6. Measurements
6.2 Measurement and evaluation methods
6.1.2 Air velocity measurements
Air velocities in test room were measured by use of air velocity transducer SensoAnemo5100SF with accuracy of ±0.02 m/s and ±1% of reading for velocities below 5 m/s [60]. Measurements were carried out in the occupied zone on positions of vertical stands at heights 0.1 m; 0.6 m; 1.1 m and 1.7 m respectively.
Sufficient times were allowed for the air in the room to stabilize before any readings were taken.
6.1.3 Tracer-gas measurements
In order to find a pattern of ventilated air distribution in test room, the thorough investigations with use of tracer gas were performed. The units from producer Innova were used for those purposes [62]. The tracer gas was dosed and sampled by unit Brüel & Kjær Innova 1303 (multipoint sampler and doser), and its concentration was measured by use of Innova 1312 (photo acoustic multi gas monitor). Dozing and sampling tubes were firstly connected to special filters in order to prevent dust entering the sampler unit which could cause its malfunction and then were connected to the units. The dosing pipes with diameter of 3 mm were used. Experiments were carried out using Freon R134a (C2H2F4) a tracer gas. Freon has advantage over other gases (such as CO2) that it is usually not present in outside air (zero concentration), which can help to avoid any discrepancy when analyzing the results of the measurements. This feature makes the results of investigations more precise. The accuracy of dosing is ±2%.
6.1.4 Pressure drop
The micromanometer FC0510 from Furness Control with the accuracy of 0.25% was used to measure the pressure drop between plenum and investigated room [63].
6.1.5 Thermo-graphic investigation
Thermo-graphic investigation with use of radiometric thermal camera Hot-Find D was carried out to investigate the proper functioning of the radiant systems [64]. Rather large temperature differences between walls with activated radiant systems and surrounding internal surfaces allowed us to uncover any problematic areas. Used thermo-graphic camera operates at temperature range from -20 °C to 250 °C with measuring accuracy ± 2 K and ± 2 % of reading.
𝑃𝑃𝐷 = 100 − 95𝑥𝑒−�0,033𝑥𝑃𝑀𝑉4+0,2179𝑥𝑃𝑀𝑉2� Eq.(4)
Figure 28: PPD based on PMV values [29]
6.2.2 Temperature measurements Operative temperature
According to ISO 7726 operative temperature can be calculated according Eq.(5) as mean value of air temperature and radiant mean temperature [59]. This is however possible only if the velocity of air in occupied zone is less than 0.2 m/s and difference between air and mean radiant temperature is less than 4 K.
𝑡𝑜 =𝑡𝑎+𝑡2 𝑟 Eq.(5)
Where: 𝑡𝑜 is room air temperature [°𝐶], 𝑡𝑎 is room air temperature [°𝐶], and 𝑡𝑟 is mean radiant temperature [°𝐶].
If mentioned conditions are not ensured then the average value of air temperature and mean radiant temperature need to be weighted by respective heat transfer coefficients ℎ𝑐 and ℎ𝑟 and Eq.(6) should be used to calculate operative temperature.
𝑡𝑜 = ℎ𝑐ℎ𝑡𝑎+ ℎ𝑟𝑡𝑟
𝑐+ℎ𝑟 Eq.(6)
Where: 𝑡𝑎 is room air temperature [°𝐶], 𝑡𝑟 is mean radiant temperature [°𝐶], ℎ𝑐 is heat transfer coefficient by convection [W/(m2·K)], and ℎ𝑟 is heat transfer coefficient by radiation [W/(m2·K)].
Mean radiant temperature
Mean radiant temperature according to ISO 7726 represents ”uniform temperature of an imaginary enclosure in which radiant heat transfer from the human body is equal to the radiant heat transfer in the actual non-uniform enclosure” [59]. Its measurement is of particular importance when using radiant
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heating and cooling systems. It is calculated according to Eq.(7) by use of surface temperatures and angle factors. Angle factor is the value which represents relation of the surface to the person present in TR and its values can be found in ISO 7726 [59].
𝑇𝑟4= 𝑇14𝐹𝑝−1+ 𝑇24𝐹𝑝−2+ 𝑇𝑁4𝐹𝑝−𝑁 Eq.(7)
Where: 𝑇𝑁 is surface temperature of surface N [°𝐶], 𝐹𝑝−𝑁 is angle factor between person and surface N.
Vertical air temperature difference
The report CR 1752 suggests to measure the temperatures at heights 0.1 m and 1.1 m, which reflects the seated person [14]. American standard ASHRAE 55 suggests that temperatures should be measured at heights 0.1 m and 1.7 m representing standing person [65]. The difference of temperature should not exceed 3 K. The idea here is to investigate the temperature difference between head and ankles of occupant. In Figure 29 is shown the percentage of occupants likely to feel dissatisfied due to vertical air temperature difference in the room.
Figure 29: Discomfort caused by vertical air temperature difference [14]
Radiant temperature asymmetry
People could experience a discomfort if large differences in temperatures of surrounding surfaces in the room are present. According to ASHRAE 55 is the maximum acceptable radiant temperature asymmetry 5 K in vertical direction (floor to ceiling) and 10 K in horizontal direction (wall to wall) [65]. Relation of dissatisfaction of occupants and radiant temperature asymmetry is shown in Figure 30.
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Figure 30: Discomfort caused by radiant asymmetry [14]
6.2.3 Velocity measurements
Draught feeling can be very annoying and therefore much attention should be paid to it. Draught is also most common cause of local discomfort. Draught rating index was used to investigate the critical areas in the room where the draught could bother occupants. Draught rating describes the magnitude of draught and is defined as a percentage of people predicted to be bothered by draught. The model applies for people at light, mainly sedentary activity with a thermal sensation for the whole body close to neutral and for the height of the neck level (1.1 m). For the abdomen and ankle level, the model could overestimate.
Draught rating was calculated according to Eq.(8) and it is recommended by ISO 7730 to be kept bellow value of 15% [29].
𝐷𝑅 = (34 − 𝑡𝑎)(𝑣 − 0,05)0,62(0,37 ∗ 𝑣 ∗ 𝑇𝑢 + 3,14) Eq.(8)
Where: DR is percentage of people dissatisfied due to draught [%], 𝑡𝑎 is temperature of air at place of measurement [°𝐶], 𝑣 is mean air velocity at place of measurement [m/s] and 𝑇𝑢 is turbulence intensity at place of measurement [%]. The turbulence intensity represents the magnitude of change of velocity at certain place and time in the space. Turbulent intensity is calculated by use of Eq.(9), Eq.(10) according to [59].
𝑇𝑢= 𝑆𝐷𝑣
𝑎𝑥100 Eq.(9)
𝑆𝐷 = �𝑛−11 ∑ (𝑣𝑛𝑖=1 𝑎𝑖− 𝑣𝑎)2 Eq.(10) Where: 𝑣𝑎 is mean velocity [m/s], 𝑣𝑎𝑖 is velocity at the time ”i” of the measuring period [m/s].
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Concerning general velocities in occupied zone, those should not exceed 0.15 m/s in winter and 0.25 m/s in summer [66]. The values could be possibly adjusted according to the activity and clothing level of the occupants. The value also depends on the designed thermal comfort class [14].
6.2.4 Tracer-gas measurements Ventilation effectiveness
Ventilation effectiveness gives us an idea about the removal of air-borne contaminant from the investigated space and also indicates the level of mixing in the test room. According to [14], “ventilation effectiveness is a function of location and characteristics of air terminal devices and of pollution sources”.
An influence on ventilation effectiveness has also flow rate and temperature of supply air. Ventilation effectiveness can be calculated according to Eq.(11).It is based on the measured values of tracer gas concentrations in supply, exhaust and room air in the occupied zone.
𝜀𝑣=𝐶𝐶𝑒−𝐶𝑠
𝑖−𝐶𝑠 Eq. (11)
Where: εv is ventilation effectiveness [-]; Ce is pollution concentration in the exhaust air [PPM]; Cs is pollution concentration in supply air (background concentration) [PPM] and Ci is pollution concentration in the breathing zone [PPM].
Ventilation effectiveness is directly linked to the position of pollutant source in the room. So if positions of future contaminants are not known, the ventilation effectiveness should not be the only indicator of ventilation performance as it could be very misleading.
Local age of air
Age of air describes a time required for a supply air to reach a particular point in the occupied zone. The air can, however, reach the point through different paths. The mean value of ages of air path is therefore used, called Local mean age of the air [67].
Local mean age of the air can be calculated according to a step up method adapted from Han [67]
according to Eq.(12)
𝜃𝑝= ∫ �1 −𝑜∞ 𝑐𝑐𝑝𝑝(∞)(𝑡)� 𝑑𝑡 Eq. (12)
Where θp is local mean age of air [s] and cp is measured concentration in point P in equilibrium [ppm].
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