Temperature Difference and Heat Transfer

Also note that you could have converted the temperature difference in d rees Rankine to Kelvin directly in the followio matmen 

The conversion ftictors among various units of heat are given in Table 11.2. 

Conversion Factors for Thermal Energy and per Unit Time (Power) 

Using the units of energy and time, show that 1 watt (W) is equal to 3.4123 Btu/h, as shown inlable 11.2. 

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These calculations should yield liquor concentrations in each effect that make possible a revised estimate of boihng-point rises. They also give the quantity of heat that must be transferred in each effect. From the heat loads, assumed temperature differences, and heat-transfer coefficients, heating-surface requirements can be determined. If the distribution of heating surface is not as desired, the entire calculation may need to be repeated with revised estimates of the temperature in each effect. 

In boiling liquids on a submerged surface it is found that the heat transfer coefficient depends very much on the temperature difference between the hot surface and the boiling liquid. The general relation between the temperature difference and heat transfer coefficient was first presented by Nukiyama(77) who boiled water on an electrically heated wire. The results obtained have been confirmed and extended by others, and Figure 9.52 shows the data of Farber and Scorah<78). The relationship here is complex and is best considered in stages. 

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 increase of the drying temperature difference (Tj t TouUet) decreases the heat requirement. The temperature differences and heat transfer rates, especially close to the nozzle are higher than in cmiventional cOTicurrent spray drying processes. And finally, an increase of the feed temperature due to higher heat transfer close to the nozzle decreases the droplet sizes and the heat transfer efficiency is increased. 

Unhke other refrigeration systems, the chiUed-water flow rate is of no particular importance in steam-jet system design, because there is, due to direct heat exchange, no influence of evaporator tube velocities and related temperature differences on heat-transfer rates. Widely varying return chiUed-water temperatures have Uttle effect on steam-jet equipment. 

Atj, = temperature difference between heat transfer surface and boiling liquid, °F... 

The driving forces, or driving potentials, for transport phenomena are (i) the temperature difference for heat transfer (ii) the concentration or partial pressure difference for mass transfer and (iii) the difference in momentum for momentum transfer. When the driving force becomes negligible, then the transport phenomenon will cease to occur, and the system will reach equilibrium. 

The higher Qin and Q0, the higher the required and associated temperature differences for heat transfer into the working fluid at high temperatures and out from the working fluid at low temperatures for heat exchangers with finite heat exchange area. If the assumption is made that these two losses... 

Bejan, A. and Tien, C.L., Natural Convection in a Horizontal Porous Medium Subjected to an End-to-End Temperature Difference , J. Heat Transfer, Vol. 100. pp. 191-198, 1978. 

Example 9.1 A refrigerated space is maintained at 10(°F), and cooling water is available at 70(°F). The evaporator and condenser are of sufficient size that a 10(°F) minimum-temperature difference for heat transfer can be realized in each. The refrigeration capacity is 120,000(Btu)(hr) 1, and the refrigerant is Freon-12. 

A vapor-compression refrigeration system is conventional except that a countercurrent exchanger is installed to subcool the liquid from the condenser by heat exchange with the v stream from the evaporator. The minimum temperature difference for heat transfer is 10(°F). Amm is the refrigerant, evaporating at 22(°F) and condensing at 80(°F). The heat load on the evapo is 2,000(Btu)(s) . If the compressor efficiency is 75 percent, what is the power requirement ... 

Temperature control for laboratory reactors is typically easy because of high heat transfer area-reactor volume ratios, which do not require large driving forces (temperature differences) for heat transfer from the reactor to the jacket. Pilot- and full-scale reactors, however, often have a limited heat transfer capability. A process development engineer will usually have a choice of reactors when moving from the laboratory to the pilot plant. Kinetic and heat of reaction parameters obtained from the laboratory reactor, in conjunction with information on the heat transfer characteristics of each pilot plant vessel, can be used to select the proper pilot plant reactor. 

In Section 1 I, we defined heat as the form of energy that can be transferred from one system to another as a result of temperature difference. A thermodynamic analysis is concerned with the amount of heat transfer as a system undergoes a process from one equilibrium state to another. The science that deal with the detenninalton of the rates of such energy transfers is the heat transfer The transfer of energy as heat is always from the higher-temperature medium to the lower-temperature one, and heat transfer stops when the two mediums reach tlie same temperature. 

This temperature drop creates temperature differences within the water at the top as well as between the water and the surrounding air. These temperature differences drive heat transfer toward the water surface from both the air and the deeper paits of the water, as shown in Figure 14-52, If the evaporation rate is high and thus the demand for the heat of vaporization is liigher than the amount of heat that can be supplied from the lower parts of the water body and the surroundings, the deficit is made up from the sensible heat of the water at the surface, and thus the temperature of water at the surface drops further. The process continues until the latent heat of vaporization equals the heat tran.sfer to the water at the surface. Once the steady operation conditions are reached and the interface temperature stabilizes, the energy balance on a thin layer of liquid at the surface can be expressed as... 

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