Energy Efficiency - Industrial Systems (Compressed Air System) Lectures Notes PDF

ENERGY EFFICIENCY INDUSTRIAL SYSTEMS (COMPRESSED AIR SYSTEM) 1  INTRODUCTION Compressed air has 3 primary uses: ▪ Power: As an energy source to perform work ▪ Process: Air becomes part of a process ▪ Control: To stop, start or regulate the operation of a machine...

ENERGY EFFICIENCY INDUSTRIAL SYSTEMS (COMPRESSED AIR SYSTEM) 1  INTRODUCTION Compressed air has 3 primary uses: ▪ Power: As an energy source to perform work ▪ Process: Air becomes part of a process ▪ Control: To stop, start or regulate the operation of a machine 12/4/2024 INTRODUCTION 12/4/2024 3 COMPRESSED AIR ENERGY FLOW 12/4/2024 4 12/4/2024 5  THE SYSTEMS APPROACH AND REDUCING ENERGY CONSUMPTION Why do factories have compressed air systems? ▪ To make a profit Compressed air cost is frequently overlooked If production is interrupted there is a little concern for cost ▪ System reliability is the primary concern; Energy Efficiency and Cost are Secondary ▪ Using the Systems Approach, well managed systems are more reliable AND achieve energy savings. 12/4/2024 6  INTRODUCTION ▪ A compressed air system includes both the supply side components and the demand side components. 12/4/2024 7 INTRODUCTION 12/4/2024 8 UNDERSTANDING COMPRESSED AIR 12/4/2024 10 UNDERSTANDING COMPRESSED AIR ❑ COMPRESSED AIR is atmospheric air under pressure. ❑ That means energy is stored in the air. ❑ When the compressed air expands again this energy is released as WORK. 12/4/2024 11  UNDERSTANDING COMPRESSED AIR The Compression Ratio (CR) of air or other gas is defined by the volume reduction ratio before and after compression (V1 / V2). Figure shows that the initial volume V1 is 7 m3, and the final volume V2 is 1 m3, giving a compression ratio of 7. Compression ratio (CR) can also be calculated based on the ratio of the absolute pressure increase (assuming constant temperature). 12/4/2024 12  UNDERSTANDING COMPRESSED AIR FAD (Free Air Delivered) volume flow rate ▪ FAD is the volume of air delivered at the discharge of an air compressor package. The volume flow rate is expressed at the prevailing ambient conditions of temperature, pressure, and relative humidity as they exist at the compressor intake. No matter what those ambient conditions are. Changes in pressure, temperature or relative humidity (changes in mass) do not change the FAD rating. This rating is, therefore, a measure of volume independent of the weight of air being delivered. The units of measure for FAD include m3/min, litre/sec, acfm (actual cubic feet per minute), and many others. ▪ Normal cubic meters/minute (Nm3/min) is a measurement of weight or mass flow rate. Although Nm3/min and m3/min sound similar, they are as different as litres and kilograms. Nm3/min refers to the weight (or mass) of air that occupies one cubic meter of space under a defined (normal or standard) condition of temperature, pressure and humidity conditions. It is used when specify mass flow. 12/4/2024 13 TYPES OF COMPRESSORS 12/4/2024 14 SPECIFIC POWER FOR VARIOUS COMPRESSOR TYPES 12/4/2024 15 SCREW COMPRESSORS 12/4/2024 17 SCREW COMPRESSORS Lubricant-injected rotary screw air compressors are the most popular industrial compressors today. They have a lower initial cost, and their installation and maintenance expenses are also significantly lower than those of two-stage double-acting reciprocating compressors. While rotary screw compressors are less efficient, at 6.0 to 7.8 kW/m³/min compared to 5.3 to 5.7 kW/m³/min for reciprocating models, their overall cost advantage makes them a preferred choice. 12/4/2024 18 SCREW COMPRESSORS Control Method Various air compressor designs have different full load efficiencies shown in the table above, expressed in terms of specific power (kW / m3/minute). For most air compressor types full load capacity is the most efficient operating point. Variable speed drive compressors are an exception where-in the most efficient operation is often in the range of 80% to 85% capacity with slightly lower efficiency at full load capacity. Few air compressors operate at full load capacity every minute of operation. Or, to put it another way, most air compressors operate for some amount of time at somewhat less delivered airflow than their 100% full load rated capacity. Rotary screw compressors have several different types of capacity control. Each capacity control method has unique characteristics and unique performance. When considering part load performance for a specific manufacturer and model of the compressor, a performance chart of power –vs– delivered airflow can be created. Another method of describing part load performance is to chart specific power –vs– delivered airflow as in Figure below When comparing different types of capacity control, actual flow and power performance data for a specific model of air compressor is the best comparison. 12/4/2024 19 SCREW COMPRESSORS PERFORMANCE CURVES 12/4/2024 20 SCREW COMPRESSORS Control Method Start / Stop Control: The simplest type of control uses a pressure switch to start and stop the compressor’s drive motor. When the pressure falls to the low pressure or “cut-in” set point, the pressure switch closes, the motor starts, and the compressor operates. As the air pressure increases eventually reaching the high pressure or “cut-out” set point the pressure switch opens and the compressor’s motor stops. While start/stop control is appropriate for a small fractional kW compressor that might be used occasionally in the home or small shop, it is not appropriate for larger kW industrial air compressors. Of greatest concern is that for high kW motors, the inrush current associated with frequent starts will eventually overheat the motor windings, causing damage or a complete motor failure. 12/4/2024 21 SCREW COMPRESSORS CONTROL METHODS Load / Unload (On-line / Off-line or Load / No Load) Control: A capacity control method that allows an air compressor to run at full load or at zero (no load) compressed air flow delivery while the main motor driver continuously runs at constant speed. Load / Unload control is used with many different types of air compressors. Load / Unload control functions with a pressure switch using cut-in and cut-out set points to control the compressor. The difference is that, unlike the start/stop control, the electric drive motor runs continuously. When the cut-out pressure is reached, the pressure switch causes the compressor’s inlet valve to close, and no air is compressed. As the pressure decreases, the cut-in pressure is reached, and the pressure switch signals the inlet valve to open, and air is again compressed. These pressure-based controls typically operate within a pressure range of about 0.7 to 1 bar. Load/unload control can effectively meet system demands if the system has adequate compressed air storage capacity. Since air demand persists even when the compressor is unloaded, sufficient storage is essential. If a compressor is applied in a system with inadequate storage, it can lead to quick cycles of loading and unloading, known as "short cycling.“ An operating condition where an air compressor with load/unload capacity control rapidly switches from a loaded state (delivering full rated airflow) to an unloaded state (delivering zero airflow) and back. During short cycling, the compressor control will rapidly change state, usually several times per minute. 12/4/2024 22 SCREW COMPRESSORS CONTROL METHODS 12/4/2024 23  SCREW COMPRESSORS CONTROL METHODS Load/No Load controls rely on a discharge air pressure swing of about 0.7 bar. This fluctuation can negatively impact efficiency by 1% to 1.4% for every 0.07 bar in supply pressure. This can lead to rapid pressure changes and increased inlet valve wear in systems with limited storage. Modulation control solves these problems by maintaining a steady system pressure with minimal valve movement, regardless of demand. Modulating controls function within a pressure range of 0.7 to 1 bar to manage the compressor's response. The inlet valve remains open when the full-load operating pressure reaches 6.9 bar. If the pressure exceeds this level, it indicates underutilisation, prompting the inlet valve to close and reduce capacity. The absolute suction pressure between the inlet valve and rotors is lowered to modulate capacity by restricting inlet flow. A 10% reduction in pressure leads to a 10% decrease in mass within a fixed volume. This modulation is smooth, and when the pressure reaches the upper limit, the inlet valve is fully closed. Operating a partially loaded compressor with modulating control is inefficient. At 40% load, it can use over 80% of its full capacity, exceeding twice its power rating at full load. Below 40% flow, most modulating compressors switch to a more efficient load/unload mode. 12/4/2024 25  SCREW COMPRESSORS CONTROL METHODS Rotor length control enables compressors to adjust their output to match system demand without increasing compression ratios. By managing the effective rotor compression length, inlet pressure remains stable, and compression ratios are consistent over the upper 50% of the compressor's capacity, providing a power advantage at part load. Several methods of rotor length control are in use today. Each type offers improved efficiency at certain part-load ranges compared to modulation or load/no-load controls, but they vary in design, operation, and efficiency. 12/4/2024 27 SCREW COMPRESSORS PERFORMANCE CURVES 12/4/2024 28 SCREW COMPRESSORS PERFORMANCE CURVES 12/4/2024 29 SCREW COMPRESSORS PERFORMANCE CURVES 12/4/2024 30 COMPRESSED AIR STORAGE 12/4/2024 31  CALCULATING USABLE COMPRESSED AIR AVAILABLE IN STORAGE 32 12/4/2024 UNCONTROLLED STORAGE 12/4/2024 33 UNCONTROLLED STORAGE 12/4/2024 35 AIR TREATMENT (IMPURITIES IN AIR) 12/4/2024 37 AIR TREATMENT 12/4/2024 38 AIR TREATMENT 12/4/2024 39  AIR TREATMENT This compressor with an air delivery of 5 m3/min (referred to +20° C, 70 % moisture carry- over and 1 bar absolute) Around 20 liters of this water transports around 30 liters of As the air cools down accumulates in the aftercooler water into the air main during further, the remaining 10 in the form of condensate (at 7 an 8 hour day liters accumulate at bar gauge working pressure and convenient points in the air an outlet temperature of +30° c main at the aftercooler) 40 12/4/2024 AIR TREATMENT 12/4/2024 41  AIR TREATMENT The term “pressure dew point” (PDP) describes the water content in the compressed air. It is the temperature at which water vapour condenses into water at the current working pressure. Low PDP values indicate small amounts of water vapour in the compressed air. It is important to remember that atmospheric dew point can not be compared with PDP when comparing different dryers. For example, a PDP of +2˚C at 7 bar is equivalent to –23˚C at atmospheric pressure. 42 AIR TREATMENT 12/4/2024 43  AIR TREATMENT For the proper application of refrigerated air dryers, the dryer performance must be adjusted to account for the worst-case operating conditions expected at the actual job site conditions in which the dryer is expected to function. The worst case is defined as follows: ▪ The highest compressed air inlet temperature to the air dryer, ▪ The lowest compressed air inlet pressure to the air dryer, and ▪ The highest ambient air temperature is at the cooling airflow intake to the refrigeration condenser. 12/4/2024 44  AIR TREATMENT Air dryer manufacturers provide correction factors that are applied to an air dryers’ catalogue performance airflow capacity rating (in accordance with ISO 7183). For proper air dryer selection, the actual airflow requirement (m3 /min) is divided by the proper correction factors for the actual job site operating condition. To determine the maximum airflow rate available for an existing air dryer at a given job site condition, the existing dryer’s catalogue performance rating is multiplied by the appropriate correction factors based on the existing job site condition. 12/4/2024 45  INTRODUCTION TO PNEUMATIC ENERGY TRANSMISSION The air velocity through the pipeline also impacts the pressure loss in a given length of pipe. Pressure loss in a fluid system is proportional to the change in fluid velocity squared. Recommended design velocity targets no more than 6 meters per second in the mainline and major branches of air distribution piping. The velocity should be designed to match the typical peak airflow rate that will occur. Velocity in piping connections leading to a point of use should be at most 15 meters per second in short runs (less than 15 meters). Another quick way of picking pipe sizes for distribution systems is to use the following chart. Keeping the acceptable pressure drop to 0.1 bar or lower will result in pipe sizes that keep velocities near 6 meters per second. 12/4/2024 46  INTRODUCTION TO PNEUMATIC ENERGY TRANSMISSION Straight-line graph for determining inside pipe diameter (steps 1 to 8) 12/4/2024 47  INTRODUCTION TO PNEUMATIC ENERGY TRANSMISSION 12/4/2024 48  INTRODUCTION TO PNEUMATIC ENERGY TRANSMISSION 12/4/2024 49  INTRODUCTION TO PNEUMATIC ENERGY TRANSMISSION Pressure drop If the normal working pressure of a pneumatic tool is 6 bar (g), any increase above that pressure costs money. Example: V = 30 𝑚3/min demand at 7 bar (g) 160 kW At 8 bar (g), approximately 6% more power is required, i.e. around 9.4 kW more Costs: 9.4 kW x 0.05 $/kWh x 4000 h/year = 1880 $/year ! Air main: On a well-designed air piping system, a pressure drop of 0.1 bar is normally expected. The maximum pressure drop in the air piping system should be no more than 1.5 % of the working pressure. 12/4/2024 50  INTRODUCTION TO PNEUMATIC ENERGY TRANSMISSION 12/4/2024 51 POINTS TO BE OBSERVED WHEN SIZING AND CHOOSING AIR SYSTEM PIPING: 12/4/2024 52 KEY ENERGY POINTS 12/4/2024 53 DEMAND SIDE: ELIMINATE COMPRESSED AIR WASTE 12/4/2024 54 FINDING LEAKS 12/4/2024 55 LEAKAGE LOSSES 12/4/2024 56 12/4/2024 57 MEASURING LEAK LOSSES BY MEASURING LOAD/UNLOAD TIME 12/4/2024 58 MEASURING LEAK LOSSES BY Pressure Bleed- Down Test 12/4/2024 59 ARTIFICIAL DEMAND 12/4/2024 60 INAPPROPRIATE USE OF COMPRESSED AIR 12/4/2024 61 12/4/2024 62 12/4/2024 63 12/4/2024 64