2017 RETA Breeze Sept-Oct

ENERGY BALANCE & SYSTEM PERFORMANCE

By Andrew Fiala, SCS Tracer

If an ammonia system is subject to the OSHA process safety management (PSM) regulations and is built after May 26, 1992 it is required to have an energy balance. Regardless of whether or not a system was built before or after that date, it is still important and useful to have a documented energy balance. Why? An energy balance can be used to show deficiencies in system capacities; are the compressor capacities greater than the evaporative heat transfer surface capacities and are the condenser capacities greater than the compressor capacities? Compiling the data necessary for creating an energy balance can be obtained from the equipment manufacturers. Once the basic data has been collected it can either be filed and forgotten, or used to produce some really amazing information about an ammonia system. The information obtained from the manufacturer is the equipment’s optimum performance data at specific temperatures/pressures. Most compressor manufacturers use 31.5 psig

Swing compressors should reflect the capacities in both high and low stage on the energy balance. Condenser heat exchange surfaces should be examined to verify the heat transfer surface is clean of build-up and the water distribution is effective at covering the coils. We’ve learned from RETA’s Industrial Refrigeration Book 1 that as little as 1 / 32 of an inch scale on a condenser coil can reduce the capacity by as much as 30%. A reduction in condenser capacity will lead to higher discharge pressures and, therefore, higher compressor horsepower being required. Non-condensable gasses in the system can also cause higher discharge pressures and must be removed. Evaporators, chillers, and other heat exchange surfaces can become oil logged, reducing the effectiveness of the heat exchange surfaces. An effective oil management system compares additions and subtractions from the ammonia system. An effective oil management system can inform the owner if more

suction pressure and 151 or 181 psig discharge pressures to produce the high side compressor tonnage rating. If the suction and discharge pressures differ from the manufacturers design, then the tonnage rating of the compressor can be adjusted to show actual performance. As the suction pressure is lowered the compressor tonnage rating also drops. With a lower compressor capacity, the upstream heat transfer surfaces can become “backed-up” affecting the room and product temperatures. Heat of compression should also be calculated into the equation—(HP x 2545)/12,000. “While an energy balance can be a daunting task to compile, it should not be just a check in the box for your PSM program.”

10 RETA.com