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.
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.
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.
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
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 .
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
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.
We spotted a good article in Plant Services this week by Ron Marshall, an independent consultant on compressed air efficiency. Ron reported on some compressed air related innovations he saw at the big International Manufacturing Technology Show (IMTS) last fall.
In addition to covering some new controls and motor designs from compressor manufacturers, he noted some other items he saw that can save energy by reducing compressed air demands.
Some plants divert compressed air into electrical cabinets for cooling effect. Ron mentions some alternatives that are much more energy efficient. On a related note, he pointed out that food processors or other plants needing frequent wash down sometimes use compressed air to create positive pressure in electrical cabinets to prevent moisture infiltration. This is effective but potentially wasteful unless the air is regulated to the minimum pressure required. Other items of interest were devices to detect leaks within pneumatic equipment and air saving nozzles for air knives. While we often emphasize methods to optimize the supply of compressed air, reducing demand-side inefficiencies is equally important.