Wednesday, October 12, 2011

On Suction

  A vacuum cleaner is basically an air moving device, in the same category as a hair dryer or a ceiling fan. In fact, Murray Spangler, who in 1907 invented what was to become “The Hoover Suction Sweeper”, used a ceiling fan with its blades cut down as the heart of his invention. Logically, then, judging the air moving capability of a vacuum is a good way to rate one against another. However, this capability as it relates to vacuum cleaning is difficult to simplify into just one or two measurements. In this article I hope to take some of the measurements used in the vacuum industry and explain what they mean, and how they connect to the actual purpose of a vacuum cleaner, that is, “sucking up dirt”.

 Amps: Simply an electrical consumption measurement. Any device that uses electrical power uses a certain amount of amps – there is no direct connection to air movement. The origin of amperage as a vacuum cleaner rating was when manufacturers would take a given vacuum design and install motors which ran at different speeds (and thus drew greater or fewer amps) to create different “models” using the same design. So, the basic model might be rated at four amps, the middle model might draw seven, and the high-end model could be rated at ten. Since the motor design and vacuum design were the same, the higher amperage did indicate higher airflow and suction. Other manufacturers, seeking to compete, only had to find a motor that drew the same amount of amperage to use in their machines, without paying attention to the efficiency of the motor or the vacuum it was put into. Thus, two vacuums drawing the same amount of amps could deliver quite different amounts of suction to the cleaning tool. Likewise, a vacuum using fewer amps has the potential to be as good or better at cleaning than one using more. Underwriters’ Laboratories limits the amount of electricity a portable vacuum can draw to 12 amps – is it any coincidence that many vacuums on the market today draw exactly 12 amps?

 Cubic Feet Per Minute (CFM): A measurement of air movement. Seen more often in central vacuum systems, this always refers to the rated maximum CFM of the motor or motors, not taking into account the orifice of the cleaning tool or the various hindrances to air movement present in the design of any vacuum cleaner or vacuum system. The maximum motor CFM indicated on a spec sheet will never be encountered during actual use – a vacuum cleaner whose motor can produce a maximum of 100 CFM may only be moving 20 CFM once the restrictions in the airflow path and filtration system are taken into account.

 Inches of Waterlift: A negative pressure measurement; indicates the “pull” of a vacuum cleaner motor. Industrial vacuum systems are commonly rated in inches of mercury (Hg). To convert an inches of Hg measurement to inches of H20, simply multiply by 13.59. In residential vacuums, this rating is nearly always taken at a seal, with no air movement. Therefore, the maximum (sealed) waterlift measurement is nearly useless as an indicator of cleaning power – the greater the orifice size, the lower the waterlift measurement. A much more useful measurement is inches of waterlift at a given orifice size, or “Working Waterlift”. This reveals the pressure at which the air is moving – not how much pressure the machine is capable of generating when it’s fully sealed.

 One big difference between CFM and waterlift is that with waterlift, as long as a vacuum has no leaks, the maximum waterlift produced by the motor is the maximum waterlift at the cleaning tool. However, the maximum CFM produced by the motor is choked off by filters, bags, hoses, etc., so that the maximum motor CFM is never seen at the cleaning tool. The most important measurement in vacuum cleaning is also one that’s very seldom seen; that is, velocity at the cleaning tool. While very difficult to measure, this reading translates very directly to the rate of dirt removal – the faster the air flows, the faster the dirt flows. Factors that influence air velocity include the distance from the vacuum motor to the cleaning tool, design of the cleaning tool (orifice size and shape), and filtration system design.

 In traditional upright vacuum designs, the fan is located before the bag, as close to the nozzle/brushroll area as possible (typically a few inches away). The dirt-handling fan moves lots of air, and doesn’t need to generate very high pressure, due to the directness of the airflow path. These uprights tend to be phenomenal carpet cleaners due to the high velocity they generate at the nozzle opening. However, they tend to be rather poor at above-the-floor cleaning, since they cannot generate the pressure needed to move air effectively through a five- to eight-foot cleaning hose. More recent upright vacuum designs include onboard hoses, which can be connected either to the brushroll housing or to smaller cleaning tools for above-floor use. To produce strong suction at the end of the hose, the fan is typically placed after the bag and made with closer tolerances to produce higher waterlift. However, since household portable vacuums are limited in the amount of current they can draw, this is done at the expense of airflow. So, once the hose (typically 1 1/4 inches) is connected to the much larger brushroll housing, air velocity drops to a level much lower than is present in the hose. Consequently, these cleaners (like canister vacuums) tend to be weaker at carpet cleaning.

Canister vacuums rely on the same high-waterlift, low-CFM motors as newer upright designs, but separate the vacuum unit and the cleaning tool by a five- to ten-foot cleaning hose. Often the carpet cleaning tool will have its own electric motor and brushroll, but the motors still may not use more than twelve amps. Thus, power used to run one motor is potentially taken away from the other motor.

The third main category of vacuum cleaners is that of central vacuum systems. These systems permanently mount the vacuum unit in an out-of-the-way area, and provide wall inlets around the home for connection of a long cleaning hose. One of the benefits of a central vacuum (there are others, like less noise, less dust, more convenience, etc.) is that the motor unit can be made as large and powerful as needed, so that the end of the hose has much greater cleaning power than the hose of a portable vacuum. However, there’s also a potential to choke off all that cleaning power the unit produces – things like restrictive filtration designs, improper power unit sizing, poor piping installation and outdated hose construction can rob a central vacuum system of its power, causing it to clean poorly.

 As you can see, the many factors at work in how well a vacuum does its job require quite a bit of attention to design. Nowhere is this more critical than in central vacuum systems. A central vacuum should be a versatile, long-lasting tool that provides intense cleaning power for any application. Many central vacuums don’t live up to their expectations, for the reasons outlined above. At JCV, ours do – if you purchase one of our systems, my promise to you is that you’ll get your money’s worth, and you’ll never regret your decision.











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