Airflow heating

It is also useful to make a steady-state balance with respect to airflow, heat flow, and mass flow of substances into and out of the enclosure. See Chapter 7 for details with respect to calculation of heat and contaminant emission. 

A simple direct-fired dryer with once through airflow heating, especially with gas, can be achieved with little noise generation. On the other hand, a more efficient air recirculation unit often requires high-noise burners and ancillaries. 

Either system requires that the resin be exposed to adequate airflow heated to the proper temperature for the time prescribed by the resin manufacturer. The equipment required for these two types of drying systems varies considerably in terms of cost and complexity (see Figs. 7.26 and 7.27). 

FIGURE 7.1 TGA of EVA, EVA microcomposite with 5 wt% Na" -montmorillonite, and EVA nanocomposite with 5 wt% organoclays under a) helium and b) airflow. Heating rate, 20°C/min EVA, Escorene UL-00328 with 28 wt% vinyl acetate content. Organoclay, Nanofil 15. (From Ref. 48, copyright 2001, John Wiley Sons, Ltd., with permission.)... 

Alternative to the airflow heating discussed in Sec. 27.8.3, heater blankets are sometimes employed on the inside or outside of the battery case. These blankets may be energized with any available electric energy source. Primarily this source will be either the DC bus of the same voltage as the battery or an aircraft AC bus of higher potential. Heaters may also be energized from an auxiliary ground supply. Heater blankets have the inherent poor thermal time constant of nonair-cooled batteries. 

For heat transfer directly to solids, predictive equations give directly the volume V or the heat-transfer area A, as determined by heat balance and airflow rate. For devices with gas flow normal to a fluidized-solids bed,... 

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. 

Circulation of air at velocities of I to 10 m/s is desirable to improve the surface heat-transfer coefficient and to eliminate stagnant air pockets. Proper air flow in tray dryers depends on sufficient fan capacity, on the design of ductwork to modify sudden changes in direction, and on properly placed baffles. Nonuniform airflow Is one of the most serious problems in the operation of tray di yers. 

Evaporative Cooling of the Turbine. Traditional evaporative coolers that use media for evaporation of the water have been widely used in the gas turbine industry over the years, especially in hot climates with low humidity areas. The low capital cost, installation, and operating costs make it attractive for many turbine-operating scenarios. Evaporation coolers consist of water being sprayed over the media blocks, which are made of fibrous corrugated material. The airflow through these media blocks, evaporates the water, as water evaporates, it consumes about 1059 BTU (1117kJ) (latent heat of vaporization) at 60 °F (15 °C). This results in the reduction... 

Nozzle Area of the First Turbine Stage Expander Stage). This is a very eritieal parameter and limits the total airflow into the turbine seetion, thus this limits the amount of steam injeetion or the amount of the heated and humidified eompressed air injeetion. 

This is the state equation of an ideal gas, where p is pressure, v is specific volume, p is density, R is the gas constant, and T is absolute temperature. In an airflow there is a transfer of heat from one layer to another. This change of... 

Above we considered the question of which temperature the damp cloth settles to when it is thermally insulated against all surroundings but the airflow, and when it can be assumed that there is no radiation heat transfer between the cloth and the airflow. In this consideration the state of the air has been constant. 

It is important to emphasize that, especially in process measurements, radiation can have an essential influence on the wet bulb temperature, and therefore generally the wet bulb temperature is dependent on the mea,surement device and the method of measurement. If the airflow is very low, the radiation can have a remarkable contribution in addition to the convective heat transfer. Basically, an equation analogous to Eq. (4.138) can be empirically determined for each wet bulb temperature and method of measurement. 

Cooling towers are commonly used for water cooling, but they can also be used for heat recovery from outlet air. If the water temperature is higher than the dewpoint of the air, water will cool in the tower. Cooling is caused by vaporization on the surface of the water drops. The vaporization energy comes from the inner energy of the water and in a certain phase, when the water temperature is lower than the dry bulb temperature of the air, also from the airflow. When the water temperature drops to near the air wet bulb temperature at the observation point,... 

Air-handling units equipped with heat recovery and sophisticated control of the key parameters of HVAC systems, such as temperature, airflow rate, and pressure difference... 

FIGURE 7.7 Schematics of air supply (o) with inclined jets toward the occupied zone (b) with horizontal jets and occupied zone ventilated by reverse flow (c) with vertical jets. Shaded areas show the effect of buoyant forces on airflow pattern when supply air is excessively heated over the room air" ... 

Local exhausts arc designed to capture air pollutants and heat at the source, and thus their location and the exhausted airflow rate should ensure sufficient capture velocity. 

Experimental studies conducted by Grimitlyn on heated and chilled confined jets showed that the airflow pattern remains the same as for isothermal... 

Similar studies were conducted by Troyanovsky, who concluded that to maintain the airflow pattern in rooms with heated or cooled air supply as in isothermal conditions, it is necessary that the rise of horizontally supplied jet does not exceed Ay = 0.1 BH at the distance from the outlet X = 0.15K BH) -. From this assumption the following equation for the maximum air temperature difference was derived ... 

Nielsen, P. V. 1980. The influence of ceiling-mounted obstacles on the airflow pattern in air-conditioned rooms at different heat loads. Building Service Engineering Research and Jeebnoi-ogy, vol. 1, no. 4. 

Thermal plumes above point (Fig. 7.60) and line (Fig. 7.61) sources have been studied for many years. Among the earliest publications are those from Zeldovich and Schmidt. Analytical equations to calculate velocities, temperatures, and airflow rates in thermal plumes over point and line heat sources with given heat loads were derived based on the momentum and energy conservation equations, assuming Gaussian velocity and excessive temperature distribution in... 

A point source has a convective heat output of 100 W (see Fig. 7, >2). Determine the airflow rate 1 m above the source. 

In reality, heat sources are seldom a point, a line, or a plane vertical surface. The most common approach to account for the real source dimensions is ro use a virtual source from which the airflow rates are calcu-lared " " see Fig. 7.64. The virtual origin is located along the plume axis at a distance on the other side of the real source surface. The adjustment of the point source model to the realistic sources using the virtual stmrce method gives a reasonable estimate of the airflow rate in thermal plumes. The weakness of this method is in estimating the location ol the virtual point source. 

The airflow rate from a heat source can then be calculated as half of the flow from a source with a heat emission of... 

Calculate maximum air velocity, airflow rate, and excessive temperature (relative to the ambient air temperature equal to 20 °C) in thermal plume above the heated cube (0.66 m x 0.66 m x 0.66 m) with convective heat production = 225 W, at heights of 2.0 m and 4.0 m above the floor level. Neglect temperature gradient along the room height. Compare the results with predictions made for the same case using CFD code (Fig. 7.80). 

FIGURE 7.80 CDF-predicted values of maximum velocity V, temperature differential, ( C), and airflow, q (Us), in the horizontal cross-section of the buoyant plume above the heated cube (0.66 m x 0.66 m X 0.66 m, 22SW).i ... 

Natural ventilation design allows one to size the inlets, and outlets, / p based on their pressure loss characteristics, Cp, and on the airflow rate, G , required to maintain the occupied zone within desired limits. The reverse design procedure is commonly used to evaluate the airflow rate through the building given the sizes, characteristics, and locations of inlets and outlets and the heat load and characteristics of heat sources. 

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,... 

Typically, the share of the static pressure across the inlets, p, is selected to be between 0.1 and 0.4. This allows one to keep a low velocity of airflow through inlets so as not to disturb thermal plumes above heat sources. 

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