Heat transfer thermal insulation

The limitation of the heat transfer (thermal insulation) by an intumescent coating is illustrated by the temperature profile in a PP-APP/PER sheet (under a heat flux of 50kW/m ) versus time and depth, which presents three successive plateaux (Figure 3). 

Heat Transfer in Insulated Pipes Solve case (b) of Problem 2.22 for a composite tube made of material of thermal conductivity kj for / , < r < Rm and of material of thermal conductivity k0 for Rm < r < R0. 

The so-called apparent thermal conductivity of insulating materials depends upon four modes of heat transfer gas conduction and convection, radiation, and solid conduction. The principles of these four mechanisms of heat transfer are fairly well understood individually but their combined effect on heat transfer in insulating materials is complicated. Nevertheless, because of the additive nature of the heat transferred by these mechanisms, the conductivities assigned to each mechanism are additive. Thus, if each of these conductivities can be evaluated under various conditions of temperature and pressure, their sum stated as an apparent conductivity may be estimated. 

In summary, data have been presented for the effect of pressure on the room temperature thermal conductivity of four Fiberglas insulating materials. The data have been interpreted in terms of the fundamental mechanisms of heat transfer through insulation. Methods are indicated for correcting the low pressure data to conductivity values at low temperature by treating separately the effect of temperature on the solid conduction and radiation contributions to conductivity. 

A guarded hot-plate method, ASTM D1518, is used to measure the rate of heat transfer over time from a warm metal plate. The fabric is placed on the constant temperature plate and covered by a second metal plate. After the temperature of the second plate has been allowed to equiUbrate, the thermal transmittance is calculated based on the temperature difference between the two plates and the energy required to maintain the temperature of the bottom plate. The units for thermal transmittance are W/m -K. Thermal resistance is the reciprocal of thermal conductivity (or transmittance). Thermal resistance is often reported as a do value, defined as the insulation required to keep a resting person comfortable at 21°C with air movement of 0.1 m/s. Thermal resistance in m -K/W can be converted to do by multiplying by 0.1548 (121). 

The optimization of heat-transfer surfaces also plays a role. At the optimum, the lifetime cost of a surface is approximately equal in value to the lifetime cost of power used to overcome the temperature differential in the condenser and evaporator. Additionally, condensation on insulation is a sign of questionable insulation (see Insulation, thermal). Frost is a certain signal that insulation can be improved. 

Materials or combinations of materials which have air- or gas-fiUed pockets or void spaces that retard the transfer of heat with reasonable effectiveness are thermal insulators. Such materials may be particulate and/or fibrous, with or without binders, or may be assembled, such as multiple heat-reflecting surfaces that incorporate air- or gas-filled void spaces. 

Low heat transfer capacity. Too much thermal insulation. 

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. 

The sum includes concentric cylinder layers, such as the layer between the outer and inner diameters of the pipe or a possible thermal insulation layer. For each layer the corresponding heat conductivity Aj is used. The outer heat transfer fac-ror is the sum of the proportions of convection and radiation. Note Very thin pipes or wires should not be insulated. Because the outer diameter of the insulation is smaller than A/a , the resistance is less than that without the insulation.)... 

Origins of the science associated with thermal insulations coincide with the development of thermodynamics and the physics associated with heat transfer. These technical subjects date to the eighteenth century. Early obseiwations that a particular material was useful as thermal insulation were not likely guided by formal theoi y but rather by trial and error. Sawdust was used, for example, in the nineteenth centui y to insulate ice storage buildings. 

Reducing heat transfer with gas fill. Conduction and convection cause heat transfer across the air spaces in multilayer windows. Although air is a relatively good insulator, other gases that have lower thermal conductivity can be sealed into the cavities... 

Transparent or translucent insulating materials (TIMs) can provide light or solar gains without view. TIMs typically have thermal properties similar to conventional opaque insulation and are thicker than conventional insulating glass units, providing significant resistance to heat transfer. 

L = wind velocity factor, Btu/hr-ft -°F ho = convective heat transfer coefficient, Btu/hr-ft °F hj = steam, heat transfer coefficient, Btu/hr-ft °F ko = thermal conductivity of insulation, Btu/hr-ft-°F L = length of pipe, ft n = number of tracers... 

A pipeline of 100 mm outside diameter, carrying steam at 420 K, is to be insulated with a lagging material which costs 10/m3 and which has a thermal conductivity of 0.1 W/m K. The ambient temperature may be taken as 285 K, and the coefficient of heat transfer from the outside of the lagging to the surroundings as 10 W/m2 K. If the value of heat energy is 7.5 x 10 4 /MJ and the capital cost of the lagging is to be depreciated over 5 years with an effective simple interest rate of 10 per cent per annum based on the initial investment, what is the economic thickness of the lagging ... 

If a layer of insulating material 25 mm thick, of thermal conductivity 0.3 W/m K, is added, what temperatures will its surfaces attain assuming the inner surface of the furnace to remain at 1400 K The coefficient of heat transfer from the outer surface of the insulation to the surroundings, which are at 290 K, may be taken as 4.2. 5.0, 6.1, and 7.1 W/m K, for surface temperatures of 370, 420, 470, and 520 K respectively. What will he the reduction in heat loss ... 

Would it be feasible to use a magnesia insulation which will not stand temperatures above 615 K and has a thermal conductivity 0.09 W/m K for an additional layer thick enough to reduce the outer surface temperature to 370 K in surroundings at 280 K Take the surface coefficient of heat transfer by radiation and convection as 10 W/m- IC... 

We have designed, manufactured and tested a prototype that may be applied in thermal control of electronic devices. It was fabricated from a silicon substrate and a Pyrex cover, serving as both an insulator and a window through which flow patterns and boiling phenomena could be observed. A number of parallel triangular micro-channels were etched in the substrate. The heat transferred from the device was simulated by different types of electrical heaters that provided uniform and non-uniform heat fluxes, defined here respectively as constant and non-constant values... 

Though short fiber-reinforced mbber composites find application in hose, belt, tires, and automotives [57,98,133,164] recent attention has been focused on the suitability of such composites in high-performance applications. One of the most important recent applications of short fiber-mbber composite is as thermal insulators where the material will protect the metallic casing by undergoing a process called ablation, which is described in a broad sense as the sacrificial removal of material to protect stmcrnres subjected to high rates of heat transfer [190]. Fiber-reinforced polymer composites are potential ablative materials because of their high specific heat, low thermal conductivity, and ability of the fiber to retain the char formed during ablation [191-194]. 

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