Mind Your Refrigeration Systems, Part II

Feb. 16, 2016
A periodic energy assessment can spur some low-cost energy saving measures

In Part I of this two-part series, we talked about fluid management. Now, lets discuss some minimal or low-cost measures in refrigeration and chiller systems that can provide some “quick wins” for better energy efficiency. Refrigeration-and-chiller-system performance and energy efficiency are not key performance indicators in most plants that I have worked. Because these systems are part of the plant’s utilities infrastructure, they typically get the least amount of attention due to other pressing issues at a plant. Hence, they continue to function year after year and the single key parameter monitored is the refrigeration temperature the process requires.

Although the actual system configuration and components vary significantly from one system to another, the over-arching principles and thermodynamics are consistent across all systems. There are two types of refrigeration and chiller systems: mechanical vapor compression systems (>95%) and thermally activated absorption systems. This article will focus on mechanical vapor compression systems, but I plan to talk about absorption systems in a future column. Nevertheless, some of the measures will apply to both systems.

The electric- or steam-powered refrigerant compressor drives the system; we spend our energy and money to run the compressor and in return get the refrigeration temperatures and heat-removal capability. Reducing the compressor’s work leads to energy efficiency and savings without sacrificing production, throughputs, run-rates, etc. The compressor energy is a function of the lift (difference between saturation temperatures of the condenser and the evaporator) and the refrigerant mass flow rate (operating load) through the compressor. The compressor has an isentropic efficiency, which depends on the type of machine, as well as its design, controls and operating load.

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One of the most common conservation measures for these systems is to reduce the entering condenser water temperature (aka floating head pressure). This allows the system to lower the compressor discharge pressure and thus reduce the compressor work. Fan power on the cooling tower or fin-fan will increase — albeit minimally compared to the compressor energy savings. Typically, reducing cooling water temperature could obtain 0.5–1.0% energy savings per °F. This depends on the refrigeration temperature; lower temperatures would provide higher savings. Main constraints to this implementation include processes with varying cooling water temperatures and the ability to transport refrigerant liquid/vapor to the end-user from the central utilities area.

Minimizing operation of multiple compressors is another low-cost energy opportunity. A compressor, being a turbo-machine, has an operating curve and an associated efficiency at part-load conditions. Centrifugal machines have inlet guide vanes, speed control and hot-gas bypass as methods to match loads and continue reliable operation. Screw machines use variable speed control, slide valves, etc. Most plants are set up without a master controller and operation is very basic with full burden on the operators to figure out sequencing of machines. The net effect is we run more compressors than needed to meet the refrigeration load. I recommend “N+1” compressor operations and doing a study to identify the best configuration match between loads and which compressors to run to minimize overall system energy usage. Developing a basic matrix for operators to follow in different load conditions can easily save 3–5% of the energy, increase system reliability and reduce maintenance costs.

The refrigeration temperature may not be something that can be changed but I would highly recommend during the periodic energy assessment you ask what is the critical process or end-user that requires the lowest refrigeration temperature. Follow up that question with a discussion on possibly raising that temperature from the design or current operating conditions. Processes change over time and as our reactors become more efficient, so the need for the refrigeration temperature may also change. It’s possible in certain systems to save as much as 1.5% per °F increase in refrigeration temperature.

To summarize, a significant amount of energy system optimization can be done in refrigeration and chiller systems, including pinch analysis, and this article just scratches the surface. Hopefully, I have stirred enough interest for you to undertake an investigation to better understand and operate your refrigeration and chiller systems so that they don’t feel neglected and run down.

Riyaz Papar, PE, CEM, is director, Global Energy Services, at Hudson Technologies Company, Pearl River, N.Y. He has more than 20 years of experience in industrial energy systems and with best practices. He also is a U.S. Department of Energy (DOE) Steam Best Practices senior instructor and a DOE steam energy expert. He has provided energy consulting services in 100+ industrial plants in the U.S. and internationally. You can email him at [email protected].

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