Absorbed heat

If there is no volume change (dV= 0), then obviously there is no pressure-volume work done (du = 0) irrespective of the pressure, and it follows from equation (A2.1.10) that the change in energy is due entirely to the heat absorbed, which can be designated as qy. 

Note that in this special case, the heat absorbed directly measures a state fiinction. One still has to consider how this constant-volume heat is measured, perhaps by an electric heater , but then is this not really work Conventionally, however, if work is restricted to pressure-volume work, any remaining contribution to the energy transfers can be called heat . 

Thus for isobaric processes a new fimction, the enthalpy H, has been introduced and its change A// is more directly related to the heat that must have been absorbed than is the energy change At/. The same reservations about the meanmg of heat absorbed apply in this process as in the constant-volume process. 

By allowing compounds to react in a calorime ter It IS possible to measure the heat evolved in an exothermic reaction or the heat absorbed in an en dothermic reaction Thousands of reactions have been studied to produce a rich library of thermo chemical data These data take the form of heats of reaction and correspond to the value of the enthalpy change AH° for a particular reaction of a particular substance... 

A change in enthalpy indicates the heat absorbed or released during a chemical reaction at constant pressure. 

Below about 0.5 K, the interactions between He and He in the superfluid Hquid phase becomes very small, and in many ways the He component behaves as a mechanical vacuum to the diffusional motion of He atoms. If He is added to the normal phase or removed from the superfluid phase, equiHbrium is restored by the transfer of He from a concentrated phase to a dilute phase. The effective He density is thereby decreased producing a heat-absorbing expansion analogous to the evaporation of He. The He density in the superfluid phase, and hence its mass-transfer rate, is much greater than that in He vapor at these low temperatures. Thus, the pseudoevaporative cooling effect can be sustained at practical rates down to very low temperatures in heHum-dilution refrigerators (72). 

For example, vaporization may occur as a result of heat absorbed, by radiation and convection, at the surface of a pool of hquid or as a result of heat absorbed by natural convect ion from a hot wall beneath the disengaging surface, in which case the vaporization takes place when the superheated liquid reaches the pool surface. Vaporization also occurs from falling films (the reverse or condensation) or from the flashing of hquids superheated by forced convec tion under pressure. 

Heat rejec ted in the condenser ( cd consists of heat absorbed in the evaporator ( Eevap. nd energy W supplied by the compressor ... 

The boiler designer must proportion heat-absorbing and heat-recovery surfaces in a way to make the best use of heat released by the fuel. Water walls, superheaters, and reheaters are exposed to convection and radiant heat, whereas convection heat transfer predominates in air preheaters and economizers. The relative amounts of these surfaces vary with the size and operating conditions of the boiler. 

The efficiency, QE, is then defined as the heat absorbed minus the heat released divided by die heat absorbed in the cycle. This expression can, in turn, be transformed to show that the efficiency is equal to tire difference between the two temperarnres of operation, divided by the upper temperamre. 

AA is sometimes referred to as the change in work function. This equation simply states that energy will be available to do work only when the heat absorbed exceeds the increase in internal energy. For proeesses at constant temperature and pressure there will be a rise in the heat content (enthalpy) due both to a rise in the internal energy and to work done on expansion. This can be expressed as... 

Put in another way, since in equation (5.3) we have in effect only added FAT to each side of equation (5.1) it follows that energy will only be available to do work when the heat absorbed (FAS) exceeds the change in enthalpy, i.e. when AF has a negative value. 

The amount of heat absorbed by a vessel exposed to an open fire is markedly affected by the size and character of the installation and by the environment. These conditions are evaluated by the following equation, in whieh the effect of size on the heat input is shown by the exponent of A, the vessel wetted area, and the effect of other conditions is included in a factor F ... 

Figure 19. Chart for determining the amount of heat absorbed by a vessel exposed to an open fire.

The heat balanee operation at eonstant pressure on the first stage CFSTR is -GAh + Q = 0, where G = pu and Q = heat absorbed to the reaetor tanks. 

Fig ure 6-20. Heat absorbed by enthaipy change and ioss to environment. 

To form a heat balance for the flows. Net sensible heat flow from the space = Sensible heat absorbed by the ventilating air. 

Heat absorbed by chilled air flowing through the room. 

The basic idea of using TCR in a gas turbine is usually to extract more heat from the turbine exhaust gases rather than to reduce substantially the irreversibility of combustion through chemical recuperation of the fuel. One method of TCR involves an overall reaction between the fuel, say methane (CH4), and water vapour, usually produced in a heat recovery steam generator. The heat absorbed in the total process effectively increases... 

The amount of heat absorbed or lo.st by a substance that causes a change in the temperature of the substance is sensible heat, ft is called sensible heat because it can be measured by the change in temperature it causes. For example, as heat is added to a piece of steel the temperature of that steel increases and can be measured. The general equation for calculating sensible heat is ... 

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