4. GENERAL METHODS OF CALCULATION OF HEAT EXCHANGERS

4.2. Method ε-NTU

In this method used the following equations:

With equations 24 and 25 can find the 2 variables necessary in order to find ε graphically.

We can find two types of problems:

- DESIGN PROBLEM

INFORMATION

With this information must be find the area (A₀ o A_i).

The steps to follow to find this area are:

1) Proposed the two heat balance equations to find q:

2) Calculate ε and z:

 

3) With the values found for z and ε, the graph must be found and NTU, as shown schematically in the following graph:

4) Calculate the area (A₀ o A_i) from NTU's equation:

Isolated area:

Isolated area:

- HEAT PROBLEM

INFORMATION

With this information and using different equations, you have to find the final temperature (T₁₀ and T₂₀).

The steps to follow to find these temperatures are:

1) Calculate Z and NTU:

2) With the values found of Z and NTU, the graph must be found ε, as shown schematically in the following graph:

3) From the value of ε found, calculate the value of heat (q):

4) Proposed the two heat balance equations to find the final temperature (T₁₀ and T₂₀):

If isolated temperature T₁₀:

If isolated temperature T₂₀: