A commonly recommended energy strategy for a lubricant-injected rotary screw air compressor is to add main storage receiver capacity to enable the switch from inefficient modulation control to the more efficient load/unload mode. A typical guideline was to add 1-2 gal of storage receiver volume for every cfm of main compressor capacity.
While this amount of storage is better than nothing, this rule represents old thinking that was used for large reciprocating compressors and might be draining your pocketbook. And even when systems have greater volumes of storage, it’s common to discover this measure hasn’t produced the expected results. Further, the cause might not be what you expect.
To understand the issue we must look inside a typical lubricated rotary-screw air compressor (Figure 1). Each unit has an air/lubricant separator and sump assembly that filters and collects the compressor lubricant that was injected into the main compression elements during normal operation.
When a compressor is loaded, this sump is fully pressurized. To gain power savings when a compressor unloads, the compressor inlet valve closes and the sump is vented to atmosphere. During this operation, the unit compresses only small amounts of air, and at a low pressure, so its power consumption is reduced to a much lower level, typically about 25% of the fully loaded power.The main compressor control initiates this load/unload operation within an adjustable pressure band. The most commonly observed pressure band width is 10 psi, measured at the outlet of the compressor package. While it would be nice to be able to load and unload the compressor instantaneously, unfortunately, it takes a few seconds to pressurize the sump fully when the compressor loads, and significantly more time to release the sump pressure when the unit unloads. While these transitions are taking place, the compressor is consuming power over and above what it takes to produce the compressed air the system actually requires
Load/unload efficiency versus storage
Storage receiver volume affects how often a compressor loads and unloads (cycles). The more frequently a compressor cycles within a given time period, the less efficient it is. Figure 3 shows how storage capacity can affect the power vs. capacity curve of a lubricated-screw compressor. A perfect compressor, with instantaneous sump pressurization and depressurization, has a power curve characterized by a straight line between fully loaded and unloaded. The actual characteristic exhibits a definite hump, the less storage, the more pronounced the shape and the lower the compressor efficiency, especially at lower load levels.
But there’s more to the story. Two other important factors affect screw-compressor load/unload efficiency: the time it takes to depressurize the sump and pressure band width. The sump depressurization time typically is fixed, a characteristic of the compressor make and model and the sump size. The longer that depressurization takes, the less efficient each compressor cycle becomes. It’s important to maintain the unload circuit in good working order, especially the blowdown vent.
Cycle frequency: It’s simple mathematics
The power/capacity relationship shown assumes a typical 10-psi pressure band and a 45-second sump blow-down time. But if the pressure band is doubled to 20 psi, the frequency of the compressor cycles will drop by half. If increased to 30 psi, the frequency will drop by a factor of three, a simple ratio. If the same average pressure is maintained, a compressor with a 30-psi pressure band with 1 gal/cfm storage will actually follow the 3 gal/cfm curve.
The same simple arithmetic applies if the storage volume is doubled or tripled or increased by a factor of 10. A compressor with storage of 5 gal/cfm will cycle at a frequency of one-fifth that of a unit with 1 gal/cfm and, as a result, will run more efficiently.
But let’s turn this around a bit and suppose the pressure band is reduced to one-third of its original value. What happens to compressor efficiency? It gets worse. And this is where we need to explore some real-life situations.
How filters can cost you big
You’ve probably heard the old story that compressed air filter differential pressure (ΔP) can cost you extra energy — about one percent more power is consumed for every 2 psi in higher compressor discharge pressure.
This relationship is true, but there’s more to the story when it comes to the interaction of ΔP with compressor control and the storage receiver. The key is the location of the compressor control sensing in relation to the main storage capacity. An unloaded compressor produces no air flow, so no pressure differential develops across the associated air filter, typically located between the compressor and the main receiver. When the compressor loads, however, it produces full flow and full pressure differential across the filter.
When the compressor cycles, the compressor control loads and unloads according to the pressure at the control sensing point. Thus, if the set pressure range is 10 psi, there will be a 10-psi band at the sensing point. But, the effective pressure band at the storage receiver is the difference between the compressor discharge and filter differential pressure. This means if the filter ΔP is 3 psi, the effective pressure band for cycle time calculation is only 7 psi, or 30% less. Serious efficiency problems can develop if the filter(s) load up to ΔPs approaching the pressure band because it consumes the entire available pressure band.
Air dryers: Go big or go home
Another common element located between the compressor and the main storage receiver is the air dryer. Often, the largest storage receiver is placed downstream of the dryer to prevent the large flow transients that excessively pulse load the dryer. Air dryers have differential pressure ratings that affect the compressor/receiver interaction in the same way as filter pressure drop. This is why it’s important to assess the rated ΔP of any air dryer carefully before you purchase and install it. It’s also important to consider if the dryer has an internal filter element that might occasionally need changing and might add to the dryer’s rated pressure drop.
The variety of compressor output capacities and dryer models often provides various sizing options. In choosing, it’s important to realize the pressure differential varies with the square of the flow. This has some benefits when it comes to oversizing components to reduce ΔP and the interaction with storage capacity. For example, if a dryer with a rated ΔP of 4 psi at full-rated flow is oversized by 50% to account for temperature conditions, it would develop only 45% of the rated ΔP (1.8 psi) during loaded conditions.
Distribution: Savings down the pipes
It’s always important to consider pipe size, length and complexity in relation to compressor efficiency and storage receiver interaction. Piping ΔP also has a square function relationship and shouldn’t be left to amateurs to install. Neither should you blindly size the piping to the same size as the compressor outlet. In some cases, the compressor discharge size is marginal. You always should check the piping size against the expected peak flow during worst-case conditions and size accordingly for minimal ΔP.
And consider this piece of piping magic: Often, increasing pipe diameter to the next size up greatly reduces the associated pressure differential. For example, a 1-1/2-inch pipe passing 200 scfm at 100 psi has more than 3.5 times the ΔP as a 2-inch line passing the same flow.
Why unload at all?
Normally, adding larger volumes of storage follows the law of diminishing returns. Under certain conditions, however, an interesting thing happens when you apply very large storage at wider than normal pressure bands.
For some combinations of pressure bands, storage and percent loading, the cycle frequency reduces to the point where it’s possible to run the compressors in start/stop mode and not accumulate any unloaded operating hours. Often, in the case of electronic “smart” compressor controls, the compressor will keep track of the cycle frequency and shut the unit off completely, when appropriate. This minimizes and sometimes eliminates the areas of unloaded inefficiency shown in the shaded areas in Figure 2. This measure can reduce the power consumption significantly, especially for lightly loaded systems such as those found in service shops.
Figure 4. Cycle frequency as a function of loading, pressure differential and storage volume. (Source: Manitoba Hydro
It’s important to check with the manufacturer to ensure the unit can operate in start/stop mode and the unit’s pressure rating isn’t exceeded. Figure 4 shows the cycle frequency characteristics of three combinations of storage bands and pressure volumes.
Of course, another more obvious way to eliminate unloaded power consumption is to use a variable-speed drive (VSD) compressor. Eliminating differential pressure and using large storage also benefits this style of operation in many significant ways, perhaps a subject for a future article. Many power utilities offer significant incentives to assist in defraying the purchase cost, which leaves you to enjoy the benefits for years to come.
The points to remember are three.
- The less frequently the compressor unloads, the more efficient it is.
- Pressure differentials across filters, dryers and piping should be considered together for better efficiency. When implementing load/unload control, it’s important to ensure differential pressure is at a minimum.
- If at all possible, turn the compressor off to minimize unloaded power consumption.