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. This blog post addresses some common approaches.
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.
By: Michael Camber
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.
This is a tip you can take to the bank.
For additional tips visit our website!
A small town faces the common dilemma of “repair or replace?” and achieves notable operational and energy efficiency gains.
By: Michael Camber
Traditionally, planning for blower system installations at wastewater treatment plants has been approached very differently than industrial compressed air system design. We have found, however, that applying techniques such as system energy audits and modern controls to blower systems yield similar benefits in terms of identifying energy reduction opportunities. That’s exactly what happened when one of our wastewater sales managers convinced management at a wastewater treatment plant to perform an Air Demand Analysis rather than simply replace aging equipment with newer versions of what they had.
Thinking outside the box and working with the local engineering company, we were able to show the town’s public utility department how they could save energy and then document the actual savings.
See the full case study published in the May 2019 issue of Water & Wastes Digest :
Fixed or Variable? Small town achieves efficiency gains with blower station options
Chapmanville Water Department in Logan County, W.Va., provides the water distribution and wastewater treatment services in the town of Chapmanville, which has a population of 1,200. It wastewater plant processes about 400,000 gal per day (gpd). The town planned a major upgrade for 2019 and 2020, including the replacement of three 40 hp multi-stage centrifugal blowers commissioned 25 years earlier.
At the beginning of 2018, one of these was no longer in service. Chapmanville planners hoped to limp along on the two remaining blowers, but in the spring of 2018, they lost another one. Down to one blower, the planners knew they could not wait for the planned upgrade, and the department had to do something.
Read the full story online here: https://www.wwdmag.com/blowers/fixed-or-variable
Pressure Problems? Check Your Pipe Size First
By: Michael Camber
When it comes to compressed air piping, there are several options. Most manufacturers or process industries use black iron, galvanized, copper, aluminum or stainless. These may be threaded, brazed, welded or connected with various proprietary fittings. Generally, we recommend copper or aluminum as cost effective upgrades to black iron and galvanized. Lighter weight makes them easier to install, and their smooth inner walls reduce pressure drop. In some cases, stainless steel piping may be required to withstand water, cleaning agents or other corrosive substances in immediate proximity.
But regardless of pipe material, they all come in a variety of sizes, and a key point to understand is the relationship of pipe diameter to how much flow it can handle. It’s quite common for users to experience low pressure somewhere in their system. In some cases, this may mean there simply isn’t enough flow to meet demand and it’s time to increase system capacity with another compressor. But in many cases, new compressors are installed and the problem persists.
Pushing too much air though a pipe causes pressure drop. If your plant has grown over time and added production capacity, chances are it has acquired more compressors too. But if the piping hasn’t been upgraded, you may be suffering pressure drop caused by internal friction. You may mitigate the problem by adding storage downstream closer to points of use with large demands, but sometimes up-sizing the air lines is the only effective solution to get reliable pressure.
If you are putting in a new air system, you have the opportunity to plan for growth. It’s common for operators to allow space for future compressors as they grow, but adding compressors is far less disruptive and expensive than trying to re-pipe later when you find you cannot stuff all the air you need into the original piping. Better to bump up the pipe a size or two to allow for additional air flow that may be added later when you expand operations.
Below are two scenarios that illustrate the impact of pipe on pressure loss and how planning ahead can help.
Small system example:
You build a shop with two 10 hp units (total flow is approx. 80 cfm at 125 psig). Using 1″ diameter pipe, you will lose about 7 psi through 500 ft of piping at full flow. If you bumped it up to 1 ¼ ” that pressure drop would be less than 2 psi.
Two years later, due to business growth, you add another 10 hp. Now you have a max flow of 120 cfm. If you built the shop with the 1″ line, you would suffer 20 psi pressure drop. In the 1 ¼” line, the loss would be only about 4 psi. If you doubled the flow from 80 to 160 cfm, the pressure drop would be 30 psi in the 1″ line and only 7 psi in the 1 ¼”.
Larger system example:
You open a plant with two 100 hp compressors (total flow is approx. 900 cfm at 125 psig). Using 3″ diameter pipe, you will lose about 6 psi through 1000 ft of piping at full flow. If you bumped it up to 4″ that pressure drop would be less than 2 psi.
Two years later, due to business growth, you add another 100 hp. Now you have a max flow of 1350 cfm. If you built the shop with the 3″ line, you would suffer 15 psi pressure drop. In the 4″ line, the loss would be only about 2 psi. If you doubled the flow from 900 to 1800 cfm, the pressure drop would be 30 psi in the 3″ line and only 7 psi in the 4″.
The calculations above were based on Table 8.15 Loss of Air Pressure Due to Friction in the CAGI Handbook, Sixth Edition. If you are suffering pressure loss, check your pressure/flow combination in this chart before thinking of adding another compressor. If it seems like you have adequate piping, the next step is to find and fix leaks. Try to get leakage under control (less than 10% of demand) before adding compressors. Pressure drop through dryers and filters may compound the problem. There will always be some drop through air treatment components, but keeping up with maintenance will minimize pressure losses.
Another home remedy to overcome pressure drop is adjusting the compressor discharge pressure higher. If this is effective and necessary, by all means do it, but keep in mind that power consumption increases by 1% for every 2 psi increase. The larger the system, the larger the additional power costs from cranking up the pressure. Adding 5-10% to the 300 hp air system above could easily add $10,000-15,000 to your power bill. And in many cases it just won’t help. The pipe may simply be too small for the volume of air going through it.
Harder to calculate —but often more costly— are the scrap, lost production and downtime resulting from equipment getting low or inconsistent pressure. While those numbers vary from business to business, we’ve read the average cost of downtime for manufacturers is $23,000 per hour. Before you dismiss the need to upgrade your air piping, you owe it to yourself to do that math .
P.S. Our website has a handy pressure drop calculator!
By: Michael Camber
Later this month we are going to Cast Expo, the trade show for metal castings producers. In talking to colleagues about foundries and their applications, it is clear that compressed air is as vital in this industry as it is to most manufacturers. Unfortunately, while they rely on compressed air, the foundry environment is often horrible for air compressors and dryers.
In metal casting, the combination of high heat from molten metal and high loads of pervasive airborne particulate including silica, fly ash, and coke dust will almost certainly increase down time for maintenance and may reduce equipment life. Filter changes, oil changes and cooler cleaning must all be done more frequently to keep the compressors running within acceptable temperature ranges. Motors and electrical cabinets don’t do well with heat or particulate either (especially if it’s combustible). Further, high compressor discharge temperatures decrease the effectiveness of dryers downstream, resulting in more moisture in pneumatics. The stakes are high. One of our foundry customers calculates losses from downtime at $22,000 per hour.
In some cases, the ambient temperature is simply too high for the compressors and dryers to work well, even with aggressive maintenance plans. It’s not unheard of for compressor rooms in foundries to be 120°F. For the health and longevity of the compressors, and continuity of operations, it’s best to place the compressed air equipment as far from these conditions as possible. This is not always practical. At the very least, it may require longer pipe runs, and if space isn’t available in another part of the plant, a new building may be necessary.
Obviously, new construction gets pricey and time consuming. Building design, permitting and construction, take time and attention. An option is modular structures vs brick and mortar. Depending on design, these can be weatherproof and well ventilated to suit the needs of compressors. In some cases, the enclosures can be fabricated off site and delivered with the compressed air equipment pre-installed. This saves time and money on installation and doesn’t disrupt operations on site. It also takes less time to commission, and often doesn’t require construction permits. We’ve had several customers take this path with great success.
Whether building a new building, using pre-fabricated enclosures, or re-purposing existing space in the plant, careful attention to proper (temperature controlled) ventilation and dust control will be vital to compressed air system reliability. Keeping maintenance costs low and extending compressor life will ultimately pay off with reduced downtime. At $22,000 per hour, it’s worth the investment.