Question

How is the NTU of a heat exchanger defined? What does it represent? Is a heat exchanger with a very large NTU (say, 10 ) necessarily a good one to buy?

Short answer

Answer: The NTU (Number of Transfer Units) in heat exchangers represents the effectiveness of the heat exchanger, which is the ratio of the actual heat transferred to the maximum possible heat that could be transferred. A very large NTU indicates high heat transfer effectiveness. However, it doesn't always mean that a heat exchanger is a good choice to purchase, as factors like size and cost should also be considered when evaluating heat exchanger options.

Step-by-step solution

Define NTU

NTU (Number of Transfer Units) is a dimensionless parameter that characterizes the effectiveness of a heat exchanger. It is defined as the ratio of the actual heat transfer rate in the heat exchanger to the maximum possible heat transfer rate: \[\text{NTU} = frac{\text{Actual Heat Transfer Rate}}{\text{Maximum Possible Heat Transfer Rate}}\]

Represent NTU in terms of heat exchanger parameters

In terms of heat exchanger parameters, NTU can be expressed as: \[\text{NTU} = \frac{U A}{C_\text{min}}\] where \(U\) represents the overall heat transfer coefficient, \(A\) is the heat transfer area, and \(C_\text{min}\) is the minimum heat capacity rate of the two fluids involved in the heat exchange.

Significance of NTU

The NTU value represents the effectiveness of the heat exchanger, which is the ratio of the actual heat transferred to the maximum possible heat that could be transferred if the heat exchanger efficiency were 100%. A higher NTU value indicates greater heat transfer effectiveness, meaning that the heat exchanger is more efficient.

Large NTU and heat exchanger performance

Although a heat exchanger with a very large NTU (e.g. 10) might have a higher heat transfer effectiveness, it does not necessarily mean it is the best choice to purchase. A very high NTU value can be achieved by using a heat exchanger with a large heat transfer area, which usually increases size and cost. Hence, it is essential to balance performance, size, and cost when choosing a heat exchanger. To summarize, the NTU of a heat exchanger is defined as the dimensionless ratio of the actual heat transfer rate to the maximum possible heat transfer rate, and it represents the effectiveness of the heat exchanger. A heat exchanger with a very large NTU may not necessarily be the best one to purchase, as size and cost should also be considered when evaluating heat exchanger options.