Maximum Velocity of Two Phase Fluid in Air-Cooled Heat Exchanger

[Bill1959]

I am presently preparing an evaluation of some bids for air-cooled heat exchangers used downstream of a natural gas compressor that is pulling gas from a salt dome. The gas will be water saturated at the equillibrium temperature; therefore, there is a potential for two-phase flow inside the tubes as the gas cools. The problem here is extremely high flow rates and I am concerned that we need to limit the velocity inside the tubes (3/4-inch x 0.060-inch wall SA-214 or SA-179 carbon steel tubes).

Has anyone seen recommendations on the maximum allowable velocity of two-phase natural gas inside the tubes of an air-cooled heat exchanger?

I am concerned that we will have erosional velocities near the tube outlet. I have witnessed tubewall thinning at the tube outlet end before. I recall seeing something in HTRI literature several years ago, but I do not recall the limiting velocity as a function of diameter.

 

[speco]

Bill1959,

Most customers do not normally specify a maximum tube velocity in gas coolers. Usually the velocity is controlled by the maximum allowable pressure drop, which is typically in the 5 psi range, but sometimes lower.

In gas compressor aftercooler applications, there are usually several cases to be considered, ranging from high pressure and low flow to lower pressure and high flow. If all these are taken into account, and the pressure drop is limited for the very high flow cases, erosion would not be likely.

It also helps to look at the Rho-V-Square at the inlet and outlet ends, including nozzles. Typically they should be under about 3500 to 4500 (depending on how conservative you want to be. Rho is density (or mixed density) in #/ft^3, and V is velocity in feet/sec.

Also, most process air-coolers have tubes 1.0" OD or larger. Where API-661 specs are required, the minimum tube wall is .083" for carbon steel tubes. These are pretty good guidelines.

Regards,

speco (

http://www.stoneprocess.com)