Buffer fluid heat transfer

[daveed007]

An engineer informed me today that we are switching to a "better" type of barrier fluid for our seal pots. We are currently using buffer fluid A. We are proposing to switch to fluid B. If fluid B's thermal conductivity is twice that of fluid A's, and its heat capacity is 30% greater than fluid A's, how much more heat dissipation will we get? To me it seems like it would absorb 30% more heat, twice as fast, resulting in about 60% greater heat dissipation, minus small losses because of faster changes in temperature gradient, etc. Is this correct? I've tried paging through a heat transfer book and can see a formula including thermal conductivity that would indicate heat would be transferred twice as fast. I don't see exactly how heat capacity would be figured in though. AlLl other parameters remain the same (flow rate, equipment, etc.) Thank you so much for your expertise. I enjoy reading these forums although I just work on air conditioners and never got an engineering degree. Thanks again.

 

[scalleke]

The increased heat capacity is not that important. het is important is the increased conductivity. You are trying to get heat away from the seats and will be able to do so twice as fast PROVIDING the heat can be carried off twice as fast in your buffer reservoir also.

Three tings are working together here:

The heat generation of the seals
the liquid that transports the heat
The way the heat is carried away from the buffer reservoir.

The heat generation from the seals is the same Except if the new buffer liquid has a different viscosity. If the buffer liquid has a different viscosity and lubricity heat input from the seals will be different. Ask the Liquid sales man if there have been tests with the same seals with both liquid A and B and which of the two liquids made the seals genarate the most heat.

The liquid will be improved and will have more storage capacity and better heat transfer rates.

The heat will need to be carried away as fast as the liquid can remove it from the seals. Will the buffer reservoir heat exchanger be able to carry away the heat faster than before?

Scalleke

 

[daveed007]

The new liquid viscosity is much lower than the old liquid's (5x lower at 100 deg. F, 2x lower at 300 deg. F). I'm assuming that leads to increased flow which would aid in heat removal. The buffer reservoir heat exchanger runs ambient temperature cooling water through it. As long as the ambient temperature is lower than buffer fluid temperature it should be able to carry away heat right?

There is probably no chance they will alter the heat exchanger set-up, but I feel the the amount of improvement in the liquid's heat storage capacity and heat transfer rates need to be analyzed more carefully before justifying the usage of more expensive fluid. What I'm trying to get at is with the same exact set-up and material properties mentioned before, how much of an improvement in heat removal can be expected?

 

[scalleke]

I would be concerned about the heat generation by the seals.

Viscosity of the new liquid is much lower than of the old. Seals will run closer together. There is a possibility more heat is generated by the seals. I have a doubt on this one because I believe friction in a liquid depends on the shear forces in the liquid and I assume these are reduced in the case of lower viscosity. I would put this as a question to the buffer liquid provider and ask for their experience:

For the same set of seals operating under the same pump process conditions

1 what heat generation will correspond with which buffer liquid?

2 what film thickness between seat faces is expected for which buffer liquid.

Less heat generation is better.
Less film thickness is better as it means less buffer liquid consumption (double seals) and less buffer liquid contamination (for Tandem seals).

Your current heat exchanger does the job for buffer liquid A.

Suppose seal heat input remains the same. The buffer liquid can store more energy so buffer liquid temperature will be reduced; differential temperature between buffer liquid and the heat exchanger water will be reduced which decreases heat exchanger effectiveness, however buffer liquid conductivity is better which most likely increases HE eff.

Bottom line is seal heat generation must be re evaluated.
Heat exchanger calcs must be verified.

 

[25362]

Physical reasoning and experience tell us that the convection heat transfer coefficient is a function of the Re and Pr numbers:

Nu=C Re^mPrⁿ, where C, m, n are constants determined from experimental data.
Aparently m = or > n. Thus, in seteris paris, a drop in viscosity is bound to improve heat transfer.

 

[longeron]

The seal face separation would be an issue if we were talking about a Barrier fluid. (greater separation means more leakage) A buffer fluid is used at a lower pressure than seen in the stuffing box/seal chamber so it isn’t lubricating the seal faces, the process fluid is still lubricating the seal faces.

Don’t forget that the major factor in the heat exchange is the flow of the buffer fluid through the support system tubing, the lower viscosity should help. The pumping ring, or the shaft windage will not generate a ton of head to move the buffer fluid. Thermo siphoning, or the changing density of the fluid as it heats and is displaced by the heavier cooler fluid, is going to be the main fluid transport method. So be sure to follow the seal mfgs guidelines for reservoir installation. You don’t want the pot to be too far away from the pump, and just high enough above the shaft centerline to generate decent static head for the return of the cool fluid. Also ensure that nice sweeping curves are used instead of sharp 90 degree bends to decrease friction losses from the tubing.