Suction line riser

I'm hoping that someone can put me on the right track here.

In the ASHRAE Handbook of Refrigeration 2006 SI Page 2.18 Table 19, they tabulate the minimum refrigeration capacity for oil entrainment up suction risers.

This might be useful if you are only using R22 or R134a but what would be more useful would be to know the basis of the table.

I would expect oil entrainment to be a function of gas velocity. So, given the cycle temperatures, I can back-calculate the equivalent suction gas velocity for each duty listed.

Now, I have a problem. The velocity is not constant!
But, there must be some reference condition that is used as a basis for generating this table.

Here is a sample of my calculated velocities.

Minimum velocity [m/s] for oil entrainment in suction risers
R22
te sh 12 15 18 22 28 35 42 54 67 79 105 130
-40 5 3.3 3.5 3.9 5.1 5.4 6.1 7.0 7.6 7.1 9.1 10.6 11.9
-40 15 3.2 3.4 3.7 4.8 5.1 5.8 6.6 7.3 8.1 8.6 10.1 11.3
-40 25 3.1 3.3 3.6 4.7 5.0 5.6 6.4 7.1 7.9 8.4 9.8 11.0

-20 5 2.2 2.4 2.6 3.4 3.6 4.1 4.7 5.1 5.7 6.1 7.1 7.9
-20 15 2.1 2.2 2.5 3.2 3.4 3.9 4.4 4.9 5.4 5.8 6.8 7.6
-20 25 2.0 2.2 2.4 3.1 3.3 3.8 4.3 4.7 5.3 5.6 6.6 7.3

-5 5 1.7 1.8 2.0 2.6 2.8 3.2 3.6 4.0 4.4 4.7 5.5 6.2
-5 15 1.6 1.7 1.9 2.5 2.7 3.0 3.4 3.8 4.2 4.5 5.3 5.9
-5 25 1.6 1.7 1.8 2.4 2.6 2.9 3.3 3.6 4.1 4.3 5.1 5.7

5 5 1.5 1.6 1.7 2.3 2.4 2.7 3.1 3.4 3.8 4.1 4.8 5.3
5 15 1.4 1.5 1.6 2.1 2.3 2.6 2.9 3.2 3.6 3.8 4.5 5.0
5 25 1.3 1.4 1.6 2.0 2.2 2.5 2.8 3.1 3.5 3.7 4.3 4.8

Do you have any ideas about this?

Designers will typically design off a minimum gas velocity, sufficient to at least entrain the oil & pull it up the riser. I have not seen an upper velocity limit, though - most of this would have pipe pressure drop as its logical constraint.

In reality - the ability of the gas to convey the oil will be affected by more than the gas velocity. The relative flow regime will be important. I suspect that most theory is built off the 'stratified' model, with wavy flow pattern. As the flow parameters change after this, the carriage of the oil may, or may not improve. Each flow regime is governed by some rather hairy equations. (Refer to "Heat Transfer and Fluid Flow in Minichannels and Microchannels", Kandlikar S.G., Garimella S., Li D., Colin S., King M.R., Elsevier, 2006).

If you'd like to plug in some of the regime-defining equations into your software, then you could begin to perhaps see what flow regime is really at work here - at least in the mind of the writers of : ASHRAE Handbook of Refrigeration 2006 SI - Page 2.18 Table 19.

I would also point you to : Convective Boiling and Condensation - 3 ed - Collier J.G. & Thome J.R - Oxford Science Publications. Thome also has some useful stuff in the Wolverine Handbook.