Airwatt

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. 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).

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)

$P = 0.117354 \cdot F \cdot S$

Where P is the power in airwatts, F is the rate of air flow in cubic feet per minute (denoted cu ft/min or CFM) and S is the suction capacity expressed as a pressure in units of inches of water. This makes one airwatt equal to 0.9983 watts.

In terms of the orifice plate,

Air watts = ​¹⁄_8.5 × vacuum suction [inches of water] × air flow [cubic feet per minute]

Air flow [CFM] = √13.35 × D² / vacuum suction

Where D is the diameter of the orifice holes.

Using coherent SI units, power equals flow times pressure by definition. That is, where the power is expressed in watts (W), the flow is in cubic metres per second (m³/s) and the pressure is in pascals. Since one pascal (Pa) equals one newton per square metre (1 Pa = 1 N/m²), then:

$1~ \frac{\text{m}^{3}}{\text{s}} \cdot 1~\frac{\text{N}}{\text{m}^{2}} = 1~\frac{\text{N} \cdot \text{m}}{\text{s}} = 1~\frac{\text{J}}{\text{s}} = 1~\text{W}$

The power of the flow times the pressure will always be less than the power applied via the voltage and current (1 W = 1 V·A). The ratio of the power produced in the flow and pressure divided by the power from the voltage and current is the efficiency.

Alternative measurement formula

$P = \frac{1}{8.5} \cdot \text{airflow} [CFM] \cdot \text{suction} [\text{inches of water}]$

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) instead of cubic metres per second (m³/s), then the pressure would be in kilopascals (kPa). Thus one watt equals one kilopascal times one litre per second: $1~\text{W} = 1~\frac{\text{kPa} \cdot \text{L}}{\text{s}}$

Ratings recommendations

Hoover recommends 100 airwatts for upright vacuum cleaners and 220 airwatts for cylinder vacuum cleaners.

References

• ASTM Standard F558 Standard Test Method for Measuring Air Performance Characteristics of Vacuum Cleaners