Kerns method

Though Kern s method does not take account of the bypass and leakage streams, it is simple to apply and is accurate enough for preliminary design calculations, and for designs where uncertainty in other design parameters is such that the use of more elaborate methods is not justified. Kern s method is given in Section 12.9.3 and is illustrated in Examples 12.1 and 12.3. 

The procedure for calculating the shell-side heat-transfer coefficient and pressure drop for a single shell pass exchanger is given below  

Calculate the area for cross-flow As for the hypothetical row of tubes at the shell equator, given by  

The term (pt —d0)/pt is the ratio of the clearance between tubes and the total distance between tube centres. 

Calculate the shell-side mass velocity Gs and the linear velocity us  

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This compares to assumed value of 0.001 + 0.001 = 0.002 The Kern method is usually easier to handle for pressure systems than for vacuum systems. The recirculation ratio is higher and, therefore, requires more trials to narrow-in on a reasonable value for the low pressure systems. The omission of two-phase flow in pressure drop analysis may be a serious problem in the low pressure system, because a ratio on the high side may result, causing a high hj value. In general, however, for systems from atmospheric pressure and above, the method usually gives conservative results when used within Kern s limitations. 

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. 

The shell-side heat-transfer coefficient in the Kern method is calculated using Equation 10.2. An equivalent diameter is calculated which is representative of the shell-side fluid passage geometry. Equation 10.2 therefore becomes ... 

The Kern method is inaccurate for calculating the shell-side heat-transfer coefficient and shell-side pressure drop (Ref. E2, p. 545). Kern makes no account of the effect of bypss and leakage in the shell-fluid passage. For these reasons the Bell Method (Ref. E9) is employed to serve as a check on the preliminary design. 

The other parameter also needing revision was the shell-side pressure drop. This was reduced from 50 kPa (Kern method) to around 6 kPa. The Kern method is subject to overestimates of the pressure drop for the reasons already discussed. 

H.4 Tube-side Heat-transfer Coefficient (The Kern method) 314 H.5 Shell-side Heat-transfer Coefficient (The Kern method) 315 H.6 Overall Heat-transfer Coefficient 316... 

H.7 Tube-side Pressure Drop (The Kern Method) 317... 

H.4 Tube-side Heat-transfer Coefficient (The Kern Method)... 

This design based upon the Kern method appears to be quite adequate, on the basis of the results obtained. 

This value is obviously much lower than that calculated by the Kern method. This means that the previously calculated value for the overall heat-transfer coefficient must also be revised. 

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