Heat transfer, units of quantities

TABLE 8.8. Dimensionless Groups and Units of Quantities Pertaining to Heat Transfer... 

Results of diying tests can be correlated empirically in terms of overall heat-transfer coefficient or length of a transfer unit as a function of operating variables. The former is generally apphcable to all types of dryers, while the latter applies only in the case of continuous diyers. The relationship between these quantities is as follows. 

Description A tray or compartment diyer is an enclosed, insulated housing in which solids are placed upon tiers of trays in the case of particulate solids or stacked in piles or upon shelves in the case of large objects. Heat transfer may be direct from gas to sohds by circulation of large volumes of hot gas or indirect by use of heated shelves, radiator coils, or refractoiy walls inside the housing. In indirec t-heat units, excepting vacuum-shelf equipment, circulation of a small quantity of gas is usually necessary to sweep moisture vapor from the compartment and prevent gas saturation and condensation. Compartment units are employed for the heating and diying of lumber, ceramics, sheet materi s (supported on poles), painted and metal objects, and all forms of particulate solids. 

In addition to volume changes the effect of temperature is also important. Thus the specific latent heat of vaporization of a chemical is the quantity of heat, expressed as kJ/kg, required to change unit mass of liquid to vapour with no associated change in temperature. This heat is absorbed on vaporization so tliat residual liquid or tlie sunoundings cool. Alternatively an equivalent amount of heat must be removed to bring about condensation. Thus the temperature above a liquefied gas is reduced as tlie liquid evaporates and tlie bulk liquid cools. There may be consequences for heat transfer media and the strength of construction materials at low temperatures. 

The quantity of heat transfer per unit surface area, per unit time. Heat transfer coefficient (U). 

To put this in words, when heat flows at constant temperature, the entropy change is equal to the heat transferred (Jt) divided by the temperature in kelvins (T). The units of A S are energy/temperature, or J/K. The subscript T in Equation is a reminder that the quantity of heat transferred depends on the conditions. This equation is restricted to processes that occur at constant temperature. 

Worz et al. give a numerical example to illustrate the much better heat transfer in micro reactors [110-112]. Their treatment referred to the increase in surface area per unit volume, i.e. the specific surface area, which was accompanied by miniaturization. The specific surface area drops by a factor of 30 on changing from a 11 laboratory reactor to a 30 m stirred vessel (Table 1.7). In contrast, this quantity increases by a factor of 3000 if a 30 pm micro channel is used instead. The change in specific surface area is 100 times higher compared with the first example, which refers to a typical change of scale from laboratory to production. 

The important quantities in this term are the heat transfer area A, the temperature driving force or difference (Tg-Ti), where Ta is the temperature of the heating or cooling source, and the overall heat transfer coefficient U. The heat transfer coefficient, U, has units of (energy)/(time)(area)(degree), e.g., J/s m K. The units for U A AT are thus... 

Constraints (11.3) stipulates the quantity of heat transferred to storage from a hot unit at the beginning of the time horizon. Constraints (11.4) and (11.5) quantify the amount of heat transferred and received from storage unit, respectively. They ensure that if there is no heat integration between a processing unit and storage, then the amount of heat related to storage is not disturbed. 

The transport process abont which most of us have an intnitive nnderstanding is heat transfer so we will begin there. In order for heat to flow (from hot to cold), there must be a driving force, namely, a temperature gradient. The heat flow per unit area (Q/A) in one direction, say the y direction, is the heat flux, qy. The temperature difference per unit length for an infinitesimally small unit is the temperature gradient, dT/dy. According to Eq. (4.1), there is then a proportionality constant that relates these two quantifies, which we call the thermal conductivity, k. Do not confuse this quantity with... 

Thermal conductivity may be defined as the quantity of heat passing per unit time normally through unit area of a material of unit thickness for unit temperature difference between the faces. In the steady state, i.e. when the temperature at any point in the material is constant with time, conductivity is the parameter which controls heat transfer. It is then related to the heat flow and temperature gradient by ... 

The surface heat transfer coefficient can be defined as the quantity of heat flowing per unit time normal to the surface across unit area of the interface between two materials with unit temperature difference across the... 

The quantity of energy transferred as heat is measured in joules, J. However, a unit of energy still widely used in biochemistry and related fields is the calorie (cal). The original definition of 1 cal was that it is the energy needed to raise the temperature of 1 g of water by 1°C. The modern definition is... 

---