Effects of microfin tubes on capacity
- Increased capacity with the use of a microfin tube
A microfin tube increases the turbulence of the fluid in the tube and therefore improves its performance.
The reason why the performance improves, but not so much, will be explained below.
The formula of the global exchange coefficient is as follows:
Where:
Ae is the area of the finned surface
αi is the tube-side exchange coefficient
Ai is the tube exchange surface
αe is the fin side exchange coefficient
Ω * is the efficiency of the finned surface
De is the external diameter of the tubes
Di is the internal diameter of the tubes
L is the length of the tube
λ is the conductivity of the tube.
In a heat exchange coil, the fin side exchange coefficient (αe) is in the order of 70W / m2K.
The tube side, on the other hand, has a partial exchange coefficient in the order of 1500 W / m2K (if it is a smooth tube) -2500 W / m2K (if it is a micro fin tube).
As for the ratio between the surfaces (Ae / Ai), in the case of smooth pipe it will be greater than in the case of finned tube.
The dominant thermal resistance is on the air side. Therefore, even though it manages to almost double the tube-side exchange coefficient, its effect at the global exchange coefficient level is small.
- Decreased capacity while using microfin tube.
There may be situations when, even though we are using a microfin tube, the capacity decreases compared to the use of smooth tubes.
This reason is due to the pipe side pressure drops.
In fact, if there are some Kelvin of pressure drops, the end-of-evaporation temperature decreases.
See the chart below:
From point 1 to 2, we have the ideal evaporation (at constant pressure).
But if there is a pressure drop of 2K or more, the evaporation temperature is represented by the trend from 1 to 2 ‘, thus causing a lower end-of-evaporation temperature than the beginning.
And this causes, therefore, a decrease in capacity.
