CIBSE NATIONAL CONFERENCE

Transcription

1 EXPERIMENTAL PASSIVE STACK SYSTEMS FOR CONTROLLE NATURAL VENTILATION Lynn M. Parkins Bilding Research Establishment, Garston. The modern trend towards more airtight energy efficient hosing reqires additional care in providing controlled ventilation. Passive Stack Ventilation (PSV) is one way of providing the necessary control. The Bilding Research Establishment (BRE) is carrying ot research into the perfo r ma n ce of PSV systems in dwellings. Sev eral different PSV systems were installed in a t e st hose and the flow ve l o city th r o gh the m me as red nder a r ange o f meteorol ogi cal conditions. The reslts of t h ese t e s ts a r e p r e sented and t he effect o f bends in the dct a n d dct diameter are discssed. INTROUCTION ntil fairly recently the majority of U.K. hoses were bilt with chimneys and many ventilation problems were with too mch rather than inadeqate ventilation. The modern trend towards more airtight, energy efficient hosing, with no open chimneys or fles, can lead to condensation and indoor air qality problems nless care is taken to provide an alternative means of ventilation. One possible soltion cold be to reintrodce the chimneys bt in a form known as a passive stack ventilation system (PSV). By placing them in the moistre prodcing rooms i.e.. kitchens and bathrooms, se can be made of the natral stack effect to ventilate these rooms, thereby removing the warm moist air, withot the se of mechanical fans. The advantages of sing PSV systems are their lack of noisy fans, little maintenance, no direct rnning costs, no moving parts to break down,and, if installed when the dwelling is bilt, cheapness of installation. The Bilding Research Establishment (BRE) carried ot some research into PSV in the late 195's, bt the systems sed very large, sqare section stacks which bore little resemblance to modern installations, hence there was not mch interest in their se. Interest in the se of PSV was revived in the 197's by de Gids and den Oden (1) and later work was carried ot in the 198's by Johnson and Pitts (), Johnson Gaze and Brown (3) and Edwards and Irwin (4). Stephen and Uglow (5) prodced an information paper based on crrent knowledge, on the design and installation of PSV systems. As part of a research programme to test their performance the Bilding Research Establishment has set p varios passive stack ventilation systems in a test hose, sing different dct materials in a range of diameters and configrations, by sing the same dwelling for all the tests, a direct comparison of systems can be obtained. This paper describes the varios systems and their respective performances. escription of test hose and ventilator systems The hose sed to test the varios passive stack ventilator systems is an end of terrace, timber framed bilding with a roof pitch of 4 degrees. It is sitated at the Bilding Research Establishment, Garston, in a position of fairly open grond to the Soth and West with trees to the North and office blocks some distance to the East see Figre 1. The room in which the systems were installed is the kitchen/diner, having a volme of 35 m 3 Two stack ventilator configrations were sed, the first was a straight dct 58

2 from the kitchen ceiling throgh the bedroom above and the attic, terminating at jst above ridge height with a weatherproof terminal. The second configration had two 45 deg. bends in the attic section to enable the dct to be connected \ 1 ' a ridge terminal, see Figre. The stack was lagged with fibreglass qilting where it passed throgh the attic to redce heat losses and possible condensation problems within ' the stack. Two stack diameters were sed, 155 mm and 1 mm, and two different materials, smooth rigid plastic and flexible plastic on i wire spiral. A nmber of different roof terminals and ceiling diffsers were sed and their performance~ compared. The systems which will be reported on here are the fo~ basic system~ sing rigid dct material in two different configrations and diameters. A comparison will also be made of the reslts obtained from two of the systems with those from similar systems sing flexible dcting. Table 1 lists the systems reported on. TABLE 1 - Passive stack ventilation systems tested. SYSTEM IAMETER (mm') MATERIAL CONFIGURATION RIGI BENS 155 RIGI STRAIGHT FLEXIBLE BENS FLEXIBLE STRAIGHT 5 1 RIGI BENS 6 1 RIGI STRAIGHT ata collection The varios parameters which were monitored are as follows - Wind speed - Wind direction - External temperatre - Internal temperatre (3 positions) - ct temperatre ( positions) - Flow velocity in dct wind speed and direction were measred adjacent to the test hose at a height or 1 metres. External temperatres were taken from inside a Stevenson Screen on the North side of the hose. Internal temperatres were taken in the kitchen, bedroom and attic. ct temperatre s and velocity were measred at the position~ shown in Figre. ct velocity was measred sing a antek low velocity flow analyser. This cannot indicate flow direction, so to detect any reverse flow pitot-static tbes were installed in the stack, one facing down to measre pward flow and another facing p to measre downward flow. (No prolonged reverse flow was actally detected by this method). Monitoring took place over several weeks fo~ each different system, the drati a 1 1 depending on weather conditions. The data covered as wide a range of wind speerl ~ and directions as possible. A range of temperatre differences between inside and otside the hose was achieved by the se of electric panel heaters. The case of the otside air temperatre being higher than inside was not considered relevant as in warmer weather windows are more likely to be open. All data was collected sing a programmable datalogger. With the exception of wind speed, all parameters were scanned once every 1 seconds and then half-horly averages calclated and logged on magnetic tape. The wind speed recorder works on the 'plse cont' principle and therefore jst the half-horl1 average was logged. 59

3 - CIBSE NATIONAL CONFERENCE Analysis of data The data from each system was transferred from tape to compter disks and then read into a spreadsheet compter program. For each system, the data was sorted into for wind direction qadrants: , 136" - 5, 6-315, See Figre 1 for these directions with respect to the test hose. Initially, graphs were drawn of the stack velocity against temperatre difference, wind speed and wind direction, for typical reslts see Figres 3a - 3c. The stack velocity wold appear to be largely dependent on temperatre difference althogh not directly proportional to it, see Figre 3a. On examination of the figres plotted in Figre 3a it is fond that the stack velocity is in fact proportional to the sqare root of the temperatre difference, indicating trblent flow see Figre 4. To see what effect, if any, wind speed and direction had, the stack velocity was divided by the sqare root of the temperatre difference and then plotted again against wind speed and direction. At wind speeds below arond m/s the wind had very little effect bt had an increasing effect at higher wind speeds see Figre 5. The data splits into two trend lines above m/s, the higher set of data occrs at a particlar wind direction, this is possibly de to the configration of the PSV ridge terminal, dwelling roof and srronding terrain. Wind direction appears to have only a slight effect, see Figre 6, the high points arond - 3 degrees being those of the pper trend line in Figre 5. Many of the higher wind speeds recorded are from the soth west, this being the prevailing wind direction for the area and also the most open in relation to the test hose. Higher wind speeds from other directions did not case similar increases in stack velocity. Figre 7 shows the relationship between the sqare root of the temperatre difference and stack velocity for wind speeds less than m/ s. In order to compare the varios systems, regressions were calclated, sing the spreadsheet, of : 1) stack velocity wi th wind speed and sqare root of temperatre difference ) Stack velocity with sqare root of temperatre difference (for wind speeds less than m/s). The predicted mean stack velocities and flows from the first regressions for a temperatre difference of 1 degrees and wind speed of 4 m/s are shown, see Figres 8 and 9 and Table. TABLE - Comparison of velocities and flows for different systems. SYSTEM VELOCITY (m/s) FLOW (m 3 / hr) W 1 WO WO 3 WO 4 mean WO 1 WO W 3 WO 4 mean o The effect of ptting bends in the system can be seen by comparing systems 1 with, 3 with 4 and 5 with 6. The flow is redced by the bends, by as mch as 5%: especially ~ith the wind in the prevailing direction. By comparing systems 1 with 5 and with 6 the effect de to stack diameter can be seen: The velocities meas7ed in the smaller diameter stack are slightly higher than those for the larger diameter. This is the opposite of what might be expected and is probably a Reynolds No. effect. In terms of volme flow rate however, the flows 51

4 \, the smaller stack fo1 in the larger stack are arond twice those obtained 11 s The flow in the straight configration and 5% higher for one with b"'~. and the pressre l " ' straight dcts is approximately proportional to the ~i«~entry and exit los ~ ~ de to friction in the dct is of the same order as 11\ me configrations /\II i.e. the dct is very smooth. Systems 3 and 4 are th 4 sai id. The flexible systems 1 and bt are of flexible dcting instead '!«~ ~igres a and dcting appears to have little or no effect on flow, l the dcts is that th" reason for this, and indeed the apparent smoo~hness ' hs in the flid flow is at relatively low Reynolds nmbers Re<lO" an 1 ls independent of s1 f.' ' dynamically smooth region where f the friction. fact o roghness. nlclated, for a sing the regression analysis, air change rates wer~.,nt wind speeds. It c;i11 1 " temperatre difference of 1 degrees and three diff P. 1. l 1 a speed for stral 1111 seen that there is a trend for flow to increase witlt wf~r bent stack syst e m ~ stack systems (,4,6) and to decrease with wind spe~ 1 n of the design an<l 1 (1,3,5), see Figre 1. This is thoght to be a fn c ' ~ls nothing to do wi ll' position of the ridge terminal sed for these tests "'' 1 1 ch may verify this. 11 h ' ' t' i ' h d / I 11 the bends as sch. Frt er investiga ion s in an kitchen diner and thp ~ shold be noted that the room sed was a fairly lar4d ve been obtained in n change rates achieved were somewhat lower than wold 1 'n smaller room. I II lght first appear, P"' The analysis of these reslts is becase of the very low Reynolds calclating wind effects. A fll a frther paper. not as simple as it m e of the nmbers bt also b4'''~~s will be analysis of the re11 11.s difficlty " 1 the sbject " CONCLUSIONS 11 ror systems has been The performance of six different passive stack vent l ' monitored in a test hose. Reslts indicate that : d mch in the large 1, - Flow rate measred p the stack was roghly twice " t configration, as w"" diameter stack as in the smaller stack for the stral'-1 1 ' s are of the same,,,.t ' h d t h t d t J 1tllie be expected for a smoot c w ere en ry an exi Lth bends. as friction losses, and only 5% higher for the star: It w the flow rate by p 1 '' - Inclding two 45 degree bends in the stack can red 111 "" 5%. ~I compared - The apparent roghness of the flexible stack mater! indicating rigid material, has little or no effect on flow rat"' fact flid dynamically smooth.,,, with the A 11 ' that it ifi.. v~ntilation for the The PSV systems tested appear to work well, providiw1. logical conditions "' kitchen nder a range of conditions, for typical me L~ "'. air change rates or 4 m/s wind speed and temperatre difference of 1 d'1'j. <.flow rate this is n., 1,.. between.6 ach and.6 ach were obtained. In terms o rltchens bt it shold I '' mch as is recommended for mechanic-al ventilation or 11. ventilation only.,11.i 1 ' noted that the recom~ended figres are for inte7mit P : 11 the time. Anoth r ' the cas~ of the passive stack systems they are in n~ ~ooking in the kitch~'' factor is that the hotter the room becomes eg. de ' 1. te at times of mo s! showering in the bathroom,the higher will be the fl ow 1 need. ACKNOWLEGEMENT of the Bilding The work described forms part of the resea~ch program~nis pblished by Establishment of the epartment of the Environment an permission of the Chief Exective. Res111t' ' jl SYMBOLS USE ach - air changes per hor f - friction coefficient (non dimensional) W 1 wind direction deg. WO wind direction deg. WO 3 wind direction deg. W 4 - wind direction deg. 511

5 REFERENCES i. de Gids,W.F.,and den Oden,H.P.L.,1974,'Three investigations of the behavior of dcts for natral ventilation, in which an examination is made of 'the inflence of location and height of the otlet, of the bilt-p natre of the srrondings and of the form of the otlet'.!~ Klimaat ventilatie verwarming in woninggen en gebowen.elft,holland,ig-tno.(in tch). Johnson,K.A.,and Pitts,G.,198,'Experiments with a passive ventilation system',proceedings 3rd AIC Conference,London. 3, Johnson,K.A.,Gaze,A.I.,and Brown,.M.,1985,'A passive ventilation system nder trial in U.K. homes',proceedings 6th AIC Conference,Netherlands. 4. Edwards,R.E.,and Irwin,C.,1988,'Frther stdies of passive ventilation systems - Assessment of design and performance criteria',proceedings 9th AIVC Conference,Gent,Belgim. s. Stephen,R.K.,and Uglow,C.E.,1989,'Passive stack ventilation in dwellings', Bilding Research Establishment Information Paper,IP 1/89. 51

6 -- CIBSE NATIONAL CONFERENCE Test hose x direction Wind Z 135 Anemometer mast Figre 1 - Test hose and srronding area dct temperatre.::=.:..-_-_-.:.::_ -== pitot-static tbes dct velocity and temperatre Figre - stack configrations and instrmentation 513

7 1.9 1.e , 1.3 '--. E 1. >- ~ 1., v Ci >.9.:s v..., o.e Vl , 4 e Temperatre difference 4 Figre 3a - Initial reslts (System 5) , ,._ 1.3 "' E, ~ >.9 - ".8 Vi O 9 J q5l Wind speed (m/s) Figre 3b - Initial reslts (System 5) 514

8 '-. E a lil ~ >.9 ~.8 Vi.7 Cl Wind direction O Figre 3c - Initial reslts (System 5) '-. "' E 1. ]:" Qi >.9 - "'.8 B (/).7.6 IOI mo o.5.4 ~B~..3 '. I.1 I 4 6 Sqare r-oot of temperatr-e difference! Figre 4 - Stack velocity vs. Sqare root temp. difference (System 5) 515

9 .9 a.a.7 <l Cl i cp CJ... Cb cr- I/).6 '-... ~.5 Qi >.4.x CJ v;.3 B CJ Wind speed (m/s) Figre 5 - Effect of wind speed (System 5) <l t'.., O" '-... Ci > CJ~.x iii.3 O Wind direction Figre 6 - Effect of wind direction (System 5)

10 , E 1. ~ 1. 1 v Qi >.9.Y. v.8 +' VJ Cl a ~ Q 4 6 Sqore root of temperatre differen c: e Figre 7 - Effect of temperatre difference at low wind speeds (System 5) "' E.9 ;:;. v.8 QJ >.7 - " v.s.6 "' c QJ E SYS1 SYS SYS3 SYS4 SYSS SYS6 Figre 8 - Comparison of stack velocities 517

11 8 7 6 L...i:; :: E 3: -~ 4 B I/) c 3 Q) E 1 SYS1 SYS SYS3 SYS4 SYS5 SYS6 Figre 9 - Comparison of stack flows I I LO. ~ ~,.8 L.. Q) O' 1.6 c,.4.i:;... c; 1. c Q) E O.B 3.4. SYS1 SYS SYS3 SYS4 SYSS SYS6 ~ O m/s ~ 4 m/s ~ 8 m/s Figre 1 - Comparison of air change rates for different wind speeds 518