Retrofitting an older refrigeration compressor with a modern microprocessor controller can increase the system’s energy efficiency, reliability, maintainability and operability.

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. Some manufacturers might disagree, but the basic technology of the refrigeration compressor has not 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 microprocessors and upgraded sensor technology.

Thirty years ago, computers were garage or laboratory toys reserved for educational, corporate or government use. Large and expensive, they had special power and cooling requirements. Compressor packages available during this time incorporated hard relay logic controls, discrete components and pressure switches.

Figure 1. A retrofit allows for tighter control of suction pressure with a proportional capacity control method, which reduces energy consumption.

The ’80s ushered in 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 or so years, it is 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 Many plants wanted to “get control” of their engine rooms but were limited as to what they could do because of the outdated electronics on their compressor packages.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.

Figure 2. Tighter suction pressure control and the ability to automatically adjust setpoints to match power company demand schedules can provide substantial energy savings.

The Anatomy of the Retrofit

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

Energy Savings. A retrofit allows for tighter control of suction pressure with a proportional capacity control method, which reduces energy consumption (figure 1).

Setpoint Scheduling. A retrofit allows suction pressure control setpoints to be adjusted automatically to match power company demand schedules, resulting in lower power bills (figure 2).

Improved Monitoring and Equipment Protection. A retrofit provides 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 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, reduces spares requirements and minimizes lifecycle costs. It also allows for swapping of control system 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.

Overcoming Challenges to Retrofitting

Slide-Valve Indicators. The vintage of some systems may, at first glance, suggest that retrofitting a microprocessor-based control system would be difficult or impossible. For example, 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, however, the original manufacturer has upgrades available to allow electronic indicators to be added if necessary, simplifying retrofit. In cases where such upgrades are cost-prohibitive, the existing equipment (reed or limit switches, for example) usually can be used in the retrofit.

Reciprocating Compressors. Microprocessors used to be too expensive for reciprocating compressors compared to the price of the compressor. As a result, most existing reciprocating compressors use only pressure switches for controls.

However, as the price of all computer components has fallen over the last several years, microprocessor controls have become more affordable. If a plant has a reciprocating compressor that is working well and there are no plans to change it out, adding a microprocessor can improve accessibility to the panel for maintenance and enhance the system’s overall operating efficiency.

Controls for retrofits typically include a microprocessor control panel with a keypad, an LCD display, terminals, circuit breakers, pressure sensors, temperature transmitters and, where applicable, a motor current sensor.

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, men 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 a few relays and pressure switches is now being handled with desktop PC-like computers, flash drives, large color displays and touch-screen 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 cannot 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.

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 is 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.