LMTD is lower and heat transfer is more

[knight92]

Hi there,

I have tested two radiators with 90 degrees celcius of hot water going through it. The radiators were placed in a wind tunnel with the ability to control air speed.

The bigger radiator cools the water more than the smaller radiator and the air temperature rise after the air has gone through the bigger radiator is also more than that of the smaller radiator.

That means I have a bigger heat transfer rate for the bigger radiator at say 10 ms-1 of air speed than the smaller radiator however calculating the LMTD and corrected LMTD shows that the bigger radiator has almost half the LMTD values for the smaller radiator. The smaller radiator however has less cooling effect on the hot water going in.

I thought the larger the LMTD value the more heat is transferred but my results are contradicting themselves. Its true the bigger radiator is dissipating more heat but LMTD values oppose the reality ?

I am confused about what is going on. Can there be any reason why the LMTD values might be lower for the bigger radiator ?

Equation for LMTD I used:

T(LMTD) = ( (Air In temp - Water outlet temp) - (Air out temp - water inlet temp) ) / LN ( (Air In temp - Water outlet temp) / (Air out temp - water inlet temp) )

One of the results for bigger radiator:

Coolant/Water side:
Water inlet temp = 363.15 K
Water outlet temp = 325.15 K

Air side:
Air in temp = 295.15 K
Air out temp = 335.15 K

Thank you very much for your help in advance.

 

[25362]

LMTD is just one factor in heat transfer according to the equation:

Q = U[×]A[×]LMTD​

Therefore, one must also consider A (area of heat transfer) and U (overall heat transfer coefficient)

 

[knight92]

It makes sense now because the CLMTD is nearly half the small radiator's CLMTD and the bigger radiator's surface area is double the surface area of the smaller radiator.