Air Compressors |
- Air compressors are normally rated to deliver 4 cubic feet per minute (CFM) to 5 CFM per horsepower (HP) at 100 PSIG discharge pressure.
|
- Depending on the size of the system, compressed air costs 25 cents to 42 cents per thousand cubic feet of free air ingested by the compressor, including operating and maintenance costs.
|
- A 50 HP compressor rejects approximately 126,000 BTU per hour for heat recovery.
|
- Motor amperage draw is as follows:
|
| |
1 Phase |
115 Volts |
10 Amps per HP |
| |
|
230 Volts |
5 Amps per HP |
| |
3 Phase |
230 Volts |
2.50 Amps per HP |
| |
|
460 Volts |
1.25 Amps per HP |
Air Receivers |
Size air receiver tanks for 1 gallon capacity for each CFM of rotary compressor capacity. To convert gallons to CFM, multiply gallons times 0.134. Following are standard sizes of receiver tanks: |
Gallons |
Dimensions (in inches) |
Gallons |
Dimensions (in inches) |
30 |
16 x 38 |
400 |
36 x 96 |
60 |
20 x 48 |
660 |
42 x 117 |
80 |
20 x 63 |
1060 |
48 x 144 |
120 |
24 x 68 |
1600 |
54 x 170 |
200 |
30 x 72 |
2200 |
60 x 190 |
240 |
30 x 84 |
2560 |
60 x 220 |
Water Content |
- The water vapor content at 100 ° F of saturated compressed air is approximately two gallons per hour for each 100 CFM of compressor capacity.
|
- For every 20 ° F temperature drop in saturated compressed air at constant pressure, 50% of the water vapor condenses to liquid.
|
- At 100 PSIG every 20 ° F increase in saturated air temperature doubles the amount of moisture in the air.
|
Water-Cooled After Coolers |
Most water-cooled after coolers will require 3 GPM per 100 CFM of compressed air at 100 PSIG. |
Compressor Discharge Temperature (Before After Cooling) |
Following are discharge temperatures (before after cooling) at 80 ° F ambient: |
| |
Pressure |
100 PSIG |
150 PSIG |
| |
Single-Stage |
510 |
615 |
| |
Two-Stage |
325 |
365 |
| |
Rotary (oil-cooled) |
180-200 |
190-205 |
Horsepower & Cost |
- Every 2 PSIG change in pressure equals 1% change in horsepower.
|
- Most air motors require 30 CFM at 90 PSIG per horsepower.
|
- At an electric power rate of 10 cents per kilowatt hour, it cost $806 per year for 1 HP running continuously for three shifts. KW = HP x 0.7457
|
Ventilation |
The following formula estimates the required ventilation air in CFM to adequately control ambient heat rejection from an air compressor. Use full absorbed horsepower. |
| |
CFM = |
(Sensible Heat BTU/Hr
(1.08) (T2-T1) |
| |
|
| |
|
|
|
| |
EXAMPLE |
When |
BHP = 110 |
| |
|
|
Start Temp (T1) = 80 ° F |
| |
|
|
Max. Allow Temp (T2) = 100 ° F |
| |
CFM = |
110 HP x 2546
(1.08) (100 ° F-80 ° F) |
| |
|
| |
= |
12,966 CFM Required Ventilation Air |
