Compressed air is likely the most expensive utility in your plant or shop. To evaluate the energy efficiency of your system, you must first examine the many factors affecting the efficiency.
Four of these factors would be: 1. The correct compressor type selection. 2. The compressor control system. 3. The correct air after treatment selection. 4. Determine the extent of your air leaks.
1. The correct compressor type selection.
There are a large variety of compressor types available for use in industry today. In the family of displacement compressors, there are:
· Reciprocating (piston type)
· Rotary screw
· Rotary vane
· ‘Roots’ type blowers.
In the family of dynamic compressors there are:
· Overhung pinion design
· Axial Flow
Each type of compressor has a range of applications where it is most suitable, in relation to final pressure, air quality and volume. Each one could have a specific control system based upon that application. It is generally accepted that multi-stage compressors are more power efficient than single-stage compressors, however, initial purchase cost for multi-stage compressors is greater. The two most common reasons for multistaging compressors are:
· To reach a higher pressure
· To increase energy efficiency
After determining which compressor design would best suit your requirements, the next question is “which control system is appropriate for my application”? The way this control system is used will determine the power efficiency of your compressors, but not your compressed air ‘system’. We will use the most common industrial compressor used in industry today as the subject of examination. That is the rotary screw type. These types of compressors are available in single-stage and two-stage versions, oil-free and oil injected types We must accept that two stage compressors are more thermally efficient, as some of the heat of compression is removed part way through the compression cycle.
2. The Compressor Control System
There are five basic control methods for these types of compressors:
2. Modulation (inlet throttle control)
3. Combination of Load/No-load and modulation
4. Rotor length adjustment (Spiral Valve, Turn Valve or Poppit Inlet Valve)
5. Variable Speed Drive/Variable Frequency Drive.
Load/no-load control is the most common capacity control available, as it is also used extensively on reciprocating compressors. This control allows the compressor to build pressure in the network until a pre-determined pressure limit has been reached in the compressed air discharge pipe (100 psig). Remember, the only way to achieve a positive pressure in a piping system of any kind is to supply more compressed air than you consume.
One Load/No-load scenario would be: The air demand in the plant is less than the supply available from the compressor, and the plant air network has risen to a 100-psig level. This is the maximum setting on the compressor pressure switch. The pressure switch senses this backpressure and allows the compressor to unload. This means that the compressor is not delivering any compressed air to the system. This does not mean that the compressor has shut down. The compressor will run for a predetermined amount of time in an un-loaded state (set on a timer) while it waits for the plant system air pressure to drop to a point. Once the system pressure drops to the low pressure set point on the pressure switch, (e.g. 90 psig) the compressor would start to load, and again start supplying compressed air to the piping network In the unloaded state, the compressor consumes between 15-25% of the installed break horsepower (BHP).
The main purpose of running the compressor unloaded is to prevent the compressor motor from starting from a locked rotor (stopped) starting position each time the network requires more air. That would have a detrimental effect on the motor in the form of temperature increase. With larger motors, the motor manufacturers have recommendations as to the number of “cold starts” that are acceptable. Locked rotor starting current is much greater and much more expensive than the current that is required by a motor which is already turning (running unloaded) In summary, the compressor consumes either the full load rating of the electric motor while at full load, and 15-25% of the full load rating at no load, using a conventional load/no-load type control. Another factor in running unloaded is the low part-load motor efficiencies while the compressor is in an unloaded state. Every time the compressor reduces the sump pressure while it is running unloaded, it takes additional HP to re-pressurize the tank back to operating pressure. The more cycles the compressor goes through, the more energy penalty there is. To avoid excessive cycling, the manufacturers recommend that a large air storage receiver tank be installed downstream of the compressor.
Same scenario with Modulation control: The air demand in the plant is less than the supply available from the compressor, and the plant air network has started to rise to the 100-psig level. As the pressure rises, the modulation control valve allows a flow of air to an inlet throttling butterfly valve. This could also be a ‘poppet’ type valve as well. The pressure signal controls the degree of closure of the inlet valve, reducing the flow of air into the compressor, in an attempt to match the air delivery to the demand. This means that the compressor is not delivering as much compressed air to the system. The compressor will run this way until the demand drops below the lowest turn down percentage allowed by the manufacturer and then turn off. Once the system pressure drops to the low pressure set point on the pressure switch, (e.g. 90 psig) the compressor would start to load, and again start supplying compressed air to the system piping network with the modulation valve fully opened. There are drawbacks to this type of control system as well. When you have a fixed pressure ratio compressor, which is set to a specific discharge pressure, the inlet absolute pressure (psia) will have a direct impact on the pressure ratio and ultimately the HP/CFM ratio.
By choking off the inlet to the compressor you have created a negative (vacuum) pressure under the inlet valve. To compensate for this negative pressure, you must consume more Horse Power relative to the delivered CFM. Therefore, operating a modulation type compressor at a high degree of turn down results in an inefficient production of compressed air. The most efficient way to run a positive displacement compressor is at full load regardless of the control methods. In almost all situations the demand for compressed air fluctuates. That is to say, as air consuming equipment start and stop, the demand increases or decreases. To meet this fluctuating demand with a control system that offers the least power penalty possible, there are three companies that have developed similar control systems to achieve this.
These control systems are called ‘Turn Valve’ or ‘Spiral Valve’ and “Poppit Inlet Control”. They work essentially the same way, and offer the same sort of savings. What is essentially happening with the ‘Spiral Valve’ and “Turn Valve” configuration is that as a valve is turned, ports in the compressor housing are opened to atmosphere on the inlet side of the screws, effectively shortening the length of the rotors. In this position the compressor produces less volume without creating the negative pressure at the inlet and consumes a lower kilowatt draw.
With the “Poppit Valve” configuration, inlet port orifices are opened to atmosphere effectively accomplishing the same shortening of the rotors.
There is a relationship between power consumption at partial load and at full load on each of the different control methods. It is important to note that the line normally representing On-line/Offline and Start/Stop on conventionally supplied graphs is usually shown as a serrated line. This line should be a solid line as there is no possible partial load air delivery with these control methods. Again, we have to remember, there is a power penalty when you run an electric motor at partial load. We must always evaluate the percentage of partial loading we can accept and seek alternatives when that limit is surpassed.
As to Variable Speed or Variable Frequency drive capacity control, this will be discussed in a future article as they are newer control methods than the ones previously described and apply to substantially fewer existing systems. Also not discussed here is the Reciprocating compressor’s variable cylinder pocket control method.
Regardless of the control method chosen, the general recommendation from the different manufacturers is that an air storage receiver tank should be located downstream of the compressor to smooth out the load cycles of the compressor.
The correct air after treatment selection.
The next consideration in evaluating your air system is the selection of an energy efficient air dryer and filter system. Why is this important?
1. Moisture in instrument air systems will foul delicate moving parts.
2. Liquid water will wash off the lubricant in pneumatic components, leading to premature wear.
3. In spray paint applications, the water will cause blistering of the paint and damage the finished product.
4. In some industries where molten metal comes in contact with compressed air, any liquid moisture or oil in the air can be disastrous, leading to serious injury, or death.
5. In cold climates, where the temperature of the ambient air coming in contact with the compressed air piping system may fall below 32 degrees F., the pipe, or other components may actually freeze due to the moisture in the system.
6. Moisture in compressed air piping leads to corrosion. These flakes of rust can be carried downstream by the compressed air, and into some of the valves and cylinders, blocking critical orifices.
7. Pipe scale and rust inside pipes creates drag (pressure drop), which must be overcome by increasing the pressure at the compressor. Increasing the pressure at the compressor will increase the horsepower consumption and lower the CFM delivered.The objective is to have the least pressure drop across the air system components as possible
To begin with, a coalescing type of filter is required to remove the water vapor from the compressed air. This filter should be located as close to the compressor after cooler’s moisture separator discharge as possible, as these coalescing type filters are designed to remove vapors and aerosols, not liquid flows.
Once this treatment has been accomplished, the decision must be made as to what degree of dryness is required by the process.
There are three basic dryer types to choose from. Each type has advantages and disadvantages.
1. Absorption (Deliquescent Type)
2. Condensation (Refrigeration Type)
3. Adsorption (Desiccant Type)
There are also a variety of ‘point of use’ dryers that are available to handle special circumstances. One would be the membrane dryer, which is a unique low capacity, low dewpoint dryer, with a high relative purge air requirement. Another would be the desiccant filled cartridge type filter/dryer. Both are typically used in small flow, critical point-of-use applications, requiring absolutely dry air.
From an energy standpoint, each of these dryer types (other than the deliquescent type) consumes energy either in direct electrical power consumption or purge air consumption. The purge air requirement for these dryer types range from 2% to 30% and this volume is far from being free.
There are many ways to evaluate the energy requirements of these dryer types, and many methods of reducing the purge air requirements of some dryer types. All of these methods should be investigated.
SOME RULES OF THUMB
· Most air compressors deliver 4-5 CFM/HP at 100 psig discharge pressure
· Every 2 psig of pressure increases or decreases the power draw of an air compressor by 1%
· Every 10° degree F. change in inlet temperature affects the efficiency about 1%. Colder temperature increases and warmer temperatures decrease efficiency.
· Power costs for 1 HP for 3 shifts, 7 days a week (8760 hours) at 10 cents/kWh =about $654.00/year.
· A 50 HP compressor rejects approximately 126,000 BTU/Hour. Approximately 119,000 BTU/Hour is recoverable.
Armed with this general information, you can estimate the cost of generating the purge air requirements of the different types of dryers. Remember to also add in the actual power costs for components such as heaters and blowers or any other power requirements of the dryer.
Determine the extent of your air leaks
As to the subject of air leaks, this example will tell the story very clearly:
If you have a 100 HP compressor, you can assume that the capacity will be about 4 CFM per installed HP, or 400 CFM at 100 PSIG as previously stated.
The average plant loses about 20-30% of their compressed air capacity to air leaks. This would equate to 80 – 120 CFM @ 100 PSIG in the above example.
Assume that the plant operates on a 2-shift, 16-hour day, 5- days per week, and 48 weeks per year. This would add up to 3840 HRS per year.
Assume that your power cost would be $0.10 per kWh Therefore: 80 CFM ÷ 4 CFM/HP = 20 HP 20 HP x.745 = 14.9 KW 14.9 kW x 3840 HRS x $0.10 = $7,449.60 / Year This is equivalent to a leak of just under ¼” 120 CFM ÷ 4 CFM/HP = 30 HP 30 HP x.745 = 22.35 kW 22.35 kW x 3840 HRS x $0.10 = $8,582.40 / Year. This is equivalent to a leak of just over ¼”
Multiply the above percentages based on your actual installed compressor H.P.
This is just an indication of the cost of your air leaks, and should be a sufficient motivational tool to justify their repair. Air audits are available to verify both the benchmark for your compressors and your air consumption through leakage. The price of this type of audit can easily be justified through cost savings, once the leaks have been identified and fixed.
One of the big concerns is the state of the compressors. Some companies have multiple compressors all running at partial load and some not delivering any air at all. We always recommend that your compressor supplier or repair facility check your compressors prior to an audit.
To see more information about audits, you may want to check out the following web site: http://www.impactrm.com
CFM/ Cubic Feet per Minute
SCFM / Standard Cubic Feet per Minute
FAD / Free Air Delivery
HP/ Horse Power
BHP/ Brake Horse Power
kWh – Kilowatt Hour
PSIG/ Pounds per Square Inch (Gauge)
PSIA/ Pounds per Square Inch (Absolute)
As previously mentioned. this article does not address all of the specifics of compressed air operation such as variable speed drive control or poppet valve rotor length adjustment. It does not go into depth on reciprocating compressors or dynamic compressors, nor does it address multiple driver options such as steam turbine drive or engine drivers. We have just touched a little on some of the most common issues faced by end-users of compressed air equipment.
The information in this article is meant as an overview of the basics and is available in the public domain, and has been for many years. The information presented is compiled in a logical format from many different sources and is not meant for specific compressor selection or system design.
For specific issues with existing equipment onsite, please contact your local compressed air supplier of choice. For specific discussions on many of these issues you may contact: Michael J. Morel
Morel Consultants Company