VMAC – Vehicle Mounted Air Compressors
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5.0 Testing Ventilation
The following procedure is recommended when a new installation
configuration is being developed. The procedure is not necessary
for repeat installations with the same configuration.
5.1 Calculating Duty Cycle
Duty cycle can be determined based on expected tool use or by
identifying the desired CFM of the system. Refer to Table 3 to
determine duty cycle from a given CFM consumption.
The rated consumption of an air tool is listed as an average based on a 25%
duty cycle. For example, a 1/2” impact might be listed by the manufacturer
as 5 CFM rather than the true consumption of 20 CFM. Do not use the
average rating in calculating duty cycle. Typical air consumption of some
pneumatic tools is:
•
1/2” impact – 20 CFM
•
3/4” impact – 30 CFM
•
Blow-off nozzle with 0.040” orifice – 2 CFM
•
Carbon arc gouger – 25 CFM
•
Pneumatic hammer – 4 CFM
Determine what tools will be used, their actual CFM, and how many
minutes in an hour they will be used. Form 1 can be copied and used to
determine duty cycle. A sample calculation is shown in Table 2.
Tool Description
CFM
x
Operating Time
(min/hr)
= CFH
x
=
x
=
x
=
x
=
x
=
x
=
Total CFH (Cubic Feet per Hour)
Duty Cycle = Total CFH
÷
3600 x 100%
= ____
÷
3600 x 100%
= ____%
Form 1 – Duty Cycle Calculation
