Charge minimization is essential for flammable refrigerant RAC system designs. One of the solutions is using small, mini or micro channel heat exchangers. Nowadays, cost-effective air coil heat exchangers with small and mini channels have been becoming widely available. However, more circuits must be adopted to reduce the pressure drop of refrigerant flow in the smaller channels.
An example from our RefrigLab cloud eTools ( www.refriglab.com ) client is used with their kind permission to show the effect of coil tube diameter on the unit charge and coil circuiting number. An air cooler unit with propane/R290 as refrigerant employs a 18cc/R290/MLBP/POE-600cc compressor operating for 3-7 deg.C of cold product storage at 35 deg.C/65%RH ambient condition. The unit cooling capacity and energy efficiency are about 1.6kW and 2.0 respectively. Four copper tube size coils were used for the exercise during the developing stage, 9.52mm (3/8"), 7.94mm (5/16"), 7mm (0.276") and 5mm (0.197") outer diameter.
The obtained results show that, with the decrease in the coil tube outer diameter, propane/R290 charge quantity (left chart in the title image) decreases linearly, but the evaporator coil circuiting number (right chart in the title image) increases sharply. In addition, the difference in the cooling capacity and COP is less than 1% among the different tube coils.
If you are interested in our cloud eTools, please Register at the website www.refriglab.com and take free trials. This document is a copy of my LinkedIn post at the following link and you may access my other posts at LinkedIn.
https://www.linkedin.com/pulse/small-tube-coils-help-propaner290-charge-reduction-rac-jeff-wang/
The results shown above and attached are for your comments. Are there any other issues with the use of mini/micro channel heat exchangers in refrigeration and air-conditioning units?
I think it depends on the location of the gas leakage, if any. Normally, to avoid the effects of surface tension, inner diameter can suggested as 6mm.
Thank you, Dr Ravindran!
We have air coil heat exchangers with tube diameter 19/16/12/10/8/7/5mm and micro channel heat exchangers with mini/micro channel sizes. I don't understand the reason that the inner diameter is suggested about 6mm as of the effect of surface tension. Can you please put a bit more detail?
Dear Jeff,
Based on my experience, along with smaller diameter tubing, if one uses smaller quantity of oil in the system, it reduces amount of charge required in the system. However, in order to reduce quantity of oil either one has to go for smaller compressor, smaller oil separator so on so forth.