Sensible heat calculations

If a phase transition takes place between the specified and datum temperatures, the latent heat of the phase transition is added to the sensible-heat change calculated by equation 3.11. The sensible-heat calculation is then split into two parts ... 

A typical heat balance for Run LSF 34 on No. 6 oil is given in Table V. The calculated efficiencies are also given in the table. Heat input terms consist of the input heat from the fuel, the fuel sensible heat, and the makeup water sensible heat. The heat available from combustion of the fuel is calculated from the measured volumetric flow rate, the measured fuel heating value, and the measured fuel density at the nozzle temperature. The fuel sensible heat contains the fuel mass flow rate, the measured temperature at the nozzle, a reference temperature, and an estimated specific heat for the oil of 0.480 Btu/lb°F. The specific heat was taken from graphical information in the ASME Power Test Code. Similarly, the water sensible heat calculation contains a tabular value... 

Solution The fraction of liquid vaporized on release is calculated from a heat balance. The sensible heat above saturated conditions at atmospheric pressure provides the heat of vaporization. The sensible heat of the superheat is given by... 

If the vapor is superheated at the inlet, the vapor may first be desuperheated by sensible heat transfer from the vapor. This occurs if the surface temperature is above the saturation temperature, and a single-phase heat-transfer correlation is used. If the surface is below the saturation temperature, condensation will occur directly from the superheated vapor, and the effective coefficient is determined from the appropriate condensation correlation, using the saturation temperature in the LMTD. To determine whether or not condensation will occur directly from the superheated vapor, calculate the surface temperature by assuming single-phase heat transfer. 

Under steady-state conditions the temperature of the evaporating surface increases until the rate of sensible heat transfer to the surface equals the rate of heat removed by evaporation from the surface. To calculate this temperature, it is convenient to modify Eq. (12-26) in terms of humidity rather than partial-pressure difference, as follows ... 

The constant-molar-overflow assumption represents several prior assumptions. The most important one is equal molar heats of vaporization for the two components. The other assumptions are adiabatic operation (no heat leaks) and no heat of mixing or sensible heat effects. These assumptions are most closely approximated for close-boiling isomers. The result of these assumptions on the calculation method can be illustrated with Fig. 13-28, vdiich shows two material-balance envelopes cutting through the top section (above the top feed stream or sidestream) of the column. If L + i is assumed to be identical to L 1 in rate, then 9 and the component material balance... 

All heat absorption from fire exposure is considered as latent heat and no credit is taken for the sensible heat capacity of the fluid in the vessel. The vapor release rate W is calculated from ... 

Ehie to the dilute nature of the stream, we may assume that the latent heat of condensation for MEK is much smaller than the sensible heat removed from the gas. Therefore, we can apply the procedure presented in Section 10.5. Once a value is selected for ATj ", Eq. (10.19) can be used to determine T and Eqs. (10.3b) and (10.7) can be employed to calculate the value of Since the bounds on p (and consequently on AT ) are tight, we will iterate over two values of 0.96 and 1.00 (Ar2 " = 90.0 and 94.5 K). The other iterative variable is AT, . Both variables are used to trade off fixed versus operating costs. As an illustration, consider the following iteration AT," = 5 K and... 

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 ... 

If a phase change occurs in the process stream for which heat duties are being calculated, it is best to perform a flash calculation and determine the heat loss or gain by the change in enthalpy. For a quick hand approximation it is possible to calculate sensible heat for both the gas and liquid phases of each component. The sum of all the latent and sen -i-ble heats is the approximate total heat duty. 

The energy term E must be defined to calculate energy-scaled standoff R. The energy term represents the sensible heat that is released by that portion of the cloud contributing to the blast wave. Any of the accepted methods of calculating vapor cloud explosive energy are applicable to the Baker-Strehlow method. These methods include ... 

Note If sensible heat, Q, is required to bring the fluid up to the boiling point, this must be calculated separately, and the area of heat transfer must be added to that determined for the fluid boiling requirement, Q,. 

Calculate the required area for sensible heat transfer. 

Select an increment of vaporization, beginning at the end of the sensible heat zone. Use an average value of x for the increment calculations. The circulation rate that must be already developed on the basis of average conditions should be used for the initial calculations. 

This process is shown as a horizontal line on the chart, since there is no change in the moisture content. The loss of sensible heat can be read off the chart in terms of enthalpy, or calculated from the dry bulb reduction, considering the drop in the sensible heat of both the dry air and the water vapour in it. 

In the previous discussion it has been assumed that the vapour is a pure material, such as steam or organic vapour. If it contains a proportion of non-condensable gas and is cooled below its dew point, a layer of condensate is formed on the surface with a mixture of non-condensable gas and vapour above it. The heat flow from the vapour to the surface then takes place in two ways. Firstly, sensible heat is passed to the surface because of the temperature difference. Secondly, since the concentration of vapour in the main stream is greater than that in the gas film at the condensate surface, vapour molecules diffuse to the surface and condense there, giving up their latent heat. The actual rate of condensation is then determined by the combination of these two effects, and its calculation requires a knowledge of mass transfer by diffusion, as discussed in Chapter 10. 

The use of mean heat capacities often facilitates the calculation of sensible-heat changes mean heat capacity over the temperature range r, to t2 is defined by the following equation ... 

REM SUBROUTINE TO CALCULATE STREAM SENSIBLE HEATS 590 PRINT... 

The program ENERGY 1 was used to calculate the sensible heat in the inlet and outlet gas streams. The composition of the inlet stream and the heat capacity data will be the same as that for the WHB outlet given above. Outlet stream flows from flow-sheet, converted to kmol/h ... 

Before equation 12.1 can be used to determine the heat transfer area required for a given duty, an estimate of the mean temperature difference A Tm must be made. This will normally be calculated from the terminal temperature differences the difference in the fluid temperatures at the inlet and outlet of the exchanger. The well-known logarithmic mean temperature difference (see Volume 1, Chapter 9) is only applicable to sensible heat transfer in true co-current or counter-current flow (linear temperature-enthalpy curves). For counter-current flow, Figure 12.18a, the logarithmic mean temperature is given by ... 

If the degree of superheat is large, it will be necessary to divide the temperature profile into sections and determine the mean temperature difference and heat-transfer coefficient separately for each section. If the tube wall temperature is below the dew point of the vapour, liquid will condense directly from the vapour on to the tubes. In these circumstances it has been found that the heat-transfer coefficient in the superheating section is close to the value for condensation and can be taken as the same. So, where the amount of superheating is not too excessive, say less than 25 per cent of the latent heat load, and the outlet coolant temperature is well below the vapour dew point, the sensible heat load for desuperheating can be lumped with the latent heat load. The total heat-transfer area required can then be calculated using a mean temperature difference based on the saturation temperature (not the superheat temperature) and the estimated condensate film heat-transfer coefficient. 

The global heat accumulation is summarized in Figure 10. The basic idea is that thermal pollution is released on Earth. Part of this heat is emitted to space as OLR. The remaining heat is first released into the atmosphere. Part of it will accumulate in the air, which in its turn warm the ground and water. In the air heat is stored as both latent and sensible heat. In the ground heat is stored as sensible heat. It is easy to calculate heat that is accumulating in air and ground. The problem is to estimate the heat that is stored in water. 

Maa (M2) developed a procedure for calculating the liquid surface temperature as a function of the time each liquid element is in contact with the vapor. He assumed that the latent heat of vaporization is transferred from the interior of the liquid to the interface by pure conduction. Consequently, the sole source of energy for vaporization is the sensible heat made available by a change in the liquid temperature. If exposure time is short, only the liquid near the surface will undergo a temperature change. The heat transfer within the liquid is modeled by... 

The equipment requirements that we have determined are well within the realm of technical feasibility and practicality. The heat transfer requirements are easily attained in equipment of this size. The fact that some of the heat transfer requirements are positive and others negative indicates that one should probably consider the possibility of at least partial heat exchange between incoming cold feed and the effluent from the second or third reactors. The heat transfer calculations show that the sensible heat necessary to raise the cold feed to a temperature where the reaction rate is appreciable represents a substantial fraction of the energy released by reaction. These calculations also indicate that it would be advisable to investigate... 

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