Fills From Compressor--Table

Hourly Fills from Compressor

	Type SCBA			Type SCUBA
Compressor FAD @ 5000 PSI	2216 PSI 45 CF	4500 PSI 45 CF	4500 PSI 88 CF	2250 PSI 72 CF	3000 PSI 80 CF
4.0	5.3	5.3	2.7	3.3	3.0
6.6	8.8	8.8	4.5	5.5	5.0
8.7	11.6	11.6	5.9	7.3	6.5
10.7	14.3	14.3	7.3	8.9	8.0
15.0	20.0	20.0	10.2	12.5	11.3
19.0	25.3	25.3	13.0	15.8	14.3
25.0	33.3	33.3	17.0	20.8	18.8
29.1	38.8	38.8	19.8	24.3	21.8
35.0	46.7	46.7	23.9	29.2	26.3
36.0	48.0	48.0	24.5	30.0	27.0
45.0	60.0	60.0	30.7	37.5	33.8
55.0	73.3	73.3	37.5	45.8	41.3
65.0	86.7	86.7	44.3	54.2	48.8

Free Air Delivery vs. Cylinder Charge Rate
Two of the major ways of expressing a compressor's flow rate are free air delivery and average charge rate. When the same compressor is rated using these two methods, the results are dramatically different--average charge rate will exceed free air delivery. Why? Following is a simplified explanation.
Free Air Delivery
Free air delivery (FAD) measures the compressors output while operating at a specific pressure. The result, expressed in actual cubic feet per minute, rates the outlet flow based on inlet conditions.
The method for measuring FAD is specified by American Society of Mechanical Engineers (ASME PTC-9) and the International Standards Association (ISO 1217).
A FAD of 15 acfm means if the out put of compressor for one minute was free to expand to the same barometric pressure, temperature and humidity that existed before it was compressed, the output would occupy 15 cubic feet of space. If the compressor were operating constantly at 5000 psig during the test, then the FAD would be 15 acfm at 5000 psig.
Cylinder Charging Rate
Charge rate measures the average output of a compressor as it charges a cylinder. Since there are no recognized standards governing how a charging rate is determined, the method is left to the imagination of the compressor manufacturer. As a result, there are numerous ways to determine charging rates.
Charging rate is usually determined by timing how long it takes a compressor to fill a BA cylinder, perhaps an 80 cf cylinder from 0 to 3000 psi. The cylinder volume (80 cf) is then divided by the filling time (e.g. 5 minutes). The result would be a charging rate of 16 cfm. In actual practice, charging rate is meaningless. To fill BA cylinders, the compressor must be fitted with a purifier equipped with a minimum pressure valve. If the minimum pressure valve is set at 2800 psi, then the average charging rate for a 3000 psi BA cylinder must be measured from 2800 to 3000 psi, not from 0 to 3000 psi. Since average charging rates cannot be duplicated while refilling BA cylinders, the use of this method to rate compressors for this application is deceiving.
Why use charging rate rather than FAD? Charging rates allow compressor manufacturers to manipulate compressor ratings and to hide compressor inefficiencies that often occur at higher pressures. Using an average cylinder charging rate masks problems that would be revealed by FAD.
For the purpose of this catalogue, both FAD and cylinder charging rates are used. However, all cylinder refill calculations and purifier process capacities are determined using free air delivery.
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