Layout specifications for thermal comfort

Transcription

1 Layout specifications for thermal comfort TB 9 E / 2008

2 Categories of thermal comfort With regard to thermal comfort in commercial buildings the European standard EN ISO 7730 defines three categories for thermal environment where the percentage of dissatisfied is expected to be under the PPD index (Predicted Percentage of Dissatisfied). Dissatisfaction may be the result of too high indoor air velocities (Draught Rating DR in %), too high vertical temperature gradient, too high radiant temperature asymmetry, and uncomfortable floor temperatures. The 3 categories are shown in Table 1. PPD % DR % Percentage of Dissatisfied because of vertical temperature gradient radiant temperature asymmetry floor temperature A < < 10 < 3 < < 10 B < 10 < 20 < < < 10 C < 1 < 30 < 10 < 10 < 1 Table 1: Three categories for thermal environment In category A, for instance, one expects that less than % of the people are not satisfied with their thermal environment. This is the case when the draught rating (DR) is < 10% and dissatisfaction is as follows: < 3% with regard to vertical temperature gradient, < % with regard to radiant temperature asymmetry, and < 10% with regard to floor temperature. These 4 criteria should be met simultaneously for each of the three categories. Which category is to be complied with will be agreed upon between the consultant and the client. Max. air velocity u in m/s DR = 20% DR = 10% 0.10 Extrapolated Activity level I: Light activity, seated (1.2 met) Draught rating and allowable indoor air velocities The draught rating depends on the indoor air velocity, the turbulence intensity and the room temperature. This relationship is defined with equations in EN ISO 7730 for room temperatures of 20 C to 2 C and activity level I, and is illustrated in Fig. 1. For higher room temperatures and activity level II, the relationship has been extrapolated on the basis of further criteria specified in EN ISO 7730 as well as from our personal experience. The following turbulence intensities have been taken as a basis in Fig. 1: Turbulent mixing ventilation Tu = 40% Displacement ventilation Tu = 2% Examples of activity levels: Activity level I: Offices, schools, theatres, (1.2 met) assembly rooms Activity level II: Laboratories, exhibition spaces, (1. met) sales spaces, museums, sports centres. If, for instance, category A at DR < 10% has been specified, then the indoor air velocity at an indoor air temperature of 2 C and activity level I should not exceed 0.1 m/s with turbulent mixing ventilation and 0.17 m/s with displacement ventilation. But, if category B (at DR < 20%) has been specified, then the indoor air velocity under like conditions of temperature and activity should not exceed 0.2 m/s with turbulent mixing ventilation and 0.30 m/s with displacement ventilation. As far as activity level II is concerned, for category A and 2 C the maximum indoor air velocity is 0.21 m/s with turbulent mixing ventilation and 0.24 m/s with displacement ventilation. Displacement ventilation Turbulent mixing ventilation Maximum indoor air velocities for category C (DR < 30%) are not indicated here as this category will be found only seldom in commercial buildings. TB 9 E Bl Activity level II: Light activity, standing (1. met) Air temperature R in C Fig. 1: Allowable range for maximum indoor air velocities to EN ISO

3 Allowable indoor air velocities in industrial plants are specified in the German standard VDI 3802 dated December 1998 (Air-conditioning systems for factories). As shown in Fig. 2, this standard defines the allowable mean indoor air velocities in relation to activity level and clothing, yet irrespective of turbulence intensity. The type of clothing or rather the clothing insulation value is expressed in clo units. The following values refer to: 0. clo: light work clothing (shirt) 0.9 clo: standard work clothing 1.3 clo: heavy work clothing (protective jacket). Industrial halls are also concerned with activity levels III and IV: Activity level III: 2 met (moderate work, standing) Activity level IV: 2. met (heavy work, standing). For example, in an industrial hall with indoor air temperature 2 C activity level III (2 met) clothing 0.9 clo, indoor air velocities up to max m/s are allowable. At 28 C indoor air temperature other conditions being the same indoor air velocities may amount to max. 0.4 m/s. 0.8 Allowable mean air velocity u in m/s met 2. met 2.0 met clo 1.3 clo 0.9 clo 0. TB 9 E Bl Air temperature R in C Fig. 2: Allowable mean indoor air velocities in factories as per VDI 3802 of December

4 Compliance with allowable indoor air velocities Turbulent mixing ventilation Compliance with allowable indoor air velocities as per Fig. 1 and 2 mainly depends on the following physical variables: 1. Maximum temperature difference J between supply air and indoor air in the cooling mode. 2. Specific air volume flow rate per m 2 of floor area. The maximum specific air volume flow rate can be read off as a function of the maximum allowable indoor air velocity and the discharge height. The allowable indoor air velocity will be taken from Fig. 1, or from national guidelines in non-european countries, or from special arrangements between consultant and client, or from Fig. 2 for industrial plants. For air outlets for turbulent mixing ventilation manufactured by KRANTZ KOMPONENTEN the following limit criteria apply: 1. Maximum temperature difference For air outlets generating three-dimensional diffuse air flow without tangential patterns: Ceiling twist outlet Radial outlet Radial slot outlet Multiplex outlet Induction outlet Opticlean Swivel jet outlet J max = 12 K J max = 10 K For air outlets generating two-dimensional diffuse air flow with tangential patterns: Linear whirl outlet Wall slot diffuser Jet nozzle Broad multiplex outlet Parapet outlet J max = 8 K J max = 10 K Maximum specific volume flow rate VSp max m 3 /(h m 2 ) l/(s m 2 ) Discharge height H in m Maximum allowable indoor air velocity u in m/s Fig. 3: Three-dimensional diffuse air flow, e.g. with ceiling twist outlet, radial outlet, radial slot outlet, multiplex outlet, induction outlet, Opticlean, and swivel jet outlet Maximum specific air volume flow rate According to Fig. 3 for air outlets generating threedimensional diffuse air flow. According to Fig. 4 for air outlets generating twodimensional diffuse air flow. The layout criterion is based on J max = 10 to 12 K If the maximum temperature difference is lower, V. Sp max can be increased by the following percentage: J max = 8 K V. Sp max 1% higher J max = K V. Sp max 3% higher J max = 4 K V. Sp max 70% higher TB 9 E Bl

5 Maximum specific volume flow rate VSp max m 3 /(h m 2 ) l/(s m 2 ) Discharge height H in m Low-turbulence displacement ventilation For low-turbulence displacement flow (generated by displacement outlets for commercial or industrial applications), other layout criteria are relevant; they are described in the technical brochures relating to the different types of KRANTZ KOMPONENTEN displacement outlets. In particular the near zone is defined, where higher air velocities occur due to physical conditions. An exception, however, is our circular displacement outlet of type VA-ZD when placed above the occupied zone. In such case and for horizontal discharge, the criteria indicated in Fig. 3 can be roughly taken as a basis for layout Maximum allowable indoor air velocity u in m/s Fig. 4: Two-dimensional diffuse air flow with sidewall air outlets, e.g. linear whirl outlet, wall slot diffuser, broad multiplex outlet, jet nozzle, and parapet outlet The layout criterion is based on J max = 8 to 10 K If the maximum temperature difference is lower, V. Sp max can be increased by the following percentage: J max = K V. Sp max 3% higher J max = 4 K V. Sp max 70% higher TB 9 E Bl For instance, if with three-dimensional diffuse air flow a maximum indoor air velocity of 0.2 m/s is allowable, the specific air volume flow rate may not exceed 10.3 l/(s m 2 ) [37 m 3 /(h m 2 ) ] at 3 m discharge height. If the maximum temperature difference is limited to 8 K in the cooling mode, the specific air volume flow rate may be 1% higher, i.e l/(s m 2 ) [43 m 3 /(h m 2 )].

6 Temperatures Indoor air temperature In the occupied zone one should consider the interaction of air temperature and radiant temperature of surrounding surfaces, in particular when rooms are fitted with chilled ceilings or buildings have large glass facades. The local temperature J o is called operative temperature and is determined from the following approximate equation: J o = 1 (J a + J r ) 2 J o = Operative temperature J a = Room air temperature J r = Mean radiant temperature J r is calculated from the surface temperatures of the room surrounding surfaces and the angles of radiation to the spot considered (the workplace as a rule). For instance, the closer the workplace is to the facade, the greater is the influence of the facade temperature on the operative temperature. With reference to EN 121 the following layout values are recommended for the operative temperature: Operative temperature in C Min. value for the heating season (winter)» 1 clo Max. value for the cooling season (summer)» 0. clo A B 20 2 C Table 2: Recommended layout values for the operative temperature at 1.2 met Radiant temperature asymmetry Thermal comfort is also affected by the radiant temperature asymmetry. Discomfort occurs when the surface temperatures of the different room surrounding surfaces differ too widely. These are influenced by active cooling or heating surfaces that can be used to make up for too great differences. It must be noted that an individual has different perceptions of the same values of radiant temperature asymmetry in different situations. To an individual, a warm ceiling is much more unpleasant than a cold one. The acceptable limits for radiant temperature asymmetry have been derived from this knowledge and are specified in EN ISO 7730 as follows (Table 3): Radiant temperature asymmetry in K Warm ceiling Cold wall Cold ceiling Warm wall A < < 10 < 14 < 23 B < < 10 < 14 < 23 C < 7 < 13 < 18 < 3 Table 3: Limits for radiant temperature asymmetry to EN ISO 7730 If these values are not exceeded, the acceptable percentage of dissatisfied with radiant temperature asymmetry as per Table 1 is kept to. In practice, where chilled ceilings are used, the limit of 14 or 18 K is never reached in the cooling mode, simply because condensation would occur much earlier. The limit of K in the heating mode, however, may be reached if the system has not been designed properly. m Unless otherwise agreed, the operative temperature stated above applies to the area in the middle of the room at 0. m above the floor. U = 1.1 W/(m 2 K) 10 C 21 C 1 m 18 C C 22 C 22 C 22 C Fig. : Example of calculation of radiant temperature asymmetry 0.9m 1.8 m TB 9 E Bl. 2008

7 TB 9 E Bl The following example illustrates the method of calculation: Upper half space: J rh1 = 2 18 C C C = 2.4 C Lower half space: J rh2 = C C C = 21.8 C The radiant temperature asymmetry is J rh = 4. K, which is acceptable for all categories. These formulas enable to determine a maximum heating capacity of approx. W/m 2 of floor area (Fig. ) for categories A and B when a chilled ceiling is used to heat a space. Radiant temperature asymmetry in half space rh in K Limit as per EN ISO Glazing over full room height Reference height 1.2 m 1 Facade with parapet Reference height 1.2 m Specific heating capacity in W/m 2 of floor area Fig. : Radiant temperature asymmetry in half space J rh (ceiling to floor) at 1 m from facade Vertical temperature gradient According to EN ISO 7730 the following maximum vertical temperature gradients are acceptable at a height between 1.1 m and 0.1 m above the floor, depending on the category of thermal comfort (Table 4). A B C Vertical temperature gradient < 2 K < 3 K < 4 K Table 4: Acceptable vertical temperature gradients as per EN ISO 7730 If this is complied with, the acceptable percentage of dissatisfied with the vertical temperature gradient as per Table 1 is kept to. With turbulent mixing ventilation and a chilled ceiling operating in the cooling mode, the limits for vertical temperature gradient are of no significance as the related values are always far below these limits. With displacement ventilation and a chilled ceiling operating in the heating mode, however, one should take care that the limits for vertical temperature gradient are not exceeded. The following experimental values are applicable in this regard: With displacement ventilation, the specific cooling load should not exceed 4 W/m 2 for category A and W/m 2 for category B. For a chilled ceiling operating in the heating mode, the specific heating output per m 2 of floor area should not exceed 0 W/m 2 for category A and 70 W/m 2 for category B. If a turbulent mixing ventilation system from the ceiling or using floor twist outlets is in operation together with a chilled ceiling, then the acceptable heating output will be raised to 100 W/m 2 of floor area. Floor temperature Table shows the acceptable values for the surface temperature of the floor, depending on the category of thermal comfort. Acceptable surface temperature of the floor in C A B C Table : Acceptable surface temperature of the floor as per EN ISO 7730 The surface temperature of the floor can be influenced only a little using HVAC systems. 7

8 Cold air drop at facades There is a risk of discomfort as a result of cold air drop at glass facades being very high or having a too high heat transmission coefficient U > 1. to 2 W/(m 2 K). The cold air flow is deflected at floor level and penetrates the occupied zone. Fig. 7 illustrates the indoor air velocities u in relation to the window/facade height. Indoor air velocity u at 1 m from facade, in m/s U value in W/(m 2 K) Outdoor air temperature: Indoor air temperature: : 12 C 22 C 34 K Window height in m Fig. 7: Indoor air velocities u above the floor as a result of cold air drop at glass facades To efficiently prevent cold air drop at high facades, one can use window air curtain units or facade heating systems; for glazing over the room height, one can also use heating elements or parapet air supply units. Aachen, April 2008 Dr. Franc Sodec TB 9 E Bl caverion GmbH Business unit KRANTZ KOMPONENTEN P.O. Box Aachen Germany Phone Fax info@krantz.de