Tips for controlling energy costs on industrial refrigeration systems

An integrated approach to optimizing the mechanical system—along with an automation strategy—is one of the most effective ways to reduce energy costs.

02/18/2014


Heavy user: Refrigeration systems account for the largest amount of energy use in a typical manufacturing facility. Courtesy: StellarIndustrial refrigeration systems are widely used in a number of manufacturing facilities from food processing to pharmaceutical production. Those refrigeration systems also account for the largest energy usage in a manufacturing facility, often up to 60 percent of a plant’s total operating expenses. As energy costs continue to increase, engineers are looking for ways to manage these costs and operate these systems more efficiently. 

An integrated approach to optimizing the mechanical system, along with an automation strategy, is one of the most effective ways to reduce energy costs. Too often, engineers look to optimize each individual component of their system rather than looking at the system as a single, integrated unit. Every time a new piece of equipment is added to the system, if it’s not properly optimized within the scope of the entire system, you’ll end up with wasted energy and operational inefficiencies. Prior to adding new equipment, it’s important to obtain baseline data on the amount of energy a system consumes. That’s the only way to accurately measure the impact a new piece of equipment has on the overall system. 

The primary factors that influence the energy efficiency of a refrigeration system are the efficiency of the system’s design and the refrigerant used; the condition of the equipment, the control strategy, and the load profile of the system (deviation of the operating cooling loads from the design cooling loads).

Compressors and condensers are the two components that account for the greatest energy usage.

Compressors

Compressors account for the majority of the energy consumption in a refrigeration system and therefore should be carefully selected to ensure they match the load and can be staged and sequenced effectively. Screw compressors are most efficient operating at full load; at part-load conditions, they become increasingly less efficient. The addition of variable frequency drives (VFD) to a screw compressor will increase the part-load efficiency. A reciprocating compressor has much better part-load efficiency and may be a good choice for smaller loads.

Condensers

Primary players: Compressors and condensers are the two components that account for the greatest energy usage in a refrigeration system. Courtesy: StellarCondensers are typically the second highest energy usage component of a refrigeration system. Adding VFDs to condenser fans can have several advantages and will give better condensing pressure control, which can smooth system operation. Condensers need to be sized for peak loads, meaning for all loads except for a few peak conditions when they are oversized. Reducing the fan speed to match the capacity will produce considerable horsepower savings.

Here are steps engineers can take to optimize a refrigeration system to achieve the greatest energy efficiency:

  1. Optimize set points, as condensing pressure should typically be run as low as possible. Suction pressure should be run as low as possible while still maintaining the desired room/product temperatures. Adjusting suction pressure up 1 degree could mean a 1.5 percent savings for those compressors. 
  2. Compressors should be sized to match the loads as closely as possible. It is good practice to include different sized machines and sequence them properly to keep the machines as fully loaded as possible. For large systems, large compressors handle the majority of the load with a smaller compressor included as a trim compressor to handle the swings. This will keep the larger compressor fully loaded at all times. The trim compressor could have a VFD to further increase savings. Two equally sized compressors, each running at 50 percent capacity, can require 30 percent more HP than one compressor running at 100 percent, so proper selection upfront and good sequencing is important.
  3. Install VFDs on screw compressors to optimize mechanical efficiencies of the machines. The best approach is to set the slide valve position at 100 percent and vary the RPM of the motor according to the refrigeration needs of the machine, which allows it to run more efficiently.
  4. Install VFDs on condenser motors to stabilize head pressure and prevent the motors from heavy repeats and intense start/stop cycles. This will allow the fans to change speeds so they don’t continually stop and start, which requires additional energy and results in mechanical wear. The biggest payback from a VFD will be on systems with variable loads (some systems run at 100 percent almost all the time, and in these cases, a VFD will actually be less efficient since the VFD has inherent losses). 
  5. Use of floating head pressure to maintain the ideal temperature for compressor and condenser operations is important. Higher condensing temperatures require compressors to work harder while lower temperatures require less power. Find the optimal break-even point where the condensers and compressors are cumulatively using the lowest overall horsepower requirements.
  6. Lower temperature systems should thoroughly evaluate the efficiency of a two-stage or possibly a cascade system. Depending on the conditions, the savings over a single stage system can be considerable.
  7. A completely integrated automation system can run a machine room to ensure efficiency and automate temperature controls within zones. Automating defrost cycles to sequence at different times can result in significant energy savings.  An automated system can make calculations and adjustments constantly, where a system operated manually requires constant operator attention, and is susceptible to human error and will react much slower.
  8. Remote monitoring can have a significant impact on the bottom line, reducing energy, maintenance and overall operating costs. If you’ve recently upgraded your refrigeration controls system or are in the process of upgrading, it’s important to ensure your new system offers remote access. This will allow for speedy diagnostics, offsite troubleshooting capabilities, and constant monitoring, all with a “big picture” view of the entire system and allow you to run reports in real-time, including trend analyses, alarm logs, and energy management data. Working with a vendor who has expertise in both refrigeration and controls will help your team identify inconsistencies and trends that could be wasting energy so you can make the necessary changes to improve efficiency. 

Luke Facemyer is director of design for Stellar, www.stellar.net, a fully-integrated firm focused on design, engineering, construction and mechanical services worldwide. Founded in 1985 and headquartered in Jacksonville, Fla., Stellar has extensive successful project experience in the healthcare, commercial, educational, institutional, office, hospitality, military, automotive, industrial, food and beverage, refrigeration, distribution, power and utilities markets. Edited for the CFE Media Industrial Energy Management section in February as a Digital Edition Exclusive. Send comments to controleng@cfemedia.com.

Key concepts 

  • Refrigeration systems account for the largest energy usage in a manufacturing facility, often up to 60 percent of a plant’s total operating expenses. 
  • An integrated approach to optimizing the mechanical system, along with an automation strategy, is one of the most effective ways to reduce energy costs, including those related to refrigeration systems.
  • There are seven steps engineers can take to optimize a refrigeration system to achieve the greatest energy efficiency. 

Consider this:

With refrigeration costs consuming so much of the typical plants operating budget, doesn’t it make sense to look at a holistic approach to cutting that expense? 



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