Heat exchangers capacity ratio

R distillation column reflux ratio (-) or heat capacity ratio of 1-2 shell-and-tube heat exchanger (-)... 

LMTD = log mean temperature difference, °F M = mass flow rate, Ib/hr Ntu = number of heat transfer units, dimensionless N = number tubes/row in direction of air flow n = number tubes/row, per ft of exchanger width, 1 /ft Q = total exchanger heat load (duty), Btu/hr R = = heat capacity ratio, dimensionless... 

This expression compares the characteristic time of runaway (TMRad) with the characteristic cooling time. Thus, knowing the mass, specific heat capacity, heat transfer coefficient, and heat exchange area allows the assessment. It is worth noting that, since the thermal time constant contains the ratio V/A, heat losses are proportional to the characteristic dimension of the container. 

Fig. 1.3. The effect of heat exchanger performance, NTU, and heat capacity ratio, h, on the normalized heat loss for heat-integrated processes in (a) countercurrent and (b) counter-cocurrent flow configuration.

If a nitrator is equipped only with a cooling or heating jacket, the ratio of the heat exchange area to the nitrator capacity depends on the latter. When the capacity is cubed the surface area is squared. Thus, when the dimensions of a nitrator are increased, the heat exchange area may no longer be adequate. Then, a stainless steel coil or a lead coil must be mounted inside the nitrator. 

In heat exchanger analysis, it is also convenient to define another dimensionless quantity called the capacity ratio c as... 

It can be shown that the effectiveness of a heat exchanger is a function of the number of transfer units NTU and the capacity ratio c. That is,... 

For a specified NTU and capacity ratio c, the counier-tlow heat exchanger has (lie highest effectiveness and the parallel-flow the lowest. 

The effectiveness of a heat exchanger is independent of the capacity ratio c for NTU values of less than about 0.3. 

These are also known as the Number of Transfer Units or NTIJ for short. We suggest TVi be characterised as the dimensionless transfer capability of the heat exchanger. Instead of N2 the ratio of the two heat capacity flow rates... 

Here P indicates the effectiveness of the heat exchanger (to be elaborated in Section 7.4) and R (from its definition) is the ratio of the heat-capacity flow rates Note the change in nomenclature from subscripts h and c to t and s, the latter two referring to tube and shell, respectively. An important fact is that whether the hot (or cold) fluid is flowing in the shell side or in the tubes has no effect on F as long as the heat transfer to the ambient is negligible. Otherwise, the cold fluid should be in the shell side to reduce heat losses Combination of Eqs (7.28) and (7.29) gives... 

In all formulas of plate heat exchangers with the number of thermal plates N — (equation numbers starting with V), the single-pass parallelflow and counterflow temperature effectivenesses are presented in implicit forms. Their explicit forms are as follows, with x and y representing the appropriate values of the number of transfer units and heat capacity rate ratios, respectively. 

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