By Timothy A. Warren, PE, LEED AP BD +C
Continuing our conversation on industrial energy reduction that started here, we will discuss energy reduction for compressed air and pumping systems.
Compressed air.
This is probably the most expensive form of energy used in an industrial plant because of its poor efficiency – with an efficiency rating of typically 10%. Because of this, compressed air should be high on your target list if it is not already. There are more than a dozen measures that can be considered to reduce energy use for compressed air plants.
Here are three major ones:
1. Reduce leaks
• Typical plants that are poorly maintained have 20-50% leak rates
• Leak repair and maintenance can reduce the leak rate to 10%
• Overall, leak repair could reduce annual consumption by as much as 20%
The most common areas for leaks are couplings, hoses, fittings, regulators, traps, valves, and disconnects. Quick disconnects are notorious for leaking and should be avoided. Detecting methods include manual inspection as well as ultrasonic acoustic detection. Leaks will continue to occur, so detection and correction programs need to be ongoing efforts.
2. Use alternate sources for specific applications:
Source: ENERGY STAR
This measure is very interesting when you think about how many processes utilize compressed air within manufacturing plants. Some industry engineers believe this measure has the largest potential for compressed air energy savings, as many operations can be accomplished more economically and efficiently by using other sources.
3. Reduce inlet air temperature
• Each 5 degree F will save 1% of compressor energy
• There is usually a quick payback of 1-5 years
• Approaches to reduce air inlet temperature include:
– Duct inlet from outside
– Duct exhaust air (cooling air) to outside
– Ventilate room
– Heat recovery
It seems somewhat obvious, but when walking through mechanical rooms I’m never surprised to find the rooms with air-cooled compressors are super-hot, with compressors utilizing room air, or worse yet, discharging the radiator exhaust air directly to the space! From a design perspective, we always try to duct inlet air directly from the outdoors. The worst case would be that we have the compressor use room air, duct the exhaust to the outside, and ventilate the space very well. In addition, we always try to include a damper arrangement in the exhaust duct to utilize the rejected heat for space heating when needed.
Pumps.
To reduce pump demand, a sound approach is to eliminate bypass loops, which can result in energy savings of 10% to 20%. Constant volume pumping was a traditional approach before the advent of smarter control strategies and capabilities. Although constant volume pumping provides very predictable controls to meet requirements, the energy use is often overlooked. Not surprisingly, there are a lot of these systems still in operation.
Bypass loops are devices that maintain constant flow in the main circuit. For example, a three-way valve serving a chilled water coil allows variable flow through the coil, but maintains constant flow in the main loop. Other similar devices can be in the main circuits. By eliminating these devices, we allow the pumps to operate closer to actual demand and at a lower brakehorsepower, basically riding the pump curve. This is not as efficient as speed drives, but certainly is a step in the right direction.
When properly sizing pumps, it is critical to trim the pump impeller and minimize throttling valves, which can result in energy savings of anywhere from 15% to 20%.
Once we know the design condition (flow and head), we select a pump with the best operating characteristics. For this example, let’s say that we have 600 gpm at 70 ft. head design condition, with a 9”diameter impeller. The balancer goes to balance the pump and realizes he’s getting 750 gpm. What does that mean? The actual pressure drop is less than expected so the pump is over-achieving. To balance to the design flow, the balancer would typically use the triple duty valve to throttle the system to induce additional pressure drop in the system.
Is this energy efficient? Of course not!
Instead, a system curve should be developed using the pump affinity laws and a pump curve to determine what impeller size is really needed. Upon doing so, we find that a smaller impeller would give us the required flow at a much lower brakehorsepower. For this example, the pump would operate at 50% of the brakehorsepower with a trimmed impeller compared to the throttling scheme. Keep in mind, this is not only for new installations. This checkup can be done with any existing pump, knowing the flow demand at full load conditions.
Variable speed drives should also be considered to address variable demand. However, this is not applicable for systems with high static head, open systems, or for systems with extended low-flow conditions.
Variable speed drives are an effective energy measure for pumps with variable demand. Rather than just riding the pump curve as a constant speed pump, pumps with variable speed drivers will continuously match speed and power to actual demand requirements, thus maintaining operational efficiency throughout the pump’s operating range.
Questions about reducing energy in your industrial facility? Contact Tim Warren at 717.434.1566 or [email protected]