cnneced he disric heaing sysem, required emperaures f supply and reurn h waer in piping newrk depend n cnnecin ype (direc r indirec). Majriy f huse heaing insallains in Belgrade fla area is direcly cnneced. As disric heaing sysem peraes wih cnsan waer flw, cnsan mixing rae f primary and secndary waer is adped. Fr described echnical sluin, general equains fr supply and reurn waer emperaure in primary lp (piping newrk) are: ' " / m ' " s = mn ( Y Z) + N N ( Y Z) + 2 i (9) 1 1 m ( ) ( ) ' " 1 / 1 " r = mn Y Z N Y Z + 2 i (10) In case f disric heaing sysem 1/70P PC under he design cndiins, msly used by Belgrade Municipal DH Cmpany, required variain f supply waer emperaure in h waer newrk vs. uside emperaure and wind velciy ( sliding diagram f primary lp) is shwn in fig. 4. This

7 diagram relaes fig. 1 - Supply waer emperaure in huse heaing insallain. Type f hea emiing unis and design cndiins crrespnd he example f waer emperaure in huse insallain. Rai f QBTN B: QBVNB is equal 50:50. Dashed line is drawn fr bundary wind velciy f 4 m/s. This udr - indr rese schedule is suiable fr he applicain in he disric heaing plans which supply he newly buil residenial areas. Applied a he perain f he disric heaing plans fr heaing f lder dwelling areas, he udr - indr rese schedule is given in figure 5. The rai f ransmissin and venilain hea lsses fr he design cndiins is QBTNB : QBVNB = 70 : W = 14 m/s W = 12 m/s Supply waer emperaure s W = 6 m/s Q TN :Q VN = 0.5:0.5 W * = 4 m/s W = 2 m/s W = 0 m/s W = 10 m/s W N = 8 m/s 30 Ouside air emperaure Figure 4. "Sliding diagram" f supply waer emperaure in disric heaing newrk, in dependence n he uside air emperaure and wind velciy, fr rai: QBTNB : QBVNB = 0.5 : 0.5 The impac f QBTN B: QBVNB rai fr bundary wind velciy n required supply waer emperaure in disric heaing sysem newrk is shwn in fig. 6. The characer f he curves is equal he nes in fig. 3; ha cncerns huse insallain, while all menined remarks are valid fr DH sysem,. Wind direcin cerainly makes impac n he air infilrain, bu i shuld be aken in accun nly if here is znal regulain (due he facade rienain). Fr pure cenral regulain f supply waer emperaure (wih n eiher znal regulain, r he lcal ne by hermsaic valves) wind direcin is meaningless. Supply waer emperaure is deermined accrding he uside emperaure and average wind velciy. Rms n leeward side will be verheaed.

8 1 130 W = 14 m/s 120 W = 12 m/s Supply waer emperaure s W = 6 m/s Q TN :Q VN = 0.7:0.3 W * = 4 m/s W = 2 m/s W = 10 m/s W N = 8 m/s W = 0 m/s 30 Ouside air emperaure Figure 5. "Sliding diagram" f supply waer emperaure in disric heaing newrk, in dependence n he uside air emperaure and wind velciy, fr rai: QBTNB : QBVNB = 0.7 : :0.2 w * = 4 m/s Supply waer emperaure s : : : :0.4 Ouside air emperaure Figure 6. "Sliding diagram" f supply waer emperaure in disric heaing newrk, in dependence n he uside air emperaure and rai QBTNB : QBVNB fr bundary wind velciy w*=4m/s

9 QBTB (W) QBVB (W) The main purpse f his par f he paper is deermine he apprpriae supply waer emperaure (udr-indr rese schedule) in disric heaing sysem wih direc cnnecin. There exiss nly cenral regulain in heaing plan wih fare r n regulain in he huse insallains. Frm he aspec f rainal energy cnsumpin, alhugh far frm he ideal echnical sluin, his ype f regulain sill exiss. I was designed sme 20 years ag, when buildings were n hermally insulaed and when rai beween ransmissin and venilain hea lsses was very high. Fr small rai f venilain hea lsses, he impac f wind velciy n he supply waer emperaure was n significan and herefre cenral regulain f hydrnic heaing sysems was adped. Fr up--dae mde f building, rae f venilain hea lsses is higher, and wind velciy is impran facr in defining required h waer emperaure a hea emiing uni inle. Diagram 2 clearly shws ha pimum peraing f hydrnic heaing sysem invlves lcal regulain. Radiar valves wih hermsaic heads, cmbined wih sligh cnrl regulain sand as ne f he bes sluin day. CONCLUSION A qualiaive regulain f heaing h waer, sliding diagrams f supply and reurn waer emperaure shuld fllw bh uside emperaure and wind velciy. Bu, while supply waer emperaure fllws he variain in he uside air emperaure hrughu he whle inerval f change (frm uside design emperaure he emperaure a which he heaing seasn is cease), supply waer emperaure shuld fllw wind velciy change up he bundary velciy w*. Fr any uside emperaure, when wind velciy is lwer han he design ne, supply waer emperaure fr hydrnic sysem may be diminished bu nly cerain level (he bundary level). If supply waer emperaure is deermined accrding very lw wind velciy, cerain prblems may arise in buildings (insufficien heaing). Bundary wind velciy causes he infilrain rae equal minimum venilain rae (fr hygienic purpse). Bundary wind velciy depends n he windw area and windw ype (air ighness and crack lengh). Bellw bundary wind velciy, hea lsses due infilrain are lwer han hea demand fr rm venilain, which is dne by her measures (pen windws, mechanical venilain, ec.). A sliding diagram deerminain, cnsidered is hea necessary fr addiinal air quaniy heaing. I shuld be pined u ha presened sliding diagrams are idealized, because cerain simplificains are made: linear building hea lsses dependence n he uside emperaure, cnsan values f hea ransfer cefficiens hrugh building envelpe, negleced hea accumulain in building envelpe and nigh heaing seback (seady sae cndiins), cnsan rm air emperaure, ideal building heaing regulain, and negleced usual ver-design f hea emiing unis. LIST OF SYMBOLS 3 2/3 a - (mp P/mh PaP P) - windw air ighness 2/3 3 H - (WhPaP P/mP PK) - building characerisic fr cerain wind velciy; Q (W) - rm hea demands (al rm heaing lsses); - ransmissin rm hea lsses; - venilain (infilrain) rm hea lsses;

10 BiB (P BB (P (P PC) - waer emperaure; PC) - indr air emperaure; PC) - uside air emperaure; (P PC) - difference beween supply and reurn waer emperaures; BmB (P PC) - difference beween mean waer emperaure in hea emiing uni and rm air emperaure; Y (-) - relaive rm hea lsses - eq. (4); Z (-) - relaive uside emperaure - eq. (3). INDEXES m - mean; N - design (nminal) cndiins; r - reurn waer; s - supply waer. EXPONENTS m - hermal characerisic f he hea emiing uni. ' - primary lp (disric heaing (piping newrk) " - secndary lp (huse heaing insallain) REFERENCES [1] Zeknja, P., Zivkvic, B.: General Frm f Equains fr Sliding Diagram Deerminain in Huse Insallain and Disric Heaing Sysem wih Direc Cnnecin, Jurnal KGH, 2/1996, SMEITS, Belgrade (in Serbian). [2] Recknagel e. al.: Heaing and Air Cndiining, Inerklima, Vrnjacka Banja, 1995 (in Serbian). [3] Rakic, M.: Rai beween Transmissin and Venilain Hea Lsses in Belgrade s Dwellings, Diplma Thesis, Faculy f Mechanical Engineering, Belgrade Universiy, 1977 (in Serbian). ABSTRACT This paper is abu he analysis relaing simulaneus impac f he uside emperaure and wind velciy n required supply waer emperaure. Inrduced is bundary wind velciy a which infilrain hea lsses are equal he necessary hea fr rm venilain. Fr h waer heaing wih radiars, presened are sliding diagrams (udr-indr rese schedule) f supply waer emperaure, regarding he uside emperaures and wind velciies fr bh huse heaing insallain and disric heaing sysem wih direc cnnecin.