Heat total area

Total heat transfer area is assumed to be divided equally between exchangers. This tends to overestimate the capital cost. 

Equation (F.l) shows that each stream makes a contribution to total heat transfer area defined only by its duty, position in the composite curves, and its h value. This contribution to area means also a contribution to capital cost. If, for example, a corrosive stream requires special materials of construction, it will have a greater contribution to capital cost than a similar noncorrosive stream. If only one cost law is to be used for a network comprising mixed materials of construction, the area contribution of streams requiring special materials must somehow increase. One way this may be done is by weighting the heat transfer coefficients to reflect the cost of the material the stream requires. 

The saturation coverage during chemisorption on a clean transition-metal surface is controlled by the fonnation of a chemical bond at a specific site [5] and not necessarily by the area of the molecule. In addition, in this case, the heat of chemisorption of the first monolayer is substantially higher than for the second and subsequent layers where adsorption is via weaker van der Waals interactions. Chemisorption is often usefLil for measuring the area of a specific component of a multi-component surface, for example, the area of small metal particles adsorbed onto a high-surface-area support [6], but not for measuring the total area of the sample. Surface areas measured using this method are specific to the molecule that chemisorbs on the surface. Carbon monoxide titration is therefore often used to define the number of sites available on a supported metal catalyst. In order to measure the total surface area, adsorbates must be selected that interact relatively weakly with the substrate so that the area occupied by each adsorbent is dominated by intennolecular interactions and the area occupied by each molecule is approximately defined by van der Waals radii. This... 

This implies that the LMTD or M I D as computed in equations 20 through 26 may not be a representative temperature difference between the two heat-transferring fluids for aU tubes. The effective LMTD or M ID would be smaller than the value calculated, and consequentiy would require additional heat-transfer area. The tme value of the effective M I D may be determined by two- or three-dimensional thermal—hydrauUc analysis of the tube bundle. Baffle—Tube Support PlateXirea. The portion of a heat-transfer tube that passes through the flow baffle—tube support plates is usuaUy considered inactive from a heat-transfer standpoint. However, this inactive area must be included in the determination of the total length of the heat-transfer tube. 

The modified Palen-SmaU method can be employed for reboiler design using finned tubes, but the maximum flux is calculated from A, the total outside heat-transfer area including fins. The resulting value of refers to A. ... 

Most efficient performance is obtained with plates having open areas equal to 2 to 3 percent of the total heat-transfer area. The plate should be located at a distance equal to four to six hole (or equivalent) diameters from the heat-transfer surface. 

The overall heat-transfer rate is almost entirely dependent upon the film coefficient between the inner jacket wall and the solids, which depends to a large extent on the solids characteristics. Overall coefficients may range from 30 to 200 J/(m s K), based upon total area if the diyer walls are kept reasonably clean. Coefficients as low as 5 or 10 may be encountered if caking on the walls occurs. 

Assume that the total area of the bottom head is effeetive for heat transfer. Therefore, the total surfaee area of tlie reaetor in Eigure 13-1 available for heating or eooling the working volume is defined by... 

Heat transfer area on the straight side of the reaetor, ft Total heat transfer surfaee area = A -i- Ag, ft ... 

The heat transfer area, A ft, in an exchanger is usually estahlished as the outside surface of all the plain or hare tubes or the total finned surface on the outside of all the finned tubes in the tube bundle. As will be illustrated later, factors that inherendy are a part of the inside of the tube (such as the inside scale, transfer film coefficient, etc.) are often corrected for convenience to equivalent outside conditions to be consistent. When not stated, transfer area in conventional shell and tube heat exchangers is considered as outside tube area. 

Add area requirements of sensible heat to the area required for vaporization to obtain the total area. 

The value taken for S is the developed area after pressing. That is, the total area available for heat transfer and due to the corrugations will be greater than the projected area of the plate, i.e. 0.17 m versus 0.14 m for an APV HX plate. 

The packet thus formed was filled with the food product to be tested and the open side was heat sealed. Heat seals were usually made at 350°F jaw temperature, 40 psi pressure with 0.5 second dwell time the exact conditions necessary depend on the materials used and caliper. This produced a packet with an 8 sq. in. total area. 

Equation (4.2) requires that the total area above 0 Kelvin be obtained, but heat capacity measurements cannot be made to the absolute zero of temperature. The lowest practical limit is usually in the range from 5 K to 10 K, and heat capacity below this temperature must be obtained by extrapolation. In the limit of low temperatures, Cp for most substances follows the Debye low-temperature heat capacity relationship11 given by equation (4.4)... 

Assuming Aq is the total heating surface in the full tank, with volume, Vq, and assuming a linear variation in heating area with respect to liquid depth, the heat transfer area may vary according to the simple relationship... 

This factor corrects for the effect of flow through the baffle window, and is a function of the heat-transfer area in the window zones and the total heat-transfer area. The correction factor is shown in Figure 12.33 plotted versus Rw, the ratio of the number of tubes in the window zones to the total number in the bundle, determined from the tube layout diagram. 

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. 

Kern recommends that the heat flux in thermosyphon reboilers, based on the total heat-transfer area, should not exceed 37,900 W/m2 (12,000 Btu/ft2h). For horizontal thermosyphon reboilers, Collins recommends a maximum flux ranging from 47,300 W/m2 for 20-mm tubes to 56,800 W/m2 for 25-mm tubes (15,000 to 18,000 Btu/ft2h). These rule of thumb values are now thought to be too conservative see Skellence el al. (1968) and Furzer (1990). Correlations for determining the maximum heat flux for vertical thermosyphons are given by Lee et al. (1956) and Palen et al. (1974) and for horizontal thermosyphons by Yilmaz (1987). 

Number of plates = total heat transfer area / effective area of one plate = 72.92/0.75 = 97... 

System of packed bed solar air heaters The system has a total solar heat collector area of 27 m2 consisting of 18 packed bed solar air heaters (Demirel... 

Example 2.1 A new heat exchanger is to be installed as part of a large project. Preliminary sizing of the heat exchanger has estimated its heat transfer area to be 500 m2. Its material of construction is low-grade stainless steel, and its pressure rating is 5 bar. Estimate the contribution of the heat exchanger to the total cost of the project (CE Index of Equipment = 441.9). 

If the heat exchange involves desuperheating as well as condensation, then the exchanger can be divided into zones with linear temperature-enthalpy profiles in each zone. Figure 15.12a illustrates desuperheating and condensation on the shell-side of a horizontal condenser. The total heat transfer area is the sum of the values for each zone ... 

Targets also can be set for total heat exchange area, number of units, and number of shells for 1-2 shell-and-tube heat exchangers. These can be combined to... 

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