Airwatt

Short description: Measurement unit of power, specific for vacuum cleaners

Airwatt or air watt is a measurement unit of the effectiveness of vacuum cleaners which refers to airflow and the amount of power (watts) a vacuum cleaner produces and uses.[1][2] It can also be referred to as a measurement of the energy per unit time of the air flowing through an opening, which is related to the energy that electricity carries through the power cable (wattage).[3]

The airwatt is a useful measurement of vacuum cleaner motor efficiency, since the power carried by a fluid flow (in the case of a typical house vacuum the fluid is air) is equal to pressure times volumetric flow rate. The airwatt relates to actual airflow, while part of the electrical power (watts) consumed by a vacuum cleaner is dissipated into heat due to necessarily imperfect efficiency; two vacuum cleaners of the same airwattage have essentially the same suction, while devices of the same electrical wattage may have a difference in efficiency and thus have substantially different airwattage.

## Definition

The formula used to compute airwattage differs between vacuum cleaner manufacturers. The standard airwatt formula is from ASTM International (see document ASTM F558 - 13)[4]

[math]\displaystyle{ P = 0.117354 \cdot F \cdot S }[/math]

Where P is the power in airwatts, F is the rate per minute (denoted cu ft/min or CFM) and S is the suction capacity expressed as a pressure in units of inches of water.

Some manufacturers choose to use the fraction ​1⁄8.5 rather than the ASTM decimal, leading to a less than 0.25% variation in their calculations.

[math]\displaystyle{ P = \frac{ \text{inches of water} \cdot \text{airflow} }{8.5} }[/math]

Where airflow in Cubic Feet per Minute [CFM] is calculated using

airflow = √13.35 × D2 / vacuum

Where D is the diameter of the orifices.[5]

CFM is always given statistically at its maximum which is at a 2-inch (51 mm) opening. Waterlift, on the other hand, is always given at its maximum: a 0-inch opening. When waterlift is at a 0-inch opening, then the flow rate is zero - no air is moving, thus the power is also 0 airwatts. So one then needs to analyse the curve created by both flow rate and waterlift as the opening changes from 0 to 2 inches (0 to 51 mm); somewhere along this line the power will attain its maximum.

If the flow rate were given in litres per second (L/s), then the pressure would be in kilopascals (kPa). Thus one watt equals one kilopascal times one litre per second: [math]\displaystyle{ 1~\text{W} = 1~\frac{\text{kPa} \cdot \text{L}}{\text{s}} }[/math]

The ratio between the Airwatt rating (power produced in the flow) and electrical watts (power from voltage and current) is the efficiency of the vacuum.

## Ratings recommendations

Hoover recommends 100 airwatts for upright vacuum cleaners and 220 airwatts for "cylinder" (canister) vacuum cleaners.[6]

## References

1. ↑ ""Airwatts" applied to carpet extractors". February 12, 2013. https://cleanfax.com/carpet-care/airwatts-applied-to-carpet-extractors/.
2. ↑ "Powr-Flite Applies Airwatts to Carpet Extractors". https://www.cleanlink.com/news/article/Powr-Flite-Applies-Airwatts-to-Carpet-Extractors--15182.
3. ↑ American Woodworker. United States: Rodale Press. 1998. https://books.google.com/books?id=DfYDAAAAMBAJ&dq=airwatt&pg=PA22.
4. ↑ "ASTM F558 - 18 Standard Test Method for Measuring Air Performance Characteristics of Vacuum Cleaners". ASTM International. doi:10.1520/F0558-21. https://cdn.standards.iteh.ai/samples/108316/aa46d5572158448c8d48c91fb9b96042/ASTM-F558-21.pdf.
5. ↑ "Ametek/Lamb Electric Technical Series". http://www.grainger.com/tps/motors_ametek_tech_cross_ref.pdf.
6. ↑ "Air Watts | Suction Power Watts vs Air Watts - Hoover". http://www.hoover.co.uk/small-appliances/vacuum-cleaners/suction-power-watts-vs-air-watts.

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