What you should know about servicing refrigeration systems with scroll compressors

Keeping up with technology does not always mean installing the latest version of Windows, surfing the Net or investing in the latest wireless phone to increase your power of communication. Sometimes it means keeping up with the latest compressor technology in refrigeration systems.

Across the United States, scroll compressors are becoming more prevalent in supermarkets, including rack applications, distributed refrigeration systems, outdoor condensing units, and air conditioning systems. In case you are not familiar with scroll technology, the concept is really very simple. Two spiral-shaped members fit together, forming crescent shaped gas pockets. One member remains stationary, while the second orbits relative to the first. As the spiral movement continues, gas is drawn in and forced toward the center of the scroll form, creating increasingly higher gas pressure and discharging the gas from the port of the fixed scroll member.

Several pockets of gas compressed simultaneously provide a smooth, nearly continuous compression cycle. This is very different from the up-and-down pumping of the pistons inside a reciprocating compressor. The continuous motion of the scroll compressor makes it very durable and reliable. However, because scroll compressors are designed differently, they require different service steps from those for reciprocating compressors.

Here are some key service guidelines to familiarize yourself with:

* A discharge line thermostat is standard on all low-temp scroll models.

* Oil is managed by measuring oil levels in scroll compressors, as compared with measuring oil pressure on a semi-hermetic.

* Whereas reciprocating semi-hermetics can operate in either direction, scroll compressors can only run in one direction.

* The smaller size and weight of the scroll compressor may change the structural requirements for applications such as rack systems.

* Scroll's simpler design requires less time to install, often making the replacement process easier.

If your store's refrigeration system contains scroll compressors, follow these key service guidelines.

Oil management: scroll oil pumps don't have accompanying oil pressure safety controls. They require an external oil level control. The control needs to combine the functions of oil level control and timed compressor shutoff should the oil level not come back to normal within a set period of time. Immediately after the system startup, the oil reservoir level will fluctuate until equilibrium is reached. The oil level has to he carefully monitored during the first 24 hours to ensure sufficient oil is available and to prevent any unnecessary trips of the oil control system.

Reverse rotation: Scroll compressors are direction-dependent, meaning they'll compress in just one direction. Three-phase scrolls will rotate in either direction, depending on power phasing. You should be aware of potential problems associated with reverse rotation. Prolonged operation in reverse results in insufficient lubrication, scroll galling, and scroll set damage. Look for the following signs to verify reverse rotation:

* Discharge and suction pressures don't change.

* Noisy operation.

* Current draw is substantially reduced compared to tabulated values

With a three-phase scroll, determine the proper rotation at start up by observing that the suction pressure drops and discharge pressure rises. Larger scrolls (7 to 15 hp) incorporate a phase monitor and timed lockout control in the motor protection module to protect against reverse rotation and power interruptions.

Injection: At low evaporating temperatures, refrigeration scrolls require liquid injection. The liquid injection function feeds a steady flow of liquid to the middle of the scroll compression cycle to cool compressor components. On older compressors, a liquid solenoid valve controls injection and cap-tube metering device that feeds more liquid as head pressure rises. On newer compressors, a DTC Valve monitors discharge line temperature and injects refrigerant through a special solenoid with an accurately sized orifice. In either case there must be a solid flow of refrigerant feeding the capillary to function properly.

Liquid handling: Scroll handles liquid better than most other compressor types, but still can require additional accessories for optimal protection. Those accessories include:

* Accumulators -- recommended for systems with large refrigerant charges or those that allow for uncontrolled refrigerant flood-back.

* Crankcase heaters -- required for three-phase systems with large refrigerant charges and in outdoor applications.

Pressure/temperature monitoring: As with all compressor types, it's important to monitor and control pressure and temperature in refrigeration scroll applications. A discharge line thermostat is required on all models from 2 to 6 hp. The thermostat must be insulated if the application is outdoors. Both high- and low-pressure controls are also required. (See Table 1)