E-ntu

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  • Publicado : 11 de septiembre de 2012
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 -NTU Method

The formal introduction of the  -NTU method for the heat exchanger analysis was in 1942 by London and Seban [4]. In this method, the total heat-transfer rate from the hot fluid tothe cold fluid in the exchanger is expressed as

where is the heat exchanger effectiveness. It is nondimensional and for a direct transfer type heat exchanger, in general, it is dependent on NTU,C*, and the flow arrangement:


These three nondimensional parameters, C*, NTU, and , are defined next.

Heat Capacity Rate Ratio, C*. This is simply the ratio of the smaller to larger heatcapacity rate for the two fluid streams so that C* ≤1.

Here, C refers to the product of mass and specific heat of the fluid, and the subscripts min and max refer to the Cmin and Cmax sides,respectively. In a two-fluid heat exchanger. one of the streams will usually undergo a greater temperature change than the other. The first stream is said to be the "weak' stream, having a lower thermalcapacity rate (Cmin) and the other with higher thermal capacity rate (Cmax)is the "strong" stream.

Number of Transfer Units, NTU. NTU designates the nondimensional "heat-transfer size" or "thermal size"of the exchanger. It is defined as a ratio of the overall conductance to the smaller heat capacity rate.

If U is not a constant, the definition of the second equality applies. For constant U,substitution of the expression for UA results in [ I ,2]:

where Rl , and R2 are the thermal resistances due to fouling on the hot side and cold side, respectively. As defined in Eq. 7. In the absenceof the fouling resistances, NTU can be given by the expression



and the number of heat transfer units on the hot and cold sides of the exchanger may be defined as follows:

Heat ExchangerEffectiveness,. Heat exchanger effectiveness, . is defined as the ratio of the actual heat-transfer rate, y. to the thermodynamically possible maximum heat-transfer rate (qmax) by the second law...
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