\[\text{NTU}=\frac{\text{UA}}{\text{c}_{\min }}\]
\[\text { If } m_{h} c_{h}<m_{c} c_{c} \Rightarrow c_{\min }=m_{c} c_{c}\] \[\Rightarrow N T U=\frac{U A}{m_{h} c_{c}}\]
\[\text { If } m_{n} C_{h}<m_{c} c_{c} \Rightarrow c_{\min }=m_{h} c_{h} \]\[\Rightarrow N T U=\frac{U A}{m_{h} c_{h}}\]