The pool and spa business primarily concerns itself with the hydraulics branch of fluid mechanics, which specializes in the properties of moving water. But when the focus is on the operation of air blowers, the pneumatics branch — the study of air and other gases — comes into play.
Fortunately, these two disciplines share many basic concepts. In simple terms, think of the air blower as nothing more than an air pump, forcing a thin fluid — air — through a piping system.In a spa, the piping system may lead to an air channel or air injectors installed in the floor, seat or walls; or, it might be connected to the hydrotherapy jets in a manner that will enhance their hydro-massaging operation. Various valve arrangements allow blowers to serve double duty, feeding an air channel for basic bubbling and acting as a hydrotherapy-jet booster, often referred to as “turbo-charging.”
Understanding air blowers
The air blower pump shares many features with its water-pumping cousin.
An electric motor — directly coupled to an impeller (or impellers, in a multistage design) — adds energy to the flow of air passing through the blower as the motor spins. Result: The velocity and pressure of the air flow increase dramatically.
Water pumps normally operate at 3,450 revolutions per minute. But air blowers typically rotate at 18,000 to 20,000 rpm. This high speed has two side effects: noise and a lot of heat generated within the air blower.Several manufacturers have successfully reduced noise by using sound-absorbing materials around the blowers and by adding mufflers to the intake and/or exhaust ports. Also, air blowers have been designed for burial in the ground as a noise-reduction measure. Blowers are available in two- and three-speed models, allowing users to select the intensity of the bubbling action, with subsequent adjustments in noise levels.
Heat generated within the air blower is a matter of concern. Air flowing through the blower must carry all the heat away from the motor immediately. Failure to do so leads to the No. 1 cause of blower failure.Finally, some blowers today feature small electric heaters that preheat the air sent into the spa. This minimizes the temperature differential between the water circulating in the spa and the (often) much cooler air being drawn into the system — increasing user comfort and easing the heating burden on the spa’s primary heater.
What is the most critical factor for a spa’s air-support system? Proper sizing of the blower to the air piping system. Nearly all blower problems can be traced directly or indirectly to incorrect sizing.
For example, if either the spa’s air holes or the blower’s distance from the hot tub are inadequate, the blower will work too hard. To remedy that, either replace the blower with a higher-capacity model, or modify the air piping system.
In replacing a blower, you must first consider certain design features: the unit’s operating voltage, the orientation of the exhaust, the need for various motor speeds and the blower’s position relative to the spa. Beyond that, effective blower replacement depends largely upon proper sizing.But you cannot always put a larger blower into a system. Oversizing can cause a blower to overheat and melt or simply fail.
Sizing the blower
Blower sizing isn’t all that complicated. Units come in three basic sizes: 1, 1½ and 2 hp. Beyond that, there’s a defined set of factors to consider in finding the best-sized blower for a given application:
The total length of the pipe run: The longer the run, the more power the blower needs. For portable spas and spa packs, it’s usually not difficult to determine. For component systems, or those in which a spa pack was installed a distance away from the vessel, you need to accurately determine the amount of pipe between the blower and the spa’s air piping. Keep the following factors in mind:
The number of 90-degree turns: Remember these turns add to the work a blower must do to force air into the spa.Water depth: Water pressure bearing down into the air channel or air injectors must be countered by the force of the air pushed through the piping. The deeper the water, the greater the burden on the blower.
The number and size of air holes or air injectors: As mentioned, too few or too small air holes or air injectors can result in an overworked, overheated unit. Manufacturers have found ways of incorporating these factors into their sizing schemes. Many offer helpful tables for proper sizing.
Most blowers in the parts world are designed for light-duty residential applications and are intended to run a maximum of 15- to 20 minutes at a time. These will not last in a commercial environment with long running times or with heavy bather loads. In heavy-use applications, it is best to install a commercial-grade air blower.
Accommodating jets
In well-planned systems, blowers make a dramatic difference in jet action. As a result, many spa and spa-pack systems include booster options that use either a separate blower or special valve configurations.
It’s a bit trickier sizing for retrofit or replacement than, say, for basic air channel or injector applications. Differences in jet orifice size make gauging power requirements difficult, but manufacturers help by including horsepower-per-jet requirements for blowers in booster applications.
For the field tech, however, there are a few basic guidelines to follow based on the number of jets in the system: • If you have four to six jets, you generally need a 1hp blower;
• Six to eight jets call for a 1½hp blower;
• Eight or more jets require 2hp units.
Beyond that, consider these critical factors: Depending on the location of the blower relative to the plumbing, water can flow back into the blower unit. Blowers installed below the water level require the installation of a check valve mounted above the waterline or a Hartford loop, or both.Some suppliers incorporate check valves in the jets and injectors themselves to help prevent backflow and keep water out of the air blower piping.
Isolating the problem
Proper sizing helps you avoid most blower problems, but other factors can have significant impact. Many problems can be traced to improper installation and can be isolated quickly with a few simple checks. Some common symptoms, causes and remedies:
Overheating: When this happens, the blower motor’s thermal-protection device usually shuts the unit off. If it fails to do so, however, you may find premature brush wear on the motor or, in extreme cases, melted blower housings. Extreme heat also can lead to bearing failure in the motor.
Overheating indicates a blower working too hard or a restricted air flow. A key sign is inadequate bubbling action. Often caused by back pressure brought on by undersized plumbing, it can usually be fixed with 1½-inch plumbing. For long plumbing runs, however, 2-inch plumbing or larger may be required; check the manufacturer’s specs.Another cause of overheating can be an improper blower check valve. Blower check valves normally have ¼-pound springs in them. Standard check valves usually have much stiffer springs and can be too hard for a blower to open correctly.
If you have an improperly sized blower, one of the best ways to determine the dynamics of the system is to get a reading with a magnehelic gauge with the blower running. The manufacturer can use the reading to help determine the right blower size.
Water damage: You often see this with units installed below the water level. Install a double loop or Hartford loop in the plumbing between blower and spa. Faulty check valves (or those installed backward) also may be the culprits.When an owner complains of a GFCI tripping, first check the air blower. Water finding a path to the innards of a blower almost always trips a GFCI.
Intermittent operation: Random overheating, poor bubbling action, arcing, sparking, smoking and excessive noise all can result from improper electrical installation of the blower.
One of your first checks here should be with a voltmeter to determine that the line voltage matches the voltage listed on the blower. Sometimes, you’ll find an overloaded circuit with too many appliances drawing down its voltage. Other times, you’ll discover a 240-volt blower limping along on a 120-volt supply line — or a 120-volt unit running dangerously on a 240-volt line.
Generally poor performance: Check the blower’s amp draw, using an ammeter. Determine the amp draw under normal operating conditions, then disconnect the air piping and take another reading to determine its “free flow” amp draw. The free-flow reading will always be higher.
Compare these two readings. For most air blowers, a difference greater than 1½ amps indicaters either a blower that’s undersized for the application or an air piping system that’s too small or partially plugged.A final note: If in doubt, call the manufacturer. They’ll know about basic blower performance sizing, replacement and troubleshooting and can help keep you out of hot water.
PSN thanks Zodiac Pool Systems, ITT Hydro-Air Industries, Air Pro and APSP for their help with this article.
