Rotary Air Compressor
There has been a trend in recent years to promote the use of rotary compressed air in automotive care applications because reciprocating technology is perceived as “dated” while rotary is perceived as “cutting edge.” Rotary technology is much quieter than recip technology, which increases the perceived value of rotary machines. It’s true that rotary technology is newer and quieter than recip, however, the initial cost for a rotary machine is typically 50% more than an UltraPack Recip machine. In addition, there are several hidden costs associated with rotary machines that need to be considered before deciding which one is the best fit for your application:
Maintenance Costs – Some manufacturers have mistakenly led customers to believe that recip machines are more expensive to maintain than rotary machines by stating that maintenance is “easier” or “less” involved in a rotary compressor. While it is true that rotary maintenance intervals (typically 2000 hours or one year, whichever comes first) are longer than recip maintenance intervals (typically 3 or 6 months), the cost of the consumables makes rotary maintenance 2 to 3 times that of recip over a 5-year period.
Misapplications – One of the great advantages of rotary technology is that it loves to run all the time. So, if you have an application requiring a continuous use of air, and you size the compressor appropriately, a rotary machine will perform well without overheating. But, this strength becomes a weakness if a rotary machine is misapplied.
Intermittent demand – If a rotary machine is placed in an application where demand fluctuates greatly, it will be underutilized during periods of low demand. This will cause the rotary machine to “short cycle” – i.e., the machine will not run long enough to build up the temperatures required in the airend to burn off water vapor in the oil. When a machine short cycles, it can lead to many issues including premature fouling of internal filtration and downstream components, pre-mature airend failure and potentially to motor failure, if an electronic controller or timer is not used to ensure that the machine does not start and stop too often. All these potential issues add up to extra maintenance and/or replacement costs for misapplied rotaries. In addition, these things may void a manufacturer’s warranty, leaving the end-user to bear the burden of all costs.
Importance of sizing – If a rotary machine is oversized to account for future growth or simply as an insurance policy to make sure enough air is supplied, all the issues outlined above come into play. An oversized machine will almost certainly short-cycle and lead to increased maintenance and service issues.
Cost of extra tank if flexibility is needed – If a rotary machine is misapplied, one of the ways to fix the application is to provide a larger storage buffer by adding a remote stand-alone tank, which will cause the compressor to run longer and achieve the required operating temperature. While this can be an effective solution, it adds extra cost to some rotary applications that should be considered up-front.
Bleeding air to the atmosphere – Another solution to short-cycling is to bleed air into the atmosphere if there is not enough demand. This requires extra equipment and results to the customer wasting money on compressed air that is vented into the atmosphere instead of being utilized in the shop. Although this can be an effective solution, it again adds over all energy costs that should be considered up-front.
Re-builds – While rotary airends typically are designed to last twice as long as the best recip pumps, it costs a great deal to have them repaired or re-built. In some cases, repairs to a rotary airend are not possible. If a rotary machine is misapplied and the airend fails prematurely, the re-build cost could be more economical than the cost of re-building a piston compressor.
While rotary machines are great in many applications, it pays to do some homework upfront to ensure the application makes sense. At a minimum, check the following before buying a rotary machine for your shop:
- Make sure it’s the right size – A walk-through assessment or air audit of your shop is the best way to tell what size rotary machine you need.
- Ensure your demand profile is appropriate – This means ensuring that air demand is consistent enough to warrant a rotary machine. Again, a walk-through assessment is a great way to determine this but engaging a local air expert is a good backup.
- And finally, once you buy the machine, make sure that it’s installed and maintained per the operator manual by a qualified rotary screw service center.
Rotary or Recip? The choice is yours. But an educated decision could save you some money. Learn more about what compressor you need for your automotive shop.
The Nx Series offers a space-saving profile that’s easy to install while accommodating for limited floor space. The integrated separator, MPV and thermostatic valve reduce connections by up to 90% -- meaning fewer opportunities for leaks. The series also comes in variable speed drive from 8-90kw for even more efficiency and energy saving opportunities. The FS-Curtis Nx Series combines efficient operations with outstanding durability, which is the perfect combination for automotive, light industrial, and other applications that require reliable performance in demanding conditions.
Learn more about this Plant Engineering 2015 & 2016 Product of the Year http://us.fscurtis.com/product-catego...
For over 160 years FS-Curtis has prided itself on being the best in air compression. Assembled in St. Louis, MO we are proud of our reputation for having quality products and great customer service has continued to be unwavering. Our Reputation Has Stayed The Same.
An air audit may be in order for your compressor system, especially if it hasn’t been carefully examined recently. While many people focus on the initial cost of purchasing an air compressor, they often forget to factor in the cost of operating the machine over the long run. Any issue within a system can reduce its efficiency, wasting air and electricity, and thereby costing you more money.
But what exactly can an Air Audit do for you? Learn more about how you can help you maximize savings. Download "What is a Compressed Air Audit"?
Learn More About FS-Curtis Products
Compressed air can be one of the most expensive uses of energy for manufacturing plants, often using more energy than other equipment. For instance, one horsepower of compressed air requires eight horsepower of electricity. With many air compressors running at efficiencies as low as 10%, there’s often plenty of room for improvement. Fortunately, 50% of compressed air systems at small to medium-sized industrial facilities have opportunity for energy conservation.
Learn how you can save money and keep your air compression working at a premium by downloading our "10 Ways You Can Optimize Your Air Compression and Save Money".
The mission of FS-Curtis has always been about the people. Some Companies are founded on hard work others are founded on ideals. FS-Curtis was founded on both. FS-Curtis is the best in reciprocating, rotary, and oil-free compressors. Assembled in St. Louis since 1854.
An air audit may be in order for your compressed air system, especially if it hasn’t been carefully examined recently. While many people focus on the initial cost of purchasing an air compressor, they often forget to factor in the cost of operating the machine over the long run. Any issue within a system can reduce its efficiency, wasting air and electricity, and thereby costing you more money.
But what exactly can an Air Audit do for you? Learn more about how an Air Audit can help you maximize savings. Download "What is a Compressed Air Audit"?
Compressed Air is an integral part of the Fabricated Metals Industry. Premier Tool Grinding manufacturer of carbide cutting tools uses our NxB22 along with our RNE Dryer and CF Filtration system. See how we assist companies like Premier Tool in the Fabricated Metals Industry.
Many processes and applications continue to use inefficient devices to deliver the compressed air, and many companies fail to recognize the simple implementation and significant payoff of improving compressed air efficiency.
Improving compressed air efficiency, or saving more of your compressed air capacity by minimizing compressed air demand, can be realized by following some simple procedures. Though there are many actions that can be taken to further improve compressed air efficiency, some simple and effective steps can be put to action quickly.
A few things you need to do to improve the demand of your air compression: Measure the air consumption to identify sources that waste compressed air Find and fix the leaks in your compressed air system. Upgrade your blow off, cooling and drying operations using engineered compressed air products. Use intermediate storage of compressed air near the point of use Control the operating air pressure at the point of use to minimize air consumption.
Why not take your compressed air to the next level by seeing the services and products that FS-Curtis can offer?
Previously we discussed The 6 Types of Basic Individual Compressor Controls to learn about the differences, but how do you determine which controls you would want for your compressor? To help with this decision here are few pros and cons of the basic air compressor controls:
- Simple control using only a pressure switch
- Motor and compressor operate only when needed, which saves energy
- Good for small compressors that are 25 HP or less (depending on application)
- Frequent starting wears down motor and compressor
- The pressure setting to stop must be higher than required system pressure to build storage. This may increase energy use.
- Loss of pressure control in the range of 35 psi
- Limited to small compressors
- Motor compressor runs continuously which reduces wear and tear associated with too many frequent motor starts
- Tighter range of pressure (approx 10 psi)
- Provides adequate storage and offers energy-efficient control of rotary screw, reciprocating and some centrifugal compressors
- If applied incorrectly short cycles cause premature wear and tear. There is minimal or no power savings on lubricant-injected rotary screw compressors
- There needs to be proper blow down time and the storage capacity required for lubricant-injected rotary compressors to achieve energy savings and prevent lubricant foaming
- Requires over-pressurizing to maintain minimum system pressure
- The motor and compressor run continuously reducing wear
- Tighter range of pressure control (10 PSI)
- Steady progressive capacity control that matches demand
- Pressure ratios increase as inlet pressure is throttled
- Inefficient at lower loads(lubricant-injected rotary compressors limited to 40-60% capacity; centrifugal compressors limited by potential surge and may require discharge blow off)
- Combines features of modulating, load/unload. and start/stop
- Shuts down compressors when unloaded for pre-set duration which in turn saves energy
- Better selects operation mode for small reciprocating compressors
- Makes the control complex
- The Over-run timer must be set to limit premature starting and stopping
- Energy-efficient control scheme that gets down to 50% of capacity
- Matches displacement to demand without reducing inlet pressure or increasing ratios of compression
- Makes the control complex
- High initial cost
- Only available for 50 HP+ compressors
- Energy-efficient and precise control
- Various rotating speeds and giving more displacement and power. These are directly proportional to speed rotation
- Makes the control complex
- High initial cost
- Reduced full load efficiency
- Efficiency of rotary screw compressor ends drop at lower or higher speeds
Compressor controls are designed to match compressor delivery with compressed air demand, by maintaining the compressor discharge pressure within a highly specified range. This discharge pressure should be set as low as possible to minimize the energy usage.
Compressor systems are typically composed of multiple compressors delivering air to a common plant air header. The combined capacity of these machines is generally sized to meet the maximum plant air demand. System controls are needed to orchestrate a reduction in the output of the individual compressors during the times of having lower demand. Compressed air systems are usually designed to operate within a fixed pressure range to deliver a volume of air that varies with system demand. System pressure is monitored so that the control system decreases compressor output when the pressure reaches a predetermined level. Compressor output is then increased again when the pressure drops to a lower predetermined level.
There are 6 basic types of individual compressor controls that a person has to take into account when looking into purchasing and using air compression:
- Turns the motor which drives the compressor on or off in response to a pressure signal (seen on reciprocating and rotary compressors)
- Allows the motor to run constantly, but unloads the compressor when a predetermined pressure is reached. The compressor reloads at a predetermined lower discharge pressure. This is also sometimes referred to as constant speed or constant run control (seen on reciprocating, rotary, and centrifugal compressors).
- Restricts passage of air to the compressor to progressively reduce compressor output to a specified minimum, when the compressor is then unloaded. This is also referred to as throttling or capacity control (seen on rotary and centrifugal compressors).
- This controller is commonly seen in small reciprocating compressors, allows the selection of either Start/Stop or Load/Unload. When used in a lubricant-injected rotary compressor it provides modulation or load/unload control to a preset reduced capacity. When unloading the addition of an over-run timer will stop the compressor after running unloaded for a preset time.
- This controller allows progressive reduction of the compressor displacement without reducing the channel opening (seen on reciprocating and rotary compressors).
- This controller adjusts the compressor capacity by varying the speed of the electric motor driving the compressor in response to system signals.
Learn more about the these six basic compressor controllers and more about other FS Curtis products
With high summer temperatures fast approaching please make sure your rotary compressor is prepared for the change in climate. Here are 4 tips to keep your rotary compression at peak performance for the summer.
- Lubricant and Coolant Levels
Is your lubricant and coolant low? Is it due for a fluid and filters service? Please note that running FS-Curtis synthetic fluids will reduce the heat generated by your compressor. The compressor oil serves as coolant in oil flooded rotary compressors. In rotary compressors, the compressor oil is circulated 7 or more times per minute through the machine. This is why oil levels become more critical in the summer. To be sure of the condition of your air-end and bearings we recommend an oil analysis by a distributor.
- Air Density
In the summer your compressor will have a lower performance. Make sure you install a new intake filter for the hot summer days. Please be aware that dirtier environments require more frequent intake air filter changes.
- Having Clean Compressor Air and Oil Coolers
A clean cooler may be the difference between normal operation and a high temperature shut down. Many times synthetic compressor oils are fruitless because the oil breaks down prematurely due to overheating. To protect from high acid numbers, loss of lubricity and increased viscosity, long oil change intervals must be monitored with frequent lube analysis, particularly when exposed to high ambient temperatures. Please contact your local FS-Curtis service company for a compressor “health” check.
- Hot Ambient Air and Moisture
Most lubricated, air cooled, rotary screw compressors run 100 to 110 degrees F above the ambient temperature. If your compressor room temperature is over 100 degrees F, you are already in trouble. Cross flow ventilation is advisable in non ducted compressors. If your compressor oil sump or air-end discharge is more than 120 degrees F over ambient temperature in the room, you need to find out why. FS Curtis has factory approved Air Treatment accessories to minimize moisture in your system and air lines.
What do you consider when purchasing a rotary air compressor? Let’s say you buy a Volvo, arguably considered one of the safest cars on the road, and you drive it 70 mph into a brick wall (by the way, don’t do this, it ruins the paint job), is it the car’s fault that it may no longer run?
The same scenario applies to equipment. If the OEMs installation recommendations are not adhered to, there will be a high degree of probability that the compressed air equipment will not be reliable, will not experience the stated longevity, may require above average maintenance, may consume more electrical energy than required and may not consistently provide the compressed air quality required for production.
While it’s impossible to cover all of the things you need to consider when installing a rotary air compressor (and we hope it’s an FS-Curtis), this short article is intended to help avoid the equivalent of driving your car into a brick wall.
1) Have your system voltage measured. The most common electrical utility voltages in the United States are 200 volt, 208 volt, 230 volt, 460 volt. Electrical utilities guarantee a voltage supply tolerance of + 10%. For example, if the voltage stated supply is 230 volts it could measure at any given time from 207 up to 243 volts. Most electrical utilities attempt to hold the supply voltage to + 5%. Operating electrical motors off design or away from their nameplate voltage will generate additional heat leading to reduced motor life expectancy. The majority of induction motors, 5 hp through 600 hp supplied to the U.S. market, are 230 & 460 volts; however, 200 & 208 volt systems are becoming more popular. If a 230 volt name plated motor is operated on a 200 to 208 volt system it will operate in an elevated heated condition leading to extremely reduced motor life. This is not a recommended practice. There are motors specifically designed for 200 & 208 volt operation. For these applications, motors name plated as such should be used.
2) Many air compressors operate into the motor service factor. As such, request the package full load operating amp draw from the manufacturer to properly size the electrical utility supply equipment (i.e., wire, electrical quick disconnect, overloads, transformers). Seek out a qualified electrical contractor in your area to assist in proper electrical utility modeling and sizing.
1) Proper placement and mounting is vital. Request a General Arrangement (GA) drawing from the OEM for each piece of compressed air equipment purchased. The GA drawing should illustrate the weight and recommended clearances to be maintained around the equipment. The floor will need to support the static weight of each piece of equipment. The equipment should be secured to the floor to prevent it from shifting around imparting stress on connected utilities such as piping, electrical and ducting. In addition, many states have adopted seismic codes mandating equipment be structurally secured to the floor. Many floors appear flat to the eye but are not flat enough to meet accepted equipment mounting practices. Many floors can be crowned (bowed) or have undulations. When securing equipment to a floor it is important to identify floor imperfections filling the air gaps with shims. When properly shimmed, the tightening of the mounting bolts will not bend or distort the equipment frame. Frame distortion will lead to excessive equipment vibration. Excessive vibration causes premature component failure which reduces machine reliability, increases the cost of ownership and could potentially create an unsafe situation.
Proper clearances should be adhered to. These recommended clearances provide boundary area around the machine for safe, effective operation and servicing of the machine. This boundary area also allows adequate cooling airflow around the machine.
All piping should be properly supported as to not impart stress on the equipment. When connecting piping to the equipment pipe connector, it should be supported with proper tooling to prevent the equipment connector from turning during the piping tightening process. If the equipment receiving connector is allowed to rotate it very well could damage internal components to the equipment. Air piping should be properly sized and configured to prevent short cycling of the air compressor controls. A rule of thumb is to always use piping at least the size of the supply connection. If the dryer connections are larger don’t worry, the reason could be that the dryer is over sized due to rerating or simply the fittings are larger to slow down the compressed air velocity through the dryer. The piping exiting the dryer can be the same size as the air compressor connection or the outlet of the dryer, which can be used to establish the piping downstream of the dryer.
If the ambient air conditions can drop below 32°F measures should be taken to prevent the compressed air equipment from being exposed to this as it will freeze. During air compressor operation the most effective way to condition the room temperature is to use the heat generated by the operating air compressor(s). Excessive heat can be discharged from the room by way of a properly sized wall fan. The wall fan can be thermostatically controlled to keep the room at a constant temperature. If there are times when all of the compressed air equipment is not operating, an auxiliary heating system should be considered.
Air-cooled rotary screw air compressors give off 2545 BTUs/hr per horsepower.
Example: A 50 horsepower air-cooled rotary screw air compressor discharges 127,250 btus/hr into its surrounding environment.
That is enough heat to keep a 1500 square foot home comfortable in the dead of winter with outdoor temperatures below 32°F. Many of these air compressors are installed with little thought of eliminating heat build-up around the air compressor. The heat discharged into the ambient air surrounding the air compressor is continuously cycled through the heat exchangers elevating the temperature. Unchecked, the temperature will rise to an unacceptable level, eventually overheating the air compressor & shutting it down. Ducting can be attached to the air compressor to directly route the cooling air discharge outdoors or away from the rotary air compressor. Wall or ceiling fans can also be strategically placed in the room to exhaust the hot cooling air outside. Either way, an adequate amount of replacement cooling air must be brought into the room.
All installations have contaminants in the ambient air. Contaminants can negatively impact an air compressor in many ways, two of which are indigestion into the air compressor itself and or fouling of the air-cooled heat exchangers. The best way to determine if the air compressor is digesting an abnormally high amount of contaminants is to frequently perform oil analysis. The oil analysis should find components not normally part of the oil make-up.
In the case of a lubricated rotary air compressor, all of the ambient air drawn into the machine for compression flows right past or through the airend bearings. Too much ingestion of particulate will damage the bearings. The oil not only cools the air during compression, but it also washes the contaminants out of the air. Over time these contaminants will build up continuing to be cycled through the bearings.
Example: A 10 micron particle should be able to flow through an anti friction bearing suspended in the oil film without damaging the bearing surfaces.
This doesn't account for multiple particles clumping together, which will damage the bearing surfaces. Two solutions for reducing digestion of contaminants are to install an air filter with a lower micron rating and higher efficiency rating. The ratings should be adjusted until the oil analysis indicates the level of contaminants is satisfactory. Remember, as the rating of the inlet air filter is increased the surface area should also be increased or the air filter may have to be changed out too frequently from excessive fouling. The second solution is to relocate the inlet air filter to a remote environment, which is cleaner than the immediate area around the air compressor. The piping used to remotely mount the air filter should be non-ferrous. PVC works great as it is light, easy to assemble and inexpensive. Connect it to the air compressor airend with an easily removable joint in case machine repairs or troubleshooting is required. The piping i.d. should be no smaller than the i.d. opening in the OEM supplied inlet filter mounting assembly. For every 10 feet of inlet pipe run, the pipe diameter should increase one size or 1". If the inlet air filter is relocated outdoors it should be protected from digestion of rain or snow.
Relocation of the inlet air filter is a recommended solution when the surrounding area around the air compressor contains hazardous vapors or fumes. There are air filters, which can remove hazardous fumes or vapors; however, they are costly to purchase and maintain.
If you have questions or would like more information or pricing, please visit our Distributor Lookup page on our website to locate a distributor in your area.
Each manufacturer’s make and model of rotary air compressor gives specific guidelines for preventative maintenance in the operations manual sent with the air compressor. You’ve invested a good deal of money in your new compressor, to protect your new investment it’s a wise idea to follow these preventative maintenance procedures. Keep in mind this is not a comprehensive list, but it’s a good start and will help give peace of mind knowing you are not neglecting your air compressor.
Every manufacturer of rotary screw air compressors provide an Operators Manual with specific guidelines for preventative maintenance. You’ve invested a good deal of money in your new compressor, to protect your new investment it’s a wise idea to follow the preventative maintenance procedures. Some compressor designs may require more steps and checks, but here is a good place to start to get the longest life out of your investment.
Daily: Check for abnormalities (unusual sound, vibration, leaks, warnings or alarms on the controller, etc.)
- Check oil level
- Monitor temperature and pressure
- Check control panel for any alarms or maintenance lights
Monthly: Perform Daily checks
- Clean inlet air filter and enclosure filters.
- Check operation (load and unload, low and high pressure set-points, maintaining pressure, etc)
Quarterly: Check hour meter and check the maintenance schedule to determine if its time for maintenance to be scheduled. Your compressor may require lubrication change at 2,000, 4,000 or 8,000 hours depending on the manufacturer’s recommendations.
- Take oil sample if needed
- Check separator scavenger line flow (if applicable)
- Check v-belts (if applicable)
- Perform daily and monthly checks
Semi-Annual or 4000 hours: Perform Quarterly Check list.
- Change air/oil separator filters (spin on or drop in type)
- Take oil sample and sent to compressor vendor
Annual or 8000 hours: Start by checking annual maintenance list for manufacturer’s recommendations:
- Change lubricant
- Change air / oil separator (drop in type)
- Perform quarterly checks
Keep in mind; these are the basic maintenance items needed to keep an oil flooded rotary screw compressor in good operation. This preventative maintenance program is designed for a standard condition, but as we all know most installations for air compressors would be considered dirty and dusty environments.
Another factor that is critical to the life of a compressor is the actual run time. All rotary screw compressors underlying guideline for maintenance is hours of operation. Most installations never operate 24 hrs per day / 7 days a week. There are only 8760 hours in a year and anything less than 24 hours will result in lower hours, which affect the amount of maintenance needed for each installation.
So, with all of these factors to consider, what does an owner of an oil flooded rotary screw compressor do to properly maintain the air compressor to ensure long reliable operation?
To answer this question we need to look at several factors:
First the environment is the one factor that governs all of the others. If your compressor is running in a dirty environment where you have to keep cleaning the coolers externally to keep the compressor cool and the air filter is continually dirty when you inspect it periodically; then you have a dirty environment. Use the monthly recommendations and not the hours of run time.
2) Hours of operation (less than 4000 hrs per year)
In this case the recommendation would be to follow the time frame set out for preventative maintenance as described in the operations manual. Use monthly intervals for filter changes and not the hours of operation as your guide. This means air and oil filters changed every quarter (or more frequently) as well as air/oil separators changed per instruction manual. Please note if you are running 24 hrs / 7 days then air and oil filters will need to be changed in accordance with OEM recommendations.
Costs of preventative maintenance should be only considered as a last resort as it has been proven in many studies that preventative maintenance saves money. A proper preventative maintenance program will translate into more reliable operation and less down times for the owner. This will create a more efficient production process for the owner which generates income on a consistent basis.
In all other instances the OEM manual should be used as a guideline for changing all filters for the proper maintenance your rotary screw compressor.
A properly maintained air compressor can be accomplished by following several practical guidelines and discussing your needs with an FS-Curtis compressed air professional. To learn more download the product information or contact your local FS Curtis distributor.
When specifying a compressed air system for optimum operation and energy efficiency, proper selection of the compressed air receiver tank(s) is one of the most critical decisions one can make. There are several different “rules of thumb” and formulas that will assist you, but finding straightforward guidance that says “use this size air receiver” is difficult if not non-existent.
Air receivers can be used in several different ways in a compressed air system:
- “Primary” receiver between the supply side (air compressor and ancillary equipment), and the demand side (your plant). Today’s air compressor controls (on/ off-line, modulation, and variable frequency) strive to maximize energy efficiency and smooth compressor operation by responding to demand side pressure changes sensed at the discharge of the package.
A properly sized air receiver acts as a “buffer” and minimizes the effect of dynamic demand side pressure changes, allowing the compressor controls to operate smoothly and consistently. The end result is less energy used, longer component life, and consistency in plant air pressure.
- “Secondary” receiver typically on the demand side, at the point of use to minimize the effect large intermittent air demands have on the overall compressed air system.
Sizing a PRIMARY receiver for general FIXED SPEED APPLICATIONS:
- The air compressor industry has widely accepted the general rule of thumb that a properly sized air receiver for a fixed speed compressor should be between 1-2 gallons per CFM output of the compressor.
Example: A 100 CFM fixed speed air compressor should have an air receiver between 100-200 gallons sitting next to it. Err on the high side if your budget permits.
Sizing a PRIMARY receiver for general VARIABLE SPEED APPLICATIONS:
- When considering air receiver sizing on a variable speed drive application, the general rule of thumb is between 2-4 gallons per CFM output of the compressor.
Example: a 100 CFM variable speed air compressor should have an air receiver between 200-400 gallons sitting next to it. Err on the high side if your budget permits.
Useful Air Receiver Sizing Formulas (Primary and Secondary):
Use this formula when you have an existing air receiver
and need to know how long you can draw CFM greater than the output of the air compressor, from the receiver, while still maintaining system pressure:
- T = R x P1 – P2 Qr – Qc 7
Use this formula to determine what receiver size to use to supply pressure for a given period of time, not allowing the system to drop below a minimum pressure. The demand of air is greater than the CFM output of the air compressor:
- R = 7 x (Qr – Qc) x T
P1 – P2
Use this formula to determine how long it will take to recharge an air receiver to P1, after demand goes back to being below CFM output of the air compressor:
- T = R x (P2 – P1)
Qr – Qc x 14.7
T = Time in minutes
R = Receiver in cubic feet Qr = CFM removed
Qc = Compressor output in CFM
P1 = Maximum air receiver pressure P2 = Minimum air receiver pressure
14.7 = Atmospheric pressure in PSI (sea level)
CUBIC FT to GALLON CONVERSION: 7.48 gallons in a cubic foot
A Variable Speed Drive compressor sounds like a smart idea, but is it really?
Let’s address the rotary compressor question that auto care facilities ask us on a regular basis: Should I spend the extra money to put an energy-saving Variable Speed Drive (aka: Variable Frequency Drive) rotary screw compressor in my facility? The short answer to this question is: probably not.
The folks asking this question have heard that VSD/VFD machines may cost more up front but that they cost less to run and thus pay for themselves in the long run. In addition, many utility companies offer rebates for VSD/VFD machines that can help offset the larger up-front cost. Just like everyone else, automotive care facilities want to save money and energy, so this supposed value proposition sounds attractive. Right?
So why then is VSD/VFD probably not a good idea for most automotive care facilities? The short answer is that most automotive care facilities typically have severe peaks and valleys in their compressed air usage due to the intermittent use of tools and equipment in the shop. This in itself does not rule out VSD/VFD. In fact, VSD/VFD is ideal for customers whose demand fluctuates, provided that it does not fluctuate too much. The problem with auto care facilities is that their valleys are too deep. Too much downtime. The rule of thumb is that when the valleys are 30% or less of the peaks, the compressor will have to shut down, essentially forcing it to run as a start/stop machine. There are several issues with running a VSD/VFD machine in this type of application:
- More expensive – First of all, the customer is paying a 30% or more premium for a VSD/VFD machine to run just like a cheaper start/stop fixed speed machine. Perhaps the more important comparison is that the customer would be paying a whopping 70% premium compared to a fully-loaded UltraPack recip, which is probably the best application in most auto care shops.
- Phantom Energy Savings – VSD/VFD machines are ideal for loads somewhere between 30 and 80 percent of full load. There are little to no energy savings otherwise. Even though very short payback periods are often cited, these are for machines in ideal scenarios with exactly the right load profile. Auto care facilities very seldom have an appropriate load profile. If one isn’t going to get the benefit of the VSD/VFD, why pay extra for it, and risk some of the potential headaches outlined below?
- Increased Maintenance -
o VSD/VFD’s require minimal incremental maintenance on the drive compared to standard fixed speed machines. These increased costs should be considered in any decision to go with a variable speed machine.
o VSD/VFD’s require a cleaner environment for the sensitive electronics of the drive. This can lead to increased maintenance costs and possibly drive replacement, which is very expensive. If the machine is going to be placed in a shop with dust and other contaminants floating in the air, then the cost of protecting the drive from these contaminants, or paying extra for maintenance and/or replacement, must be considered.
The bottom line is that most auto care applications are not ideal for VSD/VFD machines. The severe valleys in demand profiles mean there probably won’t be energy savings to offset the increased acquisition and maintenance/replacement costs. In addition, most shops are too dirty to install a VSD/VFD without increased risk and cost.