1、 - STD-BSI BS EN IS0 8990-ENGL L79b Lb24bb9 05750bb 302 BS EN IS0 8990 : 1996 BRITISH STANDARD Thermal insulation - Determination of steady-state thermal transmission properties - Calibrated and guarded hot box The European Standard EN IS0 8990 : i996 has the status of a British Standard ICs 27.220
2、NO COPYING WITHOIJT BSI PERMISSION EXCEPT AS PERMITTED BY COPYRIGHT LAW STD-BSI BS EN IS0 8990-ENGL L99b m Lb24bb9 05750b7 249 m BS EN IS0 8990 : 1996 Committees responsible for this British Standard The preparation of this British Standard was entrusted to Technical Committee RHE/s, Thermal insulat
3、ing materiais, upon which the foilowing bodies were represent The temperature nonuniformity close to the me- tering box, on the specimen surface, and in the air, respectively, define the corresponding best imbalance resolution. The apparatus shall be designed and operated in such a way as to obtain
4、optimum heat flow balance as in- dicated in a) above, .e. apparatus geometry and guard air space and air flow speed so that O3 does not ex- ceed 10 % of +,. Inhomogeneities in the specimen will enhance non- uniformities in local surface coefficients and in speci- men surfacetemperatures. Heat flow i
5、mbalance through the metering box wall and in the specimen shall be evaluated, and when necessary corrected for. For this purpose the metering box walls shall be equipped to serve as a heat flowmeter. Additionally, a thermopile across the metering area periphery can be mounted on the specimen surfac
6、es. In routine testing, imbalance detection can be simplified by cali- bration and calculation. 1.6.1.2 Size of metered area The metering area is defined: a) for a guarded hot box, as the centre-nose to centre-nose when the specimen is thicker or equal to the nose width, or if the specimen is thinne
7、r than the nose width, as the inner periph- ery of the nose; b) for a calibrated hot box, as the inner periphery of the metering box. The size of the metered area determines the maxi- mum thickness of the specimen. The ratios of the metering area side to the specimen thickness and of the guard width
8、 to the specimen thickness are gov- erned by principles similar to those for the guarded hot box. The size of the specimen can also limit possibilities for a representative section of the construction to be tested and thus allow errors and difficulties in inter- pretation of the result. Measurement
9、errors in testing to the hot box meth- ods are in part proportional to the length of the per- imeter of the metering area. The relative influence of this diminishes as metering area is increased. In the guarded hot box, the minimum size of the metered area is 3 times specimen thickness or 1 m x 1 m,
10、 whichever is the greater. STD-BSI BS EN IS0 8770-ENGL L77b 3b24bb7 057507b 251 Hot side Specimen - Page 7 EN IS0 8990 : 1996 i I i Coldside III III III II 111 I I I I II Metering box - 03 c Figure 2 - Calibrated hot box -Cold box - Specimen Isothermal Heat flow - Figure 3 - Heat flow path in specim
11、en and frame STD-BSI BS EN IS0 8770-ENGL 1996 9 1624669 0575077 198 Page 8 EN IS0 8990 : 1996 For the calibrated hot box, minimum specimen size is 1,5 m x 1,5 m. The perimeter error in the guarded hot box is due to the heat flow rate, *, along the surface of the specimen, due to imbalance between me
12、tering and guarded area, or by inhomogeneities. The perimeter errors in the calibrated hot box are due to the flanking heat flow, Q4, which includes the distortion of the heat flow rate at the edges of the specimen. 1.6.1.3 Minimum power input Total power input, d+, to the metering box is the sum of
13、 the power supplied to heaters, fans, transducers, actuators, etc. Some of these cannot be reduced to zero thus defining a minimum heat flow which has to pass through the specimen. This limit can be lowered by cooling the hot chamber, but that will cause further uncertainty connected with the measur
14、ement accuracy of the cooling rate. The minimum power is also limited by the uncertainty of total power input to the metering box including All the above factors set a lower limit for the ratio 3. ( this will affect the heat transfer mechanism of the surface. In the case of the guarded hot box decre
15、asing the specimen resistance, this imposes stricter requirements on the equivalence of convective and radiative heat transfer in the me tering and guard box to obtain a given accuracy. 1.6.2 Limitations and errors due to specimen 1.6.2.1 Specimen thickness and thermal resistance For a given apparat
16、us design, specimen thickness can be limited for reasons depending upon specimen properties and boundary conditions, an upper limit for the thickness is due to edge losses, or flanking losses, ad, which, although decreasing with increas- ing specimen thickness, can become significant in comparison t
17、o a, and degrade measurement accu- racy. 1.6.2.2 Specimen inhomogeneity Most test specimens representative of building and industrial components will generally be inhomogene- ous. Inhomogeneities in the test specimen will affect the pattern of the density of heat flowrate in such a manner that it is
18、 neither one-dimensional nor uniform. Also variations of the thickness throughout the speci- men can cause significant local modifications of the pattern of the density of heat flowrate. The effects of these are nonuniformities in temperatures and local transfer coefficients making the following mor
19、e diffi- cult or even impossible: a) the definition of a mean surface temperature; b) the detection of imbalance in the guarded hot box apparatus; c) the definition of the metering area; d) the error analysis of test results for a given in- homogeneous specimen. Specific examples include: a) facings
20、 having a high thermal conductivity. These form easy paths for imbalance heat flow rate, Q2, and flanking heat losses, 04. It can help to cut the facing along the metering box periphery. When layers are homogeneous, an alternative solution is to run independent tests on each layer with test methods
21、using a guarded hot plate or a heat flow meter; b) horizontal and vertical structurai members like studs. Their effect is in most cases symmetrical; c) sections of the specimen made of different ma- terials. The temperature differences through the materials are not the same. A heat flow exists close
22、 to the interface of the different materials. When this interface is not far from the metering box periphery, this implies a temperature nonuni- formity that affects both imbalance detection and the ambiguity in the definition of the metering area. Also, local heat transfer coefficients are af- fect
23、ed by these inhomogeneities; d) cavities within the specimen. Natural convection can create an unknown imbalance heat flow rate, 02. The effect of installing barriers shall be evalu- ated. It is not possible to provide immediate solutions to all types of problems. The operator is advised to be fully
24、 aware of te effects of anomalies. STD-BSI BS EN IS0 8990-ENGL Lb Calculations of the importance and effects if inhomo- geneities are of great help to predict the thermal per- formance of the test specimen. If significant differences exist between predicted and measured specimen performance which ca
25、nnot be explained, as a minimum requirement, where such divergences exist, a careful inspection of the specimen should be performed to identify any difference between actual and specified sizes, dimensions, materials, etc. Any irregularities from the original specification shall be reported. 1.6.2.3
26、 Moisture content in specimen Moisture transfer during the test may have a signif- icant effect on test results. It is not possible to specify a standard pretest conditioning. As a minimum re Lb24bb7 0575078 O24 Page 9 EN IS0 8990 : 1996 quirement, the method of conditioning shall be re- ported. For
27、 most specimens, it is normally impossible, without derating measurement accuracy to an un- acceptable level, to reduce temperature differences so much that moisture transfer is so slow that steady-state mass transfer can be assumed during measurement time. It should also be realized that not only m
28、oisture transfer through the specimen, but also moisture redistribution in the specimen and phase change, will affect the results. 1.6.2.4 Temperature correlation Specimen thermal resistances or thermal transmit- tances are often a function of temperature differences across the specimen itself. Care
29、 shall then be taken in reporting and interpreting measurement results. STD-BSI BS EN IS0 8990-ENGL L79b Lb2LtbbS 0575079 Tb0 Page 10 EN IS0 8990 : 1996 Section 2: Apparatus 2.1 Introduction As stated in 1.1, it is impractical to impose specific design details for an apparatus. However, this section
30、 gives mandatory requirements and the aspects which must be considered. Figures 1 and 2 show typical arrangements of the test specimen and major elements of the apparatus. Fig- ures 4 and 5 show alternative arrangements. Other arrangements, accomplishing the same purpose, may be used. The effect on
31、the heat transfer through the specimen of the box walls in figure 1 and of the frame in figure2 depends upon the wall or frame shape and material, upon the specimen thickness and resistance and such test conditions as temperature differences and air velocities. The apparatus design and con- structio
32、n should be made compatible with the ex- pected types of specimen to be tested and expected testing conditions. 2.2 Design requirements The metered area shall be big enough to provide a representative test area. For modular components the metered area should preferably span exactly an inte gral numb
33、er of modules. The ratio of metered area to perimeter of the metered area influences accuracy in both types of boxes be- cause onedimensional heat flow cannot be main- tained at the perimeter of the metered area. These error heat flows at the perimeter of the metered area, measured as a fraction of
34、the metered heat flow, will increase with decreasing metered area. Imbalance heat flow, e2, in the guarded hot box is due to nonuniformities both in surface coefficients and air temperatures close to the periphery of the metered area. An amount of heat enters the specimen through the nose of the met
35、ering box in the guarded hot box. Deviation from onedimensional heat flow is caused by the finite thickness of the nose seal. Both edge insulation and edge boundary conditions affect peripheral losses, U they shall be shielded by insulated reflective shields to minimize radiation to metering box wal
36、ls and the specimen. It is recommended that a baffle be positioned in the metering box, parallel to the surface of the specimen when forced convection is used. The baffle should extend to the full width of the metering box and have gaps at each end to allow air circulation. The baffle may be moveabl
37、e, perpendicular to its surface, to aid in adjusting the air velocity parallel to its surface. When natural convection is used, a baffle may be necessary to shield specimen surfaces from radiative heat transfer of heaters. The considerations in 2.2 regarding emissivity of sur- faces also apply to th
38、e baffle. When testing in a vertical position, the circulation re- sulting from natural convection can be sufficient to ensure temperature uniformity and the desired sur- face coefficients. When air movement is due to natu- ral convection, the distance between specimen and baffle should be larger th
39、an the boundary layer thick- ness, or no baffle should be used. When it is im- possible to achieve the desired conditions with natural convection, circulating fans should be installed. If the fan motors are installed inside the metering box, then their power consumption shall be measured and added t
40、o the consumption of the heaters. If oniy the fans are inside the metering box, the shaft power shall be determined and added to the heater power: this shall be done with an accuracy such that the error on specimen heat flow is less than 0,5 %. 2.4 Guard box In the guarded hot box the metering box i
41、s placed in- side a guard box. The purpose of this guard box is to establish such air temperature and surface coeffi- cients around the metering box that heat flow through the metering box walls, U+, and imbalance heat flow, Co, in the surface of the specimen from metered to guard area is minimized.
42、 The relationship between the metering area size and the guard area size and edge insulation shall be such Page 12 EN IS0 8990 : 1996 that when testing a homogeneous specimen of maximum expected resistance and thickness, the predicted error on specimen heat flow caused by peripheral heat loss, Os, s
43、hall be smaller than 0.5 % of the metered heat flow, O,. A procedure to quantify this error can be found in IS0 8302. The requirements concerning emissivity, shielding of heaters, and temperature stability are in principle the same for the guard box as for the metering box. Temperature uniformity sh
44、all be such that the influ- ence on imbalance error will be smaller than 0,5 % of the heat flow through the metered area of the specimen. Circulating fan(s) will normally be needed to avoid stagnant air in the guard box. 2.5 Specimen frame In the calibrated hot box, the specimen frame is a critical
45、component due to the flanking losses which for the sake of accuracy should be kept at a minimum. There is a compromise between loadcarrying capac- ity, e.g. support of the specimen, and high thermal resistance. The facing towards the specimen should have low thermal transmission. In the typical conf
46、iguration of the guarded hot box the specimen frame is omitted and lateral heat flow is minimized by edge insulation. If, however, a speci- men frame is used it shall minimize lateral heat flow, as required in 2.4. 2.6 Cold side chamber The size of cold side chamber is governed by the size of the me
47、tering box in the case of the calibrated hot box, or the guard box in the case of the guarded hot box; arrangements may be as illustrated in figures 1, 2, 4 and 5. The chamber walls should be constructed to reduce the load of the refrigeration equipment and prevent moisture condensation. The inside
48、surfaces of the chamber shall have an emittance in accordance with the desired radiative heat exchange. The require- ments concerning emittance, shielding of heaters, temperature stability and temperature uniformity are in principle the same as for the metering box. For fine tuning of the cold side
49、temperature, electric resistance heaters in the outlet from the evaporator are often useful. As mentioned under the metering box, a baffle may also be advantageous to achieve uniform air distribution. Air flow direction correspond- ing to natural convection is suggested. Motors, fans, evaporators and heaters shall be radiation-shielded. Air velocities should be adjustable to meet the re quired surface coefficients of the test and should be measured. In simulating natural conditions for building components, the range can be from 0,l m/s to 10 m/s. 2.7 Temperature measurements
