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Air Compressor

Compressor Size

Air compressors are sized by air pressure and air flow. In the US, air pressure is measured in pounds per square inch (psi). Flow is measured in cubic feet of air per minute (cfm). Air flow capacities vary from compressor to compressor—from a few cfm for small electric piston units, to thousands of cfm for high horsepower, in-plant compressors. Just because your compressor can achieve high pressure, does not mean it can provide enough flow to run a Guardair vacuum! Ensure you have proper pressure and flow.

Air Pressure

All Guardair pneumatic vacuums are designed to operate optimally at 90-110 psi. Operating vacuums at air pressures less than 90-110 psi leads to significantly lower performance. Conversely, operating vacuums at air pressures above 90-110 psi increases performance but only to a limited extent. Supplying higher pressure to a pneumatic vacuum designed to work at 90-110 psi actually de-focuses the venturi thereby degrading the performance per cfm consumed. In addition, the energy required to operate the compressor at higher pressures increases dramatically.

Air Compressor

Air Flow

Guardair vacuums come equipped with different size venturis (M, S, B, D) to provide desired vacuum lift and vacuum flow characteristics. Note that vacuums with a given compressed air consumption may always be used with a larger capacity compressor, but not with a smaller capacity compressor. Running a vacuum with a higher air consumption rating than the capacity of the compressor will result in a significant pressure drop (to less than 90-110 psi at the vacuum) and will noticeably diminish performance. Refer to the specification on each vacuum for air consumption and horsepower requirements.

Air Hose Size

Check the specifications of your Guardair vacuum and be sure the air hose supplying the vacuum is adequately sized. As a rule, the larger the air hose ID, the more air flow the hose is capable of delivering. Air hose ID may always be larger than recommended but not smaller. Operating the vacuum with smaller than recommended air hose will result in noticeably reduced performance.

Another consideration relating to air hose size is pressure loss. Compressed air flowing through air hose experiences a loss in pressure from friction and constrictions. Friction loss is proportional to the length of the hose. Operating pressure, flow rate, air hose inner diameter, and air hose smoothness also determine the loss. The table below shows the pressure loss for 50 feet of common air hose (including couplings) as a function of air hose size for each Guardair venturi type.

Pressure Loss (psi) for 50 Feet of Common Air Hose

Vacuum Generating Head
with Venturi Type
Air Flow
(cfm)
Air Hose (ID)
3/8" 1/2" 3/4"
M 20 1 1 0
S 20 1 1 0
D 47 11 8 1
B 68 N/R 20 2
2B 110 N/R N/R 6

N/R = Not Recommended

The above table can be interpreted with the following example. Assume a Guardair 55 gallon vacuum featuring a D venturi connected to an air compressor via a 50 ft long, ¾" ID air hose. From the table, the air hose will see a 11 psi pressure loss over its length. Therefore, in order to deliver 100 psi at the vacuum (for optimal performance), ideally the compressor output should be raised to 111 psi (100 + 11) to make up for the pressure loss.

Connectors and Fittings

Be sure that connectors and fittings do not restrict the air flow in any manner. Pick connectors that are rated the same size, or larger, for a given air hose size. When using vacuums and air lines equipped with Chicago "Air King" style connectors, make sure safety pins are engaged.