Radiant temperature

Fig. 4. Comfort lines, ambient air temperature equals mean radiant temperature (4). To convert watts to kcal/min, multiply by 0.143.

In buildings away from outside perimeter walls, air and surface temperatures are usually approximately equal. The heat losses from a person by radiation (q ) and convection (q ) are then flowing to the same temperature level. In such uniform spaces, the radiant and convective losses are about equal and together account for about 80-90% of the total heat loss of a sedentary comfortable individual. In the presence of hot or cold surfaces, as may occur in perimeter or other locations in a building, the average surface temperature of the surroundings (called mean radiant temperature) as seen by the person s body may be substantially different from air temperature. If the mean radiant temperature (MRT) is greater or less than air temperature (T,) the person will feel warmer or colder than in a thermally uniform space where MRT =. ... 

How is mean radiant temperature (MRT) determined One could calculate or measure the surface temperatures of the room and calculate MRT from... 

ISO EN 7730 standardizes the PMV-PPD index as the method for evaluation of moderate thermal environments. To quantify the degree of comfort, the PMV (predicted mean vote) index gives a value on a 7-point thermal sensation scale -t-3 hot, +2 warm, +1 slightly warm, 0 neutral, -I slightly cool, -2 cool, -3 cold. An equation in the standard calculates the PMV index based on the six factors (clothing, activity, air and mean radiant temperatures, air speed, and humidity). 

The PMV index can be determined when the activity (metabolic rate) and the clothing (thermal resistance) are estimated and the following environmental parameters are measured air temperature, mean radiant temperature, relative air velocity, and partial water vapor pressure (see ISO EN 7726). 

FIGURE 6.5 Local thermal discomfort caused by radiant temperature asymmeti-y... 

The three categories in Table 6.3 apply to spaces where persons are exposed to the same thermal environment. It is advantageous if some kind of individual control over the thermal environment can be established for each person in a space. Individual control of the local air temperature, mean radiant temperature, or air velocity may contribute to reducing the rather large differences between individual requirements and therefore provide fewer dissatisfied. 

TABLE 6.6 Radiant Temperature Asymmetry for the Three Categories of Thermal Environment... 

ISO 7726 provides a description of the parameters that should be measured (air temperature, mean radiant temperature, plane radiant temperature, air velocity, and humidity) together with methods of measurement... 

Figure 8.53 gives measured values of the radiant temperature, which means Ty - T, caused by an electrical heating panel. The effective surface temperature of the panel can be estimated from the curves, then used to calculate the temperature of a thermometer bulb at a few other places. These results can be compared to measured results. 

Solution The first step is choosing a reference point. At the point 1 m right below the panel, the radiant temperature is 29 °C, and if we assurte that the temperature of air is 20 °C, then Tf, = 49 °C. 

FIGURE e.53 Measured values of radiant temperature near an electrical heating panel. Length of panel 1.53 m width 0.28 m. 

We see that the agreement between measured and calculated temperatures is fairly good. Only in the right corner near the heating panel is there a big difference between the measured and calculated temperature. However, the measured results cannot be reliable here either, because it is not possible that the radiant temperature is 80 just near a surface ol 467 C. 

FIGURE 8.54 Measured and calculated radiant temperature at different locations... 

TTie ability of the ventilation system to protect the worker efficiently can readily be determined by personal samples. The PIMEX method (see Chapter 12) can be used to determine the worker s exposure during various work phases. The capture efficiency as well as the supply air fraction can be measured using tracer gas techniques. Simple evaluation is carried out visually with smoke tube or pellet tests. Daily system evaluation is recommended using airflow or static pressure measurements at appropriate parts of the system. The air velocities, turbulence intensities, air temperature, mean radiant temperature, and air humidity should also be measured to provide an assessment ol thermal comfort. 

Equivalent temperature A synthetic comfort scale that takes into account the effects of dry bulb temperature, air movement, and mean radiant temperature. 

Fanger s comfort equations The various equations devised by Professor banger relating to activity, clothing, vapor pres sure, mean radiant temperature, air temperature, and air velocity. 

Globe temperature The temperature of the surroundings (mean radiant temperature) as recorded by a black globe thermometer. 

Mean radiant temperature The average temperature of the six surfaces of a cubicle enclosure, used in thermal comfort work and in other heat-transfer applications. It is the sum of all the surface areas multiplied by the temperature of the surface divided by the total surface area. 

Radiant temperature The temperature of a surface emitting heat to surrounding bodies by means of electromagnetic radiation. 

Radiant temperature asymmetry The difference between the plane radiant temperature and the temperatures at the sides of an element. 

Effective See Operative Environmental The sum of two-thirds of the mean radiant temperature and one-third of the air temperature. 

Radiant Temperature The temperature resulting from the body absorbing radiant energy. Radiation The transfer of heat with no medium. 

Figure 2-1.10 Small coal particles are burned in suspension, but larger particles are forced against the outer wall. The resulting slag is mostly liquid because of the high radiant temperature and low fusion temperature, and is drained from the bottom of the furnace through a tap.6 Cyclone furnaces are most common in utility and large industrial applications.

Table I shows the flexibility of the computational system. Six types of frequently encountered problems are classified according to their respective boundary conditions. In each classification, one or more run options can be selected. For example, Class 1 are typical simulation problems where the reactor outlet pressure and feed conversion are specified and the inlet pressure and radiant temperature are calculated. Alternatively, the effect of fouling can be determined by calculating a coking factor from a known pressure drop. The following examples illustrate applications of the system in problems under Classes 1, 5 and 6 respectively.

Diameter of reactor Length of reactor Radiant temperature Energy requirement Product distribution... 

Figure 6 shows the variations of the independent variables in successive iterations and their effects on the dependent variables. Initial estimates of the diameter, length, radiant temperature and inlet pressure have resulted in a reactor with residence time, outlet pressure, conversion, and tubeskin temperature all higher than specified values. 

In the first iteration, a reduction in the length and diameter successfully corrected most discrepancies except those of the outlet pressure and the residence time. To meet the latter specification, the diameter was further reduced. Simultaneously, the inlet pressure had to be increased to account for the resulting increase in pressure drop. Furthermore, as the heat transfer area decreased with diameter, the radiant temperature must also increase to supply a constant total heat input. 

For a constant volume, it can be shown that the surface area is proportional to /l. With a shorter reactor, and hence a smaller area, the radiant temperature must increase in order to provide a constant heat input causing the tubeskin temperature to rise beyond the design limit. 

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