Energy Efficiency Reference/Industrial/Compressed Air/Recommendations

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Recommendations[edit]

This section contains methods for estimating savings from efficiency improvements. Overview this section at the plant to determine which recommendation are feasible and what data is required to accurately calculate savings.

Recommendation #1 - Reduce Air Leaks[edit]

Summary: Reduce the amount of compressed air lost to leaks.

When to Apply: Generally, reducing leaks to the following percentages of plant loads is possible:

  • 30% high power, dirty environments, ex. Sawmills
  • 20% medium power environments, ex. Plastic manufactoring
  • 10% light power, clean environments, ex. Electronics

If leak load is smaller than this the action will be more difficult to justify economically. On air compressors that only operate part time, the payback periods are longer.

Key Engineering Concepts:

  • Reducing airflow saves some energy.
  • Turning off a compressor provides more significant savings.
  • Avoiding the purchase of a new compressor saves both capital and energy expenses.
  • Although it is not direct, compressor energy use is related to the amount of air compressed.

Preparation:

  • Data Required
    • All Standard Air Compressor Data, including leak load. (See Section 3.)
  • Tools Required
    • DMM or Ammeter or Power Analyzer
    • Air Compressor Data Worksheets
    • Measured Operating Conditions worksheets
    • Air Compressor Savings Worksheets

Analysis Process[edit]

1. Enter Existing Operating Conditions on Worksheets.

See Section 5. Analysis Tools & Methods

2. Calculate Peak Plant Load.

The peak plant load is the peak production airflow minus air leaks.

  • Plant Load = Peak Production Load - Leak Load

3. Calculate Existing Leak Load as a percentage of the Peak Plant Load.

The Leak Load Percentage relates the air lost to leaks to air used by the plant.

  • Leak Load % = Leak Load / Plant Load

4. Enter Proposed Leak Percentage

A typical proposed leak load is 30% of plant load for a relatively leaky system, 2% is fairly average, and 10% is a tight, clean system. if the current leak load is at or below 30% of plant load, this recommendation is not likely to offer significant savings. Calculate Proposed Leak Load:

  • Proposed Leak Load = Plant Load x Proposed Leak Load Percentage

5. Calculate Proposed Airflows for each operating load.

  • Proposed Airflow = Existing Airflow - Existing Leak Load %C + Proposed Leak Load %C

6. Calculate Proposed energy use for each operating condition

Calculate proposed energy use using proposed airflows. Also, see Section 5. Analysis Tools & Methods.

7. Complete the Air Compressor Savings Worksheets for reduced air leaks.

8. Estimate implementation cost and determine payback

Leaks typically arise faulty fittings, lines, valves, and hoses. These leaks are generally easy to repair, with little or no material cost. If pneumatic rams or cylinders are leaking, they will have to be rebuilt or replaced. Rebuild kits and labor may cost severally hundred dollars, depending on the size and location of the leak. Typically repairing leaks has a payback of less than one year.

Recommendation #2 - Use Unload Controls[edit]

Summary: Use low-unload or load-unload controls with a modulating compressor to increase the part load efficiency.

When to Apply: Modulating compressor have poor part load efficiencies. Any time a modulating compressor operates at a part load there may be an opportunity for savings with unloading controls. This recommendation applies best to modulating compressors operating at part load for a significant percentage of time. Sometimes the unloading controls are already present, but are not being used.

Key Engineering Concepts:

  • Inlet and discharge pressure are reduced when a compressor unloads. Lower pressure result in lower power requirements, generally in the range of 20% of full load power when discharge pressure is reduced to atmospheric pressure.
  • Unload controls allow an air compressor to unload when airflow drops or system pressure increases to a predetermined set point.
  • Successful implementation may require adding receiver capacity. The air system must also allow pressure variations up to 10 psi typically.

Preparation:

  • Data Required:
    • All Standard Air Compressor Data. (See Section 3)
    • Proposed Unload Point and existing Unloaded Operation Conditions.

Analysis Process[edit]

1. Calculate Air Compressor Energy Use for Existing Operating Conditions.

(See Section 5)

2. Determine a Proposed Unload Point.

Choosing an unload control strategy will determine the unloading point. Load-unload controls dictate a 100% Capacity Unload Point, while Low-Unload controls will have an unload point that is adjustable or set by the compressor manufacturer. The compressor manufacturer will quote both an Unloaded Power and a range for the Unload point. Use this information from the manufacturer or dealer to choose a proposed unload point and calculate proposed power for all operating conditions of interest.

3. Calculate Proposed Power.

Airflow will not change for any loads when adding unload controls. Calculate power for typical plant operating points using the appropriate formula.

  • If airflow < Unload Point (%C<%CUL):
  • Power = (Unload Point %P - Unloaded %P) x Load %C + No Load %P

If airflow is above the Unload Point, calculate power with the appropriate formula for the appropriate type of modulating controls. Refer to Section 5. Analysis Tools & Methods.

4. Calculate Energy Use for Proposed Operating Conditions.

See Section 5. Analysis Tools & Methods.

5. Determine energy savings.

Use differences in proposed and existing Operating Conditions to calculate energy and dollar savings.

6. Estimate implementation cost and determine payback.

Costs for implementing this recommendation can include unloading controls, additional receiver capacity, and labor.

Prices for unloading controls vary from compressor to compressor. contact the manufacturer for their prices. Estimate $1,500 to retrofit, $700 option for a new compressor.

Unload controls perform better if the system has adequate receiver capacity. The receiver must be large enough to allow the air compressor to stay unloaded for at least 2 minutes per cycle. To size system storage, a rule of thumb estimate is one gallon of storage for each CFM of peak airflow. Estimate $4/gallon of received capacity installed.

Alternately, you can get vendor quotes for receivers and estimate labor time and cost to install receivers. Many plants have maintenance crews that could install additional receivers. Generally, labor costs for outside contractors run $50 to $60 an hour with the installation requiring from 5 to 10 hours.

Recommendation #3 - Reduce System Pressure[edit]

Summary: Power is required to compress air atmospheric pressure to system pressure. A lower system pressure will require less power per CFM. Set compressors to the lowest possible pressure required by plant equipment, plus line losses between the air compressors and equipment.

When to Apply: This recommendation applies when system pressure is significantly greater than required by end-uses. There may be pressure-regulators on some equipment to limit pressure. If the pressure drop across regulators is significant, reduce system pressure. This may required segregating end uses by pressure requirements.

Key Engineering Concepts:

  • Compressor power decreases approximately 1/2% for each psi reduction in discharge pressure.
  • With reduced system pressure, air lost to regulated air uses is reduced by approximately 3/4% per psi.
  • The minimum pressure for oil separators is typically 80 psi but it varies from compressor to compressor.

Preparation:

  • Data Required
    • All Standard Air Compressor Data. (See Section 3.)
    • System Operating Pressure
    • Minimum required pressure
  • Tools required
    • DMM
    • Air Compressor data sheets

Analysis process[edit]

1. Calculate Air Compressor Power for Existing Operating Conditions

See Section 5. Analysis Tools & Methods.

2. Determine Proposed Full Load Power with reduced pressure.

Full load motor power decreases by reducing pressure differential across the compressor.

  • Full Load Power = Existing Full Load Power x ((ln(proposed pressure/atmospheric pressure))/(ln(existing pressure/atmospheric pressure)))

3. Calculate Reduction in Airflow

By reducing system there will be less airflow to most loads, including leaks.

  • Proposed Airflow = Existing Airflow x (Proposed Pressure + Atm. Pressure)/(Existing Pressure + Atm. Pressure)

4. Calculate Air Compressor Energy Use for Proposed Operating Conditions

See Section 5. Analysis Tools & Methods.

5. Determine Energy Savings

Use differences in proposed and existing Operating Conditions to calculate energy and dollar savings.

6. Estimate implementation cost and determine payback

Implementation for this recommendation is often negligible, as the only requirement is to adjust the pressure of the compressor. However, if excessive system losses require high pressure then the implementation cost will include correcting this situation.

Recommendation #4 - Replace Air Dryer[edit]

Summary:Industrial processes require varying amounts and dryness of dried air. Several ypes of air dryers exist that dry air to varying dew points using different methods.

When to Apply: Dry air only to a minimum dew point required for plant operations. When the volume of air being dried or dryness is beyond what is required, there is an opportunity for savings.

Key Engineering Concepts: Atmospheric air contains water vapor. When compressed air cools, this vapor condenses and causes problems with corrosion and air uses that are sensitive to water. Drying air is an energy intensive process.

  • Data Required
    • All Standard Air Compressor Data.
    • Type of air dryer
    • Air Dryer Energy Use
    • The Required dew point for plant air
  • Tools Required
    • DMM
    • Air Compressor data sheets

Analysis Process[edit]

1. Estimate Existing air dryer operating costs.

Air dryers are expensive to operate and maintain. Costs arise from a number of sources. Calculate the cost of each of the following:

Direct Energy Requirements (DER) Refrigerated dryers use energy to cool the compressed air. Heated desiccant dryers use energy to heat the compressed air used to purge the desiccant beds. Calculate refrigeration and heating energy by measuring and recording volts and amps or power being used by the dryers.

Pressure Drop(PD) Air Dryers introduce a pressure drop to the air system that costs an extra 1/2% power from the comrpessor for each psi of pressure drop. Refrigerated air dryers will induce a pressure drop of about 5 psi. Desiccant air dryers cause a negligible pressure drop. Membrane air dryers will have a pressure drop of about (I am still looking for this). If gauges are available, compare pressures before and after the air dryer to determine pressure drop.

Air to Regenerate Desiccant Tanks (PA) Desiccant air dryers use dry air to regenerate the desiccant beds. Heated dryers heat the compressed air before passing it through the beds, while others pass unheated dry air through the beds. The added energy required to heat the air is more than offset by the amount of compressed air saved. The cost of compressed air used to regenerate desiccant beds can be calculated by estimating the airflow (acfm) required to regenerate, and referring to Section 5. Analysis Tools & Methods.

Purge frequency can be controlled by two means: capacity control (constant time interval); or moisture control.

Capacity control diverts a portion of the dried compressed airflow through the desiccant beds, usually for a fixed time interval. Then, the regenerated desiccant bed is switched on line while the other bed is regenerated.

The moisture (humidity) sensor initiates regeneration when a set level of moisture is reached in the desiccant bed, and can also terminate regeneration when the bed is dry. This type is generally more efficient as only the amount of dry air required is used. Estimate the regeneration airflow by observing the time that the desiccant beds are being regenerated and from manufacturer's specifications.

Replacing Desiccants and Filters (CP) Desiccant dryers require periodic replacement of desiccant material. Depending on the quantity and frequency of replacement this cost can be substantial. Plant personnel might have records of how often the desiccant is replaced.

Maintenance Requirements (MR) Membrane type dryers claim to be virtually maintenance free. Refrigerated and desiccant dryers all have parts that can wear and will need maintenance. The plant personnel should be able to estimate maintenance requirements.

The total cost of drying air is the sum of the following costs:

  • Direct Energy
  • Pressure Drop
  • Regeneration Air
  • Consumable Parts
  • Maintenance

2. Determine appropriate Dew Point.

In mild climates it is not often enough where airlines are exposed to outdoor temperatures to require dew points below about 35 degrees to 50 degrees F. However, some locations and applications do require dryer air. Ask Plant personnel about how dry the air needs to be. If they are drying the air much below 35 degrees F confirm that they really require air that dry. if there are only one or two pieces of equipment that require extremely dry air consider isolating those air uses.

3. Choose appropriate air dryer.

Use catalogs and distributors to select an appropriate air dryer. Estimate the operating costs for that dryer. Estimate the energy required by a new dryer from manufacturer's specifications. Available types of air dryers vary in cost and ability to dry air. Following is a brief guide to the operating costs and capabilities of various air drying methods.

Refrigerated Air Dryers are usually the best option for plants without extreme drying requirements. Refrigerated dryers are effective for drying air down to 35 degrees - 50 degrees F.

Desiccant Air Dryers are required for dew points below 35 degrees F. The most efficient desiccant air dryers include capacity controlled regeneration and a system to heat the dry air before regeneration.

Membrane Air Dryers are capable of reaching similar dew point to refrigerated dryers however they cause a larger pressure drop through the membrane.

4. Determine energy savings.

Calculate differences in proposed and existing Operating Conditions to determine energy and dollar savings.

5. Estimate implementation cost and determine payback.

Implementation cost includes the air dryer and installation. Rough installed costs/ 1000 SCFM are:

  • Refrigerated air dryer - $10,000
  • Heated Regenerative air dryers - $25,000
  • Heater-less Regenerative air dryer -= $12,500

Recommendation #5 - Isolated Air Uses[edit]

Summary:Often plants have one large air compressor providing air to all applications. Isolating high pressure, high hour or particularly low moisture requirements form the main air system can save energy.

When to Apply: When one application dictates the pressure, drying requirement, and/or the operating hours for the compressed air system while all of the other uses could be met with lower pressure air, fewer operating hours, or less energy intensive air drying processing.

Key Engineering Concepts: Power and energy are proportional to the pressure difference. Stringent drying requirements are expensive to maintain.

Preparation:

  • Data Required
    • All Standard Air Compressor Data. (See Section 3)
    • Required Airflow, Pressure and Operating Hours of processes that can be isolated.
  • Tools Required
    • DMM
    • Air Compressor data sheets

Analysis Process[edit]

1. Identify Processes that may be isolated from the main plant air system Any time there is a plant process that requires higher pressure air, dryer air, or more operating hours than the rest of the plant there is an opportunity to isolate that process. Examples are:

Fire Suppression Systems[edit]

In dry fire suppression systems large plant air compressors are often employed to keep the pipes charged with compressed air. This requires that the compressors be left on all the time although fire systems generally require no airflow. During non-production periods the air compressors may only be supplying leaks. By isolating fire systems the large plant air compressor can be turned off during down time, while a small air compressor can maintain fire system pressure.

Paint Booths[edit]

Painting applications generally require low pressures. If this pressure is significantly lower than that required for the rest of the plant, consider isolating the paint booth. use one air compressor to fulfill the low-pressure requirements of the paint booth and another to fulfill the higher pressure demands of the rest of the plant is often a good idea.

Dry air[edit]

Some material handling systems or painting applications require dryer air than the rest of the plant. Isolating these uses and only stringently dry the air that requires it.

Calculate the energy savings for the air system.

Savings result from reducing pressure, switching to less expensive air-drying, and reducing run time. If reducing pressures, refer to the Reduce Pressure recommendation guide. Id changing air-drying requirements, refer to Replace air dryer recommendation guide. If reducing run time, use the calculations below.

Calculate Energy Savings from Reducing Run Time

  • Energy Savings = Operating Power x Saved Hours
  • ES = D x SH

3. Estimate the required airflow, pressure and operating hours of the isolated process.

4. Size an appropriate new compressor to meet the isolated loads. Air compressor manufacturers publish specifications in both ACFM and pressure. To size a compressor, select one that meets the isolated load pressure and airflow requirements, or refer the requirements to a manufacturer or vendor. to be conservative in your savings estimates assume the compressor will operate at rated power, even thought this will not always be the case. If the compressor is to be used to pressure in a fixed volume of pipes. With small or no leaks in the pipes, the compressor will used little or no energy. If the compressor is to be used to run a paint booth you will need to find out pressure and airflow requirements. Use this information to estimate compressor energy use.

Calculate Energy Use for a new compressor

  • Energy Usage = Rated Power x Operating hours x Use Factor x Load Factor
  • E = P x OH x UF x LF

5. Determine energy savings Calculate differences in proposed and existing Operating Conditions to determine energy and dollar savings.

6. Estimate implementation cost and determine payback. The implementation cost includes purchase and installation of a new air compressor. Obtain these costs from a compressor manufacturer or dealer.