Compressor Sizing

 

Making sure the compressor package you purchase is properly sized for your application is always critical not only for you but for the life of your compressor. We have several tools available to aid in this process. Below is a compressor sizing guide, for both reciprocating and rotary screw compressors. Compressors are typically bought for two reasons. The first is that an older compressor has worn out or is no longer able to keep up with the application it is installed into. The second is a new building or application has come up that requires more air for it to operate. By reading below or contacting C&P, you will always make sure you have the right compressor for your application. 

 

Sizing Electric Screw Compressors.

Body Shops.


To properly size a body shop, first find out the total number of shop workers. You then take that number and multiply it by 8. The average amount of CFM used by a body shop employee through out the day.
(or any shop that does a lot of sanding and grinding)

Example:

Total Workers-10

10 People x 8 CFM= 80 CFM

For this shop to be sized effectively they would need 80 CFM of delivered air. This is to make sure that he compressor does not run fully loaded all the time.

Mechanic Shops.

To properly size a mechanic shop first find out the total number of shop workers. You then take that number and multiply is by 5. The average amount of CFM used by a mechanic throughout the day.

Example:

Total workers-12

12 People x 5 CFM= 60 CFM

For this shop to be sized effectively they would need 60 CFM of delivered air. This is to make sure that the compressor does not run fully loaded all the time.

Total CFM Usage.

To properly size a shop based in total CFM. First get the manufacturer recommended CFM for all the tools that are used on a daily basis in the shop. Second take their total CFM’s together and multiply that number by 1.3 to get the total CFM needed for the shop.

Example:

3 DA Sanders-10 CFM

3 Impact Guns-6 CFM

2 HVLP Paint Guns-15 CFM

30+18+30=78 CFM

78 x 1.3=102 CFM

For this shop to be sized properly they would need 102 CFM of delivered air. This is to make sure that the compressor does not run fully loaded all the time.

Symptoms of improperly sized Rotary Screw compressors

1. Excessive Heat

2. Low air pressure

3. Unit never unloads

4. High temp shutdown

Things to remember

When you are sizing a shop be sure and check for air leaks, in older shops there is generally more air used because of leaks than employees working.

If there are leaks then the customer will need to either upgrade to a larger compressor, or get the leaks fixed before the new compressor is installed. This is to ensure proper duty cycle on the new compressor. Always make sure that the electrical breaker for the compressor is sized appropriately according to local and state electrical code. This helps to ensure proper motor and electrical component life.

All compressors have electrical components so they must be located in a place that water, excessive heat, and heavy dust will not affect them, as this can cause pre-mature failure of these components.

Customers do not complain about too much air, but they will always complain if they do not have enough.

If a customers properly maintained compressor suddenly fails its probably due to its duty cycle being extended past what it is capable of. So be sure to check to see if new employees or equipment have been added that use air. This will ensure that the new compressor has a long and trouble free life.

Rotary Screw Compressor Knowledge.

Rotary screw compressors are constantly running machines, they do not start and stop like piston compressors.

Rotary screw compressors require a tank for storage, too ensure proper inlet valve life.

Rotary Screw compressors are temperature controlled by an oil cooler, so they must have adequate ventilation to ensure proper cooling.

The oil and filters for rotary screws are specifically designed for the units, so that they operate to certain CAP specifications

The oil in a rotary screw compressor is changed once a year in 9-5 Mon-Fri applications. It would be changed twice a year in applications that run more than one shift or put more than 4000hrs per year on the machine

Oil filters are changed 4 times a year in typical 9-5 applications, and separators once a year. In applications were the business runs more than one shift you change the oil filter 8 times per year and the separator 2 times per year.

All Compressor are pre-set and tested at the factory, contact the factory if you feel like anything needs to be changed.

 

Sizing Electric Piston Compressors.

Body Shops.

To properly size a body shop application, first find the total number of workers in the shop. Second multiply that number by 8. Which is the average amount of CFM used by any one person through out the day. This makes sure that the compressor stays within its desired 50% duty cycle. (or any shop that does a lot of sanding and grinding)

Example: Total Number of workers-7

7 People x 8 CFM=56 CFM

For this shop to operate within the desired 50% duty cycle you would need 56CFM of air delivered.

Mechanic Shops.

To properly size a mechanic shop first find the total number of workers in the shop. Second take that number and multiply it by 5. The average amount of CFM used by a mechanic through out the day. This makes sure that the compressor stays within its desired 50% duty cycle.

Example:

Total Number of workers-6

6 People x 5 CFM =30

For this shop to operate within the desired 50% duty cycle you would need 30CFM air delivered.

Total CFM Usage.

To properly size a compressor based on total tool consumption you would need the manufacture spec CFM of all the tools that the shop uses on a daily basis. First add all of these totals up and then multiply the total by 2. This makes sure that the compressor stays within the desired 50% duty cycle.

Example:

DA Sander-10 CFM

Air Impact-6 CFM

HVLP Paint Gun-15 CFM

Total 31 CFM

31x2=62 CFM

For this compressor to operate within the desired 50% duty cycle you would need 62 CFM of air delivered.

Symptoms of improperly sized Piston compressors

1. Excessive oil carry over

2. Pre-Mature Valve Failure

3. Blown Gaskets

4. Dis-Colored Discharge Tubing

Things to remember.

When you are sizing a shop be sure and check for air leaks, in older shops there is generally more air used because of leaks than employees working.

If there are leaks then the customer will need to either upgrade to a larger compressor, or get the leaks fixed before the new compressor is installed. This is to ensure proper duty cycle on the new compressor. Always make sure that the electrical breaker for the compressor is sized appropriately according to local and state electrical code. This helps to ensure proper motor and electrical component life.

All compressors have electrical components so they must be located in a place that water, excessive heat, and heavy dust will not affect them, as this can cause pre-mature failure of these components.

Customers do not complain about too much air, but they will always complain if they do not have enough.

If a customers properly maintained compressor suddenly fails its probably due to its duty cycle being extended past what it is capable of. So be sure to check to see if new employees or equipment have been added that use air. This will ensure that the new compressor has a long and trouble free life.

Compressor Knowledge.

Tanks size does not indicate how much air a compressor makes. The only way you determine volume of air is CFM.

All piston compressors should have their oil changed every three months under normal use.

It takes a car an average of 740 running hours to go 30,000 miles (10 oil changes) Under perfect conditions with a maximum of a 50% duty cycle, a piston compressor will run 240hrs in three months of 9-5 Monday-Friday work. That is the equivalent of changing your oil every 9750 miles in a car.

Over time the vibrations in piston compressors can loosen fittings and bolts, the whole compressor needs checked out when the routine maintenance is performed.

All Compressors are pre-set and tested at the factory, contact the factory if you feel like anything needs to be changed.

 

Dryer Sizing 

Recommended Refrigerated Dryer Sizing

5-7.5 HP Reciprocating/Piston Compressor-HG25

10 HP Reciprocating/Piston Compressor-HG35

15 HP Reciprocating/Piston Compressor-HG50

20 HP Reciprocating/Piston Compressor-HG75

25 HP Reciprocating/Piston Compressor-HG100

30 HP Reciprocating/Piston Compressor-HG100

5-7.5 HP Rotary Screw Compressor-HG25

10 HP Rotary Screw Compressor-HG35

15 HP Rotary Screw Compresor-HG75

20 HP Rotary Screw Compressor-HG75

25 HP Rotary Screw Compressor-HG100

30 HP Rotary Screw Compressor-HG125

40 HP Rotary Screw Compressor-HG150

50 HP Rotary Screw Compressor-HG200

60 HP Rotary Screw Compressor-HG250

75 HP Rotary Screw Compressor-HG300

100 HP Rotary Screw Compressor-HG500

(All refrigerated air dryers come standard with electronic timed auto drain)

Recommended Filter/Regulator/Coalescing Sizing

5-7.5 HP Compressor-½ inch

10 HP Compressor- ¾ inch

15 HP Compressor- ¾ inch

20 HP Compressor- ¾ inch

25 HP Compressor- ¾ inch

30 HP Compressor- ¾ inch

40 HP Compressor- 1 inch

50 HP Compressor- 1 ¼ inch

60 HP Compressor- 1 ¼ inch

75 HP Compressor- 1 ½ inch

100 HP Compressor – 2 inch

 

 

 

Compressor Efficiency  

There are lots of products currently on the market to aid in air compressors efficiency. Here are a couple of key things to look at before making a choice to add one of these products to your compressor. At Compressor And Parts we want to make sure you have all the information to make the right choice for your compressor package.

Variable Frequency Drives

These devices will keep the compressor loaded and delivering only as much air as you are using. The benefits are that the compressor will start and produce the exact amount of air being used from a storage tank and the airlines tied to it. Variable frequency drives also come with a soft start feature so the compressor starts slowly and rises to max RPM over a period of a few seconds. By doing this the compressors keeps your electrical demand rate at a minimum. There are certain states and municipal electric companies that offer rebates for installing these devices on the compressors. (Check with our electric company to find out what is available.) Variable frequency drives (VFD) do come with a large expense up front. If there is not a rebate offered it may not be a viable option. It is best to check and see if the VFD will pay for the added expense in the electrical savings over the life of the compressor. Since they are an electrical device, voltage and amp surges or drops can affect the VFD. It is a necessity to verify your incoming power before ordering this device on your compressor.

Duplex Compressors

This is the original form of efficiency. You have two compressors on one tank, when both are needed, both run, when one is needed, the other is off and not using electricity. These systems come in both reciprocating and rotary screw packages. They are great for applications that have a large air demand for a portion of the day and a smaller steady demand for the rest of the day. When the demand is large, both compressor packages can run in unison and keep the application going. When the demand dies down, only one compressor will run cutting your electrical usage in half. Another benefit is that you have two compressors instead of one so that if one compressor fails for any reason, you always have a backup. A downside to duplex compressors is that they are piped together to a common tank, so if a fitting or piece has to be replaced on the tank, both compressors packages are down. When you have two separate packages that are independent, a repair on the tank of one package will not affect the other.

Soft Starter or Wye Delta Starters

These are devices used to help with the first in rush of electricity to start the electric motor in motion. Typically those devices are used in areas where there is limited electricity available for the compressors. A soft start or WYE-DELTA  helps by keeping the startup rush of electricity to a minimum. These systems work b starting the compressor and electric motor slowly and ramping up to max RPM over a period of a few seconds. Due to the cost increase for these devices, check and see if installing one of these devices is necessary for the application, and if it will pay for itself over the life of the compressor in electrical savings.


 

 

Buying The Right Compressor For You
 

Reciprocating vs Rotary Screw

This is a common question asked by most people looking for a new air compressor. Either compressor depends in your application.

 If the application requires air all day or has multiple shifts that use air on a consistent basis, then a rotary screw is probably a better fit for the application due to its ability to run all day every day. Rotary Screw compressors are thermally controlled through an air cooled oil cooler. Some rotary screw compressor packages have built in timers that will turn the compressor off if it runs unloaded for an extended period of time. (Everyone goes to lunch, or it does not get turned off at the end of the day.)

Reciprocating compressors are best suited for applications that require air for shorter durations of time and might have 15-30-45 minutes before they need air again. Because a Reciprocating compressor is air cooled, they need to enter an application with a duty cycle of 70% or less. Either package depends on making sure you purchase a properly sized compressor for your application. If you have any questions, you can always contact CAP or one of our local distributors to make sure you have the right unit for your application.

Noise of Reciprocating vs Rotary Screw

The noise of an air compressor package depends on how it is constructed.. Most rotary screw compressors sold in the world today are built in sound dampening enclosures. These does not add to the performance of the compressor package, all they do is lower the DBA of the compressor package. Most reciprocating compressors are built open on a tank with no sound proof enclosure, which is why they sound louder than a rotary screw in a sound dampening enclosure. If you take a reciprocating compressor and build it in the same enclosure as a rotary screw compressor you will get a similar DBA reading. At CAP we manufacture a reciprocating compressor muffler that can lower the DBA of the compressor by up to 10DBA depending on the acoustics or the compressors environment. This would put the DBA of a CAP reciprocating compressor in the range of 74-82DBA. At CAP we are one of the few compressor companies to build an open tank mounted rotary screw compressor while the DBA range is 70-84 depending on the size of the compressor, an enclosed rotary screw compressor package is between 64-78DBA depending on the size. The enclosure for a rotary screw compressor is an added expense up front and to the yearly maintenance cost. It takes longer to service and enclosed rotary screw compressor than an open tank mount version.

At Compressor And Parts we have several sizing guides (above) to help you pick the right compressor. Or give us a call and we'll put you in touch with a local distributor who can size a compressor for your application.

 

 

Obtaining a reliable compressor

The key to getting a reliable compressor is making sure that the compressor is sized properly for your application. The best way to do this is have an authorized Compressor And Parts distributor come out and look at your application. We also have several sizing guide for you to use to get started. Once you have purchased your Compressor And Parts air compressor the next step is getting a routine maintenance program set up. Each compressor comes with a manual that is also downloadable from the web site. In the manual the general maintenance procedures are listed, they are also on the tank or base of your Compressor And Parts compressor. Your local distributor can get your set up on a maintenance plan at the time of your compressor purchase to make the process as easy as possible.

Redundancy

In any system where you are relying on the compressor for use daily, it is always recommended to have some form of backup compressor. As compressors are mechanical devices, they will need to be serviced and repaired over time. When installing a new system, look at your air needs and make sure you have a backup that will be able to handle at least 50% of your usage. This will enable you to continue working while the main compressor is being maintained or serviced. If you have any questions on what type of a backup compressor you should have contact CAP or your local CAP distributor.

 

 

Servicing Your Compressor
 

Reciprocating Compressors

The most common service on a reciprocating compressor is changing the oil. If it is done by the manual, and the right oil is used, you will have a longer lasting better performing reciprocating compressor. The oil in reciprocating compressors should be changed every 500hrs or 3 months. The standard oil for reciprocating compressors is a 30 weight non-detergent. While a non-synthetic will work fine, a full synthetic is recommended to deal with the harsh environment of a reciprocating compressor pump where oil temperatures can go over 300 degrees F (Just as a guide line, 500hrs on a vehicle is roughly 15,000 miles of driving, with temperatures around 210 degrees F). We recommend using only CAP oil in your compressor to ensure it is always running the right fluid. The other major component is the air intake filter. The life on this is dependent on the environment; the dirtier the air around the compressor, the more it will need to be changed. Typically the filter will last 3-6 months or 500-1000hrs of use. If there are ever any questions contact your authorized CAP distributor.

Rotary Screw Compressors

There are several things to service and change on rotary screw compressors. The oil filter is a 500hr part that is changed every three months or 500hrs whichever comes first. The air filter on the compressor is a 500hr part that is to be changed every 500hrs or three months whichever comes first. The oil in the compressor is good for up to 8000hrs if the compressors oil filter and air filter are serviced properly. We recommend using only CAP oil in your compressor to ensure it is always running the right fluid. The separator is a 2000hr part to be changed once a year or every 2000hrs whichever comes first. Rotary screw compressors compress air through their oil so it is critical to the machine that all filters are serviced on schedule to ensure proper life span of the compressor. We recommend that the oil be changed at a one year interval to make sure the unit is always running clean fresh oil. If there are ever any questions, please contact CAP or your authorized CAP distributor.

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