Condenser pressure control on refrigeration machines is often not well understood. Minimizing the pressure can result in significant savings. Most refrigeration machines are designed for worst-case condenser pressures, which usually occur during peak summer temperatures. Most are cooled be a cooling tower, which may struggle during peak summer conditions. Several key operating points can help minimize energy consumption.
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First, the lower the compression ratio is, the lower the energy consumption. Most chilled water systems operate with about 60°F temperature difference between the condenser and the evaporator. A rule of thumb is that each °F equates to 1/60 or about 1.5% energy use. Specific systems may vary from this, but this is a quick way to estimate how much energy you could save by reducing condenser pressure.
If you operate a low temperature system, the change is less dramatic. However, you have to remember that the horsepower required is also higher so a small change in temperature can result in large energy savings.
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The caveat is that you must have enough differential pressure between the condenser and the evaporator or intercooler/economizer so that the required flow will still pass from the condenser to the evaporator to maintain system capacity. Otherwise, refrigerant will stack in the condenser and tubes in the evaporator will be above the liquid level, resulting in higher superheated temperatures in the evaporator and reduced capacity in the condenser due to flooding.
Various experts have suggested variable speed fans on cooling towers to minimize their energy consumption. What these experts seem to miss is that the higher fan cost associated with the cooling tower are more than offset by the refrigeration machine’s reduced energy. Let me back up to say that there’s a tipping point in the cooling tower. At some point, more airflow and more fan power won’t decrease cooling tower water temperature proportionately and the extra power is wasted.
Jake’s Test
Jake had been called to a site for a boiler problem. The plant took its cooling water flow from the bottom of Lake Michigan. While on site, Jake also observed the operation of the low temperature chillers used to cool reflux columns in the process. The low temperature brine was supplied at about -60°F and cooled by a cascade refrigeration system utilizing two separate refrigerants in two separate refrigeration. The upper stage cooled the lower stage at about 15°F and was itself cooled by the lake water.
Jake asked why the condenser temperature on the upper stage was near summer conditions while the lake water temperature was at about 40°F. The operator indicated that he had been told to maintain design condensing temperature. This was based on peak design during the summer when lake temperatures climbed to near 70°F.
Jake visited the plant engineer and inquired why they were running such high temperatures when low temperatures were available. The plant engineer stated they needed the higher temperature because the machine wouldn’t run unless it was at the higher condenser pressure. Jake did a quick calculation. The temperature differential was about 120°F. Each 1.2°F would result in a 1% energy reduction. A 27°F reduction might result in 22% decrease in energy consumption. The plant engineer decided the savings were worth testing.
Jake and the plant engineer decided to do the test in 3.6°F increments. Because the old machines essentially were controlled manually, they decreased the condenser pressure on the upper stage and then trimmed the set point for the condenser/evaporator between the two stages. The reduced steam-driven compressor speed lessened the differential pressure between stages. This reduced and balanced the differentials on each stage. They were careful to make sure that process conditions were met at each phase of testing. While they didn’t achieve the full potential of 22%, they were able to reduce energy by 15%.
So, if you have a refrigeration machine that’s operating at a design condenser pressure even though colder water is available, start asking questions. You might be able to reduce your energy bill significantly!
