Driving change in chiller design

Don Eppelheimer

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A seasoned veteran of the HVAC industry, Don Eppelheimer has been working for Trane since 1972. As global chiller systems manager: commercial systems, he recently hosted a workshop on chilled water system design in Dubai, sharing his years of expertise with an appreciative audience.

Eppelheimer began with a brief overview of the main components of a centrifugal chiller: “There is an evaporator where the evaporation of refrigerant cools water. In the back is the condenser where hot refrigerant vapor is condensed so we can reject the heat to the cooling tower. On the top you see a two-stage centrifugal compressor and motor to spin the impellers. In the US, and in Saudi, the impellers spin at 3 600 rpm. Here in Dubai and in the UAE, the impellers spin at 3000 rpm. We need a starter to control the motor, and we need a control panel to provide the operator with the necessary level of access.”

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As to how the centrifugal compressor itself works: “There are two impellers mounted on the motor shaft. It is, of course, the spinning of the impellers that compresses the refrigerant. Upstream of the first impeller, there are wedge-shaped inlet-vanes. A centrifugal compressor works by spinning the gas, which creates velocity. The gas enters through the eye of the impeller and the spinning of the impeller sends it out through the circumference. On leaving the impeller the refrigerant enters the diffuser, where it decelerates. So gas accelerates in the impeller, and decelerates in the diffuser.”

Eppelheimer pointed out that this progression can be plotted. “When the refrigerant velocity decreases, velocity pressure is converted to static pressure. It is a combination of the impeller, the diffuser and the rotating action that allows the centrifugal compressor to create pressure.”

“A couple of features to remember about the centrifugal compressor: it is the inlet vanes that control the volume, which becomes (tons of) capacity. So it is inlet vanes that control part load. But it is the tip speed – the velocity of the impeller – that creates lift.” However, it is mass flow of the refrigerant through the compressor, in pounds per hour that becomes tons. “The pressure created by a centrifugal fan is static pressure; the pressure created by a centrifugal pump is head, and the pressure created by a centrifugal compressor is lift.”

“Lift has units of temperature. Because we are compressing a chemical with the unique property of saturation, for every pressure, there is a corresponding temperature. This makes it easy for HVAC engineers to use temperature to discuss pressure, as the type of refrigerant is not an issue. It can be a low-pressure refrigerant where the units are very low; it can be a medium or, even a high-pressure refrigerant, but the application is still the same when we discuss temperature.”

In respect of the work that must be done by the compressor we must consider leaving chilled-water temperature, and the temperature of the water leaving the condenser. Leaving condenser water temperature, minus leaving evaporator water temperature, is what we call ‘lift’. We adjust the load with inlet vanes, and we adjust the lift with tip speed. When we talk about applying drives to centrifugal chillers, a unit of measurement called NPLV, APLV or IPLV often surfaces. .”

Eppelheimer explained that IPLV is a mathematical definition of part load, created by ARI (Air-conditioning Research Institute) for the benefit of ASHRAE. ASHRAE Standard 90 assumes minimum levels of performance for various pieces of equipment in air-conditioning systems. When it came to chillers, the authors of ASHRAE Standard 90 highlighted the importance of full load. “Full load determines electrical line sizes, electrical distribution losses, the size of transformers and their efficiencies, and the impact on society: i.e how many chillers, how many power plants need to be built and what their efficiencies are going to be; thus full load is very important.”