Considerations about grooved tubes
Considerations about grooved tubes
The corrugation of the tubes can both affect dramatically the coil performances or not, dependently on the typology of the fluid inside the tubes. For example, if the fluid is an oil with high viscosity, or it is an ethylene/propylene glycol, a grooved tube can increase the coil performances, otherwise the difference will not be even visible. If in the project we are using refrigerants, the difference may not be so high. Why it happens like this?
The equation to calculate the heat exchange coefficient of a finned pack heat exchanger is the following
K = 1 / (Air Resistance + Tubes Resistance + U)
Air Resistance is the air side resistance equals to Air Resistance = 1 / (Air H * Coil Eff)
Tubes Resistance is the tubes side resistance equals to Tubes Resistance = Si / Tubes H
Where:
- Air H is the partial exchange coefficient air side
- Coil Eff is the finned surface efficiency
- Si = (Primary surface + Secondary Surface) / Primary Surface
- Tubes H is the partial exchange coefficient tubes side
- U are other resistances usually ignored like the thermal resistance due to the tubes material and the fouling factors both sides.
The primary surface is the thermak exchange surface of the tubes, and the secondary is the total surface of the fins.
Suppose that we set the Air Resistance to = 0,018
By increasing the surface of the tube using grooved type of corrugation, a double effect is achieved: the heat transfer coefficient on the tube side and the Si value both increase. If we use water with a speed limited to 3 m/s and a very high heat transfer coefficient on the side side of 4000 and suppose the Si coefficient = 20, the tube resistance will be calculated as 20/4000 = 0.005, i.e. the tube resistance is very low.
In the case of a liquid with a higher viscosity (thermal oil), the exchange coefficient on the tube side will be lower, equal to 800, thus tube resistance = 20/800 = 0.25. In this case, the resistance on the tube side prevails in the calculation of the global heat transfer coefficient, in which case it is advisable to use a grooved tube to increase both the heat transfer coefficient and the ratio between the surfaces, hence significantly increasing the coil capacity.
In this situation it is very important to set the parameter “Tube internal surface ratio” of the geometry, which specifies the increment of the surface of the grooved tube. This ratio is calculated as: (Tubes Internal Grooved Surface / Surface of the corresponding smooth tube).