Condensers with Grooved Tube

[wolfied]

Hello people,

I wonder what the capacity difference would be if I use corrugated grooved cupper tubes instead of corrugated smooth cupper tubes ?

Is there any information about the rise of the capacity when grooved tubes are used ?

Thanks everyone.

[desA]

If you provide more information on the fin you're using (type, material, louver details, thickness, spacing), your air on/off temps, face velocity, refrigerant, tube details (material, OD, fin details) I can give you some feedback on the overall impact of using this tube.

The typical information cited refers to the tube performance increase, but this needs to be integrated into the overall condenser design, to see its overall impact.

For instance Royal/Bergles [1978] - under convective condensation - "The best internal fin geometry provided 20 to 40% higher heat-transfer coefficients (total area basis) than the smooth tubes."

Please bear in mind that this refers to the tube effect only. This effect has to be worked into the overall condenser performance increase.

[US Iceman]

I think yo need to do the calculations for the tube geometry used. The tube manufacturer should have some equations for predicting the performance of the tube. In approximate terms I can tell you the internal enhancements increase heat transfer, however the higher turbulence also increases the pressure flow due to flow.

If the internal coefficient increases, the external coefficient may also increase due to lower tube wall resistance.

On the other hand, the ratio of surface areas due to the surface enhancements may also produce some interesting results which might not be beneficial. This is why you need to run the numbers and not use approx. percentages. Those rules-of-thumb for heat transfer should not be used.

[desA]

A sample (feel) calculation:

Case 1 - Low airside heat-transfer

Plain tube calculation:
h,o = 45 W/m2.K
h,i = 30,000 W/m2.K (plain tube)

U ~ 1 / [1/45 + 1/30,000] = 44.93 W/m2.K

Internally-finned tube calculation:
h,o = 45 W/m2.K
h,i = 30,000 W/m2.K * 1.4 = 42,000 W/m2.K (internally-finned tube tube)

U ~ 1/[1/45 + 1/42,000] = 44.95 W/m2.K (+0.04%)

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Case 2 - Medium airside heat-transfer

Plain tube calculation:
h,o = 100 W/m2.K
h,i = 30,000 W/m2.K (plain tube)

U ~ 1/[1/100 + 1/30,000] = 99.67 W/m2.K

Internally-finned tube calculation:
h,o = 100 W/m2.K
h,i = 30,000 W/m2.K * 1.4 = 42,000 W/m2.K (internally-finned tube tube)

U ~ 1/[1/100 + 1/42,000] = 99.76 W/m2.K (+0.09%)

Estimated increase in internal pressure drop ~ 38%.

------------------
Summary:
1. Air-side heat transfer dominates the overall desgn.
2. A huge increase (+40%) in tubeside heat-transfer coefficient makes negligible overall heat-transfer contribution.
3. Internall finned tubes suffer an increased pressure drop ~ 36%, over plain tubes.

desA

[US Iceman]

Before anyone gets too excited about this we should find out if the poster is considering the tube for refrigerant/liquid or refrigerant/air and which side each fluid is on.

Air is the one fluid where enhanced tubes do not provide much or any benefit because the air side heat transfer controls the process as desA pointed out.

The other thing necassary to know is: what are the fouling factors? By the time you make surface ratio corrections for the fouling factors it gets worse.

[wolfied]

Hello people,

Thanks all for your answers. Let me be more spesific with my post. I'm trying to find the answer of this question for universal condensers with air cooling.

Fin is made up of Aluminium, with 0.1 mm thickness. Fin spacing will be 2.1 mm for my design (İt can be 2 mm no problem tough). Fin type will be smooth grooved type. Geometry : 32x28 1/2", R404A is used, no subcooling and superheating. I hace 67 tubes, 3 rows, 32 circuits (6 passes for each circuit), 9 tubes are left empty, but you can neglect it. Length is 3600 mm, height is 2127 mm. So, currently the capacity is approximately 266 kW. Surface: 536 meter square. 6 fans are being used.

Dry bulb air temperature 25'C and DB air outlet temperature 32.5'C. Fluid velocity 5.7 m/s ; air velocity: 4.148 m/s Evaporation tem 0'C, condensing temp 40'C. Relative humidty %50.

As a result, I give you my details. Now I need to learn how groove tubing will change my current condenser capacity ?! Thanks people for your contributions.

[desA]

Wolfied - my post on the air condenser will be useful to you. Essentially it will help you little to use an internally-grooved tube.

You may want to watch your air face velocity at 4.148 m/s - it will be noisy.

[US Iceman]

Essentially it will help you little to use an internally-grooved tube.

I agree. The air-side film coefficient will control the heat transfer process. The enhanced tubes will only add cost to the heat exchanger.

[Sandro Baptista]

I agree. The air-side film coefficient will control the heat transfer process. The enhanced tubes will only add cost to the heat exchanger.

I can't agree at all Iceman. I depend of each case. If the tubes are finned the the air side have a much bigger heat transfer surface (secondary surface) compared with the primary surface and then there will not be so much difference between the U.A internall (refrigerant) and the U.A. externall (air).

Baptista

[US Iceman]

I can't agree at all Iceman. I depend of each case. If the tubes are finned the the air side have a much bigger heat transfer surface (secondary surface) compared with the primary surface and then there will not be so much difference between the U.A internall (refrigerant) and the U.A. externall (air).

Baptista

The heat transfer coefficient of the air side is much lower than the heat transfer coefficient through the tube wall. Therefore if you improve the film coefficient by using internal enhancements the overall impact is negligible to improving the overall heat rejection of the condenser.

The overall heat transferred is simply a function of providing sufficient secondary surface to match the ability of the primary surface to transfer heat to the fins. This is further complicated by the fin root resistance at the tube wall interface.

[Sandro Baptista]

Anyway the primary and secondary heat transfer surfaces play a lot influence. As you know you can have a big U on inside tube and a low U on the outside but if you have a great heat transfer area on the outside the global heat resistance will be smaller. the heat flux refrigerant to air will be bigger.



Of course the resistance to heat conductivity on the "connection" fin- tube it could have an importante impact if it isn't weel done.

[surfacehilly]

well ice man i have been making charge air coolers for tankers and navy ships for 30 years and belive me you hit the nail on the head man

[michaelm]

I agree with US Iceman. Overall Heat transfer coefficient will always be smaller than smallest of the two of the film coefficients.

[US Iceman]

One might think enhanced heat transfer tubing is a good thing to develop. However, it also has to be balanced between construction cost of the exchanger. You might make a smaller more efficient exchanger with enhanced heat transfer tubing, but the tubing is more expensive than prime surface non-enhanced tubing too.

Then if you consider the effects of fouling factor on the surface ratio corrections for the film coefficients it might be worse using enhanced heat transfer tubing. A lot of new equipment is using this tubing and most have extremely small fouling factor allowances.

The numbers and test data may prove out the heat transfer works however, if the exchanger is fouled beyond those tiny fouling factors you are screwed!

Enhanced tubes should be very carefully evaluated before you dive into a project that could prove to be unwise (using hindsight).

[desA]

Agreed - Iceman.

[Magoo]

Agreed Iceman.
Rifled tubing and fins is a fine line, water cooled with rifled tube and external rolling is a disaster waiting happen. Manufacturers quote extrordinary performance charactoristics. In essence the clearnce between gas and water is around 0.2 ish mm. I have found all manufacturers data is over-inflated and should be treated with contempt, and derate accordingly

[US Iceman]

If you think these are great, try them on some some viscous fluids in shell & tube chillers.

I got my butt kicked on one job because the tubing manufacturer correlations broke down going through the transition region. If you can't keep the Reynolds number high enough strange things happen!

Don't ask how I know this.

[Sandro Baptista]

Anyway the primary and secondary heat transfer surfaces play a lot influence. As you know you can have a big U on inside tube and a low U on the outside but if you have a great heat transfer area on the outside the global heat resistance will be smaller. the heat flux refrigerant to air will be bigger.



Of course the resistance to heat conductivity on the "connection" fin- tube it could have an importante impact if it isn't weel done.

okay guys,

But anyway I think that what I have posted before even so have reasonable validity.

Regards