Operational Efficiency: Compressors

Operational Efficiency:  Compressors

By:  Jayson Noetzelman

          If you work in the commercial or industrial refrigeration sector you know running refrigerant compressors can be awfully expensive. We all know it is expensive, but we also know our refrigeration systems wouldn’t work without them. When a facility has several substantially sized compressors it might be easy to write off that big energy bill as a necessity. If you need to keep something in your manufacturing process cold, well then, you’ve got to spend money to make money… right? While you may have to spend money, maybe you don’t have to be spending as much money. This article focuses on pressure loss and its financial implications in industrial ammonia refrigeration, but that’s not to say that the ideas to resonate with other refrigerants or refrigeration systems.

          Compressors are, at their most basic, just pressure converters that trade electricity for stored energy (in the form of increased pressure). When a system has to increase pressure more, it takes more energy, and that means a higher electricity bill. It’s not hard to see that if we could lower our pressure losses within our system, we wouldn’t have to work our compressor motors as hard. That could mean big electricity savings for our manufacturing plant. Carlson & Stewart Refrigeration (CSR) would highly recommend manufacturing plants examine their systems for sources of pressure loss and do their best to minimize pressure loss in their vapor refrigerant lines.

          One of the most common reasons for pressure loss is inadequately sized piping. The more vapor or liquid that must be moved through a fixed pipe size the faster the liquid or vapor must travel. The faster a substance flows through a pipe the more friction it encounters. Try to keep your piping sufficiently sized to keep movement speeds inside the pipe reasonable. CSR typically uses IIAR ammonia refrigeration standards to ensure pipes are sized effectively.

          Needlessly complicated piping can also be a drain on your system. It’s not uncommon to see a subheader that is still in use despite several unnecessary elbows, tees, and other fittings. This is especially rampant in older plants or areas of plants that undergo fairly consistent machinery changes. When that moving liquid or vapor has to change directions, it loses energy. Reduce your pressure loss, and subsequent money loss, by simplifying piping.

          Some problems may be found by just asking “Why do we operate like this?” If the answer is simply, “Because that is how we have always done it!”, you may want to try and find a better answer. In CSR’s experience, a manufacturing plants “House Suction” or their compressor suction setpoints are a great place to start when asking “Why do we operate like this?”. For example, let’s say your lowest temperature in your plant is a 35°F cooler that runs on evaporator coils at a 10°F temperature difference. This means your house suction has to be at least 25°F. Maybe, for the sake of example, your plant has always run a 10°F House suction. Again, asking, “Why do we run this way?” might save you some money. Perhaps you find out that you used to run an ice builder that needed a lower house suction, but that ice builder is long gone. This means your compressor is currently having to raise pressure from 23.7 PSIG to 180 PSIG. Let’s raise the suction pressure to 33.4 PSIG which correlates with 20°F. This would still give you ample room to adequately run the coolers from our example at 35°F. Now the compressors need to compress 9.7PSIG less, and your compressors don’t have to work as hard. Another example where being mindful of pressure changes will save you money.

          Your plant may be susceptible to another type of pressure loss if your house suction pressure has fluctuated in the past, operating with pressure regulators that are no longer needed. In our example above our system originally needed a pressure regulator because it was operating evaporators above house pressure. If we raise our house pressure to more closely match our lowest temperature loads, we should no longer need pressure regulators. CSR has seen several field installations where regulators are utilized but serve no function in the system. Regulators by nature impose a pressure drop of typically up to 2 psi. Utilizing a suction stop valve with essentially no pressure drop could allow you to raise house suction by up to 2 psi, meaning long term electrical savings at your facility.

          It isn’t a Carlson & Stewart trade secret that pressure loss hurts a systems efficiency. But knowing is, as the saying goes, just half the battle. It’s important to examine our refrigeration systems and see where we can make improvements. If you feel like you might need some help getting your industrial refrigeration system in top notch shape, Carlson & Stewart would be happy to help.