Oil coolers

Here to realize their full potential, they must be used together with turbulators produced by our Turbulator division.

While the basic graphs given in the basic curves section will cover the airside performance of the Heat exchangers it is very important to look at what happens inside the tube.

Here the heat transfer coefficient will depend on the following:

a) Oil viscosity.
b) Oil flow rate.
c) Type and density of Turbulator used.

Selecting the correct turbulators

Design

We can assist with the design data for the turbulators we produce on a case by case basis depending on the application, type of turbulators requested and the fluids involved. Our regular customers are also provided data which they can use for doing their own design. We encourage you to visit our website www.allturbulators.com for a better understanding of turbulators one can download our turbulator catalogue from this site. We have copied an extract of the design section of the allturbulators site here for your reference.

Key elements of good Turbulator design.

While we stand ready, and prefer to help our customers with design, we have tried to present an overview by presenting some graphical data.

To do this we have done the following:

The pressure drop and heat transfer of oil of 3 viscosities, 5CST, 15 CST and 25 CST travelling through the tubes at a flow rate of 1 to 6 feet per second. This data was prepared for the following Turbulator cases:

No Turbulator.
Rigid turbulators RT2, RT4 & RT6. (Our standards for this tube size).
Flexible turbulators FTD1, FTD2 & FTD3 (Again our standard models)
Twisted tape turbulators.

Since the possibilities here are infinite for the case of better comparing the two, we selected those twisted tape turbulators that gave with a 15 CST oil the same pressure drop at 3 FPS flow rate as the corresponding standard flexible turbulators. We must bear in mind though, that twisted tape turbulators thus selected have a very high twist ratio and are not commonly manufactured. However they were chosen for their value in comparision. Most commonly used twisted tape turbulators will have a lower performance and pressure drop.

From the curves, we can arrive at the following observations:

1) The rigid soldered turbulators overall give the best heat transfer and also the highest pressure drop. However as the viscosity goes up, the performance as compared to flexible turbulator goes up significantly. For example in the 5 cst case, RT6 is above FTD3, RT4 above FTD2 and RT2 above FTD1.However at 15 cst both RT6 and RT4 are above FTD 3 and RT2 is very close to FTD 3. In the case of 25 CST all the three rigid turbulators are all above the highest performing flexible turbulators.The pressure drop is also compared to performance much lower.

2) The performance of the twisted tape (even though the twisting selected is high to match the pressure drop of corresponding flexible wireTurbulator) is generally lower than that of the flexible but not by much.However given that we will not have such a high degree of twisting in standard available twisted tape turbulators we can say that the flexible turbulators performs better.

3) The pressure drop increase for the two wire type turbulators with increase in fluid velocity is more than linear. The increase in performance is less than linear. This tells us that after a point it is not worth trying to purchase performance with pressure drop.

4) The performance as well as the pressure drop in the case of twisted tape turbulators are more linear.

To get another angle on this performance we worked out the following table,where we have simply divided the heat transfer coefficient by the corresponding pressure drop to get the HTPD factor (Heat transfer coefficient per unit of pressure drop). We did this for the three oil viscosities for all the Turbulator models at all the studied flow rates. We have color coded the results so that it is easier to spot the best as well as the trend.

1) The data for rigid turbulators are for rigid soldered Turbulators. If the same turbulators are not soldered they perform but not as well. This is due to the better contact and so better heat transfer due to soldering.

2) Just as pressure drop is a cost we pay for performance so is the cost of material and labour. The rigid soldered Turbulator while an extremely efficient turbulator is expensive to make and install.

3) Ease of cleaning and maintenance are also key factors in turbulator choice.

4) Since we make all types, we have nothing to loose in giving the correct advice.

Customization

Research Assistance

Here we have as case 1, wire wound fin tube (a special high performance fin tube we make) with no turbulators. In case 2 we have the same wire wound fin tube with internal flexible turbulators. This allows us to see the standalone effect of the turbulators.

Number of rows drops from 20 to 12. Total tube length falls from 1164 to 636. This is a 45% decrease. Quite naturally the airside pressure drop falls with its commensurate power savings from 1.2 inches of water column to .8 inches. Even more remarkably the tube side pressure drop falls from .5 to .4 kgs per square cm. This demonstrates that though the pressure drop goes up nominally on account of turbulators the actual total pressure drop can actually fall as the number of passes reduce and the length of tube becomes less.

Here we have case 1 as a normal Helical fin tube with no Turbulator and 6 passes and Case 2 our own wire wound fin tube (a much higher efficiency fin tube we make) and our flexible Turbulator inside the tube. The results are given as under.

The surface area of fins per linear foot of wire wound fin tube is 4.02 feet against 6.16 for the normal helical fin.

The number of tube rows is reduced from 6 to 3 and tube passes from 6 to 1. Total fin surface area is reduced from 30,532 to 11,898. A reduction of 62%.

Case 1 is without turbulators and case 2 is with flexible internal turbulators. The total length of tubes is reduced from 55 lengths of 16 ft to 60 lengths of 10 ft. This reduces the total length of tubes by 32%.

In our study we have isolated the airside and tube side coefficient and dealt with how to enhance both to get the optimum result.
The relevant literature can also be downloaded from the following websites

Stream Air Heater catalog	Fin tube catalog	Air cooled heat exchanger catalog	Turbulators catalog