heat transfer promotors

[vinod68]

Hi all.
We are using tube and fin type fin fan cooler to cool the lubrication oil of our 10 MW gas turbine.
During high abient temp. the oil temp. rises very high.
Please inform if there are flow retarders or heat transfer promotors available which can be inserted into the tubes, to increase the heat transfer capacity from the oil to the air. The fin fan cooler has a tube diameter of 5/8".
regards

 

[davefitz]

Before you add internal promotoers, you need to confirm that the primary resistance to heat trasnfer is at the ID and not governed by OD resistance.

There are amy types of internal heat transfer promoters, notably internal spiral fins. But their additiona may not solve your problem.

If the main resistance to heat transfer is outside convection over the finned tube, you would be better off getting a 2-speed fan and increasing the air flow on a hot day. The fan solution is indicated if the heat exchanger's LMTD drops significantly on the hot day vs cold day, and this can be associated with a lower fan air flow on hot days ( due to the fan's cfm vs head curve) resulting in a lower outside heat tansfer coeficient on a hot day. If the LMTD remains roughly constant and the increase in oil temp is solely due to a hiher inlet air temp , then other solutions should be sought.

In general , the max practical heat exchanger effectiveness would be at an NTU of 2; typical economical heat exchangers use an NTU of only 1. If improving the NTU to 2 will result in an acceptable oil out temp on a hot day, then one would incrase the fan output plus increase the heat exchanger's surface area until a NTU of 2 is reached, and this assumes the WCp of the air is at least equal to the WCp of the oil.

Another possible option is to use an oil with a higher oxidation temperature that allows higher oil temps ( as used in gas turbines in the mideast).

 

[smckennz]

Hi Vinod68
if you have a truly clean water supply, humidification of the air by some fogging spray nozzles can help. Try not to get the fins wet though especially if they are tension wound L fins and not embedded fins.
Chances are you do not have clean water. If so,do you have a standby oil cooler? Running duty and standby at the same time will help. If you have a spare in the store, it may as well be a "hot" spare.
Its highly unlikely that improving the oil side heat transfer coefficient will help much.

Cheers

Steve

 

[speco]

Vinod68,

I think you are on the right track with the turbulence promoters. Contrary to what Davefitz would expect, in a lube oil cooler, the controlling heat transfer resistance is ALMOST ALWAYS on the tube side. Adding air flow will do very little to help the problem. The reason that the tubeside controls is that the oil properties are very poor for heat transfer. That is, the thermal conductivity is low, the specific heat is low, and, worst of all, the viscosity is high.

In doing the heat transfer calculations (based on the extended surface)the inside thermal resistance must be multiplied by the ratio of the extended surface to the inside tube surface. This ratio is typically in the range of 20 or 25:1

Going back to the viscosity question, lube oil viscosities vary all over the place, but typically run from about 10 cp to 50 cp in the usual cooling temperature range. With a "normal" pressure drop on the tubeside, this usually puts the Reynolds Number in the transition or laminar range, where the heat transfer coefficient is very low.

Now, going back to your existing unit. It may already have some kind of turbulence promoters inside the tubes. Most cooler manufacturers would do this automatically to get a reasonable design. You may want to check with the original manufacturer to confirm this, if they are still around.

If there are already turbulence promoters (sometimes called retarders or turbulators) they you probably have two choices: 1. Boost the pumping rate of the lube oil. Remember, the pressure drop will increase approximatly proportional to the square of the GPM. 2. Replace the tube bundle with one containing turbulence promoters AND with more surface. Since the inside coefficient controls, surface is the key to performance. Especially bare tube surface. The fins help a bit, but more fins or higher finse make practically NO difference.

If the existing unit has no turbulence promoters, I would contact Bastex in Houston (if they are still around) They make twisted tape "turbulators" which could possibly boost your inside coefficient significantly.

Regards,

Speco (

http://www.stoneprocess.com)

 

[speco]

Vinod68,

Sorry to mislead you. The Bastex name is apparently no longer used. It is now Brown Fintube:

http://www.brownfintube.com.

They still show "turbulators" on their website and have a list of sizes available.

Regards,

Speco

 

[srfish]

Hello Vinod68

I agree with Speco about contacting Brown Fintube. Some of the air-cool manufacturers will also sell these twisted tape turbulators. Another possibility is Fuel Efficiency, LLC.

 

[vinod68]

thanks speco,
i did try to increase the air flow by chenging the motor pulley dia and increaing the fan rpm.
but even with 20% increase in air flow there was hardly any appreciable chenge in the oil outlet temp.
I thing the coil does have turbospirals inserted into the tubes.
Are the heat transfer promotors or retarders better than the turbospirals.
I think the turbospirals are just plain enlarged springs which do not give much turbulance and more over do not provide with any additional primary heat transfer area

 

[speco]

Vinod68,

I wish I had a real answer to your question about turbospirals vs. turbulators. The only ones I have had any direct experience with were the twisted tape type turbulence promoters. They are commonly used by some air-cooler manufacturers. Possibly Brown Fintube could offer some help there. They could probably offer a solution if they have the full process data and a data sheet for the existing cooler.

They should also be able to tell you if there is enough surface there to make it work. Some manufacturers "shave" their ratings by as much as 20% as standard practice. If your existing unit is one of those, you will probably need a new tube bundle. This is one area where size does matter.

Regards,

Speco (

http://www.stoneprocess.com)