Compressor Sizing | RLD CNG

RLD CNG compressor Systems can refuel hundreds of cars a day. The sizes below can be used as a single unit or in twin pairs.

34 SCFM,  54 SCFM  and 93 SCFM Single Unit Sizes

Compressor Size            Slow Fill only   Slow fill time   Fast Fill Time   Cars Per Day   Cars Per Hour   24/7 run

34 SCFM - 4331 Model       34 cars           42 minutes       15 minutes             88                 3.67              yes

Twin Pair 68 SCFM            68 cars           22 minutes         8 minutes           163                 6.79              yes 

54 SCFM – 4341 Model       50 cars           29 minutes        8 minutes            130                 5.42              yes

Twin Pair 100 SCFM         100 cars           15 minutes        4 minutes            235                 9.79              yes

93 SCFM – 4351 Model      l93 cars           16 minutes        4 minutes            260               10.83              yes

Twin Pair 186 SCFM         186 cars            8 minutes         2 minutes            486               20.25              yes

Two of the most frequently asked questions are what size are your compressors and what size compressor do I need?  Compressor size and storage can often be over sold.  Compressor size is always oversold if storage is not utilized for a fleet. 

The above data summarizes as well as maximizes the number of cars per day for a specific size compressor within a standardized model for comparison.  A site specific filling station need can be interpolated by analyzing the number of cars per day v minutes to fill v number of cars per hour against your own goals. Interpolation of the above data, for instance, may suggest that if you felt that the average time spent refueling between vehicles is 8 minutes, a twin pair of 186 scfm compressors would support 4 refueling dispenser nozzles rather than 1 to meet your refueling station goals and so on.  

Standards for these calculations are stated below.  All refueling stations are fleet and site specific.  It is not my intent to design a filling station but discuss maximizing the capability of the compressor based on size. Refueling station design considers many more factors not discussed in this analysis.  The tables below state average and reasonable data for this analysis.  *It must be noted that out of the 10,000 refueling stations in California the upper 20% retail over 50% of the total fuel sold creating an elusive wide average range of vehicles frequenting site specific refueling stations on a daily basis.  **It must be also noted there is “tank filling time” and “just blocking the pump from being used for whatever reason time”.  Refueling time may include a breakdown, a repair, checking oil, washing windows, conversation, getting food from the mini mart, paying for the transaction, the bathroom, a map quest or talking on a cell phone. That being said it is also reported that the average time spent at a refueling station is between 10-20 minutes making this another elusive fact to narrow down.

Average fuel tank size range - 10-20+ Liquid Gallons

Average fuel tank size - 16 Liquid Gallons

Manufacturers target driving distance before refueling - 300-400 Miles

2015 corporate average fuel economy in the USA - 25 MPG

Average distance traveled between refueling stops - 300 Miles

Average fuel tank percent full before refueling - ¼ Tank

Average amount of fuel purchased when refueling - 12 Gallons

Average number of vehicles refueled per station in a day in California - 367-864*

Average time spent refueling a car at a refueling station - 3-20 Minutes**

Through extensive research, the Gas Institute of Technology developed software to help in assessing the performance of a cascade-type ground storage system for any set of fleet vehicle conditions.  It is most effectively used as a tool to determine the performance of different refueling configurations.   This allows the refueling station designer to minimize compression requirements while still attaining the necessary refueling performance.  A patented dispenser control algorithm is used to compute the required full-fill pressure and temperature within the cylinder at its rated mass content.

The dispenser flow rate, at any instant of time, to the vehicle cylinder is computed by considering the combined restriction of the dispenser and the vehicle connection receptacle as being an orifice.  The rated flow through the dispenser is calculated together with an associated rated dispenser inlet pressure.   The flow rate to the vehicle is computed based on this information as well as the instantaneous cascade bank and vehicle cylinder pressures.  Choked flow occurs through this simulated dispenser/receptacle orifice when the vehicle cylinder pressure is below the critical pressure at any instant during the fill.

The vehicle fill time is computed based on a summation of a 50 step incremental determination of the time for the vehicle cylinder to be filled to its rated mass of gas.  At each increment of vehicle fill the dispenser flow rate is computed based on the above methodology.  As a cascade bank becomes depleted the average pressure at the dispenser inlet is reduced.  This in turn results in longer vehicle fill times relative to those achieved when the cascade bank is completely filled.

RLD Performance compressor systems are designed to run 24/7.   The key to getting the maximum use out of your compressor is to properly size storage to go with it. Two standard choices are a 48” sphere or a 20” O.D. X 24’ long tube.   The volume of the two different style vessels are within a cubic feet of one another. Spheres are slightly larger, less money but take up more space.  We will use the size of 33 cubic feet or the average size between the two for calculations.  Calculations below are based on a one nozzle continuous fast fill and data from the table above.

The calculations below will give you an idea of the capability of each size compressor.  Calculations are designed so that at the end of a 24 hour period the cascade storage banks will be 100% full to start the next refueling day.   If time between a fill and number of cars is increased the cascade system will be prematurely depleted in less than 24 hours thus requiring more time, exceeding the 24 hour period, to refill the cascade storage back to 100% before the next refueling day begins.

Final refueling station designs must consider the total amount of fuel required in a 24 hour period for a designated fleet as well as the time of day this fuel is actually needed.  The expense for additional storage is not necessary in these models but less cascade storage would under utilize the compressor capabilities.


US Gas Institute of Technology,

US Environmental Protection Agency,

State of California Energy Almanac,

Website Disclaimer  -  General information and links on this website are offered in good faith to help educate the consumer on Government support, show major feasibility studies, inform on vehicle conversions and list examples of available refueling station designers, engineers and consultants to this industry.  Illustrations and information contained in this website are only from a point in time which may or may not reflect current trends or activity from State to State.  Referrals and links are listed only as a consumer aid and not meant to endorse anyone or any specific company.  RLD Performance accepts no responsibility for actions or business decisions by others

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