Windows heat transfer coefficient

As discussed in Section 9.4.4, the prediction of pressure drop, and indeed heat transfer coefficients, in the shell is very difficult due to the complex nature of the flow pattern in the segmentally baffled unit. Whilst the baffles are intended to direct fluid across the tubes, the actual flow is a combination of cross-flow between the baffles and axial or parallel flow in the baffle windows as shown in Figure 9.79, although even this does not represent the actual flow pattern because of leakage through the clearances necessary for the fabrication and assembly of the unit. This more realistic flow pattern is shown in Figure 9.80 which is based on the work of TINKER 116) who identifies the various streams in the shell as follows ... 

In Bell s method the heat-transfer coefficient and pressure drop are estimated from correlations for flow over ideal tube-banks, and the effects of leakage, bypassing and flow in the window zone are allowed for by applying correction factors. 

Baffle pitch, or distance between baffles, normally is 0.2-1.0 times the inside diameter of the shell. Both the heat transfer coefficient and the pressure drop depend on the baffle pitch, so that its selection is part of the optimization of the heat exchanger. The window of segmental baffles commonly is about 25%, but it also is a parameter in the thermal-hydraulic design of the equipment. 

The design calculations highlighted the shortcomings of the Kern method of exchanger design. The Kern method fails to account for shell-side inefficiencies such as bypassing, leakage, crossflow losses, and window losses. This leads to a marked overestimate of the shell-side heat-transfer coefficient and shell-side pressure drop. The Bell method is recommended to correct these deficiencies. 

This type is selected in the top window shown in Figure 5.25 on the Specifications page tab. The overall heat transfer coefficient (U = 142 W K-1 m 2) and the coolant temperature (400 K) are specified. Clicking the Configuration page tab opens the window shown at the bottom of Figure 5.25. The number of tubes (250), their length (10 m), and their diameter (0.1 m) are specified. 

Figure 6.36 shows the window that opens when Dynamics is selected under the reactor block. The amount of catalyst, the catalyst heat capacity, and the heat transfer coefficient between the process gas and the catalyst are specified. 

In gas-filled windows there are three heat transfer mechanisms conduction and convection through the gas layer and radiation between the surroundings and the glass surfaces. The heat flow by conduction is minimized by using a fairly thick gas layer with a low conductivity. With even thicker layers, the effect of convection becomes important. Conduction and radiation cause similar heat fluxes, with heat transfer coefficients of a few watts per square metre per kelvin. 

Consider a 0.8-m-high and 1.5-m-wide glass vyindow with a thickness of 8 mm I and a thermal conductivity of /r = 0.78 W/m K, Determine the steady rate of heat transfer through this glass v/indow and the temperature of Its inner surface for a day during which the room is maintained at 20 C while the temperature df the outdoors Is - lO C. Take the heat transfer coefficients on the inner and outer surfaces of the window to be /), = 10 W/m C and fr = 40 W/m "C, which includes the effects of radiation. 

Combined convection and radiation heat transfer coefficients at window, wall, or roof surfaces (from ASHRAE Handbook of Fundamentals. 

The glass (k = 0.78 Wm °C) is 0,5 cm thick and the heat transfer coefficient on the inner surface of the glass is S W/m- C Now winds at 35 km/h start to blow parallel to Ihe surface of this wall. If the air temperature outside is -2 C. delennine the rate of heat loss through the windows of this wall. Assume radiation beat transfer to be negligible,... 

Gases are nearly transparent to radiatioo, and thus heat transfer through a gas layer is by simultaneous convection (or conduction, if the gas is quiescent) and radiation. Natural convection heat transfer coefficients are typically very low compared to those for forced convection. Therefore, radiation is usually disregarded in forced convection problems, but it must be considered in natural convection problems that involve a gas. This is especially the case for surfaces with high emissivities. For example, about half of the heat transfer through the air. space of a double-pane window is by radiation, The total rate of heat transfer is determined by adding the convection and radiation components,... 

Heal transfer through a window is also affected by the convection and radiation heat transfer coefficients between the glass surfaces and sunound-ings. The effects of convection and radiation on the inner and outer surfaces of glazings are usually combined into the combined convection and radiation heat transfer coefficients /i,- and h , respectively. Under still air conditions, the combined heat transfer coefficient at the inner surface of a vertical window can be determined from... 

Assumptions 1 Steady operating conditions exist. 2 Heat transfer through the window is one-dimensional. 3 Thermal properties of the windows and the heat transfer coefficients are constant. 

Consider a l.2-m-high and 2-m-wide glass window with a thickness of 6 nun, thermal conductivity k = 0.78 W/m C, and emissivity e = 0.9. The room and the walls that face the window are maintained at 25°C, and the average temperature of the inner surface of the window is measured to be 5°C. If the temperature of Ihe outdoors is -5 C, determine (a) the convection heat transfer coefficient on Ihe inner surface of the window, (b) the rate of total heat transfer through the window, and (c) the combined natural convection and radiation beat transfer coefficient on the outer... 

Determine the CZ-faclors for the center-of-glass section of a double-pane window and a triple-pane window, fhe heat transfer coefficients on the inside and outside surfaces are 6 and 25 W/ni °C, respectively. The thickness of the air layer is 1.5 cm and there are two such air layers in tiiple-pane window. The Nu.sselt number across an air layer is estimated to be 1.2- Take the thermal conductivity of air to be 0.025 W/m C and neglect the thermal resistance of glass sheets. Also, assume that the effect of radiation through the air space is of the same magnitude as the convection. 

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