Breathe New Life into Old Compressors  

By Hank McConnell, M&M Systems, Inc.

In an era in which many people upgrade their cars, cell phones and computers every year or two, refrigeration compressors are still being manufactured to last decades. Many compressor packages built in the ’70s, ’80s and ’90s are still in operation several decades later. In some cases, these older units are viewed as more dependable and better made than their modern counterparts. Manufacturers might disagree, but the basic technology of the refrigeration compressor hasn’t changed much over the past 20 or 30 years. What has changed, however, is the ability to control these units in an energy efficient way using up-to-date microprocessor and sensor technology.

Getting Control of Older Equipment
Thirty years ago, computers were garage/laboratory toys reserved for educational, corporate or government use. They were large and expensive and had special power and cooling requirements. Compressor packages available during this time incorporated hard relay logic controls, discrete components and pressure switches.
The ’80s saw the introduction of microprocessor-based control panels for compressor control. Some initial attempts were successful, and others fell short. Looking at developments in computers and displays over the last 20+ years, it’s easy to understand why. The last two decades have been an amazing ride for silicon and software.
In the mid-’90s, a need for microprocessor panels for older compressor units became apparent. Many plants wanted to “get control” of their engine rooms but were limited as to what they could do because of the outdated electronics on their compressor packages. They wanted to reap the energy-saving benefits of microprocessor control systems without changing the compressors themselves. Microprocessor controls for retrofit applications were developed to fill this void.

The Anatomy of the Retrofit
Controls for retrofits typically include a microprocessor control panel with a keypad, an LCD display, terminals, circuit breakers, pressure sensors, temperature transmitters and a motor current sensor when applicable. Once the panel and sensors are mounted, the compressor will then be controlled to the same standards as new compressor systems. Some of the inherent benefits of retrofitting compressors with microprocessor controls include:

• Energy Savings – A retrofit allows for tighter control of suction pressure with a proportional capacity control method.
• Setpoint Scheduling – A retrofit allows suction pressure control setpoints to be adjusted automatically to match power company demand schedules, resulting in lower power bills. 
• Improved Monitoring and Equipment Protection – A retrofit provides all the up-to-date monitoring of sensors with alarm and failure setpoints. Unloading and limiting motor current and discharge pressure protects the motor and compressor from adverse conditions, thereby reducing maintenance costs. 
• Logging – Modern microprocessor panels typically contain an operation and trend log that records sensor values while the unit is running. This log allows the operator to review the system’s operation and proactively isolate problems. Alarm and failure logs record past events.
  
• Password Protection – Retrofits can provide the ability to have passwords for different users with their own access levels. Password protection is further segmented by different user functions instead of allowing access to the entire system. These access levels can be used to restrict users from viewing or modifying setpoints in the compressor. 
  
• Common User Interface – Upgrading the microprocessors on compressor units — whether the compressors are from the same manufacturer but of different vintages or they are from different manufacturers — provides a consistent and common user interface. Operators only need to learn one way to operate and control all compressors, thereby reducing the chances of operator errors.
  
• Commonality of Parts – Using the same microprocessor panel on all compressors in an engine room regardless of the vintage or manufacturer provides a commonality of parts and reduces spare requirements and life cycle costs. It also allows for swapping of components (hot spares) from one compressor to the other when required. 
  
• Communication – With the addition of a retrofit panel, older compressor units can communicate with newer plant controls or building automation systems. Many retrofit units support open protocols such as Modbus, RTU, Allen Bradley DF1, and BACnet, as well as specific manufacturer protocols.

The Slide Valve Indicator Challenge
The slide valve/capacity indicators on screw compressors provide a bit of a challenge for some models. Many early units have potentiometers for indication, while others only have reed switches. Often the original manufacturer has upgrades available to allow electronic indicators to be added if necessary. In cases where such upgrades are cost-prohibitive, the existing equipment such as reed or limit switches often can be used in the retrofit.

Not Just for Screw Compressors Anymore
Most reciprocating compressors ship with just pressure switches for controls.  In recent years, manufactures (M&M, Mycom, Vilter, York, Sabroe) have been offering microprocessors on their recip compressors as well.  In the past, adding a micro to a recip compressor was cost prohibitive compared to the price of the compressor. As with all computer components, the price point has come down and we are seeing micros on recips more. In the retrofit market, if a customer has a recip compressor that is working well for them and there are no plans to change it out, adding a micro can improve their accessibility to the panel and its operating efficiency

More Technology Isn’t Necessarily Better
The trick in the refrigeration industry is not to just add technology because it is there, but to add technology sensibly to reduce operating costs and operator workload. Thirty-eight years ago, man went to the moon on computers with less memory and processing power than a modern cell phone. Today, some refrigeration compressor manufacturers have taken advances in computer technology to the extreme. What was once done with some relays and pressure switches is now being done with desktop PC-like computers, flash drives, large color displays and touchscreen user interfaces. These new devices offer the operator a wealth of information, but at the cost of a significant increase in complexity.
 Because technology in the computer industry is moving so quickly, it is challenging to design and provide a controller that will meet the long lifespan requirements of a refrigeration compressor. Some compressor manufacturers are forced to release new models every few years because older models can’t be supported due to the obsolescence of hardware, software or components. The addition of a microprocessor to a compressor package needs to add value, not cause more problems and headaches.

Get Control First
Everyone is concerned about the cost of energy and how to best manage a facility’s needs. The goal isn’t just keeping things cold anymore; it’s keeping things cold for the least amount of money. The focus is on the bottom line as much as the temperature, but maintaining this balance is a constant struggle.
Modernizing an older compressor package by adding the right microprocessor controller is a cost-effective way to increase the energy efficiency, reliability, maintainability and operability of that unit. Installing a modern microprocessor controller also opens the possibility for other energy-saving improvements, such as adding variable speed motors to screw and reciprocating compressors.
Get control first; the bottom-line benefits will follow.

To discuss your application in more detail Contact Us at (866) 906-6462 or submit our on-line Information Request form.