Question

summatize the following info and break them into differeng key points. write them in yojr own words

 

apartus

6.1 Introduction—The design of a successful hot box appa- ratus is influenced by many factors. Before beginning the design of an apparatus meeting this standard, the designer shall review the discussion on the limitations and accuracy, Section 13, discussions of the energy flows in a hot box, Annex A2, the metering box wall loss flow, Annex A3, and flanking loss, Annex A4. This, hopefully, will provide the designer with an appreciation of the required technical design considerations.

6.2 Definition of Location and Areas—The major compo- nents of a hot box apparatus are (1) the metering chamber on one side of the specimen; (2) the climatic chamber on the other; (3) the specimen frame providing specimen support and perimeter insulation; and (4) the surrounding ambient space. These elements shall be designed as a system to provide the desired air temperature, air velocity, and radiation conditions

C1363 − 11

6

 

for the test and to accurately measure the resulting net heat transfer. A diagram of the relative arrangement of those spaces is shown in Fig. 1.

6.2.1 The basic hot box apparatus has been assembled in a wide variation of sizes, orientations and designs. Two configu- rations have been historically used for a majority of the designs. The first is the self-masking hot box which has a controlled “guard” chamber surrounding the metering box. An example of this configuration is presented in Fig. 2.

6.2.2 The second configuration is the masked hot box. This configuration can also be considered as a special case of the guarded hot box in which the surrounding ambient is used as the guard chamber. An additional design consideration for the masked hot box design is that the metering chamber walls shall have sufficient thermal resistance to reduce the metering box wall loss to an acceptable level. The masked design is generally used for testing of large specimens. Fig. 3 shows an example of a masked apparatus for horizontal heat transfer.

NOTE 6—The two opposing chambers or boxes are identified as the metering chamber and the climatic chamber. In the usual arrangement, the temperature of the metering chamber is greater than that of the climatic chamber and the common designations of “hot side” and “cold side” apply. In some apparatus, either direction of heat flow may apply.

6.3 Apparatus Size—The overall apparatus shall be sized to match the type of specimens anticipated for testing (see 7.2). For building assemblies, it shall accommodate representative sections. Generally, the maximum accuracy is obtained when the specimen size matches that of the metering chamber while the climatic chamber also matches or is larger.

NOTE 7—A large apparatus is desirable in order to minimize perimeter effects in relation to the metered area, but a large apparatus may also exhibit longer equilibrium times, thus, a practical compromise must be reached. Typical heights for wall hot boxes are 2.5 to 3 m with widths equal to or exceeding the height. Floor/ceiling hot boxes up to 4 by 6 m have been built.

6.4 Construction Materials—Materials used in the construc- tion of the hot box apparatus shall have a high thermal

resistivity, low heat capacity and high air flow resistance. Polystyrene or other closed cell foam materials have been used since they combine both high thermal resistivity, good me- chanical properties, and ease of fabrication. One potential problem with some foam is that they exhibit time dependent thermal properties that would adversely affect the thermal stability of the apparatus. Problems associated with the use of these materials are avoided by using materials that are initially aged prior to assembly, or by periodic chamber verification, or by using impermeable faced foam materials with sealed edges to greatly minimize the aging effects.

6.5 Metering Chamber:

6.5.1 The minimum size of the metering box is governed by the metering area required to obtain a representative test area for the specimen (see 7.2) and for maintenance of reasonable test accuracy. For example, for specimens incorporating air spaces or stud spaces, the metering area shall span an integral number of spaces (see 5.5). The depth of the metering box shall be no greater than that required to accommodate the air curtain, radiation baffle and the equipment required to condition and circulate the air. Measurement errors in testing with a hot box apparatus are, in part, proportional to the length of the perimeter of the metering area and inverse to metering area. The relative influence of the perimeter length diminishes as metering area is increased. Experience on testing homogeneous materials, has demonstrated that for the “guarded,” self- masking hot box configuration, the minimum size of the metering area is 3 times the square of the metered specimen thickness or 1 m2, whichever is larger (18). From the same experience base, for the “calibrated,” masked box

2 configuration, a minimum metering area size is 1.5 m . For

non-homogeneous specimens, the size requirements are more significant.

6.5.2 The purpose of the metering chamber is to provide for the control and measurement of air temperatures and surface coefficients at the face of the specimen under prescribed

C1363 − 11

FIG. 1 Typical Hot Box Apparatus Schematic—Definition of Locations and Areas

7

 

C1363 − 11

FIG. 2 Typical Guarded Hot Box Schematic

conditions and for the measurement of the net heat transfer through specimen. The usual arrangement is a five-sided

chamber containing airflow baffles, electrical heaters, cooling coils (if desired), and an air circulation system. At steady state

FIG. 3 Typical Calibrated Hot Box Apparatus

8

 

conditions, the heat transfer through the specimen equals the electrical power to the heaters and blowers minus the cooling energy extraction, corrected for the heat passing through the chamber walls and flanking the specimen. Both the metering box wall loss and flanking loss are determined from character- ization measurements (see Section 8 and Annex A2 – Annex A9).

6.5.3 To minimize measurement errors, several require- ments are placed upon the metering chamber walls and the adjoining ambient space:

6.5.3.1 The metering chamber heat flow corrections, which are estimated for design purpose using the equations of Annex A2 – Annex A4, must be kept small, by making the metering box wall area small, keeping its thermal resistance high or by minimizing the temperature difference across the wall (see Note 8).

6.5.3.2 With proper design, the metering box wall loss are controlled to be as low as 1 or 2 % of the heat transfer through the specimen. The metering box wall loss shall never be greater than 10 % of the specimen heat transfer. In any case, the minimum thermal resistance of the metering chamber walls shall be greater than 0.83 m2K/W.

NOTE 8—The 10 % limit is based upon design analysis of existing hot boxes. The choice of construction of the metering chamber can only be made after review of the expected test conditions in which metering box wall loss and associated uncertainties are considered in relation to the anticipated energy transfer through the metered specimen and its desired maximum uncertainty. The influence of the guarding temperature upon the ability to maintain steady temperatures within the metering chamber must also be considered in choosing between highly insulated walls and a tightly controlled guard space conditioning.

6.5.3.3 However large the metering box wall loss is, the uncertainty of the resulting metering box wall loss correction to the net heat flow shall not exceed 0.5 % of the net heat flow through the specimen. In some designs, it has been necessary to use a partial guard to reduce the metering chamber box wall loss.

6.5.3.4 For best results, the heat transfer through the meter- ing chamber walls shall be uniform so that a limited number of heat flux transducers or differential thermocouples can be used to characterize the heat flow from each representative area. This goal is best approximated by the use of a monolithic, uniform insulation uninterrupted by highly conducting struc- tural members, and by eliminating any localized hot or cold sources from the adjoining space. No highly conductive structural members shall be within the insulation. Thermal bridges, structural cracks, insulation voids, air leaks and localized hot or cold spots from the conditioning equipment inside the metering chamber walls shall be avoided.

NOTE 9—One method of constructing satisfactory chamber walls is by gluing together large blocks of an aged, uniform low thermal conductivity cellular plastic insulation such as extruded polystyrene foam. A thin covering of reinforced plastic or coated plywood is recommended to provide durability, moisture and air infiltration control. In addition to using a high thermal resistance, the designer must also recognize that wall heat storage capacity is also a governing factor in hot box wall design.

6.5.3.5 To ensure uniform radiant heat transfer exposure of the specimen, all surfaces which exchange radiation with the specimen shall have a total hemispherical emittance greater

than 0.8. Test Methods C1371 and E903 are acceptable methods to measure emittance. Typically, a flat paint will meet this requirement.

6.5.3.6 In applications where the metering chamber contacts the specimen, an airtight seal between the specimen and metering wall shall be provided. The cross section of the contact surface of the metering chamber with the specimen shall be narrowed to the minimum width necessary to hold the seal. A maximum width of 13 mm, measured parallel to the specimen surface plane, shall be used as a guide for design. Periodic inspection of the sealing system is recommended in order to confirm its ability to provide a tight seal under test conditions.

6.5.4 Since one basic principle of the test method is to measure the heat flow through the metering box walls, ad- equate controls and temperature-monitoring capabilities are essential. Small temperature gradients through the walls occur due to the limitations of controllers. Since the total wall area of the metering box is often more than twice the metering area of the specimen, these small temperature gradients through the walls cause substantial heat flows totaling a significant fraction of the heat input to the metering box. For this reason, the metering box walls shall be instrumented to serve as a heat flow transducer so that heat flow through them can be minimized and measured. A correction for metering chamber wall loss shall be applied in calculating test results. The use of one of the following methods is required for monitoring metering box wall loss.

NOTE 10—The choice of transducer types and mounting methods used to measure the heat flow through the metering chamber walls is guided by the hot box design. However, they must provide adequate coverage and output signal to quantify the metering box wall loss during testing (see 6.5.3.3).

6.5.4.1 The walls may be used as heat flow transducers by application of a large number of differential thermocouples connected between the inside and outside surfaces of the metering chamber walls. Care must be taken when determining locations of the differential thermocouples, as temperature gradients on the inside and outside of the metering box walls are likely to exist and have been found to be a function of metering and climatic chamber air velocities and temperatures. Care must also be taken when determining the number of differential thermocouples. Based upon a survey of hot box operators (18), a minimum of five differential thermocouple pairs per m2 of metering box wall area shall be used. The thermocouple junctions shall be located directly opposite each other and, preferably, located at the centers of approximately equal areas. Small pieces of foil, having surface emittance matching the remainder of the box walls, may be attached to the thermocouples to facilitate the thermal contact with the wall surface. The junctions and the attached thermocouple wires shall be flush with, and in thermal contact with, the surface of the wall for at least a 100 mm distance from the junctions. The thermocouple pairs are connected in series to form a thermopile in which the individual voltages are summed to give a single output or read out individually in cases where significant differences may occur or be expected in the local heat flow levels.

C1363 − 11

9

 

6.5.4.2 As an alternative, separate heat flux transducers are placed on the metering chamber walls. Care must be taken in choosing and installing the transducers to ensure that the thermal resistance of the wall and its surface emittance remain essentially unchanged. The transducers shall be initially cali- brated separately to ensure that the relative sensitivities are approximately the same. Since the transducer sensitivity is also temperature sensitive, temperature sensors shall be installed at the same or adjacent location. The outputs from these trans- ducers are measured separately or as a group. If measured separately, the transducers shall be detachable from the surface so their calibrations, at energy flux levels typical of use, may be checked periodically (see Practice C1130). If the measurement procedure is to calibrate the chamber with the heat flux transducers in place, the transducer outputs shall be connected in series to provide a single reading. The designer must recognize that the calibration factors for the heat flux trans- ducer will be different due to shunting effects when calibrated in-situ versus calibrated alone.

6.5.4.3 Regardless of the method of hot box metering wall instrumentation used, the metering box wall heat flow shall be correlated with the signal outputs during the characterization process. See Section 8 and Annex A5 and Annex A6 for this process.

6.6 Climatic Chamber:

6.6.1 The purpose of the climatic chamber is to provide controlled conditions on the side of the specimen opposite the metering chamber. The test conditions specified are generally those associated with standardized or normal outdoor condi- tions. The instrumentation shall be capable of the control and measurement of the air temperature and velocity and surround- ing surface temperatures in order to maintain the desired surface heat transfer coefficient. In the usual arrangement, it consists of a five-sided insulated chamber with internal dimen- sions matching or greater than the metering chamber opening and with sufficient depth to contain the required cooling, heating and air circulation equipment. An acceptable alternate is to utilize a large environmental chamber with an opening matching the metering chamber opening size. This arrange- ment is especially suited for a floor/ceiling test apparatus in which large roof/attic structures are to be tested.

6.6.2 The walls of the climatic chamber shall be well insulated to reduce the refrigeration capacity required and to prevent the formation of condensation on the outside of the chamber walls.

6.6.3 Heaters, fans and cooling coils shall be shielded or placed behind an air baffle to maintain the uniformity of the surface temperatures radiating to the surface of the specimen. The internal surfaces of the climatic chamber shall also meet the criteria of 6.5.3.4 for surface emittance.

6.7 Specimen Frame:

6.7.1 A specimen frame shall be provided to support and position the specimen and to provide the needed perimeter insulation. The frame opening shall have dimensions at least of those of the metering chamber opening. In the direction of heat flow, the frame shall be at least as thick as the thickest specimen to be tested. In the outward direction perpendicular to the normal energy flow direction, the wall thickness of the

specimen frame shall be at least equal to that of the metering chamber walls or 100 mm, whichever is greater.

6.7.2 Care must be taken in the design and construction of specimen frames so that flanking losses are minimized. Con- ductive plates, fasteners or structural members shall not be used in the flanking paths. The thickness and conductance of skins shall be limited to minimize the flanking loss potential.

6.8 Air Circulation:

6.8.1 The measured overall resistance, Ru, and, when applicable, the surface resistances, Rh or Rc, depend in part upon the velocity, temperature uniformity, and distribution patterns of the air circulated past the specimen surfaces.

6.8.2 Air temperature differences of several degrees exist from air curtain entrance to exit due to heating or cooling of the air curtain as it passes over the specimen surface. The magnitude of this difference is a function of the heat flow through the specimen and the velocity and volume of the air flow. When natural convection is desired, the temperature differences will be larger. A forced air flow reduces the magnitude of this difference. Specific airflow conditions are established by the specification requirements for the material being tested. The paragraphs below describe some specific details required for maintenance of an acceptable air circula- tion within the hot box.

6.8.3 Test specifications sometimes require that near natural convection conditions be used in a wall test apparatus or in a floor/ceiling test apparatus. When required, these tests shall be run using forced convection at near natural convection condi- tions. However, the air velocity shall be below 0.5 m/s if natural convective air conditions are to be approximated with some forced airflow to maintain temperature control.

6.8.4 The design of the air circulation system will have an impact on the entrance to exit air temperature difference. Tradeoffs during design must be made between the desired uniformity of the air curtain temperatures and the operational mode of convective flow. A velocity of approximately 0.3 m/s has proven satisfactory for a wall test apparatus of 3 m height when testing wall systems.

6.8.5 When more uniform air temperatures are desired, it is necessary to provide curtains of forced air moving past the specimen surfaces. For test purposes, the curtain air velocities shall be measured 75 mm away from the surface at the center of the specimen in the direction of airflow as specified in 6.8.11.3.

6.8.6 For uniform test results, the maximum point to point air temperature variation across the test panel, perpendicular to the air flow direction at the center of the test panels, shall be less than 2 % of the overall air to air temperature difference, or 2 K, whichever is greater.

6.8.7 The direction of airflow in a hot box apparatus is determined by the test design and may be parallel, that is, up, down, or horizontal, or perpendicular to surface. However, less fan power is required to maintain air movement in the direction of natural convection (down on the hot side, up on the cold) and that direction is recommended. In some situations the test specification requires a specific direction to evaluate the system performance.

C1363 − 11

10

 

6.8.8 Air velocities greater than 1m/s are permissible when their effect upon heat transfer is to be determined. Velocities commonly used to simulate parallel or perpendicular wind conditions on the exterior side are 2.75 m/s for summer conditions and 5.5 m/s for winter conditions.

NOTE 11—Distinction is made between the effects and requirements of air velocity parallel to the specimen surface and those for velocity perpendicular to it. Parallel velocities simulate the effect of the cross winds, and may be achieved by moving a small amount of air confined in a narrow baffle space and therefore require relatively little blower power. Perpendicular velocities, simulating direct wind impingement, require moving larger amounts of air with corresponding larger power require- ments. The baffles in the second case must be placed further from the specimen surface and should have a porous section (a set of screens or a honeycomb air straightener) that directs the air stream to the specimen surface. Fig. 4 shows an example of climatic chamber arrangement for perpendicular flow.

6.8.9 Air Baffles—For parallel flow, a baffle, parallel to the specimen surface, shall be used to confine the air to a uniform channel, thus aiding in maintaining an air curtain with uniform velocities.

6.8.9.1 The baffle thermal resistance shall be adequate to shield the specimen surface from radiative heat exchange with any energy sources located behind it. A baffle thermal resis- tance of 1 (m2 K /W) is recommended for this purpose. Other baffle designs that maintain temperature uniformity of the baffle surface seen by the test specimen are acceptable.

6.8.9.2 An adjustable baffle-to-specimen spacing is one means of adjusting the airflow velocity. For purpose of maintaining a well-mixed and characterized air curtain, a spacing of 140 to 200 mm is recommended.

6.8.9.3 A baffle also serves as a radiation exchange surface with a uniform temperature only slightly different than that of

the air curtain. The baffle surface facing the specimen shall have an emittance greater than 0.8.

6.8.10 Air Velocity Uniformity—Uniform air flow profile across the specimen width, perpendicular to the air flow direction, is achieved by use of multiple fans or blowers or by use of an inlet distribution header across one edge of the baffle and an outlet slot across the opposite. The inlet header shall incorporate adjustable slots or louvers to aid in obtaining uniform distribution.

6.8.10.1 After construction of an air circulation system, the air velocity profile shall be measured across the area perpen- dicular to the direction of airflow in the proximity of the specimen. The test shall be conducted with a flat, homogeneous panel in place so that the surface of the test panel has minimum effect on the velocity profile. The air velocity profile shall be defined as uniform if all measurements from the profile scan are within 10 % of the mean of all measurements. For parallel air curtains, the air flow measurements shall be made at 0.3 m intervals across the specimen face, perpendicular to the air flow direction, at the centerline of the metering chamber. For air flow perpendicular to the specimen face, the air flow measure- ments shall be made in the radial direction at a density of one per every 30 degrees around the outlet of the diffuser at a distance from the center of the metering area equal to the outlet diameter of the air supply diffuser. If the profile is not uniform, additional adjustments shall be made to the inlet header slot or louvers or in the placement of fans or blowers to achieve an air curtain with uniform velocity across the face of the specimen. The velocity profiles shall be verified, whenever modification or repairs of the distribution system are made that might cause

C1363 − 11

FIG. 4 Hot Box Arrangement for Perpendicular Air Flow

11

 

a change in flow patterns. Also, the profiles shall be verified during characterization checks.

NOTE 12—Linear air diffusers designed for ceiling air distribution systems have been found satisfactory to use as distribution headers. For large floor/ceiling testers it may be necessary to use more than one set of fans or inlet and outlet headers creating opposing zones to obtain the required temperature uniformity. Tangential fans have also been found to provide uniform temperatures.

6.8.11 Air Velocity Measurement—The apparatus design shall provide a means for determining mean air velocity past both the hot and cold faces of the specimen during each test. Acceptable methods are as follows:

6.8.11.1 One method is to measure the volumetric airflow in the duct to the inlet distribution header by using a calibrated orifice or other flow-measuring device. The average baffle space velocity is then calculated from the volume flow and the size of the space between the specimen and the parallel baffle. The baffle must be well sealed for this technique to work.

6.8.11.2 Another method is to calculate the velocity from an energy balance. The rate of loss, or gain, of heat by the air as it moves through the baffle space, as indicated by its tempera- ture change, will match the rate of heat transfer through the metering chamber opening, average values of which can be determined from the test data.

6.8.11.3 The best method is to locate velocity sensors directly in the air curtain. For test purpose, wind velocity shall be measured at a fixed location that represents the average free stream condition. For both perpendicular and parallel flow patterns, this location shall be a distance out in the air stream such that the wind speed sensor is not in the test specimen surface boundary layers or wakes. A distance of 75 to 150 mm out from the test specimen surface at the center point is typically used. On the room side, where low circulation velocities are generally used, a properly located sensor is also required. The operator’s experience and knowledge of the air distribution system obtained in the profiles from 6.8.10 shall be used to determine the optimum sensor location.

6.9 Air Temperature Control:

6.9.1 The temperature of the air entering the air curtains shall be within 6 1 K of the setpoint temperature across its width and, for steady-state tests, shall not change during the measurement period.

6.9.2 One method of providing controlled, heated air is to install open wire, low thermal mass electrical heaters in an insulated, low emittance section of the blower duct or other part of the air circulation system and to control these heaters using a sensor located at the inlet to the air curtain.

NOTE 13—Another method of heater control is to use several individual heaters that are switched on to provide fixed levels of energy. Fine-tuning is provided by an additional heater modulated by a controller. Another satisfactory method is to use a controller that varies the power to all the heaters.

6.9.3 Methods for cooling the climatic chamber include the installation of a refrigeration system evaporator inside the chamber, ducting in chilled air from an external source or injecting liquid nitrogen. Usually the evaporator or external chilled air is controlled at a constant temperature a few degrees (typically < 5°C) below the desired setpoint. Then, a reheat and

control system, similar to that for obtaining heated air (see 6.9.2) is used to achieve fine control of the temperature at the inlet to the specimen air curtain. When liquid nitrogen is used a valve regulating its flow is pulsed or modulated to obtain fine temperature control.

NOTE 14—One proven configuration for a climatic chamber utilizes two air circuits created by suitable baffles. The evaporator fan creates one circulation path that includes a mixing chamber from which air is circulated by a separate blower to the specimen air curtain and returned. An air reheat and control system provides fine control of air temperature at the distribution header inlet. Other proven configurations utilize only a single air circuit containing both cooling and reheat elements. Under certain conditions, a desiccant may be needed to remove moisture from the air stream.

6.9.4 Metering chamber blowers shall be small and efficient since, without cooling, they determine the least possible net energy input to the metering chamber. If large fans or blowers are necessary, then compensatory cooling with inherent loss in accuracy shall be used. Some heat is removed by locating the blower motor outside of the metering chamber and accurately measuring the heat equivalent of the shaft power. Precautions shall be taken to prevent air leakage around the shaft.

6.9.5 When cooling of the metering chamber is required, it must be done in a manner in which the amount of heat extracted can be measured accurately. One method is to circulate a chilled liquid through a heat exchanger located in the metering chamber air circuit. The rate of heat extraction is controlled by the inlet to chamber air temperature difference, the airflow rate, the liquid properties, and the heat exchanger efficiency. The amount of cooling used shall be limited to that necessary to overcome any excess blower or other heating loads since test accuracy will be lost if excessive heating must be used to compensate for large cooling. For example, assume that the heater input was 400 Btu/h out of an overall heater capacity of 2000 Btu/h and is known to within 1 % of capacity or 6 20 Btu/h. Also assume a concurrent cooling load of 320 Btu/h out of an overall cooling capacity of 1600 Btu/h which is known to within 1 % of capacity or 6 16 Btu/h. Since these loads oppose each other, the net load is 80 Btu/h but the uncertainty of the net could be as large as 6 36 Btu/h or 45 % of the net load. For this reason, care must be observed in obtaining the correct test setup.

6.9.6 Special Considerations, Humidity Control—Moisture migration, condensation, and freezing within the specimen can also cause variations in heat flow. To avoid this, the warm side relative humidity shall be kept below 15 % or the laboratory shall verify that the dew point temperature of the metering side air is 2 °C less than the minimum metering side surface temperature of the specimen.

6.10 Temperature Measurement:

6.10.1 When surface temperatures are required, specimen surface temperature sensors shall typically be located opposite each other on the two faces of the specimen. However, when placement opposite each other is not possible, the sensors shall be placed to represent the correct area weighting for each surface. These sensors shall be chosen and applied to the surface in a manner such that the indicated temperature is within 6 0.2 K of the temperature that would exist if the sensor

C1363 − 11

12

 

had not been applied. This requirement is met by thermo- couples if: (1) the wire is no larger in diameter than 0.25 mm (No. 30 AWG.); (2) the wire meets, or is calibrated to, the special limits of error as specified in the Tables E230; (3) the junctions, not larger than two times the wire diameter, are twisted and welded or soldered; (4) 100 mm of adjoining wire are taped, cemented or otherwise held in thermal contact with the surface using materials of emittance close (6 0.05) to that of the surface; and (5) they are electrically insulated, or otherwise protected, so that the electrical junction is at the location of the thermocouple bead. Application of alternate temperature sensor systems may be used if comparative measurements or calculations show that the basic requirements are met.

NOTE 15—Metal foil tape, which has been painted to make the emittance greater than to 0.80, is an effective means to attach thermo- couple sensors to most high emittance test specimens.

6.10.2 If the specimen construction, and therefore its ther- mal resistance, is uniform over its entire area, then a minimum number of sensors, spaced uniformly and symmetrically over the surface, are sufficient. The required minimum number of sensors per side shall be at least two per square meter of metering area but not less than nine (24).

6.10.2.1 If each element of the specimen construction is relatively uniform in thermal resistance and is repeated several times over the entire surface, the number of sensors specified in 6.10.2 may still be sufficient. In this case, the sensors shall be located to obtain the average surface temperature over each type of construction element and, for each type of element, shall be distributed approximately uniformly and symmetri- cally over the specimen area. The average surface temperature of the specimen shall be calculated by area weighting of the averages for the different types of construction elements.

6.10.2.2 If the surface temperatures are expected to be, or found to be, greatly non-uniform, additional sensors shall be required. Often a great number, such as three or more times the normal amount as determined by trial and error, is required to adequately sample the different temperature areas so that a reliable area weighted mean surface temperature may be obtained. Some research has been published on the subject of testing highly conductive member that might be used as guidance for this determination. For example, see the work on steel framed buildings (29).

6.10.2.3 If an accurate determination of the average surface temperatures cannot be obtained, the hot box apparatus can accurately measure only the thermal transmittance, U, or the overall thermal resistance, Ru. The average panel resistance, R, of the specimen can be estimated by subtracting off the previously determined surface film thermal resistances estab- lished using a transfer standard of equal thermal resistance, size, surface configuration and roughness. Note that the geometry, average temperatures, and energy exchange condi- tions must be similar for the calibration transfer standard (CTS) and test panel for this technique to have reasonable accuracy. (See Test Method C1199 for discussion on CTS design.)

NOTE 16—Tests on specimens containing thermal bridges require special care because of the possible great differences in thermal resistance and temperatures between the thermal bridge areas and those of surround-

ing insulated structures. Added complications arise when tests are run at higher air velocities since temperatures and energy transfer can depend significantly upon bridge geometry relative to the overall sample as well as the velocity and direction of air movement. If test results are to be comparable for competing systems, they must be run under similar conditions. This method does not attempt to standardize such conditions.

6.10.3 The temperature of the air on each side of the specimen shall be measured by thermocouples, temperature sensitive resistance wires, or similar temperature sensors.

6.10.3.1 The minimum number and locations of sensors used to measure air temperatures shall be that specified for surface temperature sensors in 6.10.2. These sensors must be radiation shielded or otherwise protected to provide an accurate indication of the temperature of the air curtain. Sensors shall be small to ensure fast response to changing temperatures. Resis- tance wires, if used, shall be distributed uniformly in the air curtain.

NOTE 17—One suitable radiation shield is made by using 12 mm diameter, 75 mm long pieces of thin walled plastic tubing covered on the outside with aluminum foil tape. The air thermocouple is placed at the center of the tube to measure the air stream temperature and yet be shielded from radiation sources.

6.10.3.2 The best location for temperature sensors depends upon the type of air curtain convection (natural or forced). In natural convection situations, it is usually possible to identify the temperature of still air outside the boundary layer. Consequently, when natural convection is established, air temperature sensors shall be located in a plane parallel to the specimen surface and spaced far enough away from it that they are unaffected by temperature gradients of the boundary layer. For minimum velocities required to attain temperature unifor- mities (see 6.8 and Note 12), the minimum spacing from the specimen surface is 75 mm. At velocities greater than 1 m/s, the required minimum spacing is greater. The boundary layer thickness increases sharply at the transition from laminar to turbulent flow. With fully developed turbulent flow, the bound- ary layer occupies the full space between the specimen and the baffle. When forced convection is established and the flow is fully developed, the sensors shall be located at a distance from the specimen surface corresponding to 2⁄3 up to 3⁄4 of the specimen-to-baffle distance. This is to detect a temperature approaching the airflow bulk temperature.

6.10.3.3 Thermocouple sensors used for measurement of air temperatures shall meet the requirements of Items (1), (2), (3), and (5) in 6.10.1. Other sensors are acceptable if they have similar time response and are calibrated so that the measure- ments are accurate within 6 0.5 K.

6.10.4 Thesurfacetemperatureofthebafflesinthemetering and climatic chambers, where required, shall be measured by placing sensors on all surfaces seen by the specimen. A minimum area density of three sensors per square meter of baffle area, but not less than one sensor per baffle surface, is required. These data (1) can be used to determine any differ- ence between the baffle surface and air curtain temperatures; (2) permits corrections to be made to the radiation component of the surface film conductance due to differences in these temperatures; and (3) is a necessary component of the data analysis for specimens such as windows which have a high

C1363 − 11

13

 

thermal conductance. (See the discussion on the environmental temperature determination in Annex A9.)

6.11 Specimen Pressure Difference:

6.11.1 For some tests, it is necessary to establish and measure the air pressure differential between the faces of the test specimen. This is especially important for window and other samples where the airflow resistance between the speci- men surfaces is low. The specimen pressure difference is defined as the difference in the local static pressure, on either side of the specimen, measured at a location at the geographic center of the metered area, at a distance 75 mm from the surfaces of the sample.

6.12 Instruments:

6.12.1 All signal conditioning and data logging instruments shall be located outside of the apparatus. All instruments shall be calibrated to the specified accuracy, traceable to a national standards laboratory, and shall meet the following additional requirements:

6.12.1.1 All instrumentation shall have adequate sensor response so that the scanning speed does not adversely effect the measurement results.

6.12.1.2 Temperatures shall be readable to 6 0.05 K and be accurate within 6 0.5 K.

6.12.1.3 Heat flux transducer outputs shall be measured to the precision required to limit the error in estimation of the metering box wall loss to less than 6 0.5 % of the specimen energy transfer. This requires a heat flux transducer calibration accuracy of 5 percent or better.

6.12.1.4 Many methods of air velocity measurement are possible depending on the specific box design and test condi- tions. However, an accuracy of 6 5 % of the reading is required. A sensor whose signal can be processed by automatic data acquisition equipment is recommended.

6.12.1.5 Pressure difference measurements shall be accurate to within 6 5 % of reading or 6 1 Pa, whichever is greater.

6.12.1.6 Total average power (or integrated energy over a specified time period) to the metering box shall be accurate to within 6 0.5 % of reading under conditions of use. Power measuring instruments shall be compatible with the power supplied whether ac, dc, on off, proportioning, etc. Voltage stabilized power supplies are strongly recommended. Metered cooling instruments shall be calibrated together as a system to similar accuracy.

6.12.1.7 Temperature controllers for steady-state tests shall be capable of controlling temperatures constant to within 6 0.25 K (see 6.9

Ambient Air Space Climatic Chamber Cuard Area Metering Chamber Metering Area FIG. 1 Typical Hot Box Apparatus Schematic-Definition of Locations and Areas C1363 - 11 Surrounding Ambient Guard Camber imate Chamber Metering Chamber Specimen Harrow FIG. 2 Typical Guarded Hot Box Schematic Surrounding Ambient

Answer 1

Metering Chamber

Its purpose is to provide for the control and measurement of air temperatures and surface coefficients at the face of the specimen and for measuring the net heat transfer through specimen.

Its arrangement is usually a five-sided chamber containing airflow baffles, electrical heaters, cooling coils (if desired), and an air circulation system and its depth should be no greater to accommodate them.

At steady state conditions,

Heat Transfer through the specimen = Electrical power Input - Cooling Energy Extraction

For non-homogeneous specimens, the size requirements are more significant as compared to homogenous specimens.

Measurement errors are proportional to the length of the perimeter of the metering area and inverse to metering area.

Minimization of error is done through following ways –

· making the metering box wall area small, keeping its thermal resistance high.

· minimizing the temperature difference across the wall.

The metering box wall loss shall never be greater than 10 % of the specimen heat transfer.

The heat transfer through walls shall be uniform so that a limited number of heat flux transducers or differential thermocouples can be used which is achieved by the use of a monolithic, uniform insulation and by eliminating any localized hot or cold sources from the adjoining space. Thermal bridges, structural cracks, insulation voids, air leaks and localized hot or cold spots from the conditioning equipment inside the metering chamber walls shall be avoided.

Basic principle of the test method is to measure the heat flow through the metering box walls, small temperature gradients through the walls cause substantial heat flows totalling a significant fraction of the heat input to the metering box. For this reason, the metering box walls shall be instrumented to serve as a heat flow transducer so that heat flow through them can be minimized and measured.

Methods to monitor metering box wall loss-

1. The walls may be used as heat flow transducers by application of a large number of differential thermocouples (minimum 5) connected between the inside and outside surfaces of the metering chamber walls at suitable places. The thermocouple junctions shall be located directly opposite each other and, preferably, located at the centres of approximately equal areas. Small pieces of foil, may be attached to the thermocouples to facilitate the thermal contact with the wall surface. The thermocouple pairs are connected in series to form a thermopile in which the individual voltages are summed to give a single output or read out individually in cases where significant differences may occur or be expected in the local heat flow levels.

2. Separate heat flux transducers initially calibrated separately are placed on the metering chamber walls. Temperature sensors shall be installed at the same or adjacent location because transducer sensitivity is also temperature sensitive, the outputs from these transducers are measured separately or as a group. If measured separately, the transducers shall be detachable from the surface so their calibrations may be checked periodically. If the measurement procedure is to calibrate the chamber with the heat flux transducers in place, the transducer outputs shall be connected in series to provide a single reading.

Climatic Chamber

The purpose of the climatic chamber is to provide controlled conditions on the side of the specimen opposite the metering chamber. It should be capable of the control and measurement of the air temperature and velocity and surrounding surface temperatures in order to maintain the desired surface heat transfer coefficient.

It consists of a five-sided insulated chamber with internal dimensions matching or greater than the metering chamber opening and with sufficient depth to contain the required cooling, heating and air circulation equipment.

Its walls should be well insulated to reduce the refrigeration capacity required and to prevent condensation on the outside of the chamber walls.

Heaters, fans and cooling coils should be shielded or placed behind an air baffle to maintain the uniformity of the surface temperatures radiating to the surface of the specimen.

Specimen Frame          

It is provided to support and position the specimen and to provide the needed perimeter insulation.

The frame shall be at least as thick as the thickest specimen to be tested. The wall thickness of the specimen frame shall be at least equal to that of the metering chamber walls or 100 mm, whichever is greater.

Conductive plates, fasteners or structural members shall not be used in the flanking paths so that flanking losses are minimized.

Air Circulation

The measured overall resistance and/or the surface resistances depend in part upon the velocity, temperature uniformity, and distribution patterns of the air circulated past the specimen surfaces.

The temperature difference between the entry and exit of air curtain is a function of heat flow through the specimen.

Free as well as forced convection is done depending upon the test specimen.

Air velocity < 0.5 m/s - natural convective air conditions are to be approximated with some forced airflow to maintain temperature control.

Air velocity > 1 m/s   - permissible when their effect upon heat transfer is to be determined.

Air Baffles are used to confine the air to a uniform channel to maintain uniform air curtain velocities.

The air velocity profile shall be measured across the area perpendicular to the direction of airflow in the proximity of the specimen.

Air Velocity Measurement methods -

1. Measure the volumetric airflow using a flow-measuring device. The average baffle space velocity is then calculated from the volume flow and the size of the space between the specimen and the parallel baffle which should be well sealed.

2. Calculate the velocity from an energy balance. The rate of heat loss or gain by air as it moves through the baffle space, as indicated by its temperature change will be equal to the rate of heat transfer through the metering chamber opening.

3. Locate velocity sensors directly in the air curtain which is the best method.

Air Temperature Control

The temperature of the air entering the air curtains shall be within 6 1 K of the set point temperature across its width and, for steady-state tests, shall not change during the measurement period.

One method of providing controlled, heated air is to install electrical heaters in blower duct or other part of the air circulation system.

For cooling the climatic chamber, a refrigeration system evaporator is installed inside the chamber.

Metering chamber blowers should be small and efficient.

When cooling of the metering chamber is required, it must be done in a manner in which the amount of heat extracted can be measured accurately. It can be done by circulate a chilled liquid.

Because Humidity can also cause variations in heat flow the warm side relative humidity shall be kept below 15 % or the dew point temperature of the metering side air should be 2 °C less than the minimum metering side surface temperature of the specimen.

Temperature Measurement

Specimen surface temperature sensors shall be located opposite to each other on the two faces of the specimen and if it is not possible then the sensors shall be placed to represent the correct area weighting for each surface.

The temperature of the air on each side of the specimen is measured by temperature sensors.

The minimum number and locations of sensors used to measure air temperatures shall be that specified for surface temperature sensors on the basis of the thermal resistance of each element of the test specimen.