How to optimise light commercial hydrocarbon systems through component design – part 1

John Prall, Technical Support Engineer, Embraco North America, discussed how to adjust a refrigeration system to handle the hydrocarbon refrigerants R290 and R600a to meet the 150g-charge limit. Prall suggested that using natural refrigerants is a prime solution to meeting pending DOE standards for commercial appliances and potential EPA bans on R507A, R404A, R407A/C/F, and R134a in new and retrofit vending machines and retail food refrigeration equipment in 2016. Prall noted that while this date is not set in stone, the industry should still move forward as if it was.

 

 

Prall highlighted that switching from commonly used HFCs to hydrocarbons R290 and R600a offers significant refrigerant charge reduction, even without adjusting other parts of the refrigeration system because hydrocarbons have more capacity per volume than the refrigerants they replace.

 

Prall gave a brief overview of the refrigeration cycle and explained in detail what adjustments to reduce refrigerant charge can be made with the different components once the compressor of a system using HFCs is replaced with one designed for use with hydrocarbons. He noted in each case, that a good starting point to the optimisation process is with components of similar size to those used with the HFC refrigerant. 

 

 

To evaluate the evaporator, one must understand the overall temperature differences (TD) of the system and superheats as well as pay attention to airflow and fin design. With regard to size of the evaporator, Prall highlighted that evaluating TD will aid in finding the right balance between efficiency and refrigerant charge. Superheat plays a role in evaporator design as well; too much superheat is an indication that the evaporator is too large and that the boil is ending too soon or that there is a restriction in the system that is increasing the pressure drop in the evaporator. Too little superheat could be an indicator that the evaporator is too small and that there is too little pressure drop in the system, in this case, increasing the length of the capillary tube could aid in increasing pressure drop.

 

If a design with a flooded evaporator is desired, it is important to note that this will increase refrigerant charge because there is more liquid in the system. 

 

Prall highlighted that airflow is critical to the behaviour of the evaporator as well. It is important to ensure as much air is moving across the evaporator as possible and that it is not spilling into other areas or bypassing the evaporator. Increased airflow creates a lower TD and allows for a smaller evaporator with reduced internal volume and increased heat transfer. 

 

Prall described fin design as the most important point in removing heat from the evaporator. If the fins are too tight, there is the risk of ice build up, requiring more defrost cycles, particularly in low temperature applications, though as fin spacing is increased, effectiveness is reduced. In addition, having cut or wavy fins allows for better heat transfer.

 

 

Embraco is a global market and technological leader in hermetic compressors for refrigeration, with their mission to provide innovative solutions for a better quality of life. As one of the pioneers in the development of environmentally friendly products, Embraco reinforces its commitment to global sustainability by offering a full range of high-efficiency compressors and condensing units. 

 

With factories in Brazil, Italy, China and Slovakia and productive capacity of 36 million compressors per year, Embraco also produces electronic systems to make intelligent household appliances.