The authoritative public forum
  for Metal Finishing since 1989
-----

Vapor bubbles in heated water preventing resistivity measurement

 

I manage a group of Manufacturing Engineers, 2 of which are assigned to install an ultrasonic wash system. The system uses DI water coming from a central facility system. Our problem is that the units resistivity meter will not work because of bubbles in the water that enters the tanks. The water goes through a water heater prior to going into the system, then thru control valving and on into the tanks. Samples of water taken before the heater appear to be very clear (no sign of bubbles), samples taken after the water heater appear very cloudy. The cloudy samples turn clear after sitting for about one hour. Is there a test that we can use to determine if the bubbles are air of water vapor (from the water heater)? How can we heat the water quickly without entraining bubbles into it? Is there an easy way to remove the bubbles in-line before the water enters the tanks?

Any help we can get would be very much appreciated.

Charles Beauvais
laboratory. - Sinking Spring, Pennsylvania, USA

Where is the conductivity probe and what is it measuring? If it is in the pipe after the water heater and it is measuring the output of the D.I. system, it sounds like you should simply move it to a position before the water heater.

But if it is in the tank, and you are using the probe to trigger additions of fresh D.I. water when the tank contents become too conductive, it sounds like there are plenty of problems :-) The cavitation of ultrasonic system will surely cause hot water to flash to steam bubbles, won't it?

I hope there is a simpler solution, but one approach would be to put the probe into an external well with a slipstream of the tank contents fed to it and overflowing back to the tank.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

---

 

I had a similar problem when designing a kidney dialysis machine. I solved the problem by using a small vortex separator.

Design is quite simple and the device is very flexible.

Construct a cylinder with a conical bottom. In principal the fluid stream is fed into the top of a cylinder from a tangential feed inserted into the wall of the cylinder. This causes the fluid to spin. The outflow is in the conical bottom of the cylinder. Blocking the entry into the conical bottom of the cylinder is a plate with the same diameter as the cylinder but with small cutouts round the circumference. At the top of the cylinder you need a small bleeder tube.

The water is thrown to the outside of the cylinder and air is drawn to the center of the device where it can exit through the bleeder tube. Separation is accomplished by centrifugal force.

In operation the center of the separator forms a vortex much like the vortex formed in the outflow of a sink.

There are other ways of providing de-aeration.

Good luck,

John Holroyd
- Elkhorn, Wisconsin

---

You need to position the sensor so that it doesn't entrain air bubbles. If it is in a pressurized pipe, mount it horizontally in a "T" with the water flow directed right at the probe. The turbulence should sweep away any ait bubbles that might otherwise collect in a vertically mounted probe.

Lyle Kirman
consultant - Cleveland Heights, Ohio
 

---

 

Don't make the error of focusing exclusively on a single possible cause, no matter how probable, if you can also test alternatives in parallel.

(1) The cloudiness could be microbubbles (OR NOT!). Have you confirmed by checking samples for upward migration (side view microscope etc on a test tube of sample). (2) You assume that the DIW system is operating nominally and contaminants are not breaking through (This can be continuous or cyclically). The cloudiness could be colloidal calcium / magnesium carbonates or silicates etc liberated by the heating process. These can be unstable and redissolve OR they can precipitate. The levels can be sufficient for visibility but insufficient to greatly affect conductivity readings. ACTIONS: Check for precipitates on a black base, check for scale on the heating elements, get AAS ⇦this on eBay or Amazon [affil links] analysis of pre & post DI treated water, regen the DIW system and see if the effect continues.

P.S. On a different note, IF the cloudiness is in fact microbubbles, I suggest that the heating element rating is incorrect. Reducing flow rates to achieve higher temperatures is inefficient. It causes overheating, vapour bubbles and resulting low heat transfer characteristics at the element surface. The best solution is to increase the surface area i.e., a second heating stage. If you wish to confirm further, there are tables available for optimal heat flux standards vs temperature differential for water.

Let me know how it goes.

John Tuohy
- Ireland