Heat exchanger effecctiveness
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The Number of Transfer Units (NTU) method is used to determine heat transfer rates in heat exchangers when information for the Log-Mean Temperature Difference (LMTD) is lacking. Effectiveness is defined as the ratio of actual to maximum possible heat transfer, relying on heat capacity rates, and is calculated differently for parallel and counter-flow heat exchangers. The method utilizes correlations in terms of heat capacity ratio and the NTU to find effectiveness and can adapt to special cases such as phase changes.
Heat exchanger effecctiveness
- 1. NUMBER OF TRANSFER UNITS (NTU) METHOD The Number of transfer units (NTU) method is used to calculate the rate of heat transfer in heat exchangers when there is insufficient information to calculate the Log-mean temperature difference (LMTD). To define the effectiveness of a heat exchanger, we need to find the maximum possible heat transfer that can be hypothetically achieved in a counter-flow heat exchanger. Therefore one fluid will experience the maximum possible temperature difference. The method proceeds by calculating the heat capacity rates Ch and Cc for the hot and cold fluid respectively and denoting the smaller one as Cmin. max min , ,h i c iq C T T Where maxq is the maximum heat that could be transferred between fluids per unit time. minC must be used as it is the fluid with the lowest heat capacity rate that would, in this hypothetical length exchanger, actually undergo the maximum temperature change. The effectiveness is the ratio between the actual heat transfer rate and the maximum possible heat transfer rate: max q q Effectiveness is dimensionless quantity between 0 and 1. If we know for a particular heat exchanger, and we know the inlet conditions of the two streams, we can calculate the amount of heat being transferred between the fluids by: min , ,( )h i C iq C T T For any heat exchanger it can be shown that: min , Max C f NTU C For a given geometry, can be calculated using correlations in terms of heat capacity ratio. min max r C C C and the number of transfer units, NTU min UA NTU C where U is the overall heat transfer coefficient and Ais the heat transfer area. The effectiveness of parallel flow heat exchanger is calculated with: 1 exp 1 1 r r NTU C C The effectiveness of counter-current flow heat exchanger is calculated with: 1 exp 1 1 exp 1 r r r NTU C C NTU C For rC =1 1 NTU NTU For rC =0 (Special case in which phase change is occurring in the heat exchanger) 1 exp NTU