CAGI Sheets tell the story!
When you buy a 100 hp compressor, how many horsepower do you think it should consume? Yes, this is a trick question.
Compressor ratings are often confusing when it comes to checking the specifications of screw compressors. The 100 hp associated with the Compressed Air and Gas Institute (CAGI) compressor data sheet shown in this post refers to the nominal rating, not the actual compressor power consumption. If we remember back to technical school, we will remember that one horsepower equals 746 watts. So, the expectation would be that a 100 hp compressor would consume 74.6 kW at full load and full pressure.
Looking at the actual rating, we can see this compressor consumes far more than this, 90.2 kW or the equivalent to over 121 hp. Why is this?
One of the reasons is this compressor is an air cooled compressor. It has electric powered cooling fans that consume extra power over and above the nominal rating. This adds to the total.
In addition to this, the main compressor motor is only 95.4% efficient and the 100 hp refers to the motor shaft output. Therefore, for rated shaft output, this motor needs to consume about 105 hp.
But the biggest reason relates to the service factor of the motor. Electric motors have a service factor rating that allows the motor to be run higher than its rated nameplate amps. More amps means more power input.
Why do the manufacturers push into the service factor? Because you, the customer, purchase based on numbers. Given the choice of Compressor A that produces 400 cfm or compressor B that produces 490 cfm, the customer will often choose the bigger number. The further into the service factor you go, the more flow a compressor can produce. But, if the customer simply compares compressors based on nominal hp, they may miss the fact that the compressor with the highest flow may consume much more power and actually be less efficient, increasing the operating costs. This is why it pays to check the CAGI sheets.
Why should you care? Well, motors that are pushed into the service factor are often less efficient and have a shorter life. Typically motors loaded to 80% of motor rating are at the most efficient point of operation. And wire size can be an issue—if the input wiring to a compressor is sized for nominal power, the wire capacity may be on the edge of acceptable, causing extra power loss due to overloading.
Sometimes the solution is to oversize the motor, if this is an option, or to purchase a compressor with a higher pressure rating than where it will be running. This will keep the motor loading low and be easier on the compressor on the whole.
Next time you choose a compressor, have a close look at its operating characteristics so you can make a wise choice.
Most, if not all manufacturers typically utilize about 10% of the 15% motor service service factor when designing their compressor packages, so nominal 100 H.P. motor will be using nominal 110 H.P. at full rated flow and pressure. when the 95.4% efficiency in the CAGI example is taken into consideration,that would equate to about 86 kW.
The nominal 3 H.P. fan motor typically is considered an an over motor (being mounted in the path of the cooling air flow created by the fan) and as such the motor will utilize a greater amount of the service factor; in this case about 25%and with an 89.5% efficiency that would equate to about 3.75 kW.
Total 89.75 kW or very close to the 90.4 kW stated in the CAGI sheet. The variation is covered by the +/- factors of the power test code.
Electric motors chosen for these compressors typically are rated for 90 deg. C rise at 40 deg. C ambient at service factor amps so maintaining proper cooling is very important to stay within motor temperature operating temperature limits so as not to cause premature motor failure ( insulation breakdown / motor burnout )
Your comment about choosing a higher pressure rating compressor and operating at a lower pressure would not be particularly wise. A fixed speed compressor would deliver less volume ( 450 CFM at 125 PSIG ) while consuming similar full load power. By reducing the operating pressure to 100 or 110 PSIG, less power would be consumed but the flow would also remain at the lesser rated level of 450 CFM so your specific power
( kW per 100 CFM ) could actually be no less ( and maybe greater ) than if you selected the 100 PSIG rated model with higher flow.