Sparger: design

[semsagro]

I'm going to sparge condensate into an atmosferic tank. I designed my piping based on wet steam (condensate in the line flashes)with a max speed of 20 m/s. However the surface area of all the holes of the sparger is smaller than the surface area of the pipe. The velocity of the wet steam through the holes goes up to 80 m/s.

A few questions:

- shouldn't I take the surface area of the sparging holes equal to the surface area of the steampipe? Or is 80 m/s ok (thinking about erosion)? If not, what about backpressure to the flashing condensate?

- can I take the surface area of the holes equal to the surface area of the steampipe or should it always have to be smaller?

- what is maximum hole size, currently i took 4mm, but if the surface area needs to increase, I also need to increase the size of the holes. I want to prevent heavy vibrations

Can anyone give me some advice on this one?

 

[TBP]

The surface area of the pipe isn't a factor - the pipe is just a carrier for the steam. It's the steam itself that provides the heat transfer. As long as the pipe is big enough to carry the required steam flow, that's all you need to consider. Many smaller holes are much better than a few larger ones. The steam velocity through the holes will be high, but the only real problem that presents is impingement on the tank walls. The distance from the holes to any side of the tank should be at least one to two feet, depending upon the differential pressure. The holes can be oriented such that this can be prevented/minimized. A double row of holes located below the centre line of a horizontal sparge line is recommended. The distance from the holes to any side of the tank should be at least one to two feet, depending upon the differential pressure. If the sparge line is too close, the steam bubbles will impinge on the sides or bottom of the tank, and the system will be very noisy when it's operating.

Don't forget to factor in what the head of tank liquid is vs the steam pressure. Also, if the steam gets shut off - say if the tank temperature setpoint is made - the tank liquid will be drawn back into the steam line by the resulting vacuum. A vacuum breaker will likely be required.

There is a rule of thumb, which is also referred to as the "rule of eights". With an 8 psi differential pressure, a 1/8" diameter hole will pass about 8 lb/hr of steam.

The metric equivalent is: At 1.6 bar, 1.6 mm diameter hole will pass 1.6 kg/hr steam.

 

[katmar]

There are two questions which will impact on your solution. Firstly, what is the quality of your steam/condensate? Putting 2-phase flow through a sparger at 80 m/s sounds scary to me.

Secondly, what is the reason for putting this condensate into the tank? Are you trying to heat the tank contents? It seems that you are not trying to recover the flash steam because you describe the tank as "atmospheric".

Katmar Software
Engineering & Risk Analysis Software

http://katmarsoftware.com

 

[JimCasey]

I understood what you meant about the combined cross-sectional area of the holes being less than the cross-sectional area of the pipe. Theoretically this keeps the velocity from increasing as the flow exits the sparger.
In reality, it's probably not that meaningful. Any steam leaving the sparger will condense nearly instantly in the liquid water outside the sparger. The steam INSIDE the sparger will likely be collapsing very rapidly as well. Calculate your heat transfer and exit quality before you really commit to having such a high velocity.

80 m/sec is a lot higher than you want to have for liquid flow in a stainless line. Published guidelines I have seen are closer to 15 m/sec max. Erosion happens at high liquid velocities. Even if the liquid is in the form of droplets.

I'm thinking you might have stable operation of the sparger if it is sloped downward, with larger holes at the end for the water to escape through. If the water begins to build up, then the internal pressure of the sparger would rise as some of the holes have to carry condensate instead of steam. It would find an equilibrium level point. High-velocity droplets could crash straight into a pool of water at the end of the tube, while steam could escape thru the initial holes along the side.

Still keep jets emitted from the sparger a couple of feet fro the vessel wall. Collapsing steam bubbles in condensate is known as cavitation, and that carries its own set of headaches.

 

[TBP]

Steam leaves the sparge line at several hundred miles per hour. The velocity is lost very quickly, and is only a problem if it impinges on the tank, as it will be very noisy, and will erode the tank in fairly short order.

If possible, folks should try to find a copy of "Direct Steam Injection - Heating Liquids With Live Steam" published by Spirax Sarco in the UK. It's literally just a small booklet - 12 pages long. It was one of the Practical Studies series. I have several, but not the whole set. I was given the few I have 25 years ago, by a Spirax rep who was originally from Scotland. I haven't seen them before or since. If it's out of print, the Spirax folks don't mind, I'd be happy to scan & post it.

 

[rmw]

I'd like to know more about your condensate conditions, temperature and from whence it came (meaning did it come from a source where some of it flashed as it came across a trap or the like or is all the flashing strictly due to pressure drop along the condensate piping?)

I think the answer to your question depends on what fraction of the condensate flashes prior to arriving at the point of exiting through the sparger nozzles.

In any case, wet steam or two phase condensate/steam flow at either 20 or 80 M/sec sounds high to me.

rmw