Sensible heat transfer

The term e/(e — 1), which appears in equations 1 and 2, was first developed to account for the sensible heat transferred by the diffusing vapor (1). The quantity S represents the group ratio of total transported energy to convective heat transfer. Thus it may be thought of as the fractional... 

The effect on the coolant temperature of latent and sensible heat transferred to the surface from the condensing vapor is as shown in equation 5 ... 

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. 

Performance testing of heat exchangers is described in the American Institute of Chemical Engineers Standard Testing Procedure for Heat Exchangers, Sec. 1. Sensible Heat Transfer in SheU-and-Tube-Type Equipment. ... 

The jacketed solid-flight type (Fig. 11-60 ) is the standard low-cost (parts-basis-priced) material-handling device, with a simple jacket added and employed for secondary-range heat transfer of an incidental nature. Heat-transfer coefficients are as low as 11 to 34 W/ (m °C) [2 to 6 Btii/(h fU °F)] on sensible heat transfer and 11 to 68 W/(m °C) [2 to 12 Btii/(h fU °F)] on diying because of substantial static solids-side film. 

The heat-transfer process involves (1) latent heat transfer owing to vaporization of a small portion of the water and (2) sensible heat transfer owing to the difference in temperature of water and air. Approximately 80 percent of this heat transfer is due to latent heat and 20 percent to sensible heat. 

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 ... 

Note that Q refers only to sensible heat transfer. All latent heat is transferred via mass transfer. Likewise, h refers only to a dry gas coefficient (no condensation considered). 

When the dry-bulb temperature of a gas is altered, a p>ositive or negative sensible-heat transfer takes place, in terms of both the dry bulb and its associated moisture content. 

H = rate sensible heat transfer to ait column (BTU/mln) ijj. = total airflow rate (ftVmin)... 

Insulation of clothing The resistance to sensible heat transfer provided by a clothing ensemble, reflecting the intrinsic insulation between the skin and the surface of the clothing, excluding the resistance provided by the layer of air surrounding the clothed body. 

For counter-current flow of the fluids through the unit with sensible heat transfer only, this is the most efficient temperature driving force with the largest temperature cross in the unit. The temperature of the outlet of the hot stream can be cooler than the outlet temperature of the cold stream, see Figure 10-29 ... 

In parallel operation (sensible heat transfer), fluids A and B (Figure 10-30) flow in the same direction along the length of travel. They enter at the same general position in the exchanger, and their temperatures rise and fall respectively as they approach the outlet of the unit and as their temperatures approach each other as a limit. In this case the outlet temperature, tg, of fluid B, Figure 10-30, cannot exceed the outlet temperature, Tg, of fluid A, as was the case for counterflow. In general, parallel flow is not as efficient in the use of available surface area as counterflow. 

The following nucleate or alternate designs procedure, suggested by Kem, is for vaporization (nucleate or pool boiling) only. No sensible heat transfer is added to the boiling fluid. 

Suggested Procedure for Vaporization with Sensible Heat Transfer... 

Calculate the required area for sensible heat transfer. 

If sensible heat exchange exists, follow steps 2-6 of the Suggested Procedure for Vaporization with Sensible Heat Transfer, previously in this chapter. 

Evaluate sensible heat transfer inside tubes as previously oudined for in-tube transfer. Determine the area required. 

Sc = Schmidt number, dimensionless Pr = Prandtl number, dimensionless Cg = gas specific heat, Btu/lb-°F a = interfacial area, fti/fti Q, = sensible heat transfer duty, Btu/hr Qj. = total heat transfer duty, Btu/hr... 

The thermodynamic properties of the refrigerant determine the suitability for a given condition of operation, particularly when compared with the same requirements or other refrigerants. The quantity of refrigerant needed for a particular level of evaporation is a function of its latent heat, except when using steam jet refrigeration, because the use of its chilled water involves only sensible heat transfer to process fluids. 

In a water cooling tower, the temperature profiles depend on whether the air is cooler or hotter than the surface of the water. Near the top, hot water makes contact with the exit air which is at a tower temperature, and sensible heat is therefore transferred both from the water to the interface and from the interface to the air. The air in contact with the water is saturated at the interface temperature and humidity therefore falls from the interface to the air. Evaporation followed by mass transfer of water vapour therefore takes place and latent heat is carried away from the interface in the vapour. The sensible heal removed from the water is then equal to the sum of the latent and sensible heats transferred to the air. Temperature and humidity gradients are then as shown in Figure 13.18 . 

If the tower is sufficiently tall, the interface temperature can fall below the dry bulb temperature of the air (but not below its wet bulb temperature), and sensible heat will then be transferred from both the air and the water to the interface. The corresponding temperature and humidity profiles are given in Figure 13.18ft. In this part of the tower, therefore, the sensible heat removed from the water will be that transferred as latent heat less the sensible heat transferred from the air. 

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 ... 

Qg = total sensible-heat transfer from vapour (gas),... 

In partial condensation it is usually better to put the condensing stream on the shell-side, and to select a baffle spacing that will maintain high vapour velocities, and therefore high sensible-heat-transfer coefficients. 

Qg Sensible-heat-transfer rate from gas phase ml2t... 

Semisynthetic metalworking fluids, 1 22, 15 240-241 Semolina, 26 284 Semon, Waldo, 25 628 Senarmontite, 3 41, 58 Sensible heat transfer, 13 196 Sensing, inclusion compounds in, 14 185 Sensit... 

The condensing temperature of the steam is 300°F. The process into which the heat is transferred is at a constant temperature of 20O°F. The overall heat transfer coefficient is 300 Btu/h °F ft. The reboiler has S09 tubes that arc 10 feet long and 1 inch inside diameter. The steam and condensate are inside the tubes. The density of the condensate is 62,4 Ib ft and the latent heat of condensation of the steam is 900 Btu/lb . Neglect any sensible heat transfer. 

This calculation is typical, in that 94% of the heat is liberated at the 320°F condensing temperature of the saturated steam. Another way of stating the same idea is that a steam reboiler depends on latent-heat transfer, and not on sensible-heat transfer. 

Heat exchange provided by sensible-heat transfer is improved when velocities are higher. Especially when the heating fluid is on the tube side of an exchanger, sensible-heat-transfer rates are always increased by high velocity. 

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