According to the Farmers’ Almanac, the 2019-2020 winter season could be colder and snowier winter than usual for much of the United States. Is your compressed air system ready for the colder temperatures?
Most commercial freight is delivered via third party carriers , and even with the best of carriers, sometimes shipments are damaged. Your freight may shift, get dropped, or even be in a traffic accident. Damage may also occur at logistics centers en-route, where items are transferred from one truck to another.
Like most other suppliers, Kaeser’s freight terms are usually “FOB shipping point” –which means that once the item leaves our possession, we cannot ensure safe handling or monitor the condition the freight arrives in. In most cases the buyer is paying the freight, and has the obligation to inspect the shipment. Kaeser will assist you in case of damage, but accepting a damaged shipment without comment will limit your ability to make a claim. Here’s an infographic with some tips for receiving commercial shipments, whether from Kaeser or any other supplier.
During the purchasing decision process, it is common for prospects to compare compressors with some sort of utility criteria. In other words, how much air will they get for their money. Below, we address some common approaches we encounter:
Compressor cost per horsepower
This is a quick comparison that can be done using basic product literature, but it is a very superficial metric for comparing compressors. Since the requirements of air tools and equipment are not rated in compressor horsepower, and since the flows among compressors of the same nominal hp can vary by 20% or more, this doesn’t tell you how much air (cfm) you are getting or whether a compressor will meet your air demands (assuming you know them). Our experience with hundreds of thousands of systems has shown that without knowing your actual system needs, you are more likely to oversize your system, which leads to higher power and maintenance costs and reduced longevity (see our blog post on oversizing).
Compressor cost per cfm
This can also be done with literature and is a step forward for basic comparison, and if you know your actual flow demands it will help avoid sizing mistakes. Like the first method, its shortfall is that it only considers initial cost. It does not reflect energy efficiency, so it is not a predictor of the largest component of compressed air life cycle costs: electricity usage.
Compressor cost and specific power (kW/100 cfm)
Specific power is the true measure of a compressor’s efficiency, so combining this with unit cost is a better indicator of compressor value. Keep in mind, however, that specific power is based on a fixed set of conditions and assumes the compressor is running at maximum capacity, which they rarely do. Nonetheless, when choosing machines it is very useful to compare the specific power (AKA “specific performance) of the compressors. Most major manufacturers provide this information in CAGI data sheets on their websites or by request (see our blog post on how to read them).
System specific power
Because most compressors run partly loaded for a variety of reasons (demand fluctuation, oversizing, changes in production), the best metric for energy efficiency (and therefore compressor selection) is system specific power. This metric reflects the ability of the total system to maintain efficiency throughout the full range of production demand and is a far better metric for operational efficiency. This is not easy to assess for new plants (unless there is a similar sister plant in operation), but it is easily done for upgrades on existing systems with tools like ADA/KESS that data log parameters including compressor run time, system pressure, power consumption and flow, and then select the best mix of machines to meet the need. We strongly recommend assessing system performance anytime you are adding or replacing compressors — even if you plan to simply replace a compressor with another of same size. This is an ideal time to baseline the system and identify inefficiencies in pressure drop, storage, sizing, and controls.
Because compressed air demand changes as plants increase or reduce production levels or upgrade pneumatic equipment, it can be a challenge to maintain optimal system performance. The best approach in multi-compressor systems is a combination of proper sizing of compressors and the use of adaptive smart controls. These learn system dynamics and switch compressors on/off in the most efficient manner while maintaining desired system pressure, balancing load hours and minimizing idle time.
By: Michael Camber, Jeff Owen (Sales Manager for Kaeser USA’s Atlanta branch), and Frank Remsik (System Specialist)
We are in the business of selling rotary screw compressors, and we sell quite a few to users that have outgrown their two-stage reciprocating/piston compressors in the 5 to 20 hp range. Sometimes demand or duty cycle has increased beyond the practical range of their piston compressors. Or they need higher air quality. Sometimes noise and vibration are the issue. But there are many cases where a reciprocating compressor is still a very good, economical fit for the shop, but service issues lead them to think they need a different solution.
Heat is often the enemy
Most small shop recips are not designed to handle 100% duty cycle. In other words, they cannot run flat out for long lengths of time without sustaining heat-related wear or damage. Generally, these small two-stage units operate at relatively high temperatures (275-350°F), so they need to stop and cool down periodically. (This is why they are typically set at 145-175 psig, even though most tools only need 90 psig.) Duty cycles vary — we’ve seen 50% to 80% –depending on the design and quality of construction.
There are a number of heat-related problems, but first let’s talk about what can cause them.
First, the compressor’s environment plays a critical role in its reliability. If the room is too hot, or doesn’t get enough ventilation it will run hotter than designed. To reduce noise, many recips are installed in out of the way locations (e.g. utility closets). Ventilation is often poor, creating more heat and higher discharge temperature.
Second, excessive run time can result in heat-related problems. There are several reasons for excessive run time, and a system can suffer from any or all of them:
The compressor is undersized for the productive demand.
More users or larger tools have been added to the demand.
Leaks have developed (leaks in fittings, hoses and tools are just another type of air user—even if completely unproductive).
Lack of storage in the tank due to water. The air leaving the compressor pump is hot and contains moisture in vapor state. In the tank, the air cools and moisture condenses into liquid. Condensate can build up quickly, especially in warmer, humid climates (gallons per day). If the tank is not routinely drained, it will fill with water leaving less room for air. Less air storage => more run time => more heat =>more problems.
Potential heat-related issues
Below are some of the mechanical issues caused by overheating. Generally, these can be repaired economically.
Piston rings no longer seal properly against cylinder walls, thus losing compression. When this happens the pump may have to run longer (and even hotter) to meet demand. Lubricating oil breaks down faster and gets past the rings more easily, requiring more make-up oil to prevent further mechanical issues (and degrading air quality).
Failed intake / exhaust valves
Oil carry-over builds up and may prevent valves from properly seating, creating blow-by through valves. This can cause the intercooler safety relief to release, and also cause the voltage supply breaker / fuses to trip due to stalling out the pump. Over time this can burn out the drive motor.
failed check valves
Recips tanks have check valves to make sure they don’t start under load. Over time the elevated temperatures along with oil carry-over can distort the nylon piston in the valve, so the piston can’t seal properly. When this happens, back-pressure from the tank will create head pressure on the pump. When the compressor tries to start, the extra amps drawn by the motor can trip breakers and burn the motors out. If you have issues with belts breaking prematurely, it could be from a failed check valve on the tank. If the compressor pump is trying to start against head pressure, the crankshaft may not move even though the motor is. Motor goes, pump won’t -> belts slip/wear/break.
Motor burn out
Motors generate heat in normal operation but will cool themselves adequately unless they are in too hot an environment or are energized and try to turn something that doesn’t want to be turned, such as a pump with head pressure (see above) or one that is not properly lubricated. Over time, the insulation on motor windings will degrade and the motor will need to be rewound or replaced.
By design, reciprocating compressors vibrate. Vibration affect many things. Vibration can loosen piping connections, as well as any threaded nut or bolt. It can loosen up electrical connections and create electrical drop-out, sparking, tripping out breakers and blowing fuses. Pressure switches can also fail due to excessive vibration. Vibration can create cracks in welds and joints at the tank feet, platform and saddles. Excessive vibration also increases noise levels, loosens safety guards, and can even break up a concrete floor.
Replace missing or cracked vibration pads. While you are at it, if the discharge piping from the tank is hard pipe, swap it out for flexible steel braided hose. Same goes for electrical supply from the wall disconnect to the starters. Make sure the belt guard is secure. A missing or loose belt guard is not just a safety issue but is an OSHA violation.
Noise can often be abated with well-placed, insulated stud walls. The key is not to restrict airflow. If you are contemplating constructing a separate room to isolate a hot, noisy compressor, it is worth doing the math to see if a rotary compressor makes sense. The quieter rotary unit may cost less that permitted construction and almost certainly take less time to install. This assumes you have a good place with good ventilation and access, and that the unit will be run enough to gain some of the energy advantage.
A note about tanks
As explained above, storage is vital to the longevity of the compressor. It’s also important for meeting demand and system performance. Tanks don’t need much maintenance but you want to keep them dry. Not just for the storage, but to minimize rust. Over time, rust will build up in the tank and plug up the drain port. This makes the case for a quality automatic condensate drain that won’t get gunked up by the oil-water-rust mixture.
An ounce of prevention
The piston style compressor is simple and requires relatively little service, but it cannot be ignored. Here are some tips, whether you are installing new or want to keep ol’ faithful going:
Ventilate the compressor room to maintain positive air flow. If the compressor is in a confined space, install louvers and thermostatically controlled fans as needed.
Routinely drain the tank. Better yet, install an automatic drain (with test function)
Check oil levels routinely. Add make up oil as needed and perform oil changes on schedule with an oil recommended by the manufacturer. Avoid automotive motor oils, which contain a lot of detergents that leave deposits.
Replace the air inlet filter routinely. You may be able to vacuum it out to extend the replacement interval. Plugged air filters restrict the performance of the compressor and increase operating temperature.
Find and fix leaks on the compressor and in the system all the way to the fittings, hoses and tools at point of use. Listen for leaks and hissing sounds while the unit is off. On the compressor, check the intercooler and its SRV, pressure switch, and the liquid drain on the tank (which some people leave cracked open to avoid liquid build up).
The belt life on most recips can be very long if you take care of them, but excess heat will reduce belt life. Look for wear and cracks that might cause them to come apart. Damaged belts can create more vibration.
Check duty cycle. If the unit is running more than it used to, you could be using more air or there could be water build-up in the tank and you have less storage. Another possibility is ring wear. A pump up test will tell you if the machine is still making air to specification and point you toward the cause.
For more information on piston vs rotary screw compressors check out our infographic or read our blog post on the subject.
During a recent set up of a new controller installed to manage three compressors (two 40 hp and one 75 hp), our field rep mistakenly set the system pressure 20 psi lower than planned. A week or so later, during a system check, the technician discovered the error. Meanwhile, the plant equipment ran fine. Nobody in the plant noticed any production issues. So in addition to a 13% power reduction from better compressor management, the customer got another 10% power benefit by running at lower pressure.
We certainly don’t recommend this approach to finding your proper system pressure, but this incident highlights a very common mistake in compressed air systems: many compressed air systems are running at higher pressures than needed. A rule of thumb for typical plant air systems is that every 2 psig increase in pressure requires 1% more power. So turning up the compressors from 100 to 110 psig increases power consumption about 5%. This practice does not increase productivity. It just uses more energy— and often causes premature wear in pneumatic equipment.
If you have any doubts at all (or even if you don’t), we advise turning down the pressure to see if it affects production, but with a conservative approach. Try 1 psi per week until someone in production complains. This is a no-cost solution that immediately saves money. And the bigger the system and the higher your utility rates, the more you save. The added bonus is reducing the volume lost through leaks, and this also reduces flow demand and compressor run time.
If you are trying to overcome pressure drop between the compressor and points of use, the ideal solution is to minimize the source (s) of the pressure drop (e.g. replace clogged filters, make sure ball valves are fully open, replace undersized piping and fittings). And if it does become necessary to set pressure higher, do it incrementally. People tend to bump up the pressure 5 or 10 psi at a time without trying to adjust it back down.