Heat exchangers correction factor

Outside diameter of inner of concentric tubes Inside diameter of outer of concentric tubes Fin efficiency Radiation exchange factor Bypass correction factor, heat transfer Bypass correction factor, pressure drop Leakage correction factor, heat transfer Leakage correction factor, pressure drop Tube row correction factor... 

The Ft correction factor is usually correlated in terms of two dimensionless ratios, the ratio of the two heat capacity flow rates R and the thermal effectiveness P of the exchanger ... 

FIG. 11-4 (Continued) LMTD correction factors for heat exchangers. In all charts, fi = (Ti — T<>y(U — t ) and S = (to — ti)/(Ti — ti). ( ) Cross-flow (drip type), two horizontal passes with U-hend connections (trombone type). (/) Cross-flow (drip type), helical coils with two turns. 

These refer to hot and cold fluid terminal temperatures, inlet of one fluid versus outlet of the other. For a cross exchanger with no phase change, the ATm gives exact results for true countercurrent flow. Most heat exehang-ers, how ever, deviate from true countercurrent so a correction factor, F, is needed. 

The price of air-cooled exchangers should be obtained from vendors if possible. If not, then by coirelating in-house historical data on a basis of /ft of bare surface vs. total bare surface. Correction factors for materials of construction. pressure, numbers of tube rows, and tube length must be used. Literature data on air coolers is available (Reference 15). but it should be the last resort. In any event, at least one air-cooled heat exchanger in each project should be priced by a vendor to calibrate the historical data to reflect the supply and demand situation at the expected time of procurement. 

In the field of heat transfer, a good example of this category of shortcut design method is the famous F correction factor to correct the log mean temperature difference of shell and tube heat exchangers for deviations from true countercurrent flow. For multipass heat exchangers, the assumptions are ... 

In the basic heat transfer equation it is necessary to use the log mean temperature difference. In Equation 2-4 it was assumed that the two fluids are flowing counter-current to each other. Depending upon the configuration of the exchanger, this may not be true. That is, the way in which the fluid flows through the exchanger affects LMTD. The correction factor is a function of the number of tube passes and the number of shell passes. 

To size a shell-and-tube exchanger, first the duty is calculated. Then ii is delermined which fluid will be in the shell and which in the tube, ami i heat dansfer coefficient assumed or calculated. A choice is made of ihc number of shell and tube passes to get a reasonable LMTD correction factor (F), and a corrected LMTD as calculated from Equation 3-1. 

To determine the true overall temperature difference, the correction factors, F, shown in Figure 10-34 are used to correct for the deviations involved in the construction of multipasses on the shell and tube sides of the exchanger. Note that R of the charts represents the heat capacity rate ratio , and P is the temperature efficiency of the exchanger. 

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. 

In air-cooled heat exchangers, the air flows upward umixed across the finned tubes/bundle, and the tube-side process fluid can flow back and forth and downward as established by the pass arrangements. At 4 or more passes, the flow is considered counter-current, and the F factor = 1 0 216 q-pg other fewer-passes correction factors are given in Figures 10-187A, 10-187B, 10-187C. 

A further advantage of the plate heat exchanger is that the effective mean temperature difference is usually higher than with the tubular unit. Since the tubular is always a mixture of cross and contra-flow in multi-pass arrangements, substantial correction factors have to be applied to the log mean temperature difference (LMTD). In the plate... 

The flow in a heat exchanger will clearly not be isothermal, and this is allowed for by including an empirical correction factor to account for the change in physical properties with temperature. Normally only the change in viscosity is considered ... 

The temperature correction factor, Ft, will normally be higher with plate heat exchangers, as the flow is closer to true counter-current flow. 

Figure 12.62. Log mean temperature correction factor for plate heat exchangers (adapted from Raju and Chand (1980))...

Fhw = the window correction factor. This allows for flow through the baffle window and is a function of the heat transfer area in the window zones and the total heat transfer area. A typical value for a well-designed exchanger is near 1.0. 

The major things to specify for heat exchangers are the materials of construction and the heat-transfer area required. Generally, streams containing materials that can precipitate out or form a scale are placed on the tube side. If this is not a factor, it is generally best to place the stream flowing at the highest velocity on the tube side. Usually a 20% improvement in the corrected mean temperature difference can be realized if this is done.2... 

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