Length of a transfer unit

Results of diying tests can be correlated empirically in terms of overall heat-transfer coefficient or length of a transfer unit as a function of operating variables. The former is generally apphcable to all types of dryers, while the latter applies only in the case of continuous diyers. The relationship between these quantities is as follows. 

Again referring to Fig. 13, the same general trend is apparent in both the pressure-drop and number-of-transfer-unit curves. This suggests that another empirical correlating procedure could be arrived at for example, an approximate relationship exists between the length of a transfer unit (LTU) and the Lockhart-Martinelli parameters, X. 

The length of a transfer unit and the number of transfer units appropriate for good design are discussed later under Drying Equipment. ... 

The unusually small volume ratio of gas to liquid makes stripping plate efficiencies low. Yokota s model of the process assumes that liquid travels across a plate in uniform flow with no transverse mixing but that vertical mixing is perfect. He then characterizes a plate by the length of linear travel of the liquid. Correlation of the experimental data is in terms of the length of a transfer unit. This is the distance that liquid must travel to undergo an amount of mass transfer equivalent to one transfer unit. A plate then provides a (normally fractional) number of transfer units given by... 

N = number of transfer units supplied by a plate Lj = length of travel of liquid over a plate Lu = length of a transfer unit... 

The length of a transfer unit generally decreases as gas flow rate increases. Figure 7.100 illustrates the effect, which tends to level off at a gas velocity of about 2.8cms . Experimentally, L j at that gas rate is proportional to the 0.6 power of liquid mass... 

For design purposes it is convenient to write equations 11.97 and 11.98 in terms of transfer units (HTU) where the value of integral is the number of transfer units, and the group in front of the integral sign, which has units of length, is the height of a transfer unit. 

The Noq in Eq. (16e) is termed the overall number of transfer units. It is dimensionless and is the ratio of the change of bulk-phase concentration to the average concentration driving force. It is essentially a measure of the ease of separation. The H0G in Eq. (16d) is termed the overall height of a transfer unit. It has the dimension of length and defines the vertical height of contactor required to provide a change of concentration equivalent to one transfer unit. 

The expression in from or the integral has the units of length and is called the height of a transfer unit (HTU) or, for this cuse, the height of a gas-phase transfer unit, H0. which is defined as... 

In analogy to similar problems in heat and mass transfer, the length of the apparatus is the number of transfer units Ng, multiplied with the height of a transfer unit Hg. From this and for the case of. 9s = const., the temperature of the gas in the dryer can be evaluated to be... 

The striking feature here is the same as that seen in Illustration 9.2 — the low value of NTU. This inevitably entails a large value for the height (here length) of a transfer xmit. /cyfl values in these units are typically of the order of 0.05 kg/m s AY, mainly because the specific surface area a (m /m ) is very low (order of 1). We obtain... 

Could the height of a transfer unit change over the length of a column ... 

Now, it is of interest to determine if either the resistance to mass transfer term for the mobile phase or, the resistance to mass transfer term in the stationary phase dominate in the equation for the variance per unit length of a GC packed column. Consequently, taking the ratio of the two resistance to mass transfer terms (G)... 

For those condensing duties where permissible pressure loss is less than 0.07kpf/cm there is no doubt but that the tubular unit is most efficient. Under such pressure-drop conditions only a portion of the length of a plate heat exchanger plate would be used and a substantial surface area would be wasted. However, when less restrictive pressure drops are available the plate heat exchanger becomes an excellent condenser, since very high heat transfer coefficients are obtained and the condensation can be carried out in a single pass across the plate. 

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