Ntu Method

8/12/2019 Ntu Method

1/11


2/11

Department of Mechanical Engineering

College of Engineering, Pune (COEP)Forerunners in Technical Education

NTU Method

The number of transfer units NTU is defined as a ratio of theoverall thermal conductance to the smaller heat capacity rate.


3/11



3

NTU designates the non-dimensional heat transfer size orthermal size of the exchanger, and therefore it is a designparameter.

NTU provides a compound measure of the heat exchanger size

through the (A *U). Hence, in general, NTU does not necessarily indicate the physical

size of the exchanger. In contrast, the heat transfer surface areadesignates the physical size of a heat exchanger.

A large value of NTU does not necessarily mean that a heatexchanger is large in physical size.

When comparing heat exchangers for a specific application,U/Cminapproximately remains constant; and in this case, a higherNTU value means a heat exchanger larger in physical size.

Hence, NTU is sometimes also referred to as a heat exchangersize factor.


4/11



4

How to increase NTU? Higher NTU is obtained by increasing either U or A or both or

by decreasing Cmin.

Whereas a change in Cminaffects NTU directly, a change in Cmax(i.e., its f low rate) affects hon the Cmaxside.

A change in the value of Rmay have direct or indirect effect onNTU.

The number of transfer units achieved by the heat

exchanger.

---------------------------------------------------------(a)


5/11



5

NTU is also variously referred to as a performance factor orthermal length in the plate heat exchanger literature, and asreduced thermal flux in the shell-and-tube exchanger literature.

With increasing values of NTU, the exchanger effectivenessgenerally increases and in the limit, it approaches athermodynamic asymptotic value.

Note that the perfect exchanger requires that NTU(because A

) for q max= Cmin

Tmax.

Automobile radiator: NTU0.5 40 %

Steam plant condenser: NTU1.0 63 %

Regenerator for industrial gas turbine engine: NTU10 90 %


6/11



6

Interpretation of NTU as non dimensional residence time

Ro=(1/UA) is the overall thermal resistance;

Is minimumside fluid capacitance and d is dwell time,residence time or transit time of a fluid particle passing through

the exchanger.Thus, NTU can be interpreted as a non-dimensional residencetime


7/11



7

Another interpretation of NTU

NTU is related toTm

NTU is referred to as a temperature ratio (TR), where

NTU=1, WhenTm is equal to Thor Tc

number of transfer units required by the heat duty

------------------------------------(b)


8/11



8

NTU is also directly related to the overall (total) Stanton numberSto

Replacing Uin place of h

NTU can also be interpreted as a modified Stanton number.

p is the porosity, a ratio ofvoid volume to totalvolume of concern.

First definition of Dh is for constant cross-sectional flow passagesand second definition is applicable to expanding/contracting flowarea across flow cross sections along the flow length.


9/11


10/11


College of Engineering, Pune (COEP)Forerunners in Technical Education10

Effectivenessnumber of Transfer Unit Relationships

is a function of NTU, R, and flow arrangement.

Counter flow exchanger as afunction of NTU and R.

1.The heat exchanger effectiveness increases monotonically withincreasing values of NTU for aspecified R.2.The exchanger effectiveness

increases with decreasing values ofR for a specified NTU.3. For 40%, the heat capacityrate ratio R does not have a

significant influence on theexchanger effectiveness".


11/11


College of Engineering, Pune (COEP)Forerunners in Technical Education11

4.Heat exchanger effectiveness increases with increasing NTU asnoted above in item 1, but at a diminishing rate.5.Because of the asymptotic nature of the -NTU curves, asignificant increase in NTU and hence in the exchanger size is

required for a small increase in at high values of .6. Alternatively, a larger increase in NTU (or the size of theexchanger) is required to compensate for the same (or small)amount of heat loss to the surroundings at high values of in

comparison to that for a lower exchanger.The counter-flow exchanger has the highest exchanger effectiveness forspecified NTU and R of that for all other exchanger flow arrangements.Alternatively, the heat transfer surface area is utilized most efficientlyfor counter-flow compared to all other flow arrangements.

It should be emphasized that many industrial heat exchangersare not counter-flow

Documents

Ntu Method