FACTS AND RULES OF THUMB

 

 

 

1. Compressed air is the most inefficient utility in the plant

–        to get 1 hp work from an air motor requires approximately 30 scfm inlet air at 90 psig

–        which requires 6-7 hp at the compressor shaft to produce this compressed air

–        assuming a 90% efficient motor, this translates into 7-8 hp of electrical power to deliver 1 hp of compressed air to the plant floor

–        Net efficiency is >12.5% of the input energy is available for useful work energy

–        But, on average half of the air is wasted

 

 

 

2. Operating costs reductions are worth more than sales and productivity dollars

•         Cost reduction vs. productivity / profitability

–        $100,000 in costs reduction  =  $2,000,000 sales at 5% net profit

 

•         Turning off 100 bhp of air compressor power @ $0.08 / kwh =

–        $56,000 year in operating costs at 8760 hrs = $1,120,000 sales

 

•         Savings potential in systems average

–        35% to 50% in smaller systems <300 bhp

–        20% to 30% in larger systems >1000 bhp

 

 

 

3. RULE OF THUMB – Compressed air costs

Total Compressed Air Costs @ $0.06 /kWh

•         A 100 scfm application costs $1.22 per hour to support or $10,655 per year at 8760 hrs

•         Or,  $0.20 per 1000 scf

•         1 HP air motor costs - $3,516 per year (8760 hrs)

•         1 HP electric motor  - $426 per year

–        Energy savings on the demand side of the system are at a factor of  > 8X -10 X if electric vs. air powered

 

 

 

 

 

 

4. Estimate energy costs by :

•         Compressors - ratio of actual amps to nameplate amps times the motor nameplate HP and convert to kW

–        or nameplate HP X .746 / .92 = kW

•         Refrig Dryers – nameplate amps X 460v X .85 X 1.732 / 1000 = kW

–        or refrig dryer capacity / 200 cfm = kW

•         Cycling refrig dryers offer significant energy savings

–        Heated desiccant dryers capacity / 60 cfm = kW

•         Dewpoint based purge control offers significant energy savings

•         Cooling systems – nameplate amps of pump motors and fans and convert to Kw

•         Or 3% of total compressor kW for cooling energy

•         Total kW   x   hours   x   $0.08 /kWh =   total energy costs

•         Total energy costs / .70  =  total operating costs  $$$

     (these calculations assume full load on each compressor but still provides an estimate of costs)

 

 

 

5. Annual Electricity Cost (measurement formula)

 

full load amps) x (voltage) x (1.732) x pf x hours x rate

                                       1,000

Where:

full load amps               = average of three phases

voltage                         = line to line voltage

pf                                 = power factor

hours                            = annual hours of operation

rate                              = electricity cost in $/kWh

The full load amps and voltage are the measured values

Get power factor (pf) from motor manufacturer (use .85 as estimate)

Example:

 

            (230) x (460) x (1.732) x (0.85) x 4,160 x $0.05

                                       1,000

= $ 32,398 per year

 

 

 

 

 

 

 

 

 

 

 

 

 

6. How to Convert Acfm to Scfm

•         SCFM = ACFM x (Actual Inlet Pressure/14.5) X (520/(Actual Inlet Temperature + 460) X RH% correction (.995 to .97)

 

•         Example: 500 ACFM compressor at 14.3 psia and 95F and summer conditions 95F and 60% RH

 

•         SCFM = 500 x (14.3/14.5) x (520/(95+460) x .97

               = 448 scfm

 

Positive displacement compressor input power increases 1% for every 2 psi increase in discharge pressure

 

 

 

7. RECEIVER FORMULA

l      Tank   =Tank Height” x (Tank Radius”)²  (Gallons)

                                         73.53

l      Tank  =Tank Height” X (Tank Radius”)²      (Cubic Ft) 

l                                          550 

l      Tank  =Pi(3.14) X (Tank Radius’)²XHeight’ (Cubic Ft)

l      Tank = 23.5 X (Tank Radius’)² X Height’  (Gallons)

Gallons X 0.1337 = Cubic Feet

Cubic Feet X 7.48052= Gallons                                                               

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

8. Compressed Air Storage Primary Formula

•         Capacitance calculations:

–        The size of the event in CF (rate of flow X duration)

–        Divided by the allowable pressure drop in PSI = the required capacitance in CF/PSI, then convert to gallons (X 14.5 psi per atmosphere X 7.48 gal/cf)

–        Or, divided by the capacitance in CF/PSI = the pressure drop in PSI

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

9. HEADER STORAGE

•         Compressed air moves at a limited velocity inside the pipe; approximately 250 linear feet per second at 1.0 psid and 100 psig

 

Header Storage Problem

•         How will the startup of a 600 scfm application located 1000 feet in header distance from the compressor room impact the header pressure? The piping consists of 600 ft of 4” and 1015 ft of 3”.  Also, assume that there is a flow controller in the system.  What additional storage is required to control the pressure fluctuations to less than 2 psi?

 

•         Remember:

–        The size of the event in CF (rate of flow X duration)

–        Divided by the allowable pressure drop in PSI = 

                        the required capacitance (convert to gallons)

–        Or, divided by the capacitance in CF/PSI = the pressure drop in PSI

 

Header Storage Solution

•         Size of the event

–        This application will remove air from the header for (1000 ft / 250 fps) =  4 seconds at a rate of 10 scf/sec (600 scfm / 60 sec) = 40 scf

 

•         Divided by the capacitance

–        with the piping capacitance at 7.25 scf/psi, the pressure will drop          

–        (40 scf / 7.25 scf/psi) = 5.5 psi.

 

Assuming we want to control the pressure drop to less than 2 psi, the storage requirement would be:

•         (40 scf  / 2 psid) X 14.5 psia   = 290 scf X 7.48gal/cf = 2,169 gallons.  You can subtract the existing volume contained in the header piping from this figure if it is significant (786 gallons in this example).