Evaluating The Problem

The Greensheet building has 52,000 square feet of office space in 4-1/2 stories. Cooling was provided by two 100-ton R-22 chillers (each with Carrier 5H120 compressors designed with unloading to 33 percent), mechanical TEVs, and variable air volume (VAV). From 1996 to 2002, the system experienced more than a dozen compressor failures that were a direct result of liquid floodback.

G and W Service Co., also of Houston, was called in to pinpoint and correct the source of the problem. In 2002 Vince Barnell at G and W Service called in Sporlan Valve for a consultation.

"VAV systems such as Greensheet's often experience wide load swings with the potential for liquid floodback," said Eric Foster, a Sporlan sales engineer.

"The TEV's function is to maintain some level of superheat as a margin of safety to protect the compressor. As flooding occurs, which it will with some regularity during extreme low-load conditions, the outcome is typically compressor breakage or failure. The result is a very expensive repair job."

Foster, along with Sporlan sales engineer Dave Demma, reviewed G and W's repair documents on the Greensheet system and personally inspected the system's equipment.

They explained to G and W that mechanical TEVs would meter the proper amount of refrigerant into the evaporator under reduced load conditions - thereby preventing floodback - but that they had limitations. If the system load were severely reduced, say in the range of 40 percent of the TEV's rated capacity, the valve would begin to have difficulty maintaining its superheat setting.

The system load at Greensheet's building would fall below this level. The mechanical TEVs simply could not maintain superheat control.

The ideal refrigerant control device, they said, would be non-refrigerant-specific, have a very wide load range, have remote setting capability, and control superheat precisely. Sporlan's electric expansion valves (EEVs) could do that, they said.

Case Solved

Greensheet agreed to have G and W Service remove the mechanical TEV and install an SEH-100 electronically operated step motor flow control valve, a chiller control, temperature sensor, and pressure transducer.

Step motor valves have a sophisticated design, according to Sporlan. A small motor opens or closes the valve port, but the motor does not rotate continually.

Instead, it rotates a fraction of a revolution for each signal sent by the controller. The number of step signals sent by the controller is remembered by the controller, which can return the valve to a previous position at any time for fine control.

Sporlan step motors are designed to run at 200 steps per second. The company said its SEH-100 electric valve has 6,386 steps of control, so extraordinary superheat control is possible. The step motor valves are suitable for liquid, suction, or hot gas bypass service on any HFC, CFC, or HCFC refrigerant.

System superheat is maintained by the chiller control, the manufacturer said. This microprocessor can control two SEH valves.

Using a temperature sensor and pressure transducer, the chiller control is designed to determine the true superheat at the chiller outlet. The EEV then modulates to the correct position to maintain the superheat setpoint. A remote panel display gives G and W technicians access to all of the controller's parameters and setpoints.

The SEH-100 valve installation was completed on one chiller in January 2003 by Mike Elsner of G and W Service, and the system has been trouble-free since, according to Stiles. During the peak summer months, the retrofitted chiller was the only one operating. In 2005, G and W plans to convert Greensheet's second chiller to the same control setup.

Stiles said his employees appreciate the reliability of the retrofitted system.

"We have dozens of people who are managers or in sales, and they are required to wear suits to work every day. They are very aware of the temperature in their offices. It's great to have a system that works right whenever we need it," he said.

Publication date: 12/20/2004