Compressor capacity controls

July 20, 2006
Evaluate them carefully to determine their effect on energy cost.

Consultants reviewing compressed air systems find that the single biggest energy recovery opportunity, particularly in applications with multiple compressor units, lies with misused or poorly applied compressed air unloading controls. We have seen many instances of a plant that embarked on a significant compressed air conservation program on the demand side addressing issues such as:

  • Identifying and repairing leaks.
  • Reducing open blows.
  • Adjusting automatic condensate drains.
  • Managing potential inappropriate use.

When the dust settles, some only find that, although they are using less compressed air for production, the actual electric energy consumed did not decrease proportionately and sometimes did not reduce at all. In one recent case, after repairing the leaks, the system pressure rose and the electric power consumed actually increased.

A common reason for this outcome is that the unloading controls are not working correctly. They are simply not responding to the system demand.

What are capacity (unloading) controls?

The two most effective ways to run an air compressor are at full load and off. Continuously-operating unloading controls restrict the air delivered to the system while the unit is still running. This is always a compromise and, on the basis of specific power (BHP/CFM), is never as efficient as operating at full load.

There has been a great deal written on how capacity controls function. Now it is time to look at what they can and can't do and some of the common problems that significantly affect their ability to perform.

Basic objectives of unloading

When working effectively, unloading controls do several things. They:

  • Match supply of air to the demand at the appropriate time and minimize or eliminate system overpressure while maintaining the necessary minimum acceptable operating pressure.
  • Reduce the input power cost to the optimum point (proportional to the air flow demand).
  • Turn off unneeded air compressors and activate them when required.

Basic types of controls

Before reviewing the types of capacity controls, we should define the operating pressure band, also called the dead band or proportional band. This is the pressure range the control can span from fully loaded flow to fully unloaded (no flow). Compressors larger than 50 hp use a 10-psi band, although others can be and are used.

There is a high cost to higher system pressure (1/2% per psi) and increased flow through unregulated users (1% per psi). Most well planned systems try to hold as narrow a band as possible unless there is a specific requirement for a larger band. Regardless of air compressor type, capacity controls fall into several basic categories. Some will only be available on certain types of compressors.

First is automatic start/stop. On any compressor, this control refers to the automatic starting and stopping of the electric motor or driver. Usually, a pressure switch shuts off the motor at the upper pressure limit and restarts the motor at a lower system pressure. Although operating either at full load or off is the most efficient way to run an air compressor, most AC electric motors tolerate only a limited number of starts over a given time for reasons of heat build up. This limits the application of automatic start/stop, particularly for motors larger than 10 to 25 hp. A large system must run above minimum system pressure to hold minimum pressure and performance is dependent on adequate effective storage.

Unloading with continuous-run controls implies the driver or electric motor continues to run while the air compressor is unloaded in some manner. The objective is to match supply to demand, usually on the basis of system pressure. Continuous run controls either can be step or modulating type.

Step controls are also called on line/off line; cut in/cut out; load/no load; two-step (or three-step, five-step). The most common is two-step control, which keeps the compressor inlet either fully open or fully shut. Over the complete operational band, the unit is at full load from the preset minimum pressure point (load point) to the preset maximum pressure (no load point). This control is available on every type of air compressor as either a primary unit or part of a dual-control system.

The basic performance is fully loaded or full flow at points throughout the operational pressure band up to the final preset maximum pressure when the air flow shuts off completely. The unit then stays at no flow and full idle until the system pressure falls to the preset minimum when the unit immediately goes to full flow capacity.

In this mode of control, the compressor runs at its two most efficient modes — full load and full idle — which represents the lowest possible input power cost. Full idle at lowest input power occurs almost immediately, except in the case of lubricated or lubricant-cooled rotary screw and centrifugal compressors.

With lubricant-cooled rotary screws, full idle and lowest input power does not occur until the oil sump pressure is bled down. This can often represent a time delay from 20 seconds to as much as two minutes. Centrifugal compressors often have some time delay built into the controls before they go to full idle. Double acting reciprocating compressors can also be equipped with three- and five-step unloading.

It is usually easy to tell if the unit is loaded or unloaded. Comparing the duty cycle gives an accurate reflection of actual flow as a percent of rated capacity.

Correct piping and adequate storage is necessary to allow sufficient idle time over the operational pressure band to generate significant energy savings. This is particularly true with lubricant-cooled rotary screws, which must cover the bleed down time before any significant power cost savings can occur.

When these controls are misapplied, particularly with regard to piping and storage, not only is there little or no power cost savings, but also short cycling can result. A duty cycle of 20 sec. on alternating with 20 sec. off damages the equipment and shortens the overall life of normal wearing parts, such as bearings, coolers, motors and air/oil separators.

Too much backpressure in the interconnecting system can lead to short cycling or ineffective unloading. At loads of 85 to 95%, these controls use some extra power since they have to compress at full capacity to a higher pressure to hold a lower design system pressures.

Modulating controls match supply to demand accurately everywhere in the operating band pressure range. Some type of regulator, which converts the operating pressure control band into a proportional band, controls most of these systems. Then, as system pressure fluctuates by even one psi, the modulating control immediately decreases or increases flow proportionally, depending on the signal. Modulating controls are generally used only on lubricant-cooled rotary screw and centrifugal compressors. In centrifugal units, modulation is usually called turndown. These controls are not used in reciprocating and non-lube or oil-free rotary screws.

The minimum system pressure generates the highest power draw. As the system demand falls, the power usage also falls compared to the two-step unloading in which the power draw actually increases as the system demand falls. There is a saving at higher demand.

At higher loads, modulation does not have to produce air at a higher pressure to maintain lower system minimum pressure. Thus, it is more power-efficient at high loads. It holds a relatively steady pressure during periods of somewhat stable plant air system demand. It offers quick response to a change in demand as sensed by system pressure. It does not depend on system storage to operate effectively, except when using the blowdown and idle mode with lubricant-cooled rotary units.

However, modulation is generally inefficient at lower loads, depending on the type of modulation, and must go to blowdown and idle for long periods at lower loads. Every type of modulation can effectively be combined with blow down and idle and auto start/stop.

Without proper capacity controls operating correctly, it is impossible to translate less air used into lower electrical input energy effectively. Sometimes it's very hard for the operator to identify the actual load condition, a situation that can always be corrected. Too much backpressure in the interconnecting piping can force units into part load — often resulting in multiple units all on at part load when one or more could be shut off.

The most commonly used air compressor larger than 30 hp is the lubricant-cooled rotary screw. A significant portion (80 to 85%) use some form of modulating control as the primary unloading control or as the upper range portion of a dual control (usually combined with two-step or blowdown and idle and automatic start/stop).

Modulating controls for oil-injected rotary screw compressors

With a throttled inlet, also called suction throttle control, the inlet valve opens and closes to match supply to demand as sensed by a pressure transmitter. The inlet valve moves continuously and immediately in response to changes in the sensed pressure to let restricting the air intake to control flow capacity. Throttled inlet is simple and relatively efficient at 60 to 100% load and it holds a constant system pressure with minimal valve movement at any system demand.

When used on a fixed displacement, lubricant-cooled rotary screw, this control reduces the absolute inlet pressure between the inlet valve and rotor intake. The reduction in inlet pressure reduces compressor efficiency. As the system demand decreases, the valve closes more, further reducing the inlet pressure so the compressor delivers less mass flow at a higher compression ratio. The net effect is a relative inefficiency at loads below 60%.

Throttled inlet controls typically rely on a ten-psi operating pressure band (some are 15 psi, or more) to control the compressor's response to changes in demand. Setting this type of control for a full-load operating pressure of 100 psi means that the inlet valve is completely open at any sensed pressure less than 100 psi.

A rise in sensed system pressure above this setting indicates the air users are no longer consuming the full capacity of the compressor. The excess compressor capacity is raising system pressure. As soon as the pressure rises a bit, a signal immediately starts to close the inlet valve to reduce and balance compressor capacity with demand. This reduced flow at higher pressure is equivalent to an overall reduction in input power.

Smooth non-cycling pressure control is easier on the power train and most unit components. It is more efficient at higher loads than two-step because:

  • Modulating controls do not have to run at full capacity at higher than the minimum system design pressure.
  • It will not short cycle regardless of storage capacity or volume in the piping.
  • It is simple to run and maintain.
  • The higher pressure at lower demand usually results in reduced lubricant carryover to the system in lubricated rotary screws.

On the other hand, it is relatively inefficient at lower demand loads, but can be equipped to blow down and idle. Also, it must overcome excessive backpressure in getting the system to full capacity. Its instant response means it will often be the first machine to back down and unload even when you want it to base load. For any type of unloading control, its sensitivity and immediate reaction makes proper piping and backpressure control a necessity for optimum operation.

Variable displacement controls are used primarily in lubricant-cooled rotary screws. The controls are also called geometric, rotor length adjustment and the like. These controls match output to demand by modifying or controlling the effective length of the rotor compression volume. Throughout the turndown range, the inlet pressure remains constant and the compressor ratio is relatively stable.

This method of reducing flow without increasing the compression ratio offers a power advantage over modulating and two-step controls in the operating range from 50 to 100% load.

There are two common versions of variable displacement unloading controls. The spiral-cut high-lead valve opens or closes selected ports in the compressor cylinder. The poppet valve control uses some form of poppet to open and close the ports.

The ports located at the beginning of the compression cycle see low pressure. Opening these ports even a small amount prevents any compression until the rotor tip passes the cylinder bore casing that separates the ports. This effectively reduces the volume of trapped air to be compressed and the horsepower required to do it.

With the spiral-cut high-lead (also called turn valve), some of the compressed air is trapped in closed pockets — openings in the cylinder. As the rotor edge passes over the opening, some of the trapped, higher-pressure air leaks back to the trailing lower pressure cell. This has a negative effect on efficiency at higher loads (up to four% claimed).

The poppet valve control operates much the same way as the spiral or turn valve except the control ports are opened and closed by a double-acting poppet valve. This reduces the amount of compressed air leakage at the higher load conditions by creating a moving seal off point without any significant cavity to hold high-pressure air.

These controls operate relatively efficiently from about 50 to 100% turndown, although some will lose efficiency at higher loads from the leakage described above.

In actual operation at 50 to 60% load, the variable displacement control usually switches to load/no load (two-step) or modulation, depending on make, model and installation considerations. Electronic control systems allow this switching and setting to be automatic for installations that allow it.

Variable displacement controls offer efficient part-load performance from 50% to 100%. They maintain set pressure at minimum system pressure, and are very responsive. However, at higher loads, some units suffer efficiency loss from increased leakage. They are somewhat more complex compared to conventional throttled inlets. They still must run a two-step or modulation to reduce operating surge.

Variable speed drive

The unloading performance curve for a variable speed drive is attractive, in theory. Depending on the compressor model and installation, variable drives can offer optimum unloading from about 55 to 100%. This means that 75% flow would occur close to 75% power input.Variable speed drives have been used for years on every type of compressor, but generally as specifically designed and engineered products. In the world of "factory standard products for general industry," the lubricant-cooled rotary screw has always been a prime target for variable drives. The most commonly applied drive for retrofits or specials has been the variable frequency drive.

Variable frequency drives convert 60 Hz alternating current to direct current and then reconvert it to the proper frequency required to turn the drive motor at the desired speed. The variable frequency drive is less efficient at full load compared to other types of modulation controls because the electrical conversions usually require an additional two to four percent more energy. There have been many variable frequency drives installed successfully on lubricant-cooled rotary screw packages over the years, but there are some areas of concern that limits their economies relative to cost and overall performance.

The first issue is air end performance. Lubricant-cooled rotary screw compressors operate on bell-shaped curve of efficiency as a function of tip speed. The speed is,in turn, a function of rotor diameter and rpm. There is an optimum speed range in which the air end is most efficient. Above or below that, efficiency deteriorates for various reasons.

If the compressor has been designed for optimum efficiency at full load, at some point in the speed turndown the efficiency falls and the torque rises. Thus, we have the average limit of 50 to 60% of full load to retain reasonable efficiency.

If the air end speed is placed on the leading edge of the bell curve, then there is probably less overall efficiency at full load, but a greater turndown range with reasonable efficiencies.

This applies to the general characteristics of lubricant-cooled rotary screw profiles. Of course, it may be possible to design air ends with flatter performance curves.

The second issue is pulsation amplification. Rotary screws are often referred to as "pulsation free" but, in reality, there are pulsations. A reciprocating unit exhibits high pulsation displacement at a low frequency. The higher-rpm screw unit pulses every time a female lobe passes the discharge port to generate low-intensity pulsation having a high frequency.

Most air ends in use today were designed for a single speed and harmonics were reviewed only at this speed. But, with a variable frequency drive, the unit can run at any speed. Experience has shown that putting a variable frequency drive on such air ends leaves one vulnerable to a speed that exhibits harmonic amplification problems. Field adjustments will be needed to avoid operating at that speed for any significant time.

The third issue is the induction electric driver limitation. Induction electric motors have many design variations and, in a retrofit situation, you should be concerned about what happens to the mechanical efficiency and power factor at lower speeds for each motor. You also have to consider heat rejection and cooling capacity. These factors affect efficiency, payback and equipment operating integrity.

A factory-designed variable frequency drive package can work with alternative air end designs more suited for variable speed operation with motors carefully selected for the application.

Switched reluctance drives

Another type of variable speed drive being developed for new lubricated-rotary screws is the switched reluctance drive system. This drive converts a standard three-phase AC power supply into a high-voltage DC power supply. High-current diodes rectify the AC power supply to produce a large DC voltage. The DC supply is connected to a bank of capacitors, which stores the electrical power required by each motor phase pulse to eliminate main power supply surge currents. The final drive is a DC motor.

Variable speed drives offer attractive unloading results that nearly make input power proportional to per cent of load. As a method of retrofitting older or existing design units, it has met with mixed results. Several major manufacturers are putting a great deal of research and development into this concept. It is something that will continue to present itself to the market in new products over the coming years.

Variable speeds drives maintain the system pressure at the minimum setpoint and modulate back as soon as the sensed system pressure increases.

Main points--rotary screws

At full load, all controls are equal.

A modulating control's highest BHP point is at the minimum system design pressure. Pressures above that are at a reduced flow and lower BHP draw.

Load/no load controls require the lower system setpoint to be the minimum system pressure. This control operates at full flow capacity until the maximum system pressure setpoint is reached. Each psi above the setpoint draws about 1/2% more horsepower.

Oil-free or non-lubricated rotary screws and reciprocating compressors immediately close off the air flow and go to full idle when the high-pressure setpoint is reached.

Lubricated or lubricant-cooled rotary screw compressors cannot reach full idle until the airflow is stopped and the air/oil separator reservoir is blown down to a significantly reduced pressure. The resultant time delay is a function of how fast this can be accomplished without causing functional problems at the air/oil separator. A small compressor (15 to 20 hp) may bleed down in 15 to 20 seconds, a mid-sized unit (100 hp) may require 30 to 45 seconds. A larger unit (300 to 500 hp) may take one to two minutes.

In any event, in a lubricated or lubricant-cooled rotary compressor, the full idle power savings of the no load mode does not start until the air/oil sump pressure is fully bled down. This time frame is critical in a power analysis and varies from machine to machine.

Variable displacement controls offer better turndown efficiencies to about 50% load followed by modulation or two-step with adequate storage. Some may lose from two to five percent of this advantage at the full load. They still require adequate storage and piping to respond and run well with significant varying demands.

Variable speed drives offer attractive unloading characteristics. They very nearly make input power proportional to percent of load. Until recently, the most common variable drive was the variable frequency drive and has been limited to retrofit existing unit and special packages using selected conventional air ends and induction motors.

This is an instant reacting modulating type control that maintains the system pressure at the minimum setpoint. At full loads, there is a power penalty of two to five percent.

This data is based on the general performance curves published by the U.S. Department of Energy, Compressed Air Challenge and reflects a consensus of the members of the Compressed Air and Gas Institute. These are general guidelines that reflect what occurs in the operating pressure band with each type of unloading control. For performance on any specific unit, consult the manufacturer.

Lubricated rotary screw compressor

At the unload pressure, two-step unloading controls immediately activate the bleed down like every other modulating control. After the bleed down of sump pressure, the BHP falls to the value corresponding to full idle — about 25% of full speed. The net effect of this on actual power cost is a function of bleed down time, effective control storage and subsequent length of idle time. The same is true of a centrifugal compressor with its bleed down time when comparing it to an oil-free rotary screw without the bleed down.

At the unload point, two-step reciprocating or two-step oil-free compressors quickly go to full idle power draw since they do not have to bleed down. Some of the newer variable speed drives equipped with custom rotary screw air ends and drives are apparently able to pull much deeper into the unload cycle (down to 20%, or less) with a proportional BHP draw. They then default to modulation or load/no load.

Throttled inlet modulation and variable displacement modulation can also default to modulation blow down or load/no load anywhere along the operating band they choose, depending on conditions.

The estimated BHP is the power required at the shaft of the compressor. To estimate actual input power you must also consider:

  • Mechanical inefficiencies between the motor and compressor.
  • Motor efficiency and power factor for the specific motor at the given conditions.

A control method not listed is "blowing off" when the compressor "makes too much air." A pressure-sensing valve opens and the excess air blows off to atmosphere. Since there is absolutely no input power savings at this point, simply blowing off is the unloading control of last resort and should be avoided whenever possible. Blow off controls are most often used on:

  • Centrifugal compressors to avoid surge when they are equipped with is simple turndown or throttled inlet modulation, with or without inlet guide valves. Centrifugal compressors are available with dual-control unloading that incorporates turndown and load/no load to avoid blow off.
  • Blow off for reciprocating compressors, usually smaller horsepower, single-acting type, is much less costly to install than either free air or total inlet closure, but there is no energy savings and it can be very hard on the unit.
  • Oil-free rotary screws are sometimes installed to correct a short-cycling problem instead of correcting the installation conditions that cause the short cycling.

Basic facts to keep in mind

With any type of step control, reduced input power only occurs when the compressor reaches full idle. The total savings are a function of the full idle BHP and time at idle. This applies to modulating controls with blowdown and idle or low-demand load/no load mode. Idle time is a function of net flow out and effective storage volume. Effective storage volume is limited to the pipe, receiver and component volume.

For example, assume a compressor system with a full-load operating pressure drop of 14.6 psid. This 1,500-acfm compressor with a 10-psi operating pressure band might see only the following effective storage volume:

  • 250 ft. of 4-inch pipe = 165 gallons
  • 114 ft. of 4-inch pipe  = 75 gallons
  • Dryer (estimated) = 20 gallons
  • Filter housing (estimates) = 20 gallons

The total effective storage volume (280 gallons) divided by the flow rate (1,500 acfm) is equal to 0.186 gal/cfm. With this installation several things are evident:

  • Two-step controls will certainly short cycle, which is hard on the equipment and offers no real savings.
  • A modulating control unit set to deliver a 100 psig system pressure will have to pull down to at least 86.4 psig to get to full load operation.

The air system these controls are sensing is between the compressor discharge and the filter. It is neither sensing nor reacting to the production air system. For all intents and purposes, this compressed air system is out of control.

Pressure driven control systems react only to the actual pressure they sense. If we want the total production air system header piping and local storage to have any positive benefit to the controlled cycle time, we must get into the main header piping well below the 10 psi operating pressure band.

Yet the scenario shown here — five psid for the dryer, eight psid for a filter and 4-inch pipe on a 300-hp system — is fairly common and somewhat of an accepted standard. But, you don't have to accept this situation. There are filters with a pressure drop of one psid or less. The dryer can be carefully selected and oversized, within limits, for less pressure drop. Larger pipe can be run at probably not a great deal more cost.

This can be combined with a large storage receiver and regulated out flow to create a very effective supply side control that supplies compressed air as required, on demand, at a steady pressure.

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