©2013 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 55, no. 11, November 2013.
By Douglas T. Reindl, Ph.D., P.E., Fellow ASHRAE
About the Authors
Douglas T. Reindl, Ph.D., P.E., is a professor and director at the University of Wisconsin-Madison’s Industrial Refrigeration Consortium in Madison, Wis..
Virtually every built-up refrigeration system used in commercial or industrial applications has multiple compressors available for operation to meet loads. One of the challenges in designing and operating a system with multiple compressors is deciding how best to sequence their controls to meet variable refrigeration loads while avoiding inefficient compressor performance at part-load to maintain high system efficiency.
This article discusses typical part-load efficiency characteristics for screw and reciprocating compressors commonly found in built-up industrial refrigeration systems. It includes recommendations for sequencing and control strategies that enable efficient system operation.
Industrial refrigeration systems come in all different sizes, configurations, and arrangements. The nature of the food production and storage facilities this technology most frequently calls “home” are quite varied, and nearly all of the refrigeration systems installed in these applications are custom-engineered to meet the specific thermal requirements of loads within the facility. Individual owners establish specific requirements for their refrigeration infrastructure, and the relative importance of these requirements vary on a plant-by-plant basis. The requirements can be grouped into two categories “non-negotiable” and “negotiable.”
Non-negotiable requirements dictate that the refrigeration system must be: safe and able to meet the magnitude and temperature requirements of coincident load(s). Safety is achieved by ensuring the systems are designed, constructed, and operated in accordance with the latest applicable standards, codes, regulations, and industry best practices. Guaranteeing the refrigeration system can reliably meet its loads requires appropriate sizing of key components including compressors. In some cases, achieving the ability to reliably meet loads may necessitate redundant compressors. Negotiable requirements can include capital cost, operating costs, system complexity, extent of redundancy, and energy efficiency.
Depending on the temperature requirements and magnitude of the loads, a refrigeration system might be configured to operate as a single-stage or multi-stage compression system. As the temperature requirements for given loads decrease, systems generally transition from single-stage compression to two-stage compression to overcome limitations of the compression machines themselves or to maintain an acceptable system operating efficiency. Jekel and Reindl1 provide more complete coverage of single-stage vs. two-stage compression systems.
Let’s look at the compression technologies most commonly used in multiple compressor industrial refrigeration systems today, and consider the factors that influence their operating efficiency. The overall objective of this article is to provide guidance on compressor sequencing and controls to maintain high efficiency without compromising the system’s ability to meet thermal loads.
Compressor Technology Options and Capacity Control
Because industrial refrigeration systems preferentially use anhydrous ammonia as the refrigerant, the compressor technology options are necessarily limited. The compressors used in ammonia refrigeration systems are an open-drive configuration to prevent ammonia’s corrosive effects to copper from damaging the electric motor’s rotor and stator windings. Industrial ammonia refrigeration systems principally rely on positive displacement compression machines including: reciprocating, rotary vane, single screw and twin screw.
By far, screw compressors are the most common technology in the industrial refrigeration market today. Reciprocating compressors can still be found in smaller systems and for specialty use in larger systems (e.g., pump-out) and the rotary vane compression technology is being phased-out of service due to its low operating efficiency and relatively high maintenance costs. Table 1 provides a summary of the compression technologies found in today’s industrial ammonia refrigeration systems.
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