The air temperature

The air temperature at the earth s surface varies over a rather wide range. Extreme values are achieved at the equator and at poles (60 and -90°C, respectively). Changes of the air temperature in the lowest layers of the atmosphere are mainly governed by changes of the earth s surface temperature, which leads to an important local variability of the temperature. 

The distribution of the atmospheric temperature, together with its continuous changes, describe the temperature regime of the atmosphere. It is the result of the exchange of heat between the earth s surface and atmosphere, atmosphere and cosmic space and mutual exchange of heat between particular air masses. 

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The Calibration of the positioning system is carried out using a bar with a given distance which is placed between the referenspoint on the microphone collar and the probe. The distance is then entered into the acquisition software together with informations of the air temperature close to the tested object, pipe dimension, type of UT-probe (probe height) and scanning direction. 

Fig. 3. Comfort 2ones at 6% of population predicted dissatisfied from ref 4. RH lines are valid only when the air temperature equals the average temperature of the surfaces, (a) Operative temperature range where ET is effective temperature as defined in text, (b) Comfort 2one detail. To convert...

An important characteristic of solvents is rate of evaporation. Rates of solvent loss are controUed by the vapor pressure of the solvent(s) and temperature, partial pressure of the solvent over the surface, and thus the air-flow rate over the surface, and the ratio of surface area to volume. Tables of relative evaporation rates, in which -butyl acetate is the standard, are widely used in selecting solvents. These relative rates are deterrnined experimentally by comparing the times required to evaporate 90% of a weighed amount of solvent from filter paper under standard conditions as compared to the time for -butyl acetate. The rates are dependent on the standard conditions selected (6). Most tables of relative evaporation rates are said to be at 25°C. This, however, means that the air temperature was 25°C, not that the temperature of the evaporating solvent was 25°C. As solvents evaporate, temperature drops and the drop in temperature is greatest for solvents that evaporate most rapidly. 

Forced and Induced Draft The forced-draft unit, which is illustrated in Fig. 11-43 pushes air across the finnedtube surface. The fans are located oelow the tube bundles. The induced-draft design has the fan above the bundle, and the air is pulled across the finned tube surface. In theoiy, a primaiy advantage of the forced-draft unit is that less power is required. This is true when the air-temperature rise exceeds 30°C (54°F). 

Operating co.sts. Power requirements for air-cooled heat exchangers can be lower than at the summer design condition provided that an adequate means of air-flow control is used. The annual power requirement for an exchanger is a function of the means of airflow control, the exchanger seiwice, the air-temperature rise, and the approach temperature. 

A further factor affecting k- is the air-sea temperature difference. When the sea is colder than the air above it, the enhanced solubility of the gas in the water (relative to the air temperature) tends to increase kj. This will occur in summer in sub-polar waters and over upwelling regions. The opposite is also found, and much of the ocean equatorward of 45"" latitude is colder than the overlying air for much of the year. However, air-sea temperature differences are generally less than 2-3 "C so that this effect results in a less than 10% modulation of k- on average. 

If an ESP is 90% efficient for particulate removal, what overall efficiency would you expect for two of the ESPs in series Would the cost of the two in series be double the cost of the single ESP List two specific cases in which you might use two ESPs in series The gaseous effluent from a process is 30 m min at 65°C. How much natural gas at 8900 kg cal m would have to be burned per hour to raise the effluent temperature to 820 "C Natural gas requires 10 m of air for every cubic meter of gas at a theoretical air fuel ratio. Assume the air temperature is 20°C and the radiation and convection Iosm s are 10%. 

This equation is good if the air temperature is 50°F or above, the cooling tower s approach to the wet bulb temperature is 5°F or above, and Hog is within a range of about 0.1 to 8. 

Local air motion is another thermal nonuniformity that can cause a local cooling of the skin and the feeling of a draft. Draft discomfort from local air motion increases as the air temperature decreases below skin temperature. Fluctuations in the local air motion increase the perception of drafts and should be avoided. The unsteadiness of air motion is often described in terms of its turbulence intensity (Tu) ... 

The air Temperature senaor shall be offeccivcly protected from -my effects of thermal radiation cxmiing from hot or C(>ld walls. 

Characteristics of the air jet in the room might be influenced by reverse flows, created by the jet entraining the ambient air. This air jet is called a confined jet. If the temperature of the supplied air is equal to the temperature of the ambient room air, the jet is an isothermal jet. A jet with an initial temperature different from the temperature of the ambient air is called a nonisother-mal jet. The air temperature differential between supplied and ambient room air generates buoyancy forces in the jet, affecting the trajectory of the jet, the location at which the jet attaches and separates from the ceiling/floor, and the throw of the jet. The significance of these effects depends on the relative strength of the thermal buoyancy and inertial forces (characterized by the Archimedes number). 

Starting of the fan when the serviced aperture is opened and when the air temperature near the closed aperture deviates from a set value... 

The use of a natural ventilation system assumes temperature stratification throughout the room height. Air close to heat sources is heated and rises as a thermal plume (Fig. 7.105). Part of this heated air is evacuated through air outlets in the upper zone, and part of it remains in the upper zone, in the so-called heat cushion. The separation level between the upper and lower zones is defined in terms of the equality of and G, which are the airflow rate in thermal plumes above heat sources and the airflow supplied to the occupied zone, respectively. It is assumed that the air temperature in the lower zone is equal to that in the occupied zone, and that the air temperature in the upper zone is equal to that of the evacuated air,... 

Tj is the surface temperature of the panel, Tj, the thermometer bulb temperature, Tj the air temperature, and T the temperature of the walls of the building. F is the view factor from the bulb to the heating panel, e is the emissivity of the thermometer bulb at temperature T cr is the Stefan-Boltzmann constant (5.67 X 10 W m K " ), and is the convective heat transfer coefficient from bulb to air. 

Determine the air temperature that is necessary to maintain a volume of 1200 m- with six air changes per hour at 20 "C, if the total heat loss is 45 kW. 

FIGURE 9.57 A ventilation system for controlling the air temperature by humidifying and dehumidifying the air. 

Experimental laboratory results and results from a full-scale modeb have shown the relation between the dispersed thermal power inside and the air temperature difference between the two sides of air curtain. The results shown in Fig. 10.67 are for different conditions. There are no other experimental data readily available, so caution is needed when applying these results to the design of an air curtain. 

The air temperature of a room at any given time is given hy a heat-balance equation which includes the heat flux exchanged by convection at each wall element (A q ), the heat flow exchanged by ventilation (4>j,), the convective part of heat flow due to internal heat gains the convective part of heat flow due to the HVAC system (< >[,(.), and the variation of energy in the room air(r M ... 

The thermal comfort was evaluated with hourly mean values of the air temperature in the occupied zone, plotted against the maximum I h mean outdoor temperature value of the day. Only the period from April 1 to October 30 and only working hours (7 a.m. to 6 p.m. are considered. 7 his evaluation method is based on the Swiss standard SIA V382/2. The minimum and maximum allowable comfort temperatures are adapted to the usual activity and clothing levels of the workers in the hall (see Figs. 11.55 and 11.56). 

The hot-wire anemometer sensor is a very fine wire with a diameter of few micrometers and length of few millimeters. This wire is connected to a measurement bridge and an electrical current is fed through the wire. The wire is heated to a temperature above the air temperature and the air velocity is determined by the cooling effect of the wire. The voltage over the wire, U, is a function not only of the velocity but also of the excess temperature and the fluid properties in the following way ... 

A calibration facility must produce the desired velocity range for the meter to be calibrated. The air temperature should be kept constant over the test to ensure constant density. For thermal anemometers, velocity calibration only is not sufficient. They should also be checked for temperature compensation. In the case of omnidirectional probes, sensitivity to flow direction should be tested. In the case of low-speed (thermal) anemometers, their self-convection error should be measured, and, for instruments measuring flow fluctuation (turbulence), dynamic characteristics testing should be carried out as well. ... 

To calibrate larger sensors/instruments such as vane anemometers, a wind tunnel is required. A calibration wind tunnel consists of an open or closed tunnel, a fan to deliver the air, a nozzle to shape the velocity profile, and a mesh arrangement to uniform and reduce the flow turbulence. It may be necessary to control the air temperature in the tunnel by means of a heating/cooling sys-... 

Expired air temperature The air temperature of the breath on leaving the nose. 

Kata cooling power The rate of cooling of a silvered or unsilvered kata thermometer due to the relationship between the air temperature and the air velocity over the bulb. 

Dry bulb The air temperature recorded by a dry bulb thermometer, a sensory device excluding any effects of moisture or radiation. 

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

Room reference The average of at least five measurements of the air temperature at a height of 1.1 m from the floor and outside the area directly influenced by the device. 

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