Calculations for oil>water exchanger?

[sprintcar]

We've built a small linear "tube inside a tube" oil to water heat exchanger and I'd like to get a better idea of the heat rejection capacity. I did some 'book calculations' and don't trust the answer.

SPECS:
Ambient air temp - 90F
Inlet water temp - 85F (city water)
Water flow was nominal 6 gpm with no measurable temperature rise out of cooler
Oil is ISO 150 synthetic with viscosity at 176F (80C) of 187.5 SUS or 40 cst.
Oil flow was around 1 gpm which is circulated back into the reservoir by gravity

CONSTRUCTION
Inner tube for water: 2" sch 40 (2.375" OD, 0.154" wall x 42" long = 2.17 sq ft surface area)
Outer tube for oil: 3" sch 40 - outside exposed to ambient air.

TEST RESULTS
Oil temps were inlet: 172F and return: 143F

QUESTIONS - what is the potential oil heat loss (or temp drop) thru the system assuming max 175F oil?
Oil flow can be increased to about 3 gpm, water to 20gpm

Thanks guys!!


Keep the wheels on the ground
Bob
showshine@aol.com

 

[siretb]

You do not state if the ambiant air is stagnant or if this is on the outside (hot climmate at 90°F !?).
If so you (we) need to know the max wind velocity to estimate the heat losses to the outside.

 

[sprintcar]

No air flow to speak of - it's inside a building.
The adjacent equipment and good ol' southern weather created the 90F ambient which actually peaked at 96F during one of the tests.
Actual units could be located outside where heat loss from wind could be counteracted by solar heat during the day. We are also not including the small radiant heat transfer from the bearing housing surface area which overlaps part of the cooler length.
THANKS!!

Keep the wheels on the ground
Bob
showshine@aol.com

 

[25362]

To sprintcar.

Thoughts in answer to your original query:

1. Because of the large difference in OHTC on the oil/air side vs the oil/water side, I pressume water will still be the main heat sink.

2. Increasing flow rates improves the OHTC, but the NTU and the H/T effectiveness are bound to come down. One can thus assume that the oil temperature drop will be inversely proportional to its flow rate.

OHTC = overall heat transfer coefficient; H/T = heat transfer; NTU = number of transfer units.

Any comments ?

 

[MortenA]

How hot does the water get?

If the dT(oil,water) in the outlet is large then i would guess thatyour limiting side must be on the oil side and that the dT(oil) will remain allmost the same (e.i. new outlet temperature around 146 deg F.



Best regards

 

[MortenA]

Just saw that the temperature increase in the water was small - so i think the dT(oil) will remain constant (allmost). Increasing the oil flow rate will most likely increase the dT(oil) due to a lower heat transfer resistance and the bulk of the total heat transfer resistance beeing at the oil side.

Best regards

Morten

 

[25362]

MortenA,

Even when assuming countercurrent flow (which, BTW, was not indicated by Sprintcar) and since

Q=UA (LTD) = MCp[Δ]T​

what you are implying is that the heat transferred Q will increase almost proportionally to the flow rate M with the same area A and ~ same LTD. This means that U (the OHTC) should increase by the same order of magnitude !

I still think that Q would stay about constant and that by increasing the oil flowrate by, say, a factor of 3 would result in an oil temperate drop of the same (inverse) magnitude, namely, 9-10 deg F. Any comment ?

 

[25362]

To MortenA, please note that my assumption is based on an oil "laminar" flow regime even at the highest flow rates.

The improvement on the convection heat transfer coefficient on the oil side by tripling its flow rate would be about 20%, and the OHTC would improve by ~15%. Thus the temperature of the oil would drop just by 11 deg F.

[δ]T_{triple flow} = [δ]T_original*1.15/3=29*1.15/3=11.1 deg F​

Kindly comment.

 

[MortenA]

My respone actually assumed that the oil flow remains constant (the original post says _can_ be increase to 3 GPM.

While Q off source will increase with increased flow i agree that this will NOT be proportionally with flow rate and that the dT(oil) thus will decrease (e.i. oil exit temperature will be higher) when increasing the flow rate.

Best regards

Morten

 

[25362]

To Sprintcar, allow me to be a bit more extended:

Having agreed with MortenA on that, in this case, an increased flow rate would reduce the oil side [Δ]T, let me add that under special conditions, when the main H/T resistance is on the oil side, doubling the oil flow rate would result in an equal or even increased oil [Δ]T (!) as in steam heaters.

This is possible when the Re number changes from, say, 1500 to 3,000, i.e., from a laminar to a turbulent flow regime. In such a case the convection HTC on the oil side could change from 350 to 950 W/(m².K)! And the resulting U could more than double.

This can be seen on the log-log graph resulting from the classical measurements by Sieder and Tate: Heat Transfer and pressure drop of liquids in tubes Ind. Eng. Chem., 28, 1429-1435 (1936).

This graph shows:

Nu.Pr^-1/3([μ]/[μ]_w)^-0.14=f(Re,L/D)​

where L is the tube length, m; D is the hydraulic diameter, m; and [μ]_w is the viscosity of the fluid at the surface temperature of the tube or pipe, N.s/m²=Pa.s. All other fluid properties are related to the bulk temperature of the fluid. For most cases ([μ]/[μ]_w)^-0.14 is practically 1.0.

On the graph one finds that for L/D=50, the values of
Nu.Pr^-1/3([μ]/[μ]_w)^-0.14~5.4 for Re=1500, and is ~11 for Re=3000. This effect is felt more on larger L/D ratios, say, 200-240 as in S&T heat exchangers.

However, in general, one can safely say that usually an increased flow rate will reduce the [Δ]T on the oil side.

 

[sprintcar]

Thanks Guys!
Due to design constraints, the water flow and oil flow are in the same direction so I lose a bit of efficiency.

Based on all your info I'm adding a 5/16" diameter wire spiral wrapped around the water line to disrupt the laminar flow of the oil. Another test cooler will be built this week - more of a production version for field testing.

The next step will be to increase oil flow which will be a major change to the unit.

Thanks again!!

Keep the wheels on the ground
Bob
showshine@aol.com

 

[siretb]

I agree with 25362 that in this case, the limiting factor is on the oil side, and that the heat duty will remain almost constant if you triple the oil flow rate.
That means that the oil Delta T will be divided by 3.
We are in the laminar regime, and the Nusselt number is about constant (about 4.5).
Due to the high viscosity, although this cannot hurt, I am not sure you'll gain a lot by adding youy wire spiral.
Your heat exchanger is very small, why don'nt you build a new one, with an annular gap smaller, and an overall diameter larger(==surface area larger). AS I'd expect the Nusselt number to remain quite constant, reducing the characteristic dimension (the gap) would help a lot.

 

[sprintcar]

Like any design, the envelope is limited and I'm pushing it with the 3" pipe. Since the water flow in the field will be above 10gpm, I can't use much more than a straight pipe for it due to customer perceptions.
I am looking at a rectangular oil tube to increase volume and air exchange surface area but this may compromise the contact area for the water tube.
The spiral wire evolved from test results showing stratified laminar flow of the hot oil along the top of the cooler directly to the outlet.

Thanks for the education too!!


Keep the wheels on the ground
Bob
showshine@aol.com

 

[25362]

Sprintcar, articles on heat transfer enhancement in tubes by twisted inserts on viscous fluids claim (probably optimistically) 4 to 5-fold increases in the HTC by convection. I wish you luck.

 

[sprintcar]

UPDATE
With your input, we built the prototype and just finished preliminary testing. The oil stayed below the design goal of 180F after 24 hrs of run time. The oil flow rate was increased and the internal spiral was built into the unit. The cooling water flow had to be increased over 50% (still below actual duty flow) to water outlet temp within 2 degrees of the inlet temp.

The final temp drop was 26 F.

The design also increased oil capacity by 50% which will extend maintenance interval.

Thanks again folks!!!

Keep the wheels on the ground
Bob
showshine@aol.com