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).

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   