Tiny Bubbles

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In the beginning, there was warm water, rushing over the outstretched legs and arms and across the muscles of the back. And that was good.

But spa makers soon discovered that air could be mixed into the stream of water to create an even more scintillating experience. Spa users quickly acquired a taste for the sensation of bubbles against the skin. It's been in the spa spec ever since.

Sometimes it's piped through holes in the floor of the spa where it bubbles up under the feet and torso, but most often it is added into the stream of water issuing from jets in the side and floor of the spa.

The trick is done in two ways , either with a blower or a venturi; two completely dissimilar methods, one result — an invigorating tickle amid the jetstream, playing across one's hide.

How Blowers Blow

The original and most obvious means of getting more (water) action in a spa, the blower is designed to take in air and inject it under pressure into the stream of water squirting into the spa.

In terms of operation, the basic function of a blower is not terribly complicated, explains Peter King, research and development, Spa King Blowers, Queensland, Australia. "It's essentially a vacuum cleaner in reverse. A typical spa blower has a housing around the motor/air pump, an air intake molded into one end of the housing and an air outlet suitable for connection to a pipe which leads to the spa at the other."

On closer inspection, however, it's a bit more complicated. The motor/air pump that supplies the air is a series electric motor, with wound coils that take AC current and transform it into mechanical power to the air pump at high speed in order to deliver up to 100 cubic feet of air per minute at pressures up to 2 psi.

The air moving swiftly through the chamber toward the spa now encounters a blower manifold, which sends air through tubing to individual jets or injectors.

At this point, a blower may inject air into the stream of water coming through the spa jets or it may outlet directly through holes in the floor of the spa or a seat to give the spa user a champagne-type effect. "There can be as many as 60 to 80 one-eighth-inch holes," King notes. "It can literally lift you right off your seat."

"The bubbles are very tingling, relaxing and therapeutic," adds Pat Scanlon, co-owner, El Nino Air, a blower manufacturer based in Anaheim, Calif. "In the early days of the spa industry, they found it a good way of giving the water jets more action.

"They've been around for a long time. In Europe it's very popular to use a blower on a jetted tub instead of a water pump — to use air instead of water. That's never caught on in the United States, although two of the biggest manufacturers, American Standard and Kohler, offer them. It's considered a gentler method; a softer bubble, if you will, as opposed to a pump and all its water pressure."

Ad Venturi

Alongside the development of the blower, both in the United States and overseas, a completely different means of bubble production has been on the rise for decades.

Today, many spa manufacturers employ a bit of physics in a device called a venturi tube to bring air into the spa. It uses the energy of flowing water to draw air into the nozzle just before spouting the mixture into the spa.

It sounds complicated, but it's just a simple tube with a small narrowing in the middle. As water runs through the constricted part, it speeds up. As the tube widens, a partial vacuum is created. And into that vacuum — and on into the spa — rushes the ambient air.

Blower Vs. Venturi

Both means have advantages and adherents.

In terms of effect, the biggest difference between blowers and nozzles with venturis is that motor-driven blowers are more powerful air movers. The passive suction of a venturi nozzle unquestionably draws less air into the spa, and fewer air bubbles trickle up the legs of the bather.

Still, for many manufacturers, it's plenty, especially considering the venturi's appealing simplicity.

As Steve Fleischer, director of product development, Bullfrog Spas, Bluffdale, Utah, puts it, the good thing about using a venturi is you don't have to have a blower. "There's the cost of the blower. It's an extra thing that you have to deal with that can fail. We want the spa to be quiet and the blower makes noise, and then there's the cost to run the blower," He says. In contrast, the venturi is extremely inexpensive to produce, costs very little to use (the constriction in the hose adds very slightly to the water pump load) has no moving parts, and is all but immune to failure.

Its performance is dependent on strong, dependable water flow through the nozzle, notes Richard Lopez, technician, Waterway Plastics, a company which produces spa nozzles (among other things) in Oxnard, C alif. The greater the flow rate through the nozzle, the stronger the vacuum produced by the venturi and the more air is pulled into the stream.

In the past, that dependence has been something of an obstacle. If a venturi-equipped spa had a weak pump or poorly installed piping, airflow would be reduced as well. But if a spa had a weak pump and a blower, between them the water still had plenty of "action."

In general, pump strength and installation savvy have improved with the maturation of the industry. Over time, that performance improvement has added weight to the venturi side of the debate. As Fleischer puts it, "Yes, the blower pushes a little more air than a venturi, but we get pretty good water flow, and it sucks enough air that we don't worry about not having a blower."

That opinion is not exclusive to Fleischer and Bullfrog Spas, notes Scanlon. "The downside for blower manufacturers is that right now, the blower is being written off a lot of spa manufacturers' equipment lists. People used to consider it an advantage to the system, and now many OEMs consider it an unnecessary addition to the equipment package. We supply the replacement market, but we're faced with a diminishing market for new products.

"The pumps have gotten so huge that the movement in the water is adequate to satisfy some of the OEM spa manufacturers; so they feel they don't need the air blower.

"I happen to disagree," he adds, "because I think there's a difference. I think blower-injected air is more subtle and relaxing. Water out of a jet can be more aggressive. That's how the blower-outfitted tub manufacturers market their product — as the gentler product."

The blower still has a place in the industry, he says, but more and more as a feature to be marketed in its own right than its traditional role as part of the water jet package.

And a lot of OEMs still use the blower on their more-expensive models. Blower-injected air allows them to market a separate system — it may be called a "foot-well relaxer," for instance. "That can add another feature to a product," Scanlon says, "and add more appeal for the consumer."

In the relaxation industry, that's always the end goal — how to tickle the customer in just the right way. And whether a spa has a venturi on the nozzle or a champagne feature in the floor, it comes down to the bubbles in the end.

Hey! That air is cold!

WHETHER A SPA EMPLOYS a blower or nozzles with venturis is a matter of marketing and perceived customer preference. But in either case, an issue for some spa customers is the contrast in temperature between the air and the water.

"With more air in the spa," notes Richard Lopez, a technician with Waterway Plastics, "and the air being cooler than the water, you can notice the feel of the cool air."

"Even though it may be warm," adds Peter King of Spa King Blowers, Queensland, Australia, "when air in the spa water touches your skin it will actually cool it. The cooling effect is mostly due to evaporation because the skin is wet. So what you've got is a lot of tiny spots all over you that are being cooled even though you're sitting in 96-degree spa water. Some people don't like that contrast."

It's a thorny problem, King explains, because to eliminate the perceived temperature contrast (i.e. to make the air feel as warm as 96 degree spa water) the air needs to be around 160 degrees. But at that temperature, the air delivery lines may become malleable and prone to collapse.

Still, the problem can be mitigated to some extent without the risk of melted pipes. "There are two ways to reduce the contrast between the air and the water," says Scanlon. "One way you can do it is add a heater element to the blower, so the air passes over a warm element. But this method is not very popular because it's cumbersome and Underwriters Laboratories doesn't like it.

"The other is to draw the air across the motor itself. The motor generates heat, and the heat from the motor transmits itself to the air flowing over it, and that helps reduce that contrast between the air and the water."

Another way of driving blower temperature up, says King, is to offer resistance to the airflow. By channeling outlet air through a tortuous series of twists and turns, its temperature can be raised, which will lessen the air/water temperature contrast.

To be fair, it should be noted that while a diligent industry may be concerned about this issue, at least some spa bathers have never noticed an air/water temperature contrast. A small, unscientific survey taken in the writing of this article found little awareness of the problem.


ONE ISSUE ALWAYS on the minds of spa manufacturers, and therefore their suppliers, is noise. The motor on a blower typically runs at around 20,000 rpm or so, which (one can imagine) produces a high frequency sound that some customers find annoying.

"The main thing the OEMs want is a quiet product," says Pat Scanlon, coowner, El Nino Air. "If you can imagine yourself sitting in a portable spa and you turn your equipment on, and you've got pumps and blowers going. You really want to reduce the noise factor as much as you can. So that becomes an important criteria for us as blower manufacturers."

As a blower manufacturer, Scanlon's company is always striving to make its products quieter. It uses insulating foam inside the blower housing, and forces the air through a maze of channels. At each turn, the amount of noise that escapes the blower unit is reduced.

After the blower has left the plant, spa manufacturers may configure the spa to lessen the audible effect of the motor through positioning, says Scanlon. "We get a lot of orders from OEMs for longer lengths of cord, and that tells us that they are positioning the blower farther away from the access door and the equipment. They tend to tuck it away in a corner, so the noise will be less noticeable there."


THE VENTURI EFFECT is a special case of Bernoulli's principle (the phenomenon of internal pressure reduction with increased stream velocity in a fluid), in the case of fluid or air flow through a tube or pipe with a constriction in it. The fluid must speed up in the restriction, reducing its pressure and producing a partial vacuum via the Bernoulli effect. It is named after the Italian physicist Giovanni Battista Venturi.

The reduction in pressure in the constriction is a result of conservation of energy: the fluid (or gas) gains kinetic energy as it enters the constriction, and that energy is supplied by a pressure gradient force from behind. The pressure gradient reduces the pressure in the constriction, in reaction to the acceleration. Likewise, as the fluid leaves the constriction, it is slowed by a pressure gradient force that raises the pressure back to the ambient level.

The Venturi effect is visible in the capillaries of the human circulatory system or in large cities where wind is forced between buildings. It is used in gas jets that mix air and flammable gas in barbecues, gas stoves, and Bunsen burners; in water aspirators that produce partial vacuum from a waterspigot; in atomizers that disperse perfume; and in carburetors that use the effect to suck gasoline into an engine's intake air stream."

From Wikipedia at answers.com

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