The assessment of freezing risk in apartment buildings after heat supply break

Transcription

1 The assessment of freezing risk in apartment buildings after heat supply break Jurate Karbauskaite, dr., assoc. Professor, Kaunas Technology University, Institute of Architecture and Construction, Vytautas Stankevicius, hab. dr., Professor, Kaunas Technology University, Institute of Architecture and Construction, Arunas Burlingis, dr., Kaunas Technology University, Institute of Architecture and Construction, Romaldas Morkvenas, M.Sc. Kaunas Technology University, Faculty of Construction and Architecture, KEYWORDS: air tightness, thermal resistance, indoor/outdoor air temperature, time period, freezing risk SUMMARY The economical casualties could be of tremendous value with the restoration of the pipelines themselves, tenant relocation, refurbishing of apartments and other additional works if the heat supply break in the district heating pipelines happens because of the severe outdoor temperature. The feasibilities of the freezing in heating system of existing apartment buildings at various duration of the heat supply break in regard to building envelope characteristics and outdoor temperature, as well as the danger of freezing in main heat supply pipes in the building are analyzed with the purpose to avoid or decrease the level of possible losses. The special calculation method is composed for the assessment according to outdoor infiltration level, thermal resistance and heat capacity of the envelope and heat supply pipelines in the building. In regard to the removal experience at accomplished heat supply breaks and with purpose to decrease risk of the future casualties, the limit period of chilling down to 0 C in the spaces of apartment buildings and in the main pipelines of the heating system is assessed due to thermal capacity level, thermal resistance of the building envelope elements, outdoor air infiltration level, insulation type of the pipelines at calculated outdoor temperature (-15, -20, -25, -30 C) during the heat supply break. Temperature descent to 0 ºC in corner rooms of apartment buildings at average quality of building structures is reliable to be h at outdoor air temperature ºC, for leaky rooms h, in basement premises the water in pipelines of heating system could freeze in even shorter period. The basic recommendations for safety measures implementation are suggested. 1. Introduction The economical casualties could be of tremendous value with the restoration of the pipelines themselves, tenant relocation and other additional works if the heat supply break in the district heating pipelines happens because of the severe outdoor temperature. The rough evaluation of indoor temperature change after the heat supply break in a big apartment building of mass construction type showed more than 3 days term of freezing period down to 0 C, which could be estimated enough satisfactorily for heat supply restoration. The sequence analysis of the real big accident in district heating heat supply pipelines in Lithuania indicated quite low time periods, during which water in building heating system was frozen. It took in from 12 to 18 hours in typical buildings. The investigation of the reasons has been carried out with the purpose to highlight the effect of the building quality on the heat storage. The feasibilities of the freezing in heating system of existing apartment buildings at the heat supply break in regard to building envelope thermal characteristics, air tightness and outdoor temperature, as

2 well as the danger of freezing in main heat supply pipes in the building are analyzed. The estimation of sequences at such heat supply break in new buildings and buildings after additional insulation is included into research volume. 2. Method of assessment and the main simulation schemes The corner rooms in side apartments of ground and topmost floor are recognized to be mostly dangerous places for freezing according to the initial consideration. Thus values of indoor temperature, temperature on inside surface of the external wall and in the middle of heating radiator have been calculated for a corner room located in the ground floor, topmost floor and in the middle floor. Room space of 3x5 m, at height 2,5 m with two external walls and two internal partitions has been selected. Window area is estimated to be 18 % of floor area. Radiator size is selected according to the heat loss calculation. The following heat flows have been estimated at the simulation of the freezing process: heat flow through the external building envelope elements (wall, roof and window and ceiling over basement respectively), heat amount accumulated in the external envelope elements and internal partitions, heat amount accumulated in the furniture and internal equipment, heat flow due to outdoor air infiltration, heat flow due to internal heat gains and heat gained from heating device (radiator) and heating system pipeline. The heat gained from insulated heating system pipeline was included at the simulation of the thermal behaviour in the premise located in basement as well as the heat flows through the walls and floor in contact with ground. The heat flow has been calculated for the least time period z of the structure layers in accordance with the essential requirement obtained from equation in detail discussed by [2],[3],[11]: a z < 0,5 d l. 2, (1) where: d l thickness of material layer, m; a temperature conductance of a material, m 2 /h, determined: a = 3,6 λ l. / (ρ l. c l. ) (2) and: λ l - thermal conductivity of a material layer, W/(mK), ρ l - density of material layer, kg/m 3 ; c l specific heat of material layer, kj/(kgk). The building element materials are divided into smaller imaginary layers according to limit indicated by (1) equation at the selected time period. Temperature on the border of layer at the certain time is calculated according to equation (3) in dependence if the both calculated layers are of the same material, if not, the equation (4) is used: θ i,j = θ i,j-1 +[(θ i-1,j -θ i,j-1 )/ R i (θ i,j-1 -θ i+1,j-1 )/ R i )] 3600 z/(d i ρ i c i ); (3) θ i,j = θ i,j-1 +[(θ i-1,j -θ i,j-1 )/ R i (θ i,j-1 -θ i+1,j-1 )/ R i )] z/(d i ρ i c i +d i-1 ρ i-1 c i-1 ); (4) θ i,j - temperature of i-th layer at time moment j; θ i,j-1 temperature of i-th layer at the time moment (j-1), that means time step before; θ i-1,j temperature of (i-1)-th layer at time moment j; θ i+1,j-1 temperature of (i+1)-th layer at time moment (j-1) that means temperature of next layer time step before; R i thermal resistance of calculated layer i; c i heat capacity of calculated layer i; ρ i density of calculated layer i. Initial temperatures of the layers at the beginning of calculation are determined according to constant condition terms indoor and outdoor air temperature and standard surface thermal resistance values. Heat amount which is transferred through a building element during time period z is determined according the equation [2],[3],[5]: Q j = A at (θ air,j - θ si,j ) z/r vp ; (5)

3 where: A el area of a considered building element, m 2 ; θ air,j indoor air temperature at the time moment i; θ si,j temperature on internal surface of building element at the time moment i; R vp corrected value of inner surface thermal resistance, in dependence to the temperature difference of air and surface. The heat accumulation by furniture and indoor equipment is presumed to have the same shape as in inner partition of a certain area. It is assumed, that equivalent area is equal to 5 m 2. Heat flow due to internal heat sources is estimated as a linear dependence due to floor area. The value could be assumed to be 0 15 W/m 2 in regard to the space destination. The value of 5 W/m 2 is taken as default. The total heat amount is calculated from the heat flow balance in the simulated room at the considered time moment. The equations system for calculation has been laid down in the spreadsheets MS Excell. The relations between the quantities included into calculation of building element with thermal mass are described in the Fig.1. Similar system is applied for every building element of the simulated room model. FIG. 1: The calculation graph for determination of heat transfer through the external wall The freezing period is determined according to the time period when pursued indoor air temperature value should be equal to 0 C. The freezing period estimated by internal surface temperature value is determined also, as well as water temperature drop period in the middle of radiator and pipeline. Outdoor air temperature during calculation was assumed to be constant. The values have been selected under following consideration: -30 C average temperature of coldest day during the recent 30 years, -25 C average temperature of coldest 3 days during the recent 30 years, probability 0,98, -20 C average temperature of coldest 5 days during the recent 30 years, probability 0,92, -15 C average temperature of coldest 10 days during the recent 30 years, -10 C average temperature of coldest month during the recent 30 years. Time step in the calculation then is assumed 0,03 h as the least value along the modeled room building elements. The thermal parameters of apartment building elements used in the calculations are indicated in Table 1. The data obtained during long-time experience by Institute of Architecture and Construction has been used for estimation of average thermal resistance values of building elements in dependence to the type of apartment building for old apartment buildings [1, 7, 9, 10]. The value of a considered building element thermal resistance for new buildings is derived according to the requirements of recent National Building Code [8] for the heat transfer coefficient of building elements. Most popular modern structure types are selected. Airtightnes in accordance with the investigations of [4, 6, 9] expressed by air change rate was assumed to be estimated from 0,2 changes per hour as minimal value for new modern windows, 0,5 1/h as normal rate for new windows

4 including ventilation, 0,7 1/h as average rate for old windows of good quality, up to 0,9 1/h as characteristic for bad old shape windows. The calculation was provided for a corner room in the top floor, the middle and ground floor with the set of building elements appropriate to the certain type of apartment building. The building elements included in calculation for every variant are indicated in Table 2, with the step-by step spread of calculation volume. TABLE. 1: Thermal parameters of mass construction apartment building elements Description of building element External walls External walls of expanded clay concrete boards External walls of ceramic brick masonry External walls with additional insulation New construction, three layered Roofs Old shape, bad quality Old shape, satisfactorily quality Renewed with additional insulation New Ceilings Thickness, mm Name Material Density, kg/m 3 Specific heat, kj/(kg K) Thermal resistance, m 2 K/W 450 1, ,94 expanded clay concrete 350 0, , ceramic brick masonry , Old shape, over basement 200 New, three layered over basement ceramic brick masonry + mineral wool layer ceramic brick masonry + mineral wool + ceramic brick masonry + cellular concrete + cellular concrete + cellular concrete + mineral wool layer+ + mineral wool layer+ + linoleum + expanded polystyrene foam + equalizing layer ,5 4,6 1,2 1,8 6,0 6,0 0, Windows Old shape, bad quality - - 2,7 Old shape, satisfactorily quality 2,4 New - - 1,6 3. Results and discussion The least time period of temperature descent down to 0 C could be observed on the internal surface of the external wall, it is 1-2 h less than the time period of indoor air temperature descent for the almost all simulated variants. The time period for water temperature descent in heating pipeline is insignificantly less (0,1 0,2 h) than in radiator itself. The time period of indoor air temperature descent is in all cases less than for water in radiator 1,34 2,66

5 by 1-1,5 h. In apartment buildings with external walls of the expanded clay concrete boards indoor air temperature is falling down to the 0 C throughout h at the outdoor air temperature of -25 C in dependence to the outdoor air infiltration rate and room location when thermal resistance value external wall is near 1,0 m 2 K/W. The effect of internal heat gains of default value is about 4 6 h. If the thermal resistance value of the external wall is near 0,75 m 2 K/W (4 row in Table 1, met in buildings of mass construction of year), the time period of indoor air temperature descent is decreased up to h, and the time period of temperature descent temperature on internal surface of the external wall is h, the time for restoration of heat supply is decreased approximately by 30 %. The time period could be expected near 6 h, according to the calculation results if the outdoor temperature level is about -30 C. The emergency heating must be provided in buildings of this type without considerations, if heat supply break happens at outdoor temperatures lower -20 C. TABLE. 2:Calculation elements for estimation of freezing danger in apartment buildings Model location Top floor Middle floor Ground floor Basement Building elements partitions, roof, window partitions, window partitions, ceiling over basement, window partitions, ceiling over basement, window, wall in contact with ground Calculation elements included Heating device Internal gains, Outdoor air W/m 2 infiltration rate Radiator and heating system pipeline Radiator and heating system pipeline Radiator and heating system pipeline Heating system pipeline The time period of indoor air temperature descent to the 0 C in brick masonry buildings in dependence to the air infiltration rate is h, and for temperature on internal wall surface is h at the outdoor air temperature of -25 C. In comparison with the values for apartment buildings of expanded clay concrete walls the values are % better. The time period of temperature descent to 0 C in the radiator estimated according to the simulation terms is close to assumption of radiator location in the middle of the room space. In reality, the cold air flow coming through the window and falling down would additionally chill the radiator which is located underneath the window. Thus it seems to be more correct to assess the danger of freezing according to the temperature descent on the internal surface of the external wall. Analysis of the results in relation to the floor location of the considered room has revealed the following dependence: at the outdoor temperature level of o C, the least temperature descent is found in the room at the top floor, and at the outdoor temperature of o C - at the ground floor. The generalized results according to least values are presented in the Fig. 2. In the summary the danger of freezing could arise after 12 h in the apartment building with walls of expanded clay concrete boards, when the thermal quality of structures is good (R 1,0 m 2 K/W, air change < 0,7 ) at outdoor temperature -30 o C. If thermal quality is bad ((R 0,7 m 2 K/W, air change 0,9 ), danger of freezing could arise approximately in 6 h from the heat supply break. The apartment in the middle floor of building when quality is recognized to be bad could be estimated in same range as the apartment in ground floor at good quality due to danger of freezing. So, this type of buildings could be assumed as very unsafe for heat supply breaks. The danger of freezing could arise after 13 h in the masonry apartment building when the thermal quality of structures is good (R 1,0 m 2 K/W, air change < 0,7 ) at outdoor temperature -30 o C, and accordingly in buildings of bad quality it could arise in 11 h. In the renewed buildings with additional wall insulation at the auspicious circumstances the danger of freeze could arise in 19 h, at outdoor temperature -30 o C. The temperature descent is less than expected because of uninsulated ceiling over basement (additional insulation here is very complicated to be installed and expensive). Danger value of freezing in new buildings under recent requirements for thermal protection could be estimated near 38 h, that is, time for heat supply restoration is 3 times longer, than for the buildings of old shape.

6 The increase of outdoor air infiltration air rate from 0,5 up to 0,9 times per hour could reduce the temperature descent period by %, and the influence is bigger if thermal resistance of the building enclosure is better. Least descent to 0, o C, time bad quality, expanded clay conclete walls bad quality, brick masonry walls good quality, expanded clay conclete walls good quality, brick masonry walls renovated new 10 Outdoor temperature, o C FIG.2: Dependence of temperature descent time to 0 C due to outdoor temperature at different quality of apartment building structure (bad quality, expanded clay concrete walls: R wall 0,75 m 2 K/W, R rof 1,2 m 2 K/W, R ceiling over basement 0,44 m 2 K/W, air change 0,9 times per hour, good quality, expanded clay concrete walls: R wall 1,0 m 2 K/W, R rof 1,8 m 2 K/W, R ceiling over basement 0,44 m 2 K/W, air change 0,5 times per hour, bad quality, brick masonry walls: R wall 1,0 m 2 K/W, R rof 1,2 m 2 K/W, R ceiling over basement 0,44 m 2 K/W, air change 0,9 times per hour, good quality, brick masonry walls: R wall 1,0 m 2 K/W, R rof 1,8 m 2 K/W, R ceiling over basement 0,44 m 2 K/W, air change 0,5 times per hour, renovated: R wall 3,5 m 2 K/W, R rof 6,0 m 2 K/W, R ceiling over basement 0,5 m 2 K/W, air change 0,5 times per hour, new: R wall 4,6 m 2 K/W, R rof 6,0 m 2 K/W, R ceiling over basement 2,66 m 2 K/W, air change 0,5 times per hour) The danger assessment of freezing could be determined according to the temperature descent on internal surface of the wall. Calculation of any other element could be omitted. The change of outdoor air infiltration rate and heat capacity of the building elements have the biggest influence on the temperature descent in a heated space at the same thermal protection level. The temperature descent of water down to 0 ºC in the main pipeline of heating system located in the basement depend upon the outdoor air infiltration rate and pipeline insulation level. The least value is obtained for wall above the ground level because of the low thermal resistance of this building element. The thermal parameters of the space in the ground floor do not show significant effect on the thermal behavior in the basement premises because of big thermal capacity here. And, in opposite, the conditions in the basement premises shall have influence on thermal behavior in heated space of ground floor. The least value of indoor air temperature descent down to 0 ºC is 10 hours, for internal surface of wall above the ground is 3 hours, for wall surface in contact with ground is 30 hours at the outdoor air temperature -25 ºC. The value for water temperature descent in insulated pipeline is 10 hours. The results are close with the freezing time values presented in the report of the real accident check-up. Usually the pipelines are laid along the side wall above the window top within the building envelope. Taking into consideration this detail, the water freezing in the pipelines could occur even faster in separate places, where the windows are leaky. In basements of renovated apartment buildings the danger of freezing is delayed by 3 hours approximately in comparison to the buildings without renovation, as the basements usually are not refurbished at all. New buildings: the temperature descent to 0 ºC for the indoor air is close to 28 hours, on internal surface of external wall above the ground 18 h approximately, on the internal surface of wall in contact with ground 39 h at outdoor air temperature of -25 ºC. The danger of freezing in pipelines is assessed as 30 h.

7 The temperature descent is more deliberated if the heating from other heat source is provided. The possible heat flow value has been estimated to be near 20 W/m 2, if the local electric heating devices are applied. The capacity of such heating is restricted by the power of old electricity supply net. Then time period could be prolonged from 3 to 6 hours in respect to external air infiltration rate and outdoor air temperature for the apartment buildings with expanded clay concrete walls when the thermal resistance value 0,9 m 2 K/W, and 2-4 hours when the thermal resistance value 0,75 m 2 K/W. The same measure could delay this period by 4 to 8 hours in dependence to outdoor air temperature and infiltration rate. If the heat supply from independent source is begun after the 6 hours break, the considered impact is not significant. In apartment buildings with walls of expanded clay concrete the time period is almost the same as in case of 5 W/m 2 default value of internal heat gains. In brick masonry apartment buildings the increase is assessed by 2 up to 6 h in regard to outdoor air temperature and infiltration rate. The windows and doors are recommended close carefully and all the air leak sources seal immediately after sudden heat supply break. The all possible heat sources shall be switched on. If the additional heating is not provided immediately in the apartment buildings with the walls of expanded clay concrete, the danger of freezing shall occur in really short time period, otherwise the additional heat flow in apartments should be increased up to W/m 2. Then electricity supply power could be insufficient. 4. Conclusions 1. Temperature descent to 0 ºC in corner rooms of apartment buildings at average quality of building structures is reliable to be h at outdoor air temperature ºC, for leaky rooms h, in basement premises the water in pipelines of heating system could freze in even shorter period. 2. The least temperature descent is obtained for rooms in the top floor when flat roof is of old shape, a little larger time period is determined for rooms in the ground floor, the largest value is determined for middle floor. 3. Temperature descent in basement premises is not significantly dependent on the thermal behavior in the ground floor, the outdoor air infiltration has the biggest impact. The danger of freezing in pipelines could occur in shorter period than in 12 h, if the outdoor temperature will be lower than the design outdoor level. 4. The thermal characteristics of windows and doors in staircases, basements or other premises of common use are dependent on the maintenance level especialy. The danger of freezing at heat supply break is increased when the maintenance is insufficient and the windows and doors are untight. 5. The municipal authorities should pay especial attention to the development of measures to be fulfilled at the heat supply breaks for the reduction of the possible damages. 5. References Bliūdžius R and Stankevičius V. (2001). Guide of building thermal physics, Technologija, Kaunas, Lithuania, p.90. (in Lithuanian) Bogoslovsky V. (1970). Building thermal physics, Strojizdat, Moscow, Rusia, p. 375 (in Russian) Fokin K.(1973), Building thermal physics, Strojizdat, Moscow, Rusia, p. 288 (in Russian) Juodis E. (2000). Energy Saving and Airtightness of Blocks of Flat in Lithuania, Indoor + Built Environment, 9/3-4/00, Basel,May August 2000, Juodvalkis J., Blaževičius E.and Vipartas R. A. (2005). The possibilities of the heat loss minimization in buildings, Proceedings of 8-th REHVA World Congress, 9-12 Oct., Lausanne, Switzerland. Jurelionis A and Karbauskaitė J. (2005). The indoor air quality in renovated Lithuanian school buildings Energetika, Vilnius, Lithuania, ISSN , No 4, National Building Code SNIP II-3-79** Building thermal physics, Gosstroi SSSR, Moscow, Russia, p.32 National Building Code STR :2005.Thermal technique of building envelope, Ministry of environment, Vilnius, Lithuania, p.133 (in Lithuanian) Stankevičius V., Karbauskaitė J.and Bliūdžius R. (2002). Analysis of heat consumption in apartment buildings Energetika, Vilnius, Lithuania, ISSN , No 2, Stankevičius V.and Karbauskaitė J. (2000). The conformance of real heat consumption in apartment buildings to the designed heat supply demand, Journal of civil engineering and management, Vilnius, Lithuania, ISSN , vol.6, N0. 5, Ulgen K.(2002). Experimental and theoretical investigation of effects of wall s thermophysical properties on time lag and decrement factor, Energy and Buildings, vol.34,