Average temperature difference

The average temperature difference between the indoor air and ambient air is... 

Since the temperature of both fluids changes as they flow through the exdianger, an average temperature difference must be used in Equation 2-3. Normally a log mean temperature difference is used and can be found as follows ... 

AT = average temperature difference between the fluid in the coil and the liquid bath, °F d = pipe outside diameter, in. 

Providing that the flow rates are steady, the heat transfer coefficients do notvary and the specific heat capacities are constant throughout the working range, the average temperature difference over the length of the curve is given hy ... 

Average temperature difference between inside and outside of house 10 °C heating period 200 days in a year. 

Average temperature difference between inside and outside of house 12 °C heating period 250 days in a year. Cost of insulation, including installation, 120/m3. Capital charges (see chapter 6) 20 per cent per year. Cost of fuel, allowing for the efficiency of the heating system, 9c/MJ. 

If both fluids had been isothermal, then the logarithmic and average temperature differences would be equal and ATh = ATc = AT. It should also be noted that ... 

T tot = 7(h - rice, average temperature difference during main drying 43.88 °C... 

Wei el al. (1984) show that the average temperature difference between the solid and gas phase is yXAA/fi for fixed bed reactors. This then defines one possible criterion for the applicability of the pseudohomogeneous model as given in Table V. Another possible criterion is that given by Vortmeyer and Schaefer (1974), discussed later in this section. 

The log mean temperature difference LMTD is simply the average or weighted temperature difference between the hot side and the cold side of the exchanger. Use Eq. (5.10) to determine this mean average temperature. It is simply, as the term indicates, the average temperature difference between the tube and shell sides of the exchanger. 

Determination of the average temperature difference, A7im, is very complex for these types of heat exchangers. In the top half of the exchanger illustrated in Figure 17, there is parallel flow and in the bottom half, counter flow. For this reason, it is common practice to introduce a correction factor, Ft, into Equation (42). The heat transfer rate is therefore given by... 

The average temperature difference along the plate, for the constant heat flux condition, may be obtained by performing the integration... 

A 1.0-kW heater is constructed of a glass plate with an electrically conducting film which produces a constant heat flux. The plate is 60 by 60 cm and placed in an airstrtam at 27°C, 1 atm with = 5 m/s. Calculate the average temperature difference along the plate and the temperature difference at the trailing edge. 

Engine oil at 10°C flows across a 15-cm-square plate upon which is imposed a constant heat flux of 10 kW/m2. Determine (a) the average temperature difference, (b) the temperature difference at the trailing edge, and (c) the average heat-transfer coefficient. Use the Churchill relation [Eq. (5-51)]. u = 0.5 m/s. 

The critical Reynolds number Reor is typically taken as 5 x 105, lie, < Re, < 3 x 107, and 0.7 < Pr < 400. The fluid properties are evaluated at the film temperature (7, + I )/2 where 7, is the free-stream temperature and 7 is the surface temperature. Equation (5-60) also apphes to the uniform heat flux boundary condition provided h is based on the average temperature difference between 7 and 7, ... 

If we consider the entire vessel as our volume element, we may as a first approximation assume that the rate of loss of heat by the gas is proportional to the average temperature difference between the gas and the vessel walls. This corresponds to the straight line in Fig. XIV. 1. At low concentrations of reactants (curve C3), the temperature will mount from To, the flask temperature, when the reaction mixture is placed in the flask. As the temperature mounts, the rate of heat loss (although initially small) increases at a faster rate than the rate of heat production until, at some temperature T the two heat terms are just equal to each other and the temperature becomes stationary. Note that this is also a stable temperature distribution, since for any slight displacements in temperature the system will tend to return to Stable distributions of this type were treated in the last section, and the mean gradients established, T. - r , are usually (juite small compared to To (about 1 to 2 per cent). 

The average value of (he overall heat transfer coefficient can be determined as described in the preceding section by using the average convection coefficients for each fluid. It turns out that (he appropriate form of the average temperature difference between the two fluids is logarithmic in nature, and its determination is presented in Section 11-4. 

The temperature difference between the two fluids decreases from AT, at the inlet to AT-i at the outlet. Thus, it is tempting to use the arithmetic mean temperature AT = (AT, + AT2) as the average icmperalure difference. The logarithmic mean temperature difference ATj is obtained by tracing the actual temperature profile of the fluids along the heat exchanger and is an exact representation of the average temperature difference between the hot and cold fluids. It truly reflects the exponential decay of the local temperature difference. 

That is, the temperature difference betv/een the. tv/o fluids varies from 8°C at one end to 16°C at the other. The proper average temperature difference between Ih. two fluids is the logarithmic mean temperature difference (not the arithm ici which is determined from... 

The surface temperature of each mold half should be held within + 4 F. At the high temperatures at which nylon RIM tools are operated, this requirement necessitates the use of more coring than is sometimes used for urethane tools. In steel tools. 75 inch diameter circulating oil core holes, spaced at 2.5 Inches on centers and located roughly 2-inches from the cavity surface have worked quite well. An average temperature difference of about 5°F can be... 

---